High torque gearmotor for upcoming project

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Spent some time prepping for a new project, and realized I haven't seen the techniques I use shown, so I figured it is as good a time as any. There are likely people that could use the information, and I have a cat on my lap, so I have time.

 

The project I am working on (to be disclosed if I get it done to m satisfaction) requires a pretty high torque, fairly fast drive. The requirement is to move about 10mm in 100mS, position withing 0.1mm, with a force of about 50N (3/8" motion to about 0.004", with a force of about 10Lbs, for those that are trapped in the world of imperial units). Not earth shaking, but beyond the cheap hobby servo.

 

The cheapest, and one of the most convenient, moderate torque drive available is the battery screwdriver. Not the big gun shaped job, but the homeowner type. The good one are about $20US new, but I usually pay $0.50 to $1.00 at yard sales and swap meets. There are millions of them. Most with little to no use. People buy them for themselves or as gifts without realizing that this little bugger in not the same as the $150 Dewalt. After they sit in the drawer unused for a few months or years, I get them cheap. They have been around for at least 30 years, with 1/4" hex drive, planetary reduction, 180RPM nominal, and design voltages of 2.4 to 4.8V.

 

The one I dissected this time tests out at about 250Ncm at 1V, with a no load speed of about 70RPM. The design voltage is 3.6V, and it will probably stall at close to 1000Ncm (10Nm) at that. If more is needed, then I can up the voltage for short bursts. I doubt that will be the case, as at 1V, I have much more torque than I need using a 15mm arm. For the output shafts and actuator levers, I use 1/4" allen wrenches, also yard sale grabs, preferably unhardened, so the cheapest sub-harbour-freight quality is fine. If they don't stay in place themselves,a little epoxy or gel CA does it.

 

For this project, the first thing I did after ditching the battery end (should have been second) was mount screw terminals. There are no electronics. The switch presses contacts directly on the motor terminals, and the battery was mounted in the swivel handle. All gone, and nice big holes (12mm dia) through the body for mounting and reaction against torque.

 

 

The first and last show the parts I used and the installed terminal. Clean and fairly neat. Last shows a sampling of the drivers in my junk box. All are black and Decker. Models are LI2000 (3.6V LIon, 330g as it sits), which is a current model,. I use this one for its intended purpose.  9078 (the one being remodeled here), which is 380g as it sits without battery or handle. Heavy. Metal gearcase. Next is a 9071, about 15 years old. 2.4V and 440g. Last is a 9018,, maybe made in 1985 or so. 2.4V and 250g. Only the first cost over $1

 

Edit: fixed image

[enl 227]

Here is why the rebuild was needed: there is a brake built in to keep the outshaft from backdriving the motor. Problem is, once engaged, the motor can't turn. The outshaft needs to go to zero torque and slightly reverse to release it. It has to go to use the drive reliably in any project.

 

IMPORTANT for ANY device with a torque limit clutch:

Before dis assembly SET THE CLUTCH TO THE LOWEST SETTING so spring pressure is released, or you will be hunting pins and springs later

 

 

The first pic shown the output shaft half of the output coupling. The six (three pairs) of jam rollers are actuated by the not quite flat faces, and are wedged against the inside of fixed ring. Much like an overrunning cluch. The three pins sticking up (at 12 oclock, 4 oclock, and 8 oclock) are the torque clutch pins. The springs are under the pins and sit on cam plates set by the torque limit control. They are what the ring gear reacts against, so if any are missing, the drive will not work.

 

Next two are the other half of the coupling, which is also the planet carrier for the second reduction. The slots clear the rollers, and the keys fit in the slots on the other half.

 

Next the jam rollers removed

 

Then the second stage carrier/half coupling installed, and the ring gear (stator for both reductions, as well as the key piece of the torque limit clutch) ready to go in next. The six bumps are the clutch. At any time, three of them are against the three spring pins. Overtorque causes them to push the pins down so the ring can rotate one sixth of a turn.

[enl 227]

Next is putting in the second stage planet gears, first stage carrier/second stage sun, and first stage gears. Liberal use of good grease is recommended. I used lithium, but any plastic and aluminum safe NLGI 1 grease is fine.

 

Last is put the pinion back on the motor, slip it in, and reinstall the spring pins.

 

Then the drive is done. I reinstalled the original case parts so I won't need to make a special mount

 

Dis-assembly is the reverse of assembly. Tweezers and needle nose pliers are key. Wipe parts and keep them clean.

 

This unit has a net reduction of 81: two 9:1 stages... 6 tooth sun and 48 tooth ring. The planet carriers float, making tolerances less critical, and the planet gears are reduced tooth count: they have 19 teeth, should nominally be 21. Doesn't effect the ratio, and is probably to make assembly easier as they are a little smaller. Grease can move around a bit more easily with the extra clearance, and 19 is prime, so the teeth share load. BandD probably did this solely to make assembly cheaper.

 

 

Experience tells me that this should last pretty much forever in intermittent use, if not driven at stall loads, and as a drive for a wheeled robot, several hundred hours.

 

But that is for another time.