Grant,
Interesting read. I'm trying to figure out where the structural
analysis (torque and bending) dictate the transition from an insulated
leg model to single insulator and pin base.
73 Martin
W2RWJ
As a side note, W8JI has a group of pictures on his site that has a
pictorial description of a 200' tall, pier base Rohn 45 tower
http://www.w8ji.com/lifting_rohn_25g_45g_tower.htm with a close up of
the locally engineered base http://www.w8ji.com/images/towers/45g-base3.jpg
On 9/1/2014 7:43 PM, Grant Saviers wrote:
Incorrect. The compressive strength of UHMW is 3000psi (and 12,500 for
nylon; 15,000 for acetal). A 4" diameter insulator less a 1" diameter
hole for the steel pin has over 11 sq in of material so the yield load
is 33,000 lbs, way more than any amateur R25 insulated tower should
generate. As I mentioned, a pier pin bottom section yaws about the Z
axis to eliminate most bending stress in the bottom section, so the
bending and compressive stress that are a big concern in small
diameter tower leg insulators is not an issue. Check out Kurt K7NV's
FEA work on tower guys. His models show that 70% of the load in the
base section of a tower embedded in concrete is bending stress.
http://k7nv.com/notebook/towerstudy/towerstudy1.html
I suggested bolting the plastic insulator to the bottom section steel
plate, but that is really not necessary if a 2" diameter piece of
plastic with the steel pier pin inside goes thru the plate.
Grant KZ1W
http://www.gplastics.com/pdf/uhmw.pdf
On 9/1/2014 12:46 PM, n8de@thepoint.net wrote:
All the materials you list are SOFT and will not be structurally
sound under load.
I prefer to use fiberglass rod, cut and drilled, to insert in the
bottom legs and base. Depending on the diameter needed, there are
fiberglass fence posts available at most farm implement dealers. WEAR
gloves when handling and wear a MASK when drilling fiberglass.
You might see the insulators I have used on my FREE-STANDING 80m
vertical tower antennas at: www.qrz.com/n8de
73
Don
N8DE
Quoting Grant Saviers <grants2@pacbell.net>:
Gordon,
I couldn't find a drawing of the Rohn 25TG pier pin base section, but
Rohn makes an insulator A4197L for the 25TGIA section per catalog, no
drawing for it either. My idea is to use a plastic (nylon, black
UHMW, delrin) block bolted to the pier pin end plate with a hole for
the pier pin, removing the center of the plate on the tower section for
adequate electrical clearance. The block could be 4" diameter rod say
3" long grooved for more path.
I also think a fiberglass solid rod would be strong enough as a pier
pin if upsized over the stock R25 3/4" steel one. Then there is no
concern about dirt etc. causing a path to the tower on the insulator.
Or a 2" od section of the insulator could go thru the plate a couple of
inches on steel pin. Of course this all depends on what the base plate
of the pier pin section looks like. The insulator pin hole should have
enough clearance for a couple of degrees of tower tilt.
However, my drawing search turned up that the 25TG section lists for
$1399, which pretty much trashes this idea!
So, having made several hinged bases for 25 plain sections, for around
70' of R25, I'd be tempted to use a 1/4" thick steel plate with a 2"
hole in the center and weld three leg stubs cross drilled for the bolts
and then use the foregoing 4" diameter insulator block bolted to it.
I bolt the stubs in a section and tack weld them to the plate to get
everything aligned before final welding. I've used 3/4" sch 40 black
(gas) pipe for R25 stubs but a sloppy fit and also made them from solid
HRS rod turned to tight fit. In this application I think either are
ok, since the idea of a pier pin base is too eliminate the bending
stresses at the base.
YMMV, this is backyard engineering judgement.
Maybe Chris KF7P might take this on as something he makes as a product.
kf7p.com
Grant KZ1W
On 9/1/2014 9:47 AM, J. Gordon Beattie, Jr., W2TTT wrote:
Grant,
So insulating a standard pier pin base is your suggestion?
How might I do that? I'm thinking that sandwiching an insulator
underneath
the base plate and isolating the pier pin from the base plate is the
approach you are suggesting. Am I right?
Do you or other have some photos or diagrams?
73,
Gordon, W2TTT
201.314.6964
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf
Of Grant
Saviers
Sent: Monday, September 01, 2014 3:13 PM
To: J. Gordon Beattie, Jr., W2TTT
Cc: towertalk@contesting.com
Subject: [Bulk] Re: [TowerTalk] [Bulk] Re: tower insulators
The max wind combined guy loads for a 100' tower can be around 4000#
downforce on the tower base (plus the tower weight) so something
more than
1" delrin will be required between the legs. That amount of force
obviously
works ok with the tapered pier pin base, but the leg span of
straight R25 or
larger will require some steel structure behind whatever insulator
is used.
I think modifying a tapered pier pin base section bottom plate for an
insulator block of delrin/nylon would be easier/cheaper.
Grant KZ1W
On 9/1/2014 7:29 AM, J. Gordon Beattie, Jr., W2TTT wrote:
Bob, Steve et al,
Any thoughts on what might be a good approach to insulating a Rohn 25
tower from the ground?
I was thinking of a pin base in a lower slab of nylon or delrin
insulating material sandwiched with another upper slab of insulating
material that has bolts into the tower legs. These could be
bolted together at the corners. When I say, "slab" I am thinking
about 0.5-1.0 inches in thickness.
My concerns are that the might be stronger and possibly cheaper. Any
thoughts?
73,
Gordon, W2TTT
201.314.6964
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