Correct term for a variable diameter cable drive spool?

Hi,
I'm trying to find the correct name for a type of drum or spool which is used to drive a cable. The one I'm thinking of is not a straight cylindrical type, it tapers in on a radius at one end and then reached the middle of the drum and then tapers back out again (radiused, not a v type). It is a variable diameter drum designed to cause the cable to bind and not slip. I would like to know the name of the type and a link to where I might buy one. I am thinking about that method as a drive for a flame cutter.
Thanks
Splint

You mean like a Convex Flat Belt pulley?
Al.

No, I think I've seen them inside a photo copier. From memory, the pitch and diameter of the groove the cable sits in varies, it starts as a larger diameter and decreases steadily then at the mid point it starts increasing again in a mirror image. I've not been able to find any images on google of what I'm looking for. Maybe a straight spool will do. I guess if I run a tensioner and v grooves on the spool it will get enough friction not to slip.
Thanks
Splint

I think you are looking for capstan or windlass depends on if the axis is vertical or horizontal.
http://en.wikipedia.org/wiki/Nautical_capstan

Thanks Andre',
yes a Capstan is what I was thinking of, I'm sure I've seen one inside a photocopier. Just looking at a capstan now though, I'm not so sure it is suited to a motion control application, the binding force created by cable tension and the conical sections of the drum may create undue wear on the cable resulting in accuracy and reliability issues.
I have some all thread which has a rough surface finish, should be able to generate sufficient traction, I think that may be a simpler approach.
Thanks for your help.
Splint

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