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APPLICATION ADVISORIES

Hot Dip Galvanized Flat Washers

Installing DTI's under an HDG flat washer will occasionally not work correctly because the ASTM F436 flat washer can be very soft, in fact softer than the DTI. ASTM F436 currently allows the hardness of hot dipped flat washers to go as low as Rc 26 compared to Rc 38 to 45 for all other flat washers. All flat washers, whether hot dipped galvanized or not, should be made to a hardness of Rc 38 to 45, in order to function correctly for all structural purposes including to compress a DTI. If flat washers softer than Rc 32 are found on a project, the DTI bumps simply dig into the soft HDG flat washer and therefore the apparent residual DTI gap will not correctly indicate tension. Also, if the flat washers are soft, the Squirt™ feature may occur late. The solution is to change and replace the HDG flat washer with an MG flat washer. Alternatively, correctly made hot dipped flat washers to Rc minimum 38 can be purchased in the marketplace and are no more expensive.

Flat washers with maximum tolerance inner diameter on a squirting DTI

First, establish the inner diameter is within the washer standard’s tolerance. If the washer exceeds the standard’s tolerance, the washer should be rejected and replaced. If the washer is within the maximum allowable limit, a baseline Squirt™ event should be determined.

Occasionally when a flat washer having an inner diameter at its maximum allowable tolerance is placed on top of a squirting DTI, not every bump of the DTI will be in contact with the underside of the washer. When this happens, the DTI may not Squirt™ at every available bump location. The best way to deal with this issue is to replace the flat washers with those with a tighter ID. If this is not feasible, a diagnostic test with a tension calibrator should be conducted to see what tension value is being reached with the existing hardware combination.

Place an assembly in the calibrator and tighten the bolt to the minimum required tension. Note the number of squirted locations. Afterward, continue to tighten the assembly until as many locations Squirt™, as possible. If the number of locations squirted are different (x number at min, & x plus more spaces above min), partial DTI Squirt™ can be used as the inspection criteria.

squirting DTIs not squirting, or partially squirting, when placed in a Tension Calibrator

First, establish the plate of the tension calibrator is in good shape and the proper plate for the bolt diameter being tested. If not, make the necessary corrections by replacing the plate and retest.

If the tension calibrator plate is in good shape and properly sized. Look closely at the inner diameter of the plate where the DTI is placed and determine if there is an ID chamfer that increases the ID at the surface of the plate. This is best performed by removing the plate and placing the DTI against the plate and looking through the ID of the plate opposite the DTI. If orange Squirt™ media can easily be seen when the two components are lined up, the issue of lack of Squirt™ is probably caused by Squirt™ media being forced toward the ID of the DTI, toward the bolt shank, and not toward the OD of the DTI.

To correct for the chamfer, place an additional flat washer underneath the DTI and retest. The issue will not occur in the steelwork since it is not typical for steelwork holes to be chamfered after drilling or punching.

If the plate is in poor shape and must be replaced, inform the supplier that the plate will be used to test DTIs and to leave the ID sharp, i.e. without a chamfer. Informing the supplier about DTIs should also trigger them to provide a plate that better performs with DTIs. DTI plates have larger flat surfaces to support the DTI than standard plates but still can be used when testing assemblies that do not include DTIs.

Difficulty feeler gauging DTI with a tension calibrator plate, recessed for short bolts

Standard calibrator plates have a recessed bore designed to permit the device to test short bolts. However, this recess’ ID is marginally larger than the OD of a DTI or flat washer. Therefore, it is a difficult to check DTI bump compression with a feeler gauge since the DTI is below the outer face of the calibrator plate.

The easiest solution is to test the DTIs on the non-recessed side of the plate, with a longer bolt. The DTI feeler gauge information (acceptances & rejections) gleaned from the longer bolt can be applied to short bolt installation & inspection.

Another option is to obtain a calibrator plate designed to be used with DTIs. Such plates have larger fastener supporting faces than standard plates and allow feeler gauge insertion regardless of bolt length. When ordering new plates, notify the supplier DTIs will be tested and request to DTI style plate.

Also, when ordering a new DTI plate, require that the plate’s ID not include any chamfer. Plate ID chamfers may interfere with squirting DTIs. See the previous advisory for more information.