Shops can save money by planning for vibration control.

Leslie Gordon, associate editor

Larger machines typically require their frame or bed being attached to a concrete foundation, often called a reaction mass or an inertia mass.

An elastomeric isolator that consists of a neoprene-elastomer vulcanize bonded to steel components, giving it stiffness in all directions.

The side bolt on a housed spring isolator provides adjustable damping by applying a compression load to an elastomer pad internal to the isolator.

This standard pneumatic isolator has a natural frequency of approximately 2.5 Hz. Systems with such isolators require a source of clean, dry gas with pressures ranging from 60 to 120 psi.

Equipment requiring isolation can be the source of unwanted vibration

...or the recipient of unwanted vibration.

Often companies neglect to account for sources of vibration when installing machines or equipment. However, by planning for vibration control, optimally before constructing a new facility or during redesign of an existing one, shops can save a lot of money in the long run. That's because isolating machines and processes from unwanted vibration reduces potential problems such as low equipment life and even physiological damage to shop personnel.

According to Fabreeka International Inc., Stoughton, Mass., all structures, including machines and equipment, vibrate, or oscillate, when displaced from their equilibrium (static) position and continue vibrating naturally until all the energy received is dissipated. Vibration is expressed in frequency, or number of oscillations per unit of time. This unit — cycles/sec — is called a Hertz (Hz).

Every physical system possesses a natural vibration-frequency property. For some, such as a piece of steel, the natural frequency is high, and for others, such as a rubber, it is low. An isolator, which serves to suppress unwanted vibration, also has a damping property that acts to decrease the frequency amplitude of an oscillating system.

Unwanted vibration
According to the company, machines and equipment can either be the source, or the recipient of unwanted vibration. For example, rotating, reciprocating, and impacting equipment all create unwanted machine-induced vibration and shock (a transient condition where a system's equilibrium is suddenly disrupted by an applied force). This vibration is transmitted into the supporting floor-slab and to the soil underneath. Here, isolation is used to reduce the vibration transmitted to the floor.

On the other hand, robotic welding systems and precision testing equipment may require protection from vibration. With them, isolation is used to keep vibration within acceptable limits to maintain system performance.

Machines can also be both the source and recipient of unwanted vibration. For example, a surface grinder typically requires protection from floor vibration. However, the grinder's heavy table reversing during operation also produces large dynamic forces that can disturb nearby equipment.

The company says isolation is not typically required for less-sensitive machines. But protection is critical when it comes to big investments such as more-precise and accurate testing equipment, or machines with long beds, which require anchoring and aligning.

Robert Haley, engineering manager at Fabreeka, explains, "To better understand vibration isolation and damping, picture a car. Its chassis rests on leaf springs. If you travel down a road without shocks and hit a pothole, the whole car will oscillate at the natural frequency of the springs for many cycles until the energy is dissipated and the car stops. However, when you put shock absorbers on, which are really dampers, the car oscillates at the same frequency, but only for maybe one cycle and at less amplitude."

According to the company, the biggest source of unwanted vibration is machines generating pulses or impacts, such as forging presses, stamping presses, impact testers, hammers, centrifugal pumps, and compressors, which typically apply severe dynamic forces to the floor.

Achieving Isolation
Isolation is achieved by placing an isolator, or elastic element, between the unit vibrating and its support structure. Francis J. Andrews, P.E., explains, "A vibration isolator acts as a mechanical filter. Isolator efficiency varies with its natural frequency, which is both a function of isolator stiffness and the mass being supported."

All vibration isolators are essentially springs with the added element of damping. In some cases, the spring and damper are separate, such as a coil spring isolator used with a viscous damper. Most isolators, however, incorporate both in one unit.

In addition to springs, other types of isolators include rubber; mats of various materials such as felt, rubber, and cork; metal coils; air bags; pneumatic cylinders, and "floating" concrete foundations. Air isolators yield the lowest natural frequency, with steel springs next, followed by elastomer (natural or synthetic rubber) pads.

Problems arise because just placing a rubber mat under a compressor, for example, can dramatically amplify vibration transmitted to the floor. In this example, the natural frequency of the isolator can coincide exactly with (resonate with) the driving frequency of the compressor. The resultant increase in amplitude of vibration is limited only by the amount of damping present in the isolators.

Thus, Fabreeka stresses that designing a vibration isolation system is not a do-it-yourself project. An expert's skill lies in selecting proper isolation systems based on their natural frequencies. Selecting effective isolators involves calculating application variables such as transmissibility — the ratio of output to input vibration.

If the transmissibility ratio is greater than one, vibration is amplified, whereas if the ratio is less than one, vibration is reduced. Vibration isolation for any isolator begins at a ratio of 1.414. An effective isolator has a natural frequency well-below the application's input vibration.

Equipment or small machines are typically mounted directly to isolators. This reduces preventative maintenance and helps ensure shops avoid emergency outages that happen when, for example, an air compressor suddenly quits. Engine-driven welders also typically require isolation from their frames so the units don't skip across the floor.

Larger systems such as a line of robots may require being attached to a properly designed floating foundation, often called a reaction mass or an inertia block. According to Haley, it takes a lot more force to move systems joined to a concrete block attached to springs than it does to move ones attached just to springs.

Making isolator system choice difficult, some equipment manufacturers provide allowable vibration specifications for their machines, but most don't. However, knowing the amplitudes of vibrations, at which frequencies, that harm machinery is key to selecting proper isolation systems. Fabreeka reports this is where savvy companies call in a consultant or company specializing in vibration protection.

Specialists will measure the vibration in a facility with highly accurate instrumentation such as real-time signal analyzers, which capture raw data, without bias, for post-processing, to quantify the amplitude and frequency of vibration. The specialist then recommends the proper isolation solution. Engineers also conduct acceptance test-measurements after installation to verify vibration amplitudes and the resultant transmitted vibration.

Reducing industrial-fan vibration An example from the Fabreeka files includes a company with an industrial fan transmitting vibration into its floor support, felt by office personnel on the same floor. The vibration is not severe, but personnel find it annoying. After a survey, Fabreeka had information including: Fan weight is 14,000 lb, uniformly distributed. Fan speed is 1,800 rpm (30 Hz). The fan is near a floor column support, meaning the support is stiff. There are no adverse environmental conditions. The fan is skid-mounted and anchored to the floor. After calculations, the solution involved isolators being placed at the anchor-bolt locations. Since vibration disturbance was not severe, only annoying, it was decided a 75% reduction would suffice. The transmissibility ratio was figured, giving the requirement for an isolator with a natural frequency of 13.39 Hz.