Having worked with vibration damping and torsonial damping on some projects
in another life along time ago when working as an "instrument man"I have the
following comments:
> Just a thought:
> Does a shock absorber in the mast like this really take any stress off the
> rotor gears? The shock absorber will wind up allowing the antenna to twist
Yes, Usually but with some cautions
> further than it would without the shock absorber and have much more
> "travel"
Yes
> when returning in the opposite direction thus increasing the force by
> allowing more travel and the antenna to whip around more than without any
> flexible joint.
Now it gets complicated. The short answer is yes, no, maybe, etc...IOW YMMV.
In this case we are talking the possibility of a low frequency resonance not
high. (Probably with a period of one to 3 seconds. With a long shaft and
coupling this could go to as much as 5 seconds.
We are working with a lossy material (hopefully) which is why we have to be
careful when using along shaft from the rotator at the bottom of the tower
to an antenna system a hundred or more feet up.
The mast is one long torsional bar. Like any spring it can resonate. Even
though lossy the rubber coupleing can resonate. There is some play in the
rotator that can resonate. The antennas and antenna system on top have
their own mechanical resonant frequencies.
IF we are lucky EACH resonance will be of a different period and probably
will unless the builder is unlucky enough to have two or more that coincide.
They *could* all cancel out, but that is hoping for a bit much <:-)) It
would be pretty much like writing checks to pay off all your bills based on
the lottery ticket you just purchased and the drawing isn't until tomorrow.
So, it is *possible* that the effect will be an increase of overall force.
It is just as possible that the overall force will decrease.
>
> The shock absorber works well in a car's drive line as the major force is
> in
> one direction.
We have to remember when working with vibration, or torsional damping that
what works well in one application may be a disaster in others. These
dampers are designed to work with a specific range of mass and a specific
frequency range. *Generally* the big problems come at the lower frequencies,
but resonance at any frequency could be a problem. That is a "could be", not
"will be". Still, the design frequency limits for the automotive damper are
well above those we should enoucnter on an antenna installation. However
being below the design limit doesn't mean they won't work. OTOH it doesn't
mean they will either.
Yes the damper is lossy, but it will be more lossy at some frequencies than
others. It may help and it may not. Systems may survive because of them
and they may survive in spite of them.
There was an aircraft called the BD-5 that used a shaft driven rear
propeller. It'd take some digging, but there is a good story on the web
about these torsional dampers and how they eventually came up with something
that didn't destroy the aircract structuraly from the engine pulses driving
the prop. Ther problem was very much like that of antennas as the pulses
were at the low frequency end of the range and occurred in both directions.
Of course we are looking at much lower frequencies than they did.
IF and I do emphasize the IF some one has the time, skill, and resources to
install transducers to measure the frequency and amplitude of the
oscillations with and without the dampers in relation to wind speed and
direction (as well as shaft length and specs, antenna mass, and system mass)
it would provide invaluable information and some "real numbers" on which to
base decisions.
There are a lot of factors that determine how much force is generated and
how well devices work to mitigate those forces.
In general, we as hams go by the trial and error method. We try it and if it
works that's fine. Of course we have no numerical information to show us how
well the trial worked or if it even worked. As I said earlier, it could be
the system survived due to the devices and it's also possible they survived
in spite of them. We really don't know.
So we try and hope for the best.
Good luck,
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: towertalk-bounces@contesting.com [mailto:towertalk-
>> bounces@contesting.com] On Behalf Of Jan Erik Holm
>> Sent: Thursday, January 25, 2007 2:50 PM
>> Cc: towerTALK@contesting.com
>> Subject: Re: [TowerTalk] Flexible coupler assembly
>>
>> Sounds like you mean a Hardydisc. I have used one on one of my rotating
>> towers for years, my unit came from a Ford Granada station wagon.
>> Sure is a good way to protect a rotor. By the way, also to
>> get away from the problem in getting everything centered/lined
>> up exactly I incorporated a u-joint from the same car, then the
>> stuff can flex or wobble around and would not hurt the rotor.
>>
>> 73 Jim SM2EKM
>> ------------------------
>> Dubovsky, George wrote:
>> > I designed one of the drive shaft couplers into my rotator system,
>> > based
>> > on the suggestion in Dave Leeson's Yagi book. Mine was intended for a
>> > BMW 1600, I think, and was bought off of the auction site for about
>> > $20.
>> > I am told that all BMWs have something like this - do a search on
>> > "guibo" and you will see what one looks like. If anyone knows where
>> > that
>> > term comes from, I'd love to know!
>> >
>> > 73,
>> >
>> > geo - n4ua
>> >
>> >> -----Original Message-----
>> >> From: towertalk-bounces@contesting.com [mailto:towertalk-
>> >> bounces@contesting.com] On Behalf Of John E. Cleeve
>> >> Sent: Thursday, January 25, 2007 1:30 PM
>> >> To: towerTALK@contesting.com
>> >> Subject: [TowerTalk] Flexible coupler assembly
>> >>
>> >> 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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