What are the problems with Grade A194 2-H and Grade A563 DH heavy hex nuts.
This edition of the Fastener Training Minute with Carmen Vertullo was originally published February 19, 2020 as “Heavy Hex Nuts” during episode 149 of Fully Threaded Radio.
Galvanized Heavy Hex Nuts come in a couple of different flavors in the inch world primarily, and at the lower end of the scale they come from ASTM A563 grade A or grade O and these are typically used with lower strength fasteners such as carriage bolts or A307 hex machine bolts, relatively low-cost, we don’t have a lot of trouble with those. On the other end of the scale at high strength levels from ASTM A563, we have grade DH heavy hex nuts galvanized, and these are used with things like ASTM A325 structural bolts.
These hot dipped galvanized nuts are over-tapped. That means that when they’re manufactured the nut is made complete without the threads then it’s galvanized and then tapped slightly oversized in order to accommodate the galvanizing that’s going to be on the threads of the mating bolt. Typically this nut is mated with a washer and a bolt and sold as an assembly, but not always. Additionally the nuts may have some wax on them. That wax is to help the torque tension relationship because of the roughness of the galvanizing on the threads. They don’t always have wax, in that the wax is not a requirement, contrary to popular opinion. It’s only required if asked for, but most manufacturers will automatically wax these A563 DH nuts because they know the application where they’re going to be used.
So what’s the problem?
The problem is that there is another standard ASTM A194 which contains a very similar nut called a 2-H heavy Hex Nut, and that nut is primarily used in its plain condition for high temperature pressure assemblies used with A193 Grade B7 bolts. Because this nut is so common and is so similar to the Grade DH nut from A563, we’re allowed to substitute it. A563 says clearly that it’s okay to use the 2H nut from A194 as a substitute.
The problem is this A194 prohibits the galvanization of this heavy hex nut, unless the customer specifically asked for it. The other issue is that the rating for load of the two nuts is different. Both in their plain form, an A194 2-H nut has a maximum load rating of 150000 PSI and the A563 DH nut has a maximum load rating of 170000 PSI, and that’s what they’re tested at. Now when we galvanized the A563 nut they make an accommodation for that and they down-rate it to 150000 PSI. So sometimes when we’re testing these nuts, whether they are over tapped or not can make a big difference in whether or not they’re able to withstand the proof load pressure that is required. When we get them out into the field, most of the time they work just fine and nobody cares or can tell the difference, but when we get them into the test lab that’s where we have an issue.
So be careful when you’re specifying a galvanized heavy hex nut. I recommend that when we are galvanized heavy hex nuts, we stay with The ASTM A563 and don’t use the A194 substitute.
I know there would be a lot of push-back on that because it’s very common out there in the world, but when we do it, let’s make sure that we understand that these nuts have different testing requirements. If I could rewrite the standard, I would say something in the A194 specification such as whenever we use this A194 2-H nut in place of the aA563 DH nut, it must only meet the requirements of a 563 DH in that it no longer needs to meet the requirements of an A194 2-H nut. They probably should consider dual marking these nuts, that would be another idea. Well, that’s what we have to tell you today.
I know it sounds very complicated. It’s actually very simple but galvanized heavy hex nuts can sometimes be a problem.