12.18 ROLLER FOLLOWERS

Cam roller followers are essentially rolling element bearings made in special configurations to suit the application. Stud and cam yoke type followers are shown in Figure 12-27. These are available with needle, caged-roller, ball, and plain bushing bearings. The most commonly used are the needle and caged-roller types. The outside diameter of the bearing’s outer race runs against the cam surface. Cam followers are typically made of tool steel, hardened to Rockwell C60-62 (HRC60-62). The cam surface should be equally hard or as hard as its material will permit.

Two types of outer surface configuration are available, cylindrical and crowned. The cylindrical type is intended to provide line contact with the cam surface. To achieve this condition requires that care be taken to guarantee that the roller axis is parallel to the cam surface at assembly. If errors or inaccuracies skew the roller axis, then it may only contact the cam along one edge. It will then have a much smaller contact patch than intended and the stresses will be much higher, leading to early failure. This condition can also be caused by excessive dynamic deflections in the links that carry the roller. The crowned roller has a large radius in the plane of the roller axis, causing its contact patch with a flat cam to be elliptical as shown in Figure 12-11 a (p. 353). This reduces the theoretical contact patch area, but in the face of any misalignment or deflection of the follower, the contact patch and thus the stresses will remain essentially the same as designed. The crowned follower should be used in any situation where good parallel alignment of roller and cam cannot be guaranteed or where the cam surface is not flat in a direction parallel to the roller axis.

FIGURE 12-27

Commercial cam rollers

A low coefficient of friction in the bearing promotes rolling at the cam-follower interface. If the torque due to the friction force at the cam-follower interface exceeds the friction torque within the bearing, it will roll; otherwise it will slide on the cam. In addition to calculating the surface stresses at the cam-follower interface, it is necessary to determine if the rolling element bearing will have sufficient life at the design loads. The Anti-Friction Bearing Manufacturers Association (AFBMA), an industry-sponsored organization, has developed test procedures and design data to allow this task to be easily done.

Types of Rolling-Element Bearings

Rolling-element bearings can be grouped into two broad categories, ball and roller bearings, both of which have many variants within these divisions.

BALL BEARINGS These capture a number of hardened and ground steel spheres between two raceways, an inner and outer race for radial bearings, or top and bottom races for thrust bearings. A retainer (also called a cage or separator) is used to keep the balls properly spaced around the raceways. Ball bearings can support combined radial and thrust loads to varying degrees depending on their design and construction. Ball bearing roller followers are available with shields to keep out foreign matter and seals to retain factory-applied lubricant.

ROLLER BEARINGS These use straight, tapered, or contoured rollers running between raceways. In general, roller bearings can support larger static and dynamic (shock) loads than ball bearings because of their line contact. Unless the rollers are tapered or contoured, they can support a load in only one direction, either radial or thrust according to the bearing design. Figure 12-27 a shows a cam follower with straight, cylindrical roller bearings designed to support only radial loads. A needle bearing cam follower, as shown in Figures 12-27 b and c , uses small-diameter rollers without retainers (meaning the rollers can rub on one another). Its advantages are higher load capacity due to the full complement of rollers and its compact radial dimension.*

* Experience has shown that despite the higher load capacity of uncaged needle bearings, the roller rubbing combined with their lower capacity for grease storage compared to caged roller bearings often leads to shorter life and higher failure rates for needle roller bearings.