The Complex Rail System of Transport - Coursework Example

Introduction The rail system is a complex system of transport that has a number of varying characteristics. The design of the railway is typically separated into two major components which are the design of the trains and the design of the supporting tracks (Boyce and Hermann, 2003). It is however very crucial that we understand the railway system as a whole. In this research paper, we shall focus on the description of the railway system, the procedure and techniques applied in the design of the rail system and the software available in the market for use in this part of infrastructural engineering. We shall also focus on the current engineering standard available in the market so that we get to understand the structural limits of the design. 2. Definition of the rail system The railway system is composed of a number of units and this can be broken down into three major sub components. i. The vehicle ii. The interface existing between the wheel and the vehicle iii. The structure of the track a. The vehicle Trains are normally referred to as rolling stocks and are usually composed of two major distinct types of vehicles: a locomotive that allows the operations of the train to occur and wagons that are used to carry a particular load (Cai, 1992). Modern locomotives are powered by electricity while the traditional diesel powered locomotives are still operational. Vehicles are usually defined by the number of axles that they have. Locomotives usually have bogies that have triple axles and wagons usually have either single, double or triple axel bogies. The body of the car is normally best defined as a container used for holding goods regardless of their nature whether human or materials (Boyce and Hermann, 2003). The motion of the vehicle is then defined in terms of the track vertical of the vehicle and both the lateral and longitudinal axes. The most basic modes off oscillation while the vehicle is in motion are: i. Pitch: This occurs either in the backward or forward direction of rotation about the transverse axis. ii. Bounce: This occurs in an up and down direction along the vertical axis. iii. Yaw: This refers to rotation about a central vertical axis iv. Lateral: This is the movement on either side along the lateral axis. v. Roll: This occurs when tipping happens on either side about the longitudinal surface. The bogie is the part that ensures that the train remains stable during its operations by guiding it on the rails. There are two main types of bogies: One with primary suspension and another one without any suspension. The suspension is normally made up of coil springs which are able to minimize the impact and hence enhance the stability of the operations of the wagons (Jenkins et al, 1974). Various springs and dampers are incorporated into the bogie to cushion the ride along the tracks. The most common type of bogie used in transportation of freight is normally referred to as a three piece bogie since it has 3 very important components: a bolster and two side frames. Figure 1: A typical bogie and its components Among all the bogies available in the market, the 3 piece bogies are the cheapest and most economical to operate and maintain (Eisemann, 1972). However the major drawback is that they only provide a low level of stability along the lateral axis and the quality of the ride in them is very poor as a result of the existence of unsprung masses. Figure 2: A typical illustration of a 3 piece bogie b. The interface existing between the wheel and the vehicle The connection of the vehicle and the track via the interface between the rail and the wheel is very important for the trains to operate successfully. If the connection is interfered with, a breakdown of either of the two systems occurs and the resulting effect is a derailment of the entire locomotive which can be very catastrophic at times. The entire load of the train is normally distributed along the system of the track through a contact area that is very small in size on each of the many wheels of the train (Boyce and Hermann, 2003). This distribution of the load can be explained from Hertz theory. On both the vertical and the lateral directions, the elastic deformation that occurs on the steel contained on both the wheel and the rail may eventually result into an elliptic contact area. The dimensions of the ellipse of contact created are then determine using the normal force that is applied on the area of contact, the elasticity and the hardness of both the wheel and the running surface. The ratio of the axes of the ellipse is usually dependent on the major curvatures of the wheel and the profiles of the rail (Boyce and Hermann, 2003). The shape of the ellipse of contact normally changes relative to the point at which the contact of the wheel and the rail is located along the railroad. Within this area of contact, the pressure of the train is distributed evenly in such a way that the shape of this area of contact takes the form of a semi ellipse with the highest contact pressure being experienced at the centre (Johnson, 1985). c. The structure of the track The most typical structure of the track that is used is the ballasted structure of the railroad. However, there are other types of structures that exist such as the Sbab ones and are also used in various parts of the world. The ballasted tracks are usually composed of structures which are grouped as either superstructures or substructures (Boyce and Hermann, 2003). The superstructure normally consists of the rail, the fasteners, the sleepers and the pads. On the other hand, the substructure is composed of the ballast, the capping layer and the sub grade. Figure 3: A ballasted rail track 3. Design of the rail track The procedure of designing tracks can be defined as shown in the flow chart below Figure 4: Flowchart indicating the procedure of rail design The concept of designing the structural components of the rail track can be traced back into the ancient times when infrastructural engineering was in its primary stages of development. During this period, the only tools that were available for designers to use were the Newton’s laws of motion and the theory of elasticity of materials. Later as more research was done in this field, these tools were unified to come up with a practical tool for structural calculations which was referred to as allowable stress design (Allen 1982). In this tool, the adequacy of any element of the structure is normally checked by calculating the elastic stresses that are present as a result of the maximum expected loads on the tracks and comparison with the allowable stresses (Johnson, 1985). The allowable stress is equivalent to the stress of the failure of the material and divided by the value of the factor of safety. An allowable stress design flowchart for the conventionally ballasted structures of the design of the track is used by the engineers involved with the design work. According to Hagaman (2001), the traditional designs of the structure that are available are empirical in nature and they normally factor in the economical considerations which actually dictate the types of components that must be selected and the size of each and every component (Allen 1982). However, the track of the railway is a complex non linear system that can no longer be solved using the empirical approach. The traditional method for designing new rail tracks uses the aforementioned allowable stress method in order to determine the load that is applied on the on the rail track. This approach actually expresses load of the wheels in an empirical manner as a function of the static load of the wheel (Allen 1982). A dynamic factor is used to account for the irregularities incurred from the vehicle and the track operations. a. The forces involved in design The railway track is normally subjected to various forces in the vertical, lateral and the longitudinal directions and they can be applied as static, dynamic or thermal forces. Both the lateral and the longitudinal forces are normally used for the design of the rail. These stresses that are realized in turn put a limit on the allowable stresses of the rail which it is able to accommodate from a position of vertical loading (Hagaman, 2001). Vehicles that are in motion along the track at a particular defined speed usually exert particular defined forces on the structure of the track due to a number of factors such as the behavior of the body of the vehicle, the bogie and the other masses. This will happen in response to the irregularities in the geometry of the track. The forces of low frequency are usually referred to as the quasi static forces and are related to the movement of the vehicle. b. The forces of the dynamic ride Dynamic forces can be defined as the sum of the static load and the effect of the static load at a particular know speed. According to the standards of dynamic forces, it also includes the effects of the geometry of roughness of the track in response to the vehicle and the end result of the effect of the unbalanced super elevation (Allen 1982). If both the wheel tread and the surface of the rail are in a good condition, the force of contact between the wheel and the rail show a great deal of similarity to the load of the static wheel. The loads applied on the tracks of the railroad from the response of the existing vehicles to the geometry of the track have frequencies ranging from a few a few cycles per seconds up to 20 Hz. This includes both the geometry of the design and the incidental geometry. Eismann (1972) proposed an empirically based means for the calculation of the dynamic increment of the quasi stake force. This new formula was able to account for factors such as the speed of the vehicle, the condition of the track and the value that is available for the statistical uncertainty. c. The dynamic forces of the wheel and the rail These forces normally result from the discrete changes that may occur in the tread of the wheel or the running surface of the rail track. Irregularities that result into high frequency reactions may be made up of periodic loads (Jenkins et al 2002). The peak forces that are created by a wheel that is travelling across a dipped rail are usually termed as P1 and P2 where P1 is a force of high frequency above 100 Hz and P2 occurs at a lower frequency of between 30 and 900Hz. 4. Dipped joints and welds The dipped joints are usually represented as the total sum of the angles of the dip that exist between each of the existing rails and the horizontal. This phenomenon is expressed in milli-radians (Eismann, 1972). It has two major components where the first is as a result of permanent deformation occurring at the end of the rails while the second one is usually as a result of the deflection of the joint that exists under the load. The total angle determines the magnitude of the track forces and the stresses that are produced (Jenkins et al, 2002). Jenkins et al also proposed a formula that can be used to calculate the value of the force P2 at the dipped joints whereby it is given by; 5. The flats of the wheels and the out of the round wheels Some other irregularities such as faults that are incurred during the process of manufacturing, the sliding of the wheel on the rail, the breaking of the tread and grinding that occurs when handling repair and maintenance works can also result into a very high frequency dynamic forces on the track. The irregularities of the wheels can be defined being: i. Out of the roundness of the wheel. ii. Tread damage from the loss of the metal. iii. Flat zones available on the circumferences resulting from sliding. 6. The standards of design of the railway tracks There are a number of standards that have been set and must be met in the design of the rail tracks. However, the rail tracks just like any other engineering infrastructure must meet certain conditions for it to operate safely. It is required to have a clearly defined maximum level of the force, a controlled design of vehicles and the tracks and a sufficient inspection, maintenance and the renewal of the tracks. The code of practice for the defined network is the most commonly used standard in the design of rail tracks and stipulates the minimum conditions that must be met in the design work. 7. The limits state design All engineering structures have some very two common fundamental requirements that they must meet. These are the safety from collapse and satisfactory performance during the operation of the structures (Allen, 1982). The limits states normally define the various manners in which a structure is likely to fail in order to satisfy these very fundamental requirements. Figure 5: Illustration of the limit states method of design The first step of limit state design is used to determine the adverse combination of loads that are likely to occur in the lifetime of the structure. The load combinations are then multiplied by the load factor and then determined from the following factors; i. The variation of the loads in space and time ii. The combination of the loads and the different types of loads that exist iii. The modeling done on the structure iv. The analysis done on the structure The nominal capacity of the material is then reduced by a factor of capacity and it is determined from the following factors; i. The variation in the defects of the strengths of the various materials ii. The deviation in the properties of various sectors of the materials iii. The correlation that occurs in the accuracy of the materials that have been fabricated. iv. The behavior of the modeling connection. In some cases, the serviceability limit states of the track may be required to be determined and therefore the deflection, the stress and the acceleration are compared against the allowable deflection and the stress. Figure: Illustration of the serviceability limit states method of design 8. The design tools available in the market The tools available in the market today for use in design of railway systems are mainly based on quasi analysis of the track. These tools normally incorporate some empirically calculated dynamic force factor. However, very few tools are available for the examination of the depth of the track. Some of the tools that are used in rail track design that are available in the market are: i. Rail selection Module version 4: This tool makes use of the general equations in order to determine the lateral and the vertical ratios of forces that are either acting or applied on the rail track. ii. Track DEM and maintenance module version 2: This tool is used for the selection of the acceptable spacing of the sleeper, the type of the sleeper and the depth that is expected for the ballast. iii. The MB design tool: This tool is normally a combination of a number of tools used for the design and the calculation of the bending stress of the rail and the ballast contact pressure. iv. CWR Buckle version 2: This tool is used to assess the stability of a track. v. ROA rail grinding model: This tool enables the calculation of the optimum grinding values for the purpose of extending the life cycle of the rail vi. Prestressed concrete sleeper design: This tool usually guides the user to go through the allowable stress design standards. 9. Conclusion In conclusion, it is important to point out that the process of designing rail tracks requires a lot of accuracy and precision in order to ensure that we minimize on the cost of installation and at the same time reducing on the cost of maintenance and repair. Engineers are still exploring more into the science of design and with the emergence of electrical trains, more development and greater changes can still be expected in this field. 10. References Allen D. (1982), Limit states design, Canadian Building digest, CBD 221, Institute for research in construction. Boyce M. and P. Hermann (2003) interview at Queensland rail regarding quasi static and dynamic track design. Cai Z. (1992), Modeling of rail track dynamics and wheel/rail interaction phd Thesis, Queens university Kingston Canada. Eisemann J. (1972), Germans gain a better understanding o the track structure Railway Gazzette international, pp. 305-308. Jenkins H, J.E Stephenson, G.A Clayton and G.W Moorland,(1974), The effect of track and vehicle parameters on wheel rail vertical dynamic forces, Rail engineering Journal. Johnson K.L (1985), contact mechanics, New York Cambridge University Press . . Read More