As the title says, I’ll try to give as much information as I can.
We are running a WCD drivetrain powered by 2 cims, with #35 chain out of Versaframe tubes, tensioned with Versablocks.
Yesterday the left side of our drivetrain locked up while testing it on the ground. We’ve loosened the chain running front and back and concluded that there is no issue with the chain or front/back axles.
We then removed both cims and spun the output shaft of the gearbox with a wrench and it rotated smoothly without resistance. When we put 1 cim onto the gearbox in the correct inboard position the output shaft spins one way with little resistance. The other way it rotates okay until it slows down, stops, and locks when trying to start back up again. We currently have both cims on the gearbox in the correct positions (1 inboard and 1 outboard). Providing force on the output shaft in the bad direction results in it locking before starting to spin. After enough force, it jumps/skips slightly(?) and then quickly locks again.
We’ve driven it both directions before yesterday, multiple times, and it rotated fine both directions.
We’re completely stumped as to what the issue may be. Both our mentors are currently unavailable (one head coach of our school’s soccer team and the other on paternity leave).
This is a slow motion video of me turning the output shaft with a wrench. Turning the spur gear towards the right is the “good” direction, turning to the left is the “bad” direction.
When the CIMs are out of the gearbox, do their shafts spin smoothly in both directions? Have you otherwise verified that they work fine (powering them in both directions while disconnected from the gearbox)?
We’ve spun each cim by hand when it’s not connected to the gearbox and there’s slightly more resistance than in a brand new cim. But they spin fine by hand in both directions.
Does this gearbox have the snowman mounting holes? Make sure the motors are in the correct position based off the pinion. There is a chance maybe one of the motors shifted and results in higher load when running one direction.
Is your shifter shaft / dog loose?
Is one of your shafts overconstrained / getting forced open or closed by something? When the motors are taken off, a lot of compression is also taken off the gearbox, and I’m wondering if that compression is driving the difference in behavior, rather than something about the CIMs themselves.
The video isn’t much to go by. Can you go through part by part and photograph how each of the interfaces are in specification / at correct location & compression?
To double check we have our cims mounted in the correct snowman holes, according to the vexpro instructions, we should have one motor mounted inboard and one mounted outboard. Is that correct?
After unscrewing and looking at it, it appears that the bolt can slip between the snowman hole. Do you any suggestions for fixing this or should we swap this plate for one that on a spare gearbox?
With proper torque and initial alignment, it’s usually never an issue. For safety’s sake, make sure the motor is shifted as far out as it can go when you screw it down, to prevent too much mesh.
On a seperate note, as grainy and low rez as this screenshot is, it looks like the middle gear is kinda messed up on the edge by the retaining clip.
One option if it needs to be brought together is using a hose clamp to draw the motor together near the case. The other option if need to be brought out is to just wedge a block of wood between the motors to keep them out. Not idea situations, really only meant for in a pinch. Best thing to do is just replace the plate.
We have 11T pinions and a 42T spur gear in our WCD. So this means that we should have both cims mounted in the inboard position. Last year we used 12T pinions with the exact same gearboxes and mounted the cims with 1 inboard and 1 outboard. We actually have this setup on our practice chassis and it spins without any significant resistance.
Also yeah, we previously put shaft clips in those pinions incorrectly and they wore down the spur gear where you pointed. Earlier today I replaced the clips and put them on correctly so now they don’t contact the spur gear.
I’ve seen similar binding in TB-minis caused by the CIM shaft being a little off square with its face plate, causing their pinions to bind with the large cluster gear; this binding would be preferential in one direction over the other. Rotating the motor 180 degrees around its axis so that the shaft bent away from the cluster gear would relieve the binding.
Thing is, we’ve rotated the output shaft by hand with each motor on individually. The problem still persists with each CIM, having only one motor on the gearbox.
With one CIM on, in the correct inboard position and with a working snowman hole, it still struggles with turning the other direction
