Belt Drive - advice needed

[Pat2000]

Folks, I would quite like to use a toothed belt drive for a wood type of router I want 1m x 1m if I can. I want to keep the rapid feed rate quite high.

I'm thinking along the lines of using say a 30mm wide belt and also using a worm drive to drive it- say something like 14:1 ratio aiming for 10mm of movement for 1 rev of stepper.

any advice on belt type? is 30mm wide enough? maybe 5 or 8mm tooth pitch?

how much tension is needed on belt?

something that is concerning me what happens if Estop is activated at high feed rate - can a deceleration be programed? or do I just hope things won't break

Pat

[ger21]

There's a lot of good info at www.sdp-si.com The GT2 belts seems to be much stronger than others, and have less backlash. The working tension on a 25mm GT2 5mm pitch belt is 712N (160 lbs)
I haven't seen a lot of belt drive machines, but the ones I have seen usually use a 25mm belt.

Depending on your control software, you should have a pause, which will decelerate, and an E-stop, which won't. I'd imagine an Estop would be hard on the belts.

A worm gear would probably have unacceptable backlash. You might want to consider reduction with belts.

[Pat2000]

Brilliant Gerry that link is just the ticket! I'm hoping to use Mach3 so I trust the pause or feed hold will use deceleration.

Pat

[HuFlungDung]

How's the 14:1 worm reduction figure into this high speed machine? The worm gearbox slows down the driver, but cannot be back driven.

I do not know if Estop is ever "power braked" on a stepper system? The motor may coast. However, the moving load cannot effectively backdrive through a worm gearbox. Thus, the momentum of a heavy load could force the worm gear to try unsuccessfully to backdrive the worm, and it might bend or break something in the process.

[Pat2000]

Originally Posted by HuFlungDung How's the 14:1 worm reduction figure into this high speed machine? The worm gearbox slows down the driver, but cannot be back driven. I do not know if Estop is ever "power braked" on a stepper system? The motor may coast. However, the moving load cannot effectively backdrive through a worm gearbox. Thus, the momentum of a heavy load could force the worm gear to try unsuccessfully to backdrive the worm, and it might bend or break something in the process.

Well yes that is part of my worry!, I was roughly working as follows 1 rev of stepped pully was to give about 200mm carriage travel (I expect I can reduce that pully size down) the worm drive as you say not allowing any back driving - which is a good thing... unless the moving mass is not given time to decelerate in particular, I reckon the worm and wormwheel may have to be of 'reasonable size' but methinks how do I allow for a rapid stop?

I was reckoning on 6m/min thats 100mm/sec (240"/min or 4"/sec) rapid movement.

[HuFlungDung]

Ok, I misunderstood when you wrote:

10mm of movement for 1 rev of stepper

as being one turn of the stepper motor, but you meant step pulley

So at 6000mm/min, that is going to require 600 rpm. At 14:1, your motor will be moving right along at 8400rpm

[ChrisJ]

Hey Gerry,

The SDP/SI site, seems to only carry GT2 pulleys & belts up to 15mm belt widths. Do you know of a online site that offers the full range of pulleys and belts up to the 25mm widths?

Also it seems that the belts come in certain lengths. If you need a specific length (for timing purposes) that is not a standard size, is there a way to make your own out of the open length stock, that would as strong as one that already comes closed? if not, would you happen to know of where you can purchase custom length belts (GT2).

Chris

[ger21]

I believe if you call them they can get any sizes you may want. I haven't come across any other online stores that carry the GT2 belts and pulleys. I too was looking for 25mm, but after reading the specs, I think the 15mm will work for me. They can carry a much greater load than the other belt types.

[HuFlungDung]

If high speed noise is a consideration (for a spindle drive), the GT2 belt is not a solution. The Eagle PD is. I found this out on my knee mill retro. Running the GT2 at 3600 rpm ( 3" driver to 6" driven) had quite an annoying whine to it that seemed to get worse over time. On a servo reduction though, you likely would not notice the noise factor as much.

[ChrisJ]

Thanks for the Eagle PD suggestion. It certainly appears to be more robust than the GT2 pulleys/belts.

The 15mm GT2's would probably be right on the fence for my application. The pulley's will be driven by a 1/3 or 1/2 HP motor.

It is not for a CNC machine, but for a pump design that requires some tricky gearing to accomplish the right rotation and timing. Motor turns at 1750 RPM (clockwise), pump impeller orbits at 1750 rpm (clockwise) but rotates at 875 RPM (counterclockwise). Add to this... there are dimensional restraints. I have it all figured out using spur gears but I was hoping to come up with a design that used timing belts/pulleys. This would improve the efficiency and would eliminate the need for gear lubrication. But I am not having any luck so far figuring it out with the belts, but I still need to exhaust the brain storming effort.

Chris

[DDM]

Does anyone know what the positional accuracy of the GT belts Are, I have been pouring over the numbers the past few days and I can't find much. There was something on the SPI tech sheets that with certian drivers and the 5mm belt there was about .010 of postional error. I couldn't determine any numbers to figure out how much error comes into the system under higher torque. I'd love to do a belt drive system but I don't know if I can get the kind of accuracy that I want for the machine that I'm designing. Over a span of 6 feet it could be quite a bit. Any thoughts.

Carl

[loves80z]

Remember that belts strech depending on the load and how long the belt is. A wider belt will strech less. We used one on a drilling machine, but we had a linear encoder rather than a rotary. Belt drive is for speed. Ball screw for accuracy. 3ft may not be too long for the speed U are after. A low cost linear encoder would be nice.

Kevin