Sunday, April 14, 2024

12 VDC Prop Pitch Motor

The aviation electrical power standard has been 24 VDC for a very long time. Since this also holds for US military aircraft, prop pitch motors require a 24 VDC power source even though their design dates all the way back to WW II. It turns out, much to my surprise, that there exist 12 VDC prop pitch motors. I first learned of their existence several years ago when a friend purchased one at a flea market. 

They seem to be quite rare. It is difficult to identify them from the outside. At the very least it is necessary to remove the motor cover and read the print on the motor. You don't even need to do that since the external appearance is quite different. I had occasion recently to become more familiar with these motors when the motor developed a fault and I offered to inspect and hopefully repair my friend's motor. The loss of any rotator is an inconvenience. Luckily he has enough antennas that the temporary loss could be tolerated.

Those of you with an interest in prop pitch motors and, like me, have never seen a 12 volt motor, this tear down and repair should be welcome. I had no information about them and I could not find any. All I had from a friend was confirmation that they exist. He was happy to receive the pictures I sent him since he had none in his files.

Separating the motor and gearbox (reduction drive)

In my workshop I carefully began disassembly. Although I have experience working on prop pitch motors, this one was a novelty. There was minor damage on the outside due to mishandling in the distant past. I filed down metal spurs on the motor body and motor retaining nut. The method for mounting the after-market rotating reed switch magnet was poor and did some damage to the exterior of the top motor bearing. I put that aside while I worked on the motor.

Pulling the motor off the gearbox was more difficult than I expected. It uses the same large threaded nut that is found on many of the small size prop pitch motors. I was surprised that the motor did not come free when the nut was removed. I carefully pried up the motor with a large gear puller to discover the reason for the resistance. 

It turns out that there are no electrical contact pins on the motor and the drive side of the gearbox. The motor wires are directly threaded through holes in the gearbox housing. Once I realized that, I removed the connectors crimped onto the wires and pushed the bare wires through the holes while pulling up on the motor. The motor and gearbox were finally separated. 

It is a good idea to label the wires at this point so that they are correctly placed for reassembly. I had to puzzle it out during reassembly since I forgot to do so. Luckily the wire arrangement is the same as the 24 volt models. This one is a right hand motor.

Note: After K7NV passed, his web site full of prop pitch motor information went offline. I have an archive as do many others, but at the time of writing there are no reliable links to point you to due to copyright and other issues. In any case, he had nothing on the 12 volt motors. I will not publish his wiring diagrams in this article. Hopefully at a later date there will be a permanently accessible archive of his material.

The next surprise was that the drive side of the motor axle was loose. That is, there is only one bearing, and that is located at the top of the motor axle. The motor cannot be spun unless it is attached to the gearbox. I thought that was very odd. On the other hand, I suppose there's some benefit in having one less bearing to deal with!

Unlike the splines on the more common 24 volt motors, the coupling is done with a blade. There is a matching receptacle for the blade on the gearbox shaft. The fit is precise so that the axle doesn't wobble. I have heard that this alternative appears on some 24 volt motors but I have never seen one.


At this point I checked the gearbox for freedom of movement. After leaving it outside during a cold spell (it was too large for my usual freezer test!) there was evidence of a poor grease choice. I didn't open the gearbox but the owner told me that he'd previously opened and lubricated what he could access. Not all parts are accessible without disassembling the planetary gears. 

I didn't open the gearbox since there was no evidence of a problem other than perhaps a poor choice of grease. I set it aside for when the motor was ready to be reassembled. Although I was curious about the design of the gearbox, that wasn't a good enough reason to open it for an inspection. Perhaps another time.

This is a picture of the drive side of the gearbox. Notice the lack of provision for electrical contacts, just the holes through which the wires are threaded. In the usual design the contact receptacles slot into holes and are held there with retaining rings. With this motor, care is needed to avoid tugging the wires and abrading the insulation during assembly and reassembly.

Since the contacts, where they exist, are part of the gearbox housing, I wasn't surprised to see that the part number was different. Other than that the housing looks the same as for the 24 volt design. The motor base is the same, and there is the same key in the gearbox housing to secure the motor position when the motor is mounted. The motor axle has to be rotated during assembly until the blade aligns with the slot on the gearbox axle.

Note the key at the bottom. It has a mating notch on the flange at the base of the motor. Its importance during reassembly will become apparent towards the end of this article.

Inside the motor

With just one bearing, it was easy to knock out the axle and armature assembly. With an appropriately sized tube, the bearing was then knocked out from the inside. This was done carefully to avoid damaging the bearing. I should say, damage the bearing further, since I already suspected that it was damaged. 

The two pictures of the armature show the axle and commutator. I later cleaned the carbon from the commutator and saw that it was in good condition. The bearing seats on the shaft up against the top washer. When disassembling prop pitch motors it is critical to take note of where the washers and shims came from so that they are put back in the same place during reassembly. A mistake can damage the motor when it is run.

In the 12 volt motor the brushes are at the top of the motor; they are at the bottom (drive side) in the 24 volt motors. You can see how the wires are routed to the field coils and brushes. The brushes appeared to be in good condition so I cleaned the interior as well as I could and set it aside. I filed down metal spurs on the exterior that may have been caused by rough handling in the past. That was done mostly for aesthetics and to avoid skin damage during handling. It also made it easier to slip the motor cover on and off.

Motor bearing

My attention next focused on the motor bearing. The symptom when it was on the tower and not turning was excess resistance to manual rotation of the motor axle. An application of modest force freed the axle and the motor worked again. When it happened again my friend brought it to the ground. This is not the first time I've dealt with bearing trouble in a prop pitch motor so I proceeded with the confidence of experience.

Since there was no sign of mechanical scraping or other damage on the armature or field coils the problem had to be in the motor bearing or the gearbox. A bearing or gear failure in the gearbox typically doesn't result in locking the motor axle. The reason is the high reduction ratio. It usually takes several rotations of the gearbox axle to take up play in the many gears before it locks up. Since the gearbox axle turned smoothly, even after the low temperature test mention earlier, the bearing was the primary suspect.

Turning the bearing by hand was smooth. At least it was at first. Eventually I noticed an intermittent roughness. I tossed the bearing in the freezer. When I retrieved it an hour later, he imperfection was pronounced. There was also a small amount of play between the inner and outer sections that indicated worn balls.

I found a compatible modern bearing for the "201" shielded bearing by perusing my catalogues. It is the same as the top bearing for the 24 volt motor. The old 201 has shallow concave races that do not support axial loads; that is, it works best as a thrust bearing. The 6201 double sealed replacement is a deep groove bearing that is suitable for high axial and radial loads at speeds greater than that of the motor. I ordered two so that I can replace the ancient top bearing on one of my 24 volt motors.

You can see the difference between them in the picture above. The larger surface of the inner section is handy for firmly securing the aluminum arm that contains the magnet for a reed switch. My friend has a Green Heron controller for his other prop pitch motor that supports this arrangement, but for direction indication with this motor he uses a 4O3A compass on the antenna. 

I use 10:1 potentiometers for my chain drive and upside down prop pitch motors. There are many ways to accomplish direction indication for prop pitch motor rotators. There are both commercial and home brew solutions.


Installation of the bearing highlighted a curious aspect of the design. Since it is installed from the top there is no resistance to axial force pushes it upward. Normally this shouldn't be a problem since the bearing should only experience radial loads. 

The method by which the direction indicator bar and magnet (parts visible at the lower right) were mounted on the axle prevented vertical migration of the bearing. In the 24 volt motor, both motor bearings are pressed in from the inside so they cannot move. 

After pressing in the bearing I pushed the axle and the attached armature into it from the open bottom end of the motor housing, taking care to have it seated against the axle washers. There is a flange on the axle that seats the washers and bearing in their correct positions. 

I had difficulty aligning the blade on the motor with the socket on the gearbox. It isn't as easy as one might expect because there are two pairs of items to align: the axle blade and socket, and the key on the gearbox body with the notch in the motor flange. What makes the task particularly difficult is the near press fit of the motor into the gearbox housing and lining up those two mechanisms must be done blind; all are invisible when the motor is pressed into the gearbox housing. Another complication is that the wires have to be inserted through the gearbox holes during this procedure, and they can't be twisted far while pushing on the motor and axle into their respective slots. Needle nose pliers come in handy.

One solution to the alignment problem is to mark the motor and gearbox and lining them up during assembly. That might work if the motor axle is simultaneously oriented correctly. After fussing with it for 10 minutes without success, I tried another method. I removed the armature and axle assembly, seated it on gearbox axle socket and then pushed on the motor housing. I did it with an aluminum tube that fit over the axle and rested on the bearing surface (not the rubber seal!) and tapped it downwith a rubber mallet. When the motor flange struck the key it was easy to slightly rotate the motor until the key fit into the slot since the axle blade and socket were already engaged.

In retrospect, it would be easier to put the bearing aside while the armature and motor housing are installed. That avoids dealing with the press fit of the bearing while simultaneously aligning the axle and key. The bearing can then be pressed in over the axle into the motor housing.


The P and Q wires are to the right in the adjacent top view. One of those is common and other is not used. Which it is depends on whether the motor is right-handed or left-handed. The R and S wires are close together on the left side. One is for CW (clockwise) and the other for CCW (counter-clockwise) rotation. Only the slots for the R and S wires are imprinted on the gearbox housing.

My initial tests with Astron 13.8 VDC linear power supplies failed. The crowbar protection circuits of the 10 A and the 25 A supplies shut down the power due to the high starting current. The protection circuit acts too quickly to permit the motor time to start. I inserted a high power resister of a few ohms in series with one lead but that dropped the voltage too much to operate the motor. It wouldn't turn at all.

I tried the same setup with a 24 VDC power supply and the same thing occurred. I dispensed with the resistor and the motor came to life. I didn't leave it running for long since the higher voltage could stress the motor. DC motors can often run quite well at lower and higher voltages than specified, provided the motor will start (low voltage) and not run too hot and fast (high voltage). Hams in decades past used this "feature" to vary prop pitch motor speed with a 120 VAC auto-transformer (e.g. Variac) on the primary side of the power transformer

My small multi-meter didn't fare well on its 10 A scale during these tests. Something sparked but it still seems to work afterward. They're cheap to replace so I was not too concerned. As you can see on the meter, the clip leads themselves lower the voltage at the motor from the approximately 26 volts measured at the power supply. The wires get quite warm, more than on the 24 volt motors I've tested. That makes sense since P = EI and the power consumption of the 12 and 24 volt motors should be similar.

My friend uses narrow gauge wire up the tower to lower the voltage from his 24 VDC power supply. It also cheaper than larger conductors! I know that he measured the current as 7.5 A, but the voltage at the top of the tower is unknown and probably has never been measured.

Although my friend doesn't need it, I plan to remount the aluminum bar and magnet on the motor axle. I prefer to have it there for two reasons. It can be used as a handy lever to test freedom of motion of the motor and gearbox, which is how my friend used it to discover the intermittent bearing problem. The other reason is for insurance against the bearing working loose. Although the risk is low, it is easy to prevent. I will change the hardware since the lock washers previously used put uneven stress on the bearing. The bearing is strong but that is no reason to take an unnecessary risk.


The motor will likely be reinstalled on my friend's tower later in the spring. I'm hoping that it will now work well for him. He had pretty much given up on the motor before I offered to work on it.

I hope you enjoyed this tour of a rare variety of prop pitch motor. I doubt that I'll ever see another like it. If you come across one, you now have an idea of what to expect.

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