The need for ballast cleaning can be recognised immediately onthe basis of inspections using track recording cars, equippedwith suitable analysing systems. Investigations using Geo-Radarmeasurements provide a quick insight into the lower layers ofthe ballast bed. Direct appraisal of the state of the ballast bytaking samples is the safest criterion. Samples can be takeneither manually or - far more economically - by using a probingdevice to obtain boring cores from the track, such as the subsoilprobing machine (UUM) (Fig. 1). When, by screening thematerial, it is found that the fouling of the ballast has reachedover 30% in weight, the ballast bed should be cleaned.

BALLAST CLEANING MACHINES (Fig. 2)Ballast cleaning machines perform the most arduous work inthe track maintenance sector. The continuous removal of theballast bed material from the track and the working thrustneeded for this, the transport of several tons of material withinthe machine, the screening process, the returning of the cleanedmaterial, as well as the removal of spoil, require huge tractionforces.

In 1959, Plasser & Theurer brought its first ballast cleaningmachine onto the market that worked off-track which, in late1962, was followed by its first fully hydraulic on-track machine.

With increasing mechanisation of track maintenance work, it became more and more necessary to coordinate the ballastcleaning machines into overall technologies. In particular,working in conjunction with track renewal trains demanded farhigher outputs from ballast cleaning machines.

The first ballast cleaning machines, developed by Plasser &Theurer, to meet these higher outputs were the machines of thetype RM 63, equipped with a large-dimensioned excavation unitand a single screening unit. Between 1970 and 1975, severalmachines of this type went into operation in France andGermany. However, these compact six-axle machines which,featuring a total mass of 120 t, an engine output of around520 kW and a nominal output of 650 m3/h, are still operatingtoday, reached the limits of the desired capacity.

New ways had to be found to achieve extensive increases and appropriate reserves of output. Thus, in the 1980s, machinedesigns were developed that guaranteed a 100% increase inoutput. The main feature of these designs was the introductionof a double screening unit. All machine components, parti-cularly the size and speed of the ballast excavation chain, andthe screening unit, have to be designed to meet the high output.

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Criteria for cost-effective ballast cleaning: machine design considerationsTraffic and a number of environmental influences cause the track to age. Even track that is used onlyscarcely, or not at all, would increase in entropy due to the general tendency in nature that highlyorganised systems transform into less organised ones. Particularly, fouling of the ballast bed leads to areduction in track geometry stability and quality, which results in the need for maintenance to be carriedout at ever shorter intervals. Timely ballast cleaning and, if needed, measures to improve the subgradeare the strategies to be adopted here.

By: Ing. Helmut Misar, Department for Advertising & Technical Information, Plasser & Theurer, Vienna, Austria.

Fig. 1: The UUM subsoil probing machine, featuring threeboring units for taking boring cores from the track

Fig. 2: Design developments of Plasser & Theurerballast cleaning machines

Ballast excavation chainOn modern heavy-duty ballast cleaning machines, the exca-vation chain takes the ballast bed material, via chain guides, not directly to the screening unit - as was the case on the earlierballast cleaning machines - but via a collection hopper on aconveyor belt unit, from where it is supplied uniformly to thedouble screening unit. The formation that is produced duringexcavation is an important feature. There should be no “watertraps” either in longitudinal or lateral direction (Fig. 3), and theresulting formation should have the correct cross-fall, in orderto allow any water entering from above to flow away.

The output of a ballast cleaning machine depends primarilyon the advancing speed and the height of the excavation chain,or the excavation depth. The volume output of the machine issimply the forward speed multiplied by the excavated cross-section (output = forward speed x cross-section). With the usualexcavation depths, the quantity of material is mostly around2 m3/m. The shovel size or the cross-section of the chain guides,the spacing between the chain shovels and the chain speeddefine the quantities of material that is taken away (shovelcross-section x chain speed). The maximum chain output liesslightly under 1000 m3/h.

However, at high chain speeds, besides an increase in noiseemissions, the following problems may arise:— an over-proportional increase in the wear of the excavation

chain (Fig. 4). The wear of the chain links and of the wearplates increases progressively. For instance, a 10% rise inchain speed results in a wear increase of 15-20%. To reducethis problem, the chain links are made of highly wear-resistant manganese hardened steel. By altering the alloyingconstituents, the service life has increased considerably overthe past years (Fig. 5). The service life of the excavation chainhas also been increased by using a special constructionalfeature, whereby hardened chain bolts are supported inpressed-in bushings in the connecting links which, if re-quired, can be reconditioned (Fig. 6);

— material could fall back into the track, thus not reaching thecollection hopper, which then has to be taken up again.

Based on experience gained to date, the chain speed should beno more than 3.5 m/s.

Screening unitThe better the quality of the screened material, the lessfrequently the ballast requires cleaning and tamping. Thequality of the screened material has to be as high as possible.Modern high-capacity ballast cleaning machines are equippedwith a double screening unit, offering a cleaning capacity of1000 m3/h and more, and a machine output of 500 m/h andhigher.

An optimisation of the screening unit of a ballast cleaningmachine with respect to cleaning quality and output requires theaddressing of the following parameters:— the size of the screen surface: the full extent of the screen

surface has to be able to easily handle the separation of spoiland ballast, thereby ensuring the quality of the reclaimedballast. When cleaning the ballast bed, all excavated materialshould be conveyed to the screening units, in order to achievea high recycling effect. Although the use of a preliminaryfilter could raise the advancing output of the machine, theeffect of a ballast cleaning machine would be lost, as thematerial would be separated without treatment.

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Fig. 3: Ballast excavation has to be performed absolutely uniformly, inboth the lateral and the longitudinal direction, so that no “watertraps” occur and the resulting formation has the correct cross-fall,thus allowing any water entering from above to flow away

Fig. 4: Chain wear curve and conveying capacity of the ballast excavation chain (see [Ref.])

Fig. 5: Ballast excavation chain featuring three adjustable scraperfingers; the chain links are made of high-alloy steel

Fig. 6: The five easily exchangeable scraper fingers of this ballastexcavation chain are designed as specially armoured round shaft chisels. The chain links feature pressed-in bushings

The size of the screen surface is, to a large extent, limitedby the clearance gauge and the setting angle of the screen. InFig. 7, the fundamental correlation between the setting angleof the screen and the possible output per square metre ofscreen surface is shown. The smaller the screen angle, thelower the screening output.

Plasser & Theurer ballast cleaning machines are charac-terised by the following cleaning outputs:— the RM 80 ballast cleaning machine, which features a

single screening unit with a screen surface of 30 m2, offersa cleaning output of 550-600 m3/h;

— the ballast cleaning machines with double screening units,such as the RM 801, the RM 900 and the RM 2002, havea screen surface of 2 x 23 m2 and offer a cleaning outputof 1000 m3/h;

— the RM 860 ballast cleaning machine, which features adouble screening unit with a screen surface of 2 x 25 m2,offers a cleaning output of 1000-1200 m3/h (Fig. 8);

— the number of screen decks: Plasser & Theurer ballastcleaning machines are equipped with screening units thatfeature three screen decks (Fig. 9), whereby the upper screendeck has the function of eliminating the over-sized ballaststones. The medium-sized and small-sized stones arereclaimed from the material, and separated from the spoil,which reaches the middle and the lower screen decks,respectively. The flow of material over three screen decksguarantees a thorough separation of re-usable ballast fromthe spoil. As a result, after ballast cleaning using modernhigh-capacity machines, the portion of fine particles in theballast bed is lower than with new ballast, which means ahigher quality of the ballast bed material;

— the mesh size: for achieving grain of Class 1, the standardmesh sizes of the three screens are, from top to bottom,80/50/30 mm;

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Fig. 7: Influence of the setting angle of the screen onthe screening performance (see [Ref.])

Fig. 9: Diagram showing a three-deck screening unit

Fig. 8: Double screening unit of the Plasser & Theurer RM 860 ballast cleaning machine

— the direction and frequency of screen vibration: the vibration ofthe screens is an important factor for achieving a goodcleaning quality, whereby the direction of vibration is ofimportance. The screens, in conjunction with vibration,achieve a higher cleaning effect as, due to the pulsingacceleration of the ballast stones, the spoil is loosened fromthe stones. The vibration also causes a good distribution ofthe material on the screen grids and a thorough separation ofspoil and ballast (Fig. 10). The direction of vibration iseffected by:— linear oscillators which, through opposite-motion fly-

weights, retain only one direction of screen vibration. Thedirection of vibration - the discharge angle - can be setexactly;

— eccentric oscillators, which produce a constant circularoscillating movement of the screening unit. As thisoscillation system operates independently of load, it ispredestined for high-volume output operation or whenthere is a fluctuating supply of material. Eccentricoscillation screens have a high self-cleaning capability.The eccentric drive with counterweights produces acomplete balancing of masses during operation and, thus,exerts low dynamic stresses on the machine construction.

The frequency of the screen vibration has an influence on thescreening output. With a screen vibration frequency of 30 Hzand an average contact time on the screen surface of circa3 seconds, the material being screened undergoes about 100acceleration pulses. Thanks to the high average contact time on the screen surface, an excellent cleaning effect isachieved.

It is an absolute necessity that high-performance ballastcleaning machines are designed, from the outset, for thedesired output capacity. Later increases in output will be atthe expense of the cleaning quality (when machines with alow-rated output drive at a high rate of revolutions, thescreen throughput will increase and the contact time of thematerial on the screen surface will be reduced substantially,leading also to a drastic reduction in the cleaning effect);

— the setting angle of the screens: on Plasser & Theurer ballastcleaning machines, the setting angles of the screens rangebetween about 10° and 20°. Eccentric oscillation screens canbe set somewhat steeper. This means that the performance issomewhat higher. Generally, however, the cleaning quality ishigher when the screen is flat. In other words, a steeplyadjusted screen can achieve higher outputs, but only with apoorer result of the screening quality.

A recently introduced technology to raise both screeningoutput and quality is the application of a grid screen,whereby metal bars (grids) are positioned in a cascadingfashion (Fig. 11). The material being cleaned is set into anadditional oscillating movement by the grids, and thisreverberation (like a tuning fork) achieves an additionalcleaning effect;

— the supply of material to the screening unit: on the Plasser &Theurer ballast cleaning machines of the first generation,equipped with a single screening unit, the excavated ballastmaterial was supplied directly from the excavation chain, via the chain guides, to the screens. On the modern high-capacity machines, equipped with a double screening unit,the excavated ballast material is first passed on to anintermediate collection hopper, from where it is taken, viaconveyor belts, to the two screening units.

Transportation from the intermediate collection hopperto the double screening unit, via conveyor belts, means thatthe belts are well filled and offer a wide supply of material tothe screens. Thus, a high efficiency of the screening unit isachieved, as all parts of the screens, over the entire width, are utilised. A further hopper behind the screens has thefunction that when the machine is shut off and the screensrun on for a few seconds, the material still present on thescreens can be collected and stored.

The conveyor belts of high-capacity machines have tohave a sturdy design and should not be friction-powered.Therefore, hydraulically-powered conveyor belts are usedwhich, together with the fact that the overall machine ishydraulically driven, enable a stop-and-go operation. When,during ballast cleaning the machine stops due, for instance,to the presence of over-sized obstacles in the ballast bed, theballast material present on the machine should not simply bepassed on. Conveyor belts and screens are switched off. Thematerial remains in the hoppers and conveyor units of themachine. Otherwise, irregular heaps of material would occurand, if work were continued, the next sections to be workedwould be characterised by a surplus and lack of ballast.

The hydraulic drive and the structural design of theconveyor belts have proven to be reliable when re-starting.There is always about 12 m3 of ballast material on themachine which, depending on the degree of fouling, consistsof about 4 m3 of spoil and 8 m3 of reclaimable ballast. Aftera machine stop, a hydraulically-powered machine can get allsystems going again when re-starting, which is not necessarilyguaranteed in the case of electric drives.

Modular design of the modern high-capacity ballast cleaning machinesThe first generation of Plasser & Theurer ballast cleaningmachines had a compact design, featuring a long spoil conveyorbelt, with a length of about 10 metres from the pivot, whichcould be slewed. Thus, for transfer travel, a match wagon wasneeded.

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Fig. 10: Screen vibration (directional and circular)has an influenceon the distribution of material. By producing an accelerationof the ballast stones, a better distribution of the material ontothe screen decks is achieved, as well as a more thoroughseparation of spoil and ballast

Fig. 11: A grid screen inside the screening unit, consisting ofmetal bars positioned in a cascading manner

The demands for increasing outputs resulted in the ballastcleaning machines becoming bigger and bigger in size, callingfor a multi-sectional machine design. Overall machine mass alsoincreased, with ballast cleaning machines designed during thepast 20 years featuring a total mass of between 220 and 280 t, 10to 17 axles, and an overall engine output, which is equivalent tothe required output, of up to 1500 kW. The ballast cleaningmachines of the later generations, thus, feature a modulardesign, consisting of an excavation section and a screeningsection, usually, with separate drive units. With only a singleexception (the Plasser & Theurer RM 802), the basic concept issuch that the spoil is transported to the front. Should any spoilbe lost during this process, it would be picked up by the machineand separated again in the screening unit. On several machinedesigns, new ballast can be supplied from the rear and installedby the machine, either separately or together with the reclaimedold ballast, whereby the height of the track can be regulatedexactly.

The modular design of the ballast cleaning machines has theadvantage that additional technologies can be built onto sepa-rate vehicle components and incorporated into the machineconfiguration, such as the application of star screens, a high-pressure ballast washing plant, including a water-recyclingsystem, a crushing plant to sharpen the ballast stones, as well asother technologies, all aimed at increasing the cleaning effecteven further.

Using wet cleaning of ballast, it is possible to reclaim aconsiderable quantity of ballast bed material. In Fig. 12, thecorrelation between the moisture content of the screenedmaterial and the possible screening output is shown. As can beobserved, when there is a high proportion of clay or loam, thescreening output is greatly reduced, even when it has a relativelylow moisture content (between 5 and 10%). Whereas, if it has ahigh moisture content, the cleaning effect increases again.

To reduce the formation of dust in the ballast excavationarea, some ballast cleaning machines are equipped with a tankwagon and a dust arresting atomiser unit. Ballast cleaningmachines are sometimes also fitted with ballast consolidatingunits, by means of which ballast is brought into the area aroundthe sleeper ends, enabling the track to be embedded firmly inthe cleaned ballast bed. A sweeper unit can also be fitted tosweep excess ballast from the sleepers, as well as equipment toplace geotextiles, membranes or insulating slabs.

Measuring equipmentThe excavation equipment of the ballast cleaning machinesneeds to be controlled in the longitudinal direction, in order toensure consistent ballast conditions. In the same way, the cross-fall of the formation produced has to be geometrically perfect,in order to guarantee that any surface water can flow off. Theexcavation depth also has to be controlled, so that no soiledlayer of ballast remains on the formation, as this would hinderthe drainage of surface water. A wet layer would soften the trackformation and, thus, reduce its bearing capacity.

Thus, the modern ballast cleaning machines are equippedwith measuring and regulating systems which, together with apersonal computer suitable for the rugged working conditionsof maintenance machines, enable automatic control of theexcavation depth and other parameters. There are cable andphase-angle sensors, an electronic pendulum, etc., mounted onthe machine that control all the required parameters. Anexternal laser reference plane can also be used to control theexcavation depth.

An electronic recorder-plotter is used for quality control ofthe work performed, whereby up to six parameters, e.g. exca-vation depth, formation cross-fall and track lowering, are re-corded.

The cost/performance ratio speaks in favour of high-techworksites whereby a high-quality result is achieved, makingfollow-up work to a large extent unnecessary. Many renownedrailway administrations demand a worksite standard whichensures that, after ballast cleaning, the track can be reopened totraffic at 70 km/h.

Logistics concerning the removal of spoilWhen using the modern high-capacity ballast cleaningmachines, considerable quantities of spoil are obtained thathave to be replaced by new ballast. Depending upon the degreeof fouling, per hour, as much as 350 m3 of spoil can occur.Today, the spoil can only rarely be left at the side of the track.Especially in station areas, in narrow cuttings and in tunnels, the spoil has to be taken away at all events.

For the transportation of spoil, the Plasser & Theurer rangeof material conveyor and hopper (MFS) units offer an idealcomplement to the machine systems performing ballast bedcleaning (and for the mechanical production of a tracksubgrade) (Fig. 13). All movements of material take place onthe track under repair, without any disruption to the output ofthe ballast cleaning machine. Since the MFS units function likea conveyor belt line, loading is performed continuously,whereby the units at the end of the loading line are used forstorage. The number of MFS units used has to be adapted to theprevailing worksite and operating conditions, whereby they canbe configured in any length required.

When a number of MFS units are full, they are uncoupledfrom the machine and taken to a suitable location for unloading.Unloading is performed via slewing conveyor belts which,depending on the MFS design used, takes approx. 3-5 minutes.In the meantime, loading of the units that remain with theballast cleaning machine continues. After the return of theempty MFS units, the material is passed on again and loadedinto these empty units. Thus, the ballast cleaning machine cancontinue its work without any interruptions.

If a ballast cleaning machine is equipped with a ballast supplyunit, new ballast material can be supplied also using MFS units,and these tasks performed continuously without disrupting theoutput of the ballast cleaning machine. Depending upon theworksite logistics, new ballast can also be placed directly byspecially-equipped MFS units whereby, in contrast to otherballast transport wagons, it is possible to accurately dosage theamount of ballast needed.

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Fig. 13: Ballast cleaning machine, together with material conveyor and hopper (MFS) unit for the supply of new ballast and the transport of spoil

Fig. 12: Screening performance in relation to the moisturecontent of the ballast bed material (see [Ref.])

CONCLUSIONSAs shown in this article, today, ballast cleaning machines form a challenge to the know-how and production capacity of themachine manufacturer. For instance, with respect to the engine,special training in hydraulics and motive power engineering isrequired. Further, the machines are controlled by newly-developed measuring systems and computer programs, in orderto guarantee the required uniformity and accuracy of the workperformed. Also, it is necessary to have specialist knowledge inthe field of excavation and screening technology and, very often,the additional requirements of ballast treatment requireextensive knowledge of processing technology.

Overall, for the production of such giant machines as ballastcleaning machines, with a total mass of more than 250 t, it isessential to have huge production areas and a great deal ofexperience in mechanical engineering.

The foundations have to be present in the minds of thedesign engineers to think in these dimensions, and also in thatof the workshop staff, in order to be able to deal with suchdimensions (Fig. 14).

Design engineers must also have visions for the future. Howdo they imagine the further developments of the machines?Without doubt, even more work functions will be incorporatedinto future heavy-duty machines and, in order to increase theoutputs, the machine systems will have to be adapted, enlargedor multiplied, and the control technology further perfected. Itwould be interesting to see how, in several years from now, theballast cleaning machines and large track maintenance systemswill have developed!

ReferenceLichtberger B.W.: ‘Track Compendium’, First Edition, 2005,ISBN 3-7771-0320-9.

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Fig. 14: The Plasser & Theurer RM 900 S ballast cleaning machine, which featuresa total mass of 345 t and a construction length of more than 100 m

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