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Belt Versus Screw Drive

Posted by maitri982 
Re: Belt Versus Screw Drive
March 02, 2010 02:57PM
Good points. Sorry Larry for using the term 'braided'. I had a brain freeze and could not remember wire rope, or if I did, I thought it wasn't specific enough. For our work, uncoated is essential. As I remember, many drafting tables, back in the day when drafting was done by hand, used the same figure eight tight cable and pulley system to keep the horizontal drafting bar perfectly parallel through out its range of travel.

Re: Belt Versus Screw Drive
March 03, 2010 08:23AM
Re: Belt Versus Screw Drive
March 03, 2010 09:25AM
This idea seems nice, workable, and seems to be something easily aquired in most world locations.

Any thoughts for the structural components necessary for pulleys, tensioning, clamps (to attach cable to movable platform), drive shaft attachment, etc?

My initial thoughts would be:

1. Larger pulleys better smaller pulleys (less stressful on the cable)
2. Tension the cable using an L-Bracket, an eye bolt, and nuts; E.G, it's not a loop, but rather, two cables that terminate on either side of the movable platform itself (And each can be tensioned separately to adjust/rotate the movable platform.)

   __. L___
  (O      O)
   \      /  
    \    /  
     \  /   
     /  \   
    /    \   
   /      \  
  (O_. L__O)

The '.' each represent a direct attachment to the platform, and L represents an L bracket with an eye bolt that can be tensioned.

I'm less certain on how to interface to the drive motor itself to get slip free operation.
Re: Belt Versus Screw Drive
March 03, 2010 09:57AM
I've used small turnbuckles available from radio control hobby shops, used for tensioning of cable runs from servos to control surfaces. The figure eight arrangement is possibly more useful as a slave linkage to maintain alignment, although it occurs to me that opposed blocks-and-tackles winding on and winding off could be used to eliminate length variation through angularity and as gearing at the same time.

Edited 1 time(s). Last edit at 03/03/2010 09:58AM by murd.
Re: Belt Versus Screw Drive
March 03, 2010 11:20AM
I like the cable drive idea.

If we only used four all under the print platform attached to the center and still support the platform like we do on the McWire, and not on the cable itself, we can just substitute the cable for the threaded rod.

Using the drawing from murd's link and stretching BeagleFury's drawing in the other dimension we have what something that might work.

I'm on my way to Lowe's right now and will pick up some stuff to try it out.

Hmmm...how do other people insert pictures? I'll attach it anyway.
open | download - cable drive but not z.PNG (4.4 KB)
Re: Belt Versus Screw Drive
March 03, 2010 02:49PM
Hi Arvin,

If you only attach at one point on the platform, the figure 8 seems overkill. A simple 'steel cable' belt drive seems simpler, and again, leads to the question, "how does the motor drive the cable?"

I believe the reason for additional complexity in the figure 8 is that you add a mechanical negative feedback loop for any perpendicular torque acting on the platform... imagine on your center drive platform that one side had a lot of really sticky bits; the table would want to rotate because as those sticky bits were hit, torque would be generated that would try to rotate the table as it moved, even when moving slowly. This would reduce the print quality because any play in the linear bearing could be amplified, and could also bind the axis completely preventing it from moving any further. With the cable attached to the sides instead, any torque will be countered by forces creating negative torque as soon as any rotation is realized.

I like the idea of springs, by the way, for self tensioning.
Re: Belt Versus Screw Drive
March 03, 2010 03:35PM
... atached is a photo of an old XY-data-plotter, where the X-axis is driven and feeded with a steel-wire.

Under the big white disk is the servomotor, the wire goes only one loop around (i made 2 or 3 loops in my setups), but is fairly strong tensioned.

Both Y-arm-brackets are fixed to the wire, so when turning the motor, the arm has strictly to follow the wire-translation.

And look at the atached images in this post - i've built some XY-plotters with steel-wires for lasercutting with a feeding strategy, where both motors were fixed to the base ...

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open | download - Gould-XY-1.jpg (261.7 KB)
Re: Belt Versus Screw Drive
March 03, 2010 04:45PM
Beagle Fury, if you overlap the lose ends on one side, then there would never be a point where the stage would go so far that the cable pulls up short. I also tend to think of bolts with a whole or slot drilled in the end and a pair of nuts to set the tension and lock it in place.

(0 0)
\ /
\ /
\ /
\ /
/ \
/ \
/ \
/ \
/ \

Drive would either be wrapping once or more around a pulley on the shaft of the motor, or wrap it enough times around that the middle of the cable never comes off. Then the mid-point can be screwed down to the pulley, to guarantee that the cable never slips.

Larger pulleys would also mean less force on the pulley, allowing for ABS to be used to make the pulleys, plus less force on the bearings. If the pulleys were made like large donuts, the inside diameter would spread the force around enough to consider making plastic bearings. In the Gould XY plotter, the pulleys are mostly plastic, but all have metal bearings. When the bearing diameter is small, the force per unit area is high and steel is required. When the diameter is large, and the pulley is tall as well as wide, the forces are much lower and plastic bearings may work. This would be a fun little test to see how much force a 2 inch diameter pulley with a 1 1/2 diameter plastic ball bearing inside.


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