One of the most common sources of noise in industry is the noise generated from vibrating machinery. For example, a 5-hp motor rotating at 1800 rpm is rather quiet, but if it is bolted directly to a metal table, the noise level can rise easily to 90 dBA at a distance of 5 ft. In this instance, the metal tabletop acts as a sounding board and radiates the vibratory energy transmitted from some form of rotational unbalance of the motor.
This sounding board effect is frequently found in industrial environments or buildings where floors or machine platforms are constructed of diamond deck, metal grills, hardwood and even concrete. The excitation can originate not only from rotating equipment but from vibrators, tumblers, shakers, pressers, etc.
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It should be emphasised that noise is not the only undesirable effect. Floor vibration, for example, can be a serious problem to adjacent equipment, where close tolerances are maintained in machine tools or especially where optical devices are used.
The simplest and most straightforward solution to this problem is to provide some sort of mechanical isolation between the machine and the structure supporting the machine. Much has been written on vibration and shock control and attention is usually focused on a high degree of isolation for areas extremely sensitive to vibration.
However, it must be emphasised that for significant noise reduction the degree of isolation can often be much less than one would require for a high degree of vibration control. In addition, the introduction of isolation is usually easy, inexpensive and often the complete solution to the noise problem. Before discussing the design guidelines of vibration control, it is important to consider the basic fundamentals of vibration and the transmission of vibrational energy.
Just as emphasised in noise control, control of vibration at its source is usually the most effective measure. For example, one of the most common sources of mechanical vibration in rotating machines is due to unbalance. Unbalance may arise from a number of sources, but the solution is simply to balance the rotating parts.
Often this is an easy matter and the result can be a dynamic reduction in vibration amplitude. Further, in some instances, minor modifications such as stiffening support brackets or flanges will radically change the natural frequencies of the system and undesirable resonance conditions can be avoided.
Often equipment such as sifters, screeners, tumblers, etc., vibrates by design at large amplitudes but very low frequencies. Generally, they are not very noisy. However, a loose sheet metal panel, door or attachment will impact against the structure with a resounding clatter. The solution here is a matter of light maintenance, often with only a screwdriver.
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As in noise control, sufficient reduction may not always be available at the source and hence some modifications must be made in the path between the source of vibration and the receiving element.
This is best accomplished by the use of vibration isolators, which will be discussed in the following are:
1. Vibration Isolators and Isolation Mounts:
There are three major types of isolators – metal springs, eleastomeric mounts and resiliant pads. Each type has advantages depending on the degree of isolation required, forcing frequency, weight of equipment, temperature, etc. In short, the relative merits of each should be considered for any particular installation.
Before discussing each type of isolator, one comment must be made which is pertinent to all types. We are dealing only with single degree of freedom systems. Since most systems cannot be reduced to such a simple model, the results and analysis should not be considered exact.
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Experience has proven and certainly the examples selected are such that the design methods outlined herein are reliable and conservative. It should also be emphasised that the basic design approach will be to select isolator devices such that the natural frequency of the system is well below the lowest applied forcing frequency. In this way, a high degree of isolation is assured.
Large heavy items of motor-driven equipment such as fans, pumps, compressors, punch presses and milling machines are prime examples of equipment that vibrates severely during operation. One method of reducing vibration transmission is to separate each machine, on mounts, so that the shock is contained.
Vibration produced is blocked from travelling beyond the machine itself by isolating the equipment so the vibration will not be transmitted to the floor, structural members or rigid sections of electrical distribution systems.
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One of the simplest methods of controlling vibration and shock from machine installations is with isolation pads made of felt, cork, elastomers, steel, aluminum or numerous combinations of these materials.
Pads can be employed by simply placing them under the legs or base of the machine. In textile mills, for example, where there is excessive unidirectional vibration, pads are cemented to the floor and absorb the energy of the low frequency vibrations that are produced. Isolating pads can also be bolted to the floor.
Some pad designs require no special attachment method, but are simply put down and remain in place. To prevent movement, the lower surface of the pad may be a waffle-like design that creates a suction with its contact surface. Elastomers are also vibration isolators that work in curtailing detrimental vibrations, which would otherwise be transmitted to floors, foundations and other adjacent machines.
These pads will protect a machine from both impact and vibration forces that are transmitted through the floor and foundation from other machines and from vibration caused by the machine itself. Isolators also provide a slight leveling action because of their resilience and their ability to keep the structure free from non-uniform stresses that might be produced by vibration from external sources. Most pads resist petroleum products, cleaning compounds and deteriorating effects of sunlight and air.
Inertia blocks are also widely used when there is a possibility of shock and vibration forces being transmitted through the foundation. These blocks are concrete sections that are set in the floors or foundations and are isolated from the surrounding material by vinyl-bonded pads, which are inserted before the concrete is poured to act as an impervious barrier.
Motors and machines can be mounted on massive inertia pads that are spring-mounted above floor slabs. Construction of pads varies; one of several possibilities consists of a reinforced concrete deck with an outer steel frame incorporating the spring housings.
With the use of relatively light-weight motors and machines, additional isolation can be affected by short rigid sections of electrical conduit stubbed down through floor slabs, passing through an amply sized sleeve. The sleeve is then packed with fibre-glass and caulked to make it airtight. The flexible conduit that extends through the lower slab continues to a remote switch gear location.
Another method for isolating vibration from rigid electrical distributor systems is to stub-up motor feeder conduit adjacent to, but not in contact with, the inertia pad, then secure a feeder terminal box to the outer steel frame of the floating slab. A large hole is cut in the bottom of the box that surrounds the conduit stub and the space between the rigid conduit and floating box is later sealed with a thick neoprene gasket.
Vibration control can be accomplished by using steel spring in combination with sound absorbing materials. Because of their effectiveness, springs are widely used in machine foundation design. Metal springs have perhaps the widest use of all isolation types. They are particularly applicable where large heavy equipment is to be isolated.
The metal springs allow for large deflections and as such are especially effective where large loads and very low forcing frequencies are present. Basically, the design consists of helical coil springs, which are fixtured in end caps. Generally, there is a central bolt which provides a tie in or fastener to the equipment being isolated. In some cases, the springs are housed or constrained to minimize any lateral vibrating motion which may occur.
The most important feature of spring mounts is their ability to withstand relatively large deflections and as such provide good low-frequency isolation.
iii. Air Spring for Vibration Control:
The ultimate design goal is for vibration less equipment, but in most cases this is unattainable. Typical problems involve isolating areas that cause undesirable forces. One of the elements available for solving this problem is the air spring.
Industrial air suspension systems are used to:
1. Isolate structures from vibrating masses.
2. Control the frequency of vibrating masses.
3. Keep a suspended platform at a given height under variable load conditions.
The air spring is basically a metal container with a rubber liner filled with air. It is a stored energy medium that is designed to support a given load.
The important characteristics of any spring in a suspension system are:
1. Transmissibility—the ratio of force transmitted through the mounting to the disturbance force. Air springs have low transmissibility coefficients.
2. Spring rate—the change in load per unit of deflection. The spring rate is directly proportional to the load on the spring and to the pressure of the air in the spring assembly. This is the result of a constant natural frequency suspension, regardless of the load.
3. Load carrying ability—the load range between maximum load and pressure and minimum load and pressure, for which the ratio is 5:1. Air springs have an inherent feature because their load carrying abilities and ratio can be varied by varying their air pressure.
Current designs in air springs are bellows, rolling lobe, bellobe (a combination of bellows and rolling lobes), restrained rolling lobe, filling supported sleeve and hydro-pneumatic.
Air suspension system ranges from simple to complex depending upon the application. If small variations in height are not important and the pressure is constant, then a tyre valve can be installed to vary air pressure. The air pressure should be checked periodically and adjusted if necessary. If the same pressure is required in all air springs of a particular system, a pressure regulator valve can be installed to keep the system at constant height.
If variable height is desired, automatic leveling valves can be installed. Occasionally it may be necessary to deflate air springs simultaneously so the entire controlled mass rests on solid supports. For this result, a series of check valves may be installed; when the main valve is opened, all the air springs deflate.
An additional advantage of air spring systems is that time has no harmful effect on them. Neither air columns do not deteriorate with age, nor do they tend to set as mechanical springs might; however, costs of these systems are somewhat higher. Pneumatic systems are easy to install because there is no need for special foundations. Devices are set in place under a machine’s support points and connected to plate air supply of 85-100 psi. Readily available units can handle equipment weights from a few hundred pounds to several hundred tonnes.
Let us now outline a systematic step-by-step procedure for selecting spring mounts for any degree of isolation:
1. Establish the total weight and lowest expected forcing frequency of the equipment to be isolated. If a motor operating at 1800 rpm drives a pump at 1200 rpm, the lowest frequency is associated with the pump (1200 rpm or 20 Hz).
2. Determine the static deflection required to provide the degree of isolation desired.
3. From the listed spring constants of the mounts (supplied by the manufacturer), select the appropriate spring mounts.
4. Since there will be four springs supporting 1200 lb, the spring constant of each mount must be 300 lb/0.23 inch = 1300 lb/inch (approximately).
In addition to the selection method just outlined, several other basic design guidelines should be followed:
1. A uniform deflection should be maintained for each mount to minimise unwanted twisting or rocking motions. In some large installations, the weight may not be evenly distributed. As such the mounts should be selected accordingly to maintain uniform deflection.
2. Driver and driven units should be mounted on a common rigid base or serious damage may occur to the drive-shaft bearings of either or both.
3. Where a wide range of temperatures may occur, spring mounts are especially stable. In addition, where acids, oils, solvents, etc., are present, spring mounts are more resistant to corrosion than most other mount types.
4. Because of the very low damping factors of metal springs, some kind of pad is usually also required in combination to achieve minimal damping and high-frequency isolation. In fact, many of the commercially available units have a resilient pad integrally attached to the mount base.
5. Often, the addition of an inertia blocks provides several desirable features. The pumps mounted on inertia blocks are isolated from the floor. This combination has the advantage of lowering the system centre of gravity, providing a firm rigid common base for the equipment and uniformly distributing the load. Inertia-blocks are usually made from reinforced concrete and should weigh at least 1.5 times the weight of the supported equipment.
Finally, it should be mentioned that spring mounts may be used in tension but most often are designed to be used in compression.
Advantages and Disadvantages of Springs:
Advantages of springs are:
1. Smoother and quieter machine operation.
2. Reduce machine maintenance.
3. Reduction or elimination of the effects of vibration on the structure and occupants.
4. A decrease in size and mass of the foundation under the equipment.
5. More accurate design because properties are known and readily predictable.
6. Considered permanent for the life of the machine; properties are not affected by time.
7. Leveling adjustment is possible in spring units.
8. Versatility of installation.
Disadvantages of springs are:
1. Higher initial cost of construction in some installations.
2. Requirements for maintenance of the spring units.
3. Noise transmission and no damping ability.
Over a limited range of stress, elastomeric isolators can also be used effectively for a wide variety of vibration isolation problems. This type of mount is at its best performance for applications that include small machines and relatively high excitation or forcing frequencies. The elastomeric medium can be easily moulded into practically any size or shape and a range of stiffness can be controlled over rather wide limits.
The most common materials selected for elastomeric mediums are natural rubber, neoprene, butyl, silicone and combinations of each. A typical mount utilising these materials generally employs the medium in shear but may also employ a compressive design. By combining both shear and compression in the design, a relatively linear deflection versus load response can be obtained, as was the case for helical springs.
Presented in Fig. 27.1 is typical load deflection curve for a line of commercially available rubber mounts. The stiffness or spring constant for this family of mounts is varied by controlling the hardness or durometer of the rubber. Intuitively, a softer rubber deflects more under load than a hard rubber. With these deflections versus load curves, one can apply essentially the same method for mount selection as with springs.
Example:
It is desired to isolate a small internal combustion engine weighing 900 lb whose normal rotational speed is 2400 rpm. An isolation transmissibility of 3 per cent or less with four elastomeric mounts is the design goal. What deflection and durometer are required?
Solution:
Recall the method outlined for spring mounts:
1. The lowest forcing frequency will be 2400 rpm = 40 Hz and the weight is 80 lb.
2. The required static deflection for 97 per cent isolation is 0.21 inch.
3. Since the load will be distributed over four mounts, a mount whose deflection is 0.21 inch for 900/4 = 225 lb is required. Looking to Fig. 27.1 a mount with durometer 40 will do nicely with a little design margin.
Elastomeric-type mounts have many advantages over springs and others, especially where space and weight are important. This type of mount is used almost exclusively in the aerospace industry for engine and delicate electronic equipment installations. Another popular application is for packaging delicate equipment is shipping containers. There seems to be no end to the configurations in which the active rubber section can be moulded. Hence, this feature gives the design engineer great latitude for innovation.
The most serious limitation of elastomeric mounts is with the life and endurance characteristics. It is pretty well acknowledged that static deflection must be kept under 0.5 inch and serious changes in stiffness can be expected if operation of outside the temperature range of-30°F to 150°F. In addition, many of the rubber compounds used will deteriorate rapidly when exposed to some highly corrosive acids, oils and solvents commonly found in industrial environments.
Probably the simplest and most often-used isolation mount is the pad type. These pads are available in natural rubber, synthetic rubber or blocks of cork, felt or fibrous glass and combinations thereof.
Several advantage of pad-type isolations are that, they:
1. Are easily inserted under equipment, eliminating tearing up the floor or inserting bolts.
2. Are available in sheets of various thickness.
3. Can be stacked to obtain large deflections and corresponding high levels of low-frequency isolation.
Let us consider briefly the basic selection criteria for the most common pad types available.
vi. Rubber Pads:
Because of economy, ease of installation and replacement, rubber isolators are much preferred.
Some important characteristics of rubber isolators are:
1. Elastic properties.
2. Damping ability.
3. Ability to reduce noise transmission.
Rubber has an excellent damping ability.
Extreme temperatures affect natural rubber characteristics, which change its isolation effectiveness. Varying temperatures between 20°F and 140°F will change the characteristics of natural rubber from 12 to 30 per cent for each 36°F change. Extremes in temperature can have permanent detrimental effects on rubber.
Rubber isolators are effective when the desirable natural frequency is above approximately 300 vibrations per minute. When natural frequencies below 200 vibrations per minute are encountered spring mountings are usually recommended.
The design selection for rubber pads follows basically the method established in the preceding section on rubber mounts. That is, load versus deflection curves are always available from the manufacturer for a variety of rubber stiffnesses (durometers) or layer thicknesses and the same approach to selection can be followed.
Generally, these pads can be bought in sheets and cut to whatever length and width desired and a table of recommended loading per unit area is usually available from the manufacturer, which also simplifies the selection. Also, these pads stack easily to achieve larger overall deflections than is recommended for a single sheet. For a single sheet, the upper load should not exceed 70 psi. Otherwise, most of the limitations listed for rubber mounts also apply to rubber pads.
Cork is perhaps the oldest isolation material and one of the most effective. The cork pads are generally available commercially in blocks 6 to 10 inch square and in thicknesses of ½ and 1 inch. A range of densities is available from some suppliers and provides a range of stiffnesses for selection.
Commercially available cork pads are made by compressing cork particles under high pressure and steam. Cork pads differ from rubber pads in that the material is porous, containing many minute air cells which become reduced in volume under compression. As such, cork exhibits little lateral expansion in vertical compression.
The stiffness of cork is relatively high for commercially available cork pads. Unfortunately, a cork pad not possesses linear deflection versus load characteristics and varies dynamically with density.
1. Rather small range of available stiffness compared to rubber.
2. Takes a permanent compression set at temperatures above 100°F.
3. Not especially resistive to industrial solvents, acids, oils etc.
4. Cannot be moulded; hence is available only in slab form.
Hairfelt pads have also been used for years as vibration isolators. Used exclusively in compression, these pads are commercially available in ½ to 3″ thicknesses. The most common thickness is 1″ and where extremely high loads are anticipated, stacks of 1 inch pads are formed to provide the required deflection.
Felt pads used as isolators are generally graded in terms of the density, i.e., hard, medium and soft. The soft grades typically can be loaded up to 50 psi, medium grades to 100 psi and the hard, most dense pads to 200 psi. Shown in Table 27.2 is a typical selection table for commercially available felt pads.
The deflection versus load curve is fairly linear up to a compression of about 25 per cent. At this point, the stiffness increases sharply with a corresponding increase transmissibility. Selecting felt pads for effective isolation again follows the method outlined for rubber pads.
Many hairfelt materials are organic and therefore deteriorate rapidly in the presence of typical industrial oils or solvents. Some suppliers have industrial-quality felt, but some caution must be given this matter for each proposed felt pad installation.
Fibrous glass pads have vibration characteristics much like felt pads. The deflection versus stated load curves is nearly linear, up to compressions of 20 per cent to 30 per cent, where further compression sharply increases the stiffness. The pads are commercially available typically in ½ and 1″ thicknesses and can also be stacked for low-frequency isolation.
One manufacturer suggests stacks up to 20 layers (10 inch thick) to achieve 1 per cent transmissibility at 40 Hz. Again, the stiffness of the mount is a function of density, but effective isolation below 20 Hz does not appear possible with the fibrous glass pads. The fibrous glass materials are basically inert and highly resistive to industrial oils, solvent, acids, etc.
One other form of isolator is the helical or steel coil isolator. These isolators are heavy-duty assemblies of stranded stainless wire cable attached between metal retainers. Each isolator type has its own stiffness characteristic depending on cable diameter, number of strands per cable, cable length, twist and number of cables per section. They are especially well suited to heavy-duty high-displacement applications such as use on shipboard and for shipping containers.
Excellent isolation is available down to the range of 5 to 7 Hz with inherent damping provided by flexural hysteresis due to rubbing and sliding friction between the strands. Another positive feature is that they can be used in many orientations and are highly resistant to industrial or other harsh environments. They are, however recommended only for applications in compression.
For equipment subject to low-frequency vibration or shock such as power presses, drop or forge hammers, tumbling barrels, etc., pneumatic-type mounts are especially applicable. Analytical methods for dealing with shock are somewhat complex, especially when applied to system response.
As such we shall say only that shock can be considered an impulsive like force and that system response resembles to the first order free vibration. Hence, an isolation system that includes a high level of damping or extremely low natural frequency or both has the primary design factors for good shock isolation.
Returning to the pneumatic isolators, their construction consists basically of an air-filled reinforced bellows with mounting plates attached to the top and bottom.
Since the stiffness of the mounts comes from an air spring, we can have zero static deflection with a degree of isolation down to 0.5 Hz. This is particularly noteworthy since a steel spring would require deflection in excess of 3 ft to provide the same degree of isolation.
The only disadvantages of these mounts are that they must be used in compression and only a single, nearly absolute vertical orientation is possible.
Before leaving the subject of isolation, it should be mentioned that prefabricated inertia bases or frames which often simplify equipment installation are readily available commercially.
An inertia base generally consists of a steel frame with isolation mounts attached. The frames are generally of steel beam construction designed to be used as is or to accept poured concrete, serving also as an inertia block.
In either case, they:
(i) Provide for easy installation;
(ii) Prevent differential motion between driving and driven members;
(iii) Reduce the system centre of gravity, thus reducing undesirable rocking; and
(iv) With concrete added, act to lower the system natural frequency.
2. Flexible Couplings:
Flexible connectors used dominantly to isolate rotating or vibrating machinery from ducts or pipes play an important role in vibration and noise control. What happens here is that ducts or pipes are connected directly to vibrating machinery and the vibratory energy is transmitted in a structure-borne manner to the ducts or plumbing with a high degree of efficiency.
Three major problems may then arise:
1. The vibratory energy tends to produce vibratory motion at joints, couplings or seals with resultant failure at these critical locations.
2. The vibratory energy is reradiated as noise throughout the pipe or duct system.
3. The vibrating pipes and ducts impact adjacent supports, pipes, walls, etc., with resultant clatter. In high-pressure fluid systems, steel reinforced rubber connectors, are generally the best isolators. These connectors are usually flanged or threaded for ease of installation and are available in various sizes of straight or elbow sections.
Design guidelines are simple:
1. The connector isolators should be as close to the pump, blower, etc., as possible.
2. Two connectors oriented 90° to each other perform much better than a single isolator.
In low-pressure air-handling system, flexible sleeves or hoses work well and are usually easy to install. These connector isolators also allow for expansion and contraction due to temperature changes.
3. Dynamic Absorbers:
In some cases, the control of vibration and the resultant radiated noise cannot be achieved using the previously described methods of vibration isolation. However, when vibratory forces are a constant single frequency, dynamic absorbers can be used to control vibration.
A dynamic absorber consists of a comparatively small mechanical system that has a natural frequency that is coincident with the frequency of the exciting forces of the vibrating source. The resultant mechanical system is schematically illustrated in Fig. 27.3. By selecting k and m properly such that √(k/m) is equal to ω, the resultant system then has a unique dynamic property.
The dynamic absorber will naturally vibrate in such a way that its spring force is at all instants equal and opposite to the exciting force P0 sin (ωt). Obviously, since these two forces are equal and opposite, the net force on mass M is zero and thus the original system will not vibrate.
Most practical dynamic adsorbers consist of weights that are attached to small beams. By adjusting the length of the beam and the size of the weight, the dynamic absorbers’ natural frequency is tuned to the exciting frequency. The actual process of selecting dynamic absorber size, mounting locations and mechanical properties is beyond the scope of this text. However, there are many practical situations where noise and vibration control can be achieved by applying this approach.
4. Adjustable Dampers:
Adjustable dampers operate on the principal that the workpiece can be considered as a part of an entire machine structure and therefore can be the source of chatter. The side opposite the cutting tool contacts the damper with the weakest portion of the workpiece. This absorbs vibratory energy and increases the dynamic rigidity of the machine structure.
The internal resistance of the machine structure is dependent upon a machine structure’s dynamic rigidity. Experience has shown that the damping ratio in machine tools is not sufficient to resist resonant vibrations. When cutting down on speed or the feed rate, productivity has to be sacrificed. With an adjustable damper this problem is eliminated. Other advantages of the external adjustable damper are the cutting process dynamics remain unchanged and the required chatter control is obtained by changing the machine structure.
The dampers can be located anywhere on a workpiece, but for a lathe they are generally hung on the frame parallel to the centerline of the spindle. Usually on a shorter piece of work only one damper is used, but more can be employed if needed. If several dampers are used on a roll and each one is manually adjusted, a skilled technician should make the adjustments.
To avoid this, sensor devices are attached to the dampers and all of the dampers can be adjusted from a centre controller. It is important to check the condition of the lubricant; stick-friction should not be used in the system in order to avoid any serious disturbances. The damper can be used in many different applications without any difficulties.
Vibration Measurement and Analysis:
When the piping is suspended from isolation hangers it is given added protection. On the discharge and intake sides of water pumps, flexible rubber hose (approximately three diameters long) should be used. On intake and discharge fans, flexible duct work should always be used.
Vibration measurements are very similar to sound measurements. However, spectral analysis and related diagnostic methods closely follow the techniques employed for sound. The basic vibration measurement system includes – a transducer, preamplifier and some means of analysing, displaying or measuring the transducer electrical output.
Transducers:
Just as in sound measurements, the transducer is the most critical element in any vibration measurement system. By far the most popular vibration transducer is the accelerometer, followed closely by the velocity pickup.
Accelerometers:
The accelerometer produces an electrical signal, the voltage of which is proportional to the acceleration experienced by the device.
Already shown in Fig. 27.3 are several commercially available accelerometers along with a schematic showing basic construction and active elements. Essentially, we have a mass suspended on a spring between two piezoelectric crystals. These components are encased in a metal housing and attached to a base.
When the assembly is mounted to a vibrating surface, the mass exerts an inertial force F on the crystals that is proportional to the acceleration. From the piezoelectric effect of the crystals, a voltage proportional to the acceleration is produced and is generally brought out through a connector to small, easily attached cables.
For accelerometers, the output voltage sensitivity is usually rated in terms of millivolts per gram, where gram is the acceleration due to the earth’s gravitational field. Typical sensitivities for commercially available, piezoelectric accelerometers range from 10 to 1000 mV/gram over a wide frequency range.
Now it must be emphasised that if the measured voltage from an accelerometer is the root-mean square (rms) value, which is most often the case, the amplitude of the acceleration will also be the rms value. Likewise, if peak voltage is measured, peak values of acceleration will be obtained.
Perhaps the most critical aspect of the use of accelerometers is the mounting. Clearly, if the accelerometer is not solidly mounted to the vibrating surface, gross errors can be expected.
Types of Vibration Mounts:
Cork has been used in many fields of industry as a vibration isolation material. A popular type of cork is made of pure granules compressed together and baked under pressure to achieve a controlled density. Cork materials are used mainly under concrete foundations.
The cork will remain reasonably durable under exposures to acids, oils and temperatures between 0°F to 200°F; however, it will be affected upon contact with strong alkaline solutions. Cork will rot, however, from repeated wettings and dryings. As a vibration isolator, cork is limited to frequencies above 1800 cycles per minute.
Because of a great degree of damping in cork, the natural frequency cannot be obtained from the static deflection. As an alternative, the natural frequency can be obtained through tests by vibrating the cork under various loads to find the resonance frequency.
Rubber has also been mentioned as a vibration isolator and is useful for frequencies above 1200 cycles per minute. Alkali solutions or acids will not affect rubber, but degradation problems could arise if it is exposed to sunlight. For natural rubber, the temperature range is from 50°F to 150°F; for neoprene from 0°F to 200°F. For applications that expose the rubber to oil, neoprene is more suitable.
As rubber ages, it gradually loses its resiliency. The useful life span of a rubber mount is approximately five years under impact applications and seven years under nonimpact applications, although it will retain its sound-insulating properties for much longer. Individual moulded rubber mounts are economical only with light and medium-weight machines because heavier capacity mounts approach the cost of the more efficient steel spring isolators.
A most efficient way to isolate vibration, however, is to use steel spring isolators, which provide greater deflection and thus higher efficiency. Some steel spring isolators can even reach deflections of ten inches. In many other materials costly trial and error tests are usually required, but this is not necessary with steel springs because they closely follow the equations of vibration control.
Steel spring isolators are usually equipped with adjustable snubbers because steel springs contain no damping. Damping is basically helpful in restraining the movement of resiliently mounted machinery but it does reduce the isolation qualities of the mount. Many of the steel spring isolators used by industry have built-in leveling bolts that eliminate the use of shims upon installation. Often, rubber sound-isolation pads are used in conjunction with steel springs because high-frequency noises have a tendency to bypass steel springs.
Basic Installation Principles:
When mountings are installed to cut down machinery vibration, the following principles should be observed to ensure maximum efficiency:
1. All parts of the unit of machinery to which the mountings will be applied must be placed on a common rigid base.
2. The entire unit of machinery should be insulated with a suitable mounting.
3. To ensure full effective insulation, elastic connections should be made to the unit.
4. If required, grounding should be installed to conduct away any static electricity.
5. If the unit of machinery is tall or unstable, it should be fitted with an enlarged base with a thick, rigid plate.
6. Coupling for elastic force transmission should be done.
Types of Industrial Mountings:
Type 1:
This mounting, consists of a cylindrical rubber body with steel plates tightly vulcanised to each end. Each plate has a threaded centre hole for attachment. Upon installation, the top steel plate is bolted to the machine base and bottom steel plate is anchored to the floor.
This type of mounting is specifically suitable for vertically loaded units of machinery that only make small sideways motions when in operation. The mounting is flexible and can be made even more so by tilting the insulator; therefore, insulation is achieved through shear and compression rather than just compression. This mounting can be applied to such machinery as converters, fans, pumps and electrical machinery.
Type 2:
This mounting, is made of two U-shaped steel components with rubber tightly vulcanised between them. There are two holes under the inner shank to improve resiliency qualities. To install, the top U-plate is bolted to the base plate of the unit of machinery and the bottom U-plate is anchored to the floor with an expansion-shell bolt.
This type of mounting is especially good for highspeed and heavy machinery because it provides a stable base. It can be applied to machinery such as punches, presses, carpentry machines, weaving machines and transformers.
An antivibration mounting is designed to give high resilience and good lateral stability. Mounting is relatively easy since installation height is low. On the bottom of the mounting is a thin layer of rubber that will prevent the machinery from moving across the floor. The friction supplied between the mounting and the floor is usually sufficient to make bolting unnecessary.
The top metal cover is designed to protect against oil spills. This type of mount is made for both light and heavy low-speed machines, which are generally difficult to insulate and can be applied to converters, pumps, fans, combustion engines, rolling, mills, presses and punches, paper machines and various domestic machines.
Type 4:
Another antivibration mounting, consists of two steel profiles of unequal size with blocks of rubber tightly vulcanised between them. When the rubber undergoes shear stress, the mounting provides resilience. This insulator is used when resilient; soft-mounting is needed for low loads. It is applicable to low-speed machines and light fans as well as various instruments.
Type 5:
Another type of mounting is constructed of two identical steel profiles with rubber snugly vulcanised between them. It provides simple resilience when the rubber undergoes shear stress. This insulator is applicable when a soft, resilient mounting is needed for low speeds.
Type 6:
This type of mounting consists of a cylindrical rubber component vulcanised to two steel intermediate plates. Installation of the mounting is not complicated and it can absorb large vertical forces without much deformation. It will also give good stability since its installation height is low.
Providing insulation against horizontal vibrations, it has a high deflection capacity in the horizontal direction, while deflection is relatively low in the vertical direction. In light of these particular deflection properties, we can see that low interference speeds can be horizontally insulated while obtaining a subcritical mounting in the vertical direction with a safe distance to the resonance point.
By connecting two parts in series, large angular movements can be absorbed and a larger vertical resilience can be obtained. This type of mounting is used for such applications as heavy machinery (crushers) and screening plants.
Type 7:
A bumper-type vibration insulator consists of a cylindrical rubber component that is snugly vulcanised to a square steel plate. There is a hole at each corner of the metal base plate for attachment. The rubber component absorbs as much energy as possible and reduces transmitted impact forces.
This isolation ensures efficient damping for moving machinery and machine parts. Considerable energy can be absorbed by the resilient rubber bumper. Two bumpers can also be used on a machine to give added resilience.
Two concentric cylindrical metal sleeves with rubber vulcanised between them are designed to take up torsional movements and radial and axial loads. These sleeves can be used to reduce bearing problems in vibrating constructions because the rubber takes the complete movements, so bearing maintenance is not needed.
Sleeves that have very good vibration and sound insulating capacity can be applied to machinery such as linkage systems in cars, tractors, railway wagons, bulldozers, agricultural machinery and vibrating screens.
Shaft Couplings:
The schematic of a vibration-insulating shaft coupling. It is made up of a U-shaped rubber ring that is tightly vulcanised to two steel rings on either side of it. When in operation, the rubber U-shaped ring will transmit torque from one of the steel rings to the other, thereby delivering the power without vibration.
In general, shaft couplings protect machinery by providing for smooth, silent and well-balanced running. They can level out any small deviations between the shafts, which could prove valuable in cases where shaft alignment is very difficult.
Vibration Plates:
There are two popular versions of this unit – the single and the double plate. The single plate version has uniform ribs on one of its sides; the double plate version has ribs on both of its sides, which are at angles to each other. These plates are made of oil-resistant rubber.
These plates are used in industry for relatively simple installation requirements. Best suited for damping a high-frequency vibration, they can be arranged in series to give added cushioning. If mounting is to be permanent, a direct contact between the base and the machinery must be avoided.
Blocks:
This anti-vibration component is just a simple rubber block with recesses in it for installation. Blocks are easy to fit because they are placed loosely between the machine and the base. The friction produced between the rubber and the base is sufficient enough to keep the machine from moving. There are four recesses in the block shown, which can be used as sockets for pins or dowels to be positioned to the machine and base.
Blocks can be used to insulate heavier machines with low interfering frequencies. Particularly useful for large composite machines that are on a common girder frame, they are also applicable to many types of machinery including converters, mixers, gear wheels and rolling mills.
Some Applications for Isolators:
Properly designed mountings allow the installation of some of the heaviest machinery in penthouses, near offices and many other areas where vibration and noise would be a nuisance. When heavy machinery is installed on upper floors of such complexes, careful consideration must be taken to prevent the transmission of vibration to several floors below when a ceiling, wall or fixture has the same natural frequency as the vibration.
In cases of such resonance vibration the result can be a disturbing noise. When mountings are selected, heavy machinery can be installed in more economically constructed and lighter structures with fewer noise problems. Machinery can also be installed in older buildings that may not have been specifically designed to handle such equipment with the aid of carefully selected mounts.
In such structures as anechoic room installations, steel spring isolators resting on rubber sound pads are used to stop the transmission of structure-borne vibration and the resultant noise to these rooms. In many air-conditioning installations, the noise and vibration that travel through the piping poses annoying problems.
However, if the refrigerating compressors are installed on mountings, provision should be made for flexibility in the intake and discharge piping to reduce the transmission of vibration. Providing flexibility in the piping itself can be achieved by running the piping a distance equal to 15 times the pipe diameter both horizontally and vertically and then attaching the piping to the structure.
Flexible metallic hose can also be used. When the piping is suspended from isolation hangers it is given added protection on the discharge and intake sides of water pumps, flexible rubber hose (approximately three diameters long) should be used on intake and discharge fans, flexible ductwork should always be used.
Velocity Pickups:
Velocity pickups provide an electrical signal whose voltage is proportional to the velocity of the vibrating surface to which it is mounted. These devices consist primarily of a seismic coil surrounded by a permanent magnet. As the coil moves due to the motion of the pickup, a voltage is induced which is proportional to the velocity.
Here again the signal is usually taken from the pickup itself through small cables. Since these devices are large compared to accelerometers and extremely sensitive to the magnetic fields common in industrial environments, they are used sparingly. As such we shall say only that, if utilised, most of the mounting guidelines given for accelerometers apply also to velocity pickups.
Finally, it should be mentioned that if integrating networks (usually available from the manufacturer) are employed in conjunction with accelerometers, the velocity and displacement of the vibration can be obtained as well.
Preamplifier:
The second element in the measurement system is the preamplifier. When using a velocity pickup, a preamplifier is not required. However, when using a piezoelectric accelerometer, a preamplifier is always required. Basically, the preamplifier serves two useful purposes: it increases the level of the transducer signal, which is small and it provides an impedance matching device between the transducer and the signal-processing equipment. Most transducer manufacturers supply an assortment of preamplifiers as optional equipment. In many cases, the preamplifier is directly integrated into the accelerometer packaging.
Processing and Analysis Equipment:
The processing and analysis equipment for vibration is essentially the same as the equipment for sound measurement. The same equipment can be used since we are using the vibration transducer signal to measure the absolute level of the vibration. Similarly, a detailed spectrum analysis of the signal can be used for diagnostic purposes. Dedicated vibration measurement instrumentation is commercially available.
Tape recording of vibration signals is also often used and detailed laboratory analysis of the signals using a spectrum analyser can be performed. Calibration of vibration measurement system can be achieved by using portable shakers which oscillate at reference amplitude, typically 1 grams rms. In this way, the vibration sensor is attached to the shaker and the signal level is measured and adjusted to meet the level associated with the vibration calibrator.