Here is a list of noise generating devices from metal industry. Also learn about how to control noise from these devices in metal industry.
1. Lathes and Milling Machines:
Noise may be generated in lathes and milling machines from three sources:
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1. Vibration of the workpiece.
2. Tool chatter.
3. Machine noise (gears, etc.).
One recent study found that the workpiece itself radiates most of the noise involved in cutting operations, especially where stainless steel and light metals are used. This noise may be reduced by loading the workpiece with damping plates.
Only the free surfaces are used for damping the workpiece. The stiffness of the plates in combination with the loading used has to be adjusted to the stiffness of the workpiece. Using this approach, a noise reduction from 108 dBA to 85 dBA was reported.
Tool chatter and also machining vibrations may often be reduced by the use of a lubricant or by modifying cutting parameters: cutting, speed, depth of cut, etc.
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Lathes and Automatic Screw Machines:
Most noise generated by a lathe or automatic screw machine comes from the gears and drive train. Noise emanating from the tool-chip interface should not be of concern in most cases. The solution to a gear noise problem is in the use of partial enclosures.
One-pound sheet lead or lead/foam material laminated to any material used for drywall construction can be a successful sound barrier built around gear cases. Hinged doors or ports should be incorporated in the design of the enclosure to provide access to speed and feed controls.
The access doors should be reasonably air tight to be an effective noise seal. The hinge side of the door should be gasketed with leaded vinyl sheet attached to the door and the jamb. The top, bottom and the side opposite the hinge should also be gasketed with a lead-loaded material. Smaller gear boxes can generally be quieted through the use of pillows made of lead-loaded vinyl and acoustical wool attached to their top.
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Heavy gears such as those on hoists and crane equipment generate high noise levels. The resonance of these huge gears can be substantially reduced by applying damping materials such as lead-loaded epoxies or lead-loaded vinyl sheets to the gear face.
2. Drills:
Generally, very little noise is generated by drill-workpiece interaction. Where excessive noise is observed due to high speed operation, the use of a lubricant is recommended to reduce cutting forces. Occasionally resonant vibrations of the workpiece will generate high noise levels; this problem may be solved by applying a vibration damping treatment to the workpiece.
In most cases where excessive noise is observed for drilling operations, the primary cause is the gear drive system. Often, the problem is worn gears and gear maintenance will solve the problem. For some operations, air is used for chip removal. This noise may be reduced by use of a thrust air silencer.
3. Grinding:
A general fundamental analysis of grinding noise is necessary in order to investigate potential modifications to the grinding operations as they may relate to noise reduction. In view of the apparent lack of published literature on the subject, a generalised analysis was made and a series of experiments were performed which are summarised here.
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The energy associated with a grinding operation is transformed directly into three basic energy forms:
The total energy required for a material removal operation may be estimated from motor power consumption. The energy required for material removal could be calculated from consideration of material strength properties.
The vibrational energy induced into the system (both grinding wheel and part) is dependent upon the following parameters:
1. Material removal efficiency. Clearly the more efficient the material removal, the lower the resulting vibrations. For conventional grinding systems, however, the influence of efficiency is slight for relatively large variations in the grinding abrasive.
2. Amount of material removed.
3. Velocity of grinding surface.
The near-field sound pressure levels due to vibrations induced by grinding may be calculated above the panel coincidence frequency from-
Lp = Lv – 20 log f + 150
where,
Lp = sound pressure level, dB
Lv = vibration level, dB re 1.0 gram
f = frequency, Hz.
The coincidence frequency for steel plates is-
fc = 500 ÷ t
Where,
fc = coincidence frequency
t = thickness in inches.
Below the coincidence frequency, the part sound radiation will be 3-4 dB/octave less than predicted by the equation. The equation is valid only for frequencies with wavelengths less than the part dimension.
Using this analysis, vibration levels of a part may be measured and the investigator may determine if the primary noise source is radiation of sound from the part or some other machine element.
An experimental was conducted on an OD grinder with a 3″ wide, 0.4″ thick strip of brake lining material to establish the influence of the amount of material removed on the sound level. An increased sound level with depth of grind was found.
An experiment was conducted on a 5″ wide strip of brake lining material to establish a relationship between surface area of the vibrating strip being ground and sound level.
The following sound levels were measured:
The lack of engineering feasibility for some grinding operations has been recognised by industry.
Examples of operations and equipment that might not be amenable to sufficient noise control, either by engineering methods or administrative controls, include certain grinding operations, such as those involved in processing structural steel.
4. Air Carbon Arc Gouging:
Air carbon arc gouging (generally called arc airing or scarfing) is a manual operation which utilises a carbon arc electrode and compressed air to remove welds and surface blemishes or to prepare surfaces for welding. The electrode holder, which is hand held, is designed to retain the carbon arc rod and expel a stream of compressed air toward the arc.
The carbon rod melts the metal and the air stream blows it away. In actuality, it is welding in reverse. The noise associated with the scarfing operation is twofold. First, there is the noise due to an electrical spark discharge in air. The second noise source is the stream of compressed air used to remove molten metal.
Noise due to an electrical spark is a complex problem, due to the complexity of the source. An electrical spark generates acoustic N waves that have non-linear propagation properties. The non-linearity of the propagation of the N waves can have serious effects on common acoustical measurements of the spark. Adjusting the voltage used to create the spark will change the noise output.
The spark gap dimension, d, will also vary the acoustical energy output. The efficiency of electrical to acoustical energy conversion increases at a rate proportional to Log (d). In industrial applications, where large voltages are required and spark gap dimensions vary, control of these variables is not feasible as a means of noise control.
The air noise is secondary to the spark noise in a scarfing operation. The air noise generation is proportional to flow velocity to the eighth power. Thus, small reductions in velocity may result in significant noise reductions. Any such noise reduction, however, would not reduce the overall noise of the operation due to dominance of the noise resulting from the electrical spark.
It may be possible to control employee noise exposure of the arc airing operations to some extent by workspace or administrative control techniques, such as:
1. Reverberation control.
2. Localised barriers or shields.
3. Reducing requirements for the process.
4. Layout.
5. Scheduling.
6. Job rotation.
5. Welding:
With the exception of carbon arc operations welding processes generally do not result in excessive employee noise exposure. This is due to the fact that welding sound levels often do not exceed 90 dBA and also because welding operations are intermittent and exposure times are low.
The mechanism of noise generation has been deduced as being rare fraction, ionisation and/or chemical and molecular decomposition of the air due to the intense localised heat associated with welding.
6. Hammering:
Impact noise is generated when metal parts are hammered to:
1. Force parts into place.
2. Finish weld surfaces.
3. Shape parts.
This noise may potentially be reduced by:
1. Reducing the vibration response of the structure being impacted by means of vibration damping.
2. Reducing the impact forces.
3. Minimising hammering requirements.
When parts are not of standard size or in a fixed location, vibration damping would generally not be practical. When operations are stationary, involve fixed dimension parts and utilise production fixtures, vibration damping may be feasible.
The use of ‘soft’ hammers, with plastic or rubber coated heads would serve to reduce impact forces. This approach has been tried in some industries. Often this substitution does not provide sufficient energy to perform the hammering function; however, it has worked in some cases. One such hammer is made of ‘compothane’ which features a unicast head which can’t fly off, mark or spark. It is said to outlast rubber, nylon, rawhide or lead hammers by 10-20 times.
In some cases, the necessity for hammering may be minimised by:
1. The use of alternate assembly methods.
2. Improvement of weld quality.
3. Designing components to assemble better by means of better control of tolerances.
7. Shot and Abrasive Blasting:
The major noise source where abrasive and shot blasting is performed is the compressed air and abrasive leaving the nozzle. The noise created by high velocity turbulent flow from an air jet is a function of air velocity and air nozzle diameter.
Until recently, there appeared to be no nozzle design modification which could be considered to provide feasible noise reduction. Differences in sound generation between ¼” and ½” nozzles are slight. Nozzles of ⅛” diameters do not provide adequate abrasive flow for most applications. Pressure reductions from 100 to 80 psi do not result in any noise reduction; below 80 psi, blasting functions are seriously hampered.
It is evident that sound levels build up due to reverberation within blasting rooms due to hard wall surfaces. To determine the effect of the installation of sound absorptive material within a blast room approximately 20′ × 10′ × 8′ in size, the ceiling was treated with a layer of 3½ ” glass fibre.
Statistical sound measurements were made for an 8 hour work day at a location approximately five feet from the operator. The mean sound level was reduced from 107 dBA to 102.5 dBA and the daily noise dose was reduced from 118 to 45 per cent. The problem is that the glass fibre must be protected from erosion by the blasting in the severe environment of the rooms.
This is most difficult, if not impossible. Placing a solid rubber sheet in front of the glass fibre would completely negate its acoustical properties. One solution is to face the glass fibre with a screen and to replace the glass fibre frequently. Other possibilities would be to face the fibre with a perforated sheet metal or perforated rubber sheet (the holes should provide a minimum 15 per cent open area).
Typical blast hoods reduce noise levels at the employee’s ear from 10-20 dBA. Thus, the proper use of ear plugs in conjunction with a blast hood would provide adequate protection of the operator in the absence of feasible methods of reducing the blasting noise.
However, noise levels generated by the respirator, measured inside the hood at maximum air flow, not to exceed 80 dBA.
8. Cut-Off Saws:
Noise from cut-off saws from two vibrating sources:
1. The saw blade.
2. The workpiece, which is usually largely unconstrained.
Since both the workpiece and blade are noise contributors, an acoustical enclosure is generally the best approach to noise control.
Workpieces pass transversely through slots in the enclosure. Flaps of lead-loaded vinyl close off the opening and reduce to a small amount the unavoidable leakage area when a workpiece is present. The front, above saw bed height, is closed by two doors whose surface is mostly ¼” clear plastic (polymethylmethacrylate).
This plastic provides very good vision. The doors close with a gap the width of the control lever. Each door has a flap of lead-loaded vinyl about 3 inches wide to close the gap. The lever pushes aside the flaps only where it protrudes. Thus, the leakage toward the worker is greatly reduced. A 13 dBA noise reduction was achieved.
For operations involving the sawing of thin-walled parts, it may be possible to design damping fixtures to be applied to the part.
Where the noise generated by the workpiece can be reduced, the blade must be treated also. This may be accomplished by installing a large stiffening collar on the blade, extending as near the base of the teeth as allowable by the stock thickness (the collar must cover 30 per cent of the blade to be effective). A layer of vibration damping material, ⅛” thick should be installed between the collar and blade.
As an alternative to the above, the centre portion of the blade not utilised for cutting may be treated with a constrained layer damping treatment. This treatment should cover as large an area as possible (30 per cent is the minimum for effectiveness) and must not come into contact with the wood being cut. The treatment consists of a sandwich structure with a viscoelastic layer and an outer layer of 18 GA steel. The construction should be bonded with an epoxy film.
A recent invention shows great promise in providing a technique for damping circular saws. The new damping means attaches to the saw table or base and employs low pressure air from a shop air supply or a small blower to force a pair of dampers against the sides of the saw in much the same way as disc brakes work.
Noise levels peaking between 103 and 108 dBA have been reported to have been decreased to approximately 90 dBA when the damper was applied to the saw. The device is still in the prototype stage and plans for commercial production are now being studied.
9. Band Saws:
Noise may be generated when cutting metal with a band saw in three ways:
1. Vibration of the saw blade.
2. Chatter between the part and saw table.
3. Vibration of the workpiece.
Generally the peak frequency of the noise is the tooth passage frequency, equal to the product of the blade speed (inches per second) and the number of teeth per inch. Thus, variation of blade speed provides one potential method of noise reduction; however, it is seldom possible to alter the speed sufficiently to achieve a significant noise reduction.
A slight (1-2 dBA) noise reduction may be achieved by the installation of rubber facings covering the pulley wheels and/or blade guide wheels. Enclosure of the portion of the blade not involved in the actual cutting may provide additional noise reduction. The saw damper is also applicable to band saws.
Noise produced by chatter between the workpiece and the saw table may be reduced by the placement of a wear resistant rubber material on the table. This phenomenon occurs most frequently in cutting large lightweight parts which bend due to lack of rigidity during cutting. The most common source of noise in band saw operations is vibration of the workpiece. This noise may be reduced by the application of a damping plate to the part.
The noise level of the shear operation is the result of four sources:
1. The hold down impact on the stock.
2. The impact of the blade on the metal.
3. The stock ‘slap’ and vibration on table after shear.
4. Part drop impact.
There are two methods for reduction of the noise from the hold down impact on the stock. The hold downs may be covered with a wear resistant rubber material or the control cylinder should be adjusted to decelerate the hold down descent.
Blade impact forces generally do not generate excessive noise on manually fed shears. Blades which cut at an angle reduce peak shear forces and are quieter.
Blade impact noise may exceed 100 dBA on high speed continuous feed shears. Since these shears do not require constant attention, noise exposure may most easily be controlled by isolating the operator. The noise may also be reduced by placing the blade at a slight angle or by means of a machine enclosure or cover.
The noise generated by stock ‘slap’ and vibration may be reduced by the shear table with a wear-resistant vibration damping material and by maintaining the hold downs in proper operating condition. Restraining rollers may also be used.
The noise producing impact forces are proportional to the kinetic energy, ½ mass (velocity), which is proportional to the drop height. Therefore, reducing the drop height will result in a sound level decrease. Effective monitoring of drop heights at both the initial cut drop and stacking drop is necessary to control the sound levels of the operation. The drop panel may also be lined with a wear-resistant rubber such as old conveyor belting.
10. Ultrasonic Welders:
The noise from ultrasonic welders may range from 80 dBA to 110 dBA. Due to the intermittent nature of this noise, welders seldom create an OSHA exposure problem. However, the very high frequency audible squeel which is emitted may require abatement due to annoyance considerations.
The first approach to sound reduction is to insure that everything possible has been done to eliminate the offending sound via control settings, fixturing or nexting. This may include everything from varying pressures, times and amplitudes to part clamps or nodally mounted horn clamps.
Where this approach does not provide adequate reduction, an acoustical enclosure may be employed. Noise reductions from 110-120 dBA to 85-90 dBA have been reported. It is often asked whether ultrasonic sounds are more damaging to humans than audible sounds.
In assessing available research on ultrasonic sound, the US Environmental protection agency concluded that –
‘Exposure to high levels of ultrasound (above 105 dB SPL) may have some effects on man; however, it is important to recognise that a hazard also arises from exposure to the high levels of components in the audible range that often accompany ultrasonic waves. At levels below 105 dB SPL there does not appear to be significant danger’.
11. Thrust Air Noise:
In applications which require a jet of air to do useful work, effective thrust must be maintained to transfer force to the object. One such application is the ejection of parts from press dies, where open air jets are typically used.
This noise may be reduced by the following methods:
1. Air ejector mufflers.
2. Directional control of air flow.
3. Regulation of air.
4. Barriers.
5. Mechanical ejectors.
With a seventh power dependency on air velocity associated with noise generation, but a direct relationship between thrust and velocity, and effective way to reduce high noise levels from nozzles is to obtain a nozzle which would exhibit somewhat lower exit velocity and reduce turbulence while maintaining high effective thrust. Several commercially available silencers have been developed to serve this function.
A recent study showed three commercially available silencers capable of producing thrusts of 32 ounces with sound levels from 5 to 10 dBA less than those associated with a free air jet of identical thrust.
Air turbulence and resulting noise is created when high velocity air passes over irregular surfaces such as the edge of a die. Air jets should be directed toward die surfaces where minimum irregularities are encountered. It should be pointed out that the use of ejector silencers will also reduce noise of this type, since air flow is more directional with the silencers.
Air jets are required to eject parts only during a portion of a press cycle; however, continuous air flow is used on many presses. Air flow should be regulated to operate only during the part ejection portion of the cycle. In addition to noise reduction, a potential annual cost savings per press may be realised.
It should also be recognised that localised shields which block the line-of-sight path between a high frequency noise source such as an air jet and an operator may reduce sound levels by approximately 5 dBA. Such shields may be constructed of transparent plastic materials.
As an alternative to part ejection by air, mechanical ejectors may be installed. This approach would require significant engineering design and may create maintenance problems, however, may achieve cost savings on a long term basis.
12. Part Cleaning and Drying:
Sound levels of 95-100 dBA are typical of tumbling barrels used to deburr small parts. The majority of the noise is due to part-part impacts. This problem may be solved by simply installing a lid or cover on the mouth of the tumbler. Typically a lid may be fabricated of ½ to 1½” plywood carefully fitted and tightly gasketed.
Plastic covers resembling kitchen bowl covers have also been used successfully. One plant incurred a serious problem in covering their tumblers: the stainless steel parts which were being deburred became tarnished. The problem was deduced as being due to moisture which became trapped in the closed system and the problem was remedied by adding corn cob to the tumbler.
Often, abrasive stone is added to the tumbler. The result is a very significant decrease in part-to-part noise; however, the barrel itself will have an increased tendency to radiate noise due to vibrations induced by the part/stone fill hitting the steel walls. This noise problem may be solved by either lining the interior with a wear resistant rubber material, applying a vibration damping compound to the barrel or by lagging the barrel.
The combinations of the two treatments listed above have reduced tumbler noise from 98 dBA to 78 dBA. Vibratory type tumblers present another potential noise problem. The part-to-part component of the noise may be reduced by using a high abrasive stone-to-part ratio. The mechanical condition, unbalances, bent shafts, loose parts etc., can cause a rise in sound levels of 5 dBA or more.
Unfortunately, in such vibratory system, this mechanical noise recurs shortly in such a vibratory system, this mechanical noise recurs shortly after maintenance and the frequency of maintenance may become greater than is justifiable for noise control. The only other solution is to use an acoustical enclosure or another type of vibrator.
In some metal fabrication operations, water is used to wash or cool parts. Generally, the water is removed by either compressed air jets or by an air curtain (a pipe with a series of air holes). Noise levels above 100 dBA at 3′ are typical. The mechanism of drying is generally blowing the water from the part rather than evaporation.
Where parts are irregularly shaped, the following approached may be considered:
1. The high pressure air system may be replaced by a high volume, low pressure system, such as supplied by a small blower.
2. A transvector air nozzle or air curtain may be used.
3. A silencer designed to provide concentrated air flow may be used.
4. The air or part may be heated to increase the evaporation rate.
5. The operation may be enclosed.
For regularly shaped parts, the above approaches may also be considered; however, the best solution is to replace the air drying system with mechanical wipers. In addition to providing virtually silent operation, this approach may save thousands of dollars in compressed air costs annually.
13. Gears:
Geared systems may be extremely noisy. Gears consist of assemblies of toothed wheels used for the purpose of torque conversion, speed change or power distribution.
The main sources of noise in geared systems are:
1. Impact caused by tooth contacts.
2. Mechanical imbalance of the gear assembly.
3. Friction due to the contact motion of the tooth.
4. Variation of radial forces.
5. Air and oil pocketing.
Principles Used for Reducing Noise in Gear Systems:
Some of the principles used for reducing noise in gear systems are:
1. Selection of a suitable type of gear (for instance, a helical gear is quieter than a spur gear and a worm gear is still quieter, but is restricted to low speeds).
2. Accuracy of manufacturing (high accuracy in all gear parameters results in quieter gear systems).
3. Detuning (when the operational frequency of the gear assembly coincides with the natural frequency of the structural members, resonance takes place amplifying the noise; to avoid resonance, the structural members are detuned to other frequencies by either stiffening or mass loading).
4. Damping (introduced by using gear material of high internal damping).
5. Vibration isolation.
6. Enclosing the gear assembly (with particular attention given to cooling and heat transfer requirements).
The use of rawhide, nylon and laminated phenolic gears is often proposed as a method of noise reduction. Non-metallic gears should not be used for applications where the above relationship yields solutions of less than 1.0.
14. Sheet Stacking and Unstacking:
The fabrication of many parts begins with a metal sheet as the raw material. Initial production operations involve unstacking and stacking of the sheets.
Noise may be generated due to the following:
1. Unstacking – Air jet used for sheet separation.
2. Stacking –
(a) Impact of sheet on rigid stop or metal inertia block.
(b) Impact of sheet on stack.
The following approaches to noise control may be considered:
An open air jet is often used to separate sheets as they are unstacked, generating noise levels from 95 to 105 dBA. The use of thrust muffler would not be effective since the dominant noise generating mechanism is turbulence generated as the air passes the sheet (dipole noise) rather than noise from the free air jet (quadrupole noise). A silencer would not significantly alter the air-sheet interaction.
A very attractive solution to this problem both from a noise control and energy conservation point of view is to replace the air separator with a magnetic fanner. The only other approach to noise reduction would be the use of an enclosure or barrier.
As the sheet leaves a conveyor line, it will be decelerated by a positioning stop prior to dropping onto a stack. Generally, the stop is metal and the impact induces noise producing vibrations into the sheet. This noise may be reduced by installing a wear retardant rubber facing on the face of the stop. The stop may also be spring loaded to provide additional cushioning.
Excessive noise may be generated as the sheet free falls onto a stack. The noise generated is directly proportional to the drop height (potential energy = weight × height). Thus, one method of noise control is to minimise this distance.
Another approach is to install a bin with four solid sides tightly around the stack. As the sheet enters the bin, air will be trapped between it and the stack, allowing the sheet to float quietly into position.
Impact noise generated when parts are dropped into tote bins may exceed 115 dBA. Although this is a short duration noise, it may add considerably to the background ambient in work areas. Many plants have hundreds of tote bins; thus any noise control solution must be applicable for implementation for a large number of units.
The following solutions may be considered:
1. The interior floor may be lined with a wear retardant material to cushion part drops. Wood and sand have also been used in some plants for this purpose.
2. Replace solid metal bins with heavy duty wire mesh bins.
This approach may have several non-acoustical benefits:
(a) They are lighter weight.
(b) Space savings (the bins may be stacked).
(c) Visibility through empty bins.
3. The exterior of the bins may be treated with a vibration damping or a spray-on or trowel-on damping compound.
4. Where hot parts are dumped into bins, items 1 and 3 above are not applicable. An alternate design is to install a wire mesh false for in the bin, to isolate the dropping parts from the bin panels.
15. Impact Riveting:
Impact riveting operations are responsible for some of the highest noise levels found in American industry; up to 120 dBA. Impact riveting involves the use of a pneumatic hammering tool with forms the rivet head by means of repeated impacts.
Three conceptual approaches to noise abatement of the impact riveting operations may be considered:
1. Substitution of fastening process.
2. Isolation by relocation or rescheduling.
3. Vibration damping of workpiece.
Noise reduction could be achieved from substitution of impact riveting processes with another fastening process, such as hot or cold squeeze riveting, Huck bolts, bolts or welding.
A new fastening system has been developed which uses an orbital and planetary action which can form flat, crown and conical heads. It is virtually silent.
Isolating riveting operations from employees not physically involved in the process may be considered by:
i. Physical location of the riveting fabrication areas within a separate building or work area.
ii. Performing riveting operations during night-time shifts when other workers are not present.
A significant noise reduction may often be achieved by positioning a vibration damping material in contact with the work piece being riveted. Partial coverage of a panel provides damping efficiencies far in excess of the percent of area covered. It is most important that each separate panel or structure attached to the part being riveted by damped in order to achieve significant noise reduction.
A limitation in noise reduction is the fact that vibrations are attributed to both forced impacts to the panels as well as resonant panel response. Vibration damping is only effective in reducing resonant vibrations and the impact mechanism will inherently result in a substantial vibrational energy which cannot be reduced by means of vibration damping in any manner.
It should be pointed out that impact riveting operations cannot feasibly be replaced or abated for some operations because of several reasons, including:
1. The superior strength of hot impacted rivets.
2. The mobility of impact riveting equipment as opposed to the very large size of squeeze riveting units.
3. The high costs involved.
16. Cranes:
The operators of open overhead cranes are often exposed to excessive sound levels by virtue of exposure to work areas over which the crane passes. Where crane operators are found to be exposed to excessive noise, it is generally the case that floor level employees also encounter noise exposure.
In many cases, a plant-wide noise programme will accomplish adequate noise reduction and the exposure of the crane operators will be sufficiently reduced. Where crane operators remain overexposed to noise after plant machinery treatment, the crane cab may be modified.
Crane operators may be isolated from exterior noise by installing safety glass or clear plastic windows in the cab. The windows should be operable or angled and extend beyond the cab for visibility. Visibility should never be sacrificed for noise control. With the use of windows, the cab should be ventilated and probably air conditioned.
Additional noise reduction may be achieved by installing a sound absorptive material on the interior ceiling and walls of the cab. A carpet may also be used. These measures would provide thermal as well as noise control benefits. Some cranes are found to generate noise themselves.
17. Warning Signals:
Sirens and whistles are utilised as warning signals throughout the mill. The sound levels of these sources must be high to insure detection and employee safety.
In many cases, the sound levels of a siren or whistle may be reduced while maintaining an adequate warning level if the following concepts were implemented:
1. The siren may be a warble or intermittent tone, similar to British police sirens. The detection limit of an oscillating tone is greater than for a steady sound and the overall sound level could be slightly reduced and the exposure time lowered.
2. The peak frequency may be reduced from 500 Hz to 250 Hz, since the ear is equally sensitive to 250 Hz as 500 Hz; however, the 250 Hz level of equal loudness would be 6 dB lower on the A-weighted scale.
3. The single siren may be replaced by two more directional sirens.
While the ear normally is most sensitive to sounds at 4 kHz, warning signals at that frequency are not recommended, since that range is also the characteristic hearing loss range due to noise exposure. Thus, workers with a PTS (permanent threshold shift) or TTS (temporary threshold shift) would not detect the high frequency siren noise.
A final point is that the siren level can be reduced as the background noise level is decreased, since the detection level is proportional to the background noise due to masking effects.
Techniques for Reducing Various other Machinery Noise:
Total elimination of noise can be very difficult, but machinery noise can be reduced to safe, livable limits. Sheet lead, lead-impregnated vinyl, sheet lead sandwiched between two layers of polyurethane foam, lead-loaded epoxy, and sheet lead laminated to a variety of substrates and other specialty lead products offer excellent sound isolation properties and can be matched perfectly for specific tasks of controlling unwanted noise.
In approaching a noise control problem, the type of machine must be considered. Fully automatic machines may be completely enclosed in a sound-insulating structure. Doors may be built into the enclosure to permit necessary manual adjustments or tooling changes. Manually-fed equipment, however, must provide for operator access so it is not possible to cover the machine completely. In such cases, efficiently constructed, partial enclosures are capable of reducing noise levels to safe limits.
Sheet lead for use in sound insulation applications is normally specified in pounds per square foot. In most cases, weights normally used for noise reduction applications are ½ lb and 1 lb. Sheet lead can be cut with ordinary scissors, formed by hand and applied to surface with elastic type adhesives. Lead can be readily laminated to many substrates, including steel and aluminium sheet. These lead laminates can be formed, drawn or otherwise shaped.
Leaded vinyl sheet is another useful material for sound attenuation. One manufacturer’s adaptation of this product comprises two sheets of lead loaded vinyl laminated to a core of glass fibre cloth to give the material more supporting strength. This material is often used in conjunction with acoustical wool for reducing noise from machinery. In addition to reducing noise, leaded vinyl sheet is also used to damp vibration in resonating structures.
Sheet lead and polyurethane sandwich material can be used for sound conditioning areas, such as the inside of existing machine shrouds or guards. Lead/foam material can also be supplied with either two or three layers of sheet lead in the sandwich.
The material can be cut with scissors or simple steel rule dies when repetitive parts are needed. It can be readily shaped to contours and held in position with adhesives. Types are available with a sealed surface that will not absorb oils or coolants.
For structure-borne noise, lead-loaded epoxies make ideal damping compounds. The compounds are made by mixing powdered lead into epoxies. The material is usually applied with a trowel and can be used on resonating structures, such as machinery guards to reduce vibration and the resultant noise.
Reducing noise from machinery is still an emerging technique. In this ‘cut and try’ period, much must be borrowed and adapted from the manner in which noise problems have been solved by others.
Pneumatic shop tools produce impact noise. By enclosing the barrel of the tool in a leaded vinyl sheet and rock wool, some quieting can be achieved by muffling exhaust noises.
In some instances, large machinery guards of sheet metal have a tendency to set up a high-pitched ringing noise if stiffening ribs have not been added. This problem may be solved by laminating pieces of sheet lead to the guard by the trial and error method until the ringing is reduced or eliminated. Self-adhesive leaded vinyl sheet may also be used to line the guard with the same result.
High Pressure Pipes and Valves:
High pressure transmission lines and valves for liquids and gases may be silenced through the application of sheet lead or leaded plastics. By first wrapping the pipe with absorbent material such as glass fibre or mineral wool to act as a thermal insulator and then covering the material with sheet lead or leaded vinyl sheet, noise levels will be significantly reduced. The final installation step should be the sealing of all joints on the line or valve with a self-adhesive lead tape.
In actual practice, noise levels of 110 to 130 dB at a gas pressure reducing station on a pipeline can be successfully muffled to 100 dB by wrapping the line with a 2-in. thickness of fine yarn glass fibre and covering it with a single blanket of reinforced leaded vinyl sheet. Greater attenuation is attained if a second layer of glass fibre and leaded vinyl sheet are used.
Noise generated by the movement of great volumes of air can reach painful and dangerous levels. A leaded vinyl bellows installed between the intake of a large air compressor and the outside of a building will reduce the noise of the air movement within the enclosure. Air ducts, which serve as pathways for the intakes and exhausts, may be enclosed in glass fibre and sheet lead to further decrease transmission noise.
Air compressors, a source of irritating noise within an industrial plant, can be silenced by enclosing them in a shroud consisting of sheet lead or leaded vinyl laminated to any material intended for drywall construction. Louvered openings with baffles should be provided for the dissipation of heat build-up within the enclosure. Lead/foam material may also be used as a lining for the enclosure.
A source of highly distracting noise in production machines devoted to automatic assembly of spark plugs can be created by air-vibrated feeds on each of the units. The noise, caused when the steel spark plug shells are vibrated into position on the metal feed track, is like machine gun fire. This unavoidable, metal-on-metal noise is a nerve-wracking distraction for workers.
Because of the construction of the feed devices, it is impossible to completely enclose them. To solve the problem, the 2 ft × 4 ft area around each vibrating feeder is surrounded with curtains made from reinforced, lead-loaded vinyl. Even though the curtained area is left open at top and bottom, the noise at the individual feeds is reduced from 93 dB to 86 dB.
The hum of transformers located at a substation can be a troublesome noise. When faced with such a noise problem, a West Coast utility used lead adhesive-bonded to steel as the noise barrier because of its weight and because it did not tend to resonate. The barrier was made by laminating 4-lb (1/16 inch thick) sheet lead to 10-gauge steel with epoxy adhesive.
The enclosure and its frame were built to be free standing and isolated from the transformer. Within this sheet, a lining of a 2-in.-thick blanket of glass fibre is spaced about 1 inch from the inner face. Pipes and fittings piercing the barrier are isolated from the walls by neoprene gaskets. After the transformer radiators are remounted, the units show a noise reduction of 17 dB.
Lead-Loaded Fabrics Keep outside Noise Out:
When production facilities are expanded in a plant, additional office space is often necessary for the resultant increased clerical work. To keep office workers convenient to the production operation, yet keep noise problems to a minimum, offices are usually constructed at one end of the production area.
Since production area ceilings are high, office walls are often constructed of brick or acoustical partitions. The ceiling is then suspended from the underside of the plant roof. The office walls will usually prove to be good sound barriers. However, the sound-deadening capacity of the ceiling will be far below what is required for conversation, even when acoustical ceiling tile is used in the T-bar suspension systems. The ceiling will control sound originating within the office, but will not serve as a barrier to external noise.
Noise from production equipment can reflect off the underside of the plant roof and focus on the acoustical tile ceiling of the office. As a result of this unique focusing problem, noise levels within the office can be more distracting than those in the plant. Constructing full-height insulated office walls to the underside of the plant roof is usually uneconomical in high-ceiling areas. High cost of materials and labour, interruption to production, and modifications to fire control sprinkler systems usually make high-wall office systems prohibitive in cost.
However, using a lead-powder-loaded, vinyl-coated fabric can help reduce outside noise substantially. The material is limp, dense and non-porous, and is easily cut with a knife or scissors. It weighs about 0.9 lb per sq ft and is approximately 1/16 inch thick. Most important, the material can be installed easily over one office in an entire group, and its effectiveness evaluated before making a commitment for a total installation.
The lead-loaded fabric can be installed by plant personnel, quickly and easily, usually needing only oral instructions to explain the installation technique. Rolls of vinyl are unrolled on the floor, cut to length and laid directly over the T-bar suspension of the ceiling. No additional support is necessary.
Material is cut to go around suspension wires and edges of the fabric are overlapped to prevent sound waves from passing through the joints. Edges can be sealed with adhesive tape; access to the sprinkler system is maintained by not sealing them. Noise levels ranging from 95-100 dB in offices have been reduced after installation of lead-loaded fabrics to 75-80 dB.
18. Pumps:
Necessity for the pump occurs in all the manufacturing plants. Raw materials may be delivered to the plant as liquids. Many materials are liquids during all or part of the manufacturing processes. Water and sanitary services may require pumps as does steam-generating equipment, cooling and washing equipment, etc.
Pumps may be classified on the basis of the applications they serve, the materials from which they are constructed, the liquids they handle, and even their orientation in space. All such classifications, however, are limited in scope and tend to substantially overlap each other.
A more basic system of classification, the one used, first defines the principle by which energy is added to the fluid, goes on to identify the means by which this principle is implemented and finally delineates specific geometries commonly employed. This system is therefore related to the pump itself and is unrelated to any consideration external to the pump or even to the materials from which it may be constructed.
Under this system, all pumps may be divided into two major categories:
(i) Dynamic, in which energy is continuously added to increase the fluid velocities within the machine to values in excess of those occurring at the discharge such that subsequent velocity reduction within or beyond the pump produces a pressure increase; and
(ii) Displacement, in which energy is periodically added by application of force to one or more movable boundaries of any desired number of enclosed, fluid-containing volumes, resulting in a direct increase in pressure upto the value required to move the fluid through valves or ports into the discharge line.
Dynamic pumps may be further subdivided into several varieties of centrifugal and other special-effect pumps. Displacement pumps are essentially divided into reciprocating and rotary types, depending on the nature of movement of the pressure-producing members.
Energy savings in pumping systems account for 40 per cent power in the industrial sector and 15 per cent of the national consumption of India.
Centrifugal pump consists of a set of rotating vanes, enclosed within a housing or casing and used to impart energy to a fluid through centrifugal force.
Thus, stripped of all refinements, a centrifugal pump has two main parts:
(i) A rotating element, including an impeller and a shaft; and
(ii) A stationary element made up of a casing, stuffing box, and bearings.
In a centrifugal pump the liquid is forced, by atmospheric or other pressure, into a set of rotating vanes. These vanes constitute an impeller which discharges the liquid at its periphery at a higher velocity. This velocity is converted to pressure energy by means of a volute or by a set of stationary diffusion vanes surrounding the impeller periphery.
Pumps with volute casings are generally called volute pumps, while those with diffusion vanes are called diffuser pumps. Diffuser pumps were once quite commonly called turbine pumps, but this term has recently been more selectively applied to the vertical deep-well centrifugal diffuser pumps usually referred to as vertical turbine pumps.
Centrifugal pump is selected on the basis of discharge requirements and total head of the system. While selecting operating speed there is certain limitations from site conditions like available submergence etc. It is recommended that the speed should be so selected that pump has maximum achievable efficiency.
Reciprocating displacement pump is one in which a plunger or piston displaces a given volume of fluid for each stroke. The basic principle of a reciprocating pump is that a solid will displace an equal volume of liquid. For example, when an ice cube is dropped into a glass of water, the volume of water that spills out of the glass is equal to the submerged volume of the ice cube. All reciprocating pumps have a fluid-handling portion, commonly called the liquid end.
Power pump is a constant-speed, constant-torque, and nearly constant-capacity reciprocating machine whose plungers or pistons are driven through a crankshaft from an external source. The pump’s capacity fluctuates with the number of plungers or pistons. In general the higher the number the less capacity variation at a given rpm.
The pump is designed for a specific speed, pressure, capacity, and horsepower. The pump can be applied to horsepower conditions less than the specific design point, but at a sacrifice of the most economical operating condition.
Diaphragm pumps are displacement pumps with flexible membranes clamped at their peripheries in sealing engagement with a stationary housing. The displacement pumps are of two types—reciprocating pumps and rotary pumps. The central portion moves in a reciprocating manner through mechanical means, such as a crank or an eccentric cam or by fluid means, such as compressed air or liquid under alternating pressure.
An inlet check valve and an outlet check valve control the flow of pumped liquid into and out of the pumping chamber. A distinguishing feature of all diaphragm pumps is that they have no seals or packing and can be used in applications requiring zero leakage. They are also self-priming and can run dry without damage. In single-diaphragm pumps, the pumped liquid can have a lot of inertia if the suction and discharge lines are relatively long.
A pumping cycle consists of an inlet stoke and a discharge stroke. A simple accumulator on the suction (inlet) side of the pump will allow the pump to draw liquid from the accumulator while it simultaneously draws liquid through the suction line. If the discharge line from the pump is relatively long, the inertia of the liquid can be great, and can impose severe loads on the diaphragm and cranking means as the diaphragm enters the discharge stroke of a complete cycle.
Screw pumps are a special types of rotary positive displacement pumps in which the flow through the pumping elements is truly axial. The liquid is carried between screw threads on one or more rotors and is displaced axially as the screws rotate and mesh. In all other rotary pumps the liquid is forced to travel circumferentially, thus giving the screw pump with its unique axial flow pattern and low internal velocities a number of advantages in many applications where liquid agitation or churning is objectionable.
The applications of screw pumps cover a diversified range of markets: navy, marine, and utilities fuel oil service; marine cargo; industrial oil burners; lubricating oil service; chemical processes; petroleum and crude oil industries; power hydraulics of navy and machine tools; and many others. The screw pump can handle liquids in a range of viscosities, from molasses to gasoline, as well as synthetic liquids in a pressure range from 50 to 5000 lb/in2 (3.5 to 350 bar) and flows up to 5000 gal/min (1135 m3/hr.).
Because of the relatively low inertia of their rotating parts, screw pumps are capable of operating at higher speeds than other rotary or reciprocating pumps of comparable displacement. Some turbine-attached lubricating oil pumps operate at 10000 rpm and even higher. Screw pumps, like other rotary positive displacement pumps, are self-priming and have a delivery flow characteristics which is essentially independent of pressure.
Rotary pumps are rotary positive displacement pumps in which the main pumping action is caused by relative movement between the rotating and stationary elements of the pump. The rotary motion of these pumps distinguishes them from reciprocating positive displacement pumps, in which the main motion of moving elements is reciprocating.
The positive displacement nature of the pumping action of rotary pumps distinguishes them from the general class of centrigugal pumps, in which liquid displacement and pumping action depend in large part on developed liquid velocity. It is characteristic of rotary pumps, as positive displacement pumps, that the amount of liquid displaced by each revolution is independent of speed.
Also, it is characteristic of rotary pumps that a time-continuous liquid seal of sorts is maintained between the inlet and outlet ports by the action and position of the pumping elements and the close running clearances of the pump. Hence rotary pumps generally do not require inlet and outlet valve arrangements, as reciprocating pumps do.
Rotary pumps are useful in handling both fluids and liquids, where fluid is a general term that includes liquids, gases, vapours, and mixtures thereof, and sometimes solids in suspension, and where liquid is a more specific term that is limited to true liquids which are relatively incompressible and relatively free of gases, vapours and solids.
Reasons for Excess Power Consumption:
1. Wrong selection of pump.
2. Over design of pump.
3. Improper layout.
4. Old inefficient pumps.
5. Multiple smaller size pumps.
6. Ad-hoc decisions.
In a vertical turbine pump, the pump assembly is submerged in water. A cone type suction strainer is provided at the suction side to prevent entry of foreign materials. The pump assembly is driven by a vertical hollow shaft motor placed at the operating floor by means of a line shaft and pump shaft.
The shafts are supported by bush bearings. Oil lubrication is provided through an oil tube. The water is pumped through a discharge column pipe. In the entire pump operation, maintaining the verticality of the line shaft is of utmost importance in ensuring reliable operation of the vertical turbine pump.
Thus, the pumping systems in metal products industry form a very important link in productivity and energy efficiency. Process pumps being mostly rotating equipment, top most priority is to be accorded in keeping these pumps in good running condition so as to maintain productivity. Hence the successful operation of pumps will depend equally on the correct selection of pumps as well as the quality of maintenance.
19. Belt Drives:
Belting is one medium of delivering a given amount of power at a low cost per unit of time which should result, in the transmission of power at a low cost per horsepower per year over a long period of useful service provided the correct type and size are employed. The function of power belting is to transmit power economically.
Therefore the user should be familiar with the characteristics and properties of the various types of belting available. Mechanical and atmospheric operating conditions varies; hence knowledge of such conditions results in low maintenance, efficient power delivery and lengthened life of the belting and other transmission appurtenances.
Most flat belts used in metal products industry are made of either duck or leather. Canvas or duck belts are protected from erosion by a rubber covering. For special conditions, flat belts can be made from almost any material that can be bent around pulleys.
Under normal conditions, flat belts operate at speeds below 3000 ft/min. However, carefully designed installations have been operated successfully at belt speeds as high 6000 ft/min. Belts experience a critical speed phenomenon which is evidenced by the pulleys ‘riding’ from side to side or by violet ‘flapping’ of the slack side of the belt. The critical speed of an installation is a function of the geometry of the installation, belt characteristics and tension. Thus, when critical conditions occur they are corrected by changing some of these parameters.
Leather has a tensile strength of 3000 to 4000 lb/sq. in and is usually designed to a safety factor of 10, making the working stress 300 lb/sq. inch. Leather belts are made in one, two, or three plies, with ply thicknesses of approximately 11/64 and 13/64 inch for medium and heavy belting, respectively.
The power capacity of a belt can be estimated for heavy ply by interpolating from the following factors:
At 600 ft./min – 1.2 hp/inch belt width
At 6000 ft./min – 8.9 hp/inch belt width
Two-ply belts carry 70 per cent more power and three plies 125 per cent more at the same speed as single-ply belts.
Duck used in rubber-covered belts is graded by the weight in ounces of a strip 36 inch wide by 40 inch long. The standard weights are 24, 26, 28, 30 and 32 oz. for such a strip. The tensile strength is almost proportional to the weight and number of plies. The effective tension of a 28-oz. belt is 10 lb/inch of belt width.
All belt drives depend on the coefficient of friction to balance the tension differential between the slack and driving sides of the pulley. The power can be calculated from the following relationships –
Where
V = belt speed, ft./min.
Ks = service factor that varies from 1 for steady loads and up to 2 or more for shock loads or loads involving stalling.
F1 = tensile force on driving side
F2 = tensile force on ‘slack’ side
The relationships between F1 and F2 depend on the angle of contact, the friction coefficient of the belt on the pulleys, and the centrifugal forces on the belt which tend to separate it from the pulley. (Gravity will also cause separation of the belt from the lower pulley in vertical drives).
The wedging action induced by the tensile forces on V-belts against the pulley grooves increases the contact load, and hence the friction of V-belts is two or three times that of flat belts, with resulting proportionally greater power transmission. In addition V-belts can use smaller pulley diameters than flat belts for the same power rating.
V-belts are constructed with an inner cord section to carry the load and an outer protective covering to form the friction surface. Load-carrying capacity per V-belt F1 varies from 35 to 400 lb; in general F2 is equal to zero when used in the equation given above for flat belts.
The theoretical speed ratio between driver and driven equipment for both flat and V-belts is essentially proportional to the diameter of the pulleys; however, because of slippage and creep, the actual speed ratio is not constant. Actual slippage between the pulley and the belt may be as high as 2 per cent for flat belts but approaches zero for V-belts. Creepage is due to elastic elongation of the belt under tension, resulting in the belt velocity on the driving side being slightly greater than on the slack side.
Belts are extremely flexible with respect to their applications in that a wide variety of spacing between pulleys can be used. Although maximum service life is obtained when the shafts are parallel, greater misalignment between shafts can be tolerated than with almost any other speed-reduction device. Belts can be used to connect shafts at an angle, provided that the belt load is reduced.
Variables-speed V-belt drives uses adjustable cones to obtain variable pulley diameters and therefore adjustable speed. For example – with the cones at their innermost position in the driving pulley and at their outermost position on the driven pulley minimum reduction is obtained. With the driver cones spaced fully apart and the driven pulley cones spaced as closely as possible maximum reduction is obtained. Such variable-speed drivers are commercially available in power ratings up to 25 hp.
Rubber belting, generally known as ‘friction-surface rubber’, is composed of two or more thickness, termed plies, of various weights of cotton duck, impregnated and frictioned with a tough slow-ageing rubber compound. Minimum and maximum plies of rubber belting are shown in Table 16.3.
The cotton duck is the element of tensile strength. Therefore the belt is graded to the weight and quality of the duck. For the average rubber belting, the duck weight varies from 28 to 32 oz. which, signifies that actual scale weight of a piece of duck measuring 36 by 42 inch when dry, is 28 to 32 oz.
The actual weight of the duck is not solely the strength factor because quality, weave, and length of the fibre have a decided effect. The average ultimate tensile strength of 32-oz duck, 1 inch wide is 440 lb. In calculating the strength of rubber belting, a factor of safety of 20 is employed. The tensile strength of the entire belt is approximately one-third less than the sum of the plies when taken separately.
The friction or the grade of the rubber compound employed governs the degree of adhesive tendency between the duck plies and also the coefficient of friction of the belt. This grading is measured in pounds and is based on the quality of the rubber compound. Frictions used are 20, 22, 24, 26 and 28 lb.
This pound friction is determined by the resistance of the rubber compound to the weight required. If a 28-lb friction is desired, a 28-lb weight or pull is applied to a peeled-back ply of the belt 1 inch wide. It must not separate longitudinally more than 1 inch per min. The duck and the friction are the grading characteristics of rubber belting as a 28-oz.-28-lb friction belt.
The manufacture of the belt is a series of operations:
(i) The duck is thoroughly inspected and dried;
(ii) It is impregnated and frictioned with the rubber compound—the rubber must be compounded so that the resultant material will be highly elastic, tough, and capable of retaining these qualities throughout the transmission life of the belt;
(iii) The cutting or sizing of the impregnated and frictioned duck to the belt width required;
(iv) Folding and placing the duck strips based on the thickness or number of plies required if of folded construction—if of raw-edge construction, folding is not employed;
(v) Vulcanising; and
(vi) Inspecting, testing, measuring, talcing and rolling.
Organic accelerators, antioxidants, flex-resisting ingredients, and many other new compounding materials have been developed by the rubber industry. All these improvements have been applied to the bonding material used in rubber-fabric belts. As a result, the quality of the bond has been steadily improved.
Conventional V-belt drives for air handling units, pumpsets, air compressors, refrigerant compressors, exhaust fans, ventilation blowers additionally consume significant amount of energy termed as transmission losses. These include a minimum of 3 per cent towards flexing the V-belts in and out of the V-grooves of the driving and driven pulleys and a further minimum of 1 per cent as slippage due to the poor coefficient of friction of the V-belt material.
Modern synthetic sandwich type flat belts overcome these problems with their flat construction and superior drive properties. The polyamide flat belt derived from the superior properties of modern synthetics, and manufactured with the state-of-the-art technology are suitable for all areas of mechanical engineering.
The flat belts are of a 5 layer polyamide construction. The main central traction layer is made of highly oriented polyamide with two covers of elastomer (NBR-acrylonitrilebutadiene rubber) and one intermediate layer of polyamide fabric on each side. The belt is further with NBsR friction cover on the outside which will ensure that the peripheral force acting on the flat pulleys is transmitted to the belt and vice versa. The layer rubbing with the pulley surface is to have longitudinal grooves for noise reduction.
When installed with a set of specially fabricated flat pulleys with crowning, these flat belt drives transmit power with a minimum of 4 per cent of energy savings when compared with conventional V-belt drives. Also the life of the flat belts is 4 to 5 times that of the V-belt and thus it is a maintenance free installation for more than 2 years in most cases.
These belts do not stretch during its life and hence slippage is totally absent assisting the maintenance crew in their jobs. In reality the energy savings are well over 4 per cent and the life of the belt more than 5 years. Flat belts are now the popular choice worldwide on many mechanical drives with the increased efficiency of transmission, long life and suitability of use for high speed drives which is not possible with conventional V-belts.