[Hi-Vac]

I have searched the “heck” out of this site looking for an answer and trying to avoid repeating a question, so if I missed something, please let me know…..

I have a question about “pressuring” the Pinion into the Rack to reduce backlash. I have read that the larger the diameter of the pinion gear, the less likely it is going to “jam” or “stick into the rack, but the trade off is the gear ratio increase. Also, the 20 degree pressure rack is better than the 14.5 degree pressure rack for this use. However, I can’t find any info on “pressuring “helical racks.

Originally, and mostly due to the weight of our gantry, we had decided on larger 34 frame motors, spinning in 25:1 gear boxes, with a 18T / 1.50” diameter “Helical Gear” pinion riding on a 26mm (1.02”) face width rack as supplied by Atlanta Drive. For the record, the motor plate is bolted to two THK Linear rails with dual cars on each rail (yes, I know….overkill). I what then planning on pushing up on the motor plate with either an Air ram or a gas shock.

1. Do / should / can we still “pressure drive” the pinion up into a Helical rack to further reduce the Backlash, and in doing so, what would be a good starting force pressure.

2. Would the use of a gas pressure shock(s) work as efficiently as an Air ram? I like the idea of two smaller force gas shocks that equal the total force needed as a redundant back-up system. I would rather not run the air lines, and pressure regulators.

3. Would I see the sticking problem with the helical pinion if the Diameter is 1.50”?

4. If the idea of pressuring the rack is not acceptable, would a “version” of a turn buckle type device that would be machined to the same size and shape of the gas shock/air ram work to adjust the pinion placement into the rack (still using the slides)?


Thanks in advance.........

[jcc3inc]

Sir,

Several comments:

1) You should strive to have the pivot line for your drive mount fall on the pitch line of the rack. This ensures that the the pinion will not try to disengage when running one direction, and try to engage when running the opposite direction.

2) For a 20 degree pressure angle set, the minimum number of teeth to avoid undercutting of the pinion is 18 teeth; more would be desireable. For 14 1/2 degree pressure angle, 27 teeth is the min. Undercut pinions do not run as smoothly.

3) The separating force between rack and pinion is = Tan(pressure angle); for 20 degrees that amounts to .364. Therefore if you anticipate a max drive force of 100#, you will need to provide 100 x .364 = 36# force. You can either use a spring for the loading or an air cylinder setup with a regulator to control force.

4) While helical gears tend to run more smoothly that spur gears, you must be careful that there is NO CHANGE in the distance from the face of your pinion to the face of the rack. A variation of this distance generates an error in positioning. (like a screw motion)

Regards,
Jack C.

[Hi-Vac]

Thanks for the quick response!

As for your first statement, I didn’t quite understand the “pivot Line” comment. We currently have it set up to run straight up and down on two sets of THK Linear rails. The helical pitch is 19 degrees.

I also am concerned with the horizontal alignment with the rack. During acceleration, the pinion will create lateral force trying to “unscrew” the pinion from the rack creating an inaccurate setup. This is why we selected 1.5” Thomson dual open bearings for the axis slide, and this is the main concern and the reason we went with the THK Linear rails to hold the gearbox steady.


Thanks Again.......

[jcc3inc]

Guess I didn't read all your first post. If you use dual slides up and down, then all you need do is compensate for the weight of your motor/gearbox AND the necessary drive force. I was assuming that you pivoted the motor/gearbox to provide engagement.

I have never used helical gearing so can't comment from experience there. You are correct that "endwise" relative motion between pinion and rack will generate errors. That is another variable to contend with!

Regards,
Jack C.

[Al_The_Man]

Did you find the link that I posted here http://cnczone.com/forums/showthread...ighlight=rosta
This shows a Rosta tensioning product, this was an a very heavy Linde machine.
Post #12 shows the Rosta at the rear, it is the Square blue pivot point.
Al.

[wildwestpat]

Hi Folks

Can I offer some observations on this as there are some basic problems with reducing backlash in gearing.

1. The mesh of the gear centres is set by the tooth form

2. The backlash is set by the free space permitted in the tooth form at the machining stage.

3. Reducing the gear centres by pressing the gear into the pinion will help but only and only if the gear od does not ride on the bottom of the rack. Obvious but easily missed.

4. Helical gearing comes into its own to help avoid the impulse ripple that is caused by imperfections in the tooth form of the spur gear. A particular problem as one tooth engages and another disengages. With a very small pinion and a coarse gear tooth this will be evident and will show in a ripple in the movement that is cyclic. Thus helical gears are of use with a precision rack and are often used in optical transports .

5. Backlash in rack pinion as well as other geared mechanisms within the design thrust rating by using two pinions that are loaded one against the other by a circumferential springing system. Coil springs are used for slow speed drives and torsion bars in the form of concentric axles for drives where more torque is required. However there is a spring involved so some form of damping may be needed. In optical drives this is achieved with hih poise grease but this is not appropriate to a machine tool. The split gear often being a helical one as smooth tooth to tooth transfer is often as important as the backlash in precision drives.

Hope this helps as I think there is a real need to consider all the factors that contribute to lack of precision between expected and actual position.

PS. Whilst posting this I had a memory flash back! I recall using two pinions with fixed axels just out of mesh with each other but correctly set to the pcd mesh with the rack teeth. The drive gear on the motor was arranged topress down and try and part the two pinions. This results in the pinions running in permanent mesh but on oposite faces of the rack teeth. The loading was I think done with an adjustabe rail mount on the motor controled by a heavy feed screw / bolt arrangement. The whole assembly was coated in anti sieze grease and was only experimental so I have no idea what the working life would be like.



Regards

Pat

[aghobby]

We use the split pinion from Atlanta drives. Expensive but worth it!!