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37 arise. Application of the guidance will require further work in Table 4-2. Situations where considerations order to develop optimum vehicle/track performance on spe- of vehicle specifications apply. cific transit systems. Ref Situation covered C Existing system, replacing vehicles with vehicle designs currently available 4.4 Vehicle Specifications D Existing system, replacing vehicles with ones of a new design E New system with new designs of vehicle 4.4.1 Who Applies These? Technical requirements for vehicles will be specified by the in the central area of the vehicle is relatively low, so the transit system and followed by the vehicle supplier, either wheelsets on the center truck do not have solid axle connec- based on those provided by the transit system or on their tions but use IRWs. The researchers are therefore assuming that own engineering and commercial strategy. The extent to this configuration is included in the vehicle specifications. The which the transit system specifies and the supplier designs remaining guidance in this subsection describes how key ele- will vary, according to the type of specification process used, ments of this configuration can be specified so as to minimize but the overall vehicle standards should not be affected by performance issues. Any basic vehicle configuration must be this as Figure 4-1 shows. tested, initially by modeling, to check that it performs satisfac- Where a performance specification is used, Guideline 1 is torily on the system(s) that might use it. particularly important, whereas where a technical specifica- tion is issued, Guideline 2 becomes more significant. 4.4.4 Wheel Profile Table 4-3 lists the main features of a typical wheel profile 4.4.2 Where Do They Apply? (Figure 4-3) and shows how each one influences the key per- Table 4-2, an extract from Table 4-1, shows the situations formance criteria. The profile must be designed to give run- in which consideration of vehicle specifications applies (i.e., ning stability at speed and must be optimized for the C, D and E). Situation C will only apply if the existing vehi- conditions of the system, including speeds and ratio of sharp cles exactly match, or can be modified to match, the vehicle curves to tangent track. requirements. In the guidelines that follow in this section, a Using the same profiles on all vehicles on a system is desir- distinction is made between vehicles being supplied to exist- able; introducing new profiles, either on new or existing vehi- ing systems and vehicles being supplied to new systems. cles, needs to be carefully programmed in order to allow a smooth transition. "Railway" type profiles may be used but this choice rules 4.4.3 Basic Vehicle Configuration out any flange tip running and it may prove difficult to main- The vehicles described here are LRVs with three-section tain the rail sections in the street, whereas transit wheel pro- articulated bodies as shown in Figure 4-2. The center section is files can use grooved rail, which is simple to maintain in that mounted on its own truck, the "center truck," which is it only requires grinding horizontally. an unpowered trailer truck. The end sections are mounted on The use of wheel profiles that match the rail profile mini- individually powered trucks, but part of their mass is also car- mizes wear (however, see Section 4.9.4. as well). ried by the center truck via the articulations. The floor height Continuously evolving contact points and contact angles reduce noise and wear and are preferable to two-point contact, which should be avoided. Contact in the flange root area should minimize noise and wear. The use of a large flange root radius between the tread and the flange can avoid the need for steep tread slopes (such as 1 in 20), which may be less suitable for light transit, but at the same time reduce flange contact. The profile should not encourage a rapid change of contact points, which will create jolts and damage wearing surfaces. Figure 4-2. Configuration of the LFLRVs used Figure 4-1. Variation in procurement methods. in the United States and Canada.
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38 Table 4-3. The purpose of the features of LFLRVs are in use, where the noise will be more apparent. a wheel profile. The need for flange-tip running can be avoided if sharp angle switches and crossings are not used, but the use of sharp angle Profile feature Influences Tread slope and shape Ride, guidance, wear, and noise crossings may be unduly restrictive on light rail transit (LRT) Flange angle Derailment protection, passage through S&C system design in some cases. Toe radius Switch safety Toe shape Flange running ability Flange height Derailment/depth of grooves Blend radius Guard rail effectiveness 4.4.5 Lubrication Flange root radius Flange wear (especially on IRW) Flange thickness Toe radius/blend radius, wear allowance. Table 4-4 summarizes the types of lubrication system avail- able. The choice of lubrication system depends on local con- On sharp curves, the wheel profile "footprint,"(i.e., a hori- ditions (e.g., a system that is largely straight with one sharp zontal section through the flange at rail level) needs to be con- curve may use a track-based solution at that location only, sidered. Clearance needs to be considered in three whereas a system characterized by many sharp curves may dimensions, rather than just the two associated with a verti- equip some or all the fleet with vehicle-mounted lubrication cal cross section. systems). LFLRVs require flange lubrication for the center Developing a suitable wheel profile for a vehicle is an issue truck because of the increased incidence of flange contact and of balancing the following main requirements: higher lateral forces compared with high-floor cars. · Satisfactory guidance on all types of track, 4.4.6 Wheel Parallelism · Safety against derailment, · Ride quality, The design of the center truck must specify sufficiently · Minimizing contact stress and avoiding rolling contact close tolerances to ensure that the parallelism of the IRWs is fatigue, maintained within close limits. In Germany, the parallelism · Minimizing wear, and of the axles in a truck should be within 1.2 mm (0.05 inches) · Minimizing the requirement for changes to rail profiles measured at the wheel. One German system that has been and wheel profiles on existing vehicles. using LFLRVs for some years found that halving this figure to 0.6 mm (0.025 inches) for the "theoretical" axles of IRWs gave Flange-tip running is a standard practice on street running a satisfactory performance. transit systems. The tip of the flange is flat so that the wheels can run on a flat surface in the frog casting rather than rely- 4.4.7 Vertical Suspension Stiffness ing on the tread, which will drop significantly in the gap required on sharp switches. The angle of switch at which The limited space available for primary suspension within flange-tip running becomes necessary depends on wheel the design of a low-floor truck creates a challenge for the width. designer in achieving sufficient flexibility to accommodate Because flange-tip running tends to cause more wear and track twist. Despite this, most solutions successfully use a noise, it should only be used when essential, especially where conventional arrangement of a rigid frame and primary sus- pension. It is possible to save space by reducing the flexibility of the suspension and providing a truck frame that is articu- lated in the twisting sense. Any such arrangement must, how- ever, ensure that parallelism of the wheelsets is maintained. 4.4.8 Vehicle Articulation Design The articulation design encompasses the mechanical arrangement by which the end sections, the center truck, and the center section are attached together. Various designs are possible with no intrinsic advantage of one arrangement over another. However, all designs must address the following basic requirements: · Safety Against Derailment. The design must not generate Figure 4-3. Main features of a wheel profile. excessive lateral forces between the wheel and rail and must
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39 Table 4-4. Types of lubrication. Location to lubricate Vehicle mounted Track mounted Flange/gauge face of rail Grease spray sticks Grease application by hand or an automatic system Wheel back/restraining rails Grease spray sticks Grease application by hand Tread-rail head Friction modifier sticks Friction modifier application by automatic system be capable of negotiating all track features without leading other. A single damper on the center line, which only to wheel unloading. Lateral forces are typically controlled reacts to sharp accelerations or decelerations, may be suf- using a pair of dampers at roof level to control center sec- ficient. Relative roll between sections must also be con- tion yaw. These only need be provided at one end of the trolled, either by linkages or by the design of the center section. articulation pivots. · Wheel Wear And Noise Generation. The design must ensure that the alignment of the center section is main- tained as close to tangential to the rails as possible, in order 4.4.9 Summary of the Proposed Guidelines to minimize noise and wear generated by flange contact. To for Basic Vehicle Parameters achieve this, the alignment tolerances must be maintained as closely as possible, and parasitic noises (e.g., such as Table 4-5 summarizes the guidelines proposed for vehicle those arising from dampers) must be minimized. parameters by heading. The final column gives the reference · Ride. Pitching of the short center section must be con- for background information on why the suggestion is made. It trolled. Typically, this requires the use of roof linkages. is only possible to give some limiting values; actual values need Alternatively, the center section can be fixed relative to to be determined for each design by the use of modeling tech- one end-section and only allowed to pitch relative to the niques. These guidelines cover the main parameters identified Table 4-5. Summary of proposed guidelines. Parameter Existing systems New systems Cross reference Floor height above rail About 350/355 mm About 350/355 mm above the center truck. (133/4-14 inches) unless (133/4-14 inches) and there are good reasons for not less than 290 mm using another height and (11 1/2 inches) never less than 290 mm (111/2 inches) Center truck wheel 26 inches (660 mm) 550 mm (215/8 inches) Section 3.6.1 diameter (Minimum) Center truck wheelbase 1800-1900 mm (71-75 inches) Section 3.6.1 Wheel gauge (see To be determined based on Nominal track gauge Section 3.6.5 note 1) giving a minimum clearance less 9 mm (3/8 inch) of 5 to 7 mm (0.2-0.3 inches) to the existing track under all conditions Wheel width To be determined based on Preferably 5.25 Section 3.6.2 track geometry inches (133 mm) or 4 inches (102 mm) if flange tip running is used throughout Wheel taper See note 2 See note 2 Section 3.6.2 Wheel flange height 24 mm (0.95 inches) with 22 mm (0.87 inches) Section 3.6.2 flange tip running, 20 mm with flange tip (0.8 inches) otherwise running, 20 mm (0.8 inches) otherwise Flange angle To suit rail profile and 72o minimum. Section 3.6.2 similar to other cars that 75o maximum (the will continue in use on the latter for lines with system sharper curves and switches) Back-to-back wheel To be determined from Wheel gauge less dimension preceding figures and based twice the appropriate on track geometry and an flange thickness appropriate flange width