TUFNOL Gears - Keyway stresses

TUFNOL Technical information

TUFNOL GEARS – Calculating Keyway Stresses



This is the preferred method of fixing a TUFNOL gear to the shaft and in the majority of cases, a keyway cut into a plain bore is adequate. Parallel keys are strongly recommended. Tapered keys, if used, may possibly be overdriven and it is advisable to fit steel shrouds or bushes to take the wedging action of the key. Bushes, shrouds or plates should also be used to reinforce the keyway when the maximum keyway stress will  be exceeded. Grub screw fixing is not usually recommended unless a metal bush or collar is fitted to take the screw. Force fitted, knurled or splined shafts require careful design but can be very successful in the right situation.



For plain gears the keyway stress should not exceed 21 MPa (3000 lbf/in2). This stress may be calculated from the following equations:


SI Units


S' =     1000 P'

             L' x H' x V'


where S'  stress on key, in MPa


P' = power transmitted, in kW


L' = length of keyway, in mm


H' = height of keyway, in mm


V' = peripheral speed of shaft, in m/s



Imperial Units


      S' =    33 000 

              L x H x V


where S'  stress on keyway, in lbf/in2


P = power transmitted, in horsepower


L = length of keyway, in inches


H = height of keyway, in inches


V= peripheral of speed of shaft, in ft/min


These equations relate only to the stress caused by transmitting the working torque. Stresses due to high interference fits in the keyway should be kept to an absolute minimum but, if particularly high, intereference must be accommodated, the use of steel shrouds should be considered.

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A full machining service is available from Tufnol for this and many other engineering plastics and composites.

Tufnol Composites Ltd, Wellhead Lane, Perry Barr, Birmingham B42 2TB, United Kingdom.

Quality Registration BS EN ISO 9001:2000, BS EN 9100:2003 and AS 9100 Reg. No. FM 90942

The information given here is believed to be correct, but completeness and accuracy are not guaranteed. The user shall be fully responsible for determining the suitability of products for the intended use.

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