Tapered roller bearings are designed to handle both radial and axial loads. They have a tapered inner raceway (called the cone), a tapered outer raceway (called the cup), and conically shaped tapered rollers. The bearing roller/cage assembly and inner ring form a non-separable unit that can be mounted independently of the outer ring. Tapered bearings find application in many automotive and industrial applications, including wheel assemblies in trucks and construction equipment, gearboxes, and worm gears. They are generally employed in pairs to counteract and balance induced thrust loads caused by the bearing tapered geometry.
A special characteristic of single-row tapered roller bearings is that their internal clearance is adjustable during assembly and installation. (Internal clearance is the total distance through which one bearing ring can be moved relative to the other in the radial direction — radial internal clearance — or axial direction — axial internal clearance. Bearing clearances can be optimized without remachining shafts or housings.
Q & A
Q: What are the effects of excessive looseness or tightness in tapered roller bearings?
A: Improper adjustment can cause either excessive tightness or looseness. Tightness results in excessive contact between bearing rollers and raceway, leading to higher temperatures, potential excessive loading, and premature bearing failure. On the other hand, excessive looseness can result in improper loading of bearing rollers, leading to more roller sliding or skidding and eventual premature failure. Looseness also affects application accuracy and noise. For example, in gearboxes tapered roller bearing adjustments can affect gear mesh; excessive looseness can lead to gear wear and tooth damage.
Q: How are induced axial forces in tapered bearings calculated?
A: Bearing theory and extensive application experience show that a “line-to-line” setting (with no endplay or preload) or a slight preload at the operating temperature provides maximum performance and bearing life. The appropriate mounted bearing preload at normal operating temperatures depends on the bearing load. Under radial loading, a tapered bearing is subjected to both radial force and force in the axial direction. These forces must be counteracted with a second bearing facing in the opposite direction. The axial force produced in the bearing can be calculated:
Fa = 0.5 × Fr/Y
Fa = Axial bearing load
Fr = Radial bearing load
Y = Axial factor for tapered roller bearings, from product tables
In a bearing arrangement with two tapered roller bearings arranged back-to-back (or face-to-face) each bearing must accommodate the axial forces from the other bearing. Therefore, when adjusting tapered bearings against each other, the bearings must be rotated so that the roller ends are in correct contact with the guide flange of the inner ring.
At initial assembly, other factors determine the non-operating clearance or preload. These factors include inner and outer ring fits to mating components, shaft and housing materials, loading cycles particularly for gearboxes, and anticipated operating temperature.
Q. How are adjustments made in tapered roller bearings?
A. The bearings has a tapered geometry, so it can be difficult to directly measure radial clearances. For this reason, radial clearances are commonly converted to axial clearances. (Radial clearance equals axial clearance times the cotangent of the bearing contact angle.)
The amount of clearance between bearing rollers and raceways measured in this axial direction is called endplay. The optimal initial endplay depends on a number of factors, including shaft and housing materials, bearing fits, and operating temperatures.
Endplay adjustments in tapered roller bearings are made during assembly or installation to compensate for the expected thermal expansion during operation and system deflections. Axial preload in tapered roller bearings is produced by displacing one bearing ring axially in relation to the other by an amount corresponding to the desired preload force. Generally, each bearing is individually and manually adjusted using shims or spacer sleeves, or by tightening torques.
In addition to manual methods, there are also several automated techniques. In the case of double-row tapered roller bearings, a preset assembly can be supplied, or the bearings can be adjusted manually at assembly by machining spacers.
Careful monitoring and observation of applications can sometimes reveal early warning signs of clearance and endplay-related problems. Vibration monitoring equipment, for example, can detect the excessive axial and radial shaft movements caused by too much endplay. Temperature sensors can detect increases in heat associated with impending bearing failure. Maintenance technicians can then intervene before bearings or other components are damaged beyond repair.
This month's handy tips provided by Dan Snyder, director of application engineering at SKF USA Inc.