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Stoneblower provides ballast maintenance solution

01 Sep 1997

INTRO: A five-year development programme culminated in Britain's infrastructure company Railtrack officially receiving the first fleet-build Stoneblower from Pandrol Jackson on June 13. Maintenance contractors are eager to receive further Stoneblowers to help them meet demands for high quality track at affordable cost. Andrew Hellawell saw the first machine in action

ON JUNE 13 RAILTRACK took ownership of its first track maintenance vehicle. Despite being charged with maintaining Britain's railway infrastructure, on the post-privatised railway it is the infrastructure maintenance companies who own and operate track machines. Until then, Railtrack owned only de-icers and similar units.

Arrival of Stoneblower 1 does not mark a change in Railtrack policy, but the culmination of work by Pandrol Jackson in developing Stoneblower under an agreement which demanded completion, commissioning and testing to Railtrack's exhaustive Group Standards before delivery. For Pandrol Jackson this was a high risk project for a demanding customer, but also one with great potential.

Pandrol Jackson was selected by competitive tender in 1992 (RG 8.92 p501) to carry out development work for a production unit with the former Network SouthEast civil engineering department, building on the PBI 84 prototype developed by Plasser and BR Research. Reorganisation of the British railway industry for privatisation saw newly-created Railtrack ratify the original agreement. BR and subsequently Railtrack was not liable to make any payment until performance targets could be seen to have been met. Stoneblower was to be able to maintain all track qualities under a tight performance specification for reliability, availability and output.

Even before SB1 was in Railtrack ownership, Pandrol Jackson began work on two more machines. These should be available by the end of this year, with the rest of the £21m fleet of eight Stoneblowers to be delivered by the end of 1998. An option for three more is expected to be exercised shortly.

Different approach

The aim of the Stoneblower programme was to find a way of improving on traditional methods of tamping. In particular, solutions were to be sought to the problem of 'ballast memory' - the tendency of track to deteriorate rapidly back to its pre-tamped condition - and the creation of dirt in the ballast through the action of conventional tampers.

Tampers operate by lifting the track and then using tines to compress the ballast into the void under each sleeper. In doing so the ballast chips are forced against each other, breaking off pieces which add to the amount of dirt in the trackbed - reducing the effectiveness of the ballast for support and drainage of the track. Each tamper pass can generate as much as 4 kg of dirt per sleeper. The effect of ballast memory amplifies this problem. While the ballast may be temporarily arranged to support the sleepers, it doesn't take many passing trains to shakedown the substructure, and with the reduced volume of effective ballast available this means that the track soon requires expensive reballasting or at least ballast cleaning.

Giving ballast amnesia

In contrast, Stoneblower works by adding new material where required, overcoming the ballast memory effect. Ballast damage is cut to a small fraction of that generated by tamping, because the existing ballast is left largely untouched.

Before treatment begins, Stoneblower passes over the section of track to be treated at 16 km/h, taking detailed alignment measurements (top and level). The data is processed by an on-board computer which runs Automatic Track Alignment (ATA) software developed by British Rail Research. A new track design is computed and displayed to the operator, and if accepted Stoneblower begins automated track treatment. Using a touch-screen interface, the operator can specify limits on changes to the existing alignment which might be imposed by the need to maintain track centre under catenary, or around level crossings.

Stoneblower then executes its treatment pass, in the reverse direction to its measurement run. The track above pairs of sleepers is raised, and stone injection nozzles pushed down through the ballast. Small stone chips up to 20mm grade are drawn from a hopper at one end of the unit, and blown through the nozzles into the cavity under the sleeper, and the track is then lowered into its new geometry.

Forces exerted on existing ballast are much smaller, resulting in generation of fewer fines. Typically 10 passes by a Stoneblower would result in less than 5 kg of dirt, while 10 tampings would damage up to 40 kg of ballast.

The process of sequencing onto the next sleepers is automatic, with an output rate of over 350m/h. This is currently of the order of that obtained from tampers running ATA, but is expected to improve with time to about 440m/h. This means that about 4 km could be treated in a single shift.

Better afterlife

The true measure of the effectiveness of a treatment is not how fast it can be executed, but how long the effects last. This is where the Stoneblower process shows its potential most clearly. Treating a poor section of track with a tamper could improve the top's typical standard deviation from 1·94 to 0·82, but following the resumption of traffic it could be back to 1·95 within just nine weeks. A similar section of track treated by the Stoneblower improved from 2·08 to 1·17, and took three years to deteriorate down to 2·10 (Table I).

Not all track conditions can be treated economically by Stoneblower. Rectification of long-wavelength vertical geometry would require impractical quantities of stone chippings, and there are limits on the amount of track lift that could be achieved - probably no more than 40mm.

Spinoff benefits

The most obvious advantage of overcoming ballast memory is the extension of time between maintenance work. More durable track geometry will cut the number of track possessions required, and thus the impact of engineering work on timetables. However, if sections of track which are presently deemed as 'untampable' can have their speed restrictions lifted, there are knock-on benefits to the whole railway network through reduced wear and tear, and faster schedules.

Reducing ballast damage has great economic benefit. About half of the damage to ballast comes from track maintenance work, a quarter from traffic, and the remainder from atmospheric action or material spillage. Initial results from Stoneblower suggest that the reduction in ballast damage is sufficient to cut the total by 50% - in effect doubling ballast life. Indeed, the places where tamping is at present needed most frequently are those where the greatest improvement is shown (Table II).

For a network with many secondary lines, such as in Great Britain, Stoneblower offers particular advantages where traffic levels do not warrant the cost of full trackbed reconstruction. Problem sections can be brought back to 'as new' condition inexpensively, and without relaying or reballasting.

In demand

As owner of SB1, Railtrack is now in the difficult position of deciding which of its infrastructure maintenance contractors can use it. 'Stoneblower is so good that as there is only one machine, everyone wants it; the others whinge when they can't have it', remarked Railtrack's Director of Civil Engineering Nigel Ogilvie.

One contractor has an advantage. During privatisation the former Network SouthEast Civil Engineering department, which sponsored early Stoneblower development, became part of Amec Rail. With it went the engineers involved, and so Amec continued as a consultant on the project, completing development and testing trials for Railtrack, and providing crew for SB1.

Amec Rail's project manager Paul Strange explained that great attention was given to ensuring the crew achieve the best performance from Stoneblower. Psychometric tests ensured people with the right aptitude for particular on-board functions were chosen. The crew consists of a captain, technician and maintainer, who work as a team following a plan-do-review approach.

Amec also examined operational issues, which have resulted in the machine being a self-contained unit designed for servicing 'in the field' rather than in workshops, and with only basic requirements for stabling points where ballast chippings and fuel can be loaded.

Where next?

Pandrol Jackson, Railtrack and Amec Rail are confident that Stoneblower will live up to the high expectations it has raised. When further machines arrive in Britain they will be in great demand from track maintenance contractors. They also expect international interest, and are confident that the option for three extra Stoneblowers will be taken up, even if these machines don't remain in Railtrack ownership in the long-term.

Five years of development have produced what Pandrol Jackson's President & CEO Alex Zaydel describes as 'probably the most sophisticated piece of maintenance machinery out there'. With Railtrack acceptance achieved, Pandrol Jackson can examine the international market.

There may be scope for bigger, faster, more complex machines able to treat switches and crossings, or a need for a simpler, cheaper Stoneblower. Development will be driven by maintenance contractors aiming to cut the whole-life cost of good quality track. o

CAPTION: The most expensive shovel in the world. Stoneblower automates the age-old function of lifting and packing under sleepers, but addition of material to an already compacted formation gives longevity to the new track geometry

CAPTION: The work-head (inset) includes a rail/sleeper lifter, and nozzles which are pushed down through the ballast (main picture). Stone chippings are blown out through the side of the nozzle into the void under the sleeper before the track is lowered into place

CAPTION: Top and alignment are measured in detail, giving data which allows Stoneblower to design the new track profile

Stoneblower is a self-contained unit, carrying up to 16 tonnes of ballast chippings in a hopper. A boom-mounted grabber allows the operator to refill on site

TABLE: Table I. Variation in standard deviation

Tamper Stoneblower

Pre-treatment 1·94 2·08

Post-treatment 0·82 -

After 5 days 1·45 -

After 9 weeks 1·95 1·17

After 22 weeks - 1·18

After 59 weeks - 1·52

After 3 years - 2·10

TABLE: Table II. Ballast cleaning frequency

With tampers With Stoneblower

18 months 3 years

12 months 3 1??2 years

6 months 5 1??4 years

Reader EnquiryNumbers

Railtrack 121Pandrol Jackson 122

Amec Rail 123

Stoneblower technical specification

Design output m/h 440

Gross weight tonnes 113

Maximum axleload tonnes 18

Length m 32·2

Maximum speed km/h 100

Engine Cummins KTTA19C

Power rating hp @1950rev/min 700

Auxiliary electrical output kW 28

Track lifting mm 0 to 80

Track slewing mm 0 to 40

Minimum working radius m 150

Fuel capacity litres 4500

Stone capacity tonnes 16

Stone usage tonnes/km 4 to 7

Stone chipping size, max mm 28

Index time (sleeper pair) sec 3

Blowing time sec 0·3 to 11

Stone delivery kg 0·5 to 22