McMasterCarr Rack-n-Pinion Problem?

[samualt]

Has anyone here ever bought a Rack and Pinion from McMasterCarr?
I got 4 foot rack lengths of 14.5 deg, 12 pitch (Part No 6295K142). I also ordered 2 spur gears that are 14.5 deg, 12 pitch, 1" diameter (Part No 6325k22).

However, when trying to roll the spur gear down the rack it seems to stick now and then. The spur gears do not always mesh smoothly with the rack. In fact, the spur gears have bulbous ends on the spurs. In a way, it looks as if they are supposed to exactly fit each other. But the snags scare me. So, is this something that "rides up with wear" or have I been cheated with bad racks or pinions?

Is this what it is supposed to look like?

[chuckknigh]

Well, I know nothing about racks, but on gears, they're not supposed to be pushed all the way together.

It's called depthing, and if you just raise the pinion off the rack, *slightly,* you may find that it runs a LOT smoother. At least that's the way it works with gears.

Clockmakers have special tools for setting the distance between the centers of adjacent gears.

-- Chuck Knight

[balsaman]

I think Chuck may be right. I would make the drive gear adjustable so you can set the gear clearance. You may need this anyways to dial out backlash over time as things break in.

Eric

[Graham S]

The bulbous nature of the gear is the correct gear form and the guys are right about the meshing, if for example you laid a piece of thin tissue over the rack you would find it would run smoothly as it would keep a meshing distance between them, it you spring load the pinion then it will ride up if it gets stuck, I have seen this done to elliminate the backlash.

Graham

[jcc3inc]

Dear Sir:

There are better choices for rack and pinion. First, 20 degree pressure angle is a better choice for your apparent application. Second, with a smaller number of teeth, there is undercutting of the pinion; you need at least 32 teeth on 14 1/2 degree or at least 18 teeth on 20 degree gearing. If you expect to load the pinion into the rack, 20 degree runs much smoother.

You might consider 24 dp instead of 12 pitch. 12 pitch, 1" diameter has 12 teeth for 3.14159 inches travel. 24 dp has 24 teeth for the same travel. Note that 20 degree requires a min of 18 teeth, so you have accomplished what you want.

Regards,
Jack C.

[HomeCNC]

For the proper meshing distance the Pitch Circles of the teeth on the Rack and Pinion are set tangent to each other, or in the case of the rack the Pitch Circle is NOT a circle it is a straight line so the pinion PC should be set tangent to the PC of the rack.

You can look up the Pitch Circle (PC) data in the machiners handbook or a gear data book.

[samualt]

jcc3inc:
I thought of using a finer pitch. But all the plans I have seen for the big 4'x8' machines use 12 Pitch, 14.5 Deg. I don't even pretend to know why. Maybe it has to do with the big honking motors we use (thus lots of torque). Or maybe it's something else, don't know. But if I ever make some money with this machine perhaps I'll try a finer pitch and see what happens. Smoother would be better.

HomeCNC:
I see, so what your saying is I should make it adustable and move it up ever so slightly till I get a smooth action (We don't need no stinking books or plans!). LOL. Seriously, I'll try to find that data so I'll at least have a good guesstiment.

Thanks everybody!
You have all put my mind at ease. I'm sure the rack-n-pinion is as it should be. I just have to be careful how I mount them. I will make it totally adjustable. Thanks again.

[Al_The_Man]

I have retrofitted machines 4'x8' and up using 12 diametric pitch 20deg pressure angle, the 20 deg is considered to have a higher load bearing capacity then the 14-1/2.
Most CNC gantry tables I have been associated with all use some kind of tension arrangement to maintain pressure of the spur gear with the rack, mainly to remove any backlash. I have seen anything from gas filled (shock absorber type) to coil spring etc. I have had success using a product from Rosta that is originally intended as a motor mount belt tensioner. But If too much tension is applied, it leads to rapid spur gear wear.
Al

[ESjaavik]

jj3inc,
Hey, you seem to know your basics on gears! Getting through such good info in so few words indicate you know a lot about this. I have a problem you may be able to help with:

My mill is now 2/3 converted to CNC. The Z-axis is still manual. There is no way I can figure out to use ball screws without introducing unwanted flex, so I was thinking whether I can improve on the existing rack & pinion. The rack is milled in as part of the quill, so no way I can change that. What I thought about is to cut a new and oversize pinion. I know this should ideally be done for both the rack and the pinion and with a modified tooth form, but as I said: not possible here. I now have around 1/4mm backlash (in rack movement), and would ideally like to have a slight preload instead, but better than 0.1mm will be OK. It's Metric Mod 1.0, 17 tooth pinion. Should I forget to even try?

If I try, should I use the #3 cutter (17-20) to cut the pinion, or will the #2 cutter be a better match to the modified form?

Or should I wait until I have the hob set up. It's not really high on my priority list.

Thanks for any input you have on this, it is quit a bit beyond my knowledge on gears.

[jcc3inc]

Einar:

Why not consider some way of modifying your pinion bearing arrangement so that you can spring load the pinion into the rack. On the plasma cutting machines which we used to make, we loaded the pinions into the racks to get rid of backlash; it was common practice to do so. All the gears were 20 degree pressure angle. You would have to be sure that your loading mechanism did not introduce errors if there was a slight movement of the pinion position due to rack wear.

A second possibility might be to redo your pinion bearing arrangement so that it is somehow eccentrically mounted, and you could set the preload by rotating the eccentric bearing. This is essentially a backlash eliminating approach; if a portion of the rack was worn, you still have a problem at the place which was most worn.

Regards,
Jack C.

[DDM]

Does anyone know what kind of accuracy and repeatability you can get with the rack and pinion setup with a preload. I'm looking at doing a small machine here and I like the idea of the rack and pinon for the X and Y but I don't know if I should go that route or if I should go with ACME lead screws. I plan to make the machine from welded steel rectangular tubing. Any thoughts?

Carl

It's late and I'm just running ideas through my head

[Al_The_Man]

I have installed machines like the LARKEN router tables and they use rack & pinion on the 12ft tables with excellent reapeatability, you can pay extra for precision rack with pre-machined ends for butting together , it depends on the accuracy you are aiming for.
Al