Productivity is a shared goal in industrial applications, involving designers, component makers, and end users. Everyone plays a role. In this report, the editors of Motion System Design polled bearing experts for their advice on optimizing productivity. Here are the responses on both sides of the issue, which we believe you'll find most helpful.
What to do
What particular design/construction features in a bearing contribute to HIGHER productivity?
Andrew/Rollon: Lifetime is the key to productivity, and the most important factor in increasing lifetime is the choice of material for the bearings themselves. The options here are actually quite varied. Most manufacturers choose a bearing steel like 52100. Running a hardened 52100 ball or roller over a hardened rail offers great lifetime possibilities.
The design of the moving element also affects lifetime. Designs based on radial ball bearings or involving ballcages that do not allow bearings to touch tend to offer greater lifetimes.
Jim/RBC: For rolling element bearings, longer running life is achieved by maximizing the internal ball or roller size within the envelope. In continuously rotating applications, improved surface finishes of the rolling element raceways help to provide a thicker lubricant film to effectively separate the rolling surfaces, subsequently extending the life of the bearing design. In some applications, bearing ring material plays a significant role. Traditional bearing rings are constructed of through-hardened 52100 steel. In applications that involve high impact loads, a case-hardened material is selected because the ductile core resists fracture under impact.
Greg/Rockwell Dodge: There are many design and construction features that impact bearing performance. Highly refined bearing steels are the cornerstone of top-quality long lasting bearings. With modern manufacturing techniques these steels can be machined to produce surface finishes and geometry which are designed to hold lubricant and promote proper rotation of internal components. Heavy contact seals to assist in retarding contamination, or labyrinth seals to decrease rotational friction, are design features that when properly specified can greatly increase bearing life. Various styles of shaft attachments can be incorporated to produce easy on/off features, higher speeds, and different loading characteristics.
What can designers do to ensure HIGHER productivity from bearings?
Jim/RBC: Designers can ensure higher productivity from bearings by first choosing the correct bearing for the application. For example, certain types of ball bearing designs accommodate different types of load conditions. A standard radial contact ball bearing has excellent radial load carrying capability but moderate axial load capability, whereas angular contact ball bearings would have excellent axial load carrying capability. Once a bearing type is defined, the bearing can then be sized for the application load and conditions. Designers should also pay close attention to the housing and shaft fits for bearings. Rolling element bearings are manufactured within a range of radial clearance. Incorrect press fits can eliminate this clearance resulting in premature failure.
Andrew/Rollon: Proper sizing and mounting are crucial. Many engineers go through the process of calculating the forces applied to the bearings without considering the importance of where and how the bearings are going to be mounted. Most recirculating ball type bearings and bushings have ground shafts. Many of the advantages of this type of bearing can be lost or severely compromised with incorrect or improper mounting. Are the mounting surfaces ground? Can you guarantee the parallelism of the rails on three axes? If the answer is “no,” self-aligning linear bearings should be considered.
Chris/Tuthill: When using a plain bearing, one of the key design parameters that should be taken in to consideration is the load direction. If possible, load should be applied radially. This is so the race takes the distributed load of the bearing. As in any formed linkage application, one should try to avoid loading a bearing in the direction against the formed material, thus trying to open up a swage or stake.
What can end users do to ensure HIGHER productivity from bearings ?
Greg/Rockwell Dodge: The number one failure mode of mounted bearings can be traced to a failure of the lubrication. Many times this is caused by poor maintenance practices, including too much lubrication, not enough lubrication, or the wrong lubricant. When lubrication becomes contaminated or stressed, the film thickness separating the internal bearing components decreases until wear of those components begins. If left unchecked, wear will continue at an accelerated rate until bearing failure occurs.
Jim/RBC: End users can ensure the highest bearing productivity by following the machine manufacturers' guidelines for the equipment. Each bearing is typically sized in a machine to operate within a load range to achieve a predictable life cycle. When equipment is run beyond its design capability this life cycle can be dramatically reduced. Lubrication filtration can also play a large role in bearing longevity. This filtration can be implemented within oil circulating systems reducing particle size or simply re-lubing through grease fittings, purging contaminants in grease applications.
Chris/Tuthill: Plain spherical bearings are never meant to be used in high rotation or spinning applications. Ideal applications are control arms, shocks and other back-and-forth type linkages as in steering applications.
What NOT to do
What particular design/construction features in a bearing can REDUCE productivity?
Tom/igus: If there isn't enough lubrication on the surface contact area, either because it has been used or the shaft is not moving fast enough to draw out the lubrication, the coefficient of friction goes up. This results in more wear. If using a metal or bronze bearing, these require constant re-lubrication, which continually interrupts production and increases cost, or if not preformed at all, will significantly lower performance and therefore production. Plastic bearings are self-lubricating and need no maintenance, ensuring a constant productivity level. Also the packaging of a lubricated bearing requires time and manpower to open and assemble. All of this leads to diminished productivity.
Pamela/Bishop-Wisecarver: Design features that affect lifetime and hence productivity include geometry and tolerances, surface finish, and hardening of the ball raceways and balls. The quality and quantity of the lubricant is also important. Uneven heat treating and inconsistent quality in raw materials can lead to a shorter life span as well.
Milton/Bosch Rexroth: The number of rows in the bearing can have a critical impact on load capacities. Four-row bearings obviously can handle greater loads than two-row, translating into longer travel life, less maintenance, and higher rigidity. Orientation of the tracks or raceways is also very critical. Some bearings have equal load carrying capacity in all directions, while others have derating factors based on the direction from which the load is applied.
Balls vs. rollers: Balls are much more forgiving in terms of system misalignment, but rollers provide much higher rigidity and better system accuracy.
Materials: Stainless steel is a common option with bushings because of the outstanding corrosion protection. But stainless bushings do not have the load carrying capacities of most manufacturers' standard options. Plating is another method of achieving similar levels of corrosion resistance, without the load derating.
What can designers do to REDUCE bearing productivity?
Pamela/Bishop-Wisecarver: Designers can underestimate the axial and/or radial loads the bearings would experience during normal operation which would result in additional unanticipated stresses and thus promote premature wear. Bearing housings not machined and/or positioned properly (causing unanticipated loads) may also lead to excessive loading, causing substantial and premature wear. Another problem is that designers may not be educated in the proper placement of bearings and the trade-offs between pre-load, capacity, and rolling resistance. Other factors designers tend to overlook when applying bearings include mis-alignment, over or undersized mounting bores, poorly engineered overhung loads, and speed and load mis-calculations.
Tom/igus: The most important factor a designer must consider is the expected lifespan of the bearing. If this isn't first determined and taken into account, then productivity levels are most assuredly threatened. It's also critical to ensure the right bearing is chosen for the application. A designer should consult a design technician and be honest about what the parameters of the application are. It does no good for anyone if the temperature will really be five times higher than what the engineer said it would be. The bearing will fail and the machine will cause havoc for everyone.
Milton/Bosch Rexroth:Improper selection of bearing preload or clearance can be a major influence on reducing productivity. Engineers sometimes request higher preloads than are really necessary because they want to ensure that the load can be stopped with precision. In applications with lighter loads, however, adding too much preload simply increases the sliding friction and can decrease machine performance.
What can end users do to REDUCE productivity fromthe bearings on their machines?
Tom/igus: End users who aren't aware of the limitations of their machine run the risk of pushing a bearing too far. This can lead to a rapid reduction in productivity. Education is key to understanding the machine's capabilities, and it will lead to longer life and maximum performance. In the case of plain bearings, choosing the wrong type of shafting is another way to reduce productivity. With plastic bearings, if the shaft is too smooth, it won't transfer the low-friction coating to the shaft; the end user, as a result, will experience stick slip and noise. On the other hand, if the shaft is too rough, the bearing will wear too quickly. With metal, a smooth shaft is imperative or the bearing will be damaged.
Pamela/Bishop-Wisecarver: Neglect is a big problem. Excessive heat build-up (insufficient cooling), environmental contaminants (dust, dirt, moisture), and improper maintenance (or lack thereof) all contribute to reduced bearing efficiency and life. End users also cause harm when they fail to account for the machine's duty cycle and environmental conditions in their maintenance regimen. Other common pitfalls include lack of lubrication, over-lubrication, pressure washing with excessive heat and pressure (even with wet-environment bearings), and not following recommended loads, speeds, and hours of operation.
Milton/Bosch Rexroth: Improper lubrication is the number one contributing factor to premature failure or excessive machine downtime. The type of lubricant, the frequency of lubrication, and the volume of lubricant can all be critical factors and should always comply with the manufacturer's recommendations. An additional consideration is that in some cases, the lubricant should be added in steps, with strokes to distribute the lubricant completely throughout the bearing. Some manufacturers pre-lube the bearings at the factory with the expectation that they will last the life of the application in many instances. Here, however, engineers and maintenance personnel should pay close attention to the rated life because “lubed for life” claims are sometimes based on tests that do not reflect real world situations.
MEET THE EXPERTS
Bosch Rexroth Corp.
Dodge Roller Bearing Development Engineer
Chris A. Kaufman
New Haven, Ind.
East Providence, R.I.
Senior Product Engineer