Two values that are very key to the performance of a polymer bushing are the speed and pressure. In this second article of the series, we are going to review how to determine them and what effect they have on bushing performance.
Running speed is also called bearing speed or surface speed. How you determine depends on how the bushing is going to be used. If it is a rotary bushing, the equation is:
where D is the dynamic diameter in inches and RPM is rotational speed in revolutions per minute giving bearing speed V in ft/min.
It makes sense that the greater the running speed, the greater the bushing wear. Polymer bushing materials have a no-load maximum dry running speed V which can help you narrow down your material choices.
Bearing pressure is the next key value that determines the performance of a bushing. It is basically the total force acting on the bearing divided by the effective area of the bearing. Its calculation is quite simple:
In the equation above, F is the total static load acting on the bearing in lbs, D is the inside diameter of the bearing in inches, and L is the length of the bearing in inches. The resulting pressure P is in psi and represents the load per unit area that the bearing needs to be able to support.
The sources of these loads could be due to the transfer of power, the weight of the rotating component, or loads generated within the mechanical system. A properly designed bearing will be able to carry these loads without excessive deflection, and certainly without failure. Note that extremely short bushings are going to result in higher pressures, and thus wear more quickly.
Polymer bushing materials have a static maximum recommended pressure that, like the no-load maximum running speed, can help you narrow down the number of polymers you have to choose from.
While pressure and velocity can be considered separately, its even more important that they be considered together. This is done by calculating the PV, which is the product of the pressure and the running speed. The resulting value has units of psi-ft/min and is used to select an appropriate bearing material.
So why do pressure and velocity need to be considered together? A bearing might function at a particular speed and a particular pressure, but when that pressure is applied to a shaft running at that speed, heat is going to be generated. The heat generated has a tremendous effect on the rate of wear, and in some cases can even result in cracking or melting of the bushing (depending on the material).
This critical value of PV where the bushing begins to experience an abrupt increase in wear is known as the PV limit. Every polymer bushing material has a PV limit which helps to guide you toward the most appropriate material for your bushing. One of the interesting aspects of the PV limit is how closely it is tied to the temperature of the bushing. Anything that can affect temperature including friction can actually affect the performance of the bushing. This includes lubrication, surface finish of the shaft, hardness of the shaft, running clearance, etc.
Pressure, running speed, and the PV for a particular application can help you to choose a bushing with the right level of performance. Higher shaft speeds and higher pressures will mean a higher rate of wear, and can make the task of selecting an appropriate bushing material more challenging.
Missed Part I in the Series? Read it here:
Factors That Influence Polymer Bushing Performance: Mating Hardware – Part I in a 3 Part Series