belt vs ball screw

I need to build a simple test bed set up and will have a small stepper motor driving a light weight carriage over a 30 inch travel. THe budget is tight of course. I am thinking a tensioned timing belt would be ok here vs a ball screw. does anyone know how the tollerences and repeatability would compare? We probably need +-.005 inches, but I am not sure of that yet.
thanks

You probably OK with the tolerances with a belt, a quick check shows that mfg spec is +/-.002 for the belt drive system.

You will need to be carefull with a screw as the length will determine the max speed at which you can drive the carriage. You can increase the lead in order to decrease the rotational speed of the screw but with a corresponding loss of positional accuracy and drive force. Cost will also be an issue but with the looser tols you may want to look at a rolled screw instead of the more expensive ground screw.

Also, if it fits in your design parameters look at the linear actuators where the drive and the guides are integrated into one unit. You'll be much happier than trying to design using separate drive and rails.

Tolomatic is a good place to start but there are many other vendors that make good product as well.

KV

as long as the stepper has ample torque for the forces encountered, a fine-tooth belt system should work, and would certainly give the accuracy you want. (most retro-fitted CNC knee mills, and alot of originals use toothed-belt drives)

Take a look inside a plotter....in fact, you may be able to cannibalize the workings out of a defunct plotter for your project.
Pick a heavy one; like a D-E size HP

5 thou should be easily doable as long as inertia is low.

You could always close the position loop with a linear encoder directly on the carriage, but probably not needed.
Use as small a pulley as you can. A larger motor will usually be a lot cheaper than a precision gearbox if torque is an issue.

Chris

surplusjohn said:

I need to build a simple test bed set up and will have a small stepper motor driving a light weight carriage over a 30 inch travel.

Click to expand...

John;

I've built a few positioning devices using timing belts. You will get much better stiffness/accuracy if you use a length of belt clamped at the ends than with an endless belt (because you only have half as much belt to stretch).

Of course this requires placing the motor on the carriage and using idler wheels.

I get my belt material from Belt Corp in Cumming, GA. They will cut it to any width you need. Thus, it is possible to use a wider than normal belt for extra stiffness. Belt Corp also has pulley stock by the inch so you can make any width.

Can't remember our contact person at Belt Corp, but I've had good luck with them. We just replaced 18 feet of 4" wide AT10 belt on a knife grinder. They got it and a special pulley to us in two days.

Of course if you can live with standard widths then McMaster is usually the simplest solution. They also have the plastic idlers in stock.

Doug

Such belt drive systems are commercially available, HEPCO in the UK is the one I've encountered. Worth spending some internet time checking design, performance and cost. Memory says that the commercial offerings were relatively inexpensive. Maybe cheap enough for you to purchase when compared to the true cost of design and build. Always nicer to buy commercial production against a specification rather than risk re-inventing the wheel and needing to do development.

Clive

thanks, I will check out