Wednesday, January 1, 2014

On Steel Rod

There is much confusion regarding the type of steel rod to use for smooth axes. Much of this confusion probably arises from the fact that most people hear steel and think of metal. Unfortunately this is a somewhat short-sighted mental model. There are in fact numerous grades of steel.

Some steel is formulated for machineability, some for corrosion resistance, some for wear resistance, etc. etc. etc. Long story short, if you bought a random grade of steel and are using LM8UU linear bearings, you may find that over a relatively short period of time your rods will show tracks or wear marks. Short Term this will cause higher friction resulting in increased motor torque and current draw. Long term this will eventually cause binding and failure of your axes.

There are two solutions to this issue.

One is to use sleeve bearings instead of ball bearings. Sleeve bearings such as those linked are made of softer material than the linear rod and thus are less likely to wear away at the rod. The other factor that helps reduce wear is the fact that sleeve bearings distribute force over a large surface area on the rod.This is in contrast to ball bearings (LM8UUs) which apply the same force to many small points on the rod. Though sleeve bearings are less likely to wear away at your linear rods, there are pros and cons to all design choices. The sleeve bearings have a larger surface area contracting the rod and thus typically will have more friction than a roller bearing on an intact rod. As such there is a greater need to maintain appropriate lubricity for sleeve bearing arrangements.

The other solution, which happens to be my personal choice, is to use a hardened grade of steel rod in conjunction with LM8UUs. With such a configuration the ball bearings will not gouge the rod, the point contacts from the ball bearings will keep friction low, and there will be less need for lubrication. The smooth shaft page of this build guide links to appropriate options for linear rods.

I recommend using hardened rods regardless of the bearing configuration that you choose. For more information on grades of steel, McMaster has a handy guide: