The BPW self-steering trailer axle contains a unique design of a centring device, which is a significant factor to its success in the UK market, particularly in running gear solutions for longer semi-trailers (LSTs).
The method of operation is really quite simple and is based on the principle of the castor. This relies upon lateral forces applied to the wheels when the trailer is turning.
If, for example, the trailer is travelling forward and the driver turns to his right, the trailer will pivot on the effective bogie centre, which, on a three-axle trailer, will be about the centre of the rear bogie.
This will cause the rear of the trailer to move to the left, and the friction between the road and the tyre will create a force that pushes the wheels on the steering axle to the right with respect to the rear of the trailer. This will cause the wheels on the steering axle to turn towards the left and follow the front of the trailer around the right-hand turn.
It is worth noting that if a trailer is fitted with three rigid axles, then the effective bogie centre line is the centre axle. But when a self-steering axle is fitted to the rear position, then the effective bogie centre line is mid-way between the first and second axles. This effectively shortens the wheelbase of the trailer, increasing manoeuvrability in the forward direction.
Returning to our analogy with a castor, anyone who has observed a castor moving at a steady speed across the ground will know that it exhibits a shimmying action. This is an oscillation about the vertical centre line of the pivot.
The cause of this is the constant hunting between the turning force (the wheel on the ground) and the restoring force (the forward motion in an undamped castor).
Obviously, this cannot be allowed to happen in an axle system mounted to a trailer, as this will cause instability when driving and contribute towards tyre wear (the avoidance of which is one of the benefits of using a self-steering axle).
You will find as many ways of preventing shimmying as you will find types of self-steering axles, but the BPW system is quite unique in its operation. It relies on corrugated washers - two nested circular plates, each with four cam lobes formed into them.
As the axle steers, the cam faces rise up against each other, similar to a rising butt door hinge. This action constantly tries to centre the steering back to the straight-ahead position.
Features
1. BPW self-steering axles operate without the need for power-assisted stabilisers. They are automatically controlled according to the load being carried. Because the design of the self-steering axle is so simple, the number of individual components required is low. This saves weight and reduces installation and maintenance costs.
2. The principle of the BPW self-steering axle is described by the abbreviation LL which stands for load-dependent steering. The axle stub is connected to the axle beam via a specially designed pressure bearing. When travelling in a straight line, the weight of the load acting on the pressure bearing keeps the wheels in the straight-ahead position.
3. When the trailer enters a bend, side forces acting on the tyres cause the wheels to turn slightly and the flanks of the pressure bearing to slide over each other, allowing the wheels to move from the straight-ahead position and creating a steering effect. The degree of steering is dependent on the axle load and the ‘tightness’ of the bend. Both wheels are connected via a tie-bar to ensure the same degree of steer on each wheel.
The trick is to design the slope of the cam faces so that the restoring forces generated balance off the turning forces. This prevents the oscillation that causes the shimmying.
The beauty of this system is that the restoring force increases with the load on the axle due to the weight of the corrugated washers. The turning forces also increase with load, and thus the two are always balanced. There is no need for systems of air bellows or dampers.
Of course, in reverse, the system will not work, as the pivot point of the castor action will now be to the rear of the hub centre line, and the self-steering system will be unstable. A pneumatic locking mechanism is provided (fitted to the track rod). This is normally activated by an electrical signal taken from the trailer’s reversing lights.
The advantages of this system are that a 10 per cent saving on tyre wear over a tri-axle bogie can be achieved. The unique, load-sensitive anti-hunting device adds further to this saving, whilst ensuring that the axle is not encumbered with heavy and bulky damping mechanisms.
27 Degrees of Steer
As well as the standard steering axle, with up to 20 degrees of steer for 14.6m trailers, the introduction of LSTs some ten years ago saw BPW launch a new self-steering axle in the UK for 15.6m trailers which enabled 27 degrees of steer.
This version can be supplied with either drum or disc brakes, and it also encompasses the BPW patented load-dependant tracking facility. As mentioned earlier, this unique feature means that it is not necessary to lock the steering axle when the vehicle is travelling forward, regardless of speed. The weight of the load acting on the unique pressure bearing keeps the wheels in the straight-ahead position until steering scrub is introduced. Equally important, considering the potential difficulties of operating longer trailers when reversing, the self-locking facility automatically locks the steering mechanism in the straight-ahead position as soon as the trailer starts to reverse.
The New Active Reverse Control
Until now, the benefits of a self-steering axle have only been available during forward travel, as the steering mechanism is locked and the axle remains rigid when reversing. However, BPW has created Active Reverse Control, an electro-hydraulic auxiliary system that automatically controls the steering axle when reversing.
Control and hydraulic elements, as well as a steering cylinder with an integrated steering angle and rotational speed sensor, make up Active Reverse Control. When a driver shifts into reverse, the steering system is engaged automatically through the reversing lights. Additionally, steering can be manually controlled with a remote control for precise movement, accurately positioning a trailer into a specific location. The direction and speed are detected by this revolutionary sensor technology, which is housed in the control unit rather than the kingpin.
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15th June 2022
For further press information, contact Rebecca Wesley, Emma Makings-Hone or
Sandie Slavin at BPW Limited:
Telephone: 0116 281 6100 | Email: [email protected] |
[email protected] | [email protected]