Tapered roller bearings can be set at initial machine assembly to any desired axial or radial clearance. This unique feature enables a designer to control bearings to meet anticipated application operating conditions, and thereby provide optimum bearing and system performance.

Some advantages of tapered roller bearings pertaining to setting include:

Longer bearing life, achieved by optimizing bearing settings while meeting application performance requirements

Increased mounting stiffness, achieved by properly set tapered roller bearings resulting, for example, in better gear contact and longer gear life

Easier assembly because cone and cup are separable

The bearings can be set at the time of machine assembly, allowing wider shaft and housing tolerances

The setting of tapered roller bearings can be readily accomplished by a wide variety of viable methods. These bearings can be set manually, supplied as pre-set assemblies, or set by automated techniques. There are a number of approaches, considerations and advantages for each, with special focus on five popular automated techniques, i.e. — set-right; acroset; projecta-set; torque-set; and clamp-set (Table 1).

Bearing Setting

With tapered roller bearings, the term “setting” simply indicates the specific amount of end-play (axial clearance) or pre-load (axial interference) within a mounted bearing. The flexibility to easily adjust and optimize setting at the time of assembly is an inherent advantage of tapered roller bearings. Unlike other types of anti-friction bearings, tapered roller bearings do not require tight control of shaft or housing fits to obtain setting. Because tapered roller bearings are mounted in pairs (Fig. 1), their setting is primarily dependent upon the axial location of one bearing row relative to the opposite row.

The three primary conditions of bearing setting are defined as:

End-play. Axial clearance between rollers and races producing a measurable axial shaft movement when a small axial force is applied — first in one direction and then in the other — while oscillating or rotating the shaft (the reference bearing load zone less than 180°).

Pre-load. Axial interference between rollers and races such that there is no discernible axial shaft movement when measured as described above. A rolling resistance to shaft rotation results, which may be measured (load zone greater than 180°).

Line-to-line. A zero setting condition; the transitional point between end-play and pre-load.

A bearing setting obtained during initial assembly and adjustment is the “cold” or “ambient” bearing setting, and is established before the equipment is subjected to service.

Bearing setting during operation is known as the “operating bearing setting,” and is a result of changes in the ambient environment bearing setting due to thermal expansion and deflections encountered during service. The ambient bearing setting necessary to produce the optimum operating bearing setting varies with the application. Application experience, or testing, generally leads to the determination of optimum settings. Frequently, however, the exact relationship of ambient to operating bearing settings is unknown, and an educated estimate has to be made. To determine a suggested ambient bearing setting for a specific application, contact your bearing representative.

Generally, the ideal operating bearing setting is near-zero, to maximize bearing life. Most bearings are set with a cold setting of end-play at assembly. This comes as close as possible to the desired near-zero setting when the unit reaches its stabilized operating temperature.

Some applications are set with cold pre-load to increase rigidity and axial positioning of highly stressed parts that would otherwise be dramatically affected by excessive deflection and misalignment.

Excessive operating pre-load must be avoided, as bearing fatigue life can be drastically reduced. Also, excessive operating pre-load can lead to lubrication problems and premature bearing damage due to high heat generation.

Load zone is a physical measure of the raceway-loaded arc and is a direct indication of how many rollers share the applied load. For a single-row tapered roller bearing, maximum life is obtained with a load zone of approximately 225°.

The ideal operating setting that will maximize bearing system life is generally near-zero to slight pre-load.

Manual Bearing Setting

Manual methods are frequently used to set bearings on a variety of equipment with low-to-moderate volume production requirements, whereby a less-than-exact, primarily end-play setting range variation is acceptable. No special tooling, gauges, charts or fixtures are typically required, but assembler skill and judgment are necessary. For example, in the case of a conventional truck non-driven wheel with a single adjusting nut design (Fig. 3), manual setting involves tightening the adjusting nut while rotating the wheel until a slight bind is felt. Then the adjusting nut is backed off 1?6-to-1?4 turn to the nearest locking hole — or sufficiently to allow the wheel to rotate freely with some minimal end-play. The adjusting nut is then locked in this position. Skill and judgment are required to determine when the wheel binds slightly in rotation. The more complicated the equipment, and the bulkier and heavier it is, so is a greater degree of skill and judgment required.

For certain complex designs, large equipment, or highproduction applications, manual setting may be too troublesome, of inappropriate accuracy and reliability, or too time consuming. The Timken Company has devised pre-set bearing assemblies and automated setting techniques as alternatives to manual setting.

Pre-set Bearing Assemblies

Many applications utilize or require the use of two-row or close-coupled bearing assemblies. This will depend upon the design and operating characteristics of the machine (e.g., thermal growth effects, high loads, etc.). To facilitate bearing settings of this type of design, pre-set bearing assemblies are frequently used. Pre-set bearing assemblies are available in a variety of forms, styles and arrangements, but for the most part are typically referred to as spacer bearings (Fig. 4). The majority of pre-set bearings are manufactured and supplied with spacer rings “custom-fitted” between the bearing rows to control the internal clearances (Ref. “2S”- and “TDI”- types). As such, these customized or “matched” spacers cannot be interchanged with any other bearing assembly. Other pre-set assemblies such as “SR”- or “TNA”-types may apply interchangeable spacers and/or bearing components. Such interchangeable assembly components are designed to hold closer control of the critical tolerances that affect bearing setting; as a result, they can be randomly selected.

Set-Right

Set-right eliminates manual setting adjustment of tapered roller bearings by controlling certain bearing and mounting system tolerances. The statistical laws of probability are applied to predict the effect of these tolerances on the bearing setting. Generally, the set-right method requires closer control of some shaft/housing machining tolerances, as well as closer control (with special class and code) of critical bearing tolerances.

The method considers that each component involved in the final assembly of a machine has a controllable tolerance range for critical dimensions. The laws of probability indicate that combinations of all low tolerances or all high tolerances will rarely occur in such an assembly. It then follows that for a “normal tolerance distribution” (Fig. 6), the overall dimensional stack-up of all parts will statistically tend to be somewhere in the middle of the total possible tolerance range.