DTIs, are single use mechanical load cells used to indicate when the required tightening torque has been achieved in structural fastener assemblies.
Direct tension indicators are hardened washer shaped components with 4 to 6 small arch-like protrusions on the bearing surface that are designed to deform in a controlled manner when subjected to appreciable tightening torque.
Basically the Direct tension indicators washers work on the principle that appreciable tightening force on the bolt nut assembly the arch like protrusions collapse under the applied load and this indicates that the required torque has been applied in tightening the assembly. The gaps in between the washer and nut are normally checked with thin feeler gauges. In some cases the protrusions are filled with certain silicone material which flows out from the protrusions and through the crevice in the washer towards the outer edge thus indicating that appreciable torque has been applied in tightening the assembly. These washers are known as Squirter Washers because the silicone is squirted out from the protusions as the washer flattens out with increasing load.
High quality alloy steel/C-65 spring steel material used in the manufacture of direct tension indicators washers shall be designed, processed, heat treated, and protectively coated as required. These direct tension indicators washers are manufactured to conform to required international standards for chemical composition, load, and dimensions.
DTIs are a function of induced tension. As the fastener assembly is tightened, the arch-like protrusions are compressed and the change in distance between the base of the protrusions of the DTI washer and the protrusion apex correlates to a value of tensile force induced into the fastener. This distance can be verified by insertion of a tapered feeler gage between the protrusions to the bolt shank. The number of instances the bolt shank can be reached by the feeler gage is then compared to either the results of a pre-installation verification procedure or the manufacturer's instructions.
DTI washers indicate the amount of torque applied to tighten the assembly is a function of induced tension. As a fastener assembly is tightened, the arch-like protrusions are compressed and the change in distance between the base of the protrusions of the DTI washer and the protrusion apex, indicate the amount of tensile force induced into the fastener assembly.
Common uses of DTI washers are in machinery and heavy structures like road and rail bridges and also building structures. Inspection entails that structural engineers randomly check the fastening units with tapered feeler gauges. Though the washers are made to international standards the same can be cross checked by using torque indicating wrenches which give a reading of the applied torque. Nowadays it is mandatory for design engineers to specify the use of DTI washers because of this inherent characteristic.
DTI Washer can sit on the structural member or be separated by a washer, but it should never be installed with the bumps facing the member and it should never be touching a rotating surface.
DTI Washer is the worldâ€™s simplest cost effective solution to control the tensioning of bolts. These unique, patented fasteners deliver consistent, reliable bolt tension, along with installation and inspection efficiency. They are used every day to achieve required tension loads on bolts in countless applications including: structural steel buildings and bridges, studs used in the petro-chemical industry, anchor bolts and SAE cap screws for automotive applications. This washer is also known as ASTM F959 Washer.