TC Bolt (Tension Control)

How TC bolts work

A TC bolt (Hobson calls them TCBOLTs) has a round dome head at one end and, beyond the thread at the other, a narrow break neck and a splined end. The shear wrench has two sockets: the outer one turns the nut, and the inner one holds the spline and takes the reaction. Hobson's data sheet describes what happens next: when the correct preload is reached the outer socket stops rotating, the inner socket counter-rotates and shears the spline off, and the wrench ejects it. The bolt is then installed at the correct tension, and the missing spline end shows it at a glance.

Hobson's article on TC bolts explains that the name is a little misleading: the break neck shears at a set torque, and that torque gives the calibrated tension only while the friction in the assembly stays as it was supplied. That is why TC bolts come as a matched bolt, nut and washer, must stay in their sealed packaging until they are used, and are never lubricated on site. Because the nut torque is reacted by the spline, the bolt is not twisted during tightening, which Hobson says makes it less prone to losing preload after installation.

Hobson lists the benefits of the system as consistent tension, one-person installation from one side of the steel, no torsional force on the bolt, reduced installer fatigue and an accurate visual inspection. HRC is the EN 14399-10 system name (bolt and nut assemblies with calibrated preload), not a hardness. Hobson's sheet gives the nut as an HRD nut and the washer to EN 14399-5, hardness 300 to 370 HV.

About the K0 on the label: Hobson's labels read TCBOLTS Structural K0 HRC, but these are EN 14399-10 assemblies, not AS/NZS 1252 K0 bolts. They take only the nut and washer packed with them: do not add loose K0 nuts or washers from the AS/NZS 1252 range, even though those carry the same K0 name.

TC bolts, AS 4100 and AS/NZS 5131

From Hobson's articles, in summary. AS 4100 (2020) permits other high strength fasteners in place of AS/NZS 1252 bolts where evidence of their equivalence is available, and Hobson's article says this allows EN 14399-10 HRC assemblies to be used with AS 4100. The conditions it lists: chemical composition and mechanical properties in accordance with AS/NZS 1252, body diameter and head or nut bearing areas no less than an AS/NZS 1252 bolt and nut, and a tensioning and inspection method that may differ from AS 4100 as long as the minimum tension is met and the procedure can be checked. AS 4100 refers bolt tensioning and inspection to AS/NZS 5131, which gives guidance on shear-off tension controlled bolts to EN 14399-10.

Hobson's TC bolt article adds that EN 14399-10 class 10.9 HRC assemblies use the same bolt thread lengths as EN 14399-3, and that a properly calibrated assembly can be used as a calibrated tension indicator device under AS 4100, providing the minimum preload required for fully tensioned (TF and TB) joints. Its AS 4100 article also points designers to a reduction factor for class 10.9 bolts in shear where the threads intercept the shear plane.

TC bolts are an engineered product: the bolt class, size, bolting category and inspection method come from the engineer's design and your project specification, not from this page.

TC bolt sizes we carry

Every TC bolt is class 10.9, hot dip galvanised, coarse thread, and comes as a bolt, nut and washer. Select a row to open that size, then pick the length.

Choosing the TC bolt length

Hobson's data sheet gives the rule: bolt length = grip length (the total thickness of steel the bolt clamps) + the added length for the nut, the washer and two thread pitches (below). That leaves at least one full thread protruding from the nut. Then pick the nearest length we carry at or above the answer.

Worked example: an M20 bolt through a 16mm plate and a 20mm plate has a 36mm grip. 36 + 27 = 63mm, so order M20 x 65mm.

How to install TC bolts with a shear wrench

From the installation guide on Hobson's TCBOLT data sheet, which follows EN 1090-2. Your project specification and AS/NZS 5131 set the inspection.

1. Check that the nut runs freely along the bolt. Discard any assembly where the nut does not run freely after initial loosening.
2. Assemble with the washer under the nut, chamfered side facing the nut (away from the steelwork), and the nut markings facing away from the steel. Use only the nut and washer supplied with the bolt.
3. Fit every bolt in the connection and bring them all to snug tight before any tensioning. The plates must be in full contact with no gap: Hobson warns that the correct tension, and so the load transmission, will not be achieved if a gap remains, so watch for distortion from welding. Snug with a podger spanner, an electric snug master (Tone SGS-5LE) or a few impacts of an impact wrench, depending on the number and size of bolts.
4. Put the spline end into the inner socket and the nut into the outer socket, and make sure the nut is well engaged.
5. Squeeze the trigger. The nut turns and slows; once the assembly is bedded the sound of the wrench changes distinctly. Release the trigger and the gearing backs off so the wrench comes free.
6. When all the assemblies are bedded, refit the wrench and keep tightening. At the designated torque the nut stops turning and the inner socket shears the spline off at the break neck.
7. Lift the wrench off. The spline stays in the inner socket until you squeeze the eject lever; drop it in a bin.

Minimum bolt tension at spline fracture

Hobson's TCBOLT data sheet gives the tension the assembly must reach when the spline breaks off, to EN 14399-10: a minimum for each bolt of 0.7 x fub x As and a minimum mean of 0.77 x fub x As, where fub is the nominal tensile strength (1,000 MPa for class 10.9) and As is the stress area. These are installation preloads for assemblies in the as-supplied condition, stored correctly and installed with the shear wrench as above. They are not a capacity for the connection, which comes from the engineer's design.

Storage, and what upsets the tension

• •Keep the assemblies protected from dirt and moisture, and take out of the carton only those you are about to install (Hobson's data sheet).
• •Never add lubrication. Hobson's article explains that extra lubrication gives a tension higher than calibrated, and a bolt that fractures in the thread during installation points to it.
• •Do not use dirty, oily or rusted assemblies: changes to the coating or foreign matter affect the tension. Hobson's article says lost lubrication, corrosion or thread damage give a tension lower than calibrated, and that an assembly changed from its as-supplied condition should be discarded or re-calibrated.
• •The free-running nut check in step 1 is Hobson's simple test for thread damage or anything else that would change the torque to tension relationship.

Hot dip galvanised, and the sheared end

• •Finish: hot dip galvanised to EN ISO 10684 (Hobson's data sheet). Hobson's finish guide rates HDG high corrosion resistance and recommends it for long term outdoor use.
• •The sheared end: once the spline is off, the end of the bolt is bare steel. Hobson's article says the zinc gives the exposed end some cathodic protection, but on larger bolts (M24 and up), in corrosive environments or where appearance matters it may be necessary to apply cold gal or another anticorrosive paint to the end after installation. Further from the ocean and enclosed from the weather, the end may need nothing more.
• •Weathering steel: Hobson's AS 4100 article says hot dip galvanised fasteners are not recommended with weathering steel, as the contact between the uncoated steel and the zinc causes galvanic corrosion. Bolting for weathering steel is specified under AS/NZS 5131.
• •Colour: hot dip galvanising now often comes out a duller matt grey than it used to. Hobson's article quotes the Galvanizers Association of Australia: brightness has no effect on performance; coating mass is what counts.

TC bolts in Adelaide

Allmat supplies TC bolts, Tone shear wrenches and sockets, and the structural bolts, nuts, washers and DTIs that go with them to steel fabricators and erectors across Adelaide and South Australia. Send your bolting list and we will price the lot.

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