The dynamic properties of urethane in combination with its high load bearing capacity make it an excellent choice for a number of vibration isolation applications. The first step in understanding how urethane can be used to isolate vibration is to look at all the factors that are involved.

Natural Frequency

The un-dampened natural frequency is expressed in Hertz (Hz) or cycles per second. It can be found using static deflection and the following formula:

Natural Frequency formula

Frequency Ratio

The ratio that indicates how effective a vibration isolator, such as polyurethane, performs is found by dividing the forcing frequency, by the natural frequency. The forcing frequency is sometimes referred to as the disturbing or driving frequency and is also expressed in Hertz or cycles per second.

frequency ratio formula

Vibration Damping

The vibration damping effect is due to the hysteresis component of polyurethane elastomers. Hysteresis converts mechanical energy into heat, which is then dissipated. In free vibration a fair percentage of the input energy is dissipated in the form of heat during each cycle causing the vibration to die out.

damping ratio formula

The Damping Ratio is used to indicate the amount of damping in a system; it’s affected by temperature and preload. Typically for most polyurethane it can be varied from a low of 0.05 for highly resilient compounds to 0.15 and higher for low resilient compounds. The transmissibility curve shown below can be used to design polyurethane vibration isolators. See the example below for more information or feel free to contact us and speak with one of our engineers.

Transmissibility and Damping Example Problem

A facilities engineer is designing a mount for a 5,000-pound rotary compressor. It will be a sandwich type mount with four urethane pads bonded between steel plates. The compressor motor operates at 1800 rpm. The engineer would like to isolate at least 75% of the disturbing vibratory forces with the urethane pads. Isolating at least 75% of the disturbing force means that the transmissibility will have to be less than 25%.

The urethane compound used for the pads has a damping ratio of 0.1. By examining the Transmissibility graph, the engineer determines that to get a maximum of 25% transmissibility with a damping ratio of 0.1, the frequency ratio will need to be 2.5 or higher.

a) The Forcing Frequency in Hertz:

forcing frequency formula

b) The Natural Frequency in Hertz

natural frequency formula

c) The Minimum Required Static Deflection:

static deflection formula

d) See the section of this guide on the load bearing ability of polyurethane for an explanation on how to properly size this mount for this deflection.