ROLLON

One of the greatest threats to the long life of a linear motion system usually occurs on its very first stroke.

How do you know when a linear bearing has reached its end of travel? When the carriage hits the wall. At least, that is how the old engineering joke goes. But in reality, crashes are no laughing matter.

Also known euphemistically as “hard stops,” crashes occur when an out-of-control pillow block slams into the bearing’s end stop or some other intermediate target. They most often occur when a linear axis is started up for the first time. Even a single crash on that very first stroke can ruin a bearing, triggering expensive repair or replacement costs.

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Engineers focused on fast, light automation systems may pay too much attention to speed specs when acceleration is what really counts.

Look through the specifications of any linear bearing, and you’ll come across a maximum speed. It’s an important spec, but not not nearly as important as many engineers think it is.

Speed does play a role in bearing life. Consistently moving loads above a bearing’s speed limit can result in premature wear. It is also possible, usually though a control error, to drive a bearing so fast that it will fail. For example, recirculating ball linear guides can experience end cap failure when driven at high speeds or accelerations. And roller bearings can freeze up at extremely high speeds. Yet these over-speed conditions are rare in well-designed motion system.

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Of all the factors that contribute to the premature failure of linear bearings, misalignment ranks near the top of the list. Here’s how to cope without breaking the bank.

When design engineers evaluate linear bearings, they always ask about performance attributes such as speed, load capacity and lifecycle. Then they want to know the price. It’s rare, however, that they ask about the bearing’s sensitivity to misalignment. And that’s a big mistake, because misalignment represents one of the leading causes of premature linear bearing wear and failure.

Linear bearings that should last for years based on expected life calculations can quit after a just few months if they are not aligned to the geometric tolerances they require to run smoothly. Usually, alignment problems begin with the design and preparation of the machine frame itself. It may not be flat, straight or parallel enough for a bearing to mounted properly. For example, mounting surfaces may have one or more high spots that will read through to the installed bearing rails. Or the frame design may make it difficult to mount bearing rails parallel to one another in the horizontal axis, vertical axis or both.

A two-rail linear motion system improves alignment and contamination resistance while lowering installed cost.

The top guide system in this design uses a Rollon T Series Rail to support the bulk of the engraving head’s axial loads from above. The bottom bearing uses a Rollon U Series Rail to help guide the head precisely while supporting the head’s moment loads. The lower rail also prevents the head from swinging under the influence of any vibration or inertia effects.

Contamination and poor lubrication practices can shorten the life of linear bearings. Here is a look at how to avoid both problems.

To get the longest working life out of a linear bearing, keep it clean and well lubricated. This common-sense advice may sound easy enough to follow. Yet in the real world of roundthe- clock, high-cycle manufacturing operations, bearings do get dirty and dry.

And when either of these conditions happens, linear bearings will wear prematurely. In the worst-case scenarios, contamination and inadequate lubrication can create metal-on-metal contact between the bearing’s rolling elements and raceway. This can cause excessive wear in the form of denting, pitting or galling results.