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| Grease Vs Silicon Grease Vs Dielectric Grease [message #368563] |
Tue, 25 January 2022 11:56  |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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Grease Vs Silicon Grease Vs Dielectric Grease
We have had Oil Wars and Tire Wars and I am going to fire a salvo into a possible Grease War.....
First, some definitions - sort of:
Grease is a lubricant with a very high pour point temperature, so high that it does not become liquid in many applications.
Pour point is an engineering evaluation of the temperature that a material will migrate (run) of its own accord with gravity only.
All normal lubricating greases are non-conductive, that is to say they are dielectric. Greases that are conductive at all are very special products. I do have some, but it is not out where unknowing people could get at it because the cost of the product will convince you that you don’t want to waste it.
Grease like you might use in most places stays below pour point for the larger part. A good and clear exception is in wheel bearings. In that application, it wants to be able to go liquid if the bearing temperature gets high. If (when) that happens, the excess grease you load into the bearing space can save the day. Most every place else, it stays its sticky self and gets pulled into loaded contact points by just being there near the moving parts. All good greases have a very high film strength, that is to say, they will keep loaded metal surfaces apart. Can you guess why this in not what you want in a connector?
Silicon Grease was a development during WWII and at first aircraft instrument makers thought they had the answer to a lot of their temperature problems. Aircraft at very high altitudes had instrument trouble because the lubricants used would go hard with the low temperatures encountered. Silicon greases did not do not this. They thought that this was the golden grease. Problem: They started doing test and yes, the new stuff did not have low temperature issues, but it was not a good lubricant at all. Things worn out fast. This was needed for control gyros that were critical to fire (as in shooting) control and attitude instruments. Things called Dynamotors were also a constant problem. A dynamotor is a low voltage DC motor turning a high voltage generator. This was a way to get the high voltages needed for the operation of the electronics of the day.
It did have a wonderful application as a film creator to keep things you might want to take apart from sticking together like spark plug boots and as a mold release agent for the manufacture of fiberglass reinforced plastic that were arriving on the scene and bringing a whole new set of manufacturing problems with them.
Now, let’s talk about Dielectric Grease. This comes in two “flavors”. First is just plain grease type. This is what you can use to prevent oxidation in connections. It does wonders for little light bulbs. As this is a non-lubricating grease, it is very effective at limiting exposure of surfaces to elements that may degrade the electrical contacts in connections. At the same time, with its almost non-existent film strength, it will not interfere with the intended connections.
Second is Heat Sink compound. This is just absolutely just silicon grease that had been loaded with something to enhance its thermal conductivity. They is most often use Barium Titanate. That is the White Stuff we put under HEI modules. This is an essential part of many semi-conductor application because the junctions that do all the important work create heat while they are doing what we need. While the heat sink compounds have thermal conductivity that is higher than the plain silicon grease, they may also still be enough dielectric to mess up a connection that is supposed to exist or create one where it should not be.
In case you did not identify that above as the situation under an HEI module, look at the parts and think about it again. Same is true of the voltage regulator in the XXSI series alternators. The mounting screws need to make essential connections to the grounded part of the system, and then there is the big metal area that is the heat sink for the electronics inside. Without that heatsink compound in there, the gap between the frame and the electronics works as what engineers call a “Heat Dam”. This is what you really don’t need to cool the inner workings of the module.
How does Matt know all about this??
His father was developing aircraft systems during that war and he often talked about the interesting discoveries that were made. All I had to do was listen and even though I did not understand everything, I retained enough to ask questions after I finished engineering school.
I will be posting this on both GMCnet and the Forum because these questions have come up both places. If I get any interesting questions (about something I forgot?), I will mirror those.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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