Safety Cable

[meca]

I have a large mass that is to be suspended in the air by a hoist. On the mass I want to attach a safety cable, which would support the mass in the event that the hoist failed. It is possible that the mass could fall some distance before the safety cable becomes engaged. My question is that given the mass, and distance of travel, how do I calculate the force that the cable must be designed to withstand?

 

[Barry1961]

I think a big question will be, what is the slow down distance? Until you know this you will be guessing. You may be able to find some info on the paticular cable you would like to use. Something that has a good bit of spring or stretch would be nice. Of course you could add a known spring mount to your cable to reduce the shock.
You may be able to use a mechanical speed limiting device also to help reduce the shock.

Barry1961

 

[desertfox]

Hi meca

Calculate the stress in the cable via the strain energy method as follows:-


the max instaneous extension of the cable = x=f*L/(E)


at the point of max extension:-

initial potential energy = strain energy in cable

regards desertfox


therefore W*(h+x)= [f^2/(2*E)]*A*L

now x = f*L/(E)


therefore W*(h+[f*l/(E)])= [f^2/(2*E)]*A*L

you can solve the quadratic equation using the standard maths formula :- -b[±][√]b[²]-4*a*c
--------------------
2*a



You will obtain 2 answers one positive and one negative you can ignore the negative one in this instant.

f= stress in cable

A = c.s.a. of cable

L = length of cable

E = modulus of elasticity of cable

h = distance weight falls before cable reacts

W = load (mass attached to cable * gravity const)

 

[ruble3]

What about a safety factor?