Safety Cable Design

[jaydigs]

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!

 

[drawoh]

jaydigs,

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

 

[MechEng2005]

I'm not sure I understand the situation exactly. You are just wrapping the cable around an angle? Sounds like it could cause the cable to kink or be damaged. You need to be very careful with the cable... Also, if the whole system is a loop, you might be able to consider that the tension in the cable at any given point is 1/2 of the force calculated. However, in a dynamic loading situation, I don't think you can count on this. How do you know one part of the cable won't get snagged and that half of the cable will end up taking the whole load before the other side has any tension? Unless the cable was looped around a pulley on the structure and a pulley on the extension tube, and the "fall" was guided and you were maintaining some tension on the cable at all times to keep the cable in the pulley, I would have big problems with this. Even if there were pulleys ect, I would be VERY wary.

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

 

[MikeHalloran]

Speaking of safety, please note that angles are not very good in bending, and are rarely intentionally stressed that way.





Mike Halloran
Pembroke Pines, FL, USA

 

[IFRs]

I think OSHA requires the anchor to hole 5000 pounds per person. Can the angle do that?

 

[MikeHalloran]

We can't answer that question.

Since you clearly can't, get some help.





Mike Halloran
Pembroke Pines, FL, USA

 

[jaydigs]

I don't think I did a good job depicting the situation. Attached is a picture of the problem. The safety cable makes a loop around 2 beams of angle iron at the top and through a support of the assembly at the bottom. This provides redundant support in case the bolts holding the assmebly to the angle iron fail. Just FYI..angle iron is just 90 degree, or "L" shaped beams of iron. The increased moment of inertia gives it more rigidity against bending than flat beams, but it's cheaper and lighter than an "I" beam.

 

[drawoh]

jaydigs,

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

 

[drawoh]

jaydigs,

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

 

[zeusfaber]

I'm picturing an application something like a large spotlight slung from the roof structure above the heads of audience or performers?

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.

 

[jaydigs]

Thanks for the feedback guys. I'm aware of design considerations for using safety cables and we've used this particular set-up for a number of other products. I think the original specs for cables were detemrined by experience and empirical testing but we've now exceeded the ability of our current Ø cable to support the fall of our newest product(new, heavier product broke cable). I'm really looking for the equations needed to ballpark a new cable Ø to test. The equations in my first post were what I used but resulted in a really large cable. I'm thinking 1/2" Ø will work but I'd like some analysis to support my theory.