Safety Cable Design
Safety Cable Design
(OP)
I'm trying to analyze a safety cable problem and need some help. Basically we have a 1/4" Ø galvanized steel cable that is wrapped around angle iron (of structure) and through an extension tube (of our propduct). If the object falls the cable will engage after a fall of 3.5ft. I calculated the kinetic energy of the object and found an equivalent force using the Fe=(2xKExK)1/2 k being spring constant. I get a huge stress in the cable when I do this so I was wondering since it is in a loop if the force would actually be 1/2 in each of the segments? It's only 1 cable but wrapped in a loop. It seems similiar to the physics problems using a block and pulley but Im not sure. Any help is appreciated!





RE: Safety Cable Design
If you are being decelerated to a stop by one cable, all the stress is in the one cable. A fancy block and tackle mechanism either attaches two cables to you, or it provides leverage for the guy who has to pull you back up.
I would look for a way to reduce k.
JHG
RE: Safety Cable Design
I get the impression that not enough caution is being taken with the SAFETY cable design... I could be wrong, and a one paragraph description of the system isn't much to base an impression on, but please be careful...
-- MechEng2005
RE: Safety Cable Design
Mike Halloran
Pembroke Pines, FL, USA
RE: Safety Cable Design
RE: Safety Cable Design
Since you clearly can't, get some help.
Mike Halloran
Pembroke Pines, FL, USA
RE: Safety Cable Design
RE: Safety Cable Design
Draw a free body diagram.
You have a mass at the end of 3.5ft+ of one cable. Any force exerted by that mass is exerted entirely on the cable. Your deceleration is a function of your mass, your velocity and the elasticity of your cable.
The elasticity of a short cable probably is mostly due to whatever it is attached to. If the cable and/or pulley attachments are elastic, your elasticity increases, and your force decreases. Please note that a broken cable is not the only way your safety system can fail.
A fancy pulley arrangement controls the force at the end of the cable or cables opposite to the falling object. It has no effect on the falling object.
JHG
RE: Safety Cable Design
I just took a more careful look at your diagram.
If your safety cable is a complete loop, and there are no weird friction issues, you have divided the stopping force between two cables. Also, you have doubled the cross sectional area of the cable. If the cable stiffness affects k significantly, then you have increased it, and your force is reduced by less than half.
If the cable is stiff enough in bending, it might act as sort of a leaf spring, which would be good.
If this is a safety harness attached to a person, what are the chances it will get in the way, and get disconnected?
The structure at both ends of the cables ought to allow a gentle bend of the cable as it is snapped tight. I don't think you want the cable bending around a sharp edge.
JHG
RE: Safety Cable Design
Yes the double strand will help in theory - but you need to be putting in huge safety factors to take account of the "foul" attachments.
Things you might do to help:
Inspect the cable regularly and replace at the first sign of damage.
If at all possible, make sure the safety cable anchor point on the structure is well above the primary fixing point for your product and design to minimise slack in the cable - this minimises distance of fall while maximising available cable to absorb energy and is going to make more difference than almost anything else you can try.
Run the cable through a stiffish sleeve (I'm thinking something like 1/2" alkathene water pipe bent into a smooth curve) where it goes over the building structure to give it a fairer approach and design the attachment point on the product to be kind to the cable.
If the peak forces are still looking big, consider fitting an energy absorbing link somewhere, if for no other reason than to reduce the damage to your product when some idiot drops it while providing incontravertible proof that it has been dropped (here, I'm thinking of the kind of device where stitching holding a folded strop together is progressively ripped out while retarding the fall with a constant force).
Consider whether steel cable is really the best material for your application (accepting that in many environments, it might be).
A.
RE: Safety Cable Design