Porcelain Insulators Cracking

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dd92251
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Porcelain Insulators Cracking

Post by dd92251 » Tue Aug 13, 2019 2:09 pm

Hi All,

Anyone who has done AM antenna work knows that porcelain insulators have a propensity to cracking. Typically this is caused by vibrations in the equipment they're used in and lateral shear. That is to say "the coil they're supporting is heavy and wiggles and breaks the insulator".

I'm curious if anyone has found a good cure for this issue. Will replacing the mounting hardware with Teflon bolts help ? Or is this something that requires an additional support to keep the Coil-or-Cap from wiggling ?

I haven't spent much time on this issue, but it seems like a good solution would really help lots of engineers. :D


Best Wishes
Dave Dybas

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RGORJANCE
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Re: Porcelain Insulators Cracking

Post by RGORJANCE » Tue Aug 13, 2019 3:01 pm

Hi Dave:

Well, I have built several DA's, worked on a lot of them and built a couple phasors and ATU's. Only time I ever had cracking was when the insulators weren't mounted with the fiber standoff washers, or caused lightning strikes. Those washers should be used on both ends.

I do recall busting a few when I snugged the bolts too tight. (tighten till it shatters, then back off half a turn)

Obviously, if there is any flexing going on, that can do it too. The only other thing I have encountered was on some crude home made stuff, where the copper pipe plumbing wasn't correctly measured to fit......that put strain against the insulator which caused cracking.

Hope this helps.

Fossil

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Deep Thought
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Re: Porcelain Insulators Cracking

Post by Deep Thought » Tue Aug 13, 2019 3:22 pm

I agree with Fossil. If you don't use the fiber washers they'll crack from thermal expansion. Also, make sure there are the proper number of standoff insulators on coils...small 10 amp coils can get away with just two--one each in the middle of the frame--since those are pretty light, but anything bigger needs four to avoid side-motion moments during vibration of the cabinet cracking the porcelain.

Don't crank the hardware down without lock washers...I prefer split washers but star washers will work fine too. The mounting hardware really doesn't need to be torqued down nearly as much as the RF connections.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: Porcelain Insulators Cracking

Post by RodeoJack » Wed Aug 14, 2019 1:32 am

Added to the above: One "lab" likes to put some kind of locktite on their standoffs in addition to tightening the screws (imo) way too much.

None of the replacements I've installed has broken, having done nothing more than just snugging them down.

At one station, I replace over 40 standoffs of different sizes. I found a group of 3 caps, hanging by their plumbing, with the lower sections being supported (and shorting out by) the cabinet wall. That left a nice burn mark. Once fixed, old 10kW Ampfet was grateful, not having to deal with a shorted ATU.

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Dale H. Cook
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Re: Porcelain Insulators Cracking

Post by Dale H. Cook » Wed Aug 14, 2019 7:23 am

I agree with Fossil and Deep. You have to use the washers, especially outdoors, to prevent cracking from thermal cycling. Over-tightening is also a major cause of cracking. Years ago I learned a trick from an older engineer when we were rebuilding a 40+ year old 5-tower DA with dozens of standoffs. Because the patterns were being modified all of the inductors had to come out. He recommended tossing all of the old fiber washers and making new washers using the thinnest cork gasket material from the auto parts store (I used my Whitney punch for the holes). Of course I replaced any standoffs that showed any evidence of damage (the station had a good stock of spares and we generated some more because the day pattern was being simplified). That was about 30 years ago and I have not had any insulators crack.
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dd92251
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Re: Porcelain Insulators Cracking

Post by dd92251 » Wed Aug 14, 2019 3:18 pm

Greetings,

Thanks to all for your suggestions. The unit with the insulator problem is a Cell Tower detuning network box made by folks who want to make your cell tower... site... safe :D The coil is mounted with the insulators secured to the back wall, I think I'm going to change that and secure the insulators to the bottom of the cabinet. That should take some lateral stress off of them. I think a little heftier diameter insulator will help as well.

All of these insulators have their fiber washers installed, so that isn't the issue. I can't tell if they were overtightened, the bolts are no longer attached.

I've been told that lightning will destroy all the insulators real quickly, and maybe that is the culprit. The Cell Tower is about 150 ft tall so it could attract lightning.

I knew something was wrong with this unit when I opened the box door and got drenched with a gallon of water that poured out from it. LoL

Best Wishes
Dave Dybas

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