--- On Mon, 2 Jun 1997 15:03:15 -0600 Rick Busche <[email protected]> wrote:
>I have a projector assembly which is essentially a large metal >structure (frame) with a metal projector platform which swivels for >purposes of maintenance. The structure is painted steel, with pressed >in bronze bushings at all motion points each of which have been >measured at significantly less than 0.10 ohms. Arguably, there is no >guarantee that these structures will remain bonded at these levels >over time no argument, guaranteed that joint bonds will get worse >and bonding wires (braid, with crimped ring lugs) have been >added across the pivot points. the generally accepted and workable solution >These wires are secured to the >structure using a toothed washer and screw. The intent of the toothed >washer is to bight through the paint to ensure conductivity. That may be YOUR intent, but the real purpose of a lock washer is to provide a method of assuring loading on the fastener. As you torque down the bolt, you compress the deformable washer. The washer, being "springy" tries to maintain its original shape, thereby loading the fastener system. The commonly used split-ring lock washer has its edge at the split point "dig" into the underside of the bolt head, thus retarding CCW rotation of the bolt. Your star or toothed lock washer deforms at each contact point, giving the same loading effect as a split ring lock washer. With more points, it may be more resistant to bolt rotation. But, each point contact is relatively small. This provides a poor path for a fault current, and an even worse path for RF current. Also, some of the points may not actually dig into the adjacent metal due to slight misalignment of the bolt in the hole. >My customer's safety officer (in Europe) is now insisting that the >paint must be ground off to bare metal and coated with Zinc paint >prior to affixing the lug to the structure. I am concerned that this >is an excessive requirement. Since your base metal is steel, then he likely is suggesting a zinc or galvanized plating over the steel. You could also tin plate or dip. Certainly not a paint. I don't think this is excessive. >It has been my understanding that the >toothed washer (and the screw for that matter) creates a gas tight >connection which is acceptable for all bonding requirements. Nope! >I would be interested in hearing from the group regarding this bonding >issue. I can find no standard or specification which disallows this >biting washer or requires Zinc paint. Rick, let's start by looking in the Don White / Mike Mardiguian EMC Handbook Series, Volume 2, Grounding & Bonding, Figure 3.9 on page 3.12. This is taken right from a more authoritative source, US Air Force Systems Command Design Handbook DH 1-4, Electromagnetic Compatibility. See section DN 5D, specifically sub-note 1(14) on page 5 of DN 5D4. And, looking at this stuff, you suddenly remember. The original source for this is Mil-B-5087B, dated October of 1964! This is a short spec, but loaded with good drawings. And look what paragraph 3.1.1.1.1 (b) says about star washers: "Prohibited for airborne use." The wisdom of all this stuff is that you need a clean, conductive area on your base metal which is larger than your washer diameter. If you were to section the fastener stack, you would see, from bottom up: plated steel lock nut, base metal sheet, wire lug, plated steel flat washer, plated steel split ring lock washer, and finally, the plated steel bolt head. The AFSC DH has a lot more to say about surface treatments, galvanic considerations, structural attachments, etc. Note that all drawings show split ring lock washers; no star washers to be seen here. Also note that the White handbook cautions against star washers, saying "...can cause long term corrosion under the washer teeth." >Any comments would be appreciated. >Rick Busche >[email protected] > > Now, of course, these citings all originate with the US military, and may be a bit overkill for commercial practice. But there's no arguing that they do point us in the RIGHT direction. It's up to you to decide how far down the path you need to go. -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA List-Post: [email protected] Date: 6/3/97 Time: 8:05:31 AM --------------------------
