help with belt or chain/sprocket driving.

[Opcom]

Has anyone good experiece with toothed belts, same pulleys, or gears, chains, whatever that will turn something that is hard? Here is what I got:

I have a couple of variable inductors that are hard to turn. Not because they are dirty but because, I believe, they are large diameter units of the type with flat wound coil (edge wound) and the wiper is made of two washers that press on opposite sides of the winding. The wiper is like in the photo, but the photo is of a much smaller coil from the proposed RF concepts Alpha 4040 tuner. These things are the kind used in a maybe 15KW BC station matcher. Too big but the right value and on-hand. This is not for a table top thing.

So, the radius on these things is about 4.5". That in combo with the high pressure of the washer-wheel contacts, makes them hard to turn. They have 3/8" shafts.

These are not easily turned by hand cranks. Not like some MFJ item.
I also want to use a motor to turn them. I have a nice little 100RPM DC gearmotor with a 9mm shaft that I am sure will do it. I ought be able to drill an off the shelf  brass coupler out to match 3/8" (9.525mm) to 9mm. - or otherwise shim a 9mm shaft to fit a coupler.

How is the chain or belt tension adjustment done? Some thing has to move, like the alternator on a car tensions the belt. But it can't be let to slip.

Any pictures or descriptions from more robust things built by others with larger parts would be really helpful for my understanding. Thanks!

[Opcom]

I was looking here:
http://www.sdp-si.com/web/html/products.htm

[KB5MD]

Use an idler pulley for adjusting the tension similar to the one on a car.
Whatever you decide, get the sprockets that have deep valleys and tall teeth so that the chain will not slip.
Something similar to a bicycle chain/sprocket

[N2DTS]

To get a symmetrical front panel layout on a push pull 812a rig I built, I used a chain and sprocket setup from Herbach and Radman (long gone I think).
The sprockets fit 1/4 inch shafts and they had a large assortment.
I still have some of the stuff I got from them.

I would look at the hobby places that sell RC cars, planes, drones and so on.

[KA2DZT]

You need a machine shop.  Too bad you're not here in NJ.  I have lots of gears, chain sprockets, timing belt pulleys, shafts, gear motors, etc.  Probably best way to do what you need is with gears.  You don't need any tensioner if you set it up right.   Check out Boston Gear.  I think they're in New England.

Fred 

[w8khk]

I agree that gears might be simpler, but they also limit the placement of the motor or other drive shaft and knob or crank.

I spent some time working with my brother-in-law on converting a milling machine to computer control using stepper motors.  Also, I joined him to mentor some high school students in building robots from scratch to enter a challenge with 60 other teams, with a meet in Orlando. 

From these two events, I learned quite a bit about using timing belts with cogs and grooves in the pulleys.  The pulleys have a lip to keep the belt in place.  Proper positioning of the stepper motor tensions the belt well.  I was amazed at the driving and holding power of the stepper motor.  I researched parts availability on the web, and learned that there are many companies offering stepper motors, drivers, controllers, along with all sizes of timing belts and pulleys, either as complete kits or piecemeal. The low cost of the complete kits was a pleasant surprise. 

If you choose to use a stepper instead of a geared DC motor, you will be pleased to know that the controllers for the steppers require just two signals, direction and pulse.  You could go with a simple shaft encoder and a bit of logic to remotely control the stepper for each inductor.  Or, you could get a controller that connects to USB or RJ-45 to control via the network.  Simple software would allow remote tuning with mouse or touch pad, or you could interface a directional coupler to the input of the controller to have fully automatic control of the inductors.

All this may seem very complex, but actually so much of the HW and SW is ready off the shelf and takes very little effort to integrate.  Look at some of the sites that provide CNC upgrades for milling machines or lathes, and also look at sites that sell steppers and drivers and controllers for robotics.  You might be very surprised how simple it is to produce a very elegant and effective solution to your challenge.

[w8khk]

To clarify the information about the timing belts and stepper motors, I am posting three pictures showing the CNC converted milling machine in operation, and the results of the first test run.

[Opcom]

I'll check out Boston Gear too.

The stepper motor idea seems very worthy. It should not be hard to work with. Seen some little kits and know how the shaft encoders work, and witnessed the real power of a big stepper. Amazing torque. No reason a bigger one could not be set up. Maybe I can avoid using a PC, I think I'm not ready for that much remote control but I'll give it fair consideration. I suppose I am afraid of having the station down because a PC is broken.

I have to do without a machine shop. I have a drill press and hand tools, and power tools, it really should be enough.

Great ideas!

[Mike/W8BAC]

I think you might be more interested in a power window motor. Lots of torque and reversible. If that isn't enough power have a look at an automotive sliding door motor. like this.

http://www.fazendomedia.com/pz1c06c4b-czf0705c-electric-rolling-door-motors-ql.html

With a limit switch or two you might have a low cost alternative to steppers.

[w8khk]

Mike's idea looks cool too.  I have been thinking more about your project, and would like to add a few more comments.....  Mike's post came in while I was typing.  Looks like you will have plenty of help with your project!

I suggested stepper motors instead of the geared DC motor because they are easier to control, and there is really no need for a PC.  But then again, once the stepper is installed, PC control is a much easier option than if a DC motor was used.

A rotary shaft encoder is small and looks very much like a pot, or variable resistor.  The output can be gated with a couple chips, so that the direction of rotation can set one signal to the stepper motor controller, while the motion of the shaft encoder could generate the pulses to cause the stepper to move.  Stepper motor controllers are cheap and very simple to interface, without the PC.  The additional PC controller, if desired, wires directly to the stepper controller, enabling full control via the PC.

We did not have a machine shop to convert the milling machine to CNC control, just common hand tools and an electric drill.  Thinking back, I recall our initial trial used stepper motors salvaged from an old HP pen plotter, I think it was a 9872 model. These steppers did not have enough holding power for the big mill, but had plenty of torque to move the table.  We purchased stronger motors for the final implementation.  But one thing came to mind regarding a source for steppers, belts, pulleys, etc.  Almost every ink jet printer, or all-in-one printer/scanner/copy machine has several stepper motors, and many include timing belts and pulleys.  A recycled one might be a source of some of these components, if large enough to deal with the force needed for your variable inductor.  Hope some of these ideas are helpful to you.  GL with your project, Patrick!

[WBear2GCR]

Some of the surplus houses, All ELectronics, Marlin Jones, etc were selling surplus window wiper motor assemblies. I'd rig up a slip clutch so that you do not drive the thing through the stops - that and a micro switch limit for both ends.

Even the big ones ought to be turnable with relative ease by hand IF a wheel the diameter of the coil were to be installed. IF it doesn't really turn right, you might want to disassemble and lube it up, check clearances, etc. They ought to be spring loaded, right - the wiper assembly?

I think you can rig a coupling with two wheels of appropriate size and type and some kevlar line, done up the way old school radio dials are run. Fishing line comes in rather heavy sizes and in Kevlar.  Thin steel wire rope will also work, and then you don't need belts or chains...

Just some thoughts.

                           _-_-

[Opcom]

I did lubricate one earlier and it was easier to turn. What I would like to do is determine the inch-pounds needed to turn it, then  know what motors may work. I need to find a fish scale. I believe the gearmotors I now have should do. They can't be stopped by hand.