[TowerTalk] Big rotators for KH6

Kimo Chun kimo at lava.net
Fri Apr 28 13:25:25 EDT 2006

Hawaii presents a problem that many other places may not have. The
prevailing weather is Trade wind. Wind oscillating from 5 to 20 MPH and
gusts to 25 MPH or higher is almost constant at times. The other pattern is
from no wind to winds up to 30 MPH plus higher gusts. This creates a
“hammering” of the hardware resisting the motion.

On a Rohn 45, star guyed, with a full size 3 element DX Engineering 40M yagi
we’ve gone through an Orion, and 2 medium sized Big Boys. One Big Boy lasted
only 6 months. We are now using a prop pitch with a Green Heron controller
supplied by Kurt Andress, K7NV. It is working well so far. We were using a
controller from Orion (before Jeff came out with his controller) but it
suffered from noise getting on the feedback circuit, causing errors. We had
to run coax for that circuit to resolve the problem.

The Big Boy’s may have worked fine but due to inadequate mechanical design
or incorrect hole size drilled by their subcontractor (as was claimed) they
cannot take the repeated twisting that the antenna places squarely on the
motor housing to base housing interface. I am not an engineer, however
hindsight (post mortem) can give one ideas on how to make a weak link better
to match the capability that is claimed and apparently delivered by the rest
of the system.

The base housing accommodates larger bolts than the bolts holding the base
housing to the main motor housing. This may be fine so long as the smaller
bolts are themselves not overtaxed. I think with the tower having much less
twist due to the star guying and the larger bolts holding the rotator
assembly to the tower rotator mounting plate allowing a smaller deflection
or movement due to higher stiffness and locking pressure. The base housing
is “stuck” to the tower plate. The rotator is “stuck” to the antenna
 “hammer”. The stainless bolts between are arbitrating forces between the
two cast aluminum housings. The bolts are almost fully threaded allowing
threads to lie on the surfaces the holes (“incorrectly sized” on that
production run?). The constant torquing of the interface allows wear that
elongates the holes of the base housing. The assembly grinds itself to
aluminum dust.

The subsequent gap and leaking at that point allows water to travel to the
center destroying the feedback potentiometer. The antenna continues to
oscillate to greater degrees as it fails. We could not get an answer as to
whether we had not applied sufficient torque to the bolts. No specifications
were put forth.

To Jay’s credit (Array Solutions), after we discussed the problem with he
and the owner of Prosistel at Dayton, he refunded the cost of the second
rotator to us.

My humble suggestions:

Redesign the rotator using a steel base housing;
Move the cast reinforcing ribs of the housings to allow more of the holes
to be used with more bolts. I recall only 4;
Allow larger bolts for that interface;
Use bolts that are not threaded where they touch the aluminum housing;
Drill out the base housing immediately upon purchase and install steel
press-in sleeves to allow the bolt threads something more tolerant to rub

OR warn prospective customers that the rotator may not perform as specified
in all applications even within those capability specifications (now that
the problem has shown itself).

OR perhaps the manufacturer is right and the holes were drilled slightly too
large allowing excessive “play” with the subsequent destruction.

Perhaps an engineer among us can explain the requirements for such designs
and applications. I don’t believe we were expecting too much and
 “undersized” the Big Boy.

For now, the K7NV prop pitch and Green Heron are a “god-send”. We already
have our second prop pitch in service on another tower.


Kimo Chun, KH7U


I'm kicking around idea right now for my relocation out to KH6...if I ever
get to go.  Right now the MonstIR for 40 is looking pretty good.  One
problem...it is a beast with about 23 sq ft of wind load.  I've always been
a T2X
guy, but that rotator would be beaten to death by this one.

Any idea for a rotator gutsy enough to handle this, besides a  prop-pitch?
No HDR-300s please.

Bill K4XS

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