Squirter® DTI FAQs
Flat Washer RequirementsDo DTI's change the flat washer requirements?
No, but occasionally yes, they can. Flat washers are made to ASTM F436, hardened, and are used to either reduce the tendency of the bolt to embed itself into the steel plates as it is tensioned, to return the apparent hole size to standard when it is larger than standard, or to reduce the turning resistance of the nut since it is turned against the hard flat washer and not the soft steel plates. They are inexpensive and NEVER affect the bolt preload as long as a .005" feeler gage is used.
The flat washer requirements of the connection are determined by the RCSC specification. The number and position of flat washers depends on bolt grade, bolt size, hole size, and bolt tensioning method, and in general they are not changed when DTI's are used. Because there are a few possible exceptions to this rule, some explanation is required.
Standard Sized Holes (Bolt Nominal Shank Dia plus 1/16" maximum)
For A490 bolts installed in steel plates of yield strength less than 40,000 psi, say in A36 steel, a hard flat washer is required under BOTH the head and the nut to prevent excessive embedment. In this case, the DTI can replace either one of these flat washers. If the DTI replaces the flat washer under the bolt head, then indeed one flat washer is saved. Note that if the DTI replaces the flat washer on the nut end, a total of two flat washers is still required because another flat washer must be installed between the nut being turned and the DTI bumps.
For bolts which were intended to be installed by the (rarely used) calibrated wrench procedure, and therefore which require a hard flat washer under the nut, when a DTI is added one flat washer is still necessary under the nut if the DTI is installed under the bolt head, and one flat washer is still needed between the nut and the DTI bumps if the DTI is installed on the nut end.
The rule of thumb here is this -- "Where a hard flat washer is needed over a STANDARD SIZED hole, it may be replaced by a DTI." Keep in mind, however, that at all times A HARD FLAT WASHER MUST BE POSITIONED BETWEEN THE TURNING NUT AND THE DTI BUMPS.
Oversized Holes (Bolt Nominal Shank Dia plus 1/8" or 1/4" depending
The flat washer requirements on oversized or slotted holes are not changed when Standard DTI's are added. It is not possible to replace or eliminate a flat washer when oversized holes are used. The DTI is added as if the flat washer (see table here) restores the hole to standard size. For Squirter® DTI's the flat washer requirements must be slightly different. See HERE
The rule of thumb here is this -- "Where a hard flat washer is to be used over a STANDARD SIZED hole, it may be replaced by a DTI." Keep in mind, however, that at all times A HARD FLAT WASHER MUST BE POSITIONED BETWEEN THE TURNING NUT AND THE DTI BUMPS
You may also be interested in viewing:
- Note -- If you have oversized holes in an outer ply, please look at the Application Advisory regarding 1 1/4" and larger diameter A490 Bolts in oversized holes
- Note -- If you are contemplating allowing hot dipped galvanized flat washers instead of mechanically galvanized only, please look at the Application Advisory regarding hot dip galvanizing of flat washers