By Leslie Lui, Mechanical Design Engineer, Bishop Wisecarver, Pittsburg, Ca.
Preloading is the practice of assembling mechanical systems in such a way that some or all components are held in a compressed or tensioned state when there are no external forces acting on the system. There are several benefits to preloading, including reduced looseness, increased rigidity, and increased integrity in the components and the system as a whole. Preloading is a common and recommended practice when assembling structural systems such as bolted joint assemblies, where screws and bolts are tightened until they are highly tensioned and the components they are fastening together are compressed.
Pre-loading requires only a few simple tools and the proper positioning of the wrenches for adjusting eccentric wheel preload.
Preloading can also be beneficial in guide-wheel linear motion systems where it increases the rigidity of the system and can help extend the useful life of the components and system, decreasing its maintenance cost and increasing its functional value. Whereas preloading in the aforementioned bolted joint assemblies involves tightening the fasteners to some suitable level of joint tension, preloading in a guide wheel-based system involves adjusting the relative positions of the guide wheels on a carriage so that all of them are in contact and suitably compressed against their mating tracks.
Properly preloading can extend the operating life of the guide wheels by reducing the variations in resultant loading on the guide wheels. When the wheels are not properly preloaded, there is a greater probability that one or more of the wheels is not in constant contact with the tracks, which could allow the carriage to freely tilt and shift relative to the tracks. This means that any applied loads on the carriage could cause higher resultant loads on the fewer wheels contacting the track, in addition to high shock loads whenever the loose wheels intermittently contact the tracks.
Proper preloading of guide wheels also has other benefits such as reducing vibration and noise because the carriage will not be able to tilt or shift relative to the tracks. In addition, when applying external loads to a preloaded carriage, they will be subject to less displacement relative to the tracks. The linear accuracy and repeatability of the carriage in a driven system are also improved. All of these attributes are beneficial in most applications and even critical in some, such as when high precision sensors are mounted on the carriage.
Guide Wheel Basics
Adjusting the preload for guide wheel-based linear motion systems is a fairly straightforward process. Some other systems, such as profile rail-based systems, can’t have their preload adjusted because it is a function of the machined dimensions of the components and, therefore, factory set. As a result, profile rail carriages (blocks) have to be carefully matched to their rails, as even the slightest dimensional mismatches can cause the carriages to jam on the rail if the fit is too tight, or allow the carriage to freely tilt and shift if the fit is too loose. In contrast, guide wheel-based carriages can easily be swapped to other track assemblies, and any minor dimensional differences compensated for in the preload adjustment process.
There are basically two types of guide wheels; concentric and eccentric. The wheel’s axis on a concentric guide wheel is concentric with the axis of its mounting stud or bushing, whereas the wheel axis on an eccentric guide wheel is slightly offset in the radial direction from the axis of its mounting stud or bushing. Carriages have two rows of guide wheels, aligned in the direction of travel. The most common carriages have three or four wheels, typically with two concentric wheels on one side and one or two eccentric wheels on the other side. When properly preloaded, the running surfaces of the guide wheels should be parallel so that all of the wheels make simultaneous contact with their mating tracks and don’t lose contact at any point over the linear motion system’s stroke length.
Preloading is done by rotating the eccentric guide wheel’s mounting stud or bushing to adjust the eccentricity of the eccentric guide wheels. Because the axes of the wheels are offset from their mounting studs, rotating them causes the wheel to move toward or away from its mating track.
A Pre-loading How-to
Preloading guide wheels requires only a few simple tools. For example, preloading Bishop Wisecarver DualVee guide wheels with integral studs requires an open-end wrench and a socket wrench. The guide wheels with separate bushings and fasteners require two open-end wrenches. Despite the slightly different tools required for each type of guide wheel, the process is the same for all of them.
Begin by placing the carriage on the track assembly with the concentric wheels fully tightened and the eccentric wheels finger-tightened within their mounting holes. Using one hand, insert the open-end wrench between the eccentric wheel and the mounting plate to engage the hex flats of the bushing or stud on the wheel. Use the other hand to hold the open-end wrench or socket wrench on the wheel’s fastener.
Use the wrench to slowly turn the hex clockwise until you feel resistance; this indicates that the wheel is contacting the track. Then, with the wrench still held in position with one hand, tighten the eccentric wheel’s fastener to lock the wheel in the adjusted position. The reason for turning the hex counterclockwise is so that the any radial load between the wheel and track will cause it to turn clockwise, creating a tightening torque on the wheel’s fastener.
To check the level of preload, hold the carriage in place with one hand and rotate the wheel with the index finger and thumb of the other hand; the wheel’s running surfaces should be able to slide on the track when the wheel is rotated with moderate hand turning effort. If the wheel can’t be rotated, adjust the wrench position to reduce the preload and try again.
Manually slide the carriage along the entire length of the system to determine whether there is any noticeable variation in rolling resistance. If there is, readjust the eccentric wheel as necessary. If the variation is unacceptably large, the tracks are likely not parallel enough and will need to be realigned.
Check the level of the preload by manually rotating the wheel.
If there is more than one eccentric wheel on the carriage, repeat the process with all of the other eccentric wheels. Once all the wheels are adjusted, recheck all the wheels, concentric and eccentric, for preload using the stationary carriage-wheel rotation and sliding resistance methods described above, and readjust if necessary.
The moderate effort required to rotate stationary wheels on the track is the preload level generally recommended for most applications. However, the preload can be reduced or increased depending on the application. For example, a system with low rigidity requirements and low payload could have less preload if less breakaway and rolling resistance is required. Conversely, a system with high rigidity and accuracy requirements could have higher preload if increased breakaway and rolling resistance are acceptable. Preload is critical and should not be set excessively and should never exceed the wheel’s specified radial load capacity.
Filed Under: Factory automation, Linear motion • slides, Motion control • motor controls