> > Not sure of the efficiency or mechanical advantages, but 
> the force is 
> > really only limited by the strength of the cable.  You don't really 
> > need a spring to keep tension, you can attach the ends of 
> the cable to whatever it is you're moving.
> 
> Sounds like what I was planning with a belt. The problem is 
> the tensioning, with both approaches. Bi-directional 
> pull/push of 1000 N... I don't understand when you say that I 
> don't need a spring to keep the tension. Of course I can use 
> a screw or something and re-tension when needed, but 
> something needs to be adjustable (or self-adjusting), doesn't it?


This difference with a cable is that you can attach it to whatever is
moving.  You still need to tension it somehow, and a screw type adjuster is
best as you don't get problems with the spring stretching.  Of course, you
can move the motor itself to adjust tension.  If the ends of the cable are
attached to the driven rod itself, then a bit of slack can be tolerated, but
you get positioning problems.  Possibly too much hassle for your case,
unless you find a similar setup running and learning about it's
advantages/failing.  

Threaded rod doesn't have those problems, of course.

 
> > As a complete aside on simple ideas, I recently discovered how the 
> > tracks are kept taut on bulldozers, not that I'd ever wondered.  On 
> > most motorcycles, to remove slack in the chain you loosen the rear 
> > axle, slide the wheel back, and re-tighten.  On a 
> bulldozer, it's hydraulic.
> > Rather than pressure being applied by air or springs, it's 
> by grease. 
> > When the track becomes loose, you just pump in more grease.  The 
> > grease can't be compressed, so the tension is maintained.  Rather 
> > neat, I thought.  I've contemplated making this work on a bike.
> 
> Calling a grease-driven mechanism "hydraulics" is stretching 
> it a bit :)
> 
> On a bike, you'd probably have the problem of having to 
> synchronize the movement on both sides of the axle. Not a 
> problem for bulldozers.
> 
> Gerhard


You're right about the bike, keeping the wheel aligned is rather important,
one of the reasons it didn't get past the  contemplation phase.  You'd need
to equalise the pressure, etc.  No doubt someone has tried it.  Can't beat a
couple of bolts on pricing though.

Hydraulic is a bit of a stretch, but that's what they called it.  A piston
with the cylinder full of grease.  Grease is a liquid, I suppose...  I asked
about it, and was told the 'getting the grease back out' bit could be fun.
It's a very old idea apparently, and no-one is in a hurry to change it.

Tony

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