26th April 2018

How RotaBolt works

A RotaBolt comprises the following precision components:

  1. Bolt or stud
  2. Mechanical strain gauge pin
  3. Rota indicator
  4. Air gap
  5. Rota indicator cap (RotaCap)

1. Bolt or stud
Conversion of a bolt/stud into a RotaBolt tension measuring fastener involves the drilling and tapping of a small diameter hole along the neutral axis of the fastener, into which a mechanical strain gauge pin can be inserted. A datum face is also machined on the head of the fastener.


2. Mechanical strain gauge pin
This is a headed steel pin located in the drilled and tapped hole along the centre of the bolt/stud and is positively anchored and secured in the thread.


3. Rota indicator
This is a small disc, retained on the stem of the mechanical strain gauge pin by a hole through its centre. A datum face is machined on the face of the indicator disc and the circumference of the disc is serrated to engage with the RotaCap.


4. Air gap
This is the gap created between the datum faces machined on the bolt/stud head and the Rota indicator during load calibration. This gap is set so that when the air gap is closed and the two datum faces are in contact, the RotaCap is no longer free to rotate by finger-and-thumb action. This signifies that the correct tension has been achieved in the bolt/stud.


5. Rota indicator cap (RotaCap)
The “thimble-shaped” RotaCap, is a press fit onto and over the serrated outer diameter of the Rota indicator enclosing and sealing all the working components of the RotaBolt system Indentations on the outer surface of the RotaCap enable it to be gripped by finger-and-thumb to test for correct tension in the system, even when wearing safety gloves.


What happens when a RotaBolt is tightened?

When a RotaBolt® is installed and tightened bolt/stud extends fractionally under the applied load. The mechanical strain gauge pin anchored within the body of the bolt/stud does not extend. As a result, as load is applied to the bolt, the datum face of the indicator disc, positioned under the head of the strain gauge pin, is drawn closer to the machined datum face on the head of the bolt/stud, closing the air gap. Initially the RotaCap, attached to the Rota indicator, can be rotated readily between finger and thumb.

However, when the bolt/stud is tightened the strain gauge pin is drawn down into the body of the bolt/stud to a point where the air gap between the datum faces on both the indicator disc and the bolt/stud head close and the two faces are forced into contact, preventing the RotaCap from being rotated by finger-and-thumb action.

When the RotaCap ‘locks’ in this way it indicates the calibrated pre-set tension of the RotaBolt has been reached and the joint is now clamped to within ±5% of the required design load. Should tension be lost across the bolted joint – for example, due to deterioration of a gasket or after thermal cycling − then the two datum faces will move apart re-creating the air gap and making it possible again to rotate the RotaCap by finger-and-thumb. This indicates that remedial action is required and the bolt/stud in question needs re-tightening until the RotaCap locks off and the required tension is once again achieved.

View the tension measurement in action

Find our more on how to bolt a flange

Inconel®, Nimonic® and Monel® are registered trademarks of Special Metals Corporation. Viton® is a registered trademark of E I DuPont de Nemours & Company or its affiliates.