Getting On Track with guide wheels

Oct. 20, 2005
High-accuracy linear motion in hostile environments? No problem.

Stephen Fournier
Chief Engineer
BishopWisecarver Inc.
Pittsburg, Calif.

An optimized wheel angle on DualVee guide wheels improves wiping action to keep rails clean.

A typical guide wheel and carriage application uses two concentric-mounted wheels and the rest eccentric. For example, a six-wheel carriage would contain four eccentric and two concentric wheels. The concentric wheels carry the primary load when the wheel carriage is loaded in the radial direction. The eccentric guide wheels act as cams to preload wheels against one side of the guide track so they roll instead of slide or skip under acceleration. Note that the location of the eccentric wheels depends on whether the track guide way locates on the inside or outside of the wheel carriage.

Abrasives used in water jet cutters quickly compromised recirculating ball-bearing guides supporting a 200-lb cutting head. The use of DualVee guide wheels eliminated contamination as a source of failure and accuracy loss. Also eliminated were failure-prone way covers. Guide wheels in this case hold system accuracy to 0.0005 in.

Linear guides find use in a variety of precision-motion applications, from machine tools to medical equipment. They come in two basic types: plain bearing and rolling element. Rolling-element types are further categorized as recirculating ball bearing and guide wheel.

Recirculating types support loads with a continuous stream of bearing balls that move in and out of contact with a guide rail. The bearings are made to follow straight sections with radii at their ends. This causes balls to vibrate and make noise as they accelerate from the straight to curved sections. The use of polymer cages that separate balls lowers but does not eliminate ball-bearing noise in square-rail designs.

Guide wheels, in contrast, react loads through wheels that run on triangular-profile rails. Guide wheels can lower noise levels by 20% compared with square-rails systems because the ball-bearing raceway path is a constant radius.

Guide wheels are also more forgiving of mounting-surface irregularities than recirculating designs. Parallel-track guide-wheel systems work fine when mounted on surfaces flat to ±0.004 in. For comparison, mounting surfaces for square rails are typically held to ±0.001 in., which can substantially raise installation costs. Still, be sure to carefully prepare surfaces for parallel-track systems needing high accuracy and repeatability.

Off-the-shelf guide-wheel systems have a positioning accuracy of about ±0.005 in. depending on the accuracy of the track-mounting surface. Tracks made of drawn steel that is hardened and ground can push that number down to about ±0.001 in.

The operating environment is another important consideration. Guide wheels better tolerate contamination than recirculating designs and don't need protective covers on track ways.

Wheels come in a variety of materials including 440C stainless steel, 52100 carbon steel, and polymer. Stainless steel makes sense for humid, liquid, and corrosive environments, though extremely harsh conditions may damage it. Polymer wheels, depending on the material, resist chemical attack and high operating temperatures but obviously handle lower loads than steel.

Stainless-steel or carbon-steel tracks work for applications with heavy concentrations of large particulates and metallic flakes. As a rule, never specify a material for tracks that is significantly softer than the wheels. Otherwise track material can gall to wheels, damaging them, the track, and payload.

Standard track materials include 1045 carbon steel and 420 stainless steel. Aluminum tracks are suitable for use with polymer guide wheels. 1045 carbon steel has good strength and hardness properties to minimize wear. 420 stainless steel contains some chromium to limit corrosion yet is hardenable to 50 HRc.

Guide wheels can tolerate temperatures to about 500°F, but with a slight loss of load capacity. High-accuracy applications should use heat-treated stainless-steel wheels that have minimal thermal growth at elevated temperatures. Carbon-steel, stainless-steel, and polymer wheels all are suitable for use in autoclaves, which reach temperatures of at least 250°F for 30 min to sterilize instruments and equipment.

Pay special attention to lubrication when specifying guidewheel systems. Rolling friction generates additional heat which, in turn, can cause bearing contact surfaces to gall, brinell, or spall and eventually fail. Proper lubrication lowers friction-generated heat buildup. Lubricator assemblies that supply bearings with ample lubricant further help prevent damage and corrosion.

Shielded bearings make sense for environments with heavy concentrations of large particulates such as metal flakes that can enter spaces between balls and bearing raceways and cause life-shortening brinelling and spalling. Sealed bearings work in environments with liquid or fine/powdery particulates that can displace and change the properties of bearing lubricants, leading to premature wear and bearing failure. In our experience, lack of or improper lubricant causes most bearing failures.

A derating factor, DF, based on empirical data, keys to the severity of operating and environmental conditions:

1.0 to 0.7 — Clean, low-speed, low shock, low duty

0.7 to 0.4 — Moderate contamination, medium duty, medium shock, low to medium vibration, moderate speed

0.4 to 0.1 — Heavy contamination, high acceleration, high speed, medium to high shock, high vibration and high duty cycle

The derating factor is used in a selection process that considers wheel size and relative spacing, as well as the orientation, location and magnitude of loads.

First, calculate the load factor, LF, by summing all external forces that react through the wheel-track interface (inertial, gravitational, payload):

LF = LA/LAmax + LR/LRmax


LA = Resultant axial load on guide wheel in a direction parallel to the axis of rotation.

LAmax = Maximum axial working load capacity of guide wheel.

LR = Resultant radial load on guide wheel in a direction perpendicular to the axis of rotation.

LRmax = Maximum radial working load capacity of guide wheel.

Bearings should be sized such that LF ≤1.

Finally, calculate system life expectancy, Le (in. of travel):

Le = DFLC/LF 3


Lc = life constant, which scales with wheel size. The above ratings and calculations are theoretical and assume ideal conditions and proper lubrication. Bearing equations for axially loaded wheels give inaccurate results because loads on guide wheels contain moments and are not uniform. Not all of the ball-bearing elements carry the same load as with thrust bearings that distribute load equally along both sides. With guide wheels, one side of the wheel is free and the other touches the track, which causes the moment action.

A typical guide wheel and carriage application uses two concentric-mounted wheels and the rest eccentric. The concentric wheels carry the primary load when the wheel carriage is loaded in the radial direction. The eccentric guide wheels act as cams to preload wheels against one side of the guide track so they roll instead of slide or skip under acceleration.

Preload equals the radial load when the system is otherwise unloaded. Boosting radial preload lets a wheel accept higher moment loads, but it also accelerates wear rate. Higher preloads as well raise breakaway force of the carriage and breakaway torque of belt and chain-drive mechanisms that move them, important metrics for sensitive medical instruments and equipment. Of course, the combination of preload and applied radial load should never exceed radial-load ratings of the wheels. Bearings with marginal capacity should be upsized to the next available size. In any case, the most heavily loaded wheel bearing limits lifetime of a properly designed guide-wheel system.

Bearing designation
Radial dynamic load, lbf
Radial static load, lbf
Axial dynamic load, lbf
Axial static load, lbf
Life constant, in. of travel
Multiply lbf by 4.449 to convert to N. Multiply in. by 2.506 10- 5 to convert to km.

Sizing Guide Wheels — By The Numbers

Move a 250-lb weight, W, which has its center of gravity at d = 12.0 in. from the guide-wheel track centerline. Move along the Y axis a distance = 72 in. in 1.2 sec with a triangular velocity profile. Wheel spacing cannot exceed 10 in. in both the Y and Z directions.

Bishop Wisecarver Inc.,

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