Exceeding PV limit of a plain bearing 1

[serdart]

Hi,

What if I exceed the recommended PV value of a bearing? Does it become useless (break, gall or seize) rapidly, or does only its life shortens?

And when these PV values are determined, are the manufacturers stick to any standard of ISO, ASTM...?

Thx

 

[caoimhin1]

Excessive heat generation basically. PV is N/mm^2.m/s, if I remember correctly if you play around with the units you get W/mm^2, energy generated per unit surface area. The higher the PV value the more heat is generated.
You may be able to exceed the limits by very carefully selecting your lubricant, cooling and filtering it as well as getting your tolerances and clearances perfect.

 

[serdart]

I am talking about not instantaneous exceeding, but a continuous operation.

I am, actually, trying to understand what is this "PV" thing analogous in strength of materials; the yield strength or the endurance limit of fatigue concept?

 

[israelkk]

P is pressure = compressive/bearing stress. If it is above the bearing material allowed compressive/bearing yield stress then it will yield the bearing material. PV is power in Watts/unit area. This power will heat the bearing material therefore, the allowed compressive/bearing stress may reduce (same as tensile stress at high temperature is lower than at room temperature).

 

[MikeHalloran]

The PV value is not directly related to the usual mechanical properties reported for a material, and as has been noted, the units are different.

The V part is a velocity, of course.
The P part is a pressure computed on the projected area of the bearing, and the allowable pressure is usually orders of magnitude lower than the compressive yield stress.

What happens to a given bearing material at PV values above the limit depends to a large extent on whether the P is excessive or the V is excessive. I.e., the resulting failure modes are usually different for each bearing material, and they are different from one bearing material to another.

That said, the usual result of excessive V is, as noted, heat, and damage normally associated with overtemperature of the particular bearing material.

The usual result of excessive P (still nowhere near the yield stress, mind you) is extreme wear, not necessarily accompanied by plastic distortion.


I am troubled because you appear to be contemplating intentional use of some material beyond its recommended PV value. That's just a waste of money and time.

The recommendations are based on experience. If you do not value experience now, you will, after you have ignored it and then acquired it for yourself.


Mike Halloran
Pembroke Pines, FL, USA