Productivity Forum: Bearings

Feb. 1, 2003
Bearings make the world go round,but what makes bearings go around,and more importantly,what do you need to do to keep them going? To find the answers, the editors of Motion System Design conducted a survey among industry experts, asking for their feedback and analysis. Here are the responses we believe you’ll find most useful

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?


• 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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What can DESIGNERS do when selecting and applying bearings to improve productivity?

CHRISTIAN/NMB: Partner with a bearing supplier. Determine the key characteristics that you expect out of the bearing. Is noise the most crucial to success? Long life at high temperatures? Extremely low torque? Once these are clear, it’ll give the bearing manufacturer a solid direction to offer the best bearing for the particular application.

MARK/TIMKEN : Improvement in productivity is attainable by considering all environmental elements when selecting bearings. Designers must also consider the reliability requirements for all applications so that the proper design can be matched to the requirements of the application.

TOM/IGUS: Choose wisely. A polymer bearing, being lightweight and maintenance-free, can increase overall machine productivity for end users, letting them produce more product cost-effectively and in less time. Also, determine early on how long the bearing needs to last. Then look at all aspects of the application and available bearing options.

DAN/SKF: Designers should take advantage of all the new technologies that bearing manufacturers offer and not remain dependent on past practices.

ROBERT/RBC: Good design requires extensive information. Designers need to know everything about the application; load, load application, duty cycle, temperature, misalignment, support structure characteristics, required stiffness, rpm, and so on.

KATE/SAINT GOBAIN: Speak with bearing manufacturers about their design to understand what type of bearing best serves the need.

CARL/REXNORD: Select the correct bearing based on load, speed, and environmental conditions.

What should END USERS do (in terms of care and upkeep) to maximize bearing life and machine productivity?

CHRISTIAN/NMB: A high percentage of bearing problems, including failures, are the result of improper handling procedures. Cleanliness prevents particles from entering the bearing; even particles as small as 0.005 mm can cause problems. Avoid exposing bearings to any environment where particles may be present. Once inside a bearing, such particles can cause raceway scratches, abrasion and shortened life, and can generate acoustic noise and vibration. Particles will also accelerate the degradation of the lubricant and cause the bearing to fail.

Care in handling and assembly will ensure the bearing raceways are not permanently damaged (brinelled). Forces, especially impact forces, placed on the inner and outer rings tend to cause the balls to permanently indent (brinell) the raceways. Brinelling can easily occur after the bearing has been properly assembled. For example, dropping a motor can induce brinelling, especially if the shaft impacted the floor. Brinelling as small as 0.1 micron in depth will have an adverse effect on acoustic noise levels as well as causing torque spikes and add high level of vibration to the system.

MARK/TIMKEN : There are several things to consider.

• Safety: Never spin a bearing with compressed air. The rollers could be forcefully expelled, creating serious risk to those nearby.
• Cleanliness:
One of the most common sources of trouble in antifriction bearings is debris that contaminates the lubrication environment. Debris can be a major cause of abrasive damage which can lead to reduced bearing life. Cleanliness doesn’t just mean clean bearings. It means clean housings and shafts, clean tools, clean solvents and flushing oils. It also means putting bearings on a clean surface and using clean rags to wipe bearings.
• Lubrication:
Guard against rust, heat, and friction. Lubrication is critical to bearing maintenance. The lubricant reduces friction between bearing mating surfaces, helps carry away heat, and protects bearing surfaces from corrosion. Whether grease or oil lubrication is used, make sure it is the proper type and grade for the application.
• Storage: Bearing packaging is designed to protect bearings against dirt and moisture during shipping and storage. Let the package do its job, and leave replacement bearings in their original containers. Store them in a clean, dry area until ready for use. Avoid temperature changes in your storage area; dramatic temperature swings can cause condensation and damage the bearing.
• Sealing:
Seals are vital to the performance of any bearing. Replace worn or damaged seals; they allow contaminants such as dirt and water to enter the bearing and they allow lubrication to escape. In most cases, it is good practice to replace all seals with new during teardown. A film of lubricant should be applied to the seal lip contact surface at assembly. This will help prevent seal damage during installation and at initial startup. It also will reduce rolling torque.
• Handling:
The less, the better. Fingerprints can cause rust, so handle bearings only when necessary. Never drop or handle these precision components roughly. Any sign of a bent cage renders replacement.

DAN/SKF: Train in the use of proper maintenance practices. Then use the training to implement a good maintenance program that includes predictive maintenance employing condition-monitoring techniques.

TOM/IGUS: With selflubricated polymer bearings, the advantage is no need for special care. They are great in dirty environments and pose no problem with washdown.

KATE/SAINT GOBAIN: Check bearings on a routine basis to ensure proper alignment, lubrication, temperature and performance. Look for visual defects on rings, balls, and cages.

CARL/REXNORD: Monitor failure trends and adjust maintenance schedules in problem areas.

How do bearing choices affect other areas of the machine or system?

CHRISTIAN/NMB: Confirm whether you have the correct size bearing to handle the applied loads. You may need to adjust the bearing envelope in order to accommodate the correct bearing size. An incorrect grease choice can adversely affect not only the noise in the bearing, but be amplified once in an assembled unit. If the bearing system is not preloaded in the unit, you will have higher vibration. If it is impossible to preload, minimize the amount of radial play to reduce the vibration.

Incorrect use or selection of contact rubber seals will increase the drag in the bearing and can slow down a motor. These seals can also slightly increase running temperature in the unit.

Incorrect grease type could have compatibility issues with adjacent plastic, rubbers, or liquids.

Incorrect seal materials may have compatibility issues with nearby liquids or solvents.

• Contact rubber seals vs. metal shields: Contact rubber seals provide the best protection against contaminants entering the bearings; however, they significantly increase the bearing’s torque. Metal shields can provide good protection for most applications, but it is recommended to work with your bearing supplier in order to achieve the right balance for your application’s needs.
• High temperature grease vs. low noise:
Most high-temperature greases increase the noise in ball bearings, so understanding which is most important for your design helps find the right balance.

MARK/TIMKEN : Bearing performance, whether fatigue life or dynamic stiffness, can affect the entire system. Likewise, system properties, including machining tolerances, the lubrication system, and housing deflections, affect bearing performance. In light of the entire system, the best bearing may not be the one with the highest fatigue life. In such cases, bearings may be selected based on the rigidity they add to the system, size (to accommodate other components), or low-torque dynamics.

DAN/SKF: Regarding performance tradeoffs, there are always tradeoffs, and they usually involve costs. Other considerations may involve liability and field maintenance requirements.

TOM/IGUS: The wrong bearing choice can literally shut down a machine because of it seizing or due to unusual wear. If a bearing’s friction increases, the same thing can happen. This leads to downtime and productivity issues for a company. All bearings are not alike, and although it can seem like a small decision in your overall design, bearing selection can create numerous problems or solve multiple problems.

KATE/SAINT GOBAIN: Bearings are a critical component to the performance of most machines as they are the “motion control” unit. If the bearing fails, the machine can seize up causing significant damage to other tools, parts, and components — not to mention scrapping the product the machine is producing. Most tradeoffs occur due to operating conditions, design choice, and of course expense to the company (hybrid bearings cost more than standard steel bearings, but they have a longer life). Other design considerations may include housing environment (what and where the bearing is contained and being operated) and what the bearing is exposed to (i.e. chemicals, other lubricants, food, salt water, electricity, magnetic environment, etc).

CARL/REXNORD: When the bearing fails, the machine stops working. Performance trade-offs arise when choosing options that increase purchase price but extend bearing life, reducing total operational cost.

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