The EF240X was up about 8 years (a couple of hurricanes, a few Nor'easters,
didn't survive the tornado). I never had trouble with the C3 I had. The
EF240X elements are a bit bigger than the C3 and have a linear loading
scheme (2 aluminum wires that go from about 3/4 of the element to tie points
about 12 inches above the boom and the tie point is about 8 inches wide
IIRC). Only one of the two elements was affected. Just a guess, but perhaps
a wind resonance problem (ever see the Tacoma Narrows bridge crash? c 1940).
Over time the repeated stress causes a failure. Of course, in your case of
only 2 months that is quite short for stress failure unless there were
defective parts to begin with. The EF240X had 4 good sized rivets on each
side of the U shaped boom to element piece. The holes on both sides of the
boom were worn to a vertical length of about 3 times the rivet diameter but
horizontally the holes were only a bit larger than the rivet diameter.
I replaced the rivets with 1/4-20 (or was it 5/16-18) SS bolts and had to
put in a new boom piece of course.
73, Larry W6NWS
-----Original Message-----
From: D. Drake
Sent: Wednesday, December 19, 2012 5:34 PM
To: 'Larry'
Subject: RE: [TowerTalk] XM-240 Failure
I had Force 12 C-3 up for over 10 years and had no trouble. I'd bet your
EF240X was up there a long time before it failed. My XM-240 was up only 2
months!
73,
Dale AA1QD
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Larry
Sent: Wednesday, December 19, 2012 3:12 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] XM-240 Failure
I had a similar failure on a Force12 EF240X. Force uses rivets. Eventually
it elongated the holes on the boom and broke the rivets and the element fell
117 feet. I fixed the element and then the following year a tornado
finished off the antenna by bending one half of both elements to the other
side of the boom.
73, Larry W6NWS
-----Original Message-----
From: D. Drake
Sent: Wednesday, December 19, 2012 2:28 PM
To: towertalk@contesting.com
Subject: [TowerTalk] XM-240 Failure
Some of you may know I recently moved to a new QTH
in New Durham, NH where I have installed a 70 ft.
Rohn tower. On the tower is my old 4 el SteppIR
and a new XM-240. The XM-240 is up at about 85
ft., above the SteppIR. The stuff all went up the
week before Sandy blew through. When Sandy
visited here she was pretty weak with the biggest
wind gusts recorded by my wx station in the 45 MPH
range. A few weeks later we had strong winds one
night when a cold front moved through. The next
day I noticed the reflector of the XM-240 hanging
at an angle. Since it seemed that we had not had
any extreme winds I assumed that I had not
adequately tightened the bolts that attach the U
channel (piece that holds the element) to the boom
clamp piece ( 4 – ¼ 20 ss hex head tap bolts). A
few days later K1RX came up to fix it and we
discovered that the bolts were not loose but in
fact the bolt heads had torn (ductile fracture)
through the aluminum U channel on one side. A
quick call to Cushcraft and they agreed to send me
a new U channel. They said they had never heard of
this failure before.
The U channel holes for the mounting bolts are
actually elongated as are the mating holes on the
boom clamp. This allows for some adjustment of
the elements to get them perpendicular to the boom
and parallel to each other. Unfortunately this
leaves little metal for the bolt head to compress
against. The instruction manual specifies a
hardware stack up of bolt head, u channel, boom
clamp, split lock washer and nylon insert locknut.
No flat washers are used. When we installed the
new U channel we added some stainless steel fender
washers under the heads of the bolts and between
the boom clamp and the lock washers. Fender
washers were also added to the driven element
hardware. This allows the forces to be spread
better over the 1/8” aluminum pieces. I know
putting a flat washer under the lock washer will
reduce the “locking” ability but with the
elongated hole the lock washers weren’t exactly
performing as intended and there is the nylon lock
nut. If anyone is interested I can email you a
photo of the damaged U channel.
The U channel is actually made from 1/8” aluminum
sheet formed on a brake. I’ve been told by a
former Cushcraft employee (when Cushcraft was here
in NH with the previous ownership) that the
antenna was originally designed using an extruded
U channel that was ¼” thick. I also suspect that
the sheet aluminum the present U channel is formed
from may not have been heat treated so it’s
toughness would be even lower than the original
design.
It was rather frustrating to experience this, not
just because of the effort in fixing it but I had
taken great pains when preparing the antenna to
try to compensate for all of the other failures
or problems that others have experienced with
these antennas. As an engineer I should have known
enough to question the assembly but hind sight is
20 20 as they say.
Dale AA1QD
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