GPO Vehicles

A prototype fitted with King Telstar aerial cabling equipment to allow cabling to continue whilst the vehicle was moving.

Pictures are taken at Bletchley Park Training School

The top two pictures show short poles which were used for overhead line training

TELSTA SPEEDS AERIAL CABLING
By B. L. NUTTALL, BA
An article from Telecommunications Journal - Spring 1967

Anew machine which, operated by only two men, can erect aerial cables four times as fast as the normal four-to-five-man gang, is undergoing extensive field trials in the Shrewsbury Telephone Area.

It is the American-manufactured Telsta T36 - more prosaically called the Aerial Cabling Unit in Britain - and is being used in rural areas to put up the aerial cables which are rapidly replacing the familiar telephone wires. It can erect aerial cables at the rate of between four to six miles a week, the machine paying out the cable in mid-air from the end of its electrically-operated boom as it is driven along at walking pace.

The Aerial Cabling Unit consists of a turret-like base and a main boom fitted with an extending inner section. The working platform, in the form of a metal basket, is supported on steel arms at the end of the telescopic boom which has a horizontal reach of 25 ft. 11 ins, and a vertical reach of 36 ft. The complete assembly is mounted on a Dodge vehicle chassis with a two-speed rear axle and a new type of differential lock consisting of two rollers which can be made to bear against the rear wheels to prevent wheel spin in mud or snow.

The boom is easily and rapidly raised, lowered or rotated by means of a hand-operated joystick, power being obtained from a propane-driven electric generating set, with an electric self-starter, mounted at the rear of the chassis. The generator drives a number of electric motors which move the boom through a series of chains and chain wheels. If a chain breaks, the working platform can be manually rotated and safely lowered to the ground.

The new unit can carry more than 30 cwt. of cable on two arms at the rear of the chassis so that additional cable-carrying transport is unnecessary. The arms can be hydraulically raised and lowered for loading and unloading cable drums. The payload of the basket is five cwt. and in addition nearly a ton of suspension wire can be carried in the well of the vehicle.

As the machine moves along its route between poles, the aerial cable can be paid out under tension obtained by electrically-operated drum brakes mounted on the cable drum spindles and controlled by the driver in his cab. The machine stops only briefly at each pole, the operator boring a hole through it with a quick-change combined electrical drill and torque wrench, passing through a bolt and tightening the clamp with the wrench. This operation takes only a few minutes.

The machine also has a small but powerful three-speed winch so that at no stage does cable have to be manhandled. This winch - called the electric towline - is used by the Americans to carry out a number of specialised tasks, including passing cable behind poles.

Shortly after the machine arrived in Britain from the United States, Mr. J. S. Kennedy, factory manager of the Telsta Corporation, was flown over to supervise the training of Post Office teams in its use and maintenance.

First reports indicate that the trials will be successful and that use of the new Aerial Cabling Unit will not only increase productivity but also take much of the hard work out of aerial cabling.

An extract from
The Post Office Electrical Engineers' Journal
Volume 62, Part 2 - Dated July 1969

An Aerial-Cabling Unit

The policy of mechanization of overhead construction methods has resulted in trials of an American aerial cabling unit which consists of a turntable and extensible boom mounted on a vehicle chassis. The trials have shown the device lo be capable of significant savings of both labour and time in the erection of the heavier aerial cables and the development of a cheaper, British-made machine, with a wider application, is now taking place.

INTRODUCTION
During the past few years there has been considerable change in the design of overhead plant, with the emphasis on the use of plastic-sheathed, self-supporting aerial cable, light weight fittings, and wider use of light poles. The size and loading of aerial routes have been much reduced. Consequently, the physical effort required at the construction stage is less and the work is carried out under much safer conditions. The erection of poles is perhaps the heaviest and most hazardous part of the job, and the first effort at
mechanizing overhead construction was made in this direction.

Some three years ago, the pole-erection unit was introduced and is now in wide use throughout the country. Attention is now being given to improving methods and techniques of erecting and recovering aerial cable. A specially designed machine for use on such work has been available in North America for a number of years. It consists of a vehicle-mounted, telescopic aerial-platform, equipped with power tools and pulling winch and is used mainly with lashed aerial-cable systems.

The general requirements of an aerial-cabling unit are that it should enable the speedy and safe erection of all forms of aerial cable with the minimum of manpower. It should be particularly efficient for the erection of self-supporting cable which has an integral suspension wire, and its use must lead to significant reductions in the overall cost of overhead construction work.

GENERAL DESCRIPTION
The aerial-cabling unit consists of a turret-like base and boom mounted on a British-made chassis. The boom has a telescopic inner section with a work bucket attached to the end of it by means of a torque tube and steel fork-arms. The maximum outreach of the unit is 24ft 7in, and it has a maximum working height of 36ft. The unit is stabilized against roll by a torsion bar on the rear axle and outriggers or stabilizing jacks are unnecessary at any time. All motions of the boom and turret are effected by electric motors, and the
current for these motors is provided by an engine generator set. This set is mounted at the rear of the vehicle body and consists of a 10h.p. 4-stroke, 2-cylinder engine running on propane gas.

All motions of the bucket can be controlled, either from the vehicle body, or from the bucket itself. A two-way loud speaker system provides communication between the cab and the bucket.

The Boom
The boom consists of two hollow, welded, rectangular sections. The outer-boom section, which is pivoted on the turret, has a reversible electric motor mounted at the end of it that extends and retracts the inner boom by means of a chain drive. Over extension or retraction is prevented by means of a double-acting limit switch mounted at the end of the main boom. Should either of these fail, additional mechanical stops are provided to prevent boom over-travel. A safety cam is also provided on the inner boom. This automatically engages the outer-boom section if the chain should fail, and locks the two boom sections together to prevent the inner boom from telescoping.

The boom is elevated by means of an electric motor mounted in the turret, which drives the end of the outer boom by means of a double chain. At maximum elevation a limit switch is operated which stops the motor, and another switch comes into operation if the elevation chain becomes slack. This will occur, for example, when the boom is seated in its carrying rest, or when the bucket rests on the ground.  Should the elevation chain fail, a hydraulic safety cylinder located on the turret locks the boom in position. After manual release of the valve on the safety cylinder the boom will descend slowly to the ground.

The boom is equipped with a lifting yoke and fixed hook, which is rated to lift a maximum load of 2,000lb at a radius of 6ft.

The Turret
The turret and boom are rotated by means of a second electric motor and gears. This motion is limited by means of a limit switch after the turret has rotated continuously for three complete revolutions from the centre position, i.e. the turret may be rotated for six complete revolutions from one extreme of its travel to the other. The motor is protected from continuous overload operation by a thermal overload switch. If this is actuated, the motor is switched off and can not be re-operated until the switch cools. A friction brake is also fitted to provide accurate boom positioning.

Should there be an electrical failure of any motor. all motions of the boom can be performed manually, although it is necessary to disengage the friction brake before the boom can be rotated.

Control switches for the turret and boom are mounted on the side of the electrical panel. These consist of a set of three toggle switches controlling elevation, extension and rotation, and they may be operated individually or simultaneously.

Also mounted on the panel is a meter for recording the running hours of the generator set, a socket for the electric tow-line ground control, and sockets for a 110-volt supply.

The Work Bucket and Controls
The bucket is fabricated from steel tubing and is of sufficient size to carry one person, his tools, and necessary fittings. The unit is rated at a safe working load of 500lb, with the boom fully extended in any position.

Free swinging of the bucket is prevented by means of a hydraulic damper between the bucket and the fork arms. The main bucket control is a single-lever, joy-stick-type directional control with a thumb-operated, recessed safety button which must be held down before any operations will take place. Moving the joy-stick in any direction causes corresponding motion of the bucket, and the three directions of motion may be employed individually or simultaneously.

For example, pushing the joy-stick simultaneously down wards, forwards and to the right will lower, extend and rotate the bucket to the right. Movement of the control operates corresponding micro-switches mounted at the base of the control, and these in turn operate 110-volt solenoids in the electrical panel on the turret.

A stop-start switch for the electric generating set is also provided on the base of the bucket control switch, as well as a toggle switch for controlling the electric tow-line.

The Electric Generating Set
The electric generating set consists of a 10h.p., Onan, 4-stroke, 2-cylinder engine running on commercial propane. The engine drives a 220-volt, 3-phase, 60hz generator with an output of 3·5kVA. The three main motors are driven from the 3-phase supply, but associated control solenoids and switches are operated by the voltage between one phase and the chassis; a nominal 110-volt. The supply is approximately balanced by using different phases for operating the separate equipments.

A self-starter, which operates from the vehicle battery, is provided for the engine.

The Chassis
The aerial-cabling unit is mounted on a British-made Dodge KP900 chassis. This has a forward-control three-man cab, power-steering, and a short wheel-base, making it extremely manoeuvrable in restricted conditions. The vehicle has a four-speed gear box and a 2-speed rear axle. When being driven with an operator in the bucket, both the low-gear and low-ratio rear axle are engaged, and the unit then has a maximum speed of approximately 5 miles per hour. The stability of the unit, with the maximum load in the bucket, is achieved by means of a very large torsion bar linking the vehicle rear springs together.

To overcome wheel spin on muddy or snowbound roads, the vehicle is fitted with a Dennis extricator, which consists simply of rollers mounted on each end of a solid shaft. By operating two hand screws these rollers can be made to bear against the vehicle rear tyres, thus effectively locking out the differential and preventing wheel spin.

Intercommunication
The aerial-cable unit is equipped with a transistor-type intercommunication system. The combined loudspeaker and microphone is mounted at the end of the inner boom, facing the bucket, so that controls are not necessary for the operator in the bucket. Normal direction of transmission is from the operator in the bucket to the vehicle driver. To transmit from the cab it is necessary to depress the talk switch.

Electric Tow-Line
This is a small electric winch with a maximum pull of 600lb that can be used for many cabling operations. It is mounted on the vehicle chassis and consists of a 110-volt motor driving a capstan through sheaves and a V-belt. If necessary, three speeds can be obtained by moving the V-belt. The capstan can either be used as a simple capstan winch, or wound with 90ft of nylon rope so that it then acts as a conventional winch.

The electric tow-line, and the direction of rotation of the capstan, can be controlled either from the bucket, or from a separate ground control. The winch is self-braking in either direction by a worm-gear drive.

Cable Storage
Cable drums of maximum diameter 5ft 6in are carried on hydraulically-controlled arms at the rear of the vehicle, and a load of up to 3,500lb can be accommodated. Additional cable may be towed on a cable trailer. The hydraulic system is powered by a 1h.p. electric motor which is located, with the pump, in a small cubicle on the near side of the vehicle.

The cable-drum spindle is equipped with a magnetic brake, and rheostat control from the cab enables the payout tension in the cable to be accurately controlled during cabling operations.

Facilities are also provided for the storage of suspension wire in the well situated behind the driving cab. Payout tension of suspension wire is controlled in a way similar to that of cable.

ERECTION OF CABLE WITH THE AERIAL CABLING UNIT
The unit was designed for the erection of lashed aerial cabling systems, which are in more general use in North America. No difficulties have been experienced, however, in applying it to self-supporting combined systems, and trials in this country have been concerned mainly with this type of cable. Two basic methods of working have been developed, the choice of system being dependent upon site conditions.

Direct Placing of the Cable
In this method, the cable drum is carried on the rear drum spindle and paid out as the vehicle moves slowly along the route. The route must permit the cable to be attached on the road side of the poles. The direction of erection must be such that the cable will be paid out to the near side with the vehicle moving on the correct side of the road as far as possible. The starting point is preferably at a stayed terminal pole but, if an intermediate pole is selected, a temporary stay may be necessary. Where an obstruction occurs within two or three spans of a section end, it may be preferable to cable towards that end, or to position the vehicle beyond the
obstruction and fleet the cable back to the terminal pole.

The vehicle is prepared by threading the cable from the underside of the drum, through the fairleads to the forward mounted bullwheel on the vehicle, and out along the boom to the fairlead on the bucket (see Fig. 3). The towline is fitted with a cam-action grip and is fed along the boom to the bucket. The operator takes with him into the bucket all the tools and pole fittings necessary for the section. He then raises and positions the bucket conveniently close to the pole and terminates the cable suspension wire. On completion, he instructs the driver to proceed along the route. The cable drum brake is adjusted to give sufficient clearance tension in the cable as it is payed out from the bucket fairlead. When travelling between poles, the boom is positioned as far as possible in line with the vehicle to avoid unnecessary strains on the machine and unnecessary extension of the boom. As the intermediate pole is approached, the operator instructs the driver to stop and positions the bucket at working height alongside the pole. The pole is then drilled and the cable supports fitted using an electric impact wrench. The cable can now be transferred to rest loosely over the pole support, the driver instructed to move on, and the bucket again moved into its paying out position. Thus, cable is erected at reasonable clearance until the terminal pole is reached. Final tensioning is carried out in two stages. After attaching the winch tow rope grip to the cable, the winch is operated to take in the cable slack up to a tension of about 600lb, i.e. the maximum pull of the winch. A Piab dynamometer with a Tirfor tensioning device is then attached to the pole and the suspension strand which has been stripped from the cable. The Tirfor is used to apply the final tension and the strand terminated. During tensioning, the driver may need to check that the cable has not fouled branches, and, since up to 20 spans may have been erected, a bicycle together with a radio transmitter, has been found useful. On completion of tensioning the vehicle returns over the route inserting the "anti-galloping" twists, and clamping or binding in the cable at intermediate poles.

Variations in the sequence of events may be necessary depending on the route conditions, and some examples are given below.

At road crossings it is usual to attach a temporary clamp to the cable close to the pole fitting. This prevents the cable dropping across the road should tension be released inadvertently. If clearance is doubtful, a temporary suspension rope, with cable pulleys attached at intervals along its length, can also be erected to support the cable across the road. Where the route is long enough to require more than one length of cable, a temporary stay is normally necessary at the jointing pole to ensure that successive cable lengths can be fully tensioned as they are erected. Alternatively, the first cable length may be made off temporarily at the jointing pole at "payout" tension. The next length can then be cabled back to the jointing pole, and both cables tensioned simultaneously, without erecting a temporary stay to stabilize the pole.

Where an obstruction occurs towards the end of a section, the cable can be temporarily clamped at the last pole before the obstruction, and the remaining cable taken off the drum and left at the pole. With the vehicle at the terminal pole a towline can be taken back over the route on foot to pick up the cable end. The winch can then be used to haul in the towline and the cable.

Drawing-in the Cable
The presence of open-wires, arms, trees, or the need to attach the cable to the pole on the field side. will prevent the erection of the cable as described. For these situations the following technique can be adopted.

The cable drum is parked on a trailer at a convenient position near the first pole with the drum-spindle braked to prevent over run and give some tension to the cable. The cable end is taken up the first pole and passed through a pulley block fitted to the pole. The towline passing through the bucket fairlead is then attached about 4 to 5 yards from the cable end, and the cable is drawn in as the vehicle moves along the road. Where the cable has to pass behind the pole, or through the arms, the vehicle is stopped and the cable is temporarily anchored using a separate line and grip. The towline may then be removed from the cable, passed round the pole, or through the arms and re-attached to the cable. After removing the temporary anchor, cabling can proceed as before.

After terminating at the final pole, the unit returns to the first pole to pull up, tension, and make-off. Since braking at the trailer is lin1ited, payout tensions are necessarily lower in this 1nethod and sag is greater. It is essential, therefore, to use the ten1porary suspension line and cable rollers where clearance is important. Temporary suspensions are recovered during the final run over the route to insert twists and bind in the cable.

Lashed Cable Systems
The unit has also been used to erect a suspension strand, and to lash polythene and lead-sheath cables to new and existing strands. Erection of the strand is a simple operation and follows a procedure similar to that described for the self supporting cables. The ease with which the boom can be positioned, and the provision of the towline and winch, greatly facilitate the use of the lashing machine so that speedy erection is possible.

CONCLUSIONS
The unit, which handles well in restricted conditions, has operated without difficulty on almost every type of road, and in most weather conditions. The problem of operating in narrow lanes has not proved a serious one as the vehicle is stationary for only a few minutes at each pole, and serious delays to other road users rarely arise.

The joystick control is simple and positive, and with very little experience the operator is able to place himself quickly and accurately in any desired working position. Training of crews for this machine has presented no problems, and there is no undue difficulty in using the unit safely and efficiently after a few hours instruction.

The trial has shown quite clearly that aerial-cabling work can be performed easily and safely by two men using the unit, with a saving of better than half the man hours. More important, perhaps, the crew experience far less fatigue than with normal manual methods of construction.

It is clear that the improvement in productivity is high, particularly on those routes carrying heavier cables of the trunk or junction type, or the longer local cables. The majority of cables erected today are, however, of the smaller local type, and the scope for this high-cost in1ported machine is limited. The experience gained has been invaluable, and work is already proceeding on the development of a cheaper, British-made machine. Although this will have rather fewer facilities, it is hoped to produce a unit which will handle the more common, sn1aller cables, and will have a wider application in the field of overhead wiring and cabling construction.

ACKNOWLEDGMENTS
The authors would like to acknowledge the generous co-operation and assistance they have received during the trials of this unit from their colleagues in the Civil and Mechanical Engineering Branch and in the Wales and Border Counties.

Last revised: February 01, 2024