A tamping machine or ballast tamper, informally simply a tamper, is a self-propelled, rail-mounted machine used to pack (or tamp) the track ballast under railway tracks to make the tracks and roadbed more durable and level. Prior to the introduction of mechanical tampers, this task was done by manual labour with the help of beaters. As well as being faster, more accurate, more efficient and less labour-intensive, tamping machines are essential for the use of concrete sleepers since they are too heavy (usually over) to be lifted by hand.
At its most basic, a tamping machine only packs the ballast. Some modern machines, sometimes known as tamper-liners or tamping and lining machines, also correct the alignment of the rails to make them parallel and level, in order to achieve a more comfortable ride for passengers and freight and to reduce the mechanical strain applied to the rails by passing trains. This is done by finding places where the sleepers have sunk from the weight of the passing trains or frost action, causing the track to sag. The tamper lifts each sleeper and the rails up, and packs ballast underneath.[1] When the sleeper is laid down again, the sagged rails now sit at the proper level. Combining tamping and lining into a single machine saves time and money, as only one machine needs to be run over the track to perform both functions.
Tampers frequently work in concert with ballast regulators, as part of a section crew.
In the early days of railroading, the ballast was maintained by hand, with gandy dancers using shovels and beaters to move, clean, and compact ballast. Rails would be jacked up with large track jacks to allow the crews to do their work. This process took quite a long time and was subject to human error.
Starting in the 1930s, automatic tools for tamping were first developed and used, though they still had to be operated by hand. In the following decades, tamping machines were developed to automate the process, using dedicated machines that travelled on the rails.
In recent years, larger tamping machines have been developed that incorporate additional functions such as lining and lifting the rails on top of their tamping function. These are known in North America as production tampers.
Tamper machines are built in many different varieties depending on their purpose:
The tamping process from any type of tamper consists of the following basic steps:
The basic principles and functions of a tamping machine remain the same regardless of manufacturer with only minor differences in design.
The majority of track machines are powered by a diesel engine. This provides power to the driving wheels via either a hydrostatic circuit or cardan shaft, allowing the machine to propel itself to and around a work-site. The engine also drives a hydraulic pump to provide power for the various tools.
The lifting lining unit of a tamping machine lifts and holds the track in corrected position while being tamped. All types of units require the following components to achieve this task:
The lifting lining unit is usually secured to the main machine chassis via a hydraulically length-adjustable trailing arm. The arm is adjustable so that the unit can be moved clear of small obstructions such as insulated joints or wires.
To ensure track geometry is corrected, most tamping machines use a two chord lining system (one for vertical alignment and one for horizontal alignment). The two chord system requires three reference trolleys fitted to the machine - usually called A point, B point and C point. Some machines will use a fourth point between B and C to carry out quality control measurements.
A point is always the front reference point and sits on uncorrected track. The anchorage for chords on A point are able to move to compensate for defects in track geometry - this is done by the "tower" operator. Depending on the chord system, the anchorages will be either a wire anchor or a light source.
B point is positioned as close as physically possible to the lifting lining unit. B point is used by the machine control system to position the track correctly using either potentiometers or optical filters, depending on type of chord system used.
C point is the rear-most measuring point and anchorage point for lifting and lining chords. Depending on type of chord system C point will be either wire anchor with a tensioning cylinder or a photoelectric light receiver.
All three points are individual rail trolleys able to freely move up, down, left and right independent of the machine chassis and hence follow any minor fluctuation in rail position. When working, the machine uses pneumatic cylinders to lightly push these trolleys into the selected datum rail both vertically and horizontally.
When using this method, the tower operator positions A point anchorages according to existing track geometry measurements taken beforehand. Once A point has been positioned, it is then assumed that both A and C are in correct position as if machine were sitting on corrected track. The machine then uses the lifting lining unit to move the rail and B point in line with A and C.
The Tamping units of most Tamping machines will consist of:
To generate the vibration needed for penetration and consolidation there are two leading methods commonly used:
A less common method more often seen on tamping head attachments for excavators is to use a motor driven vibrator assembly that is directly bolted to the support frame.
A continuous-action tamping machine (CAT) can pack between one and four sleepers at a time, with outputs between 320 m/h and 2600 m/h generally anticipated.[2]
The 'Tamping Express' is a machine developed by Plasser & Theurer, and in the UK and Europe is referred to as the 09-3X. This machine consists of a conventional CAT-style satellite with tooling for three sleepers in continuous succession, along with a full DTS stabilising unit suspended from the rear most vehicle in the machine.[3]
Tamping and cleaning operations have the adverse effect of reducing the resistance of the track to lateral movement. The resistance gradually recovers with the passage of trains, but may require a speed restriction imposed for the duration. This 'consolidation' can be achieved faster and in a more controlled manner using mechanised equipment known as the dynamic track stabiliser (DTS).
A DTS will normally be used only after a stretch of track has been tamped and aligned. D.G.S. has a vibrating unit which holds the track in position and applies a horizontal vibration and vertical load to simulate the passage of trains. The track parameters (or cross levels), before and after stabilising, can be viewed through bogies in the front and rear.
Dynamic track stabilising has the following advantages, resulting in enhanced safety:
The stabilisation achieved by one pass of a DGS is equal to that achieved by 100,000 tonnes of traffic, and allows a speed restriction of 20 km/h to be relaxed to 40 km/h
Dynamic stabilisation is usually avoided on bridges or around overhead structures since there is a risk of damage to foundations.
The appropriate tamping procedures and tamping machine depends in part on the track layout.
On plain track everything is fairly straight forward and any brand and model of machine can be used.[4]
But through tunnels and bridges with no ballast to tamp, special measures are needed to transition from the ballasted track to the unballasted track. An example would be the Glenbrook Tunnel Turnouts require a more complicated tamping machine with extra and adjustable tongs to cater for the extra rails and variable spacing of the sleepers. Similarly for dual gauge such as between Perth and Northam.
Ideally turnouts and diamond crossings should be some distance apart so the each component can be tamped without having to immediately tamp the other components. However a majority of track components are right next to each other so that these components need to be tamped as a group in multiple small stages.
Track centres determine whether one end of a crossover, consisting of two turnouts, can be tamped one at a time, with train traffic still running on the other track.