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Posted: Thursday, July 12th, 2007 at 11:51 am Author: Chris Turner

Overview

You may have noticed quite a few robots on our website use what is called an OTU, or Oven Timer Unit based gearmotor. By popular request, we have put the following information together for those of you who have an OTU on hand and want to convert it into a gearmotor. But be fore-warned, we don’t have them for sale! That said, let’s begin…

Introduction

Now, why would a person in their right mind want to find information about Oven Timer Units (OTUs), especially on a robotics website? Well, if you can find the right type, they make incredibly nice gearmotors at a very reasonable price. Have you inspected a Micro-Mo or Portescap catalogue lately? Small efficient gearmotors are easily in the US Dollar range - yowch! For the amount of money it takes to rip the OTU out of a dead oven or dishwasher in a back-alley, you can construct your own gearmotor with comparable power and efficiency.

Background

The original implementations of the Oven Timer Unit gearmotors were explored by (who else?) Mark W. Tilden of the Los Alamos National Laboratories. But this was back when he was still in Canada working at the University of Waterloo. He had found a large stack of trays in the back of a surplus store containing shaded-pole induction motors. These obviously were meant as replacement units for the ones found in the analogue (dial-type) timers found in many major house-hold appliances like ovens, dishwashers, and dryers. The truly neat things about these motors was that the gearsets were sealed in a tin container in the shape of 3 life-savers stacked on an Oreo cookie. This container was press-fit into a square iron frame which also supported a large coil of wire on the other side. When the container is peeled apart, magnetic wheels are suspended directly in the magnetic field generated by coil and magnet frame which in turn spins the gearing in the “Oreo” side of the gearset. The truly neat thing about these gear mechanisms were that they were mostly plastic gears friction-fit onto 1mm shafts riding in a brass housing.

The Pager Motor

One of the favourite “motivators” in BEAM has been (and will always be) the Pager Motor. They’re referred to as Pager Motors because that’s been their central purpose in life, as the motor that spins the eccentric weight which makes a pager vibrate to let you know you have a message waiting. Being manufactured mostly in Asia, most pager motors come with a 1mm shaft, which coincidentally is exactly the size of the shafts in our friend the OTU. Once you remove the eccentric weight, you can then pull out the shaft of the gear you want to drive, and exchange it with the shaft of the pager. This gives you the ability to use the same OTU in several different settings, being low-speed/high-torque to the other extreme of high-speed/low torque. This combination of OTU and pager motor has proven to be a dynamite configuration for making BEAMbots of varying behaviors.

Instructions

	This is what the OTU looks like when pulled out of an oven. You should find it in the middle of the mechanics driving the clock face mechanism. It’s held in place by two bolts which go through the iron core frame (you can see one of the holes on the lower left side of the core in this image).
	This is the backside, which shows the mounting holes more clearly. Also, you can see the tangs on the coil to where the 60 Hz AC power is attached. The copper squares are the “shading” aspect, which make the gears turn in only one direction.
	As you can see, you can easily pop the gearworks out of the frame. It’s just a friction fit, so just twist it back and forth as you work it out.
	Once the gearworks is free, you can pop the plastic wheel off and peel off the outer tin skin. BE CAREFUL doing this. The tin edges are sharp, and severe cuts can be inflicted (trust me). And to quote Richard Weait, who wrote a nice little synopsis of OTU construction:This also liberates some oil; mop it up. Mop up the blood from the finger you sliced open on the sharp edge of the aluminum, too.
	This is the gearworks totally separated from the tin can. You can see some of the lubricating oil that was inside the can pooling up in the corner.
	This is what the backside of the gearworks looks like after you’ve pulled it free. The circles are actually the magnetic wheels that are spun to make the gearworks move. It will take 3,600 rotations of this shaft to make the output shaft turn one rotation. As you’ll see, there’s about a 5:1 gear reduction as you work through the stages, so the next gear in line after this one will need only about 720 turns to make the output shaft turn once. Again, quoting from Richard:If everything is debris-free you should be able to turn the output shaft and make all the gears turn. “Spin gears, spin. Wheeee!”
	Pull out the center shaft, and a small plastic spur gear will fall out. KEEP IT. You can use it (as well as any of the other gears you’ll be removing) as a drive gear when you install the pager motor. You notice that the shaft is 1mm - the same size as the pager motor’s shaft. At this point, pry the tin backing off the gearworks, as you’ll be needing access to the gear shafts and brass body (for soldering to). Leave the tin shell on the output side, as it keeps the gears in better alignment. Richard says:Now if you have steady hands, and haven’t been pounding-back mugs of coffee, you can put the pager shaft into the #6 hole, press the pinion on the pager, and hold the pager in place. This gives you 3600:1; lots of torque. Power it up with a single cell (1.5Vdc) Pretty cool, eh?
	You can see the 1mm shafts poking out of the brass here. Now’s the time to select what kind of gear reduction you want to use; i.e.: what’s the application. For high-torque applications, you’ll want the original gear input or the next one in. One of our personal favourites is (counting from the output gear being #1) to drive the 4th gear. It’s a nice compromise of speed vs. torque, giving approximately a 1:125 ratio (555 - 1:5 staging, times three driven gears).
	Pull the pin out of the selected gear with a pair of needle-nose pliers or flush pliers. You may find it easier if you use a small screwdriver to push the gear as far up the shaft as you can, so when you push it down, more of the shaft comes out of the hole.
	Slide the pager motor into the same hole as the original shaft, and slip the original spur gear (remember, I asked you to save that one at the beginning, right?) onto the pager shaft. You should be able to get the gears to mesh pretty easily at this point. Some people may want to drill this hole out to make adjustment easier, but you will have to remove the next gear in the chain, or you may accidentally gouge it when drilling through.
	Slide the pager motor into the same hole as the original shaft, and slip the original spur gear (remember, I asked you to save that one at the beginning, right?) onto the pager shaft. You should be able to get the gears to mesh pretty easily at this point. Some people may want to drill this hole out to make adjustment easier, but you will have to remove the next gear in the chain, or you may accidentally gouge it when drilling through.
	Now here’s the trick - tweaking the alignment so that the pager motor draws as little current as possible. Do this by hooking the motor up to a power supply and run it through a current meter. Gently bend and position the motor (while it’s running) until you get the lowest possible current draw. We’ve built units that draw as little as 22mA. Once you get to your minimum, solder in another bridge on the other side of the motor, then reinforce the original bridge with more solder…so it looks like the next image.
	This is what you’re working for - a complete gear system for much cheaper than what you’d pay Escap for. Any question? Good - we’ll see you at the building table!

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