Servo motor with a LOT of torque

[Speakerguy]

Hi,

I have an Xmas decoration I made out of plywood. Two, actually - Homer and Barney from the Simpsons in Santa suits and drinking Duff beer.

Anyway, Barney's arm is sticking out holding a mug of Duff. It needs to go up and down like he is drinking it, pivoting at the shoulder. The arm is roughly 10" wide and about 3.5' long made out of 3/8" plywood, so it is heavy (I have not put it on the scale to weigh).

I would like the arm to cycle smoothly once every 5 seconds (up and back down at 0.2Hz). So I need to rig up some sort of motor to do this, and I assume it will need to be beefy. I am very experience with PIC's and PIC programming but I know nothing about electric motors or servos.

Please help! Barney's arm needs to be helping him gulp beer by next Xmas.

 

[3v0]

Sounds like great fun.

I would use a electric window motor setup from a car. Possibly a wiper motor.

You could get by with less if your outline allows room for a counter ballance arm.

 

[Hank Fletcher]

Like 3v0, I think a gearmotor would be the better solution. From what I've seen, you can buy new/surplus-new ones that would be ample for what you need for somewhere in the $5-$10 range. For instance:
https://www.robotobjects.com/scripts/prodList.asp?idCategory=30

The same website has worm gear sets that you might consider using. Counter-balancing the arm is a very good idea - maybe you have some old bits of metal (something compact and heavy) to fit in the shoulder?

My preference is for a continuous rotation motor, as oppose to one that's constantly changing direction. Maybe you could connect a dual-lobe cam (elongated oval shape) to the output of the motor. With the counter-balance set so that the arm part always puts just a little bit of weight on the lobe, you could get away with using the least amount of power for the effect.

Also, having the counter-balance, and the cam separate from where the counter-balance/arm is attached to the figure, you don't need to worry too much about stress on the output shaft of the motor (because you'll have the other point where the arm pivots that can be quite beefy, without the bother of figuring out how you'll connect that part to a motor).

You could make the cam out of a 1/2" to 1" thick piece of UHMW, and have it contact a thin strip of aluminum on the underside of the arm. That way, the arm will move quietly and be self-lubricating. The cam doesn't need to be big - just plan where it's positioned with the arm, and the arm balance carefully. The cam should glue easily to the motor shaft. You can get UHMW at most places that also sell sheet metal (call first!).

 

[3v0]

There are many more ways to go. Linear actuators, pneumatic cylinder, water pumps etc.

If the arm is balanced use a inexpensive fountain pump and PIC for a timer. Pump fluid into a reservoir at the hand to lower it. When the pump shuts off the arm will come back up as fluid drains out. The reservoir drain hole is open all the time. The pump is twice as fast as the loss through the drain. Pump is on and off for equal times.

People have used auto style air shocks as actuators for greenhouse vents. They are too large for what you are doing but you can get smaller surplus cylinders.

If you are OK with reversing the motor the site Hank mentions has rack and pinion gearsets.

 

[Krumlink]

Use a Window motor as already suggested.

 

[Hank Fletcher]

Here's a neat site with some explanations and animations of mechanisms that might be of interest to some robot builders out there. Robot chicken, anyone?
https://www.flying-pig.co.uk/mechanisms/pages/irregular.html

 

[ed jr]

robotobjects.com?

I ordered gears from them December 1, 2007 and have not recieved any corespondance from them. They have not replied to my emails and thier phone message system is full. beware

ed jr

 

[Hank Fletcher]

I ordered gears from them December 1, 2007 and have not recieved any corespondance from them. They have not replied to my emails and thier phone message system is full. beware

ed jr

As irrelevant as your post may be to this thread (maybe you meant to start a new thread?), I kind of appreciate your warning, since I'd actually been playing with the idea of ordering some gears from there myself.

I see there's a "we've been really busy so expect delays" message on their homepage. Does anyone else know where to get their spur gear products, or are they something they manufacture themselves?

 

[Speakerguy]

So a gear motor will vary its speed with the applied voltage?

Can I PWM a gear motor to change the speed with a single voltage rail?

 

[jpanhalt]

Does anyone else know where to get their spur gear products, or are they something they manufacture themselves?

Try these sources:
https://sdp-si.com/catalogs.htm
https://www.qtcgears.com/

https://www.bostongear.com/

Boston Gear is probably too industrial for your needs. For the SDP site, you need to scroll about 2/3 down the page. They have metric and inch gears. I've had good experience with them, but a little expensive. Small Parts is an old, established company. It's a bit of a niche company and also expensive. I have also used QTC with satisfactory results. John

John

 

[Hank Fletcher]

So a gear motor will vary its speed with the applied voltage?

Can I PWM a gear motor to change the speed with a single voltage rail?

Yes, and yes again!

Try these sources:
https://sdp-si.com/catalogs.htm
https://www.qtcgears.com/

https://www.bostongear.com/

Thanks, John, I'll check 'em out!

 

[Marks256]

a windshield wiper motor out of a car would give you a pretty good start to make a moving arm...

If you can't get a hold of one, i would be happy to get you one.