British Rail Class 74

British locomotive

Twenty four British Rail Class 71 (pre TOPS type HA) locomotives were built in 1958 at the British Rail works in Doncaster. Ten of these were deemed surplus to requirements and placed in storage in 1964. The work to convert these to electro diesels was originally to have taken place at the Southern Region works at Eastleigh, but their involvement in new build EMUs for the Bournemouth electrification meant that Crewe Works was chosen for the rebuilds. The redundant HA locomotives were moved in groups to Crewe where they were rebuilt into type HB/Class 74 electro diesels; the first rebuilt example working under its own power from Crewe to Stewarts Lane depot on 10 November 1967. Buckeye couplers and high level, or 'bagpipe', control & brake jumpers were fitted to facilitate working with other EP stock especially TC units in push pull mode. Also, two tone "raspberry" air horns were mounted on the cab roof, replacing the original air whistle of Class 71 locomotives. They were intended especially for use on the boat trains to Southampton and Weymouth, as both routes included sections of non electrified track and tramway along the public thoroughfare. The elimination of the locomotive change (at either Eastleigh (for Southampton) or Bournemouth) was envisaged and their dual power capability would greatly accelerate timings and reduce operational complexity. Originally, plans had been to number them E7001–E7010, but once rebuilt they were numbered E6101–E6110 instead. Later they were given TOPS numbers 74001–74010.

In common with the first SR DC electric locomotives (Class 70), Class 74 utilised a booster set (a motor generator and flywheel combination) to overcome the problem of gapping. Booster set 836/2D had been designed by English Electric for use in Class 71 and this was retained in the rebuild, although weight considerations meant that the flywheel assembly, separate auxiliary generator and traction motor blowers were removed (the latter leading to the decision to downrate the traction motors). The compact size of the booster permitted a small diesel engine and generator inside the body shell (only a single booster was used in Classes 71 & 74 unlike the two of Class 70). Thus, these locomotives could operate either from a third rail supply at 650 V DC (Eastern & Central sections), 750 V DC (Western section) or from their Paxman 6YJXL 'Ventura' diesel engine, downrated to 650 hp in order to extend service intervals. BR already had 77 locomotives using this model of engine; 57 Class 14s and a further 20 Class 29s. The Class 71's pantograph for overhead current collection was not retained.

The bodies of Class 71 locomotives were not designed to be structural components capable of carrying any weight, following the principle of earlier designs in that the underframe was the main structural member of the locomotive while the body served primarily as weatherproofing. In August 1966, after initial strip down and examination of E5016, engineers were forced to re assess the build when it became apparent that the planned equipment changes could not be accommodated. Although at one point it was mooted that the body would have to be divided and lengthened, the problem was eventually resolved by rebuilding the bodies with a Warren truss framework and outriggers to support the curved body skin. Translucent roof panels were fitted to increase daytime illumination in the engine room. Modifications were also made to the locations of the cooler groups, water tanks, and exhaust silencer.

Obtaining a DC supply controlled by rectifiers requires an AC power source. Consequently, the existing auxiliary generator was converted to a three phase alternator with automatic voltage regulator. This arrangement gives improved control of traction current over conventional rheostatic systems and permits fine control when starting a train. The system also functioned as form of wheel slip protection by regulating traction current voltage and supply across all the locomotive's traction motors at once. Normally a driver has to maintain tractive effort well below the rail adhesion limit to give themselves time to respond to wheel slip. The constant current systems of Class 74 were designed to allow the driver to apply power very near the limit of adhesion and the booster output is regulated in such a way that the maximum current of any traction motor group does not exceed the selected value. Thus, even when one axle starts slipping, the voltage across the whole group of motors cannot rise because current in the non slipping motors is fixed. As one motor begins to slip, its current consumption drops (electric motors use greatest current when stalled and least while free running) which provides more current for the non slipping motor. The voltage drops proportionately and the slipping motornow partly deprived of powerbegins to slow, which stops the slipping and returns the traction system to the state it was in before slip set in; thus torque can be maintained at a point just below friction break away. The power controller, though capable of infinite variability, was notched so that Class 74 locomotives could work with Class 73/1s and any electro pneumatically controlled ('Blue Star' coupling code) Type 2, 3, or 4 diesel locomotive. Two constant voltage notches were provided for slow speed control of shunting and coupling up. Three further notches spaced out over the power range corresponded to the 'Series, Parallel, Weak Field' (2, 3 & 4) progression used on power controllers for multiple unit stock. Control equipment was designed on the 'Line Replaceable Unit' (LRU) ethos and consisted of 'trays' of circuit boards and equipment arranged in two cabinets. The first contained the usual circuit breakers, relays, reversers, and so forth. The second also housed standard equipment but additionally included the complex electronic control circuits that ultimately were the un doing of the class. The equipment trays were equipped with test connectors and could be slid into and out of the relevant cabinet with ease when the locomotive was shut down. Maintenance staff were provided with testing units that would connect to the trays and compare the electrical signals and stimuli on the test socket with the design values. If any anomaly was detected, the whole tray was replaced and the locomotive couldin theorybe returned to service with minimal disruption. The theory, however, did not fully account for the fact that faults in one tray might only present themselves in combination with faults on other trays, which sometimes lead to locomotives being kept out of service for long periods of time while in depth investigations were undertaken. Nevertheless, advances in power control technologyparticularly solid state electronicsallowed similar arrangements to be adopted reliably by other manufacturers and operators within ten years' time.