Introduction to Hydraulic Bolt Tensioners
What is Bolt Tensioning
Tensioning is the direct axial stretching of the bolt to achieve preload. Inaccuracies created through friction are eliminated. Massive mechanical effort to create torque is replaced with simple hydraulic pressure. A uniform load can be applied by tensioning multiple studs simultaneously.

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

* Preload (residual load) = Applied Load minus Load Losses
https://www.enerpac.com/en-us/training/e/tensioning

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Hydraulic tensioning by elongation of the screw makes it possible to develop a great tension force for tightening bolts without any torsional stress or friction, but with high precision and very good repeatability.
https://www.skf.com/us/products/power-transmission/bolts-and...

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Advantages of hydraulic bolt tensioning
Hydraulic tensioning is the modern way of manufacturing threaded connections. It is more precise and allows higher bolt forces than conventional methods. It allows multiple bolts to be tensioned at the same time, and flanges can be tensioned more evenly and tightly.
https://smarttensioner.jimdo.com/advantages/