Using a Hydraulic Bolt Tensioner: How It Works

Why Bolt Tension Matters

A bolt provides tension to hold things in place. That tension needs to be within about 80 percent of the bolt’s maximum tensile strength to safely maintain the proper mechanical tension.

Bolt tensioning involves using mechanical means to physically stretch the stud bolt. The material becomes elongated as a nut is then placed onto the bolt. Once the nut is in place, the tension is released as the stud bolt's metal returns to its original shape before the stretching work. This process is called spring tension. As the bolt returns to its previous state, it tightens against the nut to create the appropriate force. The joint is in place for the application to prevent failure during service. A hydraulic bolt tensioner is used to stretch the stud bolt to attain the right amount of preload before the nut is set into place.

When done properly and maintained regularly, industrial bolts will hold up indefinitely. Bolt-tensioner systems ensure you have the ideal level of bolt tension and maintain them over time.

How to Use a Hydraulic Bolt Tensioner

When selecting a hydraulic bolt tensioner for the work, you need to analyze the following aspects of the tensioning application. You should evaluate the bolt diameter, nut size, bolt grade, washer thickness, washer diameter, stud protrusion length that is free and the bolt load requirement. Then you can review the hydraulic bolt tensioner's technical specifications to ensure that it will provide the appropriate hydraulic pressure to achieve the desired preload rate.

Hydraulic Tensioner Steps

1. Place the bolt tensioner over the free section of the stud bolt until it comes into contact with the flange.
2. Thread the tensioner's puller down onto each load cell so it is connected to the cylinder piston.
3. Each tensioner is connected together with the hydraulic hoses.
4. Apply the specified hydraulic pressure to preload the stud bolt as it starts to stretch.
5. The tensioner will have a locking collar that will be turned using a tommy bar to push the nut on the joint.
6. Wind the nut down the joint's face until it is in place.
7. Release the pressure on the hydraulic bolt tensioner.

Once the hydraulic bolt tensioner releases the pressure and is removed, the tension created on the stud bolt is removed. Like a spring, it returns to its normal shape by contracting. It clamps down on the joint to create the right seal.

For some applications, two tensioning pressures will need to be applied because a hydraulic bolt tensioner will not be able to fit onto each bolt at the same time. Some of the bolts will be tensioned simultaneously and then the hydraulic bolt tensioners will be moved to the remaining bolts.

When you are looking for superior hydraulic bolt tensioning devices to ensure accuracy and reliability, turn to Aztec Bolting. We offer a wide range of Enerpac hydraulic bolt tensioners for your application.

Types of Hydraulic Bolt Tensioning Systems

There are essentially two types of bolt-tensioning systems. There are systems that tension all bolts at once. Others will tension half at one time followed by the other half immediately afterward.

Tensioning all bolts at once speeds up the process but might cause a damaged bolt to fail.

When you do half and half, a damaged bolt will not cause an immediate problem because the other half are still in position. That makes it easier to identify and replace faulty bolts and attain the ideal level of tension.

Both types of hydraulic bolt-tensioner systems essentially work the same way. They fit over the stud, loosen the nut, and apply tension to the bolt. Once the ideal tension level is reached, the nut gets tightened to the proper torque specs. That gives you the ideal tension level on each bolt and the best mechanical application for your machinery.

Ask the Experts 

Our expert team provides a variety of bolting services, including tool calibration services and torque tool repair. We're happy to work with you to find the solution for all your bolting applications and processes. For more information about on-site and emergency services, talk to the Aztec team.