[ZZII 527]

Okay, so I installed new 42mm carriers cut from A2 tool steel on Thursday. But in light of Dr. Brooks' comments and my own original failure (see earlier posts), I decided not to harden them for now. As they are (annealed), they are a bit softer than the stock 42mm plates, and so any deformation to them should resemble what would happen to the stock plates. The only other notable difference is that the flats are a closer fit to the shaft (I cut them undersized and filed them out).

A picture of our arm joint, showing the 256:1 gearbox and the additional 72:10 sprocket reduction, is attached.

We tested the arm Thursday and Saturday under normal usage conditions, with and without motor braking. The arm itself is relatively light and the FP motor draws about 4-6A to lift it at the worst angle. The most load it sees is during our ramp deployment, which is initiated by the arm motor. During this maneuver, the FP motor draws approximately 10-12A, corresponding to about 60 in-lbf on the carrier plate (anyone care to check my math on this? I took the motor specs from the sticky'd 2005 post and used 0.85 as the efficiency per stage), well under stall and the theoretical limit.

The result so far has been that there is LITTLE TO NO deformation. I thought I saw a bit more backlash today, so I opened the gearbox again and took the plate out (picture below from today). It is maybe the slightest bit looser on the shaft, but most of the backlash I saw was just from the fact that there are four gear stages.

So I am confident that the 42mm plates can hold up. Ours is not the most well-designed arm for reducing torque or shock loading (as of now, there is no counterweight) and we haven't implemented any software limits yet, and yet the plate has held fairly well. The 42mm plates are harder than the 56mm ones and I think they can take the torques involved in controlling a well-built arm with some additional chain and sprocket reduction. If you treat it right, I think this problem will be less drastic than the 56mm issue was / could have been.

But I would be interested to know: How many teams are still using the 42mm, 256:1 setup and if you are, is it holding up?