Rack and Pinion question

[scrambled]

Hi everyone-

I have been looking through the past posts, an i can not find the information that I need.

I am in the process of building a cnc plasma cutter, and I have the table and the gantry finished. Now it is time for the rack and pinion. I already have nema 34 planitary 5 to 1 gearboxes.

Ok now for the question. If I want to increase the overall ratio from 5 to 1 to something close to 7 to 1 or even 8 to 1, do I need to use a pinion that is 1 inch in diameter, or bigger/smaller?

I have a Boston Gear book, and the one I am looking at is--

12 diameteral pitch, 1.000 pitch dia. 14.5 pressure angle for the pinion

thanks for the advice



Steve

[DSL PWR]

You need to go smaller. You will need a 5/8 pinion to get to 8:1

[scrambled]

DSL PWR-

That is what i figured, but I had hoped that I was wrong.

Do you know of a company that makes custom pinion gears?


Thanks for the help




Steve

[harryn]

Hi - I think your pinion size can be solved by Boston Gear, but it will not be listed under pinions - it is called something like pinion wire or similar. Basically it looks like shafting that has gear teeth cut into it. They can also cut any gear you want and advertize this service.

This may or may not solve your larger problem though, as a pinion that small will start to have somewhat of a non standard tooth shape with under cut teeth. ( I learned this from reading Boston Gear's engineering literature). Since you are trying to increase torque, then this is somwhat couterproductive, as the tooth will now be weakened.

If you really need that much ratio, it might be worth looking at their pre-built worm drives. I am (hopefully) working my way around the ratio problem by using nema 34 motors and direct drive.

[scrambled]

Harryn-

I found what you were talking about. They are called "stem Pinions". This is the link to the site. http://www.bostongear.com/products/open/stem.html

Does anyone know about how much backlash these produce? My 5 to 1 gearboxes have 8 arc-minutes of backlash. How much backlash does a regular 12 pitch gear rack produce?



Thanks for all the help


Steve

[RICHARD ZASTROW]

Backlash depends on center distance. The farther the pinion is spaced from the rack, the greater the backlash. A spring loaded pinion with no backlash machined into it running against a rack with no backlash machined into it would have no backlash. The pinion would probably wear out early and the rack a bit later. Some backlash is normal. How much is one of those compromises that must be made.

The usual method is to cut the teeth slightly deeper to create the amount of backlash wanted. It can be cut into the pinion or the rack or some in both.

The "Normal" rule is for average conditions, 0.040"/DP for minimum backlash, 0.030"/DP for max backlash 0.050"/DP. So sayeth "Gear Design Simplified" page 20.

Dick Z

[scrambled]

Thanks Richard.

So some backlash is ok, but too much is bad. Easy enough!

Now for my next set of questions. I am looking at ordering both rack gear and stem pinions this week, is there any advantages/disadvantage to buying either the rack or the stem pinion that has been hardened? I know that it would probally be bad to have both hardened, but would it be good/bad to have the rack hardened and the stem pinion not hardened. This way the stem pinion is easier to replace.

Last question.

Is there any benefit to order one size pitch over another? My gantry is going to weigh about 300 pounds. I don't
want to snap off abunch of teeth when the motors are moving full speed, then reverse direction.


Thanks for the help


Steve

[RICHARD ZASTROW]

Steve, Normally the pinion is harder simply because it cycles thru more more revolutions than the mating gear/rack.

The larger the teeth the stronger they are. However the larger teeth require a larger pinion than small teeth. Too few teeth and you will get into undercutting at the base. This will weaken the pinion teeth. Try to keep the number of teeth in the pinion above 18 to prevent undercutting.

A larger pressure angle results in a stronger tooth. The larger pressure angle also wants to push away or separate from the mating gear unless it's between gears. Not usually a problem in the applications we see here, different with heavy stuff like mining equipment where the forces are greater and cancelled by mass of supporting structure.

Hope the long answer helps,

Dick Z