Basic considerations for successful bearing upgrades

Follow this road map to a successful bearing upgrade.

By Norman A. Buchta

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In today's global marketplace the need for manufacturers to continually increase productivity or efficiency seems relentless. Quite frankly, until the 1970s, most new original equipment was slightly to moderately over designed. This was particularly true of the bearings that equipment designers selected. The cost of overdesign in that era was absolutely minimal and had many advantages. One of the advantages to the industrial user was that the inherent overdesign allowed the user to increase "speed" or "load" without the need for much redesign and retrofit of critical parts such as bearings.

For decades, it was almost a "no brainer" to increase equipment output. You simply recalculated the drive ratios, installed new sprockets, gears, or belts, and walked away to the cheers of plant management. The bearings generally were ignored as a consideration because their intrinsic overdesign made them capable of handling the added burden the upgrade placed on them. I've often envied the folks who preceded us during the 30s, 40s, 50s and 60s. I've also sometimes cursed them. Each time they increased speed or capacity on equipment that has been in service for decades, they progressively ate into the "reserve" built into the original design.

Today's equipment designs are much less conservative than older designs. The original equipment manufacturers themselves must compete in the global economy. This means that even minimal "overdesign" is too costly to be tolerated. The result is the same in either case. So, today it is no longer a "no brainer" when upgrading equipment to increase capacity or speed to increase efficiency. The bearings must be a consideration when upgrading equipment.

Where to look for assistance

Before you do anything, first call the engineering department of the original equipment manufacturer. Ask if they can provide the original design specifications. Ask if they offer assistance to upgraders. Some will and frankly some won't cooperate with you. It's worth a phone call to find out.

If they are willing to help out, explain the reasons and details of the upgrade (such as increases in speed or load). Don't neglect to tell them what your duty cycle is and what it was when the equipment was purchased. It may make an important difference if the equipment was designed to be used intermittently by one shift 8 hours per day but is currently in continual use by three shifts 24 hours per day.

Regardless of whether your OEM will be of any assistance, also contact your bearing supplier. Ask your bearing supplier for engineering catalogs, help in bearing design decisions and specifications, training for your staff, and, if warranted, assistance in bringing a factory field sales engineer to your site.

Be realistic when you ask your supplier for assistance. If you have a "system contract" or a "value-added relationship", it is more likely that you will find your supplier able and willing to provide this type of support.

Basic considerations of ball and roller bearings

An important but often overlooked fact about rolling element bearings is that ball and roller bearings are designed to wear out, but only after achieving a statistically predetermined service life. This service life may be at any point the designer wishes; however, it is most commonly calculated at a point at which either 5 percent, 10 percent, or 50 percent of a population of bearings under identical conditions are expected to fail. The most common point used is 10 percent and it is often referred to as either B<->10<-> or L<->10<->, expressed as either the number of revolutions or hours of service. In determining the appropriate service life, the original designer took into consideration the load, speed, and duty cycle of the equipment (8, 16, or 24 hours per day).

The relationship among speed, load, and bearing life for rolling was established a good number of years ago. It is not a linear relationship, but rather an exponential one. Small changes in speed or load result in large differences in bearing life. Decreases in speed or load increase bearing life, and conversely increases in speed and load decrease bearing life.

Since equipment upgrades generally involve increases in speed or load, it is most likely that such upgrades result in decreases in bearing life with more frequent downtime or overhaul.

Basic considerations of plain bearings

The considerations for plain bearings are somewhat different. The main design considerations are the compressive strength of the material used (generally expressed in thousands of pounds per square inch), the operating speed of the shaft (generally expressed in revolutions per minute), and the type of lubrication (either "boundary", "mixed film", or "full-film").

As a rule of thumb for attaining an acceptable service life, the actual load on most plain metallic bearings should not exceed 1/3 of the bearing's compressive limit. The limiting load and speed for plain bearings is often expressed in manuals as the <I>PV factor<I>. This is simply the pressure on the bearing (in PSI) multiplied by the surface speed of the shaft (in feet per minute).

It is important to note that while PV factor is a convenient way to calculate plain bearing performance, it may be misleading at times. It is possible to have an acceptable PV factor, but at the same time have either the speed or load exceed the limitations of the material.

Therefore, take into account both the compressive strength and the PV factor for plain bearings.

What is interesting about plain bearings is the large role that the type of lubrication plays in their life and successful application. Both "boundary" and "mixed film" lubrication types allow metal-to-metal contact that results in wear and, ultimately, bearing replacement. But if you can achieve "full-film" (also known as hydrodynamic) lubrication, then no metal-to-metal contact exists. This means that theoretically no replacement will ever be required as long as "full film" lubrication can be maintained. In practice "full film" lubrication is difficult to achieve and may be cost prohibitive.

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