timing pulleys and belts

[mbinsewi]

Cool, right here in Cudahy, WI.

https://www.wmberg.com/

Mike.

[woodone]

If you are looking for small gears, timming belts or drives this place will have what you need. Not cheap, but if they don't have what you need,  you really don't need it?

https://www.wmberg.com/

[SeeYou190]

It looks like you got all the right answers.

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If you have a Miller Bearing store close to where you live, they can help and answer questions about all kinds of power transmission devices.

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How about cannabalizing an old copier?

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-Kevin

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[gregc]

thanks for pointing me to McMaster-Carr.   I liked how their web pages narrowed down the selection.   I found a page listing timing belt pulleys for a specified belt widths that also specified the shaft dia. that are presumably compatible with one another.

that page showed the largest pully having 60 teeth and the smallest having 10 - 6:1.

i calculate an angle resolution of 0.05 deg for an 18" (130') turntable assuming 1/8 (1/16" / 8) track width resolution.

i happen to find some very small 15% stepper motors with 63:1 gearheads (0.3 deg)

so even low 6:1 gearing makes using timing belt pulleys feasible.

 

[wvg_ca]

McMaster Carr has a good selection .. plastic is cheaper than aluminum or steel ..

slotted holes are preferred to maintain 'slop' without it changing by direction moved ..

[Renegade1c]

gregc

as an EE, i'm looking for some help from an ME to explain what is available in the way of what i believe are called timing pulleys and belts.

i wondering if they might be suitable to drive a turntable using stepper motor.   I'm wondering if sufficient gear reduction is possible to have finer resolution than the typic 1.8 deg of a stepper motor.   I've seen Walthers turntables where a motor drives a worm that turns a gear that in turn drives a 2nd worm that drives a 200 tooth gear.

interested in hearing about sizes, gear pitch (?), shaft diameters, plastic vs metal, shaft suppots, ...   Is there stuff that's relatively inexpensive for robotics?    Or is this approach really not feasible?

 

McMaster Carr has a very good selection of these components. The hard part is where to look. If you search for Timing-belt pulleys you will get a pretty good list of parts. Next would to narrow your search down by shaft diameter of your stepper motor and your turntable. You will also need a tensioning pulley to keep the belt tight (other option is to create slot where the stepper motor is mounted and adjust it. 

Other place is amazon. They have a good selection of GT2 series components which will fit most stardard stepper motors. 

As for size. you stated a typical stepper is 1.8 deg. per step. With this resolution you will get approximately .28 inches per step for a 130' turntable (17.93 inches)

Circumference = pi*Dia. 3.14*17.93inches = 56.33 inches

Angular distance = Circumference *(step size/360 deg)*Gear ratio

Angular distance for direct connection = 56.33*(1.8/360)*1=.281 inches

For a turntable we can work backwords a little and say that we don't want to off more than .020" of an inch or a train will derail coming off the turntable. 

so lets rearrange our equation to give use the desired gear/pulley reduction we need to get there.

Gear/pulley ratio = Max Angular distance*(360/step size)/Circumference

Gear/pulley ratio = 0.020"*(360deg/1.8deg)/56.33" =.071 ~= 1:14

So you will need about a 1:14 gear/pulley reduction on the stepper motor. 

The next thing to do is translate that in something we can buy

Gear/pulley ratio = N1/N2, for a double reduction = (N1A*N1B):(N2A*N2B) with A being the first reduction and B being the second reduction. N is the number of teeth on the gear/pulley

There are standard sets of pulleys out there. the most common you will probably find is GT2 which is what is used in alot of the CNC/3D printing market. 

for the GT2 series of pulleys they come in 16,20,40,60 and 80 teeth. We need to come up with a gear a ratio at or higher than .071 (or 1:14)

if we use a single gear reduction of 16 tooth to 60 tooth  that  will give us a 1:3.75 single reduction. If we do a second reduction it becomes 1:14.06. 

Angular distance for direct connection = 56.33"*(1.8/360)*(1/14)=.020 inches

if we use a single gear reduction of 16 tooth to 80 tooth that  will give us a 1:5 single reduction. If we do a second reduction it becomes 1:5. 

ngular distance for direct connection = 56.33"*(1.8/360)*(1/25)=.011 inches

So in short if you do a double pulley reduction with a 16 tooth gear on the stepper, a intermediate pulley with a 60 tooth gear attached you will be with 0.020".

All this assumes you doing full stepping on the stepper motor and not half stepping which is all done in your control system. 

One last thing. I prefer pulleys and belts to gear simply because of backlash. Belts can have a tensioning idler added in which can removed backlash when changing directions (hence why they are used so frequently in 3D printers and laser cutters). Gears will always have a fixed amount of backlash to them. 

I hope this helps. Sorry it if fairly math intense. I happen to be an ME and deal with this pretty often. 

 

 

 

 

[rrinker]

 Pulleys work the same as gears, there are just a few more factors, like the pulley length needed, which is computed from the pulley diameters and the distance between centers. The only difference a timing belt type pulley makes is that it doesn't allow for the slippage you can get with a regular smooth belt.

Calculator here:

https://www.blocklayer.com/pulley-belteng.aspx

If the large pulley is 2x the diameter of the small pulley on the motor, it will turn at half the RPM of the motor. Convert to degrees - if the motor turns 1.8 degrees, the larger pulley will turn .9 degrees. 

 You've probably seen turntables driven like this - only they weren't for trains. A belt drive music turntable often has the big 'pulley' as the turntable platter itself, and a little tiny pulley on the motor, to get the platter to rotate at 33 1/3 RPM.

                            --Randy

 

[mbinsewi]

Interesting Greg, I'm no where near a EE and I'm just as far away from being an ME, but I do understand, somewhat, about gearing.

I had a couple of old battery screw drivers, the batterys are now obsolete, and no where to be found, so I took them apart, used some AA batteries to see if the little motor still works, and it runs great.  It spins like crazy, and the little gear on the end, meshes with 3 other bigger gears to get the driver to spin at a slow rate.  It seemed interesting.

I have to hand it to you, you are out to perfect turntable operation, going by your other threads.

Mike.

[CGW121]

Have not done it myself, but the guy at the Toy Train Barn in Wisc uses microwave turntable parts for that sort of thing with good results. You get the  parts u need wife gets a new microwave. A win-win situation.

[gregc]

as an EE, i'm looking for some help from an ME to explain what is available in the way of what i believe are called timing pulleys and belts.

i wondering if they might be suitable to drive a turntable using stepper motor.   I'm wondering if sufficient gear reduction is possible to have finer resolution than the typic 1.8 deg of a stepper motor.   I've seen Walthers turntables where a motor drives a worm that turns a gear that in turn drives a 2nd worm that drives a 200 tooth gear.

interested in hearing about sizes, gear pitch (?), shaft diameters, plastic vs metal, shaft suppots, ...   Is there stuff that's relatively inexpensive for robotics?    Or is this approach really not feasible?