How each method puts load in a bolt
Torque
A wrench rotates the nut against the stud. The thread helix converts that rotation into stretch, and the stretch becomes clamp load. Roughly 90% of the applied torque is spent on friction under the nut and in the threads, so the load you get depends on how well you know friction. See nut factor explained. Tools range from hand wrenches and multipliers to hydraulic torque wrenches for the big sizes.
Hydraulic tensioning
An annular hydraulic jack sits over the nut. A puller threads onto the exposed end of the stud, and pump pressure pulls the stud axially, stretching it directly. With the stud held at load, the operator runs the nut down snug against the joint by hand or with a tommy bar, at essentially zero torque. Then pressure is released and the nut holds the stretch.
Because the load goes straight into the stud, nut and thread friction are out of the picture. What you set on the pump gauge, corrected for load transfer, is what you get in the joint.
Side by side
| Torque | Hydraulic tensioning | |
|---|---|---|
| What is controlled | Rotational input at the nut | Axial load on the stud |
| Sensitivity to friction | High. Lubricant, coatings and thread condition all matter | Very low at load-up |
| Typical preload scatter | ±25% or more | Under about ±10% |
| Several bolts at once | Possible with multiple tools, each still friction-dependent | Yes. Tools linked on a common hydraulic circuit load studs together |
| Stud length | Any | Needs about 1 diameter of thread above the nut |
| Radial clearance | Low-profile wrenches fit tight spaces | Tool is larger than the nut; check pitch circle spacing |
| Torsion in the stud | Yes, during tightening | None |
| Thread galling risk | Higher on stainless and large studs | Lower, since the nut isn't turned under load |
| Setup and cost | Lower | Higher; tooling is sized to the stud |
Load transfer: the step people skip
When tensioner pressure is released, the nut, threads and joint deflect slightly as they pick up the load. That deflection costs some stretch, so the residual load in the stud is always less than the load the tool applied. The tool load is therefore set higher than the target, and the ratio between them is the load transfer factor.
The factor depends mostly on grip length relative to stud diameter. Short, stubby joints lose a larger fraction than long ones. Tensioner manufacturers publish methods and tables for it. A procedure that ignores load transfer will under-load every stud consistently, which is a precise way to be wrong.
Tensioning a flange: 100% vs. 50% coverage
- 100% coverage: a tensioner on every stud, all pressurized together. Every stud gets the same load in one pass, and it's the fastest and most uniform method when clearance and tool count allow.
- 50% coverage: tensioners on every other stud. Pass one loads the first half at a higher pressure, because loading the second half relaxes the first. Pass two loads the second half. A check pass usually follows. This uses half the tools at the cost of more procedure.
When to pick which
Reach for a torque wrench when
- the joint is routine, and ±25% preload scatter is acceptable with the margin designed in,
- the studs are short or cut flush and won't accept a puller,
- radial clearance is too tight for a tensioner,
- you're breaking bolts loose. Tensioners can loosen too, but a wrench is usually the faster tool for break-out.
Reach for a tensioner when
- stud diameters are large. Above roughly 2 to 3 in., torque values get enormous and galling gets likely,
- the joint is leak-critical: high pressure, high temperature, toxic or flammable service, heat exchanger channel covers, reactor heads,
- you need even load around the circle, for example on large-diameter or thin flanges sensitive to rotation,
- the work is subsea or in the splash zone, or on wind turbine, compressor and pipeline connections where tensioning is the specified method.
Not 8 threads per inch? Off-the-shelf tensioners assume standard stud series. For non-standard diameters, pitches or lengths, we work with TensionPro's engineers to build pullers and bridges that fit the stud you have. Send the stud details.
Frequently asked questions
Is bolt tensioning more accurate than torque?
Generally, yes. Tensioning applies load directly to the stud, so thread and nut-face friction drop out of the result. Typical residual preload scatter is under about ±10%, compared with ±25% or more for torque. Accuracy still depends on correctly accounting for the load lost when the tool is released.
What is the load transfer factor in bolt tensioning?
When the tensioner releases, the nut and joint take up the load and some stretch is lost to thread and nut deflection. The applied tool load is therefore set higher than the target residual load. The ratio between them, the load transfer factor, depends mostly on the stud's grip length relative to its diameter. It is larger for short grips and smaller for long ones.
How much stud protrusion does a tensioner need?
Typically about one stud diameter of thread above the nut, so the tensioner's puller can engage. The exact requirement depends on the tool. If the studs are cut flush to the nut, they cannot be tensioned without being replaced with longer studs.
When should I use torque instead of tensioning?
Torque is faster to set up, cheaper and works with any stud length. It is the right choice for most routine and non-critical joints, for small diameters and where there's no stud protrusion. Tensioning earns its place on large-diameter studs, critical or leak-sensitive joints, and flanges where you want every stud loaded at once.