Breaking Cable

[team222badbrad]

Um yeah we accually broke cable during testing in the 6 week build time. We were lifting our bot up to hang and at the time we didnt have any limit switches on the lift mechanism. This caused me to overdrive the arm and SNAP, Then .25 seconds later the robot was on the ground....

And yes we had 2 drill motors lifting us up... Once during a regional we were in a hurry and time was running out. There was very little space on the bar and we were lifting up our robot and all the sudden I see team 316 on the ground.

Then after the match was over we found out the score would have been a tie if they wouldnt have came off.. 130 to 80 eek

Sorry 316 we didnt mean to take you off the bar!

Not sure of the exact cable size we used but I thinks its over 1000 pound test.
3/16? In other words our lifting mechanism is stronger than last years drive train....

[Gary Dillard]

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.