James Walker: A bolt tension solution that can replace torque

Woking Surrey, 26.09.2011. The industry has recently recognized the importance of the tension of fasteners and this will challenge some of the torque tightening practices that exists today as the industry matures and adopts more sophisticated technology to both improve reliability and reduce maintenance costs thus improving the economies of electricity production from wind turbines. Bolts can be installed in one of two ways: by torque tightening or by hydraulic tension tightening.

Torque tightening

Torque is the rotational force applied to a lever, multiplied by its distance from the centre of the tightening end (fulcrum). The force is measured in Nm or lbs/ft. Torque tightening rotates the bolt head or nut to tighten and stretch the bolt to clamp two surfaces together. Hydraulic torque wrenches allow the operator to control the torque applied and are used extensively to tighten wind turbine bolting.

It is simple to use, however, this technique suffers from inaccuracy due to inconsistent friction. Even on lubricated threads and nuts, the level of friction is uncontrollable. This means there is a critical lack of tension control that can lead to bolt failure, and ultimately, joint integrity being compromised.

Hydraulic tensioner tightening

Hydraulic tension tightening is also used to tighten (large diameter bolts on) wind turbine boltings. The tightener pulls the bolt axially by means of a hydraulic piston. When the bolt is stretched by a specified amount, the nut inside the tensioner becomes raised off the flange creating gap between the flange faces. The gap is closed by tightening the nut and when the hydraulic pressure is released, the tension is transferred to the bolt and the flange.

During this process tension loss occurs (“load transfer relaxation”). To compensate for this loss, the hydraulic pressure originally applied is increased (“hydraulic overload value”). However, the amount of load transfer relaxation cannot accurately be predicted and varies from one bolt to another. Hydraulic tensioners are therefore unable to control, or measure the tension, that has been achieved for each bolt and also unable to provide an in-service reliability check.

A bolt tension solution

Innovative engineering has come up with a bolt that indicates when the specified load/tension has been achieved during tightening. James Walker RotaBolt developed the RotaBolt® system to provide a simple and effective measure of the real load/tension in a bolt. The RotaBolt® can be used in either torque or tension tightening. When a RotaBolt® is tightened, the cap locks at exactly the right amount of preload in the bolt. This allows for more control during tightening whilst ensuring that the customer’s design criteria are met independently of the tightening system. It also enables bolt reliability status to be instantly checked in service; something that the traditional bolt retightening checks are unable to achieve.

All RotaBolt® fasteners are guaranteed (consistently) to indicate within ±5% of the specified (selected) load regardless of environmental influences. Its accuracy is not affected by friction, as in torque control, or by load transfer errors, as in hydraulic tensioners. This bolt tension solution can ensure joint integrity on wind turbines. Joint integrity, in turn, allows for greater reliability and the more efficient production of renewable energy for the future.

Note to Editors
The James Walker Group is a global manufacturing group, with an annual turnover in excess of £160-million (€180-million), operating through two strategic business units: Sealing Products & Services and Rail Systems & Products. It has more than 50 production, engineering, distribution and customer support sites worldwide and operates in over 100 countries.