In a typical machine environment, moving from point A to point B isn't as easy as it may sound. Common obstacles include punishing loads, long spans, heat and humidity, corrosives, abrasives, noise limitations, and unforgiving tolerances as tight as 60.001 in. Such challenges demand a special type of linear guide, namely steel-on-steel guide wheel systems.
When properly matched to an application, guide wheel systems operate trouble-free over a predictable lifespan. They also assemble quickly, save on costs, and present far less friction than other guide types. Getting the right match takes practice, but the learning process can be accelerated by following the time-tested advice presented here.
Operating environment usually determines what type of bearing is best for a guide wheel system. Environments laden with liquids or powders, for example, typically require sealed bearings. The addition of seals prevents fine particulates from displacing or altering lubricants, stemming premature bearing wear and potential failure.
Environments overflowing with large particulates, on the other hand, necessitate shielded bearings. Large particles, like metal flakes, pose a danger to bearings because they can work in between the ball and bearing raceway, causing premature wear, brinelling, and spalling. Shielded bearings effectively eliminate these concerns.
Guide wheels come in a variety of materials, meeting a wide range of application requirements. The most common wheel materials are 440C stainless steel, 52100 carbon steel, and polymer. Stainless steel should be used in high humidity, liquid, and moderately corrosive environments. Polymer wheels, though they fall short of steel in terms of load capacity, save on cost. They also stand up better to certain chemicals, and are continually improving in their ability to handle heat.
When selecting track material, a general rule is to never specify a material softer than that of the wheel. Otherwise, the track can gall onto the wheel, which can damage the track, wheel, and payload. There are, of course, situations where this “rule” can be broken. Hardened track, for example, may be used with acceptable results despite its being marginally softer than the mating wheels.
Standard track materials include 1045 carbon steel and 420 stainless. 1045 is a medium carbon steel with good strength and hardness properties, which minimizes wear. 420 stainless steel contains just enough chromium to limit corrosion, yet can be hardened up to 50 HRc. Another track material, aluminum, is acceptable when used with polymer wheels.
Because guide wheel systems are self-cleaning — the wheels sweep the track constantly during operation — stainless and carbon steel track are not at risk when exposed to large particles and flakes. Some guide wheels, in fact, have been optimized to provide better wiping action for especially clean rails. This is the case with steep-angled DualVee wheels.
Standard (off-the-shelf) guide wheel systems are designed to meet positioning tolerances of about ±0.005 in. High-accuracy systems — employing track made of drawn steel, hardened and ground to specified tolerances — get into the ±0.001-in. range.
The big issue with tolerances has to do with mounting the track, which is a function of mounting-surface accuracy. If the mounting surface is properly prepared and meets specified tolerances, it is reasonable to assume that the track will meet the same. Proper surface preparation prevents the track from sagging under its own weight.
Carbon steel, stainless steel, and polymer wheels have no problem operating at 250°F for 30 minutes, the typical temperature and duty cycle of an autoclave environment. In fact, they can accommodate temperatures up to approximately 500°F. Although high heat slightly compromises load-carrying capability, appropriately selected guide wheels will minimize the effect. If thermal accuracy is an issue, then stainless steel can be heat-treated, which makes it very thermally stable.
Lubrication is another issue to consider when selecting guide wheels for high-temperature environments. Friction caused by wheels rolling across the track generates additional heat. This can lead to excessive heat buildup, causing the contact surfaces to gall, potentially leading to excessive brinelling or spalling on the load bearing surfaces. Proper lubrication helps prevent friction-generated heat buildup and associated problems.
Typically, the biggest problem encountered with guide wheel systems is a lack of lubrication. To maintain a long service life and minimize field failure, lubricator assemblies are crucial. Such devices prevent damage to bearings and help prevent corrosion, even on stainless steel. Most bearing failures are caused by insufficient lubrication, a complete lack of oil or grease, or using the wrong lube type.
The lifespan of a properly designed guide wheel system is limited to that of the most heavily loaded wheel bearing. But load evaluation can be fairly tricky, making it extremely important to understand the exact conditions under which the guide wheel will be used.
The first step is to determine if the loads are radial or axial. Radial load LR refers to a load applied in a direction perpendicular to the axis of rotation. Axial load LA refers to a load applied in a direction parallel to the axis of rotation.
It would seem reasonable to use standard bearing equations at this point, but this yields inaccurate results because of nonuniform loading on the wheels. Guide wheels experience a moment load and they make contact on only one side (unlike a thrust bearing that disperses the load equally along both sides). Because all ball bearing elements are not equally loaded, one side of the wheel is free, while the other side interacts with the track, creating 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.
To get a better grasp on moment loading, think of an airplane in flight. MP is a moment load in the pitch direction. Pitch loading can be thought of as an airplane climbing or descending. Pitch moments take place when a force wants to tilt the wheel plate up or down.
MR is called a roll moment. When an airplane banks left or right this is considered movement in the roll direction. A roll moment occurs when the wheel plate is subjected to a load that makes the wheel plate want to tilt like an airplane banking.
MY is a yaw moment. Yaw occurs when an airplane tails out to the left or right. Here, the wheel plate is subjected to loading that makes it want to rotate to the left or right.
For additional information, contact Bishop-Wisecarver Corporation (888) 580-8272, or visit www.bwc.com.