Dual Capstan Drum Dia. vs wire Dia

[ahmadsalah]

Hello everybody

I am designing a dual capstan winch that should be hauling 12-15 kips. The wire diameter is 1/2" and the capstan Diameter is approximately 8 which is consistent with ANSI B30.5. Now I need to know if the D/d ratios apply for the Dual capstan design. Basically the wire doesn't run all around the capstan pulley it runs between two pulleys back and forth unit one end exits to load and one end goes back to a storage drum.

My gut feeling says that drum crushing should be minimum since the wire isn't going round and round the drum. can anyone confirm if there is an equation for this case?

 

[dhengr]

Ahmadsalah:
What does a capstan look like in your world? Capstans don’t work if you don’t wrap the rope around them a time or two. You should provide some sketches of your whole arrangement so we can understand what you are dealing with, with all important dimensions, loads, etc. We can’t see it from here. You would probably do well to take a look at a text book/ref. book on Cranes and Rigging, because I suspect your terminology will not be understood as you discuss this problem.

 

[racookpe1978]

Drum crushing is the tendency for a round wire cable (or rope) under tension to "oblong-out" when the wire is compressed against the larger dia drum, or pulley wheel, or capstan.

SO, a center (??) capstan between two drums will also create that oblong force, as well as both drum end. If the drums and capstan were cut with rope grooves, the tendency will reduce since the grooves will force the individual cable strands back into a rounder shape as the force increases. but, much higher fabrication cost, a rope feeding mechanism is needed, and more wear on the outer fibers since there is more contact area forced against the drum.

 

[zeusfaber]

dhengr: There is a type of pulling winch called a "twin bull capstan winch" which exactly matches the OP's description.

They tend to look like this and, if you want full rated pull over an extended range (for instance on a heavy recovery vehicle), they are a thing to be coveted.

A.

 

[RolMec]

@ Zeusfaber: nice thing, good find! Sheds a light on the OP's intent. Thanks!

@ ahmadsalah: For D/d you might also want to look into the fatigue properties of your system,ie whether certain segments of rope undergo specifically more cycles than others, or specific loads. If your system is as per Zeusfabers find, you need to check your rope whether the distance between the capstans allows the rope to relax the inner torsion / bending when going from the previous half-wrap to the next.

Drum crushing a minimum (compared to full wrap), this might be, however this comes at the price of a asymmetric discontinuous shell bending problem. There's nothing in (my) Roark on this... There might be some analogy in steel cord belt (drive) pulley design, as the belts inner steel cords act in quite the same way as your rope. However, here we see direct contact rope to drum surface contact with transmission of forces, so a technical design model might not be easily available. I think FEA is your friend here, as well as some knowledgeable manufacturer.

Regards
R.

 

[ahmadsalah]

Thanks everyone for your input

dhengr: yes this is the same setup I am designing.

Rolmec: I believe the right way to phrase the question is: " Is there a difference between D/d of a wire on a Drum that will go round and round and wire on a pulley that will go only half a turn before it exits?"

I believe talking to cable manufacturer is the best option for now.

Cheers guys

 

[RolMec]

Quote "Is there a difference between D/d of a wire on a Drum that will go round and round and wire on a pulley that will go only half a turn before it exits?"

Yes, in my preferred design base (DIN 15020) I calculate the bending influences of these situations differently:
a) from a drum with a fixed point of the rope, where the rope then goes round and round until it exits: the rope undergoes one bending action (when it straightens out of the wrapped state). Prerequisite is the sufficiently large diameter of the drum.
b) the rope undergoes two bending actions when first bending from straight onto the sheave's (pulley's) radius and then bending off again into straight when leaving.
Concluding, b) receives a higher bending action factor than a).
However, there's standards out there (ISO 4308, FEM II) which do not go into such subtlety. It's your decision / depends on the safety requirements of your application. A reputable manufacturer is an excellent idea, if possible meet the practical guys from the "fire brigade". Reputable + references + practical knowledge about your application = the ticket ;-)
Regards
R.

 

[ahmadsalah]

Thanks Rolmec for your reply. If you don't mind, can I have your E-mail so I can discuss with you further? My email is andy@profoundindustries.ca

 

[RolMec]

Hello ahmadsalah, I beg your pardon but I shall prefer to stay here on Engtips. ++ next time will be quite a bit later 'course I'm logging off for some days.++
R.