When rolling element bearings are required in manufactured products, an installer on the production line is often responsible for final assembly of the bearing, and for greasing and sealing it. This assembly task includes adjusting the bearing carefully so that its internal clearances — radial and axial distances between its inner and outer rings — meet the application requirements.
At the end of the line, workers test the product. In many plants, testing consists of an experienced worker listening closely to the product as it operates. Often, this is the only way to check if bearings and other components were installed properly.
Though usually successful, this procedure is susceptible to errors. The sheer number of parts in a typical bearing arrangement requires OEMs to keep an extensive inventory on hand. The installation process is time-consuming, and with the number of tasks involved, even an experienced worker can make a mistake that later surfaces in the form of poor bearing performance. Also, the sound test is often not sophisticated enough to detect signs of a bearing problem.
Today, new bearing designs, improved installation tools, and monitoring devices offer manufacturers more reliable alternatives to traditional installation methods. These new methods take less time, reduce warranty costs, and improve the performance of the end products.
For example, pre-adjusted, pre-lubricated unitized bearings greatly simplify the installation process, decreasing the risk of costly errors. Tools equipped with pressure gages speed installation while providing feedback on the force required to assemble the bearing (drive-up force). Condition monitoring devices, such as vibration analyzers, present an electronic picture of machine health, so OEMs can detect problems before a product is shipped.
Large manufacturers using high-volume assembly line or batch manufacturing methods are most likely to use the new concepts. But smaller OEMs can also benefit. The investment in new bearing designs and equipment pays off by reducing the cost of assembly time, rebuilds, and warranty costs.
The use of unitized bearings — self-contained, pre-adjusted units — is increasing. This is especially so in the automotive industry, where unitized wheel bearing assemblies, called hub units, have become virtually standard in domestic cars.
Previously, assembly line workers collected and assembled wheel bearing components — inner and outer rings, grease, seals, spacers, and lock nut. They were responsible for adjusting bearings and lubricating them with the correct grease in the right amount. Errors could occur at each step of this procedure.
By contrast, a hub unit incorporates all bearing components. It arrives at the production line preset, greased, and sealed for life. Hub units greatly simplify the mounting process. Instead of handling many separate components, the assembly worker simply bolts the unit in place, reducing the number of installation errors. Since these units were introduced, auto manufacturers have seen a large drop in wheel assembly warranty claims.
Bearing designers are working to integrate related functions into the hub unit. For example, one type of hub unit, which is typically used in driven wheel applications, also transmits power to the wheel via a splined inner ring bore, Figure 1. Installers bolt the unit’s flanged outer ring to the suspension, a procedure that takes only a few seconds.
Other hub units incorporate a wheel speed sensor, an important part of the anti-lock braking system, Figure 2. Currently, in most vehicles, the sensor is a discrete unit that is mounted separately. After installation, the auto manufacturer must perform a diagnostic test to ensure that the sensor is sending the correct signal. Including the sensor in the hub simplifies installation. Moreover, the diagnostic check can be performed off-site by the hub unit’s manufacturer, saving time on the assembly line.
The auto industry is not alone in reaping the benefits of bearings with integrated sensors. Other companies, wanting to prevent machinery breakdowns, use sensor bearings to obtain feedback on critical machine functions, such as speed, load, force, and temperature.
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In many cases, these sensors also provide real-time feedback on the bearing’s internal clearance — a critical parameter in bearing installation. In most applications, a bearing should have a slight internal clearance after mounting. But in certain situations, a bearing is preloaded, that is, installed with a “negative” clearance. Preloading compensates for deflections and heat-related changes that occur in a bearing during operation, enhancing its stiffness and improving its running accuracy. Bearings that are commonly preloaded include spindle bearings in machine tools, pinion bearings in automotive axle drives, and printing press bearings.
To ensure proper mounting of a preloaded bearing, the bearing can be equipped with a strain gage in its outer ring. As the ring expands during installation, the gage indicates whether bearing drive-up and preload are proceeding correctly. In the printing industry, where press accuracy depends largely on proper bearing installation, engineers use cylindrical roller bearings integrated with load sensors. Here, the sensor signals to the installer when the correct preload is reached.
Installers use four basic methods, Figure 3, to mount bearings on shafts. First is mechanical mounting, which uses physical force. The other three methods rely on newer techniques to increase reliability and ease mounting of large bearings: temperature mounting, which uses heat to expand the bearing and make it easier to mount; hydraulic mounting, which uses hydraulic pressure to impart mounting force; and an oil injection method, which introduces a pressurized oil film between the shaft and inner ring to reduce frictional resistance.
Mechanical mounting is generally suitable for press fitting bearings with small bore diameters — 80 mm or less. Small, straight-bore bearings are mounted on a shaft with a hammer and an impact sleeve, which transmits force to the bearing inner ring (or outer ring if bearing is being press fitted in a housing bore). Bearings with a tapered bore are mounted with a lock nut and spanner wrench.
Many OEMs still prefer a dead blow hammer, used with an impact sleeve or spanner, for mounting bearings on a production line. But a poorly aimed hammer blow can cause brinelling — impact damage consisting of small dents in bearing balls or raceways. This condition causes noise and eventual failure. Moreover, it is very difficult to mount bearings with bore sizes over 80 mm using mechanical means. Consequently, a growing number of OEMs are replacing brute force methods with the use of installation tools, such as induction heaters and the hydraulic nut. These tools facilitate the mounting of medium and large bearings (80 to 200 mm or more) on shafts and reduce the potential for costly mistakes.
Temperature mounting uses heat to expand the bearing inner ring so it can be easily positioned on a shaft. It is best suited for straight bore arrangements. When mounting a bearing, the installer applies enough heat to raise the bearing temperature to 150 F above ambient, thus ensuring adequate bearing expansion.
One prevalent temperature-mounting device is the induction heater, which uses an electromagnetic current to generate heat. Less than 5 min is required to heat a 100-lb bearing. Some heaters have gages that let production workers monitor the heat cycle by time or temperature, preventing the risk of damaging a bearing by overheating.
Other common methods include hot plates, oil baths, and ovens. Hot plates, which are thermostatically controlled, are generally used for smaller bearings. Oil baths contain a mixture of water and oil that has a boiling point of about 230 F. Because the mixture never exceeds that temperature, an oil bath effectively exerts an automatic temperature control.
Hydraulic mounting uses a hydraulic nut, which consists of a steel ring with a groove in one side and an annular piston that rests in the groove, Figure 4. Oil pumped into the nut pushes the piston out with a force sufficient to mount the bearing. By monitoring pressure and travel gages during installation, a production- line worker applies the right amount of force to the bearing.
The hydraulic nut can also be used with an axial drive-up technique to install spherical roller bearings. Like any bearing with a tapered bore, these bearings are mounted with a shaft interference fit. Traditionally, the most reliable method of establishing the correct fit has been to measure the reduction in the bearing’s internal radial clearance.
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An improved drive-up technique provides an alternative to measuring clearance. The installer determines from a drive-up table the axial force required to reach the correct starting position. Then the installer applies this force to the bearing by means of a hydraulic nut equipped with a pressure gage. From this starting position, the bearing is driven a predetermined distance up the shaft. This technique is very reliable in establishing the proper interference.
Oil injection is well-suited for installing large bearings (bore diameters over 200 mm). It delivers pressurized oil via connecting ducts in the shaft to shallow grooves on the shaft surface. A thin layer of pressurized oil reduces the fitting pressure and simplifies mounting and dismounting. Normally, a hand-operated hydraulic pump with a 10,000-psi capability pressurizes the oil and supplies it to the bearing assembly. After mounting, oil pressure is released and the oil is allowed to drain so mating surfaces can reach the optimal interference fit.
Production line innovations aren’t limited to new bearing designs and improved installation tools. Condition monitoring devices are also gaining increased acceptance as a way to ensure proper bearing installation. These devices are generally used at the final stage of production to test products prior to shipping. By detecting the vibration frequencies produced by various machine components, they can ensure that a product meets quality standards, and pinpoint defects that would be otherwise imperceptible. They can also help an OEM to improve bearing installation practices.
In one example, a large electric motor manufacturer uses a vibration analyzer equipped with an accelerometer on a production line that produces ac fractional- horsepower electric motors for home exercise equipment.
Because quiet operation is a priority in motors destined for consumer products, the company continuously strives to reduce noise levels. As part of this effort, recent tests of motors detected a small noise emanating from the ball bearings which are press mounted. The noise was virtually undetectable without the aid of sophisticated equipment. A vibration comparison between bearings mounted with the mechanical press and bearings mounted by other means pinpointed the press as the source of the problem. The company realigned the press, after which the noise levels decreased.
Monitoring devices also identify problems in large industrial machines. One type of portable data collector acquires data and performs spectrum analyses of rotating equipment. An OEM uses this device for production-line monitoring of planing equipment — huge machines that plane the four sides of a piece of lumber. The data collector detects motor misalignment and problems with bearings or gear drives. It can even spot signs of looseness, such as an improperly tightened bolt. As a result, the company can repair defects before a machine leaves the factory.
Daniel R. Snyder is director of applications engineering, Industrial Div., SKF USA Inc., King of Prussia, Pa.