Introduction | Sleeper Factory | Daily Production Operations | Systems | Labs | History

Background

The system for pre-casting concrete railway sleepers has been continually refined and perfected over the last half-century. The development of the casting system has always focused on minimising production costs and maximising output while improving quality. Our factory uses eight-cell moulds filled by machine which is a very fast and efficient method to maximising daily output.

The production process of the sleepers follows an exact series of steps which must be precisely observed to maintain quality and efficiency standards. Our system operates ten production lines containing 35, 8 gang moulds per line which have the capability to produce 2800 pre-stressed concrete sleepers per day. These are fed with concrete prepared and mixed at the batching plant which was provided by SKAKO of Denmark.

The production of the concrete sleepers in carried out using die-cast metals moulds into which the concrete is poured by overhead machine. These moulds have to be prepared prior to casting. Their preparation consists of cleaning, oiling and fitting of the railway fastenings. The moulds are subsequently winched into the casting position where the steel strands have been tensioned; which when set in concrete form ‘pre-stressed’ sleepers of unrivalled strength and quality.

Process

Each day’s production commences with the checking of the previous day’s output. A concrete cube taken from the sleepers is tested in a ‘cube crushing test’ which ascertains the strength of the concrete. These cubes taken from one end of each line must reach a minimum strength of 32N per sq. metre before de-tensioning is carried out. If the sleepers are not sufficiently cured then the covers are replaced and steam is passed across the sleepers again for 3 to 4 hours.

On satisfactory receipt of the report; the detensioning can begin. The hydraulic jacks are released at the live end releasing the tension. The steel strands are then cut at both the live and dead ends. A cutting machine then proceeds to cut the steel strands between the moulds separating them; ready for demoulding. Once this is complete the mould lifting and demoulding machines are moved into place and the moulds are removed and placed onto flat-bed trolleys. A minimum of eight sleepers from each production line will be selected at random by the line inspector for acceptance and strength testing.

The strand running machine then drags fresh strands across to the dead end; they are secured to the jack plates at one end. As the strands are drawn through the anchor blocks, the machine picks them and drags the anchors to the live end. The anchors with the strands in place are then placed into the hydraulic jacks. An initial tension force is applied while the strands are secured.

To reach optimum tension the two hydraulic jacks pull on the strands, they have a combined maximum pulling force of 600 tonnes. The correct tension is monitored by checking the jack pressure as well as a manual test using a gauge (Vogt tension meter) placed directly on the strands.

Following on from this stage; spacer bars are placed are placed between the strands and labourers check that there are no gaps between the moulds. Steel staples are used to secure the moulds together in continuous lines.

The laying of the concrete into the moulds can begin once approximately a third of the moulds have been connected. The concrete placing machine is winched to the live end and starts depositing concrete into the moulds. The mixed and tested concrete is delivered to the machine via a moveable overhead skips which maintains a continuous supply of concrete available for casting. Following closely behind; a group of labourers ensure that there is no spillage and that the concrete is laid flat in the moulds.

Once the moulds have been cast, insulated covers are placed over them, and steam is blown across the moulds to facilitate the curing process. These sheets will remain in place until detensioning is authorised following testing (cube crushing test). Curing occurs while the sleepers are in their moulds to prevent excessive moisture loss or damage to the surface finish. Curing by heating eventually ensures that adequate detensioning strength has been reached prior to the next morning.

Conclusion

At the end of the day all machinery from the production line is washed down and inspected. Routine oiling and greasing is carried out at this point as well as replenishment of renewable stocks. The following morning the sleepers will have sample cubes tested to ensure that they have sufficiently cured and another production run can begin the following day.

The factory is currently capable of producing 2,800 sleepers per day but it is expected that production will eventually be increased to over 5,000 sleepers per day.

RMS – Machines Summary

Saw

This machine runs astride the moulds to cut the pre-stressing stands once the tension has been released, this machine has powered long-travel which is collected via bus bar running alongside the bed. The cutting blade is 600mm in diameter and is powered by a 40bhp motor. It also has power clamping onto the mould therefore the interface between the moulds and saw is essential.

Mould lift and de-mould grab machine

This machine runs astride the moulds on the plant rail, lifting one mould at a time clear of the other moulds in the line, by the use of the overhead crane and lifting grab which lifts all 8 No: sleeper out of the moulds, also turning the sleepers over the correct way for stacking and use, the grab is returned to the mould lift machine and the whole machine moves to the next mould for the next and operation. The machine is designed to be lifted on/off the bed by an overhead crane.

Cleaning and oiling machine

This machine also runs astride the mould one the plant rail, sweeps and cleans moulds also oiling and wiping the mould ready for the rail fixing and strand to be placed. The machine is lifted on/off position by an overhead crane.

Cross heads

Each bed requires one live and one dead end cross head. This is a specialist structural assembly structural assembly capable or resisting the total pre-stressing forces of approximately 500 tonnes. The 2 stressing jacks have 800mm stroke with locking off device. The working pressure is 360 bar, the rams are operated by hydraulic power pack with directional controls and pressure gauges.

Pin pusher

This machine runs up and down the plant rail astride moulds, it hydraulically clamps itself onto moulds to engage/disengage push pulls holding Pandrol shoulders in pockets. The machine is lifted on/off by an overhead crane.

Strand running machine

This special designed machine runs astride the moulds on the plant rails; the purpose is to run a full cavity of strands from one end of the bed to the other. This machine driven by one operator and has hydraulic controls and drives. This machine is designed to be lifted on/off by overhead crane.

Curing system

The curing system can be either electric or steam, electric is by transformed current though the stand and concrete, steam is by either trapped steam or steam to atmosphere. Steam systems are specifically designed to suit the special local set up, all component and controls selected and designed to suit these conditions. Finally curing covers are pulled over the cast sleepers; these polythene sheets are to cover the newly cast sleepers and keep the heat and moisture in.

Casting machine

The casting machine runs astride the moulds on the plant rails and places the concrete in the moulds vibrating to fully compact the concrete; this is done by head pressure conforming plate and poker vibrators, the concrete is delivered to the casting machine by concrete skip and overhead crane, the machine travel is powered by winch through a special gear box, the hydraulic functions are to open and close the skip doors.

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