ROLLING-ELEMENT BEARINGS - Part 2
|Figure 12 - Drawn-cup needle roller bearing. Rollers are retained by lips at the ends of the outer casing and projections of the rollers.|
|Figure 13 - Because roller ends are spherically shaped, grease-retained drawn-cup needle roller bearing has slightly higher load rating than a mechanically retained bearing.|
|Figure 14 - Drawn-cup, caged needle roller bearing, left, and heavy-duty needle roller bearing with machined, hardened and ground inner race, right.|
|Figure 15 - Stud-type cam follower, left, and yoke-mounted type, right.|
|Figure 16 - Double-row, spherical roller bearing.|
|Figure 17 - Cone-principle geometry of a tapered roller bearing results in its ability to accommodate any type of loading, operate with true rolling motion (no skidding), and provide positive alignment.|
Needle roller bearings - Similar in appearance to cylindrical roller bearings, needle roller bearings have a much smaller diameter-to-length ratio. By controlling circumferential clearance between rollers, or needles, rolling elements are kept parallel to the shaft axis.
A needle roller bearing's capacity is higher than most single-row ball or roller bearings of comparable OD. The bearing permits use of a larger, stiffer shaft for a given OD, and provides a low-friction rolling bearing in about the same space as a plain bearing.
The basic needle roller bearing is the full-complement, drawn-cup bearing, Figure 12. The outer race is a thin, drawn cup that has been surface hardened. Roller ends are shaped so that lips on the outer race keep them from falling out. Because the outer race is thin, it must be installed in a correctly sized and properly backed-up housing to transmit load effectively. In most instances, a hardened shaft acts as the bearing's inner race, although an inner race can be supplied when the shaft cannot be hardened.
The grease-retained, drawn-cup needle roller bearing, Figure 13, is not used as extensively as the basic, mechanically retained version because rollers may fall out if the shaft is removed. Also, the heavy grease that retains rollers is incompatible with some applications. The advantages of this type of bearing over the basic bearing is higher load-carrying capacity because rollers have spherical ends.
A caged needle roller bearing, Figure 14, is designed for heavier-duty, higher-speed applications. The heaviest-duty needle roller bearing, Figure 14, has a machined cage. Both machined-outer-race and drawn-cup caged bearings have sufficient voids to allow pregreasing the bearing for lifetime lubricated applications. Even with these bearings, operating life can be extended if provision is made for periodic relubrication.
When selecting needle roller bearings, consider these guidelines:
- The most compact and economical arrangement uses the equipment shaft as the bearing's inner race. In this situation, manufacturer's requirements for shaft hardness and surface finish must be followed. Typical values for hardness range from Rockwell 58C to 60C; surface finish, 16 min. or better. Keep shaft parallelism within 0.0003 in. for the full length of the bearing, or within half the shaft tolerance, whichever is less.
- Do not exceed 0.0002 in. total indicator reading (TIR) for shaft out-of-roundness or 1/2 the shaft tolerance when measured by both two-point gaging and use of a 90-deg V-block in conjunction with a dial indicator.
Cam followers - A cam follower is a special, heavy-duty needle roller bearing with a heavy outer race section. There are two basic types: one with an integral stud for cantilever mounting; the other, an integral inner race for yoke mounting. Both types, Figure 15, may have a crowned OD, which compensates for a reasonable amount of bearing misalignment with the track or cam to prevent corner loading of the outer race. This helps maintain more uniform stress distribution over the track or cam surface, and increases assembly life. Crowning also minimizes skidding of the cam follower on flat circular tracks or cams.
To select a cam follower, evaluate load, speed, alignment, track or cam design, and available lubrication. If operating speed is less than the maximum allowable speed, choose a bearing size from the given load and speed for a specific life requirement. For shock loads, consider a heavy stud cam follower or cam yoke roller.
To prevent galling between the follower OD and the track member, lubricate the track with grease of high enough consistency to adhere to the track during operation. For continuous rotation, provide continuous oil lubrication or frequent grease relubrication. Automatic lubrication devices are strongly recommended for intermittent lubrication.
Spherical roller bearings - The term spherical roller bearing generally refers to a single or double-row, internally self-aligning roller bearing. Self alignment is obtained by making one of the raceways a portion of a spherical surface, Figure 16. These bearings support high radial or combined radial-axial loads. The double-row type generally is not used for pure thrust load, but the single-row type can be used to support predominantly thrust load.
Probably the greatest advantage of spherical roller bearings is the ability to accommodate misalignment with no decrease in rating or life. They usually accept 1 or 2-deg misalignment.
As a rule, spherical roller bearings lubricated with grease are limited to a speed that produces a DN value no greater than 100,000. Oil-lubricated bearings generally operate up to 200,000 DN. Spherical roller bearings have operated successfully at 1 million DN. If speeds greater than these are expected, consult the bearing manufacturer.
Tapered roller bearings - Applications in a wide variety from appliances and aircraft wheels to machine tools, automotive transaxles, and industrial equipment of all types are served by tapered roller bearings.
In a tapered roller bearing, the rollers and races are built on a cone principle, Figure 17. Specifically, the apexes of the rollers and races meet at a common point on the bearing axis.
- Load-carrying capability: Because of this geometry, tapered roller bearings are the only type of bearing that can carry heavy radial loads, or thrust loads, or any combination of the two.
When a tapered roller bearing is loaded, the external load is transformed into three load components: a radial component perpendicular to the bearing axis; a thrust component parallel to the axis; and a smaller roller-seating force. This seating force keeps the large end of each roller in contact with a rib on the large end of the cone, providing positive roller guidance that keeps rollers aligned.
Because the tapers of the rollers, cup, and cone meet at a common apex on the bearing centerline, the rollers rotate with true rolling motion with no skidding of rollers over a raceway. Thus, tapered roller bearings perform well during the life of an application.
In addition, the race and roller angles can be matched to the loading situation - shallow angles for predominantly radial loads and steeper angles for greater thrust capacity.
- Speed capability: Tapered roller bearings can handle applications from low-speed railway axles to high-speed turbine shafts. For very high-speed applications, it may be necessary to make special lubrication and design provisions.
- Misalignment: Tapered roller bearings can be highly tolerant to misalignment and deflection for two basic reasons:
1. There is the ability to adjust internal clearance within a tapered roller bearing during installation.
2. Mounting arrangement can significantly increase stiffness of an assembly.
- Preload and end play control: A special characteristic of tapered roller bearings is that their internal clearance - or setting - is adjustable. It can be optimized for a given application without remachining shafts or housings, to provide the best performance and life in any given application.
Tapered roller bearings can be manually set, supplied as a preset assembly, or set by using one of several automated setting techniques. These methods are described later under Installation Methods.
- Precision: Tapered roller bearings in the "precision" class are produced with maximum radial runout (out-of-roundness) of 75 millionths of an inch. "Super-precision" bearings for the highest-accuracy applications such as machine-tool spindles, have maximum radial runout of just 40 millionths of an inch: 1/60th diameter of a human hair.
- Profile control: The contact geometry between the large roller end and the cone rib is closely controlled to enhance lubrication. Special attention is paid to the roller body and cup and cone raceway profiles to ensure full-line contact for maximum load capacity.
For very high loads or misalignment, or both, the contact profiles can be modified to minimize stress concentrations and maximize performance.
- Materials: Cups, cones, and rollers of most tapered roller bearings are case carburized. The case carburization process produces hard, long-lasting contact surfaces that can carry heavy loads without distress and the tough, ductile core can endure heavy shock loads.
- Types: Tapered roller bearings come in a wide variety of types. The basic single-row bearing is available in many angles and roller lengths to provide a wide range of radial and thrust ratings. For more capacity, two-row bearings are used. For exceptionally rigorous service such as rolling mills, four-row bearings are used. Also available are a variety of thrust bearings, and packaged bearings with seals, lubrication, and preset adjustment.