# Belt drive for large diameter lazy susan platform



## Reddinr (Sep 17, 2018)

I'm looking for ideas on an upcoming project I have.  I need to make  a one-off for a customer and I'm a little stuck on something.

The project needs to servo-drive a large diameter platform, something like a 24" diameter lazy-susan.  I need to turn it slowly, in one direction only, maybe up to 30 RPM at most.  It needs to be fairly precise in the commanded angle.  It needs to be very quiet and smooth when operating.  I'm thinking about using a Type L timing belt and pulleys.   I had considered gears but I think those might couple too much servo / gear noise to the platform.  The pulley on the servo might be about 1.4" diameter, the smallest I can get.  The platform pulley will be milled into the underside of the platform and will be about 16-18" diameter.  This gives me a decent reduction ratio.  The platform will either be plywood or plastic.  Undecided.

With the speeds I'm thinking about I am wondering if I couldn't just purchase a timing belt, turn it inside out and glue it to the big pulley instead of having to mill 135 or more teeth precisely into the big pulley.  Then I would run the actual timing belt over the top of that one.  I know it doesn't mesh exactly but maybe it would work since the gaps are wider than the teeth and I only have to turn in one direction.  The platform will weigh about 3-4 pounds including load and acceleration/deceleration can be kept very low.

Any pointers or ideas for the power transmission would be very helpful!  Thank you.


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## tq60 (Sep 17, 2018)

To isolate the servo and to be lazy...compound it.

First look at timing gears as the camshaft end on some are fairly large.

Crankshaft end too large center hole but smaller gear easier to make if needed. 

Use the cam gear on turntable driven by smaller gear connected to another cam gear making compound gear.

Drive that with another on the servo.

That gives 2 belts to isolate the servo and may allow more compact layout by allowing motor to be placed under turntable

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## JimDawson (Sep 17, 2018)

Don't know why that wouldn't work. As long as it only turns in one direction it should load properly.  The OD of the platform pulley would have to be exactly the correct diameter to get the ends of the belt to match up properly.

On the other hand, machining the pulley shouldn't be that difficult either, should be able to do it in 2 passes.  The HDT tooth form is a bit easier to machine.


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## FOMOGO (Sep 17, 2018)

An automotive ring gear from a flex-plate might work well with the right coged belt. Plenty available cheap in the junk yard, or direct drive with a matched starter gear. Mike


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## KMoffett (Sep 18, 2018)

I did a turntable to rotate a person for full body scanning for my son.  I used a small DC motor with a gear box.  Purchased a toothed-belt pulley to fit the motor shaft. Made a large diameter pulley out of 1/2" MDF sandwiched between two 1/8" hardboard, mounted under the platform. I added two idlers on the motor mount to take up slack and increase the amount of the toothed-belt around the motor pulley.  It worked down to a creep and up to about 30RPM with a 200 Lb load. The amount of friction contact between the toothed-belt and the large platform pulley was great enough that I never saw any slippage Powered the motor with a cheap PWM controller.  I selected the belt length by setting up the belt path in CAD.

With a "servo" are you intending to precisely control the angle of rotation?

Ken


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## Bi11Hudson (Sep 18, 2018)

As a Model Railroader, of the steam era, your project sounds a lot like a medium sized turntable. In my scale, HO, a 16 inch table would be middlin large, capable of taking a fair sized locomotive. The desired speed would be a little slower, with the "alignment" being on the order of 0.020 inch. The rail is 0.085 in height, with the width less than a third of that. Such precision is necessary because models will run across the joint, albeit at low speed.

There are many drive mechanisms for such applications. My personal favorite was one I saw in 1970 where a fellow modeler had used an audio turntable drive mechanism with a rubber wheel against a plywood disk. Alignment was controlled with a spring loaded pressure against the disk that retracted on command of a sensor that aligned the rails. For what it's worth, it was in Wellington, N.Z. that I saw this.

Granted, the travel was a bit slower than you're talking, ca. 2-3 RPM, and the load was quite low, on the order of a few pounds. Brass models in that scale, plus the weight of the structure itself. But the overall drive mechanism was fairly simple. Perhaps investigating such model turntable drives will yeild an approach that is usable for your application. There are many that are based on fancy technology, but the rubber wheel against a plywood disk gets my attention even after all those years. In most cases, quietness is secondary to accuracy but still important.

It would be a matter of the weight necessary to handle and the accuracy when stopping from speed that would come into play. I like the idea about the toothed belt inverted to provide a ridged surface to pull against. The buggest problem I foresee is the concentric rotation of the disk.

Bill Hudson​
*Edt: *The above entry is quite adaptable, an elegant solution. I may well integrate a similar drive mechanism for my own applications.


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## Boswell (Sep 18, 2018)

I am curious if this is something for a one-time use or is expected to maintain accurate positioning for years. Does it need to maintain angle registration between use or can it be "zeroed" before each use?  Does it need to maintain absolute angle accuracy over multiple revolutions or just one revolution?  What is the actual Angle accuracy needed.   I think these are important things to know to determine if a gear train or pinch-roller or something in between will meet the need.


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## Reddinr (Sep 18, 2018)

Thanks all.  
Kmoffit, you hit the nail on the head.  Pretty much what I had in mind except I didn't think about using idlers to make the belt wrap around the small pulley, which I will now do.  The motor looks like one I saw at All Electronics that I was thinking about using.  Can you recall what kind of backlash the gearbox had?  I do need to be precise with the angle. 

The rubber wheel idea might also work but I would be concerned about slippage by tiny increments over time.  BTW, I was fortunate to spend a couple of weeks in NZ myself.   Loved the country and the people I met were just terrific. 

I'll need to check out the ring gear idea too.  

Next up I'll need to figure out how to support the thing from the perimeter.  I need to keep the middle 90% of the underside of the platform clear of anything.  I'm thinking small nylon or hard rubber bearings or similar.   The nylon / ball bearing ones like for a closet door track come to mind.   The load is low, maybe 3-4 pounds total.

As far as the belt type, I picked "L" because I could easily find longer lengths of it.  Not sure about the HDT type.  Will look.  I am all for a simpler profile.


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## Reddinr (Sep 18, 2018)

The angle can be re-zeroed between uses.  The typical usage time will be about an hour and there might be 1000-2000 complete revolutions per use.  Absolute accuracy must be maintained for the entire time.  I would be shooting for +/-0.1 Degrees for one revolution and +/- 0.5 Degrees for the entire session.  I will have an encoder but not sure where in the drive train it will be placed yet.


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## macardoso (Sep 18, 2018)

Reddinr said:


> The angle can be re-zeroed between uses.  The typical usage time will be about an hour and there might be 1000-2000 complete revolutions per use.  Absolute accuracy must be maintained for the entire time.  I would be shooting for +/-0.1 Degrees for one revolution and +/- 0.5 Degrees for the entire session.  I will have an encoder but not sure where in the drive train it will be placed yet.



Encoder should be on the final load. Anywhere else may not represent the position of the load


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## JimDawson (Sep 18, 2018)

This might give you some ideas, sounds like you are doing about the same thing

https://www.cnczone.com/forums/usa-rfq-s/366624-cnc-pulley-bearing-races.html


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## Boswell (Sep 18, 2018)

If you have an encoder on the platform (final load) then I would think that it hardly maters what your drive looks like because the control computer will adjust as needed based on the actual position. Assuming you can get the accuracy you need from you encoder.


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## Reddinr (Sep 18, 2018)

Makes sense about the encoder tied to the platform.  I'll need to tightly couple that to the platform.   Sure would be easier if I had a center axle but I will work out a good encoder coupling near the perimeter


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## spumco (Sep 19, 2018)

Wheel encoders are available if you want to drive the encoder at the perimeter. 
https://www.ttco.com/encoders/wheel.html

Might also pick up an ebay shaft encoder and fit a hard rubber/urethane tire to it and have it ride on the belt OD.  At your speeds it shouldn't slip, even with low pressure (to keep loads off the encoder shaft) and with the huge table OD you'll get a very large effective PPR even with a low-PPR encoder.

You could use a servo, but that seems expensive for this application - especially if you have a load-coupled encoder.  I'm sure someone could suggest a controller to drive a simple DC motor and use the load encoder for feedback & PID to hit your target angle.

As for the inside out belt glued on, you can purchase open-length belting online in just about any profile and pitch/width.  I was about to suggest HTD or GT2 profile, but the inventor of the Bell-Everson drive system indicated that trapezoidal teeth meshed better when run tooth-to-tooth (vs. tooth to pulley) than did the curvilinear profiles.  I don't know if this holds true for tooth profiles bent back on themselves, but it's worth a look.

I do see that B-E is using an actual toothed pulley for their rotary tables and not attaching an inside out belt to a blank hub.  I'd speculate that an inside-out belt profile didn't work out for them, but your precision requirements may be less stringent.


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## KMoffett (Sep 19, 2018)

If Reddinr is planing on maintaining +/- 0.5 degree over 2000 revolutions he will need a mechanically locked system. Friction coupling between the drive and platform (like mine) or the friction encoders you linked to will not work. Maintaining +/- 0.1 degree over one rotation is even unlikely.

Ken


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## spumco (Sep 19, 2018)

KMoffett said:


> If Reddinr is planing on maintaining +/- 0.5 degree over 2000 revolutions he will need a mechanically locked system. Friction coupling between the drive and platform (like mine) or the friction encoders you linked to will not work. Maintaining +/- 0.1 degree over one rotation is even unlikely.
> 
> Ken


You're absolutely right.  I missed his post about the accuracy requirement.  Ixnay on the friction drive.

Back to low-backlash belts (AT, GT2) or a direct encoder mount on a shaft.


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## tq60 (Sep 19, 2018)

Add a self calibrating feature.

Simply place a "switch" of some type at a zero position.

Type depends on your accuracy needs but the operation is simple.

At any time the table is assigned to the zero position it must trip the switch.

Code can be written to hunt for it on approach meaning it is looking for position from wherever it is and as it approaches where it thinks zero is it now looks for the switch and modifies a "calibration factor" which is how the controller compensates for any slop or stretch.

Secondary can be "in passing compare" meaning the controller should know where the table is at all times and whenever the switch activates the controller compares its position.

Easy theory code not so much but a challenge.

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## tq60 (Sep 20, 2018)

Forgot a comment...

Without a positive feedback the system is "open loop" meaning the controller assumes table is someplace and moves it a given amount to get to someplace else.

An encoder directly on center of turntable would be "closed loop" in that the controller knows exactly where the table is.

Having the encoder any place else allows errors thus opening the loop by factor of errors.

The zero position switch above closes that opening a bit in that it allows the controller to test position with positive action.

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## Reddinr (Sep 21, 2018)

Thanks everyone.  I think my drive problem is in hand now.  I will be using a toothed belt and toothed pulleys.  I now need to figure out a good coupling for the encoder.  I had a friction wheel in mind but I agree that might lose registration fairly quickly.  I'm now thinking of coupling the encoder to the belt drive loop and just making the belt a bit longer.  I think that would achieve the accuracy I'm looking for.  For sure I'll have an index switch.


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## KMoffett (Sep 21, 2018)

Easy to add an encoder with a tooth pulley to the drive belt. But, what encoder? For 0.1 degree accuracy means you are going to need 3600 counts per revolution of the platform.  Tooth ratio between the platform pulley and the encoder pulley becomes critical.   TQ60's suggestion of a " zero position" switch will help in long term accuracy. I would suggest an optical beam-break switch. What type of motor do you intend to drive the system with? Stepper? Permanent magnet DC?

Ken


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## Reddinr (Sep 21, 2018)

The last day or so the specification has been relaxed a bit.  The absolute accuracy need is not really known and the first numbers I was given were just a  w.a.g..  It is down to "make it pretty accurate but let's not get carried away".   These types of "no spec" projects seem to go one of two ways.  It is either that I can't fail or that I can't win.

If I do go for it I will be using a servo with a built-in encoder and a stout toothed belt and both toothed pulleys.  I have a couple DC servos/drives lying around, one with a 1024 line encoder.   I think I will include an index pulse directly on the platform too.  With the pulley ratio of 8 or more, the resolution is there, maybe not the precision since it is once removed from the platform.


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## P. Waller (Oct 15, 2018)

It is common practice to produce a linear drive with a fixed timing belt, that is one fixed at both ends.
The belt is stationary and the drive moves along it, producing a continuous rotary drive this way would prove difficult to terminate the belt at the ends. If you can find an inexpensive method of doing so this will be highly sought after in the packaging and labeling industries.


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