Standard DTI FAQs
Hot Dip Galvanizing of Bolting Hardware
Galvanizing of bolting hardware is usually done by the cold mechanical galvanizing process (MG). Occasionally, projects will encounter hot dip galvanizing (HDG) applied to some or all of the bolt assemblies. DTI's are never galvanized by the Hot Dip process. DTI's themselves, when installed under the bolt head, will function correctly even when the bolt is HDG, provided the bolt can be tensioned.
HDG bolts and nuts are often very difficult to install because the correct amount of overtapping of the nut's I.D. has not been done. Also, HDG flat washers are allowed by ASTM F 436 to get as soft as Rockwell C hardness of 26, far softer than Rc of 38 for mechanically galvanized washers. The soft HDG flat washers may "dish" into the hole and cause interference between the nut hex corners and the dished washer.
A partial solution, if the HDG flat washer cannot be changed, is to double up on the number of washers so the dishing is minimized.
Caution: Installing DTI's under an HDG flat washer will often not work correctly because the ASTM F436 flat washer can be very soft, in fact softer than the DTI. If this is the case, the DTI bumps simply dig into the soft HDG flat washer and do not correctly indicate tension. The solution is to change the DTI position (put it directly under the bolt head) or replace the HDG flat washer with an MG flat washer.
