“Reducing and controlling vibration on a production machine can be frustrating. Excessive vibration can cause premature failures of bearings, seals, couplings, piping, and gears. In addition, it can cause degradation of functioning instruments and product quality. The five basic causes of the excessive vibration are poor machine function, faulty design, manufacturing defects, improper installation, and wear and abuse.

The forces that cause vibration are sometimes part of the function of the machine. However many machines generate excessive forces when operated out of their design range; e.g., a pump that is operated off its best efficiency point. Designs that are subject to resonant behavior or have flexible members are sensitive to any type of forcing function. Loose tolerances in manufacturing can cause excessive mass unbalance and subject components to failure as a result of stress. Internal and external misalignment create forces on bearings and couplings. Loose bolts and inadequate foundations can increase vibration levels. Finally wear and abuse degrade machine function and can lead to premature failures. The problem has to be identified before a method for vibration reduction is chosen.

The five basic methods available to reduce and control unwanted mechanical vibration include force reduction, tuning, mass addition, isolation, and damping. Force reduction can take many different forms: balancing, alignment, repair, and restricted operational parameters.

However in the case of reciprocating machines force reduction is normally not an option. When resonance is a problem, most analysts use tuning; that is, changing the natural frequency or the forcing frequency of the machine. If the two frequencies are close to each other, the solution can be difficult. It might be necessary to change frequencies as much as 15%. The forcing frequency is typically tied to machine performance or the speed of the driver, so that changes are not simple. It is therefore necessary to change the natural frequency. The natural frequency is related to [k/m]½. It is thus the square root of the stiffness and mass that is important.

Most analysts stiffen the structure when possible, but, if the natural frequency is below the forcing frequency, mass addition might be useful. Stiffeners add weight, which is counterproductive to raising the natural frequency. For example, horizontal beams are not an effective stiffener.

Figure 1 shows the key to effective tuning: a nondimensional plot of amplification versus frequency ratio for various damping ratios (damping/critical damping). Critical damping is the amount of damping in the system that will not allow vibration. A rule-of- thumb for changes in the frequency ratio is 15%.”