Gentlemen,
In view of the response I have received, permit me to publish a copy of the
email I sent to Dennis earlier today, also, I promise to obtain some close
up digital photographs (Santa was thoughful!!) of the intstalled flexible
coupler
assembly, when the tower is brought down for the summer check........73,
John G3JVC.
----- Original Message -----
From: John E. Cleeve
To: N6KI Dennis Vernacchia
Sent: Thursday, January 25, 2007 12:52 PM
Subject: Re: [TowerTalk] rotors, controllers, etc.
Hello Dennis,
I can recommend this way of protecting the rotor. I had several experiences
of teeth breaking in the spur gearing of the average "Jap" rotator, entirely
due to the mechanical stress, and the backlash in their poor engineering. I
have copied out the detail of the installation, originally for another
member of the group, however, it does give a good idea of how I protected my
then new, and very expensive rotator. The name of the manufacturer, of the
flexible coupler is "Metalastic", and you have to imagine a six inch outer
diameter doughnut, with a section of about 1.5 inches, with six bolt holes,
three to each coupling flange, in the transmission path of the vehicle. You
will have to do a little engineering, to couple this device into your
antenna drive shaft. If you have a lathe, or, have a local engineering
company make the necessary parts for you, then there is no problem. The
actual stores part number for this item is "Unipart" GCD 301. 62 Ne2 879407
19726, and the item is manufactured in the UK. "Unipart" is the spares
distribution system for Rover, Triumph etc. British cars. I bought the
coupler as a new replacement spare part, and the cost was about 70 pounds
sterling, plus our 17.5% value added tax, and it cost me about another 50
pounds sterling "in pound notes!" to have some of the " metal turning" done
by a local engineering company. Normally, this flexible coupler is part of
the drive train, ie drive shaft to the rear axle differential, used in the
Triumph TR7 sports car, and as such, more than able to cope with the torque
stress applied by even force 12 gusting wind conditions in our
applications......So, here you are, the detail, as I posted it to to the
reflector, I hope you find the information of some use....sincerely, John
G3JVC.
I am sorry but I do not have any digital pictures, but I can give a verbal
description, so here goes:-
The Italian worm drive rotor I have, is a PST medium range model. The rotor
is bolted to a horizontal plate, which is part of the head unit, of my 60ft
Strumech tower. The drive output of my PST rotor is a 4 inch diameter
horizontal disk. and the rotary drive shaft to my yagi, is 2 inch diameter
construction grade aluminium tube, with a 0.25 inch wall thickness. The
rubber doughnut I used in my flexible coupler, normally forms part of a
Triumph sports car drive train, coupling the prop shaft to the differential.
The outer diameter of the doughnut is about 6 inches, and its cross section,
about 1.5 inches. The doughnut has six metal sleeved holes through the
rubber, intended for three fixing bolts to each flange of the interconnected
prop shaft.
In order to make use of the doughnut, I had two aluminium discs cut, each 7
inches in diameter, and 0.5 inch in thickness ( it took about five minutes).
The discs and the doughnut were very carefully centred. The discs were then
clamped together and the three hole fixing pattern, for the concentric
rotation of the doughnut, was drilled.
One disc was then engineered to fit the 4 inch drive plate of the PST rotor,
and the other disc, which forms the top, or drive disc, engineered to couple
the yagi. In order to couple the 2 inch diameter drive shaft from the yagi
to the drive disc, a 6 inch long, 2.5 inch diameter bar of aluminium was
used, this bar was then bored 2 inches internal diameter, to a depth of 5
inches. The 2.5 inch diameter bar is then drilled and tapped with three
holes, at the blind end, and secured, concentrically, by stainless steel
screws, to the top of the drive disc. The yagi drive shaft will then slide
into the bored 2.5 inch bar, to a depth of 5 inches, this sleeve/shaft
assembly is the drilled, 0.5 inch clearance, at right angles to the axis of
the sleeve/shaft, and a 0.5 inch high tensile bolt passed through the
complete sleeve/shaft assembly. This works well, and will not "slip".
In order to assemble the rubber doughnut coupling, six spacers were made
from 1 inch diameter aluminium, bored to take the doughnut fixing bolts, and
of a length, to allow approximately 1 inch spacing between the inner faces
of the two 0.5 inch thick discs and the sides of the rubber doughnut. The
complete drive chain is now rotor/disc/doughnut/disc/driveshaft
coupling/driveshaft/yagi. The yagi drive shaft then passes up through the
collar of the tower head unit, where the vertical load is taken by a large
roller thrust bearing, rescued from the rear axle of a huge tipper truck in
our local scrapyard. The weather covering of the doughnut coupler assembly
is formed by wrapping a thin soft aluminium sheet, around the outer edges of
the 7 inch discs, this soft aluminium sheet was a litho printing plate, in
another life. The weather shield is secured only to the edge of the top
disc, by means of a stainless steel, ducting strap. the weather shield
extends down beyond the lower aluminium disc, by approximately 1 inch, thus
enclosing the entire doughnut coupler assembly. All exposed metalwork has
been given several coats of Finnigans clear Waxoyl (prevents corrosion, and
stops ice forming in winter). As the doughnut flexes in taking up the wind
load, the two coupling discs are able to gently move with respect to each
other. So, there you have it, I have done all I can think of, to relieve
the PST rotor, of any unnecessary mechanical stress, and I hope, prolonging
its working life. I hope this information is of some use to you, and also,
the group, for I am sure that many people wish to protect the investment,
both in time and money, which is exposed to the elements at the top of our
towers, and this was my way........sincerely, John. G3JVC.
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