The Current System

"Whilst we support and promote good preventative maintenance practice we believe that this is currently unacceptably intensive and prescriptive for such a safety critical but mechanically simple system."

- Institute of Road Transport Engineers (IRTE) 1998.

The current system relies on torquing nuts, but no matter how accurately the torque is set there will be a scatter (variation) in the tension achieved. This is due to a variation in the friction in the threads resulting from:

• Damaged threads
• Dirt ingress
• Variation in the lubrication used

This scatter means that the maximum tension possible at any given torque is 1.6 times the minimum tension possible at the same torque.

To be inside the 'safe tension zone' the scatter must be less than 1.2 (260kN divided by 220kN).

So no matter how carefully you set the torque using the current system, you cannot be sure that you achieve the correct tension. With the current system you must re-torque soon after fitting the wheel and re-check the torque frequently thereafter.

Products available

There are a number of products on the market that claim to solve the problems of the current system, none of them do.

Nut locking devices – which prevent nuts from rotating on the studs:

• Tension may be too high or too low
• Dangerous, as they mask any signs of a problem until wheel flies

Loose nut indicators – which show when a nut moves:

• No indication if tension too high as nut won't move
• Tend to fall off
• Extra work for wheel-fitters
• Often misused

Tension indicating washers and studs – which show when minimum tension is reached:

• No indication if tension is too high
• Increased fitting-time

Electronic monitoring equipment – which sense vibration in wheel and sound an alarm:

• Expensive
• Only alerts driver once there is a problem, does nothing to prevent a problem
• No reduction in preventative maintenance