Abstract:
Lateral pressure which develops in the front wheelset when traveling along a curved track is decreased while suppressing an increase in costs as much as possible. A two-axle truck for a railway car has a front wheelset having wheels with a first tread gradient and a rear wheelset having wheels with a second tread gradient which is different from the first tread gradient. The tread gradient of the wheels of the rear wheelset is larger than the tread gradient of the wheels of the front wheelset.

Description:
This application is a continuation of International Patent Application No. PCT/JP2008/064999, filed Aug. 22, 2008. This PCT application was not in English as published under PCT Article 21(2). 
    
    
     TECHNICAL FIELD 
     This invention relates to a two-axle truck for a railway car and a railway car equipped with this two-axle truck for a railway car. 
     BACKGROUND ART 
       FIG. 7  is an explanatory view showing the state of a two-axle truck  1  for a railway car when traveling along a curved track. 
     As shown in  FIG. 7 , when the truck  1  is traveling along a curved track, the force F 1  to the left and right (referred to in this description as the “lateral force”) between the wheel  3  on the outer side of the wheelset  2   f  on the front side in the direction of travel (referred to below as the front wheelset) and the rail  5  on the outer side of the curved track increases. 
       FIG. 8  is an explanatory view showing the relationship between the wheels  3  of the front wheelset  2   f  when traveling along a curved track and the rails  5  of the curved track. 
     As shown in  FIG. 8 , when a truck is traveling along a curved track, the attack angle α which is the relative yawing angle between the front wheelset  2   f  and the rails  5  (as shown in  FIG. 8 , on a curved track, the direction in which the wheelset faces the outer rail is given a positive value) increases. Therefore, excessive sliding of the wheels  3  in the lateral direction (referred to as lateral creep) develops. As a result, as shown by the arrow in  FIG. 7 , yawing develops in the truck  1 , and a lateral force F 1  develops in the front wheelset  2   f  of the truck  1 . 
     On the other hand, in the wheelset  2   r  on the rear side in the direction of travel (referred to below as the rear wheelset), there is almost no lateral displacement, and there is almost no difference in the radius of rotation of the left and right wheels  3 . Therefore, the necessary difference in the wheel radius between the inner and outer rails  5  (referred to as the pure rolling radius difference) cannot be obtained, and excessive sliding of the wheels  3  in the longitudinal direction (referred to as longitudinal creep) develops. The longitudinal creep force F 2  in the rear wheelset  2   r  which develops in this manner causes an increase in the lateral force F 1  in the front wheelset  2   f.    
     Non-Patent Document 1 listed below describes that there is a large possibility that firstly longitudinal creep in the rear wheelset and secondly lateral creep in the front wheelset are the primary causes of rail corrugation which develops in the inside rail of a sharp curved track. Thus, it is thought that the occurrence of this rail corrugation can be suppressed if lateral creep (lateral pressure) of the front wheelset and longitudinal creep of the rear wheelset can be decreased. 
     Patent Document 1 listed below discloses, with the intension of decreasing the lateral pressure, an invention which varies the rigidity of the front and rear axle box suspensions which rotatably support the front wheelset and the rear wheelset, respectively, and an invention in which the wheels of the rear wheelset are independent of each other.
     Patent Document 1: Japanese Patent No. 2738114   Non-Patent Document 1: “Properties of Trucks and Tracks When Traveling Along a Sharp Curve and Their Effect on Rail Corrugation”, J-Rail 1995 (1995 Railway Technology Joint Symposium in Japan)   

     SUMMARY OF THE INVENTION 
     Problem which the Invention is to Solve 
     In order to carry out the invention disclosed in Patent Document 1, it is necessary to reconstruct the axle box suspensions and the wheelsets of an existing truck, and doing so is enormously expensive. 
       FIG. 1  is an explanatory view showing in enlarged form an example of a wheel  7  having a tread  6  with a basic conical shape. As shown in  FIG. 1 , the tread  6  of the wheel  7  is provided with a tread gradient in order to provide both the ability to move linearly when traveling on a straight track and maneuverability when traveling along a curved track. The tread gradient γ is defined by the equation γ=tan θ, wherein θ is the angle of the tread  6  with respect to a horizontal plane at a wheel reference point Z which is the intersection between the tread  6  and a base line L which is set for each wheel at approximately the center in the widthwise direction of the wheel  7  for measuring the wheel radius R. The tread gradient γ is normally set to be in the range of 0.03-0.06. In a two-axle truck for a railway car, the tread gradient is set to be the same for all four wheels. 
     As a result of diligent investigation, the present inventors found that by making the tread gradient provided on the wheels of one wheelset different from the tread gradient provided on the wheels of the other wheelset, the above-described problems can be solved, and they completed the present invention. 
     The present invention is a two-axle truck for a railway car characterized by having a first wheelset having wheels with a first tread gradient and a second wheelset having wheels with a second tread gradient which is different from the first tread gradient. A two-axle truck for a railway car according to the present invention can decrease lateral pressure which develops in the front wheelset when traveling along a curved track without changing the structure of the axle box suspensions or of the wheelsets of an existing truck. 
     When using a two-axle truck for a railway car according to the present invention, of the first wheelset and the second wheelset, the wheelset having wheels with a larger tread gradient is disposed to the rear in the direction of travel. In a two-axle truck for a railway car according to the present invention, the wheels of the front wheelset have a usual tread gradient γ which is in the range of 0.03-0.06, so stable running properties are guaranteed on a straight track, while the tread gradient of the wheels in the rear wheelset is made larger than the tread gradient of the wheels in the front wheelset, whereby excessive longitudinal sliding is decreased, and lateral pressure which develops in the front wheelset when traveling along a curved track can be decreased. 
     From another standpoint, the present invention is a railway car having a front and rear two-axle truck, and the front truck which is positioned on the front side in the direction of travel which experiences a larger lateral pressure than the rear truck which is positioned to the rear in the direction of travel or both the front truck and the rear truck use the above-described two-axle truck for a railway car according to the present invention having a wheelset having wheels with a tread gradient which is larger on the rear side in the direction of travel. According to the present invention, the wheels in the forward-most wheelset have the above-described usual tread gradient, so stable running properties are guaranteed on a straight track, while the tread gradient of the wheels of the rear wheelset of the front truck is made larger than the tread gradient of the wheels in the front wheelset of that truck, whereby lateral pressure which is generated in the front wheelset at the time of running on a curved track can be decreased. 
     The present invention is also a railway vehicle having a front and rear two-axle truck characterized in that the truck which is positioned on the front side in the direction of travel is the above-described two-axle truck for a railway car according to the present invention in which the wheels of the wheelset on the rear side in the direction of travel have a larger tread gradient, and in the truck which is positioned on the rear side in the direction of travel, the tread gradient of the wheels of the wheelset on the front side in the direction of travel is larger than the tread gradient of the wheels of the wheelset on the rear side in the direction of travel. As a result, even when the direction of travel of the railway car is changed, the stability of movement on a straight track is guaranteed while the lateral pressure which is generated in the front wheelset when traveling along a curved track can be decreased. 
     The present invention can be carried out using an existing truck without modifications by simply changing the tread gradient of the wheels. Therefore, an increase in costs can be minimized, and lateral pressure which develops in the front wheelset when running along a curved track can be decreased. As a result, lateral pressure in the front wheelset and the longitudinal creep force in the rear wheelset can both be decreased, whereby rail corrugation which develops in the rail on the inner side of a sharply curved track can be suppressed. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIG. 1  is an explanatory view of an example of a wheel having a tread with a basic conical shape. 
         FIG. 2  is an explanatory view showing the state when a two-axle truck for a railway car according to the present invention is traveling along a curved track. 
         FIG. 3  is an explanatory view showing the tread of a rear wheel on which a simulation was carried out,  FIG. 3(   a ) showing the case in which the tread gradient was 0.05,  FIG. 3(   b ) showing the case in which the tread gradient was 0.10,  FIG. 3(   c ) showing the case in which the tread gradient was 0.20, and  FIG. 3(   d ) showing the case in which the tread gradient was 0.33. 
         FIG. 4  is a graph showing a comparison of the lateral pressure which developed in the wheel on the outer side of the front wheelset when wheels having various treads gradients were combined. 
         FIG. 5  is a graph showing a comparison of the longitudinal creep force which developed in the rear wheelset when wheels having various tread gradients were combined. 
         FIG. 6  is a graph showing a comparison of the overall creep force when wheels having various tread gradients were combined. 
         FIG. 7  is an explanatory view showing the state when a two-axle truck for a railway car is traveling along a curved track. 
         FIG. 8  an explanatory view showing the relationship between the wheels of the front wheelset when traveling along a curved track and the rails of the curved track. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The progress from the conception of the present invention up to solving the problem and the best mode for carrying out the present invention will be explained while referring to  FIGS. 1-6 . 
       FIG. 2  is an explanatory view showing the state in which a two-axle truck  4  for a railway car according to the present invention is traveling along a curved track. 
     When the two-axle truck  4  for a railway car is traveling along a curved track, the longitudinal creep force F 2  which develops in the rear wheelset  2   r  can be decreased by providing the necessary difference in the radius of the wheels on the inner and outer rails  5 . For this purpose, by setting the tread gradient γ 3r  of the treads  3   r  of the wheels  3  in the rear wheelset  2   r  to a large value so that the rear wheelset  2   r  is slightly displaced to the outer side, the radius difference becomes close to the radius difference for pure rolling, and the longitudinal creep force F 2  can be decreased. If the tread gradient γ 3r  of the treads  3   r  of the wheels  3  of the rear wheelset  2  is made still larger, it is possible to obtain a difference in the wheel radius which is equal to or even larger than the radius difference for pure rolling. At this time, the effect is obtained that the longitudinal creep force F 2  further decreases the lateral pressure which develops in the wheel  3  on the outer side of the front wheelset  2   f.    
     However, if not only the tread gradient γ 3r  of the treads  3   r  of the wheels  3  of the rear wheelset  2   r  but also the tread gradient γ 3f  of the treads  3   f  of the wheels  3  of the front wheelset  2   f  is set to a large value, the stability of running along a straight track is decreased. Therefore, by setting only the tread gradient γ 3r  of the treads  3   r  of the wheels  3  of the rear wheelset  2   r  to a large value and making the tread gradient γ 3f  of the treads  3   f  of the wheels  3  of the front wheelset  2   f  be a usual tread gradient in the range of 0.03-0.06, running stability along a straight track can also be guaranteed. 
     With a two-axle truck  1  for a railway car according to the present invention, the running stability along a straight track is increased more than when the tread gradients γ 3f  and γ 3r  of the treads  3   f  and  3   r  of the wheels  3 ,  3  of the front and rear wheelsets  2   f  and  2   r  are both set to a large value. As shown in  FIG. 2 , when traveling along a curved track, by slightly displacing the rear wheelset  2   r  towards the outer side, the lateral pressure which develops in the wheel  3  on the outer side of the front wheelset  2   f  is decreased. As a result, lateral pressure when traveling along a curved track can be decreased while running stability along a straight track can be maintained. 
     The present invention is based on the above-described concept. Next, the effects of the present invention will be explained. 
     In order to ascertain the effects of the present invention, a simulation was carried out of the situation in which a suburban commuter rail car is traveling at 75 km/hour along a curved track with a radius of 300 m and a cant of 105 mm. The tread gradient γ of the front wheelset on which the simulation was carried out was set to 0.05. 
       FIG. 3  is an explanatory view showing the tread  8  of the rear wheels  9  on which the simulation was carried out.  FIG. 3(   a ) shows the case in which the tread gradient γ was 0.05,  FIG. 3(   b ) shows the case in which the tread gradient γ was 0.10,  FIG. 3(   c ) shows the case in which the tread gradient γ was 0.20, and  FIG. 3(   d ) shows the case in which the tread gradient γ was 0.33. 
     In  FIGS. 3(   a )- 3 ( d ), in the same manner as in  FIG. 1 , L indicates a reference line, and Z indicates a reference point on the wheel. The dimensions shown in  FIGS. 3(   a )- 3 ( d ) vary with the specifications of the wheel and the like and they are merely examples. 
     In this simulation, in addition to the lateral force F 1  which develops in the wheel on the outer side of the front wheelset and the longitudinal creep force F 2  which develops in the rear wheelset, the sum F 1 +F 2  of the lateral force F 1  and the longitudinal creep force F 2  which develop in the wheel on the outer side was investigated. This sum, referred to below as the overall creep force, affects the occurrence of rail corrugation. When the overall creep force (F 1 +F 2 ) is small, it is difficult for rail corrugation to develop. 
     The results of the simulation are shown in Table 1 and  FIGS. 4-6 .  FIG. 4  is a graph comparing the lateral force F 1  (kN) which developed in the wheel on the outer side of the front wheelset when wheels having various tread gradients were combined,  FIG. 5  is a graph comparing the longitudinal creep force F 2  (kN) which developed in the rear wheelset in each case, and  FIG. 6  is a graph comparing the overall creep force (F 1 +F 2 ) in each case. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Front axle 
               
               
                   
                 γ = 0.05 
               
               
                 Tread 
                 Rear axle 
               
             
          
           
               
                 combination 
                 γ = 0.05 
                 γ = 0.10 
                 γ = 0.20 
                 γ = 0.33 
               
               
                   
               
             
          
           
               
                 F1 
                 10.34 
                 7.77 
                 6.6 
                 5.53 
               
               
                 F2 
                 6.26 
                 4.71 
                 0.77 
                 −3.39 
               
               
                 (F1 + F2) 
                 16.6 
                 12.48 
                 7.36 
                 2.13 
               
               
                 % Decrease in overall 
                 — 
                 Δ24.8 
                 Δ55.7 
                 Δ87.1 
               
               
                 creep force compared 
               
               
                 to when γ = 0.05 
               
               
                 for front and rear axle 
               
               
                   
               
             
          
         
       
     
     In the examples of the present invention in which the tread gradient γ of the wheels of the rear wheelset was 0.10, 0.20, or 0.33, when a truck  4  was traveling along a curved track as shown in  FIG. 2 , the lateral force F 1  which developed in the wheel on the outer side of the front wheelset decreased as shown in Table 1 and  FIG. 4 . Namely, by setting the tread gradient of the wheels of the rear wheelset to be larger than the tread gradient of the wheels of the front wheelset, the rear wheelset was positioned slightly to the outer side, and the amount of deficiency of the difference in the rolling radius difference between the wheels on the left and the right of the rear wheelset was decreased. 
     As shown in Table 1 and  FIG. 5 , as the deficiency in the radius difference decreases, the longitudinal creep force F 2  also decreases. If the radius difference becomes too large, a longitudinal creep force develops in the opposite direction. A moment due to this longitudinal creep force causes the lateral force F 1  which develops between the outer rail and the front wheelset. Therefore, a decrease in the longitudinal creep which can be achieved can result in a decrease in the lateral force F 1 . 
     According to the present invention, since the lateral force F 1  and the longitudinal creep force F 2  are both decreased, as shown in Table 1 and  FIG. 6 , the overall creep force (F 1 +F 2 ) also decreases. As a result, rail corrugation which develops in the inner rail of a sharp curve can be suppressed. 
     The present invention can be applied not only to a usual two-axle truck but also to an articulated two-axle truck which is positioned in the connecting portion of two cars connected to each other. 
     Furthermore, a truck according to the present invention can employ any type of mechanism for connecting it to the body of a rail car such as a swing bolster hanging mechanism or a bolsterless mechanism. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used in a truck for a railway car traveling along a sharp curved track where rail corrugation develops.