Rubber Shock Absorbers and Vibration Isolators: Selection Guide for Motors, Pumps and Machinery

A rubber vibration isolator converts vibration energy into a small amount of heat through elastomer deformation and reduces force transmission to the structure. A shock absorber belongs to the same family but is designed for a short impact load, not necessarily continuous isolation at a working frequency. Selection based on "apparent softness" is wrong; the correct parameters are excitation frequency, static load, allowable compression, temperature and oil compatibility. Incorrect installation of a vibration isolator can make vibration worse than a rigid mounting.

Table of Contents

• Difference Between Shock Absorber and Vibration Isolator
• Simple Physics of Vibration Isolation
• Common Industrial Types
• Elastomer Material Selection
• Shore Hardness and Load Capacity
• Selection for Motors, Pumps and Machinery
• Installation Errors
• FAQ

Difference Between Shock Absorber and Vibration Isolator

Vibration isolator (Anti-Vibration Mount): the goal is to separate a continuous vibration source (motor, fan, pump, compressor) from the chassis or building; design is built around the natural frequency of the assembly.

Shock absorber (Buffer/Snubber): the goal is to absorb the energy of an impact or to limit stroke; for example the end-of-stroke impact of a cylinder or a crane buffer. Here crush resistance matters more than natural frequency.

If your source is a motor with constant speed, a vibration isolator must be calculated; if the problem is jack end-of-stroke pounding, a shock absorber.

Simple Physics of Vibration Isolation

A mass-spring-damper system has a natural frequency that, by a common engineering approximation, is calculated as: natural frequency in hertz ≈ 15.8 × square root of (1 divided by static deflection in millimetres). The practical rule is: effective isolation begins when excitation frequency is greater than about 1.4 times natural frequency; for good isolation the ratio of excitation frequency to natural frequency is usually targeted at least 3.

For example, a 1500 rpm electric motor is equivalent to 25 Hz; to reach a ratio of 3, natural frequency should be about 8 Hz or less, which means several millimetres of static deflection, not "almost zero." If the isolator is too stiff, natural frequency rises and may lie near operating speed; the result is resonance, not quiet. Rubber damping, especially natural rubber, makes passage through the resonance region at start-up safer; a metal spring has little damping.

Common Industrial Types

The quality of the metal-to-rubber bond on stud mounts is critical; separation from the metal means sudden failure, not gradual wear. Technical details of this process are described in the rubber-to-metal bonding article.

Elastomer Material Selection

Natural rubber (NR): the best general choice for vibration isolation; high resilience, excellent fatigue resistance, low internal heat. Main weaknesses are oil, ozone and unprotected outdoor exposure. Full comparison in natural vs synthetic rubber.

Neoprene (CR): better resistance to moderate oil, weathering and flame; isolation slightly weaker than NR but balanced for outdoor pumps.

NBR: suitable for continuous hydraulic-oil or fuel splash; for pure isolation NR is better.

EPDM: for outdoors, ozone, water and steam; do not use in oil.

Silicone: very low or high temperatures; lower tear strength and load capacity and more expensive.

Shore Hardness and Load Capacity

Typical industrial isolator hardness is Shore A 40 to 70: 40–50 soft and better isolation with lower load capacity, 55–65 the common motor-and-pump balance, 70–80 for high load with risk of weaker isolation. Hardness alone does not determine load capacity; capacity also depends on loaded area, free height and part shape. Typical design static compression is 10–20 % of free height; permanent compression above 25–30 % shortens life through compression set.

Selection for Motors, Pumps and Machinery

For electric motors and gearboxes specify rated speed, weight, number of feet and presence of oil splash; variable-speed (VFD) motors are more dangerous because they may sit on mount resonance at a particular speed. For centrifugal pumps, in addition to rotor unbalance, hydraulic pulsation creates extra force; the isolator must not replace alignment correction. For diesel generators and reciprocating compressors, softer mounts and an inertia base are often needed. More detail on heavy rubber-metal parts is in the heavy rubber parts for processing equipment guide.

Installation Errors

A rigid parallel connection (pipe or duct that bypasses the mount), unequal load on four feet, installing in tension a part designed only for compression, continuous oil drip on NR, and over-tightening the mount until it "doesn't move" — all these errors destroy isolation. A vibration isolator works correctly only when it is the sole force-transmission path.

FAQ

Sources & References

This article is based on ASTM D2240 (Shore hardness), ASTM D412, ASTM D395, and common vibration-isolation engineering principles (including ISO 10846).

Summary

Select a vibration isolator by frequency and load, a shock absorber by impact energy and crush resistance. Natural rubber for isolation, neoprene or NBR for oily or outdoor environments, and attention to actual static deflection are the three main rules. Send the weight, speed and environment of your equipment so that a suitable vibration mount can be proposed and manufactured — request a free consultation and price quote.