Our two main concerns with providing galvanized bolts larger than 1-1/2” diameter with 8 threads per inch is the lack of overtap allowance standards for the mating nut and the potential for premature thread stripping due to the nut oversizing.
Overtap Allowances
All inch series hot dip galvanized hex nuts, regardless of grade, are tapped oversize to accommodate the extra 2 – 6 mils (.002 – .006) of zinc that is added to the threads of the bolt during the galvanizing process. Without this allowance, the nut would not thread onto the bolt. As you can see in the table below from ASTM A563, the diametral allowance increases with the size of the nut, maxing out at 0.050” overtap at 1-3/4” diameter and remaining the same up through 4”-4. You will also notice that the allowance for 8 TPI threads stops at 1-1/2” (0.027 for both UNC and 8-UN) and does not continue above it. The problem this causes is as follows: If you are manufacturing a 1-3/4”-8 UN nut, and are being asked to overtap it for hot dip galvanizing, how much do you overtap it? Do you assume that the pattern would continue and that the larger nut would need to be tapped 0.050” oversize, or is that too much for those finer pitch threads? That is an answer we don’t have, and that leads us into the second part of our problem, premature thread stripping.
Thread Oversizing Allowances
| Nominal Nut Size, in. and Pitch | Diametral Allowance, in. | Nominal Nut Size, in. and Pitch | Diametral Allowance, in. | Nominal Nut Size, in. and Pitch | Diametral Allowance, in. | ||
|---|---|---|---|---|---|---|---|
| 1/4-20 | 0.016 | 1-1/8-8 | 0.024 | 2-4-1/2 | 0.050 | ||
| 5/16-18 | 0.017 | 1-1/8-7 | 0.024 | 2-1/4-4-1/2 | 0.050 | ||
| 3/8-16 | 0.017 | 1-1/4-8 | 0.024 | 2-1/2-4 | 0.050 | ||
| 7/16-14 | 0.018 | 1-1/4-7 | 0.024 | 2-3/4-4 | 0.050 | ||
| 1/2-13 | 0.018 | 1-3/8-8 | 0.027 | 3-4 | 0.050 | ||
| 9/16-12 | 0.020 | 1-3/8-6 | 0.027 | 3-1/4-4 | 0.050 | ||
| 5/8-11 | 0.020 | 1-1/2-8 | 0.027 | 3-1/2-4 | 0.050 | ||
| 3/4-10 | 0.020 | 1-1/2-6 | 0.027 | 3-3/4-4 | 0.050 | ||
| 7/8-9 | 0.022 | 1-3/4-5 | 0.050 | 4-4 | 0.050 | ||
| 1-8 | 0.024 | ||||||
| Table 5 of ASTM A563: Thread Dimensions and Overtapping Allowances for Nuts Hot Dipped Galvanized per Specification F2329 | |||||||
Thread Stripping
An important fundamental of joint design is that, whenever possible, a joint should be designed so that the bolt’s breaking is the mode of failure, as opposed to the threads stripping, which can be a delayed and more catastrophic failure since it can go unnoticed until the joint is put into service. In calculating thread strength, the thread depth provides the resistance to stripping out, and that resistance must be greater than the tensile strength of the bolt or else the connection is unsafe. Two factors make large 8 TPI hot-dip galvanized fasteners less resistant to thread stripping. First, 8 TPI threads in large diameters have a shallower thread profile that of Unified National Coarse threads. A shallower profile means less thread depth, and thread depth is one of the major factors in stripping resistance. Second, when you oversize the internal threads of the nut (without any specific guidelines as per the above overtap allowance explanation), you are essentially removing more of the thread depth, depth that was already lessened due to the shallower thread profile.
Let’s look at an example for an ASTM A193 Grade B7 bolt with 3”-8 UN Class 2A threads and an ASTM A194 Grade2H nut with 3”-8 UN Class 2B threads for plain finish, galvanized with an overtap of 0.027”, and galvanized with an overtap of 0.050”.
Minimum stripping resistance for the external threads
| 3-8 UN plain finish | 855,000lbs |
|---|---|
| 3-8 UN HDG/overtap 0.027” | 678,000lbs |
| 3-8 UN HDG/overtap 0.050” | 524,000lbs |
In comparison, the minimum tensile strength of a 3”-8 UN A193 Grade B7 bolt is 748,000lbs.
So clearly, even a modest amount of overtapping drastically reduces the stripping resistance, in this case a full 70,000lbs, which is 9% below where we would normally expect the bolt to fail. As the overtap increases to 0.050”, the difference between tensile and stripping resistance is 224,000lbs, or a whopping 30%.
Another example, using a 2”-8 A193 Grade B7 bolt, breaks down as follows:
Minimum stripping resistance of the external threads
| 2-8 UN plain finish | 388,000lbs |
|---|---|
| 2-8 UN HDG/overtap 0.027” | 310,000lbs |
| 2-8 UN HDG/overtap 0.050” | 242,000lbs |
In comparison, the minimum tensile strength of a 2”-8 UN A193 Grade B7 bolt is 346,000lbs.
So, like the first example, the thread stripping resistance of a plain finish fastener well exceeds the tensile strength of the bolt, but drastically decreases with overtapping. Overtapping 0.027” decreases the stripping resistance by 78,000lbs and is 36,000lbs below the minimum failure point of the fastener, and overtapping 0.050” decreases the stripping resistance by 146,000lbs which is 104,000lbs below the minimum failure point.
In summary, ASTM A563 is the only nut specification that addresses specific modifications to thread geometry and proof load values for hot-dip galvanized nuts. Therefore, only thread and pitch combinations listed in Table 5 of ASTM A563 (above) are recommended for hot-dip galvanized fasteners.