Force or Form-Closed?

A form-closed (track or groove) cam or conjugate cams are more expensive to make than a force-closed (open) cam simply because there are two surfaces to machine and grind. Also, heat treating will often distort the track of a form-closed cam, narrowing or widening it such that the roller follower will not fit properly. This virtually requires post heat treat grinding for track cams in order to resize the slot. An open (force-closed) cam will also distort on heat-treating, but can still be usable without grinding.

FOLLOWER JUMP The principal advantage of a form-closed (track) cam is that it does not need a return spring, and thus can be run at higher speeds than a force-closed cam whose spring and follower mass will go into resonance at some speed, causing potentially destructive follower jump. High-speed automobile and motorcycle racing engines often use form-closed (desmodromic) valve cam trains to allow higher engine rpm without incurring valve “float,” or follower jump .

CROSSOVER SHOCK Though the lack of a return spring can be an advantage, it comes, as usual, with a trade-off. In a form-closed track cam there will be crossover shock each time the acceleration changes sign. Crossover shock describes the impact force that occurs when the follower suddenly jumps from one side of the track to the other as the dynamic force ( ma ) reverses sign. There is no flexible spring in this system to absorb the force reversal as in the force-closed case. The high impact forces at crossover cause noise, high stresses, and local wear. Also, the roller follower has to reverse rotation direction at each crossover which causes sliding and accelerates follower wear.

Studies in our laboratory have shown that roller followers running against a well-lubricated open radial cam can have slip rates of less than 1%.[1] Crossover shock can significantly increase this slippage on single-roller form-closed (track) cams.* Dual-roller conjugate cams do not have this roller-rotation-reversal problem.

* Hollis [2] reports that high-speed video of a track cam with roller follower in one case showed that at 1000 cam rpm the roller was always sliding. It never was able to get its rotational velocity up to a level to match the cam surface speed before the crossover event demanded that it reverse direction.

Designers sometimes add a spring to the follower train of a form closed track cam, often at the end effector, in order to bias the roller to one side of the track and eliminate crossover shock. This arrangement effectively becomes a force-closed cam with a “belt and suspenders” insurance policy in the form of the other side of the track, that will take over and guarantee that the follower provides the required motion, even if dynamic forces overcome the spring and “jump” the roller off the spring-loaded side of the track, or when a jam in the tooling tends to hang up the follower. Then the other side of the track will take over and drive the follower through at the expense of some impact force.

Another common way to deal with the crossover shock problem is to use a ribbed cam rather than a track cam as shown on this book’s cover and in Figure 1-9 (p. 8) and also schematically in Figure 13-15 (p. 419). These cams use a pair of rollers that straddle the rib and can (optionally) be spring-loaded together to pinch the rib in order to eliminate backlash. This spring needs to have a high rate to avoid resonance problems. Such an arrangement has the added benefit of providing wear compensation. Another way to achieve the same effect is to make a track cam’s slot wide enough to accommodate two rollers that are spring loaded apart to take up any backlash.

LOCKOUT If it is necessary to lock out the follower motion, then a force-closed cam is mandatory and an air cylinder will be needed as the closure spring. Moreover, the cam and follower must be arranged so that the cam rise actually returns the tooling to the rest position so that pulling the follower off the cam at the high dwell (or high point) holds the tooling out of the nest. In other words, you want the air cylinder spring to drive the tooling in and the cam to pull it out.

Radial or Barrel Cam?

This choice is largely dictated by the overall geometry of the machine for which the cam is being designed. If the follower must move parallel to the camshaft axis, then a barrel cam is dictated. If there is no such constraint, a radial cam is probably a better choice simply because it is a less complicated, thus less expensive, cam to manufacture.

Roller or Flat-Faced Follower?

The roller follower is a better choice from a cam design standpoint simply because it accepts negative radius of curvature on the cam. This allows more variety in the cam program. Also, for any production quantities, the roller follower has the advantage of being available from several manufacturers in any quantity from one to a million. For low quantities, it is not usually economical to design and build your own custom follower. In addition, replacement roller followers can be obtained from suppliers on short notice when repairs are needed. Also, they are not particularly expensive, even in small quantities.

Perhaps the largest users of flat-faced followers are automobile engine makers. Their quantities are high enough to allow any custom design they desire. Flat followers can be made or purchased economically in large quantity and can be less expensive than a roller follower in that case. Also with engine valve cams, a flat follower can save space over a roller. Nevertheless, many manufacturers have switched to roller followers in automobile engine valve trains to reduce friction and improve fuel economy. Most new automotive IC engines designed in the U.S. in recent years have used roller followers for those reasons. Diesel engines have long used roller followers (tappets) as have racers who “hop-up” engines for high performance.

Cams used in automated production line machinery use stock roller followers almost exclusively. The ability to quickly change a worn follower for a new one taken from the stockroom, without losing much production time on the “line,” is a strong argument in this environment. Roller followers come in several varieties (see Figure 1-7, p. 6). They are typically based on roller or ball bearings. Plain bearing versions are also available for low-noise requirements. Roller bearing followers are available in two types, “caged” and “full complement.” A caged roller bearing has its rollers separated by the cage so that they do not touch. A full complement roller or needle bearing has no cage and its rollers are free to rub on one another. The larger number of rollers in a full complement bearing allow higher loads, but do so at the expense of potentially higher wear. The close spacing of the rollers leaves little volume for grease and the rubbing of roller on roller accelerates their abrasive and adhesive wear. They require frequent re-lubrication. Another variety of roller follower uses dual-row ball bearings. These are typically caged and have good grease capacity. They also can take some thrust load, which roller bearings cannot.

The outer surface of the roller follower, which rolls against the cam, can be either cylindrical or crowned (i.e., an oblate-spheroid) in shape. The “crown” on this follower is slight—having a large radius, but it allows the follower to ride near the center of a flat cam, despite some nonparallel misalignment of the cam and follower axes. If a cylindrical follower is chosen, and care is not taken to accurately align and stiffly support the axes of cam and roller follower, the follower and cam will ride on one edge and wear rapidly. Crowned rollers are recommended unless the loads are so high that the stresses become excessive. A cylindrical follower will give lower stresses than a crowned follower only if the parallel alignment between cam axis and roller axis is very accurately maintained and dynamic deflections are kept very small by designing in sufficient bending and torsional stiffness in the roller support. If not, the stresses in a cylindrical follower can be higher than for a crowned follower.

Commercial roller followers are typically made of high carbon alloy steel such as AISI 52100 and hardened to Rockwell HRC 60-62. The 52100 alloy is well suited to thin sections that must be heat-treated to a uniform hardness. Because the roller makes many revolutions for each cam rotation, its wear rate is usually higher than that of the cam. Chrome plating the follower can markedly improve its life. Chrome on steel is harder than the base steel at about HRC 70. Tool steel cams are typically hardened to a range of HRC 58 - 62. Hardened ductile iron cams will be somewhat softer, about HRC 55.

Roller followers are available in two types, as shown in Figure 1-7 (p. 6). One has a stud fitted in the inner race and is intended to be cantilever mounted. The other has a hole through the inner race for an axle that will be straddle mounted in a yoke. While packaging geometry and service considerations often dictate the use of a cantilevered stud-mount follower, if the choice is available, it is preferable to use a straddle-mounted follower. This follows from simple mechanics principles. A simply supported beam of given dimension and span will have a smaller deflection than a cantilevered beam of the same dimension and extension (“span”). Consideration needs also be given to the stiffness of the link to which the follower is attached. Ideally, a straddle-mounted roller can have the axis of the link coplanar with the cam, thus eliminating torsional moments on the link. A cantilevered follower will always add a torsional moment to the bending moment in the follower arm, thus potentially increasing deflections at the cam follower and exacerbating misalignment. Note also that the torsional stiffness of an offset follower arm combines in series with the bending stiffness to give a potentially lower overall stiffness for the follower system. This can worsen the system’s dynamic response as was shown in Chapter 10. Nevertheless, cantilevered, studded followers are attractive as they are simpler to mount and use simpler follower arm geometry. As in any design, one must balance the trade-offs.