After the Fall - Tailtwister Trauma

You may recall an article in late 2023 about the 110' tower of a silent key that was cut down for scrap. Once it was down we discovered that rotator at the top of the tower was a Hy-Gain Tailtwister rotator. The rotator model and condition were unknown beforehand and deemed not worth the time and trouble of retrieving it before the tower was cut down. 

I took the Tailtwister home despite its visible damage. I wanted to learn how it had fared from the trauma of impacting the ground. If nothing else it might provide a source of parts for repairing other rotators. The investigative work served as a wintertime diversion. I still don't know whether it is worthwhile to complete repairs and put it to use. 

Readers might be interested in my description of the damage it suffered and how I dealt with it. If you are unfamiliar with the insides of these rotators you might benefit from first reading the article where I refurbished an old rusty Tailtwister.

To begin, I'll reiterate that the entire tower and antenna system was derelict for years. I suspect that the rotator was never serviced since it was installed approximately 30 years ago, nor its condition at that time -- it likely was not bought new. The silent key was not a climber and it hadn't been used for years due to damage from a lightning strike and his deteriorating health. Now let's dive in.

I didn't expect the bell housing to survive the impact, yet it did. The cast aluminum alloy cannot withstand a large bending stress. That is why it is important to place the antenna load close to the top of the rotator. Longer masts can be used if two thrust bearings are employed to protect the rotator from bending stress due to wind load of the yagi(s) and the leverage amplification of a long mast. 

There was just one thrust bearing on the tower so there must have been quite an impact shock to the bell housing when the yagi struck the ground and pushed the mast upward. In this case the thrust bearing served as the pivot of a large impact force with the mast as the lever. 

Although the bell housing passed the test, the mounting bolts did not. I don't know the grade of the bolts since they were lost when the impact sheared them off the tower's steel rotator plate. It may be that, by shearing, the mounting bolts saved the bell housing. But it was not without cost. Notice that when the bolts broke off they took a chunk of the rotator's main body with them. In only one case was a portion of a bolt left inside rotator body. 

It is lucky in a way that so little of the rotator body was lost. The mounting bolts must have been quite short. Since the mounting holes are long and threaded deep, there is no impediment to their reuse. The only new requirement is that the spacers placed on the bolts must be wider so that they bear on the undamaged metal of the rotator. Two of the 6 mounting holes were not used and were therefore not damaged. With 5 holes remaining it really isn't necessary to remove the one bolt shank still inside. I'll probably do it anyway since it isn't a difficult task.

There is a surprising amount of rust on the lower half of the rotator. As you can see in the above pictures, the bottom bearing races are rusted or covered with rust deposited from the ball bearings. The lowest ring of balls (⅜") can be replaced and the races cleaned. The brake system is very rusty but works just fine. It should be okay to leave it alone, which is good since cleaning its many components would take a long time.

The other damage you may have noticed is that the terminal strip has broken from its mounting screws. Also, the plated metal is badly rusted. Testing the rotator was difficult due to the resulting poor electrical connections. The terminal strip can be replaced or repaired. The metal can be cleaned if the screws are removed. Suitable stainless screws can be substituted if you keep in mind that the original screws have their tips crimped to make them difficult to fully unscrew, and thus accidentally fall to the ground while attaching wires on the tower.

I have one more observation to make about corrosion, involving stainless steel. It is a myth that stainless steel doesn't corrode. Depending on the alloy and the metal they are in contact with, they can indeed corrode. However the result isn't rust.

The stainless bolts that hold together the upper and lower halves of the bell housing were substituted for the original bolts. That may seem like a good idea but beware! In this case the alloy quality is suspect and the bolts were not coated with a lubricant to prevent galling and galvanic corrosion due to contact with aluminum alloy of the bell housing. The piles of oxide dust used to be metal! 

If you insist on using stainless bolts in this and other antenna system components, at a minimum please use 304 (18.8) hardware. It is also perfectly acceptable to use grease or an anti-seize coating on non-stainless steel bolts. Stick with grade 2 steel rather than grade 5 so that you are not tempted to over-tighten them. The same goes for the 6 mounting bolts. The stainless u-bolts for the mast clamp are especially prone to galling due to repeated tightening and loosening that is typical in a ham station. Coating the threads with a suitable compound is advisable.

Next is a very common problem in Hy-Gain rotators that has nothing to do with the fall. In the adjacent picture notice the small indentations in the bearing race. That is due to fretting. It can happen with similar metals but is more pronounced when metals with different hardness are in contact under load. In this case, steel balls rolling on an aluminum alloy race.

Fretting does the damage when the rotator isn't used for a long time. Continuous wind-induced rocking under vertical load is responsible. 

I encountered a similar fretting of steel headset bearings and races in older generations of bicycles (another passion of mine). The bicycle generally points straight ahead with small deviations left and right while being assaulted by minor jolts due to road imperfections and debris. These impacts are transmitted to the bearings from the high-pressure tire and wheel through the forks and steer tube to the headset bearing. Newer bicycles often avoid fretting by the use of cartridge bearings that can be easily rotated during regular maintenance. A similar maintenance procedure is possible with rotators but is rarely done.

The races must not be filed or sanded to remove the indentations since that will cause unwanted bearing play. Rotators can survive some bearing slop provided that thrust bearings are deployed in a manner that prevents radial (side to side) forces. What I do is lightly polish the races with steel wool and carefully file off the raised rims surrounding the indentations. In most cases the steel bearings do not develop flat spots, so I only replace them if they're rusted. 

When the rotator is re-assembled it is unlikely, by random chance, that the bearings will contact the indentations when the rotator is pointed in the same direction. Better yet, don't leave your rotator pointing the same direction all the time. Turn it occasionally when you're not active.

When the motor broke loose and bounced around inside the bell housin, the motor was damaged and it damaged other components. Of particular concern was the direction potentiometer bolted to the top of the motor.

There are 4 bolts that bind the steel laminations of the motor body. Two of those are extra long and serve as motor mounts. Their long shanks are mechanically connected to the top plate of the gear assembly. Rather than threads and nuts, they narrow and pass through the holes in the plate. They are "crimped" to secure them to the plate. In normal operation that is sufficient. It is not sufficient when the rotator is dropped from 100'.

I removed the long bolts from the motor and inspected the damage. It turned out to be easier to repair the damage than I anticipated. Since the bolts were able to pull out of the holes it is not a surprise that I could push them back through. The small metal lips of the crimps were easily abraded by the impact force, which allowed the bolts to pull out of the plate. They were press fit back in and lightly crimped with a hammer and the hardened tip of a flat-blade screwdriver. You can see one of the re-mounted bolts in the picture of the fretted races.

The motor resisted turning by hand. The formed sheet steel brackets on the top and bottom of the motor were bent. The black common wire of the field coils was also severed. The remaining motor and pot wires were desoldered to free the motor from the rotator. It was then disassembled by pulling the drive gear off the axle and drilling out the two rivets that secured the brackets and that served as two of the 4 bolts that held together the steel laminations. The motor armature was then easily slipped off the bushings that are fit into the brackets. Although bearings are superior, bushings are sufficient for the low torque, low speed motors used in Hy-Gain rotators.

The top bracket was bent back into shape and aligned so that the armature was properly centred within the field coils. When properly aligned the armature should spin freely. It helps that the bushings pivot within rubber shock mounts. While some care is necessary this is not a high-precision device.

A new wire was soldered in to replace the severed common wire. Stainless bolts replace the rivets that hold the brackets and bind the laminations. 

I had some concern about replacing the rivets since ferrous metals can induce circulating magnetic flux that "steals" power from the motor and can cause heating of the bolts. I couldn't discern any problems with motor torque or noticable heating after several minutes of continuous operation. Stainless 304 is not the perfect choice but it seems to work fine in this application. Non-ferrous hardware can be used instead.

The drive gear was pressed on and the motor mounted onto the rotator body. It worked fine during this bench test. There was no damage to the reduction gears. The motor was removed once more to work on the direction pot which was seriously damaged.

The impact damage to the pot is evident in the pictures. The reason there is so much damage is because it is bolted to the heavy motor: it had a lot of momentum while bouncing around inside the bell housing. The plastic body has several stress cracks (not visible in the pictures). It is possible that the cracks can be repaired with glue. The tangs on the wiper can probably be bent back into shape with care. 

There isn't too much damage to the protrusion deep inside the bell housing (centre picture) that engages the wiper tangs. This is ordinary wear for a Hy-Gain rotator. There really ought to be improvements to this weak area of the design. The hard copper alloy of the sharp-edged pot tangs abrades the aluminum alloy of the bell housing. The wear goes on 24×7 as the antenna system rocks back and forth in the wind due to the play within the brake system. Wear does not only occur when the rotator turns. 

I have seen otherwise perfectly good rotators rendered useless due to excess wear of the bell housing protrusion. Alternative aftermarket solutions are easy to imagine. I am unaware whether anyone has done so or marketed a solution. It could be done with thin spring steel covers for the pot tangs or on the bell housing protrusion.

The damaged ring of resistance wire is not as bad as it looks. There is continuity through the worst kinks. It may be possible to bend them back into shape so that the wiper glides over the damaged areas. However, there is a break near one end of the wire due to a crack in the plastic body. That will be more difficult to repair since the metal of resistance wire is often difficult to solder. 

If I do attempt repairs it will only be out of curiosity to see if it could be done. A replacement pot is the more sensible option. MFJ charges an exorbitant amount for a replacement direction pot: about US$80 the last time I checked. A friend mentioned a less expensive local source that I may pursue should I attempt to return this rotator to service.

I hope you enjoyed this look inside a damaged Hy-Gain Tailtwister rotator. I don't know how serious I am about putting it back in service. It served as an interesting and educational diversion during a few cold winter evenings.