[SOLVED] How to properly interconnect screw connected parts

[Zak28]

Many components are interconnected with screws, sot-227, fuse holders ect are some of which use screws. AFAIK oxidation will incur over a period of time, especially with substantial power, iirc there was a correct way of interconnecting such components and I believe it involved some kind of special grease. What is the appropriate way to interconnect screw attached components?

 

[Relayer]

Hello Zak28,
I am not aware of using some type of grease to contact the screw to the component/s.
If there is a worry about oxidization, then a clear spray such as circuit board lacquer
can be applied after the screw has been tied down and secured. But this will cure any
environmental contaminants only.
I have seen oxidization due to increased power usage. A good example is microwave
ovens where the contact tabs on the magnetrons eventually get oxidized.
A good contact cleaner such as CRC-26 is excellent to use, but the contacts should
be cleaned with isopropyl alcohol to remove finger prints, grease and oils, then
spray both contacts and assemble. It will leave a protective film over the sprayed
area.
You can then clean the assembled connectors and spray some circuit board lacquer
to basically seal it up entirely.
Regards,
Relayer

 

[BradtheRad]

I've seen lock washers, the kind with several blade-like teeth on the inside or outside diameter. Many points of contact between metal surfaces. Much resistance to loosening.

There are conductive "liquid silver" pens and epoxies. Sticks to anything. I've used these for low power connections. It's not robust as solder and I suspect the stuff breaks down if made to carry high Amperage.

The grease you refer to may be thermal compound, which is recommended to improve heat transfer away from transistors, etc.

 

[Kajunbee]

You might be referring to dielectric grease.

 

[FvM]

Screwed interconnects have been and are still used in a wide range of consumer and professional electric and electronic circuits without reliability problems. In the high A to kA range, torque wrenches are required.

Problems can arise in situations with high vibration (some machines, vehicles), insufficient initial torque or fading contact force due to thermal fatigue.

I believe that grease appliance is a bad idea because it prevents the desirable "galling" of contact surfaces which forms a strong, corrosion-proof interconnect. As suggested by Relayer, cleaning but no grease.

 

[KlausST]

Hi,

Metallic screwd connections are critical, because there is the risk of corrosion at the contact area of the different parts.
In my eyes the most important is to take care what materials to combine. Wrong material combination is the worst, because in combination with humidity they self destroy. To avoid this the contacts come with several platings to be combined with iron, aluminum, copper...

Brian talks about the "blade like teeth washers". They are good for dry environment, because they generate many gas-tight contact places ... when they are mounted with high force. But with high humidity, condensation or wet environment the small spaces tend to "****" the water inside due to the capillaric effect. And the water may speed up corrosion.

For wet environment I tend to use "full-screen" contact area (not teethed). High force, correct material, and special grease.
The special grease fills the spaces and thus avoids fluids to be sucked in by the capillaric effect and keeps oxigen away.
Avoid split washers to avoid loosening, because they use too low mechanic force (thus they are not allowed for safety mechanical connections anymore). Use high force "tension washers".

"Gas tight connections" means that the two contact materials are pressed with high force togeter. At the contact area they "flow" and fill each others spaces.... not leaving room for gas (oxygen).
Crimp connections work this way, und thus they are very reliable. If made correctly they will cold-weld and are forced together.

Klaus

 

[FvM]

I may be wrong, but I'm not aware of any substantiated suggestions to use contact grease for permanent connections. It may be used for sliding switches or connectors. See also https://en.wikipedia.org/wiki/Cold_welding

 

[KlausST]

Hi,

contact grease for permanent connections:
* especially in power distribution connections with Al / Cu material combination the use of special contact grease is useful.
* and in telecommunications there are permanent IDC connections filled with grease: like "3M Scotchlock".
Both are for use in free air or below ground.

Klaus

 

[FvM]

Can you refer to a product and processing instructions for the former point?

I know 3M Scotchlock, it's beyond the scope of this question, I think.

 

[KlausST]

Hi,



I´ve been working both at an energy provider and at a telecommunications company.
The use of grease at high power connections as well as the telecommunication cables made sense to me. Don´t you think so?

Klaus

I just remember that I´ve seen high power connections sealed with some kind of wax.

 

[FvM]

Thanks. According to the Klauke documents, contact grease is exclusively suggested for Al respectively Al/Cu connections.

I never faced a similar suggestion (or practice) for copper interconnects in power systems.

 

[KlausST]

Hi,

out of curiosity I´ve read some reliable (from big companies) documents about contact grease.
Indeed not much information.

I´ve never read that contact grease increases the contact resistance. (Good news)
But don´t put the grease on the thread, because it influences some mechanical effects. (increased force --> broken bolts. Unfastening bolts)
(My assumption is that the mechanical problems - caused by misuse of the grease - is one of the cause for the bad reputation)
Benfit still is the decreased corrosion.

Some "contact grease" is filled with conductive metal to actively decrease contact resistance. (Makes sense).

Interesting:
There is contact grease filled with corundum. It is especially made for aluminum contacts. It is said that it breakes the aluminum-oxide on the surface of the aluminum.
The confusing is that corundum is aluminum oxide and this aluminim oxide is a very good isolator.

My personal conclusion:
There is not much risk in using contact grease - when used correctly.
You can lower the risk for mechanical problems and increased contact resistance by:
* cleaning the contacts areas
* tighten the screw with correct Nm
* then apply grease just to reduce corrosion

Klaus

 

[Zak28]

After considering what grease is intended for I believe its far irrelevant to my very low power ~120-190amp pulsed current 400-1100W average project. That grease is intended for hugely greater power densities and is likely hugely more expensive. The ideal object which I found out from the topic would be the clear board spray lacquer.