Vibration Isolation

A Note About Load Capacity

In general, to calculate the required capacity for levelers on a machine, divide the total weight of the machine by the number of mounting points. In order to ensure a stable support base, consideration must be taken for uneven weight distribution due to motors and other heavy machine components. Non-uniform load distribution will affect both the required capacity per mount and the mounting locations. Although S&W builds in a safety factor for all leveler capacities, the stated load capacities are for static loads only. Significant shock or vibration must be compensated for with extra capacity and may require a vibration isolator mount. See our vibration control section below for more detailed information. The use of a non-skid pad will generally reduce the capacity, not because the leveling device has been weakened, but because the elastomeric pad has a limited useful range under high compressive loads.

Vibration isolation

S&W’s Anti-Vibe® and Mighty Mount™ machine mounts have an engineered elastomer base specifically designed to isolate vibration. The Anti-Vibe® leveler is a modification of the Level-It™ leveler and has a pivoting stud that is threaded directly into the machine base to provide a range of leveling heights. Mighty Mounts™ are built with a large flat surface on top that supports the machine base. The threaded stud that is provided with Mighty Mounts™ is used mainly for securing the machine to the base, but can also be used to level the machine. The leveling mechanism is internal to the mount and provides about ½” of vertical leveling capability. A typical industrial machine will have some shock or vibration associated with its operation. In many cases the vibration is minor and can be dealt with by using a light duty non-skid pad on the bottom of the leveling mount. In other cases, however, shock and vibration are severe enough to warrant isolation. The source of the disturbance can either be from the machine itself or can be from a mounting surface such as a factory floor. In both cases the object is to isolate and reduce the vibration from the source. The reduction in the transmission of vibration can be expressed in a ratio: T = fo/fi where T is defined as Transmissibility, fo is output frequency and fi is input frequency. When fo < fi, there is isolation, when fo = fi, there is resonance and when fo > fi there is amplification. The latter two are to be avoided in all cases.

freq_chartIsolators have a natural frequency, fn, which is derived from a formula that includes the spring rate of the isolating material and the weight of the equipment. The natural frequency at maximum capacity is listed in the Anti-Vibe® and Mighty Mount™ tables for each part number. The natural frequency of an isolated system and the disturbing frequency it is subject to can be used to determine the amount of isolation achieved, as shown below. If transmissibility, T, is plotted against the ratio of the frequency of a machine fo (its output frequency), and the natural frequency of the isolating mount fn, a graph as shown in Figure 1 is generated. Any values above 1.0 on the T axis indicate amplification and values below indicate isolation. Resonance occurs at a ratio of 1.414, so any ratio above that will provide isolation. From the graph you can see that at a 2:1 ratio T = 33%, at a 3:1 ratio T = 12.5% and so forth. You can use this chart as a shortcut to determine either the amount of isolation achieved or to determine the mount to use for a desired T. For example, if you have a 4200 lb machine on 6 Mighty Mounts™ with a capacity of 700 lbs each (P/N MM80136) the natural frequency, fn, of each mount would be 8Hz. If the system frequency, fo, is 35Hz, then the ratio of output frequency to natural frequency is 35/8 or 4.38. Using the chart, this translates into a T of approximately .05 or 95% isolation. In equation form:


Solving for T when fo is 35HZ and fn is 8Hz yields a transmissibility of .055 or 94.5% isolation.