9 replies to this topic

[bullwheeler]

Hey,

I am working on designing a Ski Tow for tubers as a school engineering project. We are having trouble finding the friction coefficient for the rope against the bull wheel. We will likely be using a lubricated steel rope and possibly a steel wheel. The friction coefficients listed online for greased steel on steel is 0.08 which seems very low for a drive wheel. Is there something we are overlooking? Are there sources for friction coefficients under different temperatures?

Thanks a lot,

Any help is greatly appreciated.
PS: Sorry for the double post, I realized I posted under the wrong forum. Thanks again.

[skierdude9450]

It seems to me that a lubricated rope to a bare steel wheel would indeed have a very low friction coefficient. Generally a rubber liner in the bullwheel is in contact with a non-greased rope which of course has a much higher coefficient of friction.

[FSJ]

Use .07 for steel sheave or .25 for rubber lined sheave. You can also increase the wrap angle on the drive sheave.

Are you using the band-brake of belt friction formula?

[bullwheeler]

FSJ, on 18 October 2010 - 03:22 PM, said:

Use .07 for steel sheave or .25 for rubber lined sheave. You can also increase the wrap angle on the drive sheave.

Are you using the band-brake of belt friction formula?

Great, thanks for the help guys. We were thinking of adding a rubber lining. but figured it may introduce unessential maintenance, since steel would cause the rubber to wear. Is rubber common practice? And yes we are using the band brake formula with a wrap angle of 180 degrees (for now). Would you recommend increasing the angle? The issue with this is it could again introduce wear since there is more bending of the rope. We're all completely new to ski tow design, so any basic tips or resources would be greatly appreciated.

Thanks again.

This post has been edited by bullwheeler: 18 October 2010 - 06:58 PM

[zeedotcom]

Here is a general tip regarding maintenance based on your comments regarding unessential wear.

Certain parts in any system should be designed to wear out "quickly" to make others wear out "slowly." Others should be designed to break more easily so that related parts will not be forced to. Examples:

Home example of wear: Most people use plastic or wood cutting boards. These have a twofold purpose. You could cut directly on your counter top, but this would damage a more expensive and harder to replace item. You could also use a harder material for cutting boards (and some people use granite) but these tend to wear our your knives more quickly, or at least dull them. Knives can be quite expensive. A plastic cutting board is cheap, easy to throw away, and protects both the counter top and your knives.

Lift Example of wear: You will find that bullwheels, sheaves, and other components that the haul rope touches are made of rubber. These parts are designed to wear out and be periodically replaced. This means that the haul rope wears out very little from running on these components and increases the life of the rope. Replacing a liner on a bullwheel or sheave is much easier than replacing a haul rope (and the rubber component is cheaper than replacing the entire thing.

Auto example of breakage: In most vehicles, there is some combination of belts that drive the accessories. Alternator, water pump, A/C, etc. Part of the purpose of these belts is that if one item were to seize up (water pump) it will break the belt but not your engine, unless you have a car with a different set up (bad experience with a Jetta I had that could have turned out far worse. Moving to heavier equipment, some industrial and farm equipment is equipped with shear pins. Basically hardened bolts that connect a shaft to a pulley or gear. If something gets lodged in some portion of the moving parts, the bolt breaks rather than wreck a chain, a gear, a pulley, a shaft, or some portion of the machinery being driven by them.

Lift example of breakage: The primary power source for most lifts is not hooked directly to the bullwheel via a gear box. There is generally some sort of coupling between the motor, engine, etc and the gear box, often a belt or chain. This chain or belt is going to be much cheaper and easier to replace if broken than a part in the gear box or a wrecked motor and will also help prevent such a failure.

The importance in these last two scenarios is to locate the cause of the stoppage rather than to simple replace the (belt/chain/shear pin) and hope that it was just a fluke.

Hope this helps.

[FSJ]

bullwheeler, on 18 October 2010 - 06:51 PM, said:

Great, thanks for the help guys. We were thinking of adding a rubber lining. but figured it may introduce unessential maintenance, since steel would cause the rubber to wear. Is rubber common practice? And yes we are using the band brake formula with a wrap angle of 180 degrees (for now). Would you recommend increasing the angle? The issue with this is it could again introduce wear since there is more bending of the rope. We're all completely new to ski tow design, so any basic tips or resources would be greatly appreciated.

Thanks again.

If your using a wire rope, I would assume your looking at a handle tow. Trying to get more than 180d would require some sort of guidage for the handles and would most likely be more cost than benifit. With a bare rope, you can get 225d with a deflection sheave or 360d of wrap with a two groove drive sheave.

The rubber liner on the drive sheave will allow for a lower initial tension and reduce rope wear. Plus you get a replaceable wear item.

[bullwheeler]

Thanks, we'll use a liner. Is there a source where you found .25.

[FSJ]

bullwheeler, on 19 October 2010 - 03:49 PM, said:

Thanks, we'll use a liner. Is there a source where you found .25.

.25 is from "AERIAL ROPEWAYS & FINICULAR RAILWAYS" by Zbigniew Schneigert, Warsaw 1966

in ANSI B77.1-2006, table 4-4, design friction is given as .205 for rubber/neoprene

[bullwheeler]

Perfect, thanks a lot. You've been a big help.

[blick]

FSJ, on 20 October 2010 - 09:51 AM, said:

.25 is from "AERIAL ROPEWAYS & FINICULAR RAILWAYS" by Zbigniew Schneigert, Warsaw 1966

in ANSI B77.1-2006, table 4-4, design friction is given as .205 for rubber/neoprene

in the french regulation, and in the european regulation, for bare cable on bullwheel with rubber, the friction coefficient is 0.30
T/t <= exp(0,9*0,3*alpha)
T : higher tension
t: lower tension
with 0,9 : safety factor
0,3 friction coefficient
alpha : 3.14 radians=180 deg
exp(0,9*0,3*alpha)=2,34 with these values