Many motion designs include straight sections that are traversed by linear components. One common element here is the slide or linear guide — an unpowered anti-friction track and its mating carriage — also called a car or saddle. This car is usually coupled to a power-transmission device (a motor-driven screw or rack-and-pinion set) and supports loads to be transported while the actuator pushes both carriage and load along.
Most anti-friction slides are profile-rail or linear shaft bearings: Here, the car's actual rollers ride directly on the track, which in turn is precision ground and mounted to a machined frame surface, to allow motion accuracy to within a few thousandths of an inch. While highly demanding applications sometimes require such engineering, for most machinery, accuracy to within a 64th of an inch is sufficient. In fact, it's often more important that a guide last for the life of the machine.
For these situations, newer cam-follower-based slides now exist. This alternative design includes a high-strength aluminum saddle, to which an array of opposing cam followers is mounted — which in turn locks the saddle to the track. The rail is hardened and ground, and the follower's bearings have high-capacity needle rollers, to make for a durable slide suited to harsh environments that do not require precision mounting surfaces.
A linear bearing's rollers are exposed to all of the force vectors that an application generates. This leaves them vulnerable to damage when the application subjects the slide to axial load, as most bearings have quite high radial load capacity, but little axial load capacity.
Even V-shaped slides that use bearings embedded in wheels have lower moment capacities than cam-follower-based slides. V-shaped slides isolate the bearing rollers from the track as follower-based slides do, so they share that ruggedness; their angled, V-shaped surfaces also provide reliable side-to-side support. However, the bearings in V rollers must carry axial and radial load, which compromises capacity when the applied load is cantilevered or causes twisting.
Cam-follower-based slides are more robust and better at carrying moment and cantilevered loads. As in profile-rail slides, their integrated rollers cannot withstand much axial load. However, because an opposing twin mirrors each cam follower in the carriage, the bearings only carry radial load, which is far easier on these subcomponents. For this reason, cam-follower-based slides perform equally as well in any orientation — whether the saddle or the rail is the traveling component.
Because cam-follower-based slides require no precision mounting surfaces, milling of mounting surfaces is not required. This saves money and boosts design flexibility, particularly on large welded frames. Here, the engineer simply specifies that a frame be welded to certain dimensions, and then the linear guide's rail is mounted to it.
Unlike the small mounting holes provided on most profile rail and linear shaft slides, follower-based slides come with larger mounting holes and bolts — for a 1/4-in. minimum bolt diameter. This addresses the tendency of engineers to base bolt selections on holding capacity. While an acceptable approach for smaller designs, this causes Number 8 or 10 screws to be installed on a design. These can hold the load, but are physically tiny and difficult to handle. In a typical heavy-manufacturing environment, such small parts are often overtightened, broken, or lost. Small bolts also vibrate loose.
Bowing and binding
As mentioned, some of the most common profile rail is quite precise. Part of the reason is that the bearings in these slides are preloaded, so that there's an interference fit between the carriage's rollers and the rail. This is helpful where zero clearance is necessary to maintain the accuracy of a high-tech piece of motion equipment.
However, any mounting-surface imperfections or imprecise installation can cause the carriage's rollers to bind. Why?
Bolting a profile-rail slide to an unmachined surface can cause that rail to bow or flex, which in turn causes the carriage to bind. At best, this situation still allows the carriage to travel — but introduces load in the carriage bearings that has nothing to do with the machine's ultimate function. This in turn reduces the carriage's available load capacity for the application at hand. A further drawback is that binding-associated load is incalculable.
When an assembly is dramatically out of dimension, it's possible that the rail's bow can be extreme enough to cause the carriage bearings to fully bind. In this case, one solution is to shim the rail back into straightness. However, this compromised approach degrades machine appearance and requires costly technician labor. In addition, no level of shimming can produce flatness that prevents all binding.
Contamination causes bearing failures. In profile-rail guides, the rail itself serves as the inner race for the bearings; in other words, the bearing balls engage directly with the profile rail and shafting. Because this subcomponent is exposed, it presents a possible point for debris to enter the carriage. Sealing is required, though isolating carriages in dirty industrial environments can be difficult. When contaminants get past the seals, the bearings are corrupted.
In contrast, cam-follower-based slides inherently separate contaminants that collect on the rail in manufacturing environments from the roller's bearing needles; only the outer race of the sealed needle-bearing roller engages the rail.
Disassembly and repair
Designs that are resold must not fail or cause unreasonable warranty claims, even when subject to abuse. Consider the servicing of commonly specified linear guides: When disassembled, the bearing balls fall out of the saddle. This can lead to both contamination and missing balls. The former accelerates wear, while a carriage with missing balls offers reduced load capacity. The design's engineers cannot control the treatment of components in the field. Then, when these guides fail, sometimes the ball-bushing pillow block must be replaced; other times, the entire unit must be replaced. This is costly.
In contrast, when removing the cam-follower-based saddle from the rail, all components remain attached. If a cam-follower-based slide fails, a single subcomponent can be replaced with a readily available stock cam follower.
In one application for Chrysler, 1.75-in. diameter shafting from a major manufacturer was originally used with a pair of $1,200 pillow blocks. The bearings failed about every six months. Those were eventually replaced with a modified cam-follower-based slide; here, the cam-follower slide has never failed, even after three years. Cam followers are much better suited to this environment.
Single and split
Standard cam-follower-based slides have one integral saddle that includes all 12 rollers. A split version, on the other hand, has two saddles, each with six of the 12 rollers. For applications where a compact self-contained saddle and a single rail are preferred, the standard saddle is recommended. When applications require a longer mounting surface and better aspect ratio, the split design is preferable. Here, two two split saddles span one rail, and engineers put their own plate across the span; moment load capacity is thereby increased as well. In addition, engineers may choose two pairs of split saddles joined by one plate on two rails.
R.P. Gatta has a U.S. patent pending for HD Slides; the component is sold through Bearing Distributors Inc. (BDI) in the U.S. and Canada. For more information, call (330) 562-2288 or visit rpgatta.com.
Cam-follower-based slide capacity
|Model||Static moment capacity||Dynamic moment capacity|
|MA, ft-lb||MB, ft-lb||MC, ft-lb||MA, ft-lb||MB, ft-lb||MC, ft-lb|
Slide saddles are steel or high-strength aluminum. Rails are hardened and ground steel (for high duty cycles) or cold-rolled steel standard bar stock (for low duty cycles and lower-budget applications.) Stainless assemblies are available when corrosion-resistant machine components are specified.
|Model||Static load capacity||Dynamic load capacity|
|FA, lb||FB, lb||FA, lb||FB, lb|
Some slides are available in a 1/2-in. cam-follower size and up. The 1/2-in. diameter roller is the smallest standard cam follower available, which prevents smaller designs.
Cam follower quick facts
First patented in 1937, cam followers were originally developed to perform the function implied by their name: When attached to a movable frame and pressed against the profiled edge of a mechanical cam, cam followers roll along that edge while transmitting tailored X and Y motion to output precisely engineered reciprocating motion.
Cam followers resemble a traditional needle bearing mounted on a bolt. However, their outer race is thicker than that of a standalone bearing to be mounted in a larger piece of equipment; their inner race is one piece with the stud that allows mounting.
Today, cam followers find use in myriad applications — including the one described here.