Using a bolt tensioner, specifically a hydraulic bolt tensioner which is common, involves placing the tool over the bolt and stretching it with hydraulic pressure, then tightening the nut while the bolt is elongated, and finally releasing the pressure.
Bolt tensioning is a method used to apply a specific and controlled tension to a bolt or stud. Unlike torqueing, which tightens the nut and relies on friction, tensioning directly stretches the bolt itself.
What is a Bolt Tensioner?
A bolt tensioner is a specialized "Tool" designed to apply an axial load to a bolt or stud. Hydraulic tensioners use hydraulic pressure acting on a piston to generate this load. The tool typically consists of a cylinder, piston, puller bar, and a reaction nut or bridge.
Why Use Tensioning?
Bolt tensioning offers several advantages over traditional torqueing, especially for critical applications and larger bolts:
- Accuracy: Provides a more accurate and consistent method for achieving target bolt load.
- Simplicity: Does not require recalibration for different bolt sizes once set up.
- Reduced Friction: The applied load is independent of thread or face friction.
- Safety: Operators are not exposed to reaction forces as with torque wrenches.
- Simultaneous Operation: Multiple tensioners can be linked and pressurized simultaneously for uniform joint compression.
Steps for Using a Hydraulic Bolt Tensioner
The procedure for using a hydraulic bolt tensioner is straightforward and systematic.
- Preparation: Ensure the bolt threads are clean and free from damage. Select the correct size tensioner for the bolt and the required bolt load. Ensure washers (if used) are in place and flat.
- Positioning the Tool: Place the bolt tensioner "Tool" over the bolt stud. The puller bar inside the tensioner screws onto the threads protruding above the nut. The base or bridge of the tensioner rests on the nut or the joint surface around the nut.
- Connecting the Pump: Connect the hydraulic hose(s) from the hydraulic pump to the tensioner(s). Multiple tensioners can be connected in a series or parallel arrangement for simultaneous operation.
- Applying Pressure: Activate the hydraulic pump to apply pressure to the tensioner cylinder. The hydraulic pressure pushes against the piston, pulling the puller bar upwards. Since the puller bar is attached to the bolt threads, this action stretches the bolt.
- Securing the Nut: While the bolt is held under tension by the hydraulic pressure, manually rotate the nut downwards. This is usually done with a simple turning bar or socket that fits over the base of the tensioner. The nut is turned down until it is snug against the joint face, closing the gap created by the bolt elongation.
- Releasing Pressure: As mentioned in the reference, "Pressure is then released." The hydraulic pressure is relieved from the tensioner cylinder.
- Procedure Complete: "And the bolt tensioning procedure is complete." As the hydraulic pressure is released, the bolt attempts to return to its original length. However, the tightened nut prevents it from fully retracting, thus locking in the desired tension (preload) in the bolt.
After releasing the pressure, the tensioner tool can be removed from the bolt. This leaves the bolt correctly tensioned to secure the joint.
Key Considerations
- Safety: Always follow safety guidelines provided by the manufacturer. Keep hands clear of the tool when under pressure. Ensure the hydraulic system is in good condition.
- Target Load: The hydraulic pressure required to achieve the desired bolt tension is typically provided in tables or charts by the tensioner or bolt manufacturer.
- Simultaneous Tensioning: For large flanges or critical joints, using multiple tensioners simultaneously ensures uniform bolt loading and even compression of the joint.
Bolt tensioning provides a reliable and accurate method for achieving precise bolt loads, crucial for the integrity of bolted connections in various industries.
