Diagnosing Prop Pitch Motor Trouble

Last winter I mentioned that the prop pitch motor for the upper yagis of the 15 and 20 meter stacks would not turn when the temperature was below 0° C. That this is the freezing point of water was suggestive but not definitive. With an upside side down prop pitch motor there is a risk of water infiltration through the oil seals. Despite the care I took to block water from getting into the hub there remained a risk. Now that warm weather has returned it is time to investigate.

The motor was previously repaired and thoroughly cleaned when a bearing malfunctioned. Whoever had refurbished the motor for rotator use in the distant past used a combination of open and shielded bearings. There is no reason to do this unless absolutely necessary (e.g. RPM rating). The default choice should be fully (double) sealed bearings. They cost only a little more and they are far more reliable in a dirty environment and in our climate with extremes of temperature and humidity.

I suspected the same inappropriate choice in the gearbox, but at the time I was loathe to undertake the more difficult disassembly and reassembly of the gearbox. As with most equipment, the former is always easier than the latter and many have run into trouble putting them back together. There is ample if incomplete information to be found online, and the variety of motor designs can make every rebuild unique to the hobbyist who may only ever see just one of these units.

The time has come to dive in. The motor had been greatly modified by an unknown ham long ago, as they must be, from its virgin condition. From an exterior inspection I could see that the brake and its wiring had been removed (very good), the wires to the motor contacts redone (good), and the oil seals replaced (very good). The rest was unknown.

Before beginning disassembly I read all that I could about reassembly. I did not want to find myself with a repaired motor that I could not reassemble! Satisfied with what I learned I proceeded.

In this article I will describe what I discovered inside and how I went about diagnosing the motor's performance troubles. The rebuild and reassembly are yet to be done. 

I am no expert on these motors so this and subsequent articles are not intended as an instruction guide. All I can hope is that my limited experience may help those willing to get their hands dirty. I have not found a definitive guide, just a diversity of incomplete and disjointed articles. Perhaps what I have to say can fill in a few of the gaps that puzzle others.

Removing the motor from the tower

The previous times I removed the motor I left the mast free spinning. I cannot easily do that now that the yagis are connected. The coax rotation loops would have to be removed or they'd be torn apart. The simple means I previously used to keep the mast from spinning failed: rope to tie the 20 meter yagi boom to the tower shredded after several days; and, a thin wall steel angle bracket from the mast to a tower leg fatigued and broke. The continual abuse of wind torque from large HF yagis should not be underestimated.

I designed a more substantial grip this time around. After two weeks it is continuing to perform very well. A ¼" steel plate is attached to the boom with ½" u-bolts and a 1.9" aluminum pipe ties the plate and the tower. Steel chain supplemented by steel wire, a rope and shackles tie the pipe to a tower leg.

The cradle I originally built to support the motor under the plate during installation and removal procedures was left in place last year. All the steel is galvanized so it can handle the weather. This is easier than carrying it up the tower every time it's needed. 

My helper that day was surprised that I didn't lose so much as a washer when removing the motor. That's unusual! Knowing my track record, he wore a hard hat.

My improvised water seal passed a quick visual inspection. I revised that opinion when I discovered that the motor exhibited obvious signs of rust and water fouling of grease inside the crown gear hub.

How much water got past the oil seals and into the gearbox could not be determined until it was opened. I hoped for the best.

Opening it up

The first step to disassemble the gearbox is to remove the 9 bolts that join the hub with the bell gear, the ring gear and the planetary gear assembly. In an original, unmodified motor the 9 nuts are wired to the bolts to prevent movement in the high vibration environment of an aircraft engine. Ordinary grade 5 ⅜" bolts and nylocs were all I had to deal with (see lower right corner below).

In this picture you can see the separated halves. Notice the excess grease and the multitude of water beads on the grease. Rust stains were visible in multiple locations, but all were easy to wipe off. The base metal was unaffected. The accumulated rusty and oily water was poured out.

The ring gear flange between them has gaskets on each side so it may need to be pried loose. Repair the gasket if necessary. The ring gear and gaskets looked good so I put them aside.

It is worth noting, should you ever get confused during reassembly, the larger gear of each of the 3 low-speed planetary gears engages the ring gear and the smaller gear engages the bell gear. There is an alignment procedure to reassemble the gears, which I'll defer to a future article about reassembly.

In an unmodified prop pitch motor it is necessary to remove a variety of electrical and mechanical parts in the hub assembly. Since it can be difficult, I was happy that I didn't have to deal with that. Removing the bell gear and hub from the housing was easily accomplished with a mallet firmly applied to the hub with blocks of wood supporting the housing flange. The rubber oils seals have a firm grip on the hub. That grip makes it quite difficult to turn the bell gear by hand so don't be concerned by that.

The housing, hub and bell gear were cleaned of grease and most rust stains. In the picture the outer seal has been pulled from the housing. It looks and feels like an original. The inner seal is visible inside the hub well. It is clearly not the original. Since it's in good condition and is difficult to remove I left it alone. I may need to replace the outer seal.

I had to go back up the tower to retrieve the drive shaft to investigate how water got past the seal that I made. There's a picture of how I designed the seal in the original article. An inspection revealed no mechanical deterioration since it was installed. Water got past it anyway and I needed to know how.

I repeatedly poured water down the top end of the pipe to see where it went. It all exited from the side drain holes and down the bevel gear splines. That's as it should be. No water made it past the welded barrier inside the bevel gear's inner cavity.

My suspicion is that the water that flowed off the splines and onto the plastic collar would, at least some of the time, travel inward rather than outward. Wind may be a factor. There is some evidence of debris and rust stains on the hub's crown gear that is consistent with the possibility. 

I will look into a better way to spill the water off the splines to ensure water can't flow inward towards the hub. I will also cap the top end of the pipe so that less water can follow this route. There is more design work to be done but it's a solvable problem.

Inspection and initial cleaning

When it was refurbished, an extraordinary quantity of grease was packed into the gearbox. You can see some of that excess in a picture above. That is never necessary. A light coating on moving metal is all that's needed. The rest is wasted, and I wasted time and material removing the excess. All the rust found inside was residue when the rusty water that got in evaporated. The base metal in a prop pitch motor is highly resistant to rust. The stains were easy to remove.

Staring at the gearbox is not a good way to diagnose problems. Neither is moving the parts by hand. About all I got out of it was greasy fingers. I continued the work wearing latex gloves. At this stage I did not use solvent since the bearings are all open and removing grease from critical areas would reduce mobility and mask the underlying problems. I learned to live with the greasy mess.

Deep freeze

The weather is warm and the problems only manifest at freezing temperatures. Every home, including mine, has a suitable test facility: the kitchen freezer. With the major components separated, I wrapped each one in a plastic bag and left them to cool overnight in the freezer. I suggest you check with your family first before doing this to prevent domestic conflict!

The components sent into the deep freeze were the motor, high-speed planetary gears and low-speed planetary gears. The motor was first disassembled to check for water damage. Happily there was none other than rust deposits on the sealed bearing closest to the gearbox. The motor performed perfectly after being in the freezer overnight.

Before separating the planetary gear assemblies I cooled it in the freezer as a single unit. The next morning it was not possible to manually turn the main spindle. The problem was definitely in the gearbox, as I had suspected.

The planetary gears were separated by removing the pinned nut from the spindle (mentioned above). I used trick I found in an article. I loosened the nut by restraining the shaft with a long screwdriver inserted though an opening in the low-speed planetary gear housing and into the low speed planetary gear. This is easier than making a special tool to grip the other (drive) end of the shaft.

Problems were found in both planetary gear assemblies. In the picture, the low speed unit is on the left and the high speed unit is on the right.  The labels show what I found. Notice how frost formed on the bare surfaces of the cold metal. Even with perfect water protection there will inevitably be some water inside due to condensation.

I concluded that the planetary gears need to be fully disassembled for cleaning and repair.

Repair plan

As I write this, I have removed a few of the suspect bearings. As expected, several of them are rough or fully seized. The only reason they turned at all was because the high motor torque made them spin on the supporting pinions. The well lubricated pinions appear to have survived the abuse.

Whether good or not, I will remove and replace all the bearings with sealed units of suitable specifications. The bearings on the shaft and the high-speed planetary gears are the most critical selections since they spin as fast as 9,500 RPM.

Suitable grease for the shaft, pinions and gears must be found. The grease must adhere well, have a high temperature range and (for the high-speed planetary gears) perform well at high speeds. More than one grease may be appropriate depending on where in the gearbox it is used. There is no shortage of available products and I just need to conduct a few hours of research.

Grease, water and rust residue must be removed from all parts. A pail of solvent should suffice for most of the parts. Once cleaned, grease can be applied and the new bearings installed. I'll come up with a better waterproofing design and then test it rather than leave it to chance, the way I did last time. I may need a garden hose to simulate wind driven rain.

All of this work will take time, and it is unlikely to be completed until June. This time of year that is not a problem since there are few contests of interest to me and I am more focussed on 6 meter DXing and not HF. I have enough flexibility with the antennas on 15 and 20 meters to satisfy my short term operating needs without being able to turn the upper yagis of the stacks.

Expect another article when I'm done with the repair and reassembly.