Where to purchase just a few gears?

I'm an Electrical Engineer who's creating a one-off piece of instrumentation that needs to include two adjustment potentiometers that are controlled by a single shaft. These are multi-turn pots and they need to be "perfectly" synchronized throughout their 10-20 turn range over a long period of time and many adjustments. To avoid the complications of backlash, I think I need just three gears (rather than belts or chains), one on a master shaft and two slaves -- although I could probably also do it with just two gears, the mechanical construction is a bit simpler if I have identical mounting for the two pots. This isn't a production item, it's one-off. The pots are just under 1" in diameter and the shafts are round and 1/4" diameter. I'd appreciate a URL or three for some place(s) to purchase these gears and similar things like this which come up from time to time.

TIA Norm

Reply to
Norm Dresner
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In North America a few suppliers of precision gears:-

WM Berg Allied Devices Stock Drive Products

But for a one-off, check Smallparts.com first- they are easy to deal with (unless they washed away in one of the hurricanes in FL). You probably want "spur gears". You can also buy anti-backlash gears. Note that if you gear two shafts together one will turn in the opposite direction of the other- so maybe you just swap ends on the element, but it's not as symmetrical. You can also buy anti-backlash spur gears which essentially have two thin gears with a spring that causes them to exert force on both sides of the mating gear- eliminating backlash if the force of the shaft being driven is less than the force required to overcome the spring.

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

Spehro already listed some gear suppliers. For an alternative to gears, see if your pots have removable backs and can be ganged on one shaft. Also, if you have a microprocessor in your system, you could use digital potentiometers, like DS1267, DS1802, DS1803, DS1868, ADN2860, AD5290, etc. Eg:

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Reply to
James Waldby

You might give an appliance/electronics repair shop a look. The one near me carries quite a few little nylon gears from the guts of VCRs and such.

Reply to
B.B.

Greetings Norm, If you use timing belts there will be no backlash. I'd use small toothed, ie, timing belts. Small Parts will have what you want. As will Wm. Berg. CNC machine tools use these belts and the only backlash in the system will be the ballnuts on the leadscrews. Cheers, Eric

Reply to
Eric R Snow

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Reply to
Bill Chernoff

You'll be getting requests to quantify "prefectly", as real perfection is not possible.

If these are wirewound pots, depending on the resistance range, your backlash for a fine-toothed set of gears may well be smaller than the motion needed to step from one turn of the resistance winding to the next (some small fraction of a degree).

Carbon film or metal film pots have no such steps, but they will wear over time, so while the pots shafts may be well synchronized mechanically, the resistance may not track nearly as well as the resistance material wears.

However, I would suggest that each pot be equipped with an anti-backlash gear (two gears, one firmly locked to the pot shaft, and the other free to turn a little (about 1/2 tooth) with a spring biasing it. This handles small loads with no effective backlash.

Also -- you should avoid having the same metal on the driving and the driven gears -- you will get galling (transfer of metal from one to the other) making it rougher and rougher as time goes on. Common design is stainless steel meshing with aluminum or brass.

With two gears, you have two options that I can see:

1) The pots *must be linear taper, and the fully CW position on one pot would equate to the fully CCW position on the other. 2) Have the input shaft couple to the shaft on one pot and one gear (probably using an Oldham coupler to allow for sight shaft misallignment), and mount the other pot facing the other way -- shaft away from the input shaft and body parallel to the input shaft. (Obviously, the mechanical construction is a bit more complex for this.

Are the terminals on the ends of the pots, or the sides. If on the sides, you have a bit more work to do to assure that the zero position of both pots corresponds, as some orientations will not be possible.

I particular liked PIC:

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back when I did lab work. They have some very nice prototyping kits which will give you parts to work with as you discover other (unanticipated) needs. But they *are* expensive.

You can download catalog sections (in .pdf form) from their web site (without price list). Looking at the gears .pdf, it looks as though the finest pitch available with 1/4" shaft holes is 72 diametrical pitch. The smallest number of teeth in the anti-backlash gears in 72 diametrical pitch is 100 teeth, which works out to 1.417 OD, so your minimum spacing between the two pots would be about 1.5" center to center.

You want the anti-backlash gears on the pot shafts, not the input shaft, as one does not work as well when meshing with two plain gears -- and not at all with another anti-backlash gear.

Now -- you might be able to reduce the overall size of the assembly if you don't need a 1:1 ratio between the input shaft and the pot shaft rotations. Otherwise, you will wind up with a triangle of respectable size, with two anti-backlash gears, and one plain spur gear, all of the same size.

Remember aluminum meshing with Stainless Steel, not all one metal for maximum life.

Another vendor is Borg, but I don't know their line as well as that of PIC.

Good Luck, DoN.

Reply to
DoN. Nichols

Slight refinement on the above, Assuming linear taper pots so they can track effectively when turned in opposite directions, if you couple the potshafts so the pots are facing each other, then there is no backlash between the two pots. Drive the combined shaft however you like, If its just 'twiddle till tuned' a friction drive off the shaft with the knob would be fine (or even use a thumbwheel), if you need to run a multi turn dial, I'd use a toothed belt with some pullies out of a couple of VCRS or VCR service kits or the anti-backlash gears.

Alternatively have you considered a cord drive. 4 identical drums of at least 3" diameter, and 2 Vectran cords (available by the metre from any decent yacht or dinghy chandler, better than kevlar for repeated use round pullies) or glassfibre dial cord if you can still get it, identical tension springs and a symetrical layout either side of the ganged master drums on the input shaft with the cord anchored to each drum so any creep cannot accumulate and they should track very nicely with good feel. The tension springs must of course be within the drums attached to the cord anchor points. 10 to 20 turns is going to require long drums and carefull layout, but assuming you use 1.5mm cord, and allowing 20 turns either side of the anchor point in the middle of the length of the drum, you are only looking at 2 1/2" on each drum. If you get 'cute' and arrange it so that as one side of the cord winds off, the other side winds back on in the same place, with the drum drilled at each end and the cord led through the middle, inside the drum but under all the turns to the anchor point, you could halve the length of each drum, but you cant mix that with the simpler threading above to get away with three identical drums. The ganged drums for the input shaft could be turned as a single unit. Its probably best to drill and tap for the anchor points.

Dont forget, the shafts need to run in bearings or at the least bushings attached to the chassis with the pot bodies 'floated' with mounting brackets on fairly hard rubber grommets bolted down to the chassis to accomodate any slight misalinement or whatever system you use will get very jerky as the bearings in the pots wear as they are not intended to cope with any significant radial load.

Reply to
Ian Malcolm

Don and others

Thanks for the great advice. I've already dug out an ancient (1998) Small Parts catalog and I'm going to the PIC website to download their catalog.

Norm

Reply to
Norm Dresner

I hadn't considered coupling the two pot shafts together and driving them as one with a belt but it should also work since the pots are linear. IN this case, as long as calibration isn't critical, you're right that a belt should be sufficient.

The pots mount to panels directly and the shafts are left floating so this should be okay too.

Thanks Norm

Reply to
Norm Dresner

?

They're called ganged pots and you can find them just about anywhere. It's got to be easier than gears, belts, etc.

Reply to
James B. Millard

Why not simply get a "stacked" pot? This is two or more pots on the same shaft?

Gunner

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Reply to
Gunner

I don't think I've ever seen ganged 10-turn precision pots.

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

Beckman Instruments made them, I'm not sure if they still do. They were the larger (appx 1.5 inch diameter) black phenolic cased ones, and were manufactured stackable, so you could add sections.

If he could find some surplus that would be the way to go.

Otherwise I would not use gears, but rather the steel cable cogged belts that small parts sells. Aside from the smoothness issue then both pots would be going in the same direction, without need for an idler gear.

Jim

Reply to
jim rozen

On Tue, 05 Oct 2004 08:25:43 -0400, Spehro Pefhany calmly ranted:

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hits
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hits

I love 'em! (motorized volume controls on stereos.)

Reply to
Larry Jaques

Okay, I've never seen them but I'll definitely check them out -- it's a whole lot easier than the alternatives.

Thanks Norm

Reply to
Norm Dresner

I have a few of these, some with a tiny swiss made geared motor and slip clutch so the motor or pot doesn't get damaged at the end of travel. A while back I used these as a very simple very long period d/a converter driving the pot with a stepper. The purpose was for a particle feeder for sapphire growth machines. Replenishing crucibles on the rate of a few grams per hour.

Earle Rich Mont Vernon, NH

Reply to
ERich10983

When I worked for Beckman Instruments in one of their potentiometer plants, I often saw ganged pots: two or more potentiometers on a common shaft. I suggest that you contact your supplier and see what they can offer. This will save you a *lot* of effort. For a one-off, they may be able to sell you an over-run from a previous job.

Alan

Reply to
Alan Frisbie

Hi Norm,

Alternately, one might also use a link bar on two hubs mounted on the pots. I recently made a pot test fixture that linked 10 pots together with a single bar on hubs.

The motion was similar to a locomotive motion - it took a rotational motion from a motor and wiped the pots back & forth over about 90 degrees of motion. This mechanism could also accomplish a bunch of full turns if you drove it by hand.

You could to this with three bars and two shoulder screws.

Just an idea.

Later,

SMA

Reply to
Sean-Michael Adams

We're presently using the clarostat ten-turns driven by stepping motors, to control precision current sources through a screen room wall. The motors have almost no overtravel protection other than software flags.

*Except* I machined the phenolic driver rods with a narrow section, they neck down from 1/4 inch diameter, to about 1/32 dia for a short bit. That's the mechanical fuse, not the helipot innards.

Jim

Reply to
jim rozen

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