Productivity Forum: Bearings and bushings

March 1, 2002
Components ride on bearings and bushings for reduced friction. But what does the effectiveness of a bearing or bushing ride on? We asked several people in the field how to increase end-user productivity using these components. Here’s what they said

What is the best physical design of a bearing or bushing? Different setups have varied benefits.

Roger: Bushings offer several advantages over ball bearings, including cost, space savings, self-lubrication and compatibility with adverse environments. The nature of any bushing, whether polymer, Teflon lined fiberglass, manganese bronze or wood, means that two surfaces rub against each other, with resultant heat-generated friction.

Andrew: Beware of “self-aligning” linear bearings that adjust only to other bearings on the same shaft. They won’t work right if you try to use them in tandem across two rails not mounted in parallel. Here, you need to make sure the sliders in each rail can adjust to non-machined mounting surfaces, even if they are out-of-parallel in multiple axes.

Roger: Housed plane bearings are often dimensionally interchangeable with most industry standard housed units, including self-aligning plane bearing inserts. The inserts have a spherical OD, allowing them to “align” inside the housing, ensuring uniform full contact with the shaft.

Warren: A keyless bushing eliminates the problems associated with a keyed connection, such as fretting corrosion, backlash, key failure, etc. It eliminates the time-consuming, permanent and expensive task of machining a keyway into the shaft. This allows less downtime for installations, as well as less money spent on machining the shaft. In addition, a smaller shaft can be used, since no keyway is required, which can reduce the weight of a piece of equipment.

Andrew: Sliders that rotate in their rail during mounting can absorb parallelism errors. Once fixed, such sliders will run along a nonparallel path without binding or causing additional preload or play. The slider in one rail guides the movement, carries load, and absorbs structural and assembly errors while the slider in the other shares its part of the radial load (Mz moment) while also absorbing structural or assembly errors.

Warren: Lost time on installation or adjustment of mounted components can be detrimental to the bottom line. Obviously, the more time equipment is out of service, the less uptime. And as far as productivity is concerned, uptime is everything. If a bushing is simpler and quicker to install or adjust, you’ve saved money, no question.

Andrew: One of the important factors in choosing a linear bearing is the amount of linear precision needed. Higher precision bearings need to be mounted to more precise surfaces, and need to be mounted perfectly parallel to each other. Does the application need this precision? Does it gain something from the time invested in mounting the bearings parallel? Can those costs be passed on ...? For some machinery applications (like the moving heads of a machine tool, or a laser-measuring table) the answer is clearly yes. In most cases, however, the precision isn’t needed; the answer to these questions is no.

How do bearings increase productivity? Bushings indeed reduce friction, making movement easier. Where else can their designs increase — or decrease — performance? Several of our sources named contamination as a major challenge.

Pete: For many applications, failure is the result of contamination entering the bearing and causing dents on the raceways that lead to premature flaking or wear. This is more predominant in applications that use a common oil system to lubricate different components beyond just the bearing.

Andrew: Radial ball bearings in linear motion offer many advantages. Among them are higher speeds and less noise. Radial bearings are also larger than simple ball bearings used in recirculating ball sliders; because of this, they tend to work much better in dirty or contaminated environments. Where a grain or particle may stop a ball bearing, it would simply be pushed out of the way of the radial bearing.

Pete: Bearing life can be extended by keeping contamination out of the bearing. This may be done by eliminating debris from other components that are part of the assembly, or keeping the assembly work environment clean. If the level of contamination can not be reduced to low enough levels, special bearing materials and heat treatments are probably in order.

Andrew: Bearings that run inside the rails work well in dirty environments especially if they have spring-loaded wipers protecting them.

Big bearing buzzwords right now: self-lubricating, specially-coated, synthetic.

Ben: Self-lubricating bearing design has been fundamentally improved by a combination of three things: new PTFE mono-filament technology (higher tenacity fibers), improved wear-liner construction, and enhancements of the PTFE fiber geometry in relation to the wear surface. PTFE in the form we use is actually a mono-filament composite as opposed to a random filler in a thermoplastic.

Roger: We use two oil-filled polymers for inserts: nylon and Tivar (UHMWPE). The Tivar comes in powder form; under heat and pressure, it is molded into sheets, or ram-extruded into rod or tube. Then oil is added to the Tivar base resin and mixed... the oil is evenly dispersed throughout the resin; therefore the shaft is exposed to a continuous supply of oil as the polymer wears.

Ben: ... as a pin moves at pressure it builds up heat due to friction. This friction-related heat ultimately produces a phase-change in the PTFE, causing it to migrate from the bearing ID to the pin OD. This smearing takes place over 650°F and is a gradual process...

Roger: The use of lubricant-filled bushings can increase productivity by eliminating the need for additional lubrication and maintenance. Our standard materials for housed plane bearings include oil-filled polymers and a patented FDA-compliant lubricant-filled porous strip that “bleeds” lubricant under the load and movement of the shaft.

Ben: ... PTFE transfer creates what are essentially fillers of very low friction materials that embed themselves in the valleys of the pin material, thus equalizing the surface profile between the pin and the bearing and creating a low friction interface. The PTFE fibers we use are actual mono-filaments — long para-aramid molecular strands that are unbroken. These long chain polymers have a high degree of order, which means they can be either oriented linearly (and create monofilaments) or in a more random fashion as a resin structure material (... a sintering process). The difference is the difference between a ball of hair and an individual hair — both similar constituents, but one has a better chance of holding a needle. Increasing bearing contact is good because the larger the contact area, the more PTFE transfer is going to take place, and the lower the friction will be. This contact area issue must also be balanced against pin selection as the pin hardness, finish, etc. has a lot to do with break-in time and break-away frictional values.

Pete: Future developments in bearings will continue to evolve around improved materials and heat treatment.

Plenty can be done to improve bearing and bushings. What can designers do, in their selection and application of bearings and bushings, to optimize productivity?

Pete: Quantify the improvement if possible. Get the bearing company’s application engineers involved at an early stage and also have clearly defined design criteria; for example: minimum life at a given load and speed. It is also helpful to compare the life of the current design to the new design. This is usually the best benchmark, making efforts to compile the necessary information for the current design worthwhile.

Jeff: We take a proactive approach to training distributors and end-users so that lubrication, adjustment, handling, and damage analysis techniques are fully understood. Another important component is teaching proper maintenance practices. We’ve found that regardless of the operation, if a detailed maintenance manual and program is in place, the more productive the equipment is. There really is a direct correlation — that’s why we have services in place that help bring end-users upto- speed.

Ben: The most important thing to know is that answers can be provided for most applications when we have the time to properly identify performance drivers, and design a product to suite those needs. We also believe that the more attention designers pay to their total cost of ownership, the easier it will be to expand features and benefits while holding down costs.

Andrew: We find that many people spend a lot of time on their linear bearings. First they spend too much time and effort mounting them. Then they take too much time to maintain them. Neither of these should be happening. The bearings are simply a component of a larger machine and should not be demanding attention from the engineers.

Jeff: Education is important for design engineers too — not necessarily in the context of schooling, but more so in terms of knowing what new products, services, and programs are available.

Pete: When making changes to a current system, once again — get the bearing company’s application engineers involved. Also, clearly summarize the historical performance and any changes that have occurred.

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