Chain slipping and more - advice needed for rookies

Our team needs your help with our Hab3 lift mechanism. We are a rookie team and I’m the main mentor, but my expertise is software, not mechanical. Our competition is 3/21, so we have about a week left to get this working on our practice robot if we can.

The issue we have at the moment is the #25 chain is slipping. From reading on CD, we think the advice you’re going to give is to upgrade that to #35 chain and looking at sprocket availability and sizes, that will also change our gear ratio. The reason we went with #25 is that we had it on hand already, so it was easier on the budget.

I could see that we need a better way to keep the chain tight. It’s loose in the pictures because our tensioning mechanism is failing, the motor/gearbox mount is sliding on the rails. Any advice on how to improve this is appreciated.

What other issues do you see with our design? We are running the CIM motor at 50% currently. The first gearbox is a 12:1 Versaplanatary which in turn drives a 48:1 CIM Sport. The drive sprocket is 24 tooth and driven is 48 tooth, although with #35 chain I think that will change to 15 and 36 tooth. We’re expecting total weight (with battery and bumpers) of the robot to be just shy of 100lbs.

I’m advising them to upgrade the center driven leg from one to two 1x1’s to give it more strength. The plywood base will be replaced by something more light weight.

Another question, if when sitting on Hab3, if the legs are touching the corner between the horizontal platform and vertical wall, is that counted as a successful climb? If not, we need to adjust a little because backing up even the slightest bit too far and our rear wheels come off the platform and we are then supported by Hab1.

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If im reading this correctly, the tensioning method is moving the motor back on the rails, i think can see how its done from the picture, and the main problem is that it is aluminum on wood. That is going to slide a lot more than other things. My recommendation would be to put grip tape on the WOOD so when you tighten it down, it actually has more friction and will hold in place better. make sure to put it on the wood, with the sand paper side grinding against the aluminum. If the sand paper side is on the wood side, it will just sand the wood
grip tape will stop it from slipping and sliding

also, please please please, get a nut on the end of that bolt, it could be disastrous if that bolt were to slide out

Two things here:

1: Chain Tensioner.
If you know the exact distance between center of the two shafts, you can cut a 1x1 piece of tube with holes at that distance. You should use bushings in that as well, and place it as close to the sprockets as you can. The 1x1 will act as a very stiff member to prevent the shafts from flexing. I already see a little slack in your chain, so make sure your get that chain fairly tight before you take your measurement.

2: LV3 Climb Criteria.
The only two things you need to do to get the LV3 climb are to have your bumpers be fully about that platform, and your robot needs to be fully supported by that level. You can have part of your robot hang below it, but it cannot touch anything below the horizontal surface of LV3. If your foot looks like it’s touching LV1, the ref will not give you the LV3. You should make it abundantly clear that you are only touching LV3.

In regards to the bolt/axle missing the nut, totally understand that it needs it. We were just anxious to try it out and didn’t put that on for the initial lift attempt.

Thanks for the grip tape idea, hadn’t thought about that as an option.

figured it just wasnt on at the moment.
We are using grip tape for drive chain tensioners this year and they work amazing

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How far back can you move the motor? Misumi makes some cool chain turnbuckles if there’s enough chain that some of it will never go over a sprocket.

We can’t move the motor any further away from the driven axle without significant work to move the electronics board. I don’t think we can tackle moving that at the competition. Our hope/plan was to just take a bolt on mechanism that is under the 30lb limit.

Is the battery tray pretty rigid? I wonder if you can add a compression member (e.g. a spacer) that pushes between the blind tapped hole on the left and the through hole on the right, reducing the deflection of the motor mount.

is what we did, to keep the chain tight. There’s a lot of load, things move around a lot more than you’d expect.

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A few things. You are likely to do bad things to that CIM Sport by trying to drive it like that. The gearbox is probably designed for a CIM’s worth of torque (2.41 N*m) on the input side and you are feeding it from a 12:1 Versa planetary. I don’t know that the input toques limits on the gearbox are, they don’t seem to be published on Andymark’s website. Maybe, @Andy_Baker, @Nick_Lawrence or @Ruth_T can help with that one.

Just a quick check to see if #25 chain is viable:

At the listed 18.36 amps, a single CIM is producing 0.28956 N*m of toque = 2.56282 in *lbf. (Number came from the table here:

Putting that torque through the two prior gearboxes yields 1476.18 In*lbf of torque at your sprocket.

The pitch-Diameter of your sprocket is 1.915", thus the radial lever-arm is: 0.9575"

The tension in the chain is: 1541lb = 1476.18 in* lb/ 0.9575in

Vex has a good load rating guide here related to chain:

#25 Standard Roller Chain:

Max Working Load: 154 lbs

Average Tensile Strength: 1,058 lbs

#35 Roller Chain:

Max Working Load: 419 lbs

Average Tensile Strength: 2,601 lbs

You can see that you are trying to use the #25 chain well beyond it’s limits. Depending on tensioning system, number of cycles and allowable stretch under use, you may be able to get away with using a chain beyond it’s working load limit. I recommend using 2x or 3x #35 chain runs.

Your tensioner is operating under relatively high loads and is relying on clamping force (friction) to hold it in place. You are unlikely to be successful with this strategy, as you have already learned. Consider a strategy where you thru-bolt the L-beam that currently slides into the farthest physical position (bolt shown in RED). You can then use the bolts that connect that L-beam to the motor mounting beam as your tensioning device (BLUE). As you tighten the bolts, the two beams get closer to each other and the tension on the chain increases. Be sure to use nylon-insert lock nuts on the bolts to avoid them backing out under load and vibration. You might need to introduce a second plate in there so the gearbox and bearing holder are mounted to a single interface.


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That’s a cool design.

Another way to stop the motor/gearbox assembly from slipping would be to attach a piece of aluminum angle or a block of plywood to the top surfaces of the 1" x 1" tubes pressed against the vertical surfaces of the motor/gearbox assembly. These would be screwed into the 1" x 1" tubes with sheet metal screws.

I would be concerned about the hole for the bolt with the big sprocket deforming under load. This would cause the slack to increase with use. If the slack grows slowly enough, you could add non-compressable shims between the blocks and your motor/gearbox assembly. Changing the shaft to a piece of round stock may make the wearing proceed slower since I see that the threaded part of the bolt is running on the tube wall.

@Dan_Karol thanks for the calculations and tensioning design idea. I like that design and the idea of multiple chain runs., I hadn’t thought of either. As for the cim sport and possibly bad things happening, unless someone says for sure it will break, we’ll just try it and see what happens since the only other option at this point is to completely change course which isn’t viable.

Thanks for the ideas everyone.

Taking a deeper look at it, the gearbox is certainly designed for the stall torque of the motor, 2.42 Nm and you are only planning on using about 0.289 Nm of available torque** from the CIM, you only require 1.43% more torque than what the gearbox is guaranteed to provide. You have a reasonable chance of being OK.

You could greatly improve your chances by using a shaft coupler ( or and providing a 2nd gearbox on the other side with less reduction going through the independent Versa Planetary transmissions.

**good choice

A tensioning idea I haven’t seen posted yet… Use a WCP CAM in front of the motor mount. It’s relatively simple to implement, you’re just drilling a hole for a 10-32 bolt through the tubing and screwing it in. That’ll let you rotate the CAM to give you tension, while putting something solid in front of the mount to keep it from moving. I would also ditch the wood in there so you have metal on metal contact, both between the bar/mount and CAM/mount.

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i cant tell you anything about climbing but without proper tentioners on those chains they will slip and continue to hinder your climbing abilities

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