2 CIM Motor Winch

[nakzz]

So my team has been trying to build a low-bar robot which can scale and we were thinking of having a winch mechanism to lift the robot up we grab the bar with a "elevator" mechanism delivers the hook.We have all the part pretty much designed and built, but we are having trouble with the winch.

We were using 2 CIM ball shifter gearbox from vex pro with 60 24 ratio. But the winch seems to fail when lifts the prototype robot (aprox. 50lbs). The motor stop and the robot hangs on the bar about 20 inches high from the ground. But the actual robot would be at least 100 lbs.

So is there any suggestions we can get? Maybe try changing the gear ratio? Different winch mechanism?

[Jon Stratis]

Your ratio is waaaaay too low. When we climbed on the pyramid, we used two mini-CIMs in a toughbox mini with a 12.75:1 gear ratio.

[Kevin Sevcik]

2 CIMs should be plenty to lift a robot, as long as they're geared properly. What's the diameter of the pulley on your winch? Is the 60-24 ratio an additional gear down after the ball shifter?

Originally Posted by Jon Stratis

Your ratio is waaaaay too low. When we climbed on the pyramid, we used two mini-CIMs in a toughbox mini with a 12.75:1 gear ratio.

*poke* winch diameter is part of the gear ratio. If they're using the 60:24 3rd stage, then low gear is 20.8 to 1. Which would be plenty on a 1" radius pulley, but completely hopeless on a 6" radius pulley.

[jijiglobe]

Don't know anything about your design but keep in mind that if your robot turns sideways on it's way up, your "height" is now your frame perimeter extension and it can't exceed 15 inches. If you have a low bar robot that's climbing with a winch you will most likely need a dedicated solution to staying level when you climb.

If your team has already considered this and designed around it, I apologize for wasting your time.

[kitare102]

I would be easier to help you if you gave us the full reduction of the gearbox and the diameter of the pulley that spools whatever line, strap, etc. is doing the winching.

With our current design, we have one CIM, a 10.71:1 reduction and a .5" diameter pulley (the shaft itself) which seems to perform admirably, though we could probably work it around to climb even faster if need be.

A wonderful resource which you should take advantage of is JVN's mechanical design calculator (http://www.chiefdelphi.com/media/papers/3188). In the linear mechanism tab, you can set motor, gearbox, pulley, and distance traveled parameters and get a basis estimate of what you should expect from your system. I'd put what you have currently in and see if the calculated numbers are in line with your expectations. If not, play around with stuff you can change like gearbox rations and pulley diameter to see what would work best for your team's resources.

[Jon Stratis]

Originally Posted by Kevin Sevcik

2 CIMs should be plenty to lift a robot, as long as they're geared properly. What's the diameter of the pulley on your winch? Is the 60-24 ratio an additional gear down after the ball shifter?

*poke* winch diameter is part of the gear ratio. If they're using the 60:24 3rd stage, then low gear is 20.8 to 1. Which would be plenty on a 1" radius pulley, but completely hopeless on a 6" radius pulley.

My assumption, based on their post, was that the total active gear ratio they were using to climb was 60:24, as no other information was given. Even with just a 1/2" diameter, that wouldn't suffice - the ratio is still way off unless there's another stage he's not telling us about. Yes, winch diameter plays a big part in the overall gear ratio, but one thing at a time... get the gearbox to an acceptable range, and then you can size your winch according to that.

This is going to be the third time I've built a winch mechanism to climb with my team - Ultimate Ascent and BreakAway being the other two. In designing them, it all starts with the proper gear ratios in the gearbox, then moves on to the winch. It's just easier to size the winch appropriately than it is to change a gear ratio by just a little bit.

[kitare102]

Originally Posted by Jon Stratis

My assumption, based on their post, was that the total active gear ratio they were using to climb was 60:24, as no other information was given. Even with just a 1/2" diameter, that wouldn't suffice - the ratio is still way off unless there's another stage he's not telling us about. Yes, winch diameter plays a big part in the overall gear ratio, but one thing at a time... get the gearbox to an acceptable range, and then you can size your winch according to that.

This is going to be the third time I've built a winch mechanism to climb with my team - Ultimate Ascent and BreakAway being the other two. In designing them, it all starts with the proper gear ratios in the gearbox, then moves on to the winch. It's just easier to size the winch appropriately than it is to change a gear ratio by just a little bit.

They say they're using a two-CIM vex ball shifter, which leads me to believe that they're referencing a 3rd-stage when talking about a 60:24 ratio. Check out the reduction stages tab at this link for more info: http://www.vexrobotics.com/ball-shifter.html