[TowerTalk] tower insulators

[Martin A Flynn]

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 at 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 at 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 at contesting.com] On Behalf 
>>>> Of Grant
>>>> Saviers
>>>> Sent: Monday, September 01, 2014 3:13 PM
>>>> To: J. Gordon Beattie, Jr., W2TTT
>>>> Cc: towertalk at 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