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Old 21st March 2009, 10:48 PM   #1
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Default Mounting hardware for potted toroids?

I have some toroidal transformers which are center potted with epoxy. A screw goes up through the bottom panel of the power supply chassis, through the potted center, then a flat washer, then a toothed lock washer, and finally a nut. It seems no matter how tight I make it without deforming the chassis, the transformer will be loose after going through FedEx or UPS's hands. I think there is also a significant difference in tightness depending on if the transformer is hot or cold.

What is the best way to fix this?

Some possibilities I want to try are:
- Change the tooth lock washer to a split (spring) lock washer
- Add loctite on the threads
- Add a 2nd nut on top
- Add a fender washer below the screw head to help prevent deforming the aluminum chassis bottom
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Old 22nd March 2009, 01:09 AM   #2
Chartal is offline Chartal  Canada
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You can try a nut with nylon insert.
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Old 22nd March 2009, 03:16 AM   #3
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some manufacturers use rubber pads on top of and under the transformer with a large steel washer about 80% of the diameter of the transformer. it seems to work well for pro audio amps that get lots of road abuse. the fender washer under the screw head is a good idea. loctite? depends on what type you use. the type used for scope ring screws on firearms is probably good (it's green or blue) because you can always remove the bolt or nut if you have to. the red stuff isn't easy to get apart again. one thing i'll mention that's not always obvious when you build a project using a toroid... make sure you avoid electrical contact between the top cover and bottom cover through the mounting bolt. it acts like a shorted turn and blows the line fuse
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Old 22nd March 2009, 03:41 AM   #4
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I prefer split ring lock washers to toothed washers. Using a nylon-insert (nylock) nut will also help the nut stay on, though you will want to minimize the length of the screw or you will be causing yourself undue frustration; there is no such thing as a free-spinning nylock.

If you really want to nip this thing in the bud, stack flat washers* around the stud below the toroid until the toroid is no longer resting on the aluminum. The potted center of the toroid will then be resting on the washers. Now you can wrench the $$$$ out of the nut(s) without damaging the aluminum, since the bending stress will be removed. Do add the fender washer underneath the head. You definitely won't need an annoying nylock this way.

If the nut still goes loose even after you torque it 'til she squeals, you need to work on your packaging methods.

*As wide as you can fit inside the toroid; they don't need to fit the screw properly.
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Old 23rd March 2009, 01:17 AM   #5
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I forgot to mention something. It'll help if you add a fender washer underneath the last stacked washer that makes the transformer "lift off". So:

nut
split ring lock washer
fender washer (and/or whatever goes on top of the toroid)
potted center
washer
...
washer
fender washer
chassis
fender washer
screw head
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Old 23rd March 2009, 04:41 AM   #6
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and glue a piece of "fish paper" (that grey epoxy filled cardboard used to isolate high voltage) on the top cover above the nut if the bolt or nut are within 1/4" (6mm) of the top cover. this will preclude any mysterious blown fuses if something heavy gets stacked on top of the amp while it's running.
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