Extension lift mechanics

As realtives newcomers to the world of robot building, we need some advice about extnesion lift cabling. We are interested in a continuous-cabling extension lift, but are having difficulty getting our heads around the details of the cabling - attachment points and all that. WE have a Power point from JVN that helps, but nothing that gives us the ability to sort it out in hardware and strict mechanical terms. Any help out there with pictures or documents that are explicit?


when you say extension arm, do you mean sevral arms which slide up each other to get to the top, like our robot last year( but didn’t actually make it due to weight limits :o )

From what I understand, you have:

Continuous design-
Pro: Moves evenly but slowly, as more cable must be moved in order to fully extend. Has been recommended over cascading by two engineers. Identical speed up and down, so one drum (of a larger diameter) may be used.
Con: More cable must be moved in the same amount of time, requiring a faster motor to keep the extension speed the same.

Cascade design –
Pro: Moves evenly, less likely to jam, less cable needs to be moved in order to fully extend.
Con: Lots of tension in the cable, extension and retraction cables both have different speeds requiring two different drums of differing diameters. Require lots of crimps that can potentially go wrong.

The black dots are where the cable is fixed, and the green circles are pulleys. The large green circle is a drum powered by the motor(s).

That is a good example and probably the least complex, I would just more suggest not using a cable in the location of your yellow cable as gravity would pull it back down.


the continuos one is what we used last year (well, after the compo… weight limit etc)… You want to be careful. You want to make sure the wire is strong and preferably plastic or something. We used a metal wire thing and its really not nice at all. It curls back up and is really unpleasent to try and fix, even with gloves etc.
You dont need a yellow cable at all. the continous system will pull the top bit with the lifter to the top as it is. We used some extrusion (iirc thats what its called) with plastic runners, runs fine except the pullies are a bit cheap and nasty sometimes.

I would not be inclined to trust gravity becuase dirt will build up in you mecanism as it is used and all of the sudden it might just cease to work. :yikes:

DIRT??!!?? :ahh: Ok, true, that could happen but I have personally not needed to worry about it in the last 3 versions of this lift I’ve made.

Okay I guess I overdid it. From past experience members of our team insist on never trusting gravity.

Gravity is generally pretty reliable, but in this application you need to ensure that it is able to do it’s thing. The key thing is to make sure that the sliding sections cannot tilt sideways enough to lock up. Gravity can only pull so hard, if it takes too much force to clear the jam then you will be stuck in the up position. If you want the belt and suspenders approach then you ensure that you pull down as well.

Our lifts have always been powered both ways. Keeping the lift from jamming up was one reason. But another is that gravity is not always pointing the same direction relative to the robot. :eek:

If, for example, your robot should fall over sidways with its lift extended then gravity is now pulling 90 degrees from the direction you originally intended. Your robot is now blocking a large part of the field, a situation which may not be desireable. If you power the lift down, then you can probably retract it and clear the field. It would also be easier for another robot to help you back up on your wheels. But if you are relying on gravity to lower the lift, then it is already as low as it will go.