THE MILNER PARK AERIAL ROPEWAY

 ABSTRACT

The various types of aerial ropeways are given. The Milner Park Showground mono-cable ropeway is described, details being given of the end stations, the portal supports and the gondolas. The method of operation and all the various devices and ancillary apparatus to ensure safe operation are set out. The modifications found necessary after operating experience are mentioned.

 INTRODUCTION

Early ropeways before the advent of the steel wire rope

The earliest recorded ropeways, dating back to the fourteenth century, consisted of a fixed carrying rope along which the passenger slid in a sling, the sling being propelled either by the passenger pulling on the fixed rope or by a second rope attached to the sling pulled by helpers at either end of the travelling way. As ropeways developed, heavier objects were transported using oxen as the motive power. By the seventeenth century, mono-cable ropeways were used in the building of town fortifications.

The advent of the steel wire rope

Wire ropes made from bronze wires were used at Pompeii, but it was not until the nineteenth century that drawn steel wire ropes were successfully made and used by mines in the Harz Mountains, Germany. Subsequently, mono-cable aerial ropeways were constructed in England and America using steel ropes. These were followed by bi-cable ropeways in Germany, Austria and Russia. These earlier ropeways were solely designed for the transportation of goods, but passenger-carrying ropeways were soon developed and by the end of the nineteenth century, ropeways were in use throughout the world. During the 1914 to 1918 war, rope transport was used on a wide scale, 2 680 ropeways being erected during this period. This trend continued during the 1939 to 1945 war when ropeways were used to transport munitions and wounded soldiers.

TYPES OF AERIAL ROPEWAYS

1. Continuously operating

(a) Mono-cable. (Fig. 1)

The mono-cable ropeway is generally made with a single rope running continuously with cars fixed to the rope either permanently, or so that they can be detached at the stations.

Fixed grips

Mono-cable ropeways employing the fixed-grip principle are more usually used for ski-lifts where loading and unloading can be undertaken with the car in motion. The fixed grip is clamped securely onto the rope in such a way that it can be. tightened from time to time to allow for a decrease in rope diameter. To the best of our knowledge, there are no ropeways of this construction in South Africa.

Detaching grips

Where a car has to come to a standstill for loading or off-loading, a mono-cable system with a detachable grip is used. This grip falls into three general categories, viz: spring-loaded, gravity loaded and hook-cm type. The spring grip and the gravity loaded grip are made with jaws which clamp onto the rope and are held in position by a load of a spring or by the weight of the car. The hook-on type grip hooks either onto a hanger which is permanently fixed to the rope or onto the rope itself.

The detaching grip is automatically detached from the rope as the car enters the station, allowing the car to be slowed down and stopped for the loading operation. When loading has been completed, the car is' accelerated to rope speed when the grip automatically engages with the rope and the car proceeds to its destination.

Examples of this type of ropeway are to be found at the Milner Park Showgrounds, Johannesburg, at the Klip River Power Station, Vereeniging, and at the Pretoria Portland Cement Company, Loerie, in the Cape.

(b) Bi-cable.

The bi-cable ropeway consists of two sets of ropes, one set comprising the stationary track rope and the other the moving hauling rope. The car hanger is fitted with grooved wheels which run along the fixed track rope while a detachable type grip is used to connect the car to the hauling rope. As with mono-cable ropeways, the detachable grip is either spring-loaded or gravity loaded.

Examples of this type of ropeway can be found at the Havelock Mine in Swaziland, at the Pretoria Portland Cement Company at Loerie and at the Swartkoppies Pumping Station near Johannesburg.

2. Intermittently operating

(a) Mono-cable

This type of ropeway has the car permanently fixed to the single moveable rope through a hanger and clamps. The whole system is stopped each time a car enters a station for the loading or off-loading operation. These ropeways are usually used in conjunction with a fairly large car which is able to accommodate a number of persons at a time.

(b) Bi-cable

Intermittently operating bi-cable ropeways fall into two categories viz:

The single track rope type, and The double-track rope type.

(i) The single track rope type, which is extensively used in South Africa for bridge and dam building, consists of a track rope which supports a carriage, a hauling rope which moves the carriage to and from and the crane rope which passes over pulleys on the carriage to a hook, from which is suspended the load.

This type of ropeway, more commonly known as the cable crane, has been used on such sites as The Storms River bridge, the Klaver River bridge, the Vaal River bridge at Standerton, the Kyle Dam in Rhodesia and the Kat River Dam in the Eastern Province. (See Figs. 2 and 3.)

(ii) The double-track rope type of ropeway is mainly for passenger carrying and consists of two-track ropes spaced some distance apart, each carrying one car. The cars run in opposite directions and are attached to hauling ropes driven by a double drum winding engine situated at the higher terminal station. Where it is required to transport ore down from the higher station, the engine can be dispensed with. The heavier car raises the empty car by gravity and the speed is controlled by a brake.

An example of this type of cableway can be found at Table Mountain, Cape Town.

 ROPEWAY TRANSPORT

In general, ropeway transport is feasible where more conventional means are hampered by inaccessibility. The ropeway is able to operate directly from one point to another which means that in a mountainous country the distance traversed is considerably reduced, the necessity for a good road is obviated and the human element is reduced to a minimum.

A large number of ropeways in use in Europe give an indication as to the efficiency of the system and the day is dawning in our country where rising labour costs will cause more and more engineers to favour this form of transport.

THE MILNER PARK CONTINUOUSLY OPERATING MONOCABLE ROPEWAY

The passenger-carrying mono-cable ropeway at Milner Park Showgrounds, Johannesburg, was constructed at the beginning of 1964 by a consortium of Johannesburg industrialists who realized the need in this country for ropeways and determined to illustrate the simplicity of this type of transport to the public. The ropeway has now operated successfully for two seasons during which time 185 000 persons have been transported, if not without some heart palpitation, at least with complete ease and comfort. The system, which covers a distance of 1 700 ft between stations and which has seven intermediate support points, cost in the neighbourhood of R100 000. Much effort was made to make this ropeway as aesthetically pleasing as possible and to ensure that its operation in no way hindered the passage of traffic on the ground. The two-terminal stations were elevated and were constructed of reinforced concrete which met with modern concepts of streamlined beauty. This increased the initial cost considerably.

End stations and supports

Tower of Light or South station.

This end station was built onto the existing Tower of Light and the platform is 26 ft above ground level. The station has two decks, the upper deck containing the return sheave, accelerating and retarding rollers and the fixed track rails, while the lower deck forms a platform to enable passengers to board and alight. Due to constructional difficulties, the dimensions of this station were kept to a minimum.

North Station

The north end terminal, which is 110 ft lower than the south terminal, consists again of two platforms which are built onto and around a tower containing the drive, the tensioning gear and the control room. The lower platform again serves for the despatch and receipt of passengers. The accelerating and retarding rollers and fixed track rails are supported on the upper platform which is 40 ft above ground level.

Intermediate supports.

The rope supports were constructed of precast reinforced concrete for aesthetic effect and are spaced roughly 220 ft apart. They are in the form of arches, each arch having been made in two sections on the ground and then raised into position on foundation pads. (Fig. 4.) Midway between the two terminals, the existing Everite tower was used to attach seventh rope support. The profile of the system is such that the rope rises from the north or lower station, fairly steeply, to the first portal or arch, and then continues to rise but at a much less acute angle until it reaches the upper portal from whence it enters the south station almost horizontally (Fig. 5). The two ropes are parallel to each other and are supported at 12 ft centres on sets of four balancing wheels. These balancing wheels are arranged in pairs so that they can articulate vertically in line with the rope. (See Fig. 6.) Each balancing wheel is made of two steel discs, the outer periphery of which form flanges. A special rubber tread is clamped between the discs by bolts. The groove of the wheel is machined into the rubber so that the rope is supported on the rubber and never touches the steel flanges.

Each pair of balancing wheels is mounted and high tensile steel spindles between side plates. Midway between the wheels, there is a spindle passing through bearings in the side plates and connected to the outer ends of the equalizing arms. These arms are centrally pivoted on bearings supported by a box section hanger which is supported on trunnion bearings in a fabricated steel plate frame attached to the concrete portal. The balancing wheels are therefore free to form whatever profile is necessary for a vertical plane.

The general method of operation

The rope has a diameter of 1 in, is 3400 ft long and is 6 x 19 (9/9/1) construction. It has only one splice. It is driven by an 8 ft diameter drive wheel round which it laps for 180 degrees. The rope runs at a speed of 420 ft/min and, under normal circumstances, is never stopped. It goes from the drive wheel over a pair of deflecting sheaves near the top of the north station tower and then rises to grip level. It is gripped by one of the detachable grips on the car, rides under a hold-down roller at the extremity of the station and takes the car with it up and over the balancing wheels at intermediate portals, eventually entering the south station. Here the rope is deflected back to the horizontal line over a single pair of balancing wheels, the grip on the car is released and the rope is deflected down and away from the grip. The rope then passes around the return sheave and up slightly to meet an outgoing grip. It is gripped by another car and passes from the station through the various portals and carries the car on the return journey, to the north station where the grip is again released and the rope deflected down slightly away from the grip, around the two sheaves in the north tower and back to the drive wheel. One of the most important aspects of this operation is the gripping and releasing of the rope at the appropriate moment. This has to be done with the grip travelling at exactly the same velocity as the rope. One of the simplest methods of accomplishing this is by using gravity to accelerate the car and the grip. The gripping action is then accomplished by using the mass of the car.

This system, used extensively by Bleichert, has one serious drawback in that the inclined rail is set for the full speed of the rope. If it is required to run the rope at some other speed, the car is caused to sway uncomfortably at contact and the action of grip on the rope produces severe abrasion.

The accelerating and retarding devices

The acceleration and retardation of the grip is done at the Milner Park installation by means of a bank of 10 inch diameter rubber-tyred rollers set at 10 ? inch centres. The drive for the rollers is taken from the hold down rollers at the north station and from the return sheave at the south station by means of a chain to the middle roller of the bank. There are twenty in each bank at the north station and at the receiving side at the south station, and fifteen rollers in the despatch bank at the south station, each roller being coupled to the adjacent roller by means of individual chain drives. Each roller thus has two sprockets of such a size as to make it run slightly faster than the roller preceding it. The rollers are individually spring loaded vertically and set so that they will bear down on the top of the grip, causing it to move and accelerate or retard at the required rate. Because the rollers are driven off the rope, their speeds are always proportional to rope speed thus obviating any difficulty caused by variation of the speed of operation.

The grips

The grip (See Figs. 9 and 10) is made of cast steel, is spring-loaded and consists of eight parts, viz:

1. The fixed grip and body

2. The movable jaw

3. The helical springs

4. The grip actuating rollers

5. The fixed roller

6. The track rollers

7. The overlock

8. The grip lever system

As a car enters the path of the accelerating rollers, It’s suspended by the track rollers on a square section rad which runs below and parallel to the rope. (Figs. 7 and 10.)

The accelerating rollers make contact with the top of the body of the grip and move it forward to engage with a linear cam or spear which enters the grip between the fixed and moving rollers and at the same time another cam engages the overlock roller unlocking it. As the grip is moved forward, the spear forces the moving rollers inwards against the pressure if the two springs, causing the movable jaw to move inwards away from the fixed jaw, thus opening the grip. At this stage, the rope is deflected upwards to enter between the fixed and movable jaws. By this time the accelerating rollers have brought the car up to rope speed and the spear begins to taper back to zero, with the result that the springs close the movable jaw and the grip is attached to the rope.

 As this happens, the overlock cam loses contact with the overlock roller allowing it to snap into the locked position. With the overlock down, the grip cannot be opened even though the springs break. The grip now passes through a series of safety flaps (Fig. 12) which ensure that –

1. the grip has closed properly and is lying in the correct position relative to the rope, and

2. the overlock lever is in the correct position.

The grip then runs for a further 8 ft on the steel track, being now propelled by the rope directly. Over this section it may still be stopped if one of the safety flaps indicates incorrect attachment to the rope. Having passed the flaps satisfactorily, the grip leaves the rail and the station and proceeds on its way safely secured to the rope. The gripping force of the springs is designed to allow no slip between rope and grip at a pull of 800 lbf measured parallel to the rope axis.

All pivot points in the grip have sealed bearings of the roller or needle type. The track rollers have grease points. To ensure that the faces between the fixed and the movable jaws are always parallel, two sets of linkages, one near the grip end and one at the roller end, are used. These links having the same radial movement, constrain the movable jaw without sliding friction and yet provide the required parallel action. The ends of both the fixed and the movable jaws are belled to prevent sharp bends in the rope when the grip and car are passing points of maximum deflection such as at mid span and on approaching the portals.

The spears

The spear, which is actually a linear cam, forces the grip open and allows it to close at the appropriate moment. It is the length of the accelerating or retarding roller bank and is tapered on the inside from zero to 24 in and back to zero. The outside of the spear is straight and is parallel to the fixed track rail. The lower face of the spear is also straight and is horizontal, acting as a location for the outer end of the grip. The spear is made from high tensile steel plate.

The drive and counterweight

The drive and counterweight assembly are located in the tower at the north end and are suspended on the rope. The total weight of the assembly is 15 ton which is twice the tension in the rope. Because there was not sufficient space available at the south end for All pivot points in the grip have sealed bearings of the roller or needle type. The track rollers have grease points.

To ensure that the faces between the fixed and the movable jaws are always parallel, two sets of linkages, one near the grip end and one at the roller end, are used. These links having the same radial movement, constrain the movable jaw without sliding friction and yet provide the required parallel action. The ends of both the fixed and the movable jaws are belled to prevent sharp bends in the rope when the grip and car are passing points of maximum deflection such as at mid-span and on approaching the portals.

The spears

The spear, which is actually a linear cam, forces the grip open and allows it to close at the appropriate moment. It is the length of the accelerating or retarding roller bank and is tapered on the inside from zero to 24 in and back to zero. The outside of the spear is straight and is parallel to the fixed track rail. The lower face of the spear is also straight and is horizontal, acting as a location for the outer end of the grip. The spear is made from high tensile steel plate.

The drive and counterweight

The drive and counterweight assembly are located in the tower at the north end and are suspended on the rope. The total weight of the assembly is 15 ton which is twice the tension in the rope. Because there was not sufficient space available at the south end for either the drive or the take-up mechanism, it was decided to combine these devices and position them at the north end. This has been effectively accomplished by having a deadweight made of concrete blocks held in a steel box below a platform on which is supported the motor and gearbox which drives the main friction wheel through a spur wheel and pinion. The whole arrangement can move up or down and is guided by steel rollers on two steel boxed columns running vertically from floor to sheaves.

The main drive is by a 25 hp 3-phase slip ring motor through the gearbox. An auxiliary petrol engine can be quickly connected by means of a chain coupling should the electric power fail. It is not usual to drive a cableway from the lowest point, because, by so doing, the effective pull due to the weight of the rope and car is not utilized at the friction sheave. However, in this installation, because the difference in level between the upper and lower station is not great, the loss in the effective pull is negligible.

The motor speed is 1420 rev/min which is reduced by a worm gearbox to 117 rev/min. The speed is further reduced through an open pinion and spur wheel to 17 revs/min which is the speed of the friction wheel.

Two sets of brakes are supplied, one of which acts at the coupling of the motor and is used as the 'normal brake'. The other set acts on a 4 ft dia brake race attached directly to the friction wheel. Both sets of brakes are spring-loaded. Nitrogen under pressure from a portable gas container is used to release the brakes. The material used on the tread of the mam friction wheel is PVC and it has been found to withstand wear and pressure satisfactorily to date.

The gondolas

Each gondola or car (Fig. 13), accommodates three persons and is constructed of a tubular framework with a seat, sides and roof made of fibreglass. The construction is light in weight in spite of being robust. The car has one door which opens outwards and which when closed, is locked with two catches for safety: The car is suspended from a central point in the roof by means of a curved fabricated box section member which is connected to the grip by a bushed pin, thus allowing the car an unrestricted swing through an angle of 110 degrees in the plane of travel.

Safety devices

Safety devices are provided to prevent the following eventualities:-

The grip not gripping the rope correctly on exit.

The overlock not falling into place on exit.

A car entering a station when the preceding car is not clear of it.

The overlock not being disengaged on entry to a station.

The grip not being disengaged from the rope on entry to a station.

The rope coming off the balancing wheels at a portal.

The grips being derailed at a station. In addition, emergency stop buttons and tripwires are provided. The electrically - operated safety circuits are initiated at the south station where two twelve-volt lead-acid batteries supply the power. This power is fed through safety switches in two separate circuits to the north station, through the safety switches there, and to four relays which switch off the power to the motor or cut the ignition circuit of the petrol motor and apply the brakes.

Further details of the above-mentioned safety devices are given hereunder:-

1. Having closed onto the rope, the grip passes through two flaps which have an internal profile such that only a correctly closed grip can pass through. Should the grip strike either flap, a switch is tripped which (a) applies the grip brake and, (b) applies both brakes to the driving engine and stops the motor. The grip brake consists of a heavy channel section beam with a friction lining on its lower face. This beam is normally held up by a strong flap held in Fig. 13: A gondola. Grip leaving retardation roller" position by a solenoid. When power to the solenoid is cut, the flap falls into the path of the grip and allows the brake beam to press onto the grip. Thus the grip is initially retarded by the beam and then stopped, if necessary, by the flap. The flap is spring-loaded so that in the event of a 'false alarm', when the mechanism is tripped although the grip is in fact correctly secured to the rope, the grip will be forced through the grip brake and past the flap by the rope. The circuit for this trip has been arranged so that should a grip cause the south end grip brake to operate, although the brakes of the system will be applied, the grip brake at the north end will not come into action.

2. Having closed onto the rope and the overlock having snapped into place, the grip passes a second flap which monitors the position of the overlock lever. If this should not be in the correct position the normal brake only will be applied to the system.

3. To prevent two. cars colliding under the retarding rollers, two switches, one placed at the entrance and one placed at the exit of the bank of retarding rollers are fitted. If a car enters the station before the preceding car is clear of the retarding rollers, the normal brake is applied to the system.

4. Should the overlock not be disengaged on entry to a station, the grip would jam on the spear. A flap is provided to monitor the incoming grip overlock lever position and to cause the normal brake to be applied if the overlock is not disengaged.

5. The rope is guided downwards and away from the grip after disengagement. In the event of a grip not becoming disengaged, the grip would tend to lift the rope from its normal path. A counterbalanced wheelset below the rope will rise and trip a switch, so applying both the normal and the emergency brakes.

6. The balance wheels at each portal have a plate set close to the inner rim so as to prevent the rope from coming off the wheels on that side. Should the rope slip off on the outside of the wheels, a flap situated on that side is positioned so as to trip a switch connected in series with one of the safety circuits. The normal brake to the system is then applied.

7. In passing from the receiving side to the despatch side of the station, the cars are pushed by hand on the fixed rail around the tower. Attendants in their enthusiasm have been known to push the cars so fast as to cause them to derail with severe consequences. A guard rail has been fitted above the track rail which stops the grip lifting off the rail.

8. Trip buttons are arranged at strategic points by means of which attendants can stop the system. In addition, a tripwire is placed along the full length of the peripheries of both upper platforms.

Communications

Two methods of communication are used when the ropeway is operational, one consisting of a set of telephones with their own connecting cables, the other being a telephone connected to the main ex-change and situated at either station. For maintenance and for use by the responsible engineer during operation, two walkie-talkie radio sets are used which make it possible to stop and inspect points along the line.

Operating experience

The writers have been responsible for the operation of the Milner Park ropeway for the last two seasons and the following remarks concerning operation may be of interest.

The 1965 season.

The ropeway had been inspected after installation by an expert from Germany who made numerous suggestions for improvement. A local engineering firm, who were the original designers and manufacturers of the mechanical portion, had been made responsible for the implementation of these suggestions and on completion, the ropeway was handed over to the writers. It was decided to start by running the ropeway, fully loaded, for at least a week. This was done by loading each car to its rated capacity with ingots of iron and letting the installation run every day at full speed. At intervals, each of the safety circuits was tested to ensure that they were all operative. It was during this period of 'running in' that the following were noted:-

(a) The grip, if it was secured to the rope correctly at the exit but due to vibration or some other effect caused the grip brake to trip, would strike the safety flap at the end of the brake beam, so causing damage to both the flap and the rope. This was remedied by spring-loading the flap so that a force of 700 lbf was required to force the grip through the flap.

(b) The chains driving the acceleration and retarding wheels showed a tendency to stretch during service. This was thought to be part of the normal bedding in process.

Other than these two points, the ropeway was considered safe and ready to run for the show. The ropeway operated successfully for the season of 13 days, no mishap of any consequence occurring. However, a few further points were noted, viz:-

(a) The chains driving the acceleration and retarding wheels were too weak and several broke during use.

(b) The acceleration and retarding wheels had no vertical adjustment to ensure that each exerted sufficient pressure on the grip, although they were spring-loaded.

(c) The horizontally disposed deflection sheaves at the north station became badly worn in the tread.

(d) The timer device then used to ensure that cars did not collide under the rollers was not satisfactory. This device operated on a time cycle which did not allow for the ropeway being run at slow speed.

(e) At the south station, the lower platform was too short. There was the danger of attendants slipping in rainy weather and falling off.

Maintenance and modification

Before the start of the 1966 season, the following items were attended to:-

(a) The chains and sprockets on the accelerating wheels on either side of the central drive were increased in size.

(b) A simple bracket was fitted to each acceleration and retardation wheel so that the position of the wheel could be adjusted vertically to ensure that each wheel exerted an equal effort on the grip, irrespective of the variation in height of the grip.

(c) The tread on the horizontal deflection sheaves was machined.

(d) The rope which had stretched slightly was shortened.

(e) The timer device was altered to operate only when a car entered the station while the preceding car was still under the retarding rollers, irrespective of the time factor.

(f) A steel platform, 26 ft long and 19 ft wide was erected in front of the existing lower concrete platform at the south station. It was suspended from steel rope tendons attached to the upper platform. This allowed for easier maintenance and ensured that an attendant was never near the edge when pushing a car.

(g) All the grips were taken down and tested for cracks. In addition, a few were stripped to check for lubrication in the sealed bearings. No deficiencies were found.

(h) The grips and cars were each runs through the stations at slow speed and the spears were carefully aligned with the fixed track and rope.

(i) The rope was lifted at each of the balancing wheels on the portals in turn and the wheels were carefully checked for freedom of movement.

The 1966 season.

The 1966 season of 11 days went without mishap except for one long stoppage which was caused by a combination of a power failure and inexperienced operators. During this period of running the following points were noted for improvement:-

(a) The operation of the chains on the accelerating and retarding wheels was improved but was still not correct.

(b) The treads of the horizontal deflection sheaves were again badly worn.

(c) From time to time and for no apparent reason, a grip would cause the grip brake to trip. Whenever this occurred, the grip was always carefully examined but not once was it found to be incorrectly attached to the rope.

Further modifications

After the 1966 season it was decided to use ? inch pitch chain to drive the accelerating wheels and to fit sprockets of greater size. The chains were installed slightly slack to allow for the fact that two wheels running at different peripheral velocities contact the grip simultaneously.

The horizontal deflection sheaves are to have tread inserts made of the same material as is used on the friction sheave.

The grips have all been re-machined on the relevant faces to allow them to present identical profiles to the trip flaps and in addition, the track rail has been made to fit exactly the wheel flanges at this point.

CONCLUSION

The advantages of aerial ropeways in the industry are rapidly being recognized, more especially in the civil engineering profession where cable cranes are now looked upon as a necessity in the building of bridges and dams. In the other sectors of the industry, there are signs of increasing awareness of the possibilities presented by this form of transport. The Milner Park prototype has indicated that this type of ropeway can be most successful provided that cognisance is taken of the experience to date with this installation. Any saving made in the capital cost of installation is usually only a fraction of the cost of breakdown and of maintenance. This point has been made previously by Mr H. P. Stromsoe in the discussion on the paper presented to this Institute, 'Notes on the operation and maintenance of an aerial ropeway', in July 1946.