Late last year I was engaged by a truck fleet operator to investigate an apparent failure of the park brake on a Daf eight-wheeled tipper. The results should serve as a serious warning to operators and vehicle manufacturers.
The incident occurred when a laden truck was driven up a ramp to the loading area of a cement plant. The driver set the park-brake. The park-brake lever remained where the driver had set it. The vehicle was on a slight incline.
The driver then headed for the rear of the vehicle to release the tailgate-retaining hoop. As he began to return to the cab, the vehicle began to roll back. Mercifully, the driver was able to jump clear. Only when it struck a wall did the truck come to a standstill.
The Daf dealer which had supplied the vehicle 18 months earlier insists that the driver must have failed to set the handbrake correctly. This was why I was called in to inspect the vehicle, a 2002-registered 340hp Daf CF 85 eight-wheeled tipper fitted with a Knorr-Bremse DPM 92D park-brake valve.
The valve is designed to supply a precise and gradual inverse air pressure to operate spring-brake actuators. The handle is spring-loaded and should automatically return the brakes to the “off” position.
When the lever is pulled fully back to the “park” position it should automatically lock into place. The lever should remain in this position until the driver lifts the collar, releasing the locking mechanism.
If application of the brake lever is incomplete and the driver fails to ensure that it is locked in the park position, the return spring should cause the lever to return to the “off” position. This evidently did not happen on this occasion.
When the park-brake lever is in the central position, only partial (secondary) braking is being applied. Held in this position by friction, the lever could give a driver the impression that the park-brake is fully applied.
I carried out a close examination of the park-brake valve assembly and found that the lever mechanism’s protective plastic dust cover was damaged. I took the valve to the IRTE head office in London for a second opinion. It was agreed that there may be a fault with the plastic dust sleeve which could affect application of the brake.
I then visited Knorr-Bremse in Bristol where a bench test of the valve assembly was carried out. During the bench test we were able to replicate the seized position while the valve was connected to a supply of compressed air. In this position the hand valve would seize with between 35 psi and 60 psi of pressure applied through it.
It was considered that with this amount of pressure still being applied through the braking system to a vehicle in good working order there would have been enough braking force to hold it on a slight gradient.
With the hand valve in this position we simulated the normal vibration that would be expected with the engine left running while the driver climbed out of the cab. To our surprise, the hand-valve sprang off after one or two minutes. I was also able to create the same effects with another valve of the same design.
I believe the evidence points to the incident at the cement plant being the result of a faulty dust-cover holding the brake lever in the secondary position. The forces acting on the brake system created by the vehicle resting on an incline, coupled with the internal return spring overcoming the friction forces holding the lever in the secondary position, caused the park-brake lever to move to the “off” position with a snap.
I also believe that the park-brake was set to the secondary position and not to the park position. The defective dust-cover and not driver error is to blame here. Had the dust cover not gripped the shroud, it would not have remained in the secondary position, and the driver would have applied the park-brake to the correct position.
All park-brake levers like this, and their dust-covers in particular, should surely be inspected carefully as a matter of urgency, and routinely from now on.
I would like to see a recall of all trucks with this kind of park-brake valve to establish whether this is an isolated incident or there is indeed a potential life-threatening fault with this valve.
Graham Doughty
Transport Management Consultants
Chislehurst
Kent
Some people’s minds are so small they have no chance in seeing the big picture.
and anyway they reckon it’s a pain getting past the truck when they are driving their children a mile to school and all freight should go by train. One person even suggest that all vehicles over 7.5t should be banned from North Norfolk.