Thank You All for your thoughts on this project. I do, indeed, wonder why all specs were given or were later obtained except thread pitch for the lug screws.
When I first received the project data, I inquired about fastener diameter, alloy & pitch. The diameter was given as #10 (I would have used 1/4 by default on this job but they don't want the lugs re-drilled, removing the zinc plating in the hole.)
Alloy was specified as zinc-steel Grade 1 or better (I didn't know machine screws had grades).
Pitch is the big mystery that you all are helping me with. (And I appreciate the new knowledge I'm gaining.)
Thanks Tony for the silicon bronze idea. I presented it early in the project, but it was shot down due to cost. The justification for zinc steel was that the bars & lugs are both zinc plated. But what about inside the tapped holes? No plating's there & I can't use paste due to no back-up nut.
I use silicon bronze all the time on medium & higher voltage components. To me, the cost would not be prohibitive in this project because the fasteners are quite small. And, if someone works on these devices later, the torque value is high enough that the risk of snapping a fastener will be much less than with alloy steel and certainly brass.
You gave me good food-for-thought, Eddye, about torque & fit being more critical than pitch. But, I'd stop at NF- NEF I think would be too shallow of thread depth for soft copper. (If NEF even exists in #10).
Thanks, Dave, for your stainless steel comment. You have me giving stainless more consideration. I may go ahead and request a change order to 304 if I can get appropriate paperwork to substantiate the alloy.
Usually zinc-steel is specified in this type of application (cheap), but the added cost of stainless in a 300 series would be very minimal and less than silicon bronze. It is proven to play-well-together with copper bus bars, even unplated, in the worst of environments. I often used Types 304 & 308 stainless on open bus in (hot & humid) areas such as mines and it is ideal on poles where road salt sprays 15 feet up. (And it does spray that high & coats everything.)
A locktite type of thread sealant would be ideal, but it is prohibited in this project. A toothed lock washer has to be used instead.
It is, indeed, amazing how many zillions of times I've re-torqued fasteners (large & small) on bus bars, switch gear and in CT cabinets. They do, indeed, loosen from expansion & contraction, as you stated. I used to get service calls quite often for burned bolt-on circuit breakers in panelboards. The tabs burn when the screw loosens from heat-cool cycles. Often, the bus bar is damaged, requiring a costly replacement. I re-torque mine once per year. Transformer lugs have the same problem.
The expansion-contraction and corrosion effects are really manifested when people with aluminum wiring use switches and receptacles that are not rated for direct connection with aluminum wire or when electricians do not properly prepare the aluminum wire before terminating it in the approved devices or adapter pigtails. Big fire hazards to be sure.
If there is no place for longitudinal expansion, bus bars they tend to twist as they heat (thus the stringency about the arc gap space). When they twist, any pressure plate bolted to them gets a gap under it, resulting is a potential for arcing. The bolt holes in this project (1/2-13 studs) are 0.625" oversize to help prevent pushing to the attachment studs' limits of movement. (The studs are mounted on resilient stand-offs with 0.125" allowed movement.)We do have double bolted lugs to help prevent gaps upon twisting.
In real life, these bars & lugs should not get very hot at all, but it doesn't take much heat to cause loosening. Calculations show a load of 400 amps (project max) at unity power factor (pure resistance) will cause <1 degree Celsius rise in 3 hours. But, add harmonics on the 'neutral' bus link from non-linear loads- Who knows how hot that material will get. (Thus the generous over-sizing of the bus. The rated load for this bus is 860 amps at 65-C rise.)
I'm thinking about submitting for a change order to allow me to install GPO-3 in the arc gap clearance under the bus. That would then let nuts be used on the lug fasteners as extra insurance. I'd still tap the bus', but install nuts, too. The GPO-3 would void the gap requirement. This is a very common practice on medium & high voltage equipment. I'd sleep better at night. The downside? GPO-3 eats tooling. A good excuse to buy colbalt if I ever heard one.
Thanks Again everybody for helping. I learned quite a bit here today.
I always learn on this user group. It is a friendly place loaded with good teachers. You all will make a machinist out of me yet!
Paul
