What applications present the biggest challenge in terms of machine productivity?

CHRISTIAN/NMB: Applications where temperatures run above 120°C are the most challenging. High temperatures reduce the life of the bearing by degrading the grease at an accelerated rate. Because high temperature greases tend to be noisier, your challenge is to decide between noise and life. In addition to lubrication choice, elevated temperatures usually require a review of the enclosure, retainer, and cage.

High-speed applications are also very challenging. “High speed” is a relative term, referring to any speed that approaches the bearing’s “critical speed.” In most cases, critical (maximum) speed will be spec’ed in the bearing manufacturer’s catalog. Usually, the value relates to a unit equipped with a common two-piece, metallic ribbon retainer. Not all retainers are made of metal, however. For higher speeds, many bearings employ glass-fiber reinforced nylon retainers; bronze, plated-steel, and phenolic retainers are also used, particularly in extreme cases (2X critical speed). Beyond that, you’re probably looking at ceramic balls because of their lighter weight and lower moment of inertia.

MARK/TIMKEN: Generally speaking, any application that contributes to inadequate lubrication, poor maintenance, and debris ingress can pose a challenge for machining operations.

KATE/SAINT GOBAIN: Most applications where hybrid bearings turn up are the tough ones, requiring higher speeds, longer life, less wear, and less maintenance.

DAN/SKF: Applications involving low speeds and high loads are also a challenge. Environmental contamination, both liquid and solid, can be a problem as well.

TOM/IGUS: The biggest challenges for polymer bearings include high-speed, continuous-duty, extreme-temperature, and heavy-load applications.

What are the most common pitfalls and how can they be prevented?

CHRISTIAN/NMB:

• Lack of preload: Since ball bearings have internal clearance (or looseness) between the balls and raceways, they need to be preloaded in order to eliminate major vibration and noise in the system design. The easiest fix here is to simply offset the inner and outer rings axially with a wave washer, spring, or shim, or add a deadweight and glue in place.
• Improper lubrication: Choosing the right lubricant for an application is the most crucial step in the implementation of ball bearings in a design. Lubrication affects lifetime, bearing noise, and torque (drag), and is a potential source for compatibility issues with plastics and elastomers.

Oil is a basic lubricant for ball bearings, but if an application requires thousands of rpm, the bearing will quickly fail because oil readily disperses away from the balls and raceways. Greases hold oil in place, allowing the rotating components to continually have a reservoir of lubricant during rotation. Selecting the right lubrication is a matter of defining temperature range, speed requirements, lubricant type (grease or oil), viscosity (oil), and base oil (grease).
• Fitting procedures: How a system runs can be dramatically affected by the way ball bearings are handled and mounted. Bearings that are damaged due to excessive force or shock loading during assembly, or improperly fitted (too tight or too loose), will cause problems. Excessive interference fit, for example, results in a ratchety and bumpy rotation.

Bearing radial play (internal clearance) and seat tolerance (location of bearing bore and OD) also need to be reviewed. Tightening or shifting tolerances and/or increasing radial play is usually sufficient. It gets more complicated when the application sees a wide range of temperatures or if the bearing mates up with dissimilar materials.

MARK/TIMKEN : There are several factors that can limit the life of a bearing, including heat, speed, load, sealing, adjustments, lubrication, handling, mounting, and maintenance. It is imperative that the proper bearing be selected for the given application. Knowing the necessary performance criteria, such as speed capabilities and load capacity, will aid in choosing the right bearing for the job.

TOM/IGUS: To properly design a polymer bearing, one needs all relevant technical data up-front. Type of motion (linear, rotating or oscillating), shaft material, and surface finish also are relevant and have an effect on bearing life.

KATE/SAINT GOBAIN: Some of the worst cases of failure result from improper alignment. Misalignment can place excessive loading and torque on bearings, causing stress, fatigue, spalling, and eventually failure.

What are considered “best practices” when designing with bearings?

CHRISTIAN/NMB: Several considerations must be evaluated simultaneously when selecting a bearing. Miniature and instrument ball bearings are normally made of either stainless steel or chrome alloy steel. Loads and life calculations are affected by bearing material as well as lubrication selection. Selection of the ABEC grade is also a factor to consider.

Ball cages or retainers are another consideration. Most common are metallic retainers — either crown type or two-piece ribbon type. For some sizes, cages of molded and machined plastic parts are also an option.

Shields and seals are another consideration. These help keep particulate contaminants out and lubricants in. The effect of metal shields on bearing torque or noise is insignificant. Rubber seals do have an effect on bearing torque although they provide good protection against contaminants and come in various designs.

Preload levels are an important consideration. For miniature ball bearings, preloads of 1% of a bearing’s dynamic load rating are typically sufficient.

The application itself also determines best practice. Vacuum cleaners and power tools, for example, are exposed to high speeds (30,000 rpm), necessitating plastic retainers and channeling greases. Plastic retainers increase bearing life; channeling greases (stiffer grease) basically help prevent the lubricant from migrating away from the rotating components. The operating environment usually will dictate the necessary enclosure, metal shields or rubber seals. This includes internal contamination like carbon brush powder from the motor as well as external contamination. Bearings in power tools, for instance, are likely to see a lot of sawdust and/or drywall dust; in vacuum cleaners, carpet fibers and dirt are the primary contaminants. If the environment is relatively clean, then metal shields are all you need. For bearings in general-use motors, low noise, low torque, and long life are the usual requirements.

For low noise, focus on the raceway finish level and grease choice. There is not a standard in the bearing industry for noise, like the familiar ABEC grade for tolerance. “Electric Motor Grade” has been used widely by motor manufacturers in an attempt to call out a noise level, and usually bearing manufacturers will recognize the term, but technically there is not a specification that goes with it.

Grease choice can affect noise level in a bearing as well. Bearing manufacturers and suppliers have a lot of experience with various greases and can help to avoid some common pitfalls. In the case of high temperature applications (above 120°C), grease noise may be unavoidable since most high temperature greases tend to be noisier.

DAN/SKF: Look beyond the bearing itself to the intended performance requirements of the machine in which the bearing will be used. Be aware of how the selected bearing can influence overall machine parameters. Do not think of the bearing as a separate component. Think of it as part of a bigger system that involves a shaft, housing, lubricant, seals, and so on.

TOM/IGUS: The best practice for proper bearing design is to provide as many application details as possible when discussing material selection. Selection criteria include temperature, mating surface, loads, speeds, and miscellaneous items such as chemical or water exposure. Each is equally important in designing the best bearing for the job.

KATE/SAINT GOBAIN: Provide as much detail as possible for the specific application (conditions of operation, speed, temperature, load, etc.) to the bearing manufacturer to understand how the bearing’s performance could be affected. Hybrid bearings work especially well in high-speed, high-temperature, high-precision applications because they run cooler longer, with less friction and less lubrication.

What can bearing MANUFACTURERS do to offset limitations?

CHRISTIAN/MNB: Develop new sealing designs — whether to block airflow, prevent water ingression, or achieve lighter drag — and offer various seal materials beyond the common NBR, such as ACM, HNBR, PTFE, EPDM, and others.

Use cleaner raw materials in steels to improve noise performance by allowing the raceways and balls to be polished to an even smoother level.

Take advantage of new oils and greases and work with suppliers to develop custom formulations such as quiet clean-room greases with fewer and smaller particles.

Offer ceramic balls (hybrid bearings) that achieve higher speeds with less wear.

Partner with customers to perform application-level life testing, noise and vibration testing, simulation, performance analysis, environmental, chemistry and metallurgical testing, as well as dimensional testing (form, surface finish, roundness).

DAN/SKF: Besides helping with bearing selection, bearing manufacturers can offer valuable information and training in relubrication, friction reduction, mounting techniques, maintenance intervals, and operating temperature considerations.

TOM/IGUS: Manufacturers can research materials and test bearings to predict ahead of time what a bearing will do on a machine. Predictability is possible.

KATE/SAINT GOBAIN: Use hybrid bearings in applications that could benefit from it. Longer life bearings provide higher throughput.

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