Shear load and tensile load

  • The theory of bolted joints
  • Torque Tightening
  • Bolt Tensioning
  • Advantages of bolt tensioning technology
  • Bolt tensioning solutions by Atlas Copco
  • Bolt Tightening Solutions literature




The theory of bolted joints

The GOAL of tightening is Clamping force. The clamping force is sufficient if it can bear the load for which the joint is designed, and if the joint will remain tight enough to prevent loosening when exposed to pulse loads and other forces that can act on a joint.

An incorrect clamping force can be a source of flange leakage and material separation, may ultimately lead to losses, damage, risks and failures.

A bolted joint works by elastically elongating the bolt (creating a tensile load) which results in a corresponding clamping force that holds the joined parts together in the way required for the specific joint.
Achieving an even and accurate clamping force requires the application of a tightening method that compensates for secondary factors and “losses” so that the actual clamping force corresponds to the design force. The degree of complexity and difficulty in practice depends on many factors, such as the material characteristics, surface quality, tool accuracy, friction control (friction under the nut and bolt head and friction in the thread), etc.

To minimize these effects we also design and modify joints to achieve more stable and more repeatable and predictable conditions.

Torque Tightening

A nut travels over the bolt thread by nut rotation. When both the nut and the bolt head are supported by the joint members (e.g. the bolt head on one side of a flange and the nut on the other), further nut rotation is possible in an ideal situation only thanks to material elasticity (for simplicity we shall skip the effects of plasticity and relaxation).

RT Torque wrench, RTP, Tensor Revo

Achieving the correct clamping force by torque tightening (nut rotation) requires proper handling of the friction under the nut and the friction under the bolt head and in the thread, and also requires correct and stable material characteristics. When tightening torques increase it is also more difficult to absorb the reaction force of the tool that is rotating the nut.

Examples of sources of friction modification include rust, paint, grease, water, material deformation, use of washers, bolt coatings, etc.

Torque Tightening is usually measured in values of Nm or ftlb which are parameters we can monitor on tools. By using this in combination with previous tests and calculations, we can estimate the clamping force in a joint.

Typical tools used in torque tightening are click-wrenches, hydraulic wrenches, pneumatic nutrunners and electric nutrunners, with or without a clutch or sensor to stop the tool when torque is applied.


Bolt Tensioning

Tentec bolt tensioners

In situations where bolted joints need a very accurate clamping force, where the clamping force becomes very high, where it is impossible monitor the influences on torque tightening due to the size of the bolts and components, where we need to develop the clamping force evenly on many bolts at the same time, where it is difficult to absorb the reaction force, and in many other critical and technically demanding situations – bolt tensioning is a solution for the limitations of torque tightening!

A bolt tensioner is a piece of tightening equipment which works as a Hydraulic Cylinder mounted on a Bolt above the Nut.

With the tensioner, we elongate the bolt by pumping hydraulic oil into the cylinder. From the hydraulic cylinder area and the oil pressure we calculate the force elongating the bolt – and from the force elongating the bolt we obtain a precise measurement of the Clamping Force.

Bolt tensioners

The influence of friction is eliminated.

During bolt elongation a free space it created between the nut and the joined part. When the desired bolt elongation is achieved we rotate the nut close to joined part (to touch the surface of joined part) to fix the bolt elongation. Once the bolt elongation is locked by the nut we depressurize the tensioner and dismantle it from application. The joint is tightened with high accuracy in terms of clamping force, in the minimum space and without the need for reaction force absorption from torque tightening.

Advantages of bolt tensioning technology

1. Simultaneous tensioning– Tightening of every bolt together with all other bolts at the same time
2. Consistent Retained Bolt Load – Even clamping force (very low scatter)
3. Accuracy – No variables of lubricant, rust, paint, mechanical damage etc.
4. Longs Bolts – No torsional twist.
5. Reaction Force – No rotational reaction force as the reaction is absorbed directly on the surface of the joined parts (the reaction force goes into the bolt axis) and goes directly into the clamping force.

Most of advantages listed above are very difficult or impossible to handle with torque tightening.

The disadvantages of bolt tensioning relate to short bolts – which sometimes preclude the use of bolt tensioners. A specific length of bolt protrusion is required for grip and the final critical parameter is whether the Tool will fit the application given its size and shape.

Bolt tensioning solutions by Atlas Copco

Bolt tensioners

A wide portfolio of bolt tensioning products is available from Atlas Copco. All of the products do the same thing – they stretch bolts – but there are variations in terms of product automation, ergonomics, accessibility, size and shape, etc.

Our products are based on a modular design concept making it easy and fast to change tool properties and respond to unexpected requests immediately. For those applications where no existing product fits, we have a strong team of design engineers who create special bespoke products exactly to match customer needs. Product availability and speed of delivery is crucial, so Tentec tensioners are kept on stock in large quantities to serve customer needs worldwide in the shortest possible time.

With our bolt tensioning product portfolio, we focus on the following segments:

Bolt tensioners

Bolt tensioners

  • Wind Turbine Industry
  • Oil&Gas
  • Chemical
  • Energy and Power
  • Construction
  • Subsea (water applications operated by scuba divers)
  • General purpose tools for most common application
  • Hydraulic nuts replacing standard hexagon nuts
  • Special bespoke customized tools

The acquisition of UK based manufacturer Tentec by Atlas Copco in late 2013 was another step in expanding the portfolio of Atlas Copco premium industrial tools intended for the Fabrication and Maintenance business.





Bolt Tightening Solutions literature

A set of new printed materials related to bolting is now available. A marketing brochure introducing our solutions (9833 2031 01) and a sales catalogue (9833 2030 01) can be now ordered from the Print Shop. For more details do not hesitate to contact us.










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