[Antennas] RE: Fasteners

2002年6月25日 19:51:42 -0400

Frank, <[email protected]>, gave some excellent advice on the 
choice of fasteners and bolts.
"There is no substitute for quality fasteners....
 Get the SAME (SAME meaning IDENTICAL) ones speced by 
 the tower manufacturer and DO NOT SKIMP....
 Same grade
 Same size
 Same thread (and a lock nut if speced)
 Same washer & lock washer and nuts as specified
 Some manufacturers spec a torque setting for a 
 reason
Buying from the tower manufacturer directly may cost a bit 
more, but may save you a lot of headaches in the future... 
Almost every tower will eventually need to come down. It's 
easier to unscrew a bolt than cut it with a torch!"
People should file this advice away for the future! Frank
then wrote:
"Stainless steel:
 IS NOT rust proof, just very rust resistant
 Can very in strength and quality
 Can gall - you may need to use some sort of anti-
 electrolysis compound, especially if within 20 
 miles of salt water.
 Is usually not as strong as similar grade carbon steel 
 bolts
 Imported bolts & nuts are frequently mis-labeled as to 
 grade
 Same is true of units of unknown origin
 Lowes and Home Depot stress price! Read what you will
 into that."
I am not a metallurgist, but I am a PhD Chemical/Electrical 
Engineer with lots of experience with corrosion and 
alloys. I have taken several courses on metallurgy - enough 
to know when a real metallurgist is needed, but the 
questions here are pretty straightforward. So let me add 
a little here.
There are well over 70 standard types of stainless steel, 
but most high-strength stainless fasteners we are likely to 
encounter are made from the 400 series of stainless steel. 
These are the strongest stainless alloys and they can be 
hardened by heat treatment. They are somewhat corrosion 
resistant, but this property is inferior to the 300 series 
of stainless. This series is difficult to machine because 
it work hardens and galls; it cannot be heat treated to 
increase its tensile strength. So going back to Frank's 
advice, first you have to know which alloy is actually 
being used. The galling, however, is not a result of 
corrosion; it relates to work hardening and the fact that 
the 300 series is malleable until it is work hardened. 
Proper lubrication is very important with the 300 series.
Just as with the stainless steels, the tensile strength of 
carbon steel depends greatly on the heat treatment and cold 
working (for example, rolled threads are stronger than cut 
threads). The over-the-counter bolts are not strong at 
all. If you need the strength, you will have move to 
higher grades - and the bolt heads should be clearly marked 
to denote the grade. Counterfeit bolts with improper 
ratings are sadly all too common these days, so buy from a 
reputable supplier.
Going back to the corrosion issue for a moment, galvanized 
steel bolts will be the most compatible with galvanized 
steel towers. The various stainless alloys are far enough 
away from zinc in the electrochemical series to promote the 
corrosion of the tower zinc.
So to join in with the crowd, I would stick with what the 
manufacturer recommends. Skimping on safety is not real 
savings in the long run.
 73, Barry L. Ornitz WA4VZQ [email protected]