Load Indicating Washers
DTI Washer Basics
Direct Tension Indicating (DTI) Washers are washers that have collapsible protrusions capable of measuring how much bolt stress, or tension, has been developed in a fastener. They function independent of torque values and friction factors and indicate load directly when a nut is torqued along the threads of a stud or bolt. Once the induced tension or bolt stress has reached a predetermined minimum, the washers’ protrusions collapse. Standard DTI washers confirm fastener tension by use of feeler gauges. DTI Squirter washers have protrusions that are filled with orange silicon that is expelled when the target tension is reached, allowing for visible confirmation that the fastener is tightened to the required tension. The tool operator moves the tool to the next fastener, in the joint, and repeats the tightening & orange material expulsion process, until all bolts within a joint have been tightened. DTI washers are made from heat treated steel.
Common Finishes of DTI Washers
DTI washers are available in black, or a mechanically galvanized finish, which plays a role in the suitability of the fasteners in different applications, such as enhancing resistance to corrosion from environmental elements or chemicals. Coatings also greatly effect consistency of torque values for installation, and ease of disassembly at the end of service.
Black or Plain Finish DTI Washers
DTI washers are unfinished or will have a minimally protective finish. Black hex nuts offer near zero corrosion resistance, though black oxide coatings or oiled coatings do offer increased corrosion resistance over unfinished DTI washers.
- Black DTI washers are commonly referred to as plain finish, black, black oxide, black phosphate, bare or light oil coated.
Galvanized DTI Washers
DTI washers are coated with a sacrificial zinc coating that acts as an anode to prevent the fastener underneath from corroding. This includes the ASTM F595 DTI washers.
- Mechanically Galvanized: DTI washers are tumbled in a barrel with a mixture of water, zinc powder, other chemicals and glass beads. Although mechanical galvanizing results in a cleaner stud, it is an uncommon choice since it does not provide as much corrosion resistance as hot dipped galvanizing, due to the weaker bond between the steel and zinc.