The amount of wear a bearing is going to experience is related to the bearing pressure and the running velocity. The more pressure applied to the bearing, the faster it will wear; similarly, the faster the bearing surface speed, the more wear will occur. Did you ever wonder why? One clue lies in the amount of heat that is generated as a result of interaction between the bearing and the shaft. The more heat that is generated, the faster the bearing will wear. One value that represents this relationship between heat generated and the resulting wear is the PV value. You will see this value used as a key part of the material selection process when specifying a bearing.
Here are some other articles on Polymer Bushings and Bearings from Advanced EMC Technologies Blog:
- Polymer Bushings Taking a Beating: Two Important Concepts behind the Wear and Tear
- 3 Things About “Self-Lubricating Bearings” Your Boss Might Want To Know
- Plain Bearings VS. Plane Bearings
P, Bearing Pressure
The bearing pressure, P, has units of psi and is a function of the total load acting on the bearing and the effective area over which the load is acting. It represents how much load per a given unit of area that the bearing needs to be able to support.
There are several different sources for these bearing loads, such as the dead weight of the body that is rotating, loads generated by the working forces acting within the mechanical system, and the loads that arise as power is transferred.
The bearing pressure is calculated using the effective area of the bearing. For a plain journal bearing, the equation would be
where F is the total load in lbs acting on the bearing, D is the inside diameter of the bearing (in inches), and L is the length of the bearing (in inches).
V, Running Velocity
The running velocity, V, is also called the bearing surface speed. It calculates differently depending on the bearing application. For example, rotary applications use
where D is the dynamic diameter in inches and V is in ft/min.
For thrust applications
where LS is the length of stroke in inches and C is the number of cycles per minute. Again, V will be in ft/min.
For reciprocating applications, we would use
where R1 is the inner radius of the thrust bearing, and R2 is the outer radius of the thrust bearing.
PV is the product of the bearing pressure P and the running velocity V, with units of psi-ft/min. This value is related to the amount of heat generated as a combination of loading and speed.
Bearing materials have PV limits, which serve as a guidelines for the maximum recommended loading and speed combinations recommended. Above the recommended limiting PV, the rate of wear increases exponentially and some materials will crack and others may actually begin to melt.
The chart below shows the maximum dry PV (assuming room temperature) for some common polymers used as bearing materials.
Its important to remember, however, that you cannot depend just on the maximum PV when selecting a bearing material. There is also a maximum recommended static pressure, Max P, and a maximum recommended no-load velocity for dry running, Max V. These are summarized in the table below for the same four materials.
Lets look at a very simple example for calculating PV for a rotary application.
Assume that we have a bearing with an inner diameter of 1 and a length of 2 in. It is going to be used with a shaft that rotates at 1,200 rpm. The maximum load acting on the bearing has been estimated at 200 lbs.
P and V are calculated as follows
The resulting PV is then
This would be within the range for Torlon 4435 and Torlon Fluorolon 3015 PEEK BG, based on the chart and table above.
Variations in Maximum PV
There are things that can affect the PV limit for a material. Appropriate lubrication can increase the maximum PV, for example, making a bearing capable of running at a higher speed or under a higher load. The type of mating surface used can also affect the PV. For example, a mating surface like stainless steel will exhibit much less wear than a softer metal like aluminum or brass. PV limits can also be affected by variations in temperature, speed, surface finish, and pressure.
When selecting an appropriate polymer for a bearing application, it is important to start by determining the bearing size and length, then estimating the maximum load acting on the bearing, and the speed involved. From there you calculate P and V, and then the PV. This helps you to quickly eliminate potential bearing materials that wont provide sufficient wear resistance for your application.
Need more information, download the Polymer Bearing Design- 7 Step Process in Bearing Selection from Advanced EMC Technologies.