Are you greasing your sliding linear bearings? Breaking the 2:1 rule? If so, you’re probably experiencing binding, chattering or other irregular motion in your application. Below are five tips to help you set your application up for success.
You may be accustomed to greasing your linear bearings, but that’s not necessary anymore. igus® linear bearings are designed with wear-resistant plastic liners that are embedded with solid, dry lubricants, such as PTFE and silicone. These lubricants are released when the bearing moves inside an application, allowing it to run dry and without external lubrication.
Should you add regular linear bearing grease, more friction could occur and cause binding. Most lubricants (especially ones that contain propellants) absorb dirt and become more viscous over time.
If you must apply a lubricant, we recommend lightweight petroleum-based oils. For more about the hidden costs and pitfalls of lubricated bearings, read our whitepaper: The True Cost of Bearing Lubrication.
Avoid over-torquing the bearing’s guide rails into plastic or aluminum extrusions because it may cause the carriage to bind in those areas.
If you are working with welded frames or uneven equipment assemblies, we recommend using bearings with extra clearance. Typically, square-shaft bearings are more forgiving than round-shaft types because they are better able to float when in use. igus® offers its drylin® T, drylin® W and drylin® N linear guides in square styles.
Do what you can to improve the system tolerance. If the tolerances cannot be improved, use bearings that float in the direction of the error (refer to our 2:1 rule and Fixed and Floating Bearings tech talk). Alternatively, use a linear guide system, like igus®’ drylin® WS-10-40, to minimize assembly tolerances.
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To ensure that the bearing sets do not chatter or stop moving, increase the center-to-center distances of the bearings along one rail and in the same direction as the axis of travel. You may use a design tool, such as the drylin® linear guides configurator, to determine whether the bearings are properly spaced, among other important considerations.
We also recommend designating one side of the system as the "fixed" side, one as the "floating" side, and following the 2:1 rule. This rule states that if either the drive force (Fa) or applied load force (Fs) is a greater distance than twice the bearing length (wx), then a binding or chattering of the system can occur. The 2:1 rule is specific to igus® bearings because of the increased coefficient of friction of the materials in our linear bearing systems. (Bearings that slide have a higher coefficient of friction (COF = 0.25) compared to bearings that roll).
For in-depth information on this topic, read an archived installment of our tech talk: The 2:1 rule and Fixed and Floating Bearings.
If you want to move the system by hand while avoiding the maintenance and higher costs that come along with using ball bearings, a hybrid bearing that rolls and slides is an ideal solution.
When using a traditional plastic sliding bearing, a minimum of 2.5 pounds of push-force (this will be higher with cantilevered loads) is needed for every 10 pounds of radial load. If you choose a hybrid bearing, you can reduce the push-force by 0.5 pounds for every 10 pounds of radial load.
Twenty-five pounds of push-force is the upper limit of an average person’s capacity. Therefore, with regular linear bearings, approximately 100 pounds of radial load would be the upper limit of a person’s capacity to push manually. With hybrid bearings, 120 pounds would be achievable as a radial load with a 25-pound push-force limit.
Irregular or chattering motion may occur when the rails are too close together for a load-offset in either the y or z axis (perpendicular to the axis of travel, which is typically designated the x axis).
For every inch between the rails, you need to make sure that your COG is within five inches. This is the 5:1 rule for the center-of-gravity-to-rail spacing. Therefore, if you have five inches between rails, your COG needs to be less than 25 inches or chattering may occur. If you have room, make the spacing even wider -- and remember, if you were making a triangle, you'd want to make it equilateral, not isosceles.
The rails pictured left have been installed incorrectly, as the COG is greater than five times the distance between the rails. The rails pictured right, on the other hand, are correctly spaced due to the COG being less than five times the distance between the rails.