Millbrook Testing - September 2007

A key design specification of the product is that the VisiLok nut should be capable of running in a deployed condition for a distance of 300 miles while maintaining the visible warning flags and locking the nut to the stud to prevent rotation. This design requirement is to ensure that, in the event of a nut losing clamping force and rotating, the driver of the vehicle has a safety window of 300 miles in which to check his vehicle and identify the loose nut.

To validate that this design objective had been met, tests were undertaken at the Millbrook Proving Ground using a Volvo FM9 26 tonne refrigerated box van with the rear lift axle running in the raised condition to increase the load on the other wheels.

The nearside drive wheel of the vehicle was fitted with VisiLok nuts and one nut was deliberately loosened to deploy the flags and engage the locking mechanism. The nut was sufficiently backed off that the washer was loose. In this condition the vehicle was then driven on a specially prepared 30 mile circuit of the proving ground which consisted:

• Hill Route Including transient ridges, forward and reverse starts on severe gradients and rough track.
  
  
• Speed Circuit Urban cycle incorporating sleeping policemen road humps and steady state running at 55mph.
  
  
• Twist Humps Staggered and angled humps, which twist the rear of the vehicle in the opposite direction to the front.
  
  
• Special Surfaces Sine waves of different lengths.
  
  
• Figures of 8 and Sprag-spike braking forward and reverse in a straight line, repeated on full lock left and right
  

The 30 mile circuit was repeated 10 times with interim inspection stops so that the design specification was met. The severity of the test was considerably in excess of even hard operational usage. The sleeping policeman humps were hit at a speed of 25mph and throughout the test, the brake overheating warning light was regularly triggered.

The result of this first part of the test cycle was that the VisiLok nut maintained its full flag deployment and locking function. It was also observed that none of the other 9 nuts on the wheel were loosened.

On completion of the first test cycle of 300 miles, a second adjacent nut on the wheel was loosened leaving 20% of the wheel unsecured. A second cycle of 300 miles was then undertaken with the 2 loose nuts. At the end of the test both nuts were still showing full flag deployment and were securely locked. The first nut had also completed 600 miles in this condition, twice the design requirement. None of the remaining 8 nuts were loosened.

The successful completion of the test fully validated the design requirement to give the vehicle operator a safety window of 300 miles in which to identify a loose wheel nut problem.