[FTC]: Using TETRIX work gear assembly.

Other FIRST Tech Challenge
1

Hi

Do’h… The title should read WORM Gear, not work gear…

anyway…

We just started using the TETRIX worm gear assemblies this year.

Worm gears are great for producing compact high gear-ratios, and they can’t be back driven, so they seemed a great choice for an arm gear drive.

However, we’re finding that the coupling between the brass worm gear (not the worm, but the gear) and the main axle is not particularly robust.

There are two set screws for holding the axle in place, but these seem to come loose at the drop of the hat. Granted this is a high torque point, but even still.

After many attempts just to tighten them, we moved on to using blue lock-tight (We don’t use red any more… long story). Even with this, and ensuring that the screws are set on the flat, after some moderate use the shaft is slipping again.

Is this a lost cause?

The torque requirements aren’t huge. We are using the 4:1 worm. followed by a 2:1 gear reduction, and the arm move quite quickly under full power. We have prevented running the arm into stops at each end so there is no huge stopping force involved.

At this point we’re considering epoxying the axle to the gear, and making the entire gear/axle/channel a single replaceable assembly. (since there would be no way to remove the gear/axle at that point).

We know how to get a similar reduction using gears or sprockets but just thought it was worth a try with the worm.

Any magic insights available?

Phil.

2

We are seeing the same problems. Those set screws are not very strong. Did you ever find a better solution than glueing them all together?

3

No, and in the end we abandoned the worms altogether. No matter what we did the gear/shaft coupling ended up failing. My motto has always been never to use the shaft to transfer the high torque, and this turned out not to be an exception to the rule.

The alternate solution was to use smaller pinion gears on the motor to direct drive a tetrix gear.

Servo city (among others) has some compatible 32 pitch gears that fit nicely on the Motor shaft, with very low tooth counts. Specifically 16 - 24 tooth.

https://www.servocity.com/html/32_pitch__6mm_bore__gearmotor_.html#.UyO65ibD-70

By combining one of these with an 80 or 120 tooth gear you can get some high ratios… eg: 16 - 80 tooth is 5:1 16-120 is 7.5:1

On our arm, we used the 16:80 and then followed by as 40:80. The 10:1 was enough to lift our robot.

4

My solution to that would be to drill small holes, in the axle for the set screws to sit in so it transfers torque between the gear and shaft better, and then you can combine that with loctite.

5

Strange… we are also using 4:1 worm gear and had no issues with set screws - I do not think we had to tighten them once.