Maximum Allowable Angle to prevent cable from coming off from the sheeve pulley

[strainstress]

Ideally the cable should be pulled in line with the sheeve, however the case is such that cable has to be pulled at an angle. My question is, how do I calculate the maximum allowable angle before the cable slips off from the sheeve ? Any help/pointers will be greatly appreciated.

-
-
- Sheeve pulley
-
-
- Cable
-
-

 

[racookpe1978]

First, accurately measure all of the problem. (rope diameter, pulley shape, pully od, id, angle the rope will be offset, vertical angle the rope be offset (if any), exact pulley shape.) Draw those shapes accurately and to scale.

How much load -> over (sideways) AND how much in-line? What speed is the rope moving - that will have a big effect if the force sideways (up and across the pulley walls) is being applied while the rope is moving.

 

[MikeHalloran]

You don't pull the rope off the pulley at an angle, ever.

You adjust the orientation of the pulley axis.

See 'mule drive' and 'Corvair cooling system fan drive'.
Yeah, those are v-belt applications, but the rules are pretty much the same for wire rope, hemp rope, v-belts, and cable festooning.





Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Mike is right, but when you have to, you need to know the coefficient of friction of the cable against the sheave, and then rac's method is useful.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ztengguy]

wouldnt friction cut into the rope, also side loading the pulley? Sounds bad idea to me.

 

[racookpe1978]

Oh, it's a cr*ppy idea. The pulley rope will crawling (will be pulled) up and out of the throat of the pulley as it turn where the rope exits, and also by the "rolling effect" of a round cylinder being pulled sideways up a larger round-edged hole. I had not remembered the wear-and-tear on the rope itself over time, thank you.

 

[dik]

It's called the right and left fleet angle and is sometimes present when winding cable on a drum with a fixed sheave. This is typically 1-1/2 degrees max and depending on the drum manufacturer, it may be as much as 2 degrees.

It is a consequence of the drum being wider than the sheave. Sheave and drum centres are generally on the same axis and the angle right and left of the centre is the fleet angle.

Dik

 

[dik]

Should have added to check with the sheave supplier to see what the max fleet angle is.

Dik

 

[strainstress]

I agree with 'dik' regarding the fleet angle (2 deg recommended). However, in my case the cable is not winding on a drum, but is going through another sheeve pulley whose is axis is offset by 10 degrees. Does 2 deg allowable is still the maximum allowed angle or can it be allowed more than 2 deg ?

'racookpe1978' Can you please elaborate more on the method you discussed. Once I calculate the force, What should it be compared against to find the allowable angle?

@All: I agree this is not an ideal case.

 

[dik]

I think the 1-1/2 to 2 degree is similar, but you should check with the supplier; they may permit more or less...

Dik

 

[dhengr]

Strainstress:
Those kinds of bends or fleet angles cause wear on the sheaves and the wire rope, and of course the potential of the wire rope climbing out of the sheaves; they also cause spooling problems on winch drums. These are the kinds of things the various parts manufacturers will talk about. So then, put two turning sheaves (deflecting sheaves) in the system. Assuming your two sheaves are on horiz. axles, thus the sheaves are in vert planes; but they are offset so the wire rope has a 10̊ angle btwn. them. Then, btwn. your sheaves, install two sheaves with vert. axles, thus the new sheaves are in a horiz. plane, the same plane as the wire rope. These two new sheaves allow the wire rope to make the 10̊ bends, to line up with your original sheaves. We would have to see more of the entire wire rope system to offer much more. Sometimes you can use fleeting sheaves to help solve the problem, they slide laterally on their shafts, maybe that’s what MikeH and GregL are talking about. In some instances you can change the orientation of your original two sheaves to accommodate this direction change.

 

[MikeHalloran]

Yeah, if the cable/whatever just has to be pulled off at 20 deg, or 10 deg, you will have to put up with a lot of chafing or worse, or interpose a swiveling snatch block or equivalent.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

There are a lot of assumptions in many of these answers. Conveyor rollers are often driven by o-ring from a drive shaft that runs the length of the conveyor. This is a case of two pulleys with their axes at 90 degrees, and it works very well. So there is no simple answer to the original question and certainly not a simple calculation. In addition, alignment can never be perfect, so we go back to the original question: how far can you be out of alignment. The answer is in some cases you can be out by 45 degrees and it would be okay. In other cases one degree would not be acceptable. Cable type and tension are big factors.

 

[3DDave]

"Ideally the cable should be pulled in line with the sheeve, however the case is such that cable has to be pulled at an angle. My question is, how do I calculate the maximum allowable angle before the cable slips off from the sheeve ? Any help/pointers will be greatly appreciated."

Geometrically: look at the angle of the sheave tangential to the cable contact diameter along the cable centerline. When the angle becomes larger than this the cable will start to apply a side load on the sheave groove and will slide across it (welcome to the involute function) and begin to wear the sheave. If the load is never relieved I picture the twist in the cable carving/wearing a shape into the groove.

If the angle is large enough, the side load will cause enough friction to exceed the load due to direction change around the sheave, causing the cable to walk off the sheave - a bad thing to a very very bad thing, possibly a killing people bad thing. Which is why most say No-Angle. Not only is there likely to be damage due to the sudden change in radius as the cable leaves the sheave groove, it can form a kink in the cable that will significantly decrease the strength of the cable. A walk-off is an almost zero radius bend followed by a potential impact load, which is as detrimental as it gets.

There's got to be a more mathematical analysis somewhere. At least a free-body diagram.