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In the same way that the inflexible old oak trees get uprooted in a strong storm in Aesop’s fable, a track support with too high track stiffness will cause premature failure and result in higher maintenance costs than anticipated. Track support that is too stiff does not spread the load sufficiently, resulting in extremely high localised impact loads and stresses (in track, sleepers, and fasteners). This may result in microcracks in rigid sleepers, failure of screws or clips, ballast attrition or ground vibrations. Causes for too high track stiffness include rigid subgrade (including concrete bases such as bridges, viaducts, tunnels) and overly stiff sleepers, such as concrete sleepers.
Likewise, track support with too low track stiffness will allow excessive deflection of the track and will result in the loss of the required geometry. This can lead to failures like rail fatigue, excessive wear on wheels and/or rail head, strained or broken fasteners, broken rails, and ground vibrations. Causes for low track stiffness include sleepers that are not stiff enough (insufficient reinforcement), a too soft natural geology, high water table and poor drainage.
Selecting a sleeper within a suitable stiffness range (for its given context) is vital in minimising future maintenance costs and efforts. It is here that Lankhorst’s KLP Hybrid Polymer Sleeper provides a range of design options to ensure a well-suited stiffness. These options include the selection of different polymer base materials, variations in encapsulated rebar diameter, and variance in height of the sleeper.
A fundamental design philosophy of Lankhorst is to provide engineered solutions, with suitable sleeper stiffness to achieve the required track stiffness. A supporting and unique feature of polymer, when compared to both wood and concrete, is its response to impact loads. Polymer acts as a shock absorber, spreading the impact over a longer time, and reducing the peak force.
This is of particular benefit to unballasted track on rigid sub terrain, such as bridges, viaducts, and tunnels. The force on the sleepers can be much higher than in track, specifically for the first sleeper on the bridge. Sleeper compressive flexibility on a bridge should be much higher than in track to compensate for the missing flexibility of the ballast. KLP sleepers have significant higher compressive flexibility than wood or concrete and therewith a good choice for a solution. An added benefit of KLP Hybrid Polymer Sleeper in these applications is its lower weight, compared to concrete sleepers. This can significantly reduce the static load on bridges and viaducts. Damping also significantly reduces sound.
Force and impact responses: Polymer outperforms oak and concrete.
Although KLP turnout sleepers are currently mainly used to replace wooden turnout sleeper, Lankhorst expects a higher demand for KLP sleepers for new tracks, partly due to the aforementioned low maintenance, sound & vibration reduction, the Design life span of 50 years, the use of recycled raw materials and the fact that KLP sleepers are 100 per cent recyclable.
Recently KLP Hybrid Polymer Sleeper were approved by Transport for London for use in the London Underground.
LEVEL TRACKS ON SAGGING BRIDGES
Rail maintenance managers in Europe were facing a challenge: High speed rail requires an even track, but some tracks on existing bridges had settled and sagged over the years. “Should these bridge structures be replaced or repaired?” was the natural question. Fortunately, Lankhorst Engineered Products developed a creative solution, which is much cheaper and faster to implement.
Milling of KLP Bridge Transoms
Lankhorst modified their well-established KLP® Hybrid Polymer Bridge Transom by incorporating a 25mm milling zone at the bottom of the transom. This transom combines high strength polymers and strong rebar that can cater for track/girder offsets of up to 400mm on bridges. These transoms can be milled to the required height to compensate for the girder sag and ensure a level track. The required milling amount and angle is obtained by measuring the bridge to determine the required height variances at each transom position.
KLP Bridge Transoms milled to required height
Lankhorst offers a service where sequentially numbered transoms are individually milled to the right height, to compensate for height differences in the girders. This allows for smooth and level high-speed travel over sagging bridges, without the use of shims. This customised solution is now in use in various countries.
DIFFERENTIATED SLEEPERS FOR SPECIFIC APPLICATIONS
Lankhorst has deliberately chosen to develop different types of sleepers for specific applications. Why not one type for all? The reason is quite simple: it is senseless to offer oversized or undersized solutions, a generic design will be sub-optimal in all the respective applications. Based on the required track specification and particular application, Lankhorst will offer the most suitable sleeper type. Our respective brochures (available upon request via firstname.lastname@example.org) explain the unique aspects and features of the 100-series, the 200-series and the 400-series.
The 100-series is designed for main track and uses up to 30 per cent less material than a solid sleeper, allowing the sleeper to be completely embedded within the ballast. This also offers a high level of vertical stability against uplift. Its specially designed bottom profile provides high resistance to both lateral and longitudinal displacement. Due to weight saving, the 100 series is often installed on viaducts. The choice of Type 101 or Type 102 is determined based on axle load and climatic conditions.
KLP sleepers on viaduct.
The 200-series includes traditional rectangular shaped sleepers for main track and turnouts. The most suitable combination of rebar size and polymer type provide the required bending stiffness for given axle load, stiffness requirements and climate conditions. This delivers the best composition for the specific application.
The 400-series includes bridge sleepers suitable for offset applications, and available with 25 mm milling zone for adjustable height.
Link Asia Pacific is the sole agent for KLP Sleepers in Australasia. Datasheets per sleeper are available upon request via email@example.com.
This article first appeared on www.railexpress.com.au
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