Hy-Gain Rotator Mast Slippage

As the temperature crept above freezing this week I made my first tower climb of 2022. It was to re-align the antennas (yagis for 6 and 40 meters) on the 21 meter high Trylon. The rotator is a Hy-Gain T2X (Tailtwister). This is periodic maintenance that is needed due to mast slippage in the rotator mast clamp. It happens approximately once per year. It has already happened once on the TH6 side mount and Ham-M rotator with the nearly identical mast clamp.

My story is familiar to owners of Hy-Gain rotators turning medium and large antenna arrays. The mast clamp has never been improved over the decades of production by Hy-Gain (now MFJ). After all, why would they incur the cost of risk of a major product change when there is no impact on their business? Hams keep buying them despite the flaws.

The number of rotators on the market is small and at least Hy-Gain is a known quantity despite the products' flaws. There are few cost-effective alternatives. I use them but I never buy them new. They are only worth the far lower price they command on the used market.

Here's a question about these rotators that is worth pondering: When the mast slips in the clamp, is it a sign that the clamp is deficient, improperly adjusted or that the the load exceeds the capacity of the rotator?

If you have a quick answer to the question, I suggest that you think again. Let's consider the capacity specification for the T2X for the common case where the rotator is largely isolated from bending stress by one or two thrust bearings. From the Hy-Gain documentation, as extracted above:

• Braking strength: 750 ft-lb (9000 in-lb)
  
• Antenna wind area: 20 ft²
  
• Maximum effective moment: 3400 ft-lb
  

It is interesting that the several manuals of Hy-Gain rotators I perused for this article do not mention braking strength or vertical dead load. It is shown on the MFJ product page. The vertical load capacity is sometimes quoted as ~700 lb, but I can't confirm that value.

The precise values are somewhat tangential to the main thrust of this article. However, note that the braking strength is less than 25% of the MEM. Is the brake deficient or are the specs less than enlightening? It's the latter, and I have more to say about it further below.

Many hams worry more about the dead load weight than the braking torque limit. So much so that, in my experience, they insist that the thrust bearing take the weight of the mast and antennas to protect the rotator beneath! This is despite the facts that the rotator is designed to support the vertical load and the widely used thrust bearings are incapable of supporting any vertical load. The most common explanation I've heard is that the rotator is expensive and must be "protected". It is a peculiar belief with no basis in fact.

This may seem to be an irrelevant observation in the context of the present article. It is relevant since most hams are not as mechanically adept as they believe and the same lack of understanding appears in discussions about mast slippage.

For example, assuming the 9000 in-lb braking strength is valid, it can be reached by applying 75 lb of force to a 10' long lever attached to the mast, such as the antenna boom of a yagi. Failure would be catastrophic should the bell housing fracture. In practice it is more common for the ridges in the lower bell housing to wear or for the solenoid to bend or break, both of which will allow the mast to spin freely. The reduction gear train of a Hy-Gain rotator has little braking resistance on its own.

For the sake of discussion, let's assume that we install an absurdly large antenna on the mast. One so large that we are certain the braking strength will be exceeded in a moderate wind. If the mast clamp is perfect (no slippage possible) the brake or bell housing will fail. That would be an expensive failure. There is the immediate risk to the tower, antenna and property below, and the cost of a new rotator. You never want this to occur.

When the mast clamp is imperfect the mast will slip and the rotator will not suffer a catastrophic failure. The torque at which slippage occurs is far too difficult and imprecise to calculate. We can only hope that slippage occurs well below the threshold for catastrophic failure. Absent clamp or bell housing failure it is also important that the mast slip before there is sufficient force to damage the bell housing "teeth" and brake solenoid. Under the stress of continuous assaults of a too large antenna array the brake will eventually fail.

What I've described is a mechanical fuse. The slippage, although annoying, reduces the risk of catastrophic failure. Unfortunately that slippage often occurs with torque that is well below the braking strength of the rotator. Hy-Gain recommends pinning the mast to the rotator clamp (part 136, above). They helpfully mark a blank for the hole on the bell housing and provide a hole on the clamp plate. Following that advice can be a costly mistake.

With a pin through the mast and clamp there is no give. When a sufficiently large force occurs, and it will occur, something will break. In a typical installation the weakest link in the chain is the bell housing. The mast, pin, u-bolts and the exterior mast clamp plate are stronger. You can choose a bolt grade of lower tensile strength for the pin, however there is no good way to calibrate it to break at just the right amount of mast torque to protect the rotator. It will almost certainly break at too low a torque or not break when it should. A weak bolt will fatigue and fracture over time and it will then break at a low torque.

There is also a maintenance issue with pins. To insert or remove the pin the holes in the mast and clamp halves must be perfectly aligned. That can prove very difficult unless the holes for the pin are punched with the mast fully seated in the rotator clamp and when the wind is calm. Otherwise the weight of the mast and antennas must be precisely manipulated to align the holes to remove or insert the pin. One instance of removing and re-installing the rotator can make you regret using a pin.

There are less extreme methods to increase resistance to slippage. A few common ones are:

• Increase contact area between the clamp and the mast
• Teeth or grit to penetrate the surfaces of the mast and clamp
  
• Set screw

Like many universal clamping systems the surfaces have a V shape. A range of mast diameters can be accommodated at the price of a low contact surface area. The ridges on the bell housing side of the clamp reduce the contact area and provide no sharp striations to penetrate the mast. The other surface is smooth. Even that might be enough if the clamping material was steel. Unfortunately, the clamp plate relaxes over time no matter how much the nuts are tightened, and aluminum striations will not penetrate a steel mast.

Last year I inserted a thin galvanized mesh to see if it would be gritty enough to bite the aluminum and reduce slippage. It made no noticable improvement.

The clamp hole for the pin can be threaded for a large set screw to penetrate the mast at one point. I doubt that would work because the flex in the clamp would relax the hold of the set screw as it already does for the u-bolts. I've seen it done but I have never bothered.

One thing you should not do is over-tighten the nuts on the u-bolts. The clamp will relax regardless and you risk breaking or galling the stainless bolts or, worse, cracking the bell housing.

It is possible to machine a one or two piece mast shim with an inner surface that conforms to the mast and an outer surface that conforms to the V shaped clamp surfaces. The increased surface area will increase the mast torque required for the mast to slip. I've never seen one but I have heard of it being done. I doubt that it worth the effort.

I earlier put aside the "maximum antenna size" specs and it's time to delve deeper. How did Hy-Gain arrive at the maximum square footage and MEM limit and what do they mean? This is less than obvious. For example, 20 ft² of antenna surface area tells us little without a great deal of context. Let's provide some.

For the 135 kph (85 mph) wind zone which is most common, at that wind velocity the lateral force on a cylindrical 1 ft² of projected area the static load is 20 lb. For the listed 20 ft² the lateral force is 400 lb. That's a lot, however none of it is seen by the rotator when the mast, tower and thrust bearing(s) are doing their job. The rotator will only be sensitive to torque. 

Torque is roughly in linear proportion to both antenna surface area and MEM. That tells us little since yagis are balanced by weight (CoG is at the boom-to-mast clamp), and by the cross-wind principle the torque should be zero. It isn't zero because the boom and elements are not in a laminar wind stream. At high wind velocities the air flow and mechanical oscillations are non-linear, 3-dimensional and can create brief periods of high torque. The 6° of motion the T2X brake allows adds to the peak strength of oscillations and mast torque. It is the peak torque that the rotator and mast clamp need to resist.

The MEM spec is no more than an heuristic tool to guess at what the rotator can deal with. As mentioned earlier, that the MEM is so much greater than the T2X braking strength makes it clear that the weight of the antenna is never applied to a lever equal to the length of the turning radius. The spec is little more than a rough guide, and that guide is of little value without more precise conditions where it applies. The lack of those conditions in the specs is telling: don't trust MEM!

There is no simple or straight-forward calculation, and two balanced yagis with the same surface area and turning radius will give different answers. Tube diameters, alloy and wall thickness, along with taper schedule, truss and element placement are all factors that make for the difference. So how do we interpret and what are we to do with Hy-Gain's specs?

MEM is defined by Hy-Gain as the product of antenna weight (lb) and turning radius (ft). This is a rough analogue for the expected torque under undocumented conditions. For example, a Hy-Gain TH7 has a cylindrical surface area (elements broadside) of 12 ft², weight of 75 lb and a 19' to 20' turning radius. 

MEM of the TH7 is 75 × 20 = 1500 ft-lb. This is less than half of the specified braking strength of the T2X rotator. Is this reliable? Can the T2X handle an antenna of the same size but twice the surface area (40 ft²)?

Certainly my recently installed 3-element 40 meter yagi is too much for a T2X with its MEM of ~8000 ft-lb. The wind load is 27 ft² but the weight is 4× that of the TH7. Doubling antenna size typically requires 2² the weight to be structurally sound.

My T2X survived moderately high winds with a TH7 and 6 meter yagi on the mast (total MEM of 1700 lb-ft), but that is hardly definitive. It's like proclaiming that I plan to live forever -- so far, so good! Tomorrow the T2X could shatter in a wind storm. There will always be a storm that will destroy any installation. It's a matter of probability: is that storm likely to occur once a year or once a century? We can only estimate, build and hope that century storm doesn't happen during the life of the installation. Building for a century storm may be unjustifiably costly for the ham budget.

With this in mind, is pinning the mast all that bad? We eliminate slippage and assuming our antennas are within the rotator's specs the rotator will survive. There is also the matter of the tower. Even if the rotator can survive the torque with a pinned mast the tower must also survive the torque. I have personally witnessed a tower twist and break in a strong wind, and the Ham II rotator survived. In that case both the tower and rotator were overloaded, and the mast was not pinned.

If we don't pin the mast, the rotator and tower can survive many severe wind storms. But we must climb the tower after those storms to realign the antennas. It would be nice to avoid this maintenance chore since I have so many others with my large antenna farm. I have already been up once to realign the side mounted TH6 after a wind storm. That is likely to be a regular chore since the antenna is not centred on the mast.

In the face of the several uncertainties I choose not to pin the mast. I prefer the maintenance over the relatively low risk of rotator or tower failure. I'll continue to grumble about mast slippage despite my choice. For those who must hire a tower rigger to do the job the balance may shift in the other direction. 

Alternatively, buy a rotator that is less prone to slippage and that has no brake mechanism. For example, one with a worm gear drive and a better mast clamp (e.g. AlfaSpid). Of course there are the matters of cost and the trading of one set of issues for another since every product has shortcomings.

There is no universal answer. But that is no reason to shrug and be complacent. Make a sensible choice based on calculations and facts. If you live in a hurricane zone or you have doubts about your tower, your choice should take that those facts into account. Rotator and tower failures are costly, and there is the risk of high liability for those living in a densely populated urban or suburban area.