Twin ferrule compression fittings: incorrect tightening – part 1

Published:  10 November, 2015

This is the first in a series of articles based on guidance provided in the British Fluid Power Association’s newly introduced training course titled ‘Small Bore Tubing Integrity Course – using twin ferrule compression fittings’. This month, we present the first of a two-part series looking at the problem of incorrect tightening.

Tube nut not being turned the recommended number of turns

The correct assembly procedure for twin ferrule compression fittings is that the tube nut should be turned ¾ of the turn from finger tight position for tube sizes smaller than ¼” and 6mm. 1 ¼ turns from the finger type position for tube sizes ¼” – 1” and 6 mm – 25 mm.

Gap gauge

Gap gauges are supplied by many manufacturers and should only be used on the initial make-up of the compression fitting. Please note: these gauges should not be used for reassembly purposes. Where a manufacturer offers gap gauge this should only be used when assembling product from that manufacturer (i.e. gap gauge from ‘manufacturer A’ should not be used to assemble product supplied by ‘manufacturer B’.)

Typically, different sized gauges are used for different sizes of tubing and are used to determine that the tube nut has been turned the correct number of terms on the fitting body as per the assembly instruction given by the manufacturer.

The gauge is then used as a ‘go’/’no-go’ gauge for the length between the tube nut and the fitting body to confirm that the correct number of turns has taken place. If the gauge goes between the tube nut and the body of the fitting as per the image below further tightening is required.

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During initial installation the tube nut should be tightened until the correct size gap gauge no longer fits in the gap between the end of the tube nut and the fitting body.

The images below show the tube nut being progressively tightened – a gap gauge is being used to verify correct tightening has been achieved. If the gap gauge fits between the end of the tube nut and the fitting body then the tube nut is not sufficiently tightened. This can be seen in the photograph below.

[insert pic 4 and caption] Gap gauge fitting in between the end of the tube nut and the fitting body – tube nut not tightened sufficiently.

Correct tightening has been achieved once the gap gauge no longer fits between the end of the tube nut and the fitting body. This can be seen in the photograph below.

More information

The above information has been edited from course booklet for the British Fluid Power Association’s new training course titled ‘Small Bore Tubing Integrity Course – using twin ferrule compression fittings’. For more information about the new course and how you or your staff can enrol, please contact the Association at: info@bfpatrainingacademy.co.uk. For general enquiries, please contact: enquiries@bfpa.co.uk, or Tel: 01608 647900.

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