SUBJECT : Why do pump bearings fail? 4-12

What do we mean by good bearing life? Most of us change the bearings every time we disassemble the equipment to replace the mechanical seal or the packing sleeve. Is this really a sensible thing to do? If you think about it for a minute there is nothing in a bearing to wear out; there are no sacrificial parts.

Bearing life is determined by the number of hours it will take for the metal to "fatigue" and that is a function of the load on the bearing, the number of rotations, and the amount of lubrication that the bearing receives. Pump companies predict bearing life measured in years. As an example, the Duriron pump company anticipates a three hundred year life for the radial bearing on their 3 x 2 x 10 pump ( 75 mm. x 50 mm. x 250 mm.) when pumping a liquid with a specific gravity of "one" (fresh water).

To understand the term "fatigue" we'll conduct an experiment:

When a pump is operating at its best efficiency point (bep) the only loads the bearing have to carry are:

The fact is that most bearings become overloaded because of:

This overloading will cause heat to be generated, and heat is another common cause of premature bearing failure.

Heat will cause the lubricant to:

In addition to the heat generated by overloading we get additional heat from:

A leading bearing manufacturer states that the life of bearing oil is directly related to heat. Non contaminated oil cannot wear out and has a useful life of about thirty years at thirty degrees centigrade (86 F.). They further state that the life of the bearing oil is cut in half for each ten degree centigrade rise (18 F.) in temperature of the oil.

This means that oil temperature regulation is critical in any attempt to increase the useful life of anti friction bearings.

Probably the major cause of premature bearing failure is the contamination of the bearing lubrication by moisture and solids. As little as 0.002% water in the lubricant can reduce bearing life by 48%. Six percent water can reduce bearing life by 83% percent.

There are several methods used by pump companies to keep this water and moisture out of the bearing housing:

The moisture comes from multiple sources:

The moisture causes several problems:

We get solids into the lubricant from several sources:

How to keep solids and moisture out of the bearing housing.

When do you go from anti-friction ball and roller bearings to hydrodynamic (sleeve) bearings in a centrifugal pump?

The past several years have seen a decrease in the quality of the bearings available for rotating equipment. We find pre-packed bearing being shipped with too much, or no grease at all. Stabilization temperatures have changed and overall quality has diminished. If you adopt the above suggestions you should not have to be changing your bearings as frequently as you do now.

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