I’m in a position where I need to order high volumes of nuts and bolts for a project.
I read through a South African National Standard, SANS 2001-CS1, in which it states that when I use hot dip galvanized fasteners the nut should be of a higher class than the associated bolt.
Could you please specify whether this is accurate and what the reason will be for this?
Your assistance will be greatly appreciated.
I'd like to preface this with the statement that I am entirely unfamiliar with any South African standards, so take my educated guess for what it is.
I was able to find what I think is the correct version of SANS 2001-CS1:2005 online. I'm assuming your confusion stems from the seemingly contradictory nature of Section 4.5.1.1 with Section 4.5.1.6, which are copied below:
4.5.1.1 Bolt/nut Combinations
The combination of bolts and nuts shall not be less than the following:
a) class 4.8 bolts or screws with class 4 nuts in accordance with SANS 1700-5-2; or
b) class 8.8 bolts with class 8 nuts in accordance with SANS 1700-5-2.
Any bolt assembly which seizes when being tightened shall be replaced.
4.5.1.6 Galvanized nuts
Nuts that are to be hot-dip galvanized shall be of a higher class than the associated bolt or screw. Nuts shall be checked after being galvanized for free running on the bolt and shall be re-tapped to the permissible deviations if necessary to ensure a satisfactory tightening performance.
My guess is one of two things:
either the hardness of the nut is expected to decrease after the hot-dip galvanization process (which dips the nut in molten zinc, thereby tempering it a little), thus "downgrading" it, or
galvanized nuts are sold to a different class system, so you have to make sure your nut class exceeds the bolt class (I don't know how things work in South Africa, so this is a wild guess).
Section 4.5.1.1 shows that a "matching" nut is used with the corresponding bolt. You want a nut that's stronger than the bolt, such that if the bolt fails (fractures), it's very apparent that the failure has occurred. If a thread fails, it's much less noticeable, and hence more dangerous since that bolt assembly cannot be counted on. See this PDF for more information on the topic.
