Load-Indicator (DTI) Washer

What DTI washers do

A direct tension indicating (DTI) washer is a hardened washer with bumps on one face. As the bolt is tensioned, the bumps are squashed to a set amount, which shows that the bolt has reached the required tension. Hobson's Squirter troubleshooting article calls them mechanical load cells, and its FAQ notes they are not affected by the things that upset torque, such as weathering, poor lubrication and damaged threads. Hobson's articles stress that torque is not tension: its troubleshooting guide says torque values are not published for structural assemblies, as torque cannot be used for installation.

Hobson's FAQ points to AS 4100 for tensioning with a direct tension indication device. Its DuraSquirt data sheet says they are used with K0 structural bolting assemblies to confirm minimum tension while complying with AS/NZS 5131, and its load indicator washer sheet lists the method as accepted under AS 4100 and AS 5131. Which joints are fully tensioned, and with which bolt assembly, is set by the engineer.

DuraSquirt, Squirter or load indicator washer?

• •DuraSquirt DTI: squirts bright orange indicator from under the bumps when the bolt reaches tension. Hobson's Did You Know sheet says it replaced the original Squirter in class 8.8 and that its indicator is made to last on the connection, so it can be inspected any time after installation. We carry it for class 8.8 (M12 to M36) and class 10.9 (M12 to M36), mechanically galvanised.
• •Squirter DTI: the original squirting DTI, with orange silicone. Hobson's FAQ says the pressed-out silicone is a powder that clings to the steel for up to 3 to 4 weeks, depending on wind and rain; after that a feeler gauge is used. We carry it for class 8.8, class 10.9 (galvanised and plain) and imperial grade 5.
• •Load indicator washer (LIW): works the same way without the indicator. Hobson's sheet says to use feeler gauges to verify that minimum tension has been reached. We carry it for class 8.8, M12 to M36, mechanically galvanised.
• •Hobson's DuraSquirt and load indicator washer sheets say those washers carry a 3-character trace code for traceability.

DTI washer sizes we carry

Choose the type made for your bolt's property class (8.8 or 10.9), then pick the Type (DuraSquirt, Squirter or load indicator washer) in the selector. Each row below opens that size, class and finish; where it lists more than one type, choose the Type in the selector.

How to install DTI washers

From the installation guide on Hobson's DuraSquirt DTI data sheet (220902DS) and its Squirter troubleshooting article. Use the method in your project specification.

1. Assemble the bolt in the connection with the DTI's bumps facing out, away from the steel, and bearing against a hardened structural washer. Never let a turning nut or bolt head bear directly on the bumps: it grinds them off instead of squashing them.
2. Recommended: DTI under the nut, with the hardened washer between the DTI and the nut, and turn the nut. Alternative: DTI under the bolt head, which needs an additional hardened washer under the head.
3. Snug the whole array of bolts first to bring the plies into firm contact: a few strikes of an impact wrench or the full effort of someone on a standard podger spanner. Do not fully compress the DTIs on this pass.
4. Tighten each bolt, working from the stiffest part of the connection out to the free edges, until the orange indicator shows at all bump locations on a DuraSquirt (full indication; completely flat is acceptable after a full squirt). For the Squirter only (not the DuraSquirt), Hobson's older Squirter sheet says to stop when the number of squirts is at least the number of bumps less one.
5. Inspect: orange at the bumps is the visual check. On a class 8.8 DuraSquirt where the orange has weathered off, check the gap between the bumps with a 0.13mm feeler gauge, the inspection gap on Hobson's sheet. For the Squirter, the class 10.9 DuraSquirt and the load indicator washer, the Hobson documents we hold give no inspection gap, so do not use the 0.13mm figure for them: follow the instructions supplied with those washers, Hobson's procedure for that type or your project specification for the gap and how many spaces it must refuse.

DuraSquirt class 8.8 data and the 0.13mm gap check

Hobson's DuraSquirt data sheet (220902DS) gives these values for its class 8.8 mechanically galvanised DuraSquirt DTIs only, with an inspection gap at bolt tension of 0.13mm (measured between the bumps) and a maximum hardness of 37 HRC. They do not apply to the class 10.9 DuraSquirt, the Squirter or the load indicator washers; use each washer's own data. Our 0.13mm feeler gauge is the blade for that gap.

Pre-installation verification

Hobson's DuraSquirt sheet notes that AS/NZS 5131 requires pre-installation verification on at least 3 bolts per diameter and length of each heat number, by the installation crew using the tools they will install with, on bolt assemblies in the same condition as those going in (not fresh bolts if the ones being installed have been out in the weather). Its purpose is to confirm the tooling, assemblies and installers, not to pass or fail the product.

In a tension calibrator: assemble the hardware, tighten until the orange shows at all bump locations, and confirm the tension is at or above the requirement, then confirm the gap is smaller than the calibrated gap provided by Applied Bolting Technology, as Hobson's sheet requires. The visual inspection method is then confirmed for use in the steelwork. Hobson notes each of its branches has a Skidmore tension calibrator and can run demonstrations and training.

When a DTI will not squirt

Hobson's troubleshooting article says a DTI that does not squirt properly is telling you something is wrong. The common causes and fixes:

• •Oversize, slotted or chamfered holes: the indicator moves inward into the hole and shows nothing outside. Hobson's troubleshooting article says to fit an additional round or square washer over the large or slotted hole to provide the correct hole size, and to review the bolt length for it. The same article gives the hole as no more than 2mm over the bolt diameter for M12 to M24 and 3mm over above M24, as per AS 4100.
• •Paint on the steel: it can fill the channels in the washer; an extra washer between the steelwork and the DTI helps.
• •A soft flat washer: the hardened washer must be at least 320 HV (35 HRC), or the bumps dig into it and the gap looks closed without a squirt.
• •Plates out of square or not in full contact, bumps facing the wrong way, or the DTI spinning against the hardened washer (it wears off the indicator).
• •Dry, dirty or rusty bolts, dried-out lube, bolts twisting off or tools stalling: friction is eating the torque. Lubricate the bolt thread and the nut's washer face with stick wax, and store assemblies dry.

Galvanised or plain

• •Mechanically galvanised: Hobson rates it high corrosion resistance (50 micron), similar to hot dip galvanising, with better abrasion resistance and no risk of hydrogen embrittlement. Hobson recommends hot dip galvanised products for long term outdoor use. Use these on galvanised and exposed steelwork.
• •Plain (class 10.9 Squirter only): no coating, supplied lightly oiled. It has no corrosion protection of its own, so use it only where the project specification calls for plain bolting.
• •Storage: Hobson's FAQ says to keep DTIs in their cartons in dry conditions, protected from heat, moisture, rain and dust, not left out in the weather on site. The silicone has a shelf life of over 5 years.

DTI washers in Adelaide

Allmat supplies DTI washers, structural bolts, nuts and washers, feeler gauges and stick wax to steel fabricators and erectors across Adelaide and South Australia. Send your bolting list and we will price the lot.

Questions