How to disengage gear at limit, What to use?

[bigalexe]

I have a design question, I am not sure if this is the right place but it's the closest I could find.

I have a heavy object being pulled, specifically it's a miniature weight sled intended for truck/tractor pulls. For those that aren't familiar here is a quick description of the function:

The sled has 1 solid rear axle and 2 skids or a skid plate in the front. The weight is placed on a track on top of the sled. At the start of a pull the weight is over the rear axle so the truck only has to pull the sled+weight over the axle. Then as the sled moves forward the weight is drawn forward eventually ending up over the skids, so as the weight slides forward it goes from being a rolling load (easy) to being a friction load riding primarily on just the skids. The weight is moved forward by a cable that is drawn forward in some manner by the rear axle rolling.

What I have designed is using bike/motorcycle sprockets around the rear axle, then using that to drive a spool that will wind up the cable. However tying the weight directly to the rear axle presents an issue at the end of travel for the weight where the wheels can no longer turn because there is no more cable to pull.

HERE's the question!

Considering that I am using surplus lawnmower/bike parts which are small axles and sprockets (1" axle and under), what can be used as an overload or torque limiting clutch to allow the rear axle to continue to spin freely past the point where the weight has ceased to move? The problem I am having in finding parts is that I cannot find a clutch mechanism that is small enough for this application and still has a high enough let-off for this job.

Also there is a bonus question:

How would you setup a return system to let the cable unwind that is disengaged from the rear-axle travel? Just a straight engage/disengage setup that utilizes gravity to return the weight could be dangerous because the return would be uncontrolled and therefore dangerous.

THIS IS NOT HOMEWORK, I was at an Academic EV Race recently and their sled is very old and is in need of improvement so me and a friend are working on a new design and getting it built.

[keebler303]

I'm sure you could find pretty good details of professional sleds with a quick google search. I think you would want to have a continuous chain as the drive, not a cable and drum. When the sled got to the top, the chain would drop down, which would release the carriage from the chain, so the chain could just keep turning and turning. You would need some type of mechanism to keep it from rolling backward at this point. All of this is quite similar to a roller coaster lift, if you are familiar with that. For lowering the weight again, you could engage the carriage with the chain again, disengage the chain from the drive axle and put a brake somewhere in there to keep things under control.

Matt

Edit:

Here is some useful info: http://gardentractorpullingtips.com/sledinfo.htm

[bigalexe]

Using a Roller-Coaster-Style chain lift is a great idea I hadn't though of. That way it will disengage the weight. I am thinking that may be very workable.

Thanks.