Cable rigging: Cascade vs. Continuous

[ChrisH]

Originally Posted by M. Krass

It seems that the Whitepaper discuss links aren't yet working, so for reference, this thread is with respect to Christopher Hussman's Whitepaper here.

Specifically, it talks about methods of rigging cabling to raise and lower arm/lift mechanisms. I thought that, given this year's game, it might be useful to discuss the pros and cons of the two cabling systems discussed; cascade and continuous.

Cascade, apparently, has a higher cable tension than continuous. In teams' experiences, does it end up being significantly higher such that one can't switch from continuous to cascade without redesigning a mechanism?

I've been helping prototype a telescoping device and we've noted that the cable tension in our continuous-style rigging seems very high and that there's increased resistance to moving the middle stage -- just as the whitepaper suggests. We were considering adapting this design to use cascade style rigging to, perhaps, make moving the middle stage a bit less...eratic, but were hesitant to do so without learning more about cascade-style rigging and its problems.

I thought that it might be valuable for teams to share some of their uncommon wisdom about these varied sorts of cable rigging systems. Share away

(I've been away from CD and without internet access, since kickoff. If there's already been a thread that discusses cable rigging specifically, please kindly point it out. Thanks.)

First of all, I'm glad somebody is dredging that whitepaper out and making use of it. It was originally intended as an oral presentation so the text and explanations are a little thin. I'll be happy to answer any questions people might have about the whitepaper and lifts in general. Historically, lifts are something the BeachBots do very well and we've built several over the years.

Typically we use a grade of cable that can handle the tension in either configuration. I like 3/32" stainless with a nylon coating. It doesn't corrode, lays nicely, and is very strong. But it will go nicely over a 1"dia pulley. As I recall it is rated to 900 lbf. It is also thick enough that it doesn't kink very easily. Kinking can cause real problems with trying to keep a cable where it belongs. This is especially true if you are silly enough to let your cable get slack. In fact the kink resisitance was why we started using it in the first place, not because we needed the load capacity. But it does give a lot of extra strength at a very small weight penalty.

For multi-stage lifts and other telescoping devices I prefer the cascade style rigging because it forces the segments to move evenly relative to each other. This is especially important if there is going to be a heavy load on the device. This is because the moments generated by your load are all transmitted to the rollers or other devices you are using to make it easy for your mechanism to slide. The greater the overlap between segments, the lower the forces on your rollers. By ensuring that the segments move evenly you also maintain an even load on your rollers. Even movement also helps prevent jamming between segments.

Double rigging your lift, so it is pulled down as well as up, is also very helpful in both preventing slack and preventing or clearing jams. While I don't think we've ever had a jam with a completed lift, if one should happen simply reversing the lift will often clear the jam IF you are actually pulling down. If you try and lower a jammed lift that doesn't have a downgoing cable system, then you have a high probability of having a tangle of cable when you get off the field. If you use small diameter wire then it will probably be kinked as well.

The high resistance you are experiencing is probably related to some binding somewhere in the system. This could be from a number of sources. The fit between the segments is one of the first places I'd look. The pulleys or guides for the cable are other sources of binding. One thing to consider is how large are your pulleys? The FAA recomends (and the BeachBots practice) using pulleys with a diameter no less than 8 times that of the cable. This helps keep the cable from being bent too far and helps it run smoothly.

[FDSailor]

You might want to take a look at the new high-tech lines instead of wire for this kind of application. It does not kink, it's lighter and stronger, it turns a sharper radius, you can tie knots in it, it stretches a very slight amount more than wire but not noticablely and it costs about the same. Check out most any sailing catalog and look for brand names like Vectran, Technora, Dyneema and Spectron. www.westmarine.com and www.layline.com are a couple I use.

John Sayles, PE