Direct Tension Indicators (Load Indicating Washers)
Direct Tension Indicators (also known as Load Indicating Washers) are defined in BS 7644: Part 1: 1993, the associated products; Bolt-Face and Nut-Face washers are specified in BS 7644: Part 2: 1993. They are used in conjunction with High Strength Friction Grip fasteners (BS 4395: Part 1 & 2: 1969) to indicate when the required minimum shank tension has been achieved during tightening. They are simple to use, take the guess-work out of bolt tightening and provide a positive means of inspection to confirm correct tension.
In appearance the DTI is washer-like with protrusions on its upper surface. In use it is placed on the bolt with the protrusions against the under-side of the bearing-face of the bolt head (this is the preferred method of installation). Prior to tightening small gaps can be seen between the protrusions, as the bolt is tightened the height of the gaps reduce progressively until a specified height is reached at which point minimum shank tension has been achieved. Measurement is with a supplied feeler gauge used as a “no-go” gauge. This method of tightening bolts is entirely independent of the torque tension relationship.
The original DTI design, introduced in the 1960’s, had a performance variation of 15%. A “New Generation” design by Turnasure, introduced into the UK market in 2001, has significantly reduced the variation in pre-load with HSFG bolting, tests show a spread of results of +/-1%. Turnasure DTIs are manufactured using statistical process control techniques and checked for accuracy by a leading independent testing laboratory. They are test-certified and indented with a batch-trace identity number.
We stock Turnasure DTIs in the size range M16 thru M36 for both standard and higher grade HSFG bolts. They are available with Sherardized and Mechanically Galvanized finishes and to special order in weathering steel.
Mechanical galvanizing of Direct Tension Indicators is an acceptable coating, which does not affect their use or function. The coating has been used both in the USA and UK where long term protective coatings are required; the coating, at 50 – 60 microns more closely matching the thickness of hot dip galvanizing than Sherardizing. The processes are similar with zinc particles being impacted on to the surface to be coated. The major difference is that Mechanical Galvanizing is carried out at room temperature, while Sherardizing is processed at a temperature of 350 – 400 °C. The nature of the surface provides a mechanical key for the adherence of paints, such as Intumescent paint, in a similar manner to Sherardizing. Both Sherardized and Mechanically Galvanized DTIs can be used with self-colour bolts and nuts, tightening to an average gap of 0.25mm will generate the required bolt preload.
