Thread: Breaking Cable
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Unread 24-05-2004, 12:52
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Re: Breaking Cable

Here's a couple other things to consider:

The 260# you mentioned is the vertical component of 2 robots static weight. If you are pulling at an angle, you will need a higher load to get that amount (divide 260 by the cosine of the angle).

That load is also the tension in the cable between the hook and your robot. If the cable passes over a sheave or pulley anywhere then you will have an additional friction load in the cable leading to your winch.

The "bouncing" load is almost entirely a function of the stiffness of the cable - this is an energy balance where .5mv^2 input equals .5kx^2 stored in the cable. A cable without much flex (such as spectra, which creeps over time but doesn't really stretch statically) will take very high loads dynamically versus something like sailing cord that we used. It's hard to quantify but it can be done - physically measure the spring rate of the cord by measuring the deflection under load, assume some distance of drop, calculate the velocity at that distance and you'll find the g-load by velocity*sqrt(k/m)/g.

You know, you gotta admit that watching any match where the cord broke and a robot fell to the floor with a crash was pretty exciting - kinda like going to NASCAR and waiting for a wreck. Lots of moans and groans, but I was impressed with how many robots came right back after a big fall. Some pretty resilient designs.
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