Managing Bearing Installation

A bearing must perform efficiently while withstanding the combined effects of speed, load , and environmental. But before a bearing is pressed into service, it passes through a critical period- installation. Improper installation can frustrate subsequent efforts to improve rotating equipment performance and lead to costly breakdowns and a loss of productivity. Therefore, bearing installation is an important issue for maintenance managers. Installation is one of four critical factors- along with bearing quality, operating environment, and maintenance- that affect bearing performance and longevity.

Poor installation practices can have numerous adverse effects, including a decline in the quality of the manufactured product. In the printing industry, for example, an improperly mounted bearing may allow a roll to shift, resulting in blurred type or poor color registration. In a machine tool, it can cause runout and loss of accuracy in a precision grinder's cutting wheel. In some pressing or rolling operations, a problem in bearing installation results in a inability to control the thickness of the material being processed.

Improper installation can even lead to outright bearing failure. SKF engineers recently completed a 3 year study of bearing failures. Of the 872 bearings analyzed, 74 percent failed because of operational factors such as overloading or environmental contamination. But 17.7 percent failed because of flawed installation procedures. These mistakes included bearing misalignment (for instance bearing cocked on the shaft or not seated properly), improper shaft and housing fits or improper mounting of tapered-bore bearings, brinelling (denting) of the bearing through mishandling or a poorly aimed hammer blow, backward installation, or exposure of the bearing to contamination during mounting.

Unlike environmental factors, poor installation practices can be corrected through training and workplace diligence. The following six guidelines provide a solid basis for effective bearing installation management.

1. Establish proper shaft and house fitting practices.

Correct shaft and housing fits should be determined before actual installation. Therefore, fitting practices should be recognized as a priority by all personnel involved in bearing mounting. Correct fits support the bearing and prevent movement between parts under load. In most applications, the shaft is the rotating component; there is an interference fit between the bearing bore and shaft, causing friction and wear. Wear particles may contaminate the bearing and eventually lead to failure.

In arrangements using straight bore bearings, shaft size directly determines the amount of interference. When first sizing a shaft, maintenance technicians must use the right specifications and machine the shaft correctly. If evidence of wear is spotted while a shaft is in service, the shaft should be remeasured, and if necessary repaired, before a new bearing is mounted. A worn shaft will not provide the necessary interference, resulting in further wear due to bearing movement on the shaft.

The shaft's geometric shape or roundness also is important. Because a bearing's inner ring is relatively thin sectioned, it conforms to the shape of the shaft. If a bearing with correct geometry is mounted on an out-of-round shaft, the entire arrangement will be out of round. The result will be poor running accuracy and premature failure.

If shafts are machined onsite, the plant's machining equipment must hold shaft tolerances and roundness within acceptable ranges. If not, the shafts should be made at a qualified machine shop.

In most bearing arrangements there is a free bearing that accommodates shaft expansion and a fixed or held bearing. Technicians should be aware of the difference between the two. Generally the housing configuration determines whether a bearing is free or held. However, with split pillow block bearings, the installer has to make this determination by consulting design drawings. In some cases, split pillow blocks with spherical roller bearings come with stabilized rings, equipping them to be held bearings. If a pillow block is to accommodate expansion, the installer must discard the ring before mounting.

2. Ensure that manufacturer's recommendations for reducing bearing clearances are observed.

Bearing internal clearance is the total distance through which one bearing ring can be moved relative to the other in the radial or axial direction. For tapered bore bearings, as the bearing is moved up a tapered shaft or adapter sleeve, the reduction in internal clearance indicates whether sufficient interference has been established.

Errors in clearance reduction are a common cause of bearing problems. Insufficient reduction indicates that the bearing has not been driven far enough up the taper; under load, the bearing will tend to move relative to the shaft, causing wear.

Excessive drive-up, on the other hand, leaves the bearing with insufficient clearance or even preload (zero or negative clearance). In most cases, excessive drive-up causes the bearing to run hot and eventually degrades the bearing lubricant. Excessive drive-up also increases hoop stresses in the inner ring and can lead to fracture.

Technicians should follow the manufacturer's guidelines in reducing clearances rather than workshop rules of thumb. One rule of thumb, for example, holds that initial clearance should be reduced by half during mounting. However, following this rule can result in improper installation (see accompanying table). When bearing installers have questions about recommended clearances in any application, they should contact the bearing manufacturer.

3. Provide technicians with mounting tools designed for the job

Proper installation tools can reduce the risk of costly errors and speed the mounting process. Small straight-bore bearings are usually mechanically mounted with an arbor press or a hammer and an impact sleeve. Force should be transmitted through a sleeve to the bearing ring that has the interference fit, in most cases the inner ring. If force is mistakenly applied to the outer ring, the force is transferred through the rolling elements and can dent the raceways. As a result, the bearing will run noisily and fail prematurely.

Incorrectly sized sleeves can come with a bearing's rolling elements, cage, or seal and cause damage. A fitting tool kit with sleeves that properly fit many standard bearing sizes is available for maintenance personnel who use these methods.

The force needed to mount bearings greatly increases with bearing size. Ideally, bearings with bores over 2 in. or 50 mm should be temperature-mounted. There are several important guidelines to follow when heat mounting. Bearings should be heated uniformly to minimize the risk of localized "hot spots." In most cases, heating a bearing to 230° F will expand the inner ring enough so it will fit over the shaft's interference. If a bearing must be heated to more than 230° F for successful mounting, the shaft should be measured, it might be oversized.

Overheating bearings during installation should be avoided. Overheating alters a bearing's metallurgy and causes it to lose hardness, which affects bearing strength or ability to carry load. Moreover, if a bearing is heated beyond to recommended temperature, permanent distortion and a loss of geometry can result.

Induction heaters provide environmentally clean, uniform heating by means of an electromagnetic field. They heat bearings for mounting in minutes- much more quickly than an oven or oil bath. An induction heater should have built-in temperature controls and include a thermostat or temperature probe that can be attached to the bearing inner ring. Induction heaters magnetize bearings as they heat them. Because a magnetized bearing may attract metal particles during operation, a heater should have an automatic demagnetization cycle.

The hydraulic nut simplifies mounting of large tapered bore bearings. It consists of a steel ring with a groove in one side face and an annular piston that rests in the groove. During installation, a hand-operated hydraulic pump energizes the piston, developing sufficient force to mount a bearing. The nut can be used with the oil injection method for mounting large tapered-bore bearings. The shaft is pre-drilled with distribution ducts, allowing the introduction of pressurized oil between the shaft and bearing bore. This method reduces friction and makes mounting easier. The same feature can be applied to adapter sleeves and removal sleeves. The oil injection technique is most commonly used for dismounting bearings.

4. Use condition-monitoring instruments to locate bearing damage.

Monitoring devices enable maintenance departments to analyze bearing condition and even pinpoint problems stemming from installation procedures. Through multiparameter condition monitoring, technicians can obtain a revealing picture of operating status, including temperature, speed, vibration, and lubricant condition. Vibration frequencies produced by various machine components, including bearings, and provide an early indication of machine anomalies.

Even when condition monitoring does not provide conclusive evidence that the problem resulted from faulty installation or some other specific factor, it can induce technicians to make a thorough, hands-on examination of the bearing and aid them in conducting root-cause failure analysis.

Click here to view the RESULTS OF RULE-OF-THUMB PRACTICE table.

5. Conduct a failure analysis program.

Failed bearings provide an indelible record that can be interpreted by experienced technicians. For example, brinelling in a raceway corresponding to the distance between rolling elements indicates that the bearing suffered impact damage during handling or mounting. Fretting of the inner ring bore is evidence that movement occurred between the shaft and bore because of improper shaft fit. Cracks in a bearing ring or flaking (spalling) in the load zone may also be signs of an installation problem. However, some wear patterns or damage may point to a cause unrelated to installation.

Failed bearings should be retained and examined by plant maintenance personnel. In the event of recurring failures, it is wise to seek expert advice. Bearing manufacturers often provide damage analysis services including sophisticated testing of damaged components. If a cause is found, the manufacturer can recommend methods for solving the problem.

6. Train technicians in proper installation procedures.

Training is an important component of bearing maintenance. Apprenticeship programs can cover bearing installation and offer new technicians opportunities to practice techniques. Refresher courses can hone technician's skills.

Manufacturers of bearings and other components often provide training sessions at their facilities or at customer locations.

Proper installation of rolling element bearings involves many factors, including training, mounting techniques, and tools. Incorporating these six guidelines into a plant maintenance program can reduce the risk of unplanned downtime and maximize an investment in quality bearings.