Selecting and sizing guide wheels

Stephen Fournier, Chief Engineer at BishopWisecarver, explores the guide wheel properties best-suited to a given application in order to reduce design, assembly, installation and mounting costs.

Although much attention has been focused on the precision and speed of other linear guides, guide wheel systems have their niche too.

Guide wheel systems routinely operate in environments with high humidity, high concentrations of liquid and solid particulates, operating temperatures up to 260C (autoclavable), noise level limitations, very long travels, and meet tolerances as high as +/-0.03mm.

Compared with other systems, guide wheels have less friction, are much faster to assemble and very cost efficient.

By matching the component properties of a guide wheel system to a given application, engineers can ensure trouble-free operation during predicted lifespan, as well as reduced costs, lead time and field failures.

The bearing type, wheel and track material must be matched to the environment and loads, as well as accuracy, lifecycle and cost requirements.

Based on engineering expertise and empirical experience, we have developed a process for ensuring the best match of guide wheel system to application, beginning with the operating environment.

The following is a brief primer based on the best practices for bearing selection we have found useful for satisfying customer requirements.

The environment determines what type of guide wheel bearing is required, eg sealed or shielded.

Environments with heavy concentrations of liquid or fine/powdery particulates can displace and/or change the properties of the bearing lubricant, causing premature wear and failure of the bearing.

Specifying a sealed bearing for this operating environment can prevent damage to the ball and raceways, ensuring the predicted lifespan of the system.

Generally, shielded bearings are used in environments with heavy concentrations of such large particulates as metal flakes that can work in between the ball and bearing raceway.

The larger debris can cause premature wear and such damage as brinelling or spalling.

Wheels are available in a variety of materials for a wide range of applications, however, the most commonly used materials are 440C stainless steel, 52100 carbon steel and polymer.

Stainless steel materials should be used in humid, liquid and corrosive environments.

Although corrosion resistant, stainless steel can experience corrosion depending on the severity of the environment.

The properties of some polymers include high chemical and heat resistance.

Polymer wheels offer guide wheel benefits with reduced load performance versus steel, but a much lower price point.

Because the operation of guide wheels sweeps the track providing a self-cleaning action, either stainless steel or carbon steel track can be used in environments with heavy concentrations of large particulates and flakes.

The angle of some guide wheels, such as the DualVee guide wheel, has been optimised to provide better wiping action for especially clean rails.

When selecting the track material, a general rule is never to specify a material softer than the wheel material, such as using aluminium track with steel wheels.

This can result in the track material galling onto the wheel, damaging the track, wheel and payload, which can be time-consuming and expensive to repair.

However, hardened track material can be used with acceptable results despite having marginally less hardness than the wheels themselves.

Standard track materials include 1045 carbon steel and 420 stainless steel.

Other track materials include aluminium, which can be used with polymer guide wheels.

The 1045 is a medium carbon steel with good strength and hardness properties, which minimises wear.

The 420 stainless steel contains just enough chromium to limit corrosion, yet can be hardened up to 50HRc.

Generally, an-off-the-shelf guide wheel system should be able to meet positioning tolerances of +/-0.13mm depending on the track mounting surface.

However, by using track made of drawn steel, which is hardened and ground to specified tolerances, a guide wheel system should be able to meet positioning tolerances of +/-0.03mm.

The tricky issue with specifying tolerances has to do with mounting the track, which is limited to the accuracy of the mounting surface.

If the mounting surface is properly prepared and meets specified tolerances, it is reasonable to make an assumption the track will meet the same tolerances.

However, the track may sag under its own weight without proper mounting surface preparation.

Guide wheels can accommodate up to approximately 260C for operation in environments with high temperatures.

The heat slightly compromises load-carrying capability, but appropriately selected guide wheels will provide good performance characteristics at elevated temperatures.

If accuracy is a crucial issue, then stainless steel can be heat treated to the point where it becomes very thermally stable, which minimises growth.

Carbon steel, stainless steel and polymer wheels all can withstand the temperature and duty cycle of an autoclave.

(To sterilise instruments and equipment, an autoclave must reach a minimum of 121C for 30 minutes).

Lubrication is another issue to consider when selecting guide wheels for high temperature operating environments.

Friction caused by wheels rolling across the track generates additional heat at their interface.

This can lead to excessive heat buildup, causing the contact surfaces to gall, potentially leading to excessive brinelling or spalling on the bearing surfaces, eventually resulting in failure of the system.

Proper lubrication helps prevent friction-generated heat buildup.

Typically, the biggest problem encountered with guide wheel systems is a lack of lubrication.

To maintain a long service life and minimise field failure, lubricator assemblies are crucial.

Such devices prevent damage to bearings and help prevent corrosion, even in stainless steel systems.

In our experience, most bearing failures are caused by not enough, complete lack of and/or wrong type of lubricant.

The lifespan of a properly designed guide wheel system is limited to that of the most heavily loaded wheel bearing.

Therefore, loads must be evaluated to predict the lifespan, minimising warranty and in-field repair costs.

However, loads evaluation can be fairly tricky, so it is extremely important to understand exactly the conditions under which the guide wheel will be used.

However, using standard bearing equations for wheels that are axially loaded will yield inaccurate data because the loading is not uniform.

It is a moment load and the wheel makes contact only on one side (unlike a thrust bearing that disperses the load equally along both sides).

Since all the ball bearing elements are not equally loaded, one side of the wheel is free and the other side is interacting with the track, which creates the moment action on the wheel.

By increasing the radial preload, the wheel can accept higher moment loads, but higher radial preload results in a much higher wear rate.

Generally wheel preload is used to eliminate play between the wheel and track.

Preload can be increased as necessary for a given application but not to exceed the radial load capacity of the wheel otherwise premature wheel life failure may occur.

Preload equals the radial load when the system is not loaded by another outside force.

Preload can be determined by dividing the breakaway force by the coefficient of friction and subtracting the applied load.

Caution must be used when applying preload because too much preload on the wheels can cause premature failure.

The rated radial value should never be exceeded by the preload and subsequent radial loads applied to the wheel when in service.

Note that in a four guide wheel assembly sustaining a load that runs along a long beam, preload on the wheels cannot compensate for deflection of the beam.

Typically in a guide wheel and carriage application, there should be two concentric mounted wheels and the rest of the wheel should be on eccentric mounts.

The eccentric type guide wheels are used to create a camming action to preload the guide wheels against one side of the guide track.

Industrial environments tend to be forgiving where noise is concerned.

However, noise is an issue where the general public is concerned.

For example, patients can be unnerved when in contact with noisy medical devices.

Noisy guide way systems for CAT scan and magnetic resonance imaging equipment can make patients needlessly uncomfortable.

Guide wheel technology can result in a 20% noise reduction compared with a square rail system.

Guide wheel technology ensures that the ball bearing raceway path is a constant radius instead of widely varying radii, such as with a square rail.

A square rail has straight sections with radii at the ends, which make a 180-degree arc.

The ball bearing elements move along a straight line and then another arc to complete the profile path.

This causes noise and vibration at the ball returns where there is a change in acceleration of the bearing element from the straight section to the curved section.

Sometimes polymer cages are used to reduce noise of the ball bearings, but they are not completely effective.

Guide wheel track mounting does not require as much surface preparation as with other linear guide systems.

With a square rail, the mounting surface is very flat and very straight.

A guide wheel system can be bolted to a semi uneven surface and a carriage will function properly, providing a nice rolling movement.

However, for systems requiring high accuracy and repeatability, better surface preparation will be required.

When running parallel track systems, alignment is forgiving.

For example, if only +/-0.1mm tolerances are needed instead of +/-0.03mm, a substantial amount of surface preparation expense can be reduced.

In designing a wheel carriage it is important to use the right combination of eccentric and concentric guide wheels depending on the configuration.

The linear systems should always have two concentric wheels and all the other guide wheels should be eccentric.

The eccentric wheels are used to remove the play between the wheels and track (commonly known as wheel preload) equally loading all the wheels so they roll instead of sliding or skipping on the track due to acceleration.

When the wheel carriage is loaded in the radial direction, the concentric wheel should carry the primary load.

Several factors influence the service life of guide wheel systems.

We have devised a simple method for estimating the load/life relationship for guide wheel systems by defining the loading conditions.

The process accounts for the size of the bearing elements, relative spacing; and the orientation, location and magnitude of the load.

The curve is based on clean and well-lubricated track conditions.

Hepco Slide Systems and Bishop Wisecarver, are both leading lights in the field of linear motion technology.

There is a synergy not only in the way the companies work but also in their respective product lines.

Indeed since the 1980s BWC's Dual-Vee system has been an important and complementary part of the HepcoMotion product programme.

Correspondingly BWC acts as factory representative in North America for HepcoMotion's systems.

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