Prop Pitch Antenna Rotators
My main 30 m tower supports a KLM KT34XA tri-bander and an M² 3el 40 M antenna that was rotated by a Hy-Gain T2X Tailtwister rotator. I soon had to buy a spare to replace it when it went faulty. The casing bolts would rattle loose due to wind vibration but I solved that with "Locktite" and double nuts the gears still had their teeth go missing. The constant rotator replacement and the nagging for someone to come and do it for me, made me think of a heavy-duty Italian ProSistel rotator. Bernie ZS4TX warned me that two of them he knew of, had suffered cracked worm wheels and steered me in the direction of an aircraft prop pitch motor (PPM). I obtained one that I modified and paired up with a Green Heron controller and both have been performing faultlessly over the past three years.
After I consulted with professor Google about PPM I quickly came to realize that this is not a Mickey Mouse machine intended to swing your 2 m beam. The PPM possesses tremendous turning torque and braking power and is used to rotate the largest beams, including stacks, available on the amateur market, entire towers, and even moon bounce arrays.
Google will also steer you to K7NV.com. Kurt has assembled a wealth of information, excellent photographs, and diagrams on his site and I would consider it the “bible” of prop pitch motors (PPMs). Unfortunately, the site is no longer available as Kurt became a silent key in August 2022.
The fact that I live in South Africa and was removed from Kurt by 17,000 km, forced me to embark on my own adventure with PPMs. First my own, and then later on with several more that followed when it became known that I possessed the skills and tooling to rebuild them.
A complete rebuild is the best way to go as this will guarantee that your PPM will give you years of trouble-free service. It is no small matter to replace this beast on a 30 m tower.
I replaced the input and output seals with double-lip metric seals and as they are slightly larger in diameter both housings had to be machined. These double lip seals are just what we are looking for as we are now able to spread some grease in between the lips so that they do not run dry on the shafts. The large output shaft seal must be installed so that the lips face outwards as we want to prevent any possible ingress of water.
The bearings are freely available in 2RS format (double rubber seals) with the exception of the six bearings in the secondary planetary assembly and the two identical bearings on the output shaft and bevel gear. Do not despair even if these six bearings mentioned show signs of rust and are pitted, as they turn slowly. Kurt K6NV bought up all available stock of the two special large thin-wall bearings but understandably will not part with them unless he does the actual rebuild of the unit. Do not skimp on the quality of the 608-2RS, 6200-2RS, and 6201-2RS bearings as some of these motor and gearbox bearings spin at 10,000 rpm.
I had to devise my own solution that is well worth the extra effort. The end product is far superior to that if both original bearings had been used. All we have to do is salvage one of these bearings, and if worse comes to worst, you can always machine a bush from an engineering plastic such as “Vesconite” that hardly requires lubrication. The only purpose of this bearing in a vertically mounted PPM is to keep the output shaft, which turns at only 1 rpm, centered with hardly any lateral forces to take into account. I then used a 6817-2RS bearing, measuring 85 x 110 x 13 mm, that required both the bevel gear and the adapter plate to be modified.
Originally the PPM is lubricated with oil which in the case of a vertically mounted PPM would cause the top section to run dry. Grease is a much better option as it will also not leak past the input shaft seal. I used Mercury Marine’s gearbox lubricant which is water-resistant. Do not slather it all over the place - just apply sparingly to all the seal lips, shafts, gears, and the six un-sealed bearings in the low-speed planetary assembly.
Do it my way!
What follows is what I did and what you have to do if you also want to be the owner of the best rotator that money can buy - that is to say if you can find one.
Dismantling the PPM:
- Remove the aluminum dome cover on the motor and unscrew the castellated sleeve nut using a small blunt chisel. Remove the motor.
- Carefully remove the power jacks, brushes, and insulators from the input housing and retain them for later use.
- Remove the bevel gear. There might still be a snap ring on the inside retaining it to the output tube.
- If the bevel gear is not present remove protrusions welded to the splined shaft of the large bell gear.
- Remove the large adapter plate.
- The mechanical limit stops, if still there, now have to be removed and discarded. The cam is secured by three screws that can be accessed through the holes on the side just below the mounting flange. If stuck you can always drill them out.
- Split the housings after removing the bolts that secured them. Remove the gear assemblies but do not attempt to disassemble any of them as we will only do this after a thorough cleaning of all the components.
- Depending on the lubricant used and the amount thereof it might be a good idea to start the cleaning process right now.
- Remove and discard the three semi-circular stainless steel covers.
- Remove the limit stop lever and the brass spring-loaded contacts protruding from the output housing flange. The contacts are secured with tiny brass screws accessible from the side openings. Remove and discard all Bakelite insulating material found in both housings.
The two special narrow output bearings might have come apart in the disassembly process and even if the ball bearings are scattered all over the place it is no cause for alarm as they are available from your bearing supplier. Remove the bearings, or their components, from the housing and from both the output shaft and bevel gear. Pick the best one and if rusted use a mild rust remover to clean it. Take note that the bearings have a thrust side. If the bearing has come apart make use of this opportunity to assemble it with new ball bearings. They have a diameter of 3/16” and number 63??
- Remove and discard all oil seals.
- Make sure that the output housing is completely gutted and scrupulously clean so as not to hinder the welding process.
- Get a specialist welding shop to TIG weld up all the side apertures. The welding should afterward be dressed for a neat appearance.
- For a really neat appearance have the outside of housings bead-blasted – not sandblasted as that roughens the surface. Alternately brighten them by dipping them in a propriety aluminum cleaner, brush well, and rinse thoroughly to remove all traces of this aggressive cleaner.
- Close all openings on the mounting flange except for the six bolt holes and rebuild the indentations left by the limit stop lever with a "Prattley" compound to create a smooth mounting surface.
- Machine the housings to suit the new seals. The recess for the top seal must be deepened as the new seal is slightly thicker.
- The mounting flange also requires a 117 mm diameter X ?? mm deep recess to accept the now modified adapter plate if the new larger bearing is to be used.
Power jacks
We are only going to use three of them; remove the wires attached to the brass terminals and replace them with suitable lengths of black and red silicon wire that will be terminated in a waterproof cabinet mounted to the tower. Their position is shown in the pdf document.
Bevel Gear modification with original bearing:
The bevel gear may have had various protrusions welded to it that have to be removed. We are going to fit a modified Fenner Taper Lock WH-20 weld-on hub. The taper bushes that mate with the hub are available in different imperial and metric sizes to suit various shafts.
- Machine the lower part of the gear to create a shoulder for the weather cover as per the illustration.
- Clean the inner surface of the upper part of the inside of the gear and machine if badly pitted.
- Press-fit the modified "Fenner" hub into the bevel gear and weld.
- Heat shrink the weather shield onto the bevel gear and secure it with a couple of welding tacks.
- Have the bevel gear electro-galvanized.
- The splined part of the large bevel gear might have to be slightly shortened by removing the circlip groove.
Bevel Gear modification with larger 6817-2RS bearing:
- Machine the lower part of the gear to suit the new bearing and create a shoulder for the weather cover as per the illustration.
- Clean the inner surface of the upper part of the gear and machine if badly pitted.
- Press-fit the modified "Fenner" hub into the bevel gear and weld.
- Heat shrink the weather shield onto the bevel gear and secure it with a couple of welding tacks.
- Have the bevel gear electro-galvanized.
Adapter plate:
- Remove the plating with hydrochloric acid
- Machine the adapter plate center opening to 116 mm diameter to accept the new bearing adapter sleeve.
- Weld up all holes except the six mounting and the three perimeter holes.
- Machine the lower surface so it will lay flat on the tower support plate.
- Fit the bearing sleeve and TIG weld it onto the top surface only.
- Have the modified plate electro-galvanized.
- Machine the bearing sleeve recess for a press-fit as it will have distorted during the welding and galvanizing process.
The article continues as an MS Publisher pdf document that can be found here
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