Basic considerations for successful bearing upgrades

Follow this road map to a successful bearing upgrade.

By Norman A. Buchta

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.

Bearings systems not just bearings

Bearings do nothing by themselves, a fact regrettably often ignored in the real world.

Together, with housings and shafts, they form a <I>system<I>. The best bearing in the world will not achieve its optimum service life if the shaft and housing are improperly or poorly made.

It is generally agreed that the vast majority (80-90 percent) of ball bearings fail prematurely for preventable reasons. Of those that fail prematurely, approximately 25-27 percent fail because either the fit or mounting was improper.

I do not know of any corresponding statistics for plain bearings, but in over 20 years of consulting on bearing related issues, I believe that improperly machined or finished shafts and housings account for a high percentage of premature plain bearing failures. The good news is that knowledge about bearing failures can be leveraged when considering how equipment upgrades may affect bearing performance or life.

High performance requires high precision tolerances

In maximizing bearing performance, every mechanical component of the bearing system probably requires machining to higher precision tolerances than most maintenance people would intuitively believe. The minimum tolerances for shaft diameter and out-of-round for the most basic precision ball bearings (ABEC 1) are specified in the tens of thousandths (0.0001) of an inch, with a surface finish of 30 microinches, or better.

Still higher precision ball bearings (ABEC3 and above) and other types of rolling element bearings require even closer attention to shaft tolerances, surface finishes (16 microinches, or better), parallelism in the tens of thousandths of an inch, and hardness of Rockwell 58C to 62C. For plain bearings, some shaft specifications such as surface finish (10 microinches, or better) are even more stringent than for rolling element bearings and are more critical to their successful application and life.

While housing tolerances and specifications are somewhat less stringent than shaft specifications, they are nonetheless equally important to long life and successful application. Rolling element and plain bearings take the shape of their housings. If the housing is out-of-round, it forces the bearing ring seated into it to become out-of-round as well. Out-of-round bearings neither perform to their maximum design capability nor live up to their expected service life. Ask your bearing supplier for engineering manuals that contain a wealth of information on shaft and housing tolerances.

My experience has been that few internal maintenance departments have machine tooling capable of producing the necessary combination of geometry, tolerances, surface finishes, and hardness the bearing applications require. I also observed that some commonly used reference books commonly seen in machine shops do not have the appropriate AFBMA or ISO bearing shaft, housing, and fit tables. The correct specifications are found in any bearing engineering manual obtained from your bearing supplier. Given this information, if re-machining of bearing shafts and housings is necessary, it is likely to require outsourcing to a properly equipped and capable machine shop.

If the equipment being upgraded is small or can be disassembled into relatively small parts, then most organizations are probably already working with a machine shop capable of meeting the more stringent specifications. If the equipment is large, it may be possible to have the machining done "in place" by an outside vendor. A good place to start looking for capable vendors is to ask the maintenance department of your nearest "major" electric power plant.

Points to remember about assembly and disassembly

The best bearing systems are doomed to failure if assembly and disassembly do not follow proper protocols. Improper assembly or disassembly causes somewhere around 27 to 30 percent of rolling element bearing failures. It will do little good to endure the expense and trouble of redesigning and re-machining during an upgrade, only to have the bearings fail prematurely because of improper mounting or dismounting techniques.

While it is true that a knowledgeable individual can mount and dismount bearings with a hammer and a few specialized tools, most individuals cannot. Properly sized and used pullers and presses are the safest methods, but are not, unfortunately, a guarantee of success. Regardless of the method, direct force (hammer blows or otherwise) should only be applied to the <I>ring being mounted<I>. In other words, you never push on the inner ring to force the outer ring into a housing. You never push on an outer ring to slide the inner ring onto a shaft.

In addition to proper use of force, cleanliness in mounting and dismounting bearings is vitally important. Contamination introduced during assembly or disassembly is another significant cause of premature bearing failure. Try leveraging the old design first. Before you undertake major bearing redesigns, begin boring new holes, turning and grinding a new shaft, or purchasing different bearings, carefully examine whether all that is really necessary. You may be able to simply leverage the existing design by bringing the existing bearing system back to its original specifications. This, together with changing some of the bearing's internal specifications might thereby maximize its inherent capabilities.

Rolling element bearings must have the housing and shaft properly inspected and, if necessary, restored to the proper tolerances. Consult your bearing supplier to determine if changes in retainer type or material, internal clearances, lubrication type or amount, or any other internal specification can increase bearing performance without changing the actual bearing size or type.

For plain bearings, have the housing shaft properly inspected and restored to the tolerances specified in bearing engineering manuals. Next, consider improving the shaft surface finish if it was originally greater than 10 microinches and, if at all possible, have the shaft finished by "plunge grinding" to avoid machine lead. Finally, consider consulting with your bearing and lubricant suppliers to determine if increases in lubricant viscosity are possible.