Some answers to your questions: - Star washers (internal, not external) will cut through any normal paints, including powder paints. I have never seen a situation where fully tightening a screw would fail to make a good connection if an internal tooth lock washer was used. But having said that, there are those who would say that this cannot be definitely proven, and so paint masking under the screw will be necessary, if only to make people (i.e. UL etc.) feel more comfortable.
- Standard internal tooth lock washers are as good as it gets. - "Star washers" do make a gas-tight seal when the screw is tightened properly. Light oxidation becomes irrelevant, as does re-oxidation at the contact points. - Because of the aforementioned gas-tight connection, special surface treatments are not beneficial. - PEM nuts ARE "self clinching". Because they cause metal to flow under very high pressure, they make a gas-tight connection to the metal they are inserted into. Any type of "PEM" (or similar other brand) nut is as good as any other in this regard. The differences are in the "nut" part, not the way that metal is displaced when inserting. - The common type of captive lock washer screw is called a "SEMS" type, and is simply a normal screw with a normal internal tooth lock washer held captive by an undercut below the head. They function identically to screws with separate internal tooth lock washers. - If a lock washer is used, there is no point in using an anerobic screw locker like Loctite. Nonetheless, because of the gas-tight nature of the lockwasher interface, the use of Loctite will not affect the electrical connection between washer and screw or part. The pressure at the "points" of the lockwasher amounts to many tens of thousands of psi. Any liquid (i.e. loctite) will be squeezed aside. As for "preferred" types. Anerobic thread lockers are all essentially the same (chemically), being based on a methacrylate ester formulation. Some are more viscous, others less so; some are more reactive (harden faster) others less so, but this matters little when they are subject to the extreme pressure at the points of the lockwasher. - Use of a passivation coating like chromate conversion coating on zinc-plated steel (yellow, clear, or whatever), or "Alodine" on aluminum (also simply a type of chromate conversion coating) is a very good way to assure a stable interface under medium contact pressure. But its basic purpose is to ensure a consistent surface before connection is made. Since it is a sort of "gel", literally mere MOLECULES thick, it is not going to affect a high pressure electrical connection in any way. It is common to hear people say that chromate coatings are "conductive". This is completely false. Chromates are insulators. It is just that they are so thin that under influence of a high pressure connection, they are cut through with only minimal pressure. - Plating or coating treatments under aluminum or steel are useless as an aid to connection, since a) it is likely that it will be required to clear the paint under the screw head/lockwasher anyway, and b) the extreme pressure presented by the points of the lockwasher will cut through the coating and into the base metal. The main purpose for these coatings is paint adhesion. Proper paint adhesion to metal requires a conversion coating. For steel, phosphatizing is commonly used. For aluminum, Alodine chromate conversion is one of the common ones. For zinc or cadmium plated steel, a chromate conversion coating is used. But this is a paint adhesion issue, and as pointed out above, these have no effect on a high pressure connection. - Basically, as far as the electrical interface is concerned, plating on the screw and the lockwasher is irrelevant. The interface is gas-tight. An unplated screw will look like hell after a while, but that is another issue. When tightened, the points of the lock washer will cut through ANY plating present and make a gas-tight interface, so your plating choice should be made based on galvanic compatibility issues, not electrical connection issues. But all this above assumes that paint is masked under the screw/lockwasher interface. It is not a matter that with any paint system, and any reasonable sized screw/lockwasher (say, #6 or larger), there is no way that a good electrical connection cannot be made if the screw is fully tightened! And if the screw is NOT fully tightened, then you cannot guarantee a good connection, no matter what interface arrangement is used. It is simply that regulatory agencies are unlikely to accept this relatively obvious reality, and that they will insist that paint masking is needed. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Chris Wells [mailto:[email protected]] Sent: March 21, 2002 4:40 PM To: 'emc-pstc' Subject: chassis bonding - star washers enough? Looking for some feedback on bonding various parts of a painted metal chassis together for safety and EMC performance issues. I know we have a lot of experts out there on this topic and can remember a number of threads on the topic. Please help me pull it together: * When screwing the cover on the base will star washers cut through paint, even epoxy paint and provide a reliable connection? * Are there special versions of electrical star washers that work better than others? * What about oxidation of the mating surface? I understand star washers can make a gas tight seal. * Should there be a special treatment to the painted area where the screw/washer come together? * Do PEM nuts act as a good mating mechanism for the screw? There are many types are some better than others? What about the self clenching types? * One can get screws with captive washers. Are there any issues with these? Preferred Vendors and types? * There are various chemical materials to keep screws from backing out and are applied to the threads of a screw such as those from Loctite. Do these interfere or help? Are some formulas better than others? * Under the paint if the base metal is steel are there any treatments or plating that help? For Steel, for Aluminum? * If the chassis is plated steel instead like zinc chromate yellow. What changes? * Proper plating on screw and star washer? (assume painted steel) Any advice on the topic welcome! Thanks in advance! Chris Wells Senior Design Engineer Cutler-Hammer [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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