Bearings, motors and grease

by Drew D. Troyer

As an instructor and consultant in the field of machinery lubrication, one of the more common questions I’m asked concerns the decision to employ motors with sealed bearings. It’s a fair question, but one for which there is no clear answer. Product literature is populated with recommendations based upon horsepower or shaft diameter — a reasonable approach intended to define the point at which a motor is deemed “disposable.” 

While I won’t give you a black and white answer, I will help you understand the differences between the various motor bearing configuration options, make you aware of the advantages and disadvantages, and give you some guidelines on making the right decision.

Motors come equipped with one of three rolling element bearing configurations — open, shielded or sealed. Furthermore, there are three options for employing shields — dual shields, single shield facing the motor and single shield facing the grease supply. There are several key advantages and disadvantages associated with each option.

Open bearings, which have no bearing-mounted sealing or shielding mechanism, dissipate heat better than either sealed or shielded bearings, so they tend to run cooler, are easy to relubricate and there is no risk of the shield collapsing and entering the bearing.

Shielded bearings incorporate one or two shields (typically steel) mounted to the outer race. The shields have a gap, or annulus, of approximately 125 to 375 microns (0.005 to 0.015 inches) through which grease can enter the bearing from the housing’s reservoir. The bearing may be equipped with a shield on either side of the bearing or both sides. For bearings with shields facing the motor, less grease enters the motor’s windings, where it causes damage to the insulation and early failure of the motor. Bearings with shields facing the grease supply help keep contaminants out of the bearing and serve to regulate the supply of grease to the bearings (reduces churning). However, absent another pressure relief, a grease gun, which can produce more than 10,000 psi of pressure, can push the overhung shield into the bearing.

Great care must be taken when greasing bearings with grease supply-facing or dual-shield configurations. Shielded bearings run cooler than sealed bearings but hotter than open bearings, particularly dual-shielded bearings.

Sealed bearings are assembled with elastomer seals on both sides of the bearing. Lubed for life, these bearings can’t be relubricated. In addition to a low preventive maintenance cost, these bearings seal contaminants out of the bearing. However, sealed bearings run hotter than either open or shielded bearings and they can’t be relubed, so the life of the bearing is typically limited to the oxidation life of the lubricant’s base oil.

Generally speaking, the bearings in a motor lubricated with the appropriate volume of grease at the appropriate interval using effective procedures will last longer than a motor equipped with sealed bearings. The reason is simple. Without the ability to get fresh grease into the bearing, the initial charge of lubricant in a sealed bearing oxidizes, which causes the base oil’s viscosity to increase and sludge. This leads to increased churning and heat generation within the bearing that, eventually, causes bearing failure.

Often, the life of motors with sealed bearings is less than four years (it depends on speed and conditions, of course). On the flip side, poorly lubricated open or shielded motor bearings fail for other reasons (contamination, overgreasing, undergreasing, pushing the shield into the bearing, etc.).

Some organizations specify sealed bearings in all motors below 10, 25 or even 50 horsepower, which, in my opinion, oversimplifies the decision. I encourage you to consider all of the following factors that influence the decision:

1) Mission criticality: Sometimes even small motors are critical to production, safety or environmental objectives. Choose the option that provides the greatest degree of reliability for mission-critical applications.

2) PM vs. replace cost: If the motor is hard to access for routine lubrication PMs, sealed bearings may be a good option, particularly if the motor replacement cost is low.

3) Operating speed: Speed will, to a large extend, dictate the life of grease in sealed motor bearings. As speed increases, the attractiveness of sealed bearings decreases.

4) Temperature: Likewise, as temperature (ambient and operating) increases, the life of grease in sealed motor bearings, and thus the life of the bearings, decreases.

5) Condition monitoring effectiveness: If sealed bearings are employed in critical or important applications, you must effectively monitor the motor’s condition with vibration analysis, ultrasonic acoustic emissions analysis, etc., in order to effectively plan and schedule motor replacement.

6) Lubrication program effectiveness: Too often, organizations opt for sealed bearings, or even choose not to lubricate motors with open or shielded bearings, because their lubrication program is ineffective. This is a poor reason to give up lubricating bearings. But until the lubrication program can be set right, it is a consideration.

The decision to go with sealed bearings is an important one. I encourage you to consider it carefully, then be consistent. I’ve worked in plants where identical motors in identical service have differing motor bearing configurations — some greasable, some not — which complicates lubrication program management.

A final tip: Where you do employ motors with sealed bearings, specify the best, longest-lasting grease you can, particularly one made with a base oil that is highly resistant to oxidation. It is the life of the grease that limits the life of the bearings and, hence, the motor. Give yourself the best chance for long life by specifying top-quality grease in motors with sealed bearings.

Drew Troyer is the senior editor of Machinery Lubrication Magazine. If you have a lubrication or oil analysis question, contact Coach Troyer at 800-597-5460 or e-mail dtroyer@noria.com. 

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