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Selecting the Right Vibro-Isolators®


Types Of LoadingKarman Rubber Vibro-Insulators® come in a variety of styles and sizes in order to handle most vibration isolation problems. The product guide on page one shows the different styles of mounts that Karman Rubber offers. Most of these mounts are designed to be used in either the compression or shear direction as indicated in Figure 1.

Karman Rubber Vibro-Isolators® are produced with either Natural Rubber or Neoprene elastomers depending on the application.

  • Natural Rubber has a maximum operating temperature of 150°F and is good for most industrial applications. Natural Rubber is affected by immmersion in oil.
  • Neoprene has a maxiumum operating temperature of 180°F and is resistant to oil.

To select the proper Vibro-Insulator® for a specific application, it is necessary to know the following information:

  1. The maximum load that must be supported.
  2. The number of mounts supporting the load.
  3. The frequency of the disturbing vibration. (Note: If there is more than one frequency involved, the lowest frequency is the determinilng one.)
  4. Any restrictions on the size or style of the mount based on space limitations or assembly considerations.

Karman Rubber will help you to select the proper Vibro-Isolators® based on the information that you provide. You may also select your own mount based on the following calculations.

Step 1: Calculate the load on each mounmt. If the load is evenly distributed, divide the total load by the number of mounts.

 

Step 2: Calculate the lowest disturbing frequency (fd) based on the operating speed in cycles per second (Hz). Convert fd from revolutions per minute (RPM) to cycles per second (Hz).

 

Step 3: Calculate the natural frequency (fn) that the system needs for 80% isolation.

 

Step 4: Calculate the requied static deflection (ds) to obtain the desired natural frequency (fn).

 

Step 5: Calculate the required spring rate (K) to obtain the desired natural frequency (fn).

 

Step 6: Select a mount that has a maximum load rating equal or greater than the calculated load per mount and a spring rate (K) less than the calculated spring rate. Please note that overloading Vibro-Insulators® is not recommended, since it may lead to premature failure. However, excessive under loading will not deflect the mount enough to provide satisfactory isolation.

Step 7: After you have selected the Vibro-Isolators® you should calculate the transmissibility based on the actual spring rate for the selected mount (see example below).

EXAMPLE CALCULATION

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6. Select a mount with a maximum load rating of at least 45 lbs per mount and a spring rate of 1,216 lbs/in or less. The 50 durometer K47 on page 22 has a maximum load of 50 lbs with a K Value of 675 lbs/in. in the compression direction.

7. Calculate the actual isolation based on using the K47 Vibro-Isolators® at these operating conditions.

TERMINOLOGY

Compression - Deflecting a mount in the direction perpendicular to the mounting surface (see Figure 1).

Damping - The reduction of vibrational amplitude through the conversion of vibrational energy into heat. Damping reduces the amount of amplification at resonance.

Disturbing frequency (fd) - The frequency of vibration caused by a source such as a motor expressed as the number of oscillations that occur per unit time (RPM or Hz).

Durometer - The hardness of an elastometer based on a numbering scale that indicates the resistance to indentation of an indicator point.

Isolation efficiency - The percentage of vibration that is absorbed by the rubber mount that protects the equipment. It is equal to (1 - Transmissibility). An isolation efficiency of 80% is generally considered satisfactory.

Load deflection curve - A plot of the applied load versus the distance that a mount is deflected.

Natural frequency (fn) - The frequency at which a system will vibrate freely if deflected from a static equilibrium position.

Resonance - The condition existing when the disturbing frequency is the same as the natural frequency of the system. At this condition the amount of vibration transmitted will be amplified instead of isolated.

Shear - Deflecting a mount in the direction parallel to the mounting surface (see Figure 1).

Static deflection (ds) - The deflection of a mount at a given static load.

Spring rate (k) - The ratio of the imposed static load to the resulting deflection of a particular rubber amount expressed in lbs per inch.

Transmissibility (T) - A measure of the amount of vibration transmitted through a mount. A transmissibility of 20% or less is normally desired.