Spring loading a helical rack, Is it possible or is it a good idea

[johnansaro]

Hi there

I am trying to put together a 2 meter by 4 meter 3 axis wood cutting CNC Machine, to power them
I am using 4 step motors 45 kg/cm each. I am using helical rack and pinion system for my 4 meter long
Axis (Y axis). Here is my question: Some people have told me that I cannot Spring Load A Helical Rack and pinion system,
What I want to know is that, How true is this statement? Has anyone had some experience on doing so?

Regards
Johnansaro

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[louieatienza]

Well in my opinion a straight cut rack and pinion shouldn't be spring-loaded either, though it is done. That said if the spring force is perpendicular to the rack then I don't see a problem as far as displacement. You may look into a spring-loaded pinion...

[mactec54]

Hi there

I am trying to put together a 2 meter by 4 meter 3 axis wood cutting CNC Machine, to power them
I am using 4 step motors 45 kg/cm each. I am using helical rack and pinion system for my 4 meter long
Axis (Y axis). Here is my question: Some people have told me that I cannot Spring Load A Helical Rack and pinion system,
What I want to know is that, How true is this statement? Has anyone had some experience on doing so?

Regards
Johnansaro

Yes that is true, you can not spring load a Helical Rack successfully, when it is working in one direction the rack is trying to pull the pinion into the rack, in the other direction it is trying to push the pinion away from the rack, so you need a solid adjustable mounting, to adjust for the correct clearance only

[johnansaro]

Hi mactec54

thanks again for your respond. I should say that I have searched the net on this issue, But i have not been able to find any thing in support of your view.
Could you send me some links or supporting documents on this issue/
and if that is true, is there any quick fix for this Problem? or do I need to change my Rack and pinions to straight ones?

regards
jhonansaro

[ger21]

But i have not been able to find any thing in support of your view.
Could you send me some links or supporting documents on this issue/

Can you find anything that says it's OK to do it?

As Louie said, spring loading is really not the best way to use a rack and pinion drive. It's commonly done because it's easier than machining an accurate mounting surface.

[mactec54]

Hi mactec54

thanks again for your respond. I should say that I have searched the net on this issue, But i have not been able to find any thing in support of your view.
Could you send me some links or supporting documents on this issue/
and if that is true, is there any quick fix for this Problem? or do I need to change my Rack and pinions to straight ones?

regards
jhonansaro

Ask a manufacture that make these Helical Racks, they will tell you the same thing

Quick fix, there is no quick fix if you have already set them up with spring loading, they won't work well like that, Engineer / Design your machine to use what you have correctly, Helical Rack when setup correct is way better than a straight tooth Rack

[louieatienza]

The manufacturer will state the center distance between pinion and rack - this is the optimal distance designed to give proper "clearance." Clearance because for one, everything is machined to a tolerance, and two, the lubrication is used to take up some of the clearance. A precision rack and pinion setup is done with precision milling of mounting surface for the racks, which is checked against an indicator, and kept in position with dowel pins. On a large industrial machine today, there are actually two motors, master and slave, driving two pinions. And the motors are electronically controlled to both turn together when moving, and preload against each other when at standstill. For smaller machines, a split pinion may be used that adds a preload against itself, though I'm not sure how small they make them.

As to helical rack and pinion - you have greater tooth engagement, so it should run quieter and potentially be stronger than a comparable straight cut rack. Of course if you spring load your axis is only as stiff as the rating of the springs. The best thing you can do in your situation is buy thin shim stock of different thicknesses, and get a test indicator, and measure out the center distance at each bolting point, then shim as necessary to get the specified distance of the manufacturer.

[1Jumper10]

Hi there

I am trying to put together a 2 meter by 4 meter 3 axis wood cutting CNC Machine, to power them
I am using 4 step motors 45 kg/cm each. I am using helical rack and pinion system for my 4 meter long
Axis (Y axis). Here is my question: Some people have told me that I cannot Spring Load A Helical Rack and pinion system,
What I want to know is that, How true is this statement? Has anyone had some experience on doing so?

Regards
Johnansaro

As has been posted already, its not a good way to do it. The pic is one of CNC Router Parts motor mounts for a standard rack and pinion drive. It uses a spring on one end and a pivot on the other and the spring pulls the pinion into engagement with the rack. It works because the only forces acting on the pinion are trying to push it away from the rack. And these are overcome by the force of the spring tension. Its pretty simple and effective way to mount a motor to a standard rack and I used it on 2 of my DIY machines and they were trouble free. But a helical rack and pinion drive has additional forces acting on the pinion that try to move it in and out relative to the face of the rack. This in and out motion has to be constrained or it will translate into positional errors. This would be commonly referred to as backlash.

I just made a router using helical rack. I used a mount that pivoted my pinion into engagement with the rack somewhat like the one pictured, but it was then secured fore and aft with bolts.

[johnansaro]

Hi 1Jumper10

Thanks for the reply.
But I just cannot see the spring that you are mentioning in this picture, Could you clarify that spring and pivot that you are mentioning in this Pic.

also If possible could you send me a Picture of your helical Pivoted mount pinion?

regards
jhonansaro

[ger21]

But I just cannot see the spring that you are mentioning in this picture,

The spring attaches to the machine frame.

http://www.cncrouterparts.com/images...00-01_1920.jpg

[1Jumper10]

http://www.cnczone.com/forums/cnc-wo...are-forum.html

This is my build thread. Start around post #132 to see pics of how I built mine.

Sent from my Pixel XL using Tapatalk

[johnansaro]

Hi 1Jumper10

All I can say is Awesome, it will take me a long time to read them all. Excellent

Regards
Jhonansaro