APT-P Tilt System

Introduction

The main objective in any modern railway system is to reduce journey times as much as possible whilst improving, or at least maintaining, passenger comfort. British Rail had reached the best possible journey times with conventional rolling stock and motive power. Further improvement to maintain a competitive edge in the face of increasing use of the private car and airlines was necessary.

Three options were open:

Use conventional rolling stock and straighten the existing track on the main lines.
Build new high speed Inter-City lines.
Use existing track and in some way maintain passenger comfort for the increase in speed necessary around curves.

The first two options were not viable due to the enormous costs involved and the amount of new land acquisition required whilst (c) could be achieved by tilting the body of the vehicle at varying angles dependent upon the speed and severity of the curve. The existing transition curves between straight and constant radius track are relatively short for high speed operation, resulting in a demand for a tilt response of up to 5° per second. Such rates cannot be achieved by a passive (gravity operated) system as used on the Italian Railways in the 1950's so each vehicle has to be provided with its own self contained electro-hydraulic servo-system which responds to accelerometers measuring the lateral accelerations experienced by passengers. It is essential that higher curving speeds do not cause excessive overturning moments. The design of APT-P is such that the maximum transfer of load from inner to outer wheels at 9° cant deficiency is 33% leaving an adequate margin for side winds and other dynamic forces. In addition, APT-P has a low centre of mass which is kept over the centre of the track by the tilt system on straight track. The predicted overturning cant deficiency for APT-P is 25°. This was tested on a sharp curve near Dover by running a scrapped coach with suitably modified mass and suspension at successively higher speeds until it overturned - at a cant deficiency of 24.3°. The tilt system uses a tilting bolster and this tends to be self righting if weight distribution is even. Additional measures have now been taken to cope with the righting of the vehicles in the event of tilt failure. The tilt components, now fitted to the train, are collectively known as the Mk3 or Precedence Tilt Control System.

Principle of the Mk3 or Precedence Tilt System

All APT-P vehicles, including Power Cars are now fitted with the Mk3 system. It is completely automatic and has been tuned to meet the high performance requirements for fast and smooth operation. Each vehicle contains its own hydraulic pack and control system designed to tilt the vehicle to the angle demanded by the tilt accelerometer which is mounted on the leading bogie of the vehicle preceding the one to be tilted. Obviously there are interlocks to change over the accelerometers when the train reverses direction. This new tilt control ensures that the vehicle is already starting to tilt as it enters a curve giving the passenger a smoother ride and. less curving sensation. On curves, the outer rail of the track is usually laid higher than the inside one. This produces a slope across the track known as its cant, and measured as an angle from the level. If the train goes around the curve at the correct speed, the passengers feel no sideways (lateral) force. If the train goes around the same curve at a slower speed, the passengers will experience a force towards the inside of the curve, and the vehicle is said to have cant excess (also measured as an angle). If the train goes around the same curve at a faster speed, passengers will experience a force towards the outside of the curve, and the vehicle is said to have cant deficiency. On APT, the tilt system adjusts the angle to which the vehicles are tilted to suit the speed at which the train is travelling. This means, whatever the speed of the train, the passengers do not experience any cant deficiency or excess.

Basic Principle of Operation

When the APT negotiates a curve it experiences a lateral (sideways) force. This is detected by the tilt sensor mounted on the bogie frame and is converted to an electrical signal which is proportional to the amount of tilt required to counteract the lateral force. This electrical signal is fed to the electronic control box in the Tilt Pack. The Pack then supplies oil, which is metered through two servo valves to the tilt jacks at either end of the vehicle to tilt the bolster and hence the body of the vehicle. The angle between the body and bogie is monitored by a Linear Variable Differential Transducer. This converts the angle to an electrical signal which is fed back to the electronic box. When this signal matches the original tilt demand signal the servo valves cease to supply the tilt jacks and the system remains static until the next change in lateral force is detected by the tilt sensor on the bogie. In order to accomplish this. a system of extremely fine electrical and mechanical tolerances is required.

Extract from APT-P Course Notes 5 - Tilt, March 1981.