double rack on single pinion to remove backlash?

Imagine having two pinions riding on two side by side racks. You tighten down the inside rack and leave the outside rack slightly loose. Push the gantry on one side (with the power to the motor on so they are locked) tight against the tightened rack. Now with a bolt mounted to push on the end of the loose rack you tighten the bolt to push the entire rack to pinch so to speak the tooth on the pinion. Sort of like having a double pinion that the two pinions are counter rotated to do the same. Bear with me I have a reason for this! The reason being to use helical rack and have a herring bone rack to alleviate sideways thrust. Or would it be better to just fix the inside pinion and rotate the outside pinion to pinch the teeth?

Or is the whole idea useless?

To clarify: you are using two helical gear racks in parallel but with opposite hand or a helical rack in parallel with a herringbone rack?

In the first case, with opposite hand racks, tightening the screw on the second rack will apply a thrust force in the same direction on your shaft. Play will be eliminated once the shaft loads up the thrust bearing. Gear forces will always act against the thrust bearing so in theory you only need one thrust bearing. In practice your racks will have some pitch error which may not match between the two.

In the second case, a helical plus herringbone rack, rotation in one direction will force the gear to center on the herringbone and rotation in the other direction will move the gear towards the thrust bearing. There will be additional error from pitch differences between racks.

RT

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Originally Posted by Black Forest

Imagine having two pinions riding on two side by side racks. You tighten down the inside rack and leave the outside rack slightly loose. Push the gantry on one side (with the power to the motor on so they are locked) tight against the tightened rack. Now with a bolt mounted to push on the end of the loose rack you tighten the bolt to push the entire rack to pinch so to speak the tooth on the pinion. Sort of like having a double pinion that the two pinions are counter rotated to do the same. Bear with me I have a reason for this! The reason being to use helical rack and have a herring bone rack to alleviate sideways thrust. Or would it be better to just fix the inside pinion and rotate the outside pinion to pinch the teeth?

Or is the whole idea useless?

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Using 2 racks is not really the right way to approach your backlash problem

There are some setups already that have 2 pinions on the same rack to eliminate the backlash here is one that works there are other types like this also

Split-Pinions
The simplest zero-backlash drive system utilizes a split-pinion, which consists of two pinion halves and an axial spring pack.
The pinion halves mesh with opposite tooth flanks on the same rack, eliminating the backlash.
One pinion half drives the axis while the second pinion half is "preloaded" to remove the backlash.
The preload setting of the second pinion half is fully adjustable on the machine by turning the axial spring pack at the end of the pinion shaft.

https://www.youtube.com/watch?v=d1sHaF7PYwY

The above helical double pinion should work well provided the pinion is fixed axially by bearings and linear rails that resist thrust loads. Any movement of the carriage or gantry that causes the pinion to translate perpendicularly to the rack will result in perceived backlash even if there is no play between the gears and rack. This effect would be more pronounced at higher loads/accelerations.

RT

Thanks ! I had a brain fart.....it was late here in Germany when I posted this question. The DamenCNC double pinion solution while interesting is not as robust as I would like. The belt inside the housing doesn't hold up under heavier routing on a CNC gantry type router is what I have been told. One user at least has told me this has happened to him.

Build one with a bigger belt?

Quote:

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Originally Posted by Black Forest

Thanks ! I had a brain fart.....it was late here in Germany when I posted this question. The DamenCNC double pinion solution while interesting is not as robust as I would like. The belt inside the housing doesn't hold up under heavier routing on a CNC gantry type router is what I have been told. One user at least has told me this has happened to him.

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Yes everything has it's limits, that part could be redesigned to suit what you need for the load and inertia of your gantry, the split Gear is easier to make