Uneven Wear on CNC Racks and Gears

Here's a thoughtful look at the causes of various wear patterns on CNC equipment, and appropriate maintenance measures. June 17, 2009

Question
The CNC router I have has a rack and pinion. The X rack and gear wore twice as much as the Y. Does anybody know why? We are replacing both X and Y rack and gears but would like to know the possible cause so the X would last longer? There is one motor on the X and one motor on the Y.

Forum Responses
(CNC Forum)
From contributor O:
Is it the case that the X axis in your machine has to haul the Y axis gantry around, as well as the Z and router load, whereas the Y only carries the Z axis and the router load?



From contributor M:
Several variables can play into this. As stated, the X axis is typically carrying/driving more load than the Y. Typically it will also traverse more distance. I have encountered some machines, that even though this is the case, the machines were built with the same components in both axes (motor, racks, pinions). This is more economical from a machine manufacturing standpoint, but not from a wear standpoint. The X axis typically requires more "beef".

Another point to watch out for is area specific wear. Often I encounter machines that have specific wear "patterns" on rack and pinion set ups, primarily due to using specific origins repeatedly or similar nests repetitively. This can be overcome by alternating like origins or mirroring nests.

Periodic maintenance on the rack itself is a critical component. Clean, lubricate, inspect and remove any encrustations with a bronze bristled brush as soon as they become evident!



From contributor F:

Since rack and pinion systems must have the pinion preloaded against the rack uneven wear can be caused by different preloads between the 2. This will cause gantry skew on routers that have a rack on each side when one of them wears out faster than the other. Thatís the primary reason the DMS 3 axis router model D3 uses a center driven 40mm fixed screw spinning ball nut arrangement.

From contributor M:
This is also why most gantry machines for production applications are built with dual drive rack and pinion gears in the X axis ( a drive system on each side of the gantry) due to the weight they must carry.


From contributor B:
Like contributor M stated our machine has two X drive motors because it has to tote the gantry around and this helps keep it square to the Y axis. We have XA and XB and a Y and Z. When homing the machine it synchronizes the two X drive motors. Never run these machines performing the same operations in the same area all the time as this will wear the rack out in that area and not wear evenly. I remove the racks every six months and thoroughly clean and grease them up but I do apply grease every 80 hours with a tooth brush to the rack gears. The grease I use is white lithium.


From contributor F:
Keeping the rack clean and well lubed is great but you are only delaying the fact that the racks begin to wear out from the first day the machine starts to operate. Once the pinion gear begins to wear out and the teeth push further into the rack your days are numbered. Since each side has a slightly different preload they never wear exactly even.