Bolt tensioning is a tightening method that elongates a bolt or stud without twisting it, pulling it directly along its axis with hydraulic pressure to reach the target pre-tension load. It is the method of choice on critical flange and equipment connections, wherever accuracy and repeatability beyond what torque tightening can deliver are required.
For a bolted joint to do its job, the stud has to be stretched to a specific pre-tension (preload) load. In the conventional method, this load is created indirectly by turning the nut with a torque wrench; but most of the energy applied is lost to friction at the thread and nut-bearing surfaces. Depending on lubrication condition, surface quality, and the operator, the resulting pre-tension can deviate from the target by as much as ±25–30%.
In bolt tensioning, a hydraulic cylinder (the tensioner) grips the threaded end of the stud, and when pressure is applied it elastically elongates the stud along its axis. Because the stud is under tension and carries no load at this stage, the nut can be run down freely by hand; once the pressure is released, the load is transferred permanently to the nut. With friction removed from the equation, the resulting pre-tension is directly proportional to the hydraulic pressure — calculable, repeatable, and independent of the operator.
The hydraulic tensioner is threaded onto the stud's protruding thread, over the nut. The bridge surrounds the nut, leaving it accessible through a side window.
Pressure from a 1500 bar class pump drives the piston, elongating the stud elastically along its axis. The force acts purely axially — there is no torsion and no friction.
While the stud is under tension, the nut carries no load; it is run down to the bearing surface by hand, using a turning pin through the bridge window. No torque is applied at any point.
As pressure is released in a controlled manner, the stud tries to shorten; the seated nut prevents this, and the pre-tension is locked permanently into the joint. The tensioner is then removed.
| CRITERION | TORQUE TIGHTENING | BOLT TENSIONING |
|---|---|---|
| Force transmission | Indirect — the nut is turned, via friction | Direct axial — hydraulic pulling |
| Pre-tension accuracy | ±25–30% deviation typical | Repeatable within a ±5–10% band |
| Effect of friction | Decisive; highly sensitive to lubrication | Effectively negligible |
| Torsional stress in the stud | Present; reduces fatigue life | None — pure axial load |
| Simultaneous tightening | Bolt by bolt, in a cross pattern | Simultaneous at 100%/50%/25% coverage |
| Speed on large diameters | High torque requirement; slow | Advantage grows as diameter increases |
| Typical use | General-purpose joints | Critical flanges, turbines, pressure vessels |
Bolt tensioning is the standard on any joint where leaks, loosening, or uneven load distribution are unacceptable. The most common applications include wind turbine foundation and blade bolts, steam and gas turbine casing joints, petrochemical and refinery flanges, heat exchangers, pressure vessels, marine shaft couplings, and large press columns.
The Hydro Bard HRT series — 11 models, M20–M150 stud range, 160–6,400 kN tensioning force, 1500 bar. Domestically manufactured, with custom-size capability.
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