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One of the key characteristics in selecting a material for a polymer sleeve bearing or bushing is the PV value, a combination of the bearing pressure and the bearing surface speed.  This value is directly related to how much wear the sleeve bearing is going to experience, and is quite easy to calculate.  

The topic of this blog post is how to determine the PV value for bushings and sleeve bearings.

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Bearing Pressure, P

The “P” in the PV value refers to the bearing pressure.  It represents a combination of the total load acting on the bearing and the effective area over which that load acts.  The bearing load itself can involve the loads that arise as power is transferred, the dead weight of the component that is rotating, or loads that are generating by working forces acting within the mechanical system.

The standard expression for pressure, P, is used, where F is the load acting on the bearing and Aeff is the effective area upon which the load acts.

Standard Expression for Pressure

In the case of a polymer bushing or sleeve bearing, the effective area is the diameter of the bearing multiplied by its length.  The basic equation for bearing pressure is given below, where D is the inside diameter of the bearing and L is the length of the bearing.  

Basic Equation for Bearing Pressure

If you are working in English units, then force should be in pounds while diameter, diameter and length in inches, and the resulting pressure in psi.

Bearing Surface Speed, V

The bearing surface speed, also known as the running velocity, is dependent on the bearing application.  For sleeve bearings and bushings, the applications are usually going to be rotary or oscillating.  For rotary applications, the equation is given below where d is the diameter of the shaft in units of inches and RPM is the rotational speed.  

Bearing Surface Speed for Rotary Applications

The resulting bearing surface speed V will be in units of ft/min (sometimes reported as sfpm for surface feet per minute), and represents the speed of the surface that will be in contact with the inside of the bearing or bushing.

For reciprocating applications, the surface speed can be found using the equation below where LS is the length of stroke in inches and N is the number of cycles per minute.  The resulting units are again ft/min.

Bearing Surface Speed for Reciprocating Applications

The PV Value

The PV value is the product of the bearing load P and the bearing surface speed V.  The resulting units are going to be in psi – ft/min.  However, the PV value alone is not enough.  These PV values are usually associated with a maximum bearing pressure and a maximum surface speed.  The table below shows some examples of values for PV, Max P, and Max V for several different polymer bushing materials.

PV Value of Polymer Bushing Applications - Chart

When selecting a material, therefore, it is necessary to check the PV value, then verify that the Max P and Max V limits are not exceeded.

Conclusion

Calculating the PV value for a plain sleeve bearing or bushing is a simple matter once you have an idea of the expected loads and the bearing dimensions.  However, when it comes time to select a polymer, make sure that the PV value is within range for the material and that the P and V values do not exceed the Max P or the Max V for the material.

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