Bolt Tensioning Theory

What is Bolt Tensioning?

Tensioning is the vertical and direct axial stretching of the bolt to achieve a specified bolt stress preload. Depending on the application and the recommended bolt stress on the bolt, every different material will have a different required tensioning value. Inaccuries created through friction are eliminated. Massive mechanical effort to create torque is replaced with simple hydraulic pressure. A uniform load can eb applied by tensioning multiple studs simultaneuously.

Tensioning requires longer bolts, and a seating area on the assembly around the nut. Tensioning can be done using Boltite Bolt Tensioners.

How a Bolt Tensioner Works

What is Load Loss?

Load loss is a loss of bolt elongation depending on factors such as thread deflections, radial expansion of the nut, and embedding of the nut into the contact area of the joint. Load loss is accounted for in calculation and is added to the preload value to determine the initial Applied Load.

Glossary of Terms

• Yield Strength

  The point at which a bolt begins to plastically deform under tensile loading.

• Ultimate Strength

  The maximum tension which can be created by tensile load on a bolt.

• Torque Tightening

  The application of Preload to a bolt by turning of the bolt’s nut.

• Tensile Point

  The point at which the tensile loading on a bolt causes the bolt to rupture.

• Proof Load

  Proof Load is often used interchangeably with yield strength but is usally measure at 0.2% plastic strain.

• Preload

  The load in a bolt immediately after it has been tightened.

• Plastic Range

  The range on a stress/strain curve where the tensile load applied to a bolt results in permanent deformation.

• Load Scatter

  The spread of differing loads in a sequence of bolts after they have been loaded. It is mostly due to the elastic interaction of the bolts and the joint member; as subsequently tightened bolts further compress the joint, previously tightened bolts are subject to some relaxation.

• Load Loss

  The losses in a bolt which occur on transfer of load from a tensioning device to the bolt assembly (these may arise from phenomena such as thread deflection and embedding of the nut to the contact area of the joint, and is calculated as a factor of the length to diameter ratio of the bolt).

• Elastic Range

  The range on a bolt’s stress/strain curve where stress is directionally proportional to strain.

• Breakout Torque

  The amount of torque required to loosen a tightened bolt. (Usually more torque is required to loosen a bolt than was used to tighten it).

• Applied Load

  The load applied to a bolt during tensioning.