12.18 ROLLER FOLLOWERS
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.
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
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.
Commercial cam rollers
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
bearings can be grouped into two broad categories, ball and roller bearings,
both of which have many variants within these divisions.
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.
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
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
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.