Rack and pinion

[arp customz]

I built a 5x5 table using 20/20 spur gears. My gantry is a duel servo set and on the heavy side.. any ways its jumps alot. If I go with a lower pitch like 16 or 12 will that help out?

[Konstantin]

A picture will help a lot. Perhaps its the rack not making good contact with the spur gear.

[jcc3inc]

Sir,

As the previous guy suggested, pictures of your drive showing the pivot point of your drive mount would be a great help. There are some things that *many* overlook in designing/making their machines.

Rergards,
JC

[arp customz]

some

[arp customz]

more

[jcc3inc]

Sir,

Your belt reduction looks well made.

1) I couldn't tell for sure where the pivot point for your drive is located. Ideally, the pivot lies in the plane of the rack teeth or as close as possible to that plane.

2) The pivot point could be about 4" from the drive pinion. Being too close is poorer.

3) Would it be better to have the rack teeth pointing down rather than up, so as to keep dirt from falling into the rack and pinion?

4) Is your pivoting mechanism as friction-free as possible? It appears that there are several points of contact that might hinder free motion. We used disc type (flat) needle bearings with their associated hardened washers on each side of the moving member.

The separating force using 20 degree gears is Tan 20 degrees times the force; therefore if you expect a maximum of 100# of drive force (accelerating + frictional drag), the spring must have a tension of 100 x .364 or 36# force. For 14 1/2 degree gears, Tan 14 1/2 is .258. The 20 degree gears run more smoothly, however.

Regarads,
JCC

[Dustin407]

arp customz,

Your gear reduction housings are very well made, quite similar to mine. I do not see a pivot point on your reduction housing to the gantry. Think of your spur gear as a tire, your rack as the road and your pivot and spring as shocks on a car. If your rack is not perfectly straight the spur gear can ride out of the rack so the pivot point keeps this from occuring and keeps them together with force usually a 40-50 lb spring. I do see your problem very clear now and you are going to have to make a small fabrication change. The rack needs to be flipped over pointing down. You have no way of putting a spring device to pull the spur gear to the rack right now, so you are relying on the weight of the gantry to keep the spur gear down and it is not, thats where the vibration is coming from ( even with all that weight it will spool right out with the torque of the motor and gearbox plus the anlge of the gear). Here is a pic of mine to help you get a better idea of the pivot point (old pics)... Basically I have a small shaft inside of the two alum. plates. the 1/2" bolt is not tight just snug to allow for small movements....

[Dustin407]

Another pic to help.....

[arp customz]

I dont have a pivot point. the gear box is on shoulder bolts with slotted holes allowing the gear box to go up and down. Ive notice that it has been sticking.

[jcc3inc]

Sir,

I have attached 3 pix of a small machine (32 x 36) that I made as a test bed wherein I have used this pivoting technique.

You can see the arm to which the motor is attached in #034 as well as the spring. #343 shows an end view revealing the shoulder screw, the angle to which the assembly is attached and the arm that supports the motor. You can also see two needle roller thrust bearings sandwiched between two hardened flat washers for each needle thrust bearing. #344 is another view where the tension spring is not shown.

Torrington makes these bearings. For your situation, I'd consider a Torrington NTA-3244 with its associated washers. That OD is 2.75, ID = 2.00, thickness .0781. You would need some spacer to go from the shoulder bolt diam to the 2" ID of the brg. A larger brg diam might be better. There is minimum friction in a well made pivoting mechanism, and very little lost motion, and minimum error if the rack plane is not everywhere parallel to the rail associated with it.

Regards,
Jack C.