pic: The Infamous Shorn Shaft of PARC VII

I talked with Joe after the match and he was saying how he was spinning the motor… and spinning the motor… but nothing was happening… but you can see why now.

so how exactly did that happen?

btw, it’d be nice if you posted a lower res picture so it would fit on most screens lengthwise. I’d say limit it to 1280 wide (why? cause its my horizontal resolution :slight_smile: )

This happened in the quarterfinals at PARC. We went to try to grab the 2x off the mobile goal, but couldnt get a good grab on it because the ball had sunk to far into the goal, as it had probably deflated slightly over the course of the day. So I brought the arm underneath the ball and tried to lift it out, but in doing so the arm just collapsed to the ground. This was followed by my quote “Uh Vince (our driver, cocothemonkey), I don’t think we can use the arm anymore”.

so that shaft actually broke from the torque load? Thats pretty cool. Seems like something else usually gives before the shaft.

On second thought, it doesnt look like the shaft itself broke but the weld to that sprocket did, right?

whoa. I knew you had a problem but i didnt realize it was THAT bad.

Seems kinda like a freak thing to me.

gl fixin it and gl in the offseason


That’s a pretty bad break there. I wish you much luck in fixing it, hope that job didn’t cost too much.

The shaft was scheduled for repair yesterday. I haven’t heard if that was accomplished or not. We need it in working order quickly, because we will be displaying the robot at many local high schools to drum up FIRST fever. All our students are required to make arrangements to display GizMOE at their respective high schools (14 this year).

The shaft was probably saying "I can’t take it anymore! :eek: "

Poor shaft…but I do have to say is the pic is awesome. I dont mean that I’m happy that it broke, I never wish that anyone’s robot would have problems, but that picture of the shaft is amazing. Anyone want to estimate how much torque it took to do that?

What’s the diameter where the shaft broke.

The shaft was 1/2". I’m not quite sure how much torque it took to snap it but I do know that when we were testing in the workshop we put a kink in the arm because of so much torque. It’s connected to 2 Fisher Price motors.

Yeah, to tell the whole story on this. There was a long progression of arm-related failures that occured from week 4 of build until this past weekend.

First, we have the two F-P motors and gearboxes powering a further 16-1 reduction that goes out to the arm.

  1. Thursday night, end of week 4, we go to hang for the fist time. Arm buckles.

Result: Thicker tubing on the arm

  1. Sunday night, end of week 5, we go to hang, gearbox mounts tear through chassis.

Result: Have the gearbox contain the tension of the chain. (larger gearbox), and beef up the chassis in the area.

  1. End of week six, split a copy of the sproket you see on right there in half when going to hang.

Result: Replace with a less lightened copy.

  1. NJ Regional, Arm is keyed to that 1/2 shaft. A battle is fought when we try to hang for the first time between the arm, shaft, and key. The arm lost, and the keyway was destroyed.

Result: Increase contact area on the keyway from .5" at both ends of the arm (left and right) to 2" on both ends of the arm attaching the the shaft.

  1. Testing on practice 'bot before Philly Regional, strip teeth off many 'a gear.

Result: Double up gears to decrease the point load put on each tooth.

  1. Testing before nationals on the practice 'bot, buckle the shaft near where that shear happened at PARC. When going for the hang.

Result: Add more bushing blocks for support.

So, this has been a year long developing problem, where it seems we just keep moving the weak link. Finally we got to a point where I dont think that we will improve the design, but rather replace that shaft and be more gentile with it from now on, as we cant go to a stiffer material (It’s already steel) and to increase the diameter would require a redesign that just isn’t worth the work at this point.

But there you go. No you have all been fully informed.