Valves on cue at Royal National Theatre

Proportional valves from Rexroth have replaced servo technology in a hydraulically driven stage at the Royal National Theatre, London.

Servo technology has, in the past, been the first choice for electronically regulating the direction and volume of flow in a hydraulic drive. Designed to hydraulically amplify a small electrical input signal by means of a delicate flapper jet system, servos are suitable where high levels of response are required. Modern proportional hydraulics however, has advanced to the point where all but the most dynamic applications can be controlled using proportional valves.

The Royal National Theatre’s 890-seat Lyttelton Theatre features an unusual hydraulically driven proscenium stage, which had employed servo technology to raise, lower and tilt different sections.

The retrofit installation was specified and fulfilled out of Rexroth’s South East Area Office in Woking, Surrey. The order comprised four pilot-operated proportional valves, two direct-operated versions, a six-station manifold for mounting purposes, six electrical amplifier Euro-cards for controlling the valves and an associated Euro-rack, for installation in the theatre’s control desk.

The stage consists of four hydraulically-powered elevator sections, one main platform with a 40 tonne capacity, a central forestage with a 3.5 tonne capacity and two side 1.25 tonne forestages. A hydraulic screw ram drives each of three forestage elevators, which are interconnected by synchronising shafts driven by hydraulic motors. The shafts are clutched together for raising/lowering all three forestages together, with a 3.75m range of movement, and can be de-clutched to operate each section independently.

The main stage elevator is driven by four screw rams, one at each corner, with the downstage pair permanently connected by a synchronising shaft and the upstage pair similarly linked. The upstage and downstage rams may also be connected by shafts, enabling the whole stage to be raised or lowered as a whole, up to 2.5 metres, or raked to a gradient of one in eight, by operating the rear rams only.

The hydraulic rams for the two smaller forestage elevators are now each controlled by a proportional directional valve, the Rexroth Type 4WRA direct-operated model being specified for the 43 l/min duty flow required. On the main elevator, each pair of rams is driven by a single proportional valve and, here, Type 4WRZ pilot-actuated valves are used for the higher 120 l/min flow needed. A similar 4WRZ model, rated at 102 l/min, is used for the larger central forestage, whilst a fourth was also supplied for controlling auxiliary hydraulic equipment functions.

‘When the original hydraulic system was installed servos were the obvious choice, since proportional valves were not really around,’ says Frank Stevens. ‘They worked very well, but when it came to replacing them, proportional valves proved to be cheaper and gave us the accuracy we required. They are operating successfully and should stand us in good stead for the next twenty years.’

Although not a problem in the theatrical environment, servo valves are susceptible to hydraulic fluid contamination, which can result in damage to the delicate flapper mechanism or blocking of the internal jets. Having no small jets or other delicate parts, proportional valves are less vulnerable to contamination and are generally more robust, making them suitable for industrial and rugged applications where a high degree of accuracy is not essential.