Special-Purpose Maintenance Vehicles
This article describes briefly two special-purpose mechanical aids which have been developed for cable maintenance on the 60MHz route.
Much of the 60 MHz cable system will be laid across country or in situations where normal road access cannot be relied upon. The cable will be laid in nominal 500 m lengths supplied on large drums which are beyond the capacity of existing British Post Office (B.P.O.) drum trailers and the special techniques developed for jointing require electrical power greater than that available from the portable generators in general issue.
These requirements have led to the development of three special-purpose vehicles capable of travelling across country and negotiating soft and uneven ground.
The Self-Mobile Winch is based on a 4-wheel drive, forward-control Model FC 1004 County Tractor chassis and is equipped with an hydraulically-driven bull-wheel type winch similar to that fitted to Heavy Cabling Equipment. The winch can exert a maximum line pull of 35 58 kN (8,000 lbf) at a maximum rope speed of 4572 rn/mm. A 1,200 m length of steel wire rope, 11 mm in diameter, is fitted and is automatically wound, in controlled layers, on to a storage reel after passing around the winch capstans. To resist the pull of the winch, an earth-anchor system is fitted at the rear of the vehicle.
The unit carries duct-motor rodding equipment to provide rodding capability over 500 m lengths for use in the event of a drawline or rope failure. This equipment is powered by a hydraulically-driven air compressor of the same type as those fitted for similar purposes to B.P.O. Rodding and Light-Cabling Vehicles.2 Tools, rope-guiding and guarding equipment and other small ancillary items are carried in lockers ranged along the sides of the vehicle. The tractor is fitted with a British Standard Safety Cab and no bodywork extends above cab height.
Power for the winch hydraulic system is taken from the vehicle engine through a specially designed power take-off to which is fitted a reversible-flow, variable-delivery axial piston pump which is driven at constant speed. The winch hydraulic circuit is a closed-loop system and winch speed and direction are controlled by a manually-operated servo system which regulates pump output. Maximum flow is 159 litres/mm (35 gal/mm) and maximum system pressure is 151-75 bar (2,200 lbf/in2). Temperature control is effected by bleeding off oil from the low-pressure side of each motor and returning it to a reservoir via an oil cooler. A maximum of 18 litres/min is taken from the system in this manner and is replaced with filtered oil from the reservoir by a boost pump which is in unit with the main pump.
Hydraulic power for the air compressor is provided by a separate fixed-displacement pump driven from the standard agricultural power take-off at the rear of the tractor. This pump also supplies power for raising the earth anchor. All special controls and instruments are on the right-hand side of the vehicle at the rear. There are three instruments showing line pull, length of rope out from the winch and tractor engine speed and four controls, one each for winch, air compressor, earth anchor and manual engine-speed control. An automatic interlock ensures that the tractor cannot be driven until the manual engine-speed control is restored to idling-speed position.
The unit is designed to travel over soft and uneven ground and to traverse steep slopes. The minimum turning circle is under 6 m and ground loading for a total weight of 10 16 tonnes (10 tons) will be about 103 kN/m2 (15 lb/in2) at 25 mm sinkage. The machine is statically stable at angles in excess of 30 degrees when tilted sideways, and, greater than 50 degrees, when tilted to to the front or rear. The vehicle can also be used on the public highway, but, in this case, its speed is restricted by legislation to 12 miles/h.
The Cable Carrier is an articulated-type vehicle consisting of a motive unit and semi-trailer connected by a standard fifth-wheel coupling. A 3-line, air-pressure braking system is fitted.
For cross-country operation, the motive unit is a specially adapted 4-wheel drive, forward-control, Model FC 1004 County Tractor and provision is made for driving the front pair of wheels on the semi-trailer hydraulically in order to improve traction under difficult conditions. Power for the trailer wheel drive is supplied by the motive unit and the trailer wheel speed is synchronized with the tractor ground speed. The loaded weight of the complete unit will be approximately 15 tonnes and the ground loading, at 25 mm sinkage, will be about 138 kN/m2. The minimum turning circle will be under 9l m. For normal road operation, the semi-trailer can be connected to a standard road-going motive unit and, in this case, the trailer wheel drive is inoperative. The carrier is designed to carry a single drum of cable having maximum dimensions of 2,800 min diameter by 1,360 mm wide and a maximum weight of 8 tonnes. The unit is self-loading and provision is made for driving the drum at a maximum speed of 2 rev/min so that cable can be taken off for feeding through a guide system, or short lengths of cable remaining can be wound back on to the drum. Drum-braking facilities are also provided to stop the drum rotating during transit and to give control during cabling to ensure that the drum does not overrun and throw loose turns of cable.
A draw rope winch, capable of exerting a line pull of 445 kN, at rope speeds up to 55 m/min, is fitted and is used to pull the cabling winch rope through the duct from the Self-Mobile Winch Unit.
Drum-loading and driving mechanisms, draw rope winch and
stabilizing jacks are all hydraulically operated by means of a power pack on
the semi-trailer. The engine on the power pack is an air-cooled diesel
engine and it provides power for all the functions needed during a cabling
operation so that the motive unit may be disconnected. The
drum-braking facility is manually operated and the drum-loading arms can be
moved independently to allow levelling of the drum if the trailer is set up
on sloping ground.
This is not a GPO vehicle but shows what the basic vehicle looked like
Last revised: April 21, 2023