Chain Drive Prop Pitch Rotator

Even if you never lay eyes on a prop pitch motor and would never dream of or need a rotator of its capability it can teach lessons useful to many hams. I first saw a prop pitch motor up close and in action almost 35 years ago yet did not have one operating in my own station until last year. For what it's currently turning it is not strictly required but I expect that to change.

In this article I'll describe my prop pitch rotator system. I claim no credit for its design or construction since I purchased it secondhand. My job was to understand its mechanical and electrical details, then install and adjust it. I quite like it in all its "retro" awesomeness. I did have to design and build a direction indicator attachment to work with the home brew controller because most of it was missing.

While many hams who use a prop pitch motor opt for a simpler rotator system this one has several notable advantages. There are many ways to turn one of these beasts into a rotator.

Outboard mount

Use of a chain drive allows the motor to be installed with the shaft pointing down. This is advisable for unmodified prop pitch motors since the original gearbox lubrication is oil which can foul the electric motor and leave the gears dry when mounted with the shaft pointing upward under the mast. Replacing the oil with grease is strongly recommended for these reasons and because the oil will have dried or seeped out over the decades. The newest prop pitch motor was manufactured over 50 years ago!

With the shaft pointing down it is necessary to couple the motor shaft to the mast using a more elaborate system. Refer to the adjacent picture as I describe the system. For other pictures you'll be directed to follow the links to previous articles.

Unlike the typical rotator designed for amateur radio use a prop pitch motor is not designed to support the weight of a large mast and antenna system. When it is mounted under the mast it is mandatory to use a thrust bearing to support that weight. That leaves the motor to solely deal with torque, a job at which it is supremely capable.

It is not enough to simply wrap a chain around the mast and motor shaft. That would place an enormous radial load on the motor shaft which it cannot handle. A feature of the outboard mount is the elimination of these forces from being transferred to the motor.

Hanging beneath the top shelf (more detail on this will be shown below) is a mechanically robust open box that places a thrust bearing (for radial loads) above and below the gear that drives the chain. The bearings transfer the radial force to the tower and remove bending force from the motor shaft. The platform also supports the dead weight of the motor sitting on top.

The drive shaft in the tower has a similar arrangement with a bearing below the chain drive and another at the top of the shaft; the mast couples to the top of the drive shaft. I use this terminology to distinguish the mast (rotating antenna support) and drive shaft (part of the chain drive), although both can be though of as part of the mast.

The bearing under the drive shaft supports the full dead weight of itself, the mast and the antennas. It is a deep groove bearing designed for both axial and radial loads.

The collar on the drive shaft contains the cog gear for the chain and sits on top of the bearing. The drive shaft extends below the collar through the bearing for lateral support. See the article on mast construction for pictures.

All bearings are sealed. This is absolutely necessary since they won't last long in the weather without this feature. Additional weather covers are not required but can be helpful since sealing is never perfect and we want the bearings to last outdoors for many years of trouble-free service.

The photo shows the outboard mount from below, complete with an early version of the direction indicator subsystem. It gives a better idea of how massive and rigid it is. The system with its support struts weighs more than I do! That's without the motor attached. All the metal plate is ¼" galvanized steel. Joins are welded or made of ¼" angle stock. The top plate has an opening for the crown gear to mate with the prop pitch motor.

Chain system

The chain drive is quite simple. Standard size gears are welded to the motor shaft and the drive shaft and wrapped with a #60 (1") chain. The chain is stainless steel, and is highly recommended since access is limited for regular oiling. It's totally exposed to the elements. The rust you see in the pictures dripped from elsewhere. I cleaned and repainted the gears and other steel components that are not galvanized.

The number of links in the chain is adjustable. The chain has a master link. Without a master link a pin tool is needed to open and close the chain. A master link is the easier system to use. I did not need to change the number of links since my tower is the same as the previous owner's.

To avoid mishaps with the master link clips on top of the tower I raised and installed the chain with it wrapped around the motor shaft. When the drive shaft was dropped into place the chain was left loose around the bottom bearing. Only then did I wrap the chain onto the gears.

In the top picture you can see a ½" threaded rod on the bracket at the top of the platform. That and slots in the bracket are used to adjust the distance between the platform and the tower. Chain tension is determined by the distance.

I levered the chain onto the gears bicycle derailleur style after moving the platform inward as close as possible. It was then moved outward until the chain was taut. Looseness in the chain results in unwanted play in the mast that allows the wind to push the antennas back and forth several degrees. When properly adjusted the entire massive system can be easily spun by hand. When the motor is bolted on manual rotation is impossible.

Vertical alignment was a challenge. There is little room to adjust the vertical position of the motor platform. My first attempt resulted in chain binding on the drive shaft gear (see above). The only adjustment is the tower leg pinch clamp. Since there are no slots for the bolts on the platform's vertical struts the adjustment range is narrow. With some effort I was able to push the top of the platform down enough to allow the chain to move freely on the gears.

Control unit

The control unit is home brew, and although it shows its age it works well. Eventually I would like to replace it with software driven unit and only use the 24 VDC power supply to power the motor. My objective is a small desktop controller head, or eliminate it entirely with a PC application For the next year the project will remain low on my priority list.

The meter is obviously scrounged from an old CDR (now Hy-Gain/MFJ) rotator. There is an op amp circuit powered by +12 and -12 VDC that requires a linear potentiometer coupled to one of the rotating elements up the tower. The op amp circuit has two calibration pots: one to centre the meter and another to set the circuit's gain (degrees of rotation per ohm). Their settings are determined by the pot on the tower. The advantage is that almost any linear pot can be used. Mine is a multi-turn pot, identical to the one used by the previous owner, so that setup is not critical and allows > 360° of rotation. But don't try it unless the coax rotation loops are suitably designed.

I use Cat5 cable for the run up to the pot and 10/3 electrical cable to the motor. Except for part of the pot wiring both are burial grade and UV resistant.

Direction indicator

Of the two most common methods for sending direction data from the rotator to the control unit -- motor pulse generator and potentiometer -- a pot is the simpler method and will work with a greater variety of commercial control units. On the other hand a pulse generator has better long term reliability. A third method I learned about is a processor-based compass module to send direction data over wireless to a ground-based receiver.

I chose a pot since it's simple and my current control unit requires it. The first system I built (seen in an earlier picture) didn't survive long. First the wiring broke and later the ancient pot failed. The poor weatherproofing I hoped would last the winter didn't. But it was December, it was windy and cold on the tower, so I took my chances.

These 10K 10-turn linear pots are not cheap. Instead I ordered several from overseas at 10% the price. The downside was the long wait for the slow boat to arrive from China. When they arrived in August I got to work. This time I took the opportunity to improve the unit. After a couple of weeks in service it continues to work well. We'll have to wait and see how it does over the long term.

Some detail can be better seen in the earlier picture so refer to that as necessary. I chose the drive shaft to turn the pot. Common hardware store perforated steel strap holds the unit and has plenty of holes for attachment to the motor platform, bolt on the pot and tie the wires. The drive shaft coupling is made from strips of aluminum flashing (~0.03") and a hose clamp, with a ¼" screw to couple to the pot shaft.

The flashing was chosen so that perfect alignment between the drive shaft and pot is unnecessary. It will flex as the shaft turns. Silicone caulk keeps water out of the shaft coupler. Below that is a 1" circular plastic rain/snow shield cut from a food container and drilled at 15/64" to fit tight on the pot shaft. The shaft opening to the pot is coated with dielectric grease and the nut and lockwasher are caulked. The enclosure for the pot may look familiar: it's a cap from an empty WD-40 spray can.

The wiring tail is connected to the main cable run with crimp connectors. There are weatherproof commercial connectors available but this is easier to install and the wires can be cut and re-crimped easily. I enjoy the challenge of improvising with commonly available materials.

Lifting

Due to the weight and awkward dimensions of the motor platform I had to resort to alternative rigging. When all the tower sections were up the only parts of the platform installed were the support struts. These had to be installed during tower construction because they attach to two tower legs at the splice at the bottom of the top section. Once the top guys are tensioned those 4 bolts cannot be touched.

A heavy gauge steel strut was extended outward from the girt at the middle of the top section, which is several feet above the struts. The gin pole pulley was transferred to the lifting strut and the end of the strut was cable stayed. The rigging was tested to ensure it would support the load.

Using this rigging the bulky platform was lifted without being scraped along the side of the tower and tangling in the lower 3 sets of guys. The lifting strut swings laterally to help with levering the heavy platform around and onto the top surfaces of the support struts.

Despite the special rigging there was some difficulty maneuvering the platform into position. It required adjusting my safety restraints so that I could lean outward from the tower and apply the required muscle.

Once in position I secured the top of the platform to the tower legs with the pinch clamp. During motor installation, chain adjustment and electrical work the platform served as a handy shelf for tools and parts.

Spare motor

I was fortunate to be given a second working prop pitch motor when I purchased the system described in this article. My dilemma is whether to keep it as a spare or to custom build a clone of the motor platform for the second LR20 tower I am planning to raise.

Spare prop pitch & drive shaft (right) posing with a Ham motor

The platform design makes it convenient to replace the motor. You remove the 6 bolts holding the motor to the platform and drop in the substitute. The chain must be tied when no motor is present to prevent the mast from freewheeling.

Wire splices between the motor tail and the main run are silicone-filled wire nuts designed for outdoor use. They are inexpensive and easy to use, and can be reused at least once if you are careful. You can see the nuts in the picture at the top of this article, before a weather cover was installed.

Building a platform for the second tower will require the services of a machine shop since I do not have the tools needed. Material, manufacture and galvanizing could be very expensive. I would also need to acquire a third prop pitch motor as a spare to ensure minimal disruption when a motor requires service.

A modern, high-capacity rotator may be the better choice. It will certainly work out cheaper. The decision can be put off for only a couple of months.