Linear bearings based on steel guide wheels often complement motor-based linear actuation in two challenging application types:
- Long-stroke linear-motion axes as those driven by belts, linear motors, or rack-and-pinion sets
- Linear systems in harsh environments subject to hot and humid conditions, abrasive materials, extreme loading, and corrosive chemicals.
Recall that wheel-based linear guides are systems that pair profiled wheels with precisely mating tracks — usually rounded, flat, or V-shaped — to impart guide and load-bearing functions to actuated linear axes. Much like the cam-follower assemblies or trolley wheels for square or hollow box-track linear rail, the profiled wheels contain at their core precision rotary bearings employing needle rollers or other high-load rolling-element variation for long life. Most variations include twin concentric wheels to bear load and eccentric wheels to hold the load-bearing wheels firmly against the working track — so all the wheels roll without skidding or slipping.
The environment dictates which wheel bearing type is most suitable. Where a linear axis is subject to light debris or washdowns, sealed bearings are necessary. Machine-tool applications and other designs subjecting the linear-guide wheels to even more copious or detrimental debris (such as metal shavings) necessitate wheels with fully shielded bearings to prevent brinelling, spalling, and premature failure.
Steel and polymer profiled-wheel linear systems work in especially hot applications (to 120° C as in autoclaving) or even to a sustained 260° C with heat-treated stainless wheels and specialty lubrication. Stainless steel subcomponents are also indispensable for humid and corrosive settings. Steel wheel-based linear guides also excel in actuated applications consistently exposed to debris during operation, as they’re materially rugged and have geometry that tends to plow contaminants off the tracks. Guide wheels with steep side angles are especially good at clearing debris from their rails. Note that to prevent galling, the linear rails are usually harder than the wheels with which they pair. Many standard tracks are 1045 medium-carbon steel and 420 stainless hardened to 50 HRc. Otherwise (where the profiled wheels are made on engineered polymer) aluminum tracks are acceptable.
The place and way in which the linear track is mounted greatly impact accuracy. Machine surfaces prepared to proper tolerances help the linear systems meet the same. Consistency helps prevent binding, sagging, and variable output. That said, profiled-wheel linear guides require less surface preparation than other linear systems based on square rail, for example — which necessitate flat and straight mounting surfaces. Though well-prepared machine surfaces boost accuracy and repeatability, the tracks of profiled-wheel guide even bolt to moderately inconsistent surfaces without causing running issues — even if the axis necessitates twin tracks in tandem. Off-the-shelf wheel-based linear guides support positioning to ±100 µm … and high-accuracy systems with drawn, hardened, and ground steel track support positioning to within ±25 µm.
As mentioned, preload can eliminate play between load-bearing wheels and track. The main rule is that preload never exceed the design wheels’ radial load capacity (minus any other radial load from the installation on the wheels). Preload equals the ratio of breakaway force to the coefficient of friction — minus applied load. Two other caveats are that applying preload can in some instances shorten linear-guide wheel life … and preload should never be used to compensate for assembly deflection.