When it comes to moving loads along straight or curved paths, steel-on-steel guide wheel systems are tough to beat. They are unaffected by most types of industrial fluids and debris, withstand temperatures to 500°F, set up easily on less-than-perfect surfaces, and transport variously sized masses at speeds to 8 m/sec and accelerations of up 5 g's.
Properly sized, guide wheel systems operate trouble-free over long, predictable lifespans. They also run quiet and smooth, present less friction than other guide types, and save on costs. Although it takes practice to configure an optimum solution for a specific application, the learning process can be accelerated by following the advice presented here.
Wheel preload is often used to eliminate play between the wheel and track. It can be increased as necessary for a given application as long as it doesn't exceed the radial load capacity of the wheel. In systems free of other outside forces, preload accounts for the entire radial load on the wheel. In systems carrying loads, preload can be found from the following equation:
preload = breakaway force/coefficient of friction - applied load
Applying preload is somewhat risky, however, as too much can cause premature wheel failure. Another concern: Radial load capacity should never be exceeded by preload and the subsequent radial loads that the wheel expects to see in service. It is also worth noting that, in a four guide-wheel assembly sustaining a load along a beam, wheel preload cannot compensate for possible beam deflection.
Typically, in a guide wheel and carriage application, there should be two concentric mounted wheels, with all other wheels on eccentric mounts. Eccentric type guide wheels create a camming action to preload the wheels against one side of the track.
In many applications, guide wheel technology excels because it reduces noise by 20% compared to square rail guides.
Guide wheel technology ensures that ball bearing paths are a constant radius, in contrast to the widely varying radii encountered in square-rail systems. Bearing paths in square rails consist of straight runs with radii at the ends, each forming a 180°arc. Ball bearing elements following such paths cause noise and vibration at the returns, where there are associated changes in acceleration.
The good news about guide-wheel track mounting is that it doesn't require as much surface preparation as other linear guide systems. With a square rail, the mounting surface must be very flat and straight. A guide wheel system, on the other hand, can be bolted to a semi-uneven surface and a carriage will function properly, providing a nice rolling movement. Naturally, the better the surface preparation, the better the accuracy and repeatability.
When running parallel track systems, alignment is very forgiving as well. Therefore, surface preparation expense is minimized.
Sizing a guide wheel
Step 1: Calculate radial LRand axial LA loads on each bearing element in the guide wheel system. This is determined by applying statics to the application.
Step 2: All standard considerations involved in statics calculations must be taken into account, including inertial forces, gravitational forces, and external forces such as tool pressure, bearing element spacing, and magnitude and direction of the payload. Any external forces that generate a reaction through the wheel-track interface must be considered:
LF = LA/LAmax + LR/LRmax
LF = load factor
LA = resultant axial load on guide wheel
LAmax = maximum axial working load capacity of guide wheel
LR = resultant radial load on guide wheel
LRmax = maximum radial working load capacity of guide wheel
One important note is that bearings should be sized such that LF • 1. Also, it is often necessary to apply a safety factor to the maximum axial LAmax and radial LRmax working load capacities to account for variations in load, speed, shock, vibration, contamination, and duty cycle.
Step 3: Apply the load factor to the lifetime equation to determine system life expectancy. And count on getting that much if not more service from your guide wheel system.
For additional information, contact Bishop-Wisecarver Corp. at (888) 580-8272 or visit www.bwc.com.