Bianchi and Servettaz Hydraulic Interlocking

Hydraulic Power Systems

The distribution of power from a prime mover has long been a technological problem. Means that have been applied are rotating or oscillating shafts or cords, compressed air (pneumatic), liquids under pressure (hydraulic) and electricity. Today, electricity is the primary means, though pneumatic and hydraulic actuation are used where advantageous. There are even small prime movers, internal combustion engines small enough to be used on hand tools (chain saws, for example). Because of the lack of practical small electric motors before 1900, the electrical option was ruled out for most power applications, even after electric lighting became widespread.

A pioneer of hydraulic technology was Joseph Bramah (1748-1814), inventor of the toilet flap valve, an almost unpickable lock, and, most famously, of the hydraulic, or Bramah, press in 1795. The press was a demonstration of Pascal's Principle, that the pressure in an enclosed fluid is everywhere the same. Water, with a modulus of compressibility of 300,000 psi, is practically incompressible, as are most liquids. The volume of water decreases only by about 0.25% when compressed to 750 psi. It is the combination of Pascal's Principle and incompressibility that is the basis of the hydraulic transmission of force.

District hydraulic power was introduced in a few areas where there was a demand for a convenient power source for lifts, cranes and machinery at separated sites without the expense and inconvenience of an individual steam engine at each one. The London Hydraulic Power Company was established in 1883 on the banks of the Thames upriver from roughly Tower Bridge. This company distributed water at 800 psi through about 180 miles of pipes, with a capacity of 7000 hp. Hydraulic power was used at the Tower Bridge opening in 1894 for operating the bascules and other purposes. It had its own steam engines, but the hydraulic power company provided a backup. Hydraulic power was also distributed in the Bristol Harbour area, and at many other ports, mainly for operating cranes and water gates. Tower Bridge and Bristol Harbour used a nominal 750 psi pressure. The London company closed in 1977, and Tower Bridge changed to a new power source in 1974, though hydraulic actuation is still used.

An essential feature of hydraulic power systems is the accumulator that maintains a constant pressure and supplies a reserve of fluid. It was invented around 1850 by W. G. Armstrong of Newcastle on Tyne (1810-1900) as a substitute for a water column, for supplying hydraulic cranes. In the systems we are considering, the accumulator was a cylinder with a loaded piston. Tower Bridge had 6 accumulators of 20 in. diameter. To maintain a pressure of 750 psi, the load on each piston was about 100 (long) tons. Modern systems have less impressive accumulators, often simply a closed volume with gas under pressure, or spring loading. The pressure of 750 psi may seem rather high, but it was not dangerous. In case of rupture, the pressure would simply be rapidly reduced, with no explosion or other dangerous condition, as there would be with steam or even compressed gas. Hydraulic "fuses" are now available that block the flow if a sudden drop of pressure is detected, preventing loss of pressure and fluid in the rest of the system. The pump maintaining the pressure need only work at the average discharge.

The force exerted on a piston of 1 in. diameter by 750 psi is 589 lbs. Hydraulic actuators are quite compact, which is one of their principal advantages. The work done on a piston of area A when it moves a distance d under a pressure p is W = pAd = pV. Therefore, pV is a measure of the energy available. The force is actually transmitted from the accumulator to the piston by fluid as the intermediary, so pV is not really part of the internal energy of the fluid in the thermodynamic sense.

The Origin of Power Interlocking

The first plant to operate switches and signals by power was the experimental Burr pneumatic interlocking at Mantua Junction (later Zoo) in West Philadelphia in 1876, operating during the Centennial Exposition from July to November. Signal cabins and signals from the Pennsylvania Railroad's block system were used. Points were worked by a double-acting cylinder with a locking mechanism in which a loaded plunger fit into holes at each end of the stroke. The piston first raised the locking plunger and closed an exhaust valve with a port near the middle of the clinder. The points were then moved, and the opening of the exhaust valve sent pressure down an indication pipe to the controller, where it automatically closed off the air supply and displayed an indicator. Signals were operated by a single-acting cylinder with an indication pipe returning to a cylinder in the cabin that displayed a vane showing that the signal had cleared properly. The signals returned to normal by gravity. The operating pressure was 20 psi, reduced from a reservoir pressure of 60 psi supplied by a steam-operated Westinghouse airbrake pump. Interlocking between the rotary control valves was by means of cams that moved locking bars back and forth to interfere with the motion of other valves. The indication of switches and signals was observed by the operator, but did not affect the interlocking. The greatest distance to a switch was 630 ft, to a signal 1135ft.

The Burr plant attracted the attention of George Westinghouse, the primary proponent of pneumatic systems, who initiated an effort to create pneumatic and hydraulic interlocking plants. Westinghouse aquired rights to the Burr patents as well as those of the Wuerpel Switch and Signal Company of St. Louis, which had erected three hydraulic plants in the St. Louis area for the Bridge and Tunnel Railroad, as well as one in Wellington, Ohio for the Chicago, Cleveland, Cincinnati and Indianapolis company. Westinghouse was mainly interested in pneumatics, his forte, but all his patents envisiged hydraulic applications as well, and were carefully couched in terms of "fluids" without specifically identifying them. Electricity did not play a role initially, but the development of the electropneumatic semaphore around 1882 demonstrated the efficiency of electric control. Westinghouse began with straight pneumatic control, then pneumohydraulic, in an attempt to speed up actuation, which was always too slow when air had to be pumped from the command cabin through long pipes to relatively large actuation cylinders, as in the Burr system.

By 1884, there were 9 pneumatic and 4 hydraulic interlockings in the U.S., all attributed to Westinghouse's Union Switch and Signal Company. Most interlockings were, however Saxby and Farmer type machines installed by that company. Finally, electric control was introduced, which culminated in the electropneumatic system in 1892, a thoroughly excellent machine in which electrical commands controlled compressed air at the points of execution by means of solenoid valves. The actual interlocking in this case was by means of a miniature tappet locking frame, like those used in manual plants but considerably reduced in size. Tappet locking was efficient, well-understood and flexible, and was used in all power interlockings, even the Taylor "all-electric" machine of around 1900 that led to future developments. An all-pneumatic Low Pressure Interlocking was eventually perfected and saw some important applications at about the same time. It, too, was criticized for long actuation times and bulky equipment.

The Bianchi-Servettaz ACI

The engineers Riccardo Bianchi and Giovanni Servettaz designed the Apparato Centrale Idrodinamico or ACI in the mid 1880's. It was manufactured in Savona on the Ligurian Coast by the engineering construction firm managed by Servettaz. The first installation was at Abbiategrasso, 22 km south of Milan, in 1886. This plant had 10 levers. The first plants outside of Italy were those erected at Bourges (22 levers) on the PO and Nice (6 levers) on the PLM in France in 1888 and 1889. In 1890, the firm of B. Trayvou near Lyon acquired the rights for France, and supplied all later French installations. In Britain, Saxby and Farmer acquired the rights, but the Bianchi-Servettaz machine was not used on British or American railways. It was, however, used at Tower Bridge to operate hydraulic functions.

The peak year for the ACI in Italy was 1936, when 14,800 levers were in service. Outside of Italy, it seems to have been used to any extent only in France. All of the main-line companies except for the Est employed it. It was most popular on the Midi (15 plants) and the PO (10 plants), but also existed on the Nord (13 plants), where only 4 survived destruction in World War I. Of these, the one at Sannois lasted until 1956. The Ouest-Etat installed one frame at the old Paris Montparnasse, and four frames at Le Mans. The Le Mans frames succumbed in 1941, but the Montparnasse frame lasted until 1962. Besides the early installation at Bourges, the PLM only had one other example, at Paris-Bercy, which served from 1905 until 1976. Finally, the Paris Ceintures had one frame. The long life of many of these frames is remarkable. The large Midi frame at Bordeaux St. Jean 2, 80 levers, was in service from 1896 until 1980 at a very busy location. The final year of service for Bianchi-Servettaz plants in France was as late as 1988 (at Sète), 23 years after the last Westinghouse electropneumatic. These survivals attest to the basic soundness of the design.

The principal criticism of the machine was its abrupt actuation (termed brutale in French). Another was the necessity to add antifreeze to prevent the formation of ice. For some reason, mineral oil was not used in any of the early hydraulic installations. Glycerine was normally used, but other additives were possible. It was necessary to consider any effects on the lubrication of the parts or survival of nonmetallic parts such as gaskets and packing, as well as cost.

A cross-section of the operating console of the interlocking is shown at the right. The levers were 30 cm long. The normal position was back in the frame at an angle of 30° with the horizontal. When reversed, a lever stood vertical, moving through an angle of 60°. The levers were 65 mm apart, which made the operating panel quite compact. The levers directly moved tappets in the vertical interlocking frame through a link. An electric lock could be applied to the lever as shown (this was not, of course, an original feature). The hydraulic valve, or distributor, was driven by a vertical rod moved by an eccentric. Illustrations of the machine usually show a D-type slide valve, but this would not seem possible at the high pressure used. The sketch shows a spool valve in which the pressures are balanced.

The operating pressure was 808-880 psi (50-60 atm), maintained by an accumulator, usually located outside beside the cabin. In France, accumulators were usually inside the cabin in view of a more rigorous climate. In any case, a graduated scale inside the cabin showed the fluid reserve. A pump driven by a three-phase electric motor was used, but a manual pump was provided for emergencies, operated by a long lever at one end of the panel. Accumulator capacities were 5, 10 or 20 litres. Fluid pipes were 7 or 9 mm ID.

When the lever was moved from the normal position enough to apply pressure to the reverse pipe and move a switch, it was stopped short of the vertical by a movable piece. When the actuator had reached its full reverse position, it operated a valve sending pressure back to the control station through the reverse indicating pipe, where it drove the piece upward, permitting the lever to move to the full reverse position and releasing the interlocking. The same process took place when the lever was moved toward the normal position. On receipt of the normal indication pressure, the movable piece was driven downward to its original position, and the lever could be fully reversed. The first motion in each case moved the locking to the proper intermediate state, while the final motion released the locking. This action proved to be essential in any practical power interlocking. The use of tappet interlocking was another feature of all successful power interlocking of the time.

Points were originally moved by a distinctive fluid machine that was also patented by Westinghouse and used on his first power interlockings. This machine had two opposing cylinders, one of twice the area of the other. The smaller cylinder was under a constant pressure, while the actuating pressure was applied to the other cylinder through a valve that would connect it either with fluid under pressure or atmospheric pressure. Not only was the control somewhat simpler than for applying pressure or exhaust to one cylinder or the other, but the switch points would always be under pressure, whatever the state of the control. This was an attractive idea, but it was generally accepted that switch points had to be bolt locked, so it did not prevail. Later machines used a switch-and-lock mechanism (similar to that developed by Saxby and Farmer) and an actuator with two equal cylinders.

Signals were operated by single cylinders, relying on gravity to restore them to normal. Signal operation was not indicated, but signal repeaters were installed so that the operator could verify that a signal had returned to normal, or Stop after a lever had been restored to normal. It would only have been easy to indicate the reverse position, but indication is not considered essential in this case. A signal could be operated by a cylinder acting directly on the counterweight lever, lifting the counterweight and moving the signal to Clear as long as pressure is applied. This was used only for signals close to the cabin, within perhaps 300 yards. Signals further away were operated by wire moved by a single cylinder. The piston rod ended in a pulley, around which the signal wire was wrapped. This gave a wire motion twice the piston stroke. This mechanism was placed in the lower floor of the cabin.


A. Gernigon, Histoire de la Signalisation Ferroviaire Française (Paris: La Vie du Rail, 1998). pp. 273-276.

C. Zenato, Evoluzione Storica e Tecnica del Segnalamento Ferroviario Italiano (Salò: Editrice Trasporti su Rotaie, 2006). pp. 113-125.

D. Wurmser, Signaux Mécaniques, Tome I (Grenoble: Presses et Editions Ferroviaires, 2007). pp. 64-65.

Return to Railways Index

Composed by J. B. Calvert
Created 5 April 2008
Last revised