In today’s session of “Mech Minutes”, we will break down another topic related to Linear Guides, clearance selection. This video will focus on selection of a clearance or a preload and how it can affect your application. Choosing the right preload can affect the rigidity of a machine as well as the movement of the linear guide.
We will begin this lesson by discussing what preload is and how it is applied. When talking about clearance on a linear guide, we are referring to the space between the ball and the raceways of the linear guide rail and block. When a gap exists between the ball and the raceways, the linear guide has positive clearance, or, simply, clearance.
On the other hand, if no gap exists and the ball is being compressed by the raceways, the linear guide has a negative clearance or preload. The clearance range is set by matching the appropriate ball size to a pre-determined drag force measurement by the manufacturer.
Product literature for a linear guide will display the physical space between the ball and raceways as a radial clearance range for each preload type. Select the preload type that best suits your application after reviewing this data. One consequence of using a linear guide with a preload is a greater drag force required to move the block.
A linear guide with a clearance will move with less force and has less of a tendency to have sticky motion during travel. Another potential pitfall of using a preloaded linear guide is a decrease in the L10 life of the linear guide. Because the ball is under compression, the load applied to a linear guide block will be greater for a preloaded linear guide compared to one with a positive clearance.
Now that we’ve covered some potential reasons to avoid a preloaded linear guide, let’s discuss the reasons you might want to select a preload. The main advantage of using a preload linear guide is to increase the rigidity or decrease the deflection of block under a load.
When a block with clearance receives a load, the initial deflection follows a logarithmic-like curve before it stabilizes to a linear trend. When a preloaded block first receives a load, it has already reached the linear trend and will deflect less under the same load and follows an easy to predict linear deflection curve.
If your machine has a varying degree of load applied to the linear guide, or if your load varies in direction, a preloaded block will result in a more accurate, predictable deflection allowing more precise operation. We hope you are now better informed when choosing a clearance level for your linear guide application.
Join us next time for another entry of Mech Minutes where we discuss mounting and assembly considerations for linear guides.