What are Bolt Tensioners? What is the significance of the components involved?

December 4, 2019by admin0


With very limited articles shedding light on bolt tensioners, we will be elaborating in brief about hydraulic bolt tensioners and the role of its components involved in assembling a tool. This will include us talking about the functionality of the tool as well as the components of the tool to compare between conventional and hydraulic tools.

Introducing Bolt Tensioners and how do they function

Hydraulic tools like tensioners tighten large diameter bolts faster with high accuracy i.e. it applies accurate pre-loads. Unlike conventional manual torquing, tensioning does not imply forceful turning of the bolt or nut, like hydraulic wrenches, impact wrenches, and flogging spanners is not required.

The common enemy of all these methods is “FRICTION”. To overcome the friction between the washer and nut and the thread friction uses up more than 80% of the torque applied on the fastener, which leaves less than 20% of the energy to tension or elongate the shank of the bolt. The variation in friction loss, from one bolt to the other causes inconsistent tension in bolts that have been tightened to the same impact wrench or torque setting.

A hydraulically powered bolt tensioner is nothing but an annular jack that fits over the fastener to be tightened. The jack pushes the bolt down and pulls up the end of the bolt. To accommodate the bolt tensioning tool, the end of the bolt needs to be at least one diameter long. As force produced by the jack is directly applied to the end of the bolt, the load generated by the jack is equal to the tension developed to shank the bolt. Post-tensioning the bolted joint, it is possible to tighten the nut with no torque until it achieves joint-integrity. The load applied by the jack is eventually reduced to retain a high percentage to shank the bolt, which shall depend on the diameter and length of the bolt.

Components involved in a Bolt Tensioner


To suit the bolt, the PULLER has an internal thread. To assist rotation by hand, the outside edge is knurled. Additionally, tommy bar holes are present for final tightening. The nose of the PULLER helps locate the centre of the CYLINDER to the bolt. In short, the force developed by the CYLINDER is transferred into tension in the bolt by the PULLER.



The CYLINDER can be defined as an annular hydraulic jack.  The PULLER and bolt go through the centre of the CYLINDER.  A recess is located at the base of the CYLINDER to accept the BRIDGE. To connect the CYLINDER with the hydraulic hoses, one or two self-sealing quick disconnect nipples are provided.



The BRIDGE offers support to the CYLINDER over a SOCKET, nut, and bolt.  In order to fit a socket retaining ring that retains the SOCKET optionally, a circular groove has been added within the BRIDGE.  There are flats on both sides of the BRIDGE that give space to fit adjacent nuts.  The welding neck of a flange or other obstruction is cleared by an angled flat at the rear.  A cut out is provided at the front of the BRIDGE to allow access for the SOCKET with a tommy bar. While tensioning the bolt, the access gap helps turn the nut.



The SOCKET is designed to fit over a hexagon nut and inside the BRIDGE.  Tommy bar holes are designed to eliminate the need to drill holes in the flats of the hexagon nut.



To allow the tensioner to screw onto and apply tension to the bolt, an extra length of thread must protrude through the nut.  It is important to measure the length of bolt.  Details are given in the instructions to follow. Tensioning operations can be completed with high accuracy and faster if the application involves good quality fasteners. As friction is not a variable factor, bolt tensioners eliminate the requirement of washers under the nut.


Bolt and Nut

This diagram is an assembly of all the components displayed above. This is a tensioner with single hydraulic connection that is completely assembled onto a nut and bolt that is ready for tensioning once connected to the hydraulic hose. The tommy bar holes in the SOCKET are seen through the BRIDGE.


Assembled Tensioner Tool

We are sure that, now you know the role of every component and tool involved in delivering precision bolting. We will be talking more on the operation and design of the tool in our next article. Visit us next time to know more about bolt tensioners.

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