Comparison and Load Analysis for No.12 Monobloc Manganese Frog and Its Improved Type

Yang CAO, Ping WANG, Xiaoping CHEN

School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, Sichuan, China

Abstract: Monobloc manganese frog is made by casting nose rail and wing rail together, and its load analysisis very complicated under train load. This kind of frog has several weak points, so repeated train impacts lead to different failure consequences, including contacted fatigue wearing and fatigue crack. So analyzing stress characteristics for the normal operation of frog is very essential. Taking No.12 Monobloc manganese frog of 75kg/m rail as an example, a Monobloc frog model was made and then existing frog and two other kinds of improved frogs were analyzed by finite element method for comparison. The analysis shows that the first scheme has higher load resistance ability and strength margin However, the strength of flangeway versus direct load should be studied under dynamic situation.

Key words: No.12 Monobloc manganese frog; improved scheme; stress characteristics; finite element method

Frog is the equipment for the wheel to cross from one rail to another, which is composed of casting nose rail, wing rail and connection components. Monobloc manganese frog is made by casting nose rail and wing rail together, and is fully machined. Rigorous requirements are imposed to railway equipments due to Chinese railway transportation conditions: high density, large capacity, big axle load and mixed transportation of passengers and goods, which lead to different failure consequences of Monobloc manganese frog including contact fatigue wear failure and fatigue crack failure.

Since it was observed horizontal crack, vertical crack and serious vertical wearing in No.12 Monobloc manganese frogs for 75kg/m rails laid on Da-Qin heavy haul railway, it is necessary to analyze stress of Monobloc manganese frog and to study its stress characteristics under different working conditions so as to ensure the safety and stability of train passing and the normal operation of frog. This article analyzes and compares in detail the mechanical performance of No.12 Monobloc manganese frog for 75kg/m rail and that of two other kinds of improved manganese frogs by finite element method.

1． Calculation Model

1.1 Existing Monobloc Manganese Frog Model

A 3D model of frog is established by using plotting sofeware. Considering the complicated structure of frog and only stress is analyzed in this article, some minor factors are ignored in the model such as small chamfer. The model is shown in Figure 1.

Figure 1 (a) Top of Frog Model

Figure 1 (b) Bottom of Frog Model

1.2 First Scheme for Frog Improvement

First scheme contains frog improments as below: the thickness of wall is increased from 22mm to 25mm; the thickness of top surface is increased from 32mm to 40mm; the bottleneck is changed from transition of 22mm section and the straight line of throat area to double throats; use thicken ridge from theoretical point of nose rail to 50 mm section inner cavity of nose rail so as to raise impact resistance; the previous lowering values for nose rail are 10 mm for 10 mm section and 0 mm for 20 mm section and now are 8 mm for 12 mm section, 1 mm for 30 mm section and 0 mm for 50 mm section.

1.3 Second Scheme for Frog Improvement

Second scheme contains frog improments as below: the thickness of two sides of wall is increased to 25mm and that of top surface is enhanced to 35mm so as to improve the overall performance of frog; change the bottleneck into double-bottleneck structure in order to reduce the distance of harmful spaces; the 20~80mm sections of nose rail are widened in order to enhance its impact resistance.

2. Calculation Parameter and Design Load

2.1 Calculation Parameter

After water treatment, tensile strength of manganese is not less than 736 MPa; yield strength is 440 MPa; during long-term operation, fatigue strength is around 40% of yield strength, namely 176MPa; elongation is not smaller than 35%; hardness is between 170HB and 229HB. The computation in this article adopts elastic modulus of 2.06 * 1011N/m2 of manganese and Poisson’s ratio is 0.3.

2.2 Design Load

The maximum operating axle load of line should be 30 t and corresponding static wheel load is 150kN. In frog section, because of interaction between intensive wheel forces, dynamic wheel load is increased dramatically reaching 1.5~3 times of static wheel load. To avoid manganese frog from crack during long-term using, stress under the maximum wheel load and long-term dynamic load action shall be kept below the yield strength of materials. Therefore, it is necessary to implement strength test for its weak part.

Under ultimate load, vertical force is considered to be 450 kN and 70 kN for lateral force ; under fatigue load, vertical force is considered to be 250 kN and 30 kN for horizontal force. Load is applied under 4 types of working conditions: (1) nose rail is lowered 3mm, 2/3 of vertical force is allocated on nose rail and and 1/3 on wing rail and lateral force acts on nose rail; (2) both of vertical and lateral force act on the nose rail on the section in the middle of the first sleeper behind 50 mm top width position of nose rail; (3) vertical force acts on wing rail of throat area and lateral force acts on wing rail; (4) vertical force acts on flange groove of the section at 20 mm top width position of nose rail and lateral force acts on nose rail. Positions of different working conditions are shown in Figure 2.

Figure 2 Four Working Conditions: Forces Applied Positions

3. Result Analysis

3.1 Result Analysis under Ultimate Load Action

Analyzed by finite element software, calculations of existing and improved manganese fixed frogs under ultimate load under different working conditions can be obtained. Equivalent stress, vertical and lateral displacement are shown from Figure 3 to Figure 5. Maximum equivalent stress and maximum lateral and vertical displacement are shown in Table 1.

Table 1 Comparison of Results of All Types of Manganese Frogs under Ultimate Load Working Conditions

It is can be seen from comparison of maximum equivalent stress and displacement of all sorts of manganese frogs under 4 kinds of working conditions that,

(1) The vertical displacement of frog is all below 5mm and the lateral displacement is below 2mm; the maximum equivalent stress is less than the material’s allowable tensile strength before and after improvements.

(2) Under ultimate heavy load, there are vertical and lateral displacements which are more important than that in ordinary track, which may result in geometrical change of turnout and obviously increased impact when heavy haul train passes. That leads to frog defects.

(3) The first scheme for frog improvement shows better mechanical performance under all kinds of working conditions and stress is averagely distributed, which achieves the expected effect of the improvements.

3.2 Analysis under Fatigue Load

Under fatigue load, with the same working conditions, the distribution of stress and displacement of frog are similar to those under ultimate load. Results of existing and improved manganese fixed frogs under fatigue load under different working conditions can be analyzed. Equivalent stress of the 3 sorts of frogs under working condition 1 is shown from Figure 6 to Figure 8.The maximum equivalent stress and maximum lateral and vertical displacement under different working conditions are shown in Table 2.

Table 2 Analyzing and Comparing the Results on All Types of Manganese Frogs under Fatigue Load Situation

To compare maximum equivalent stress and displacement of 3 sorts of schemes under each working condition, it can be seen that:

(1) For existing manganese frog, it is pretty similar for maximum equivalent stress under the first 3 kinds of working conditions. It’s a little less than fatigue strength of material, which means that strength reserve is insufficient and fatigue stress limit of manganese frog can be easily reached with frog wear and lower inertia moment, which finally leads to frog crack; with working condition 4, flange way can not bear directly wheel load, because maximum equivalent stress exceeds fatigue strength of material.

(2) The two improved manganese frogs under working conditions 1 and 3 have similar maximum equivalent stress which is more than the value under working condition 2; the two scheme’s maximum equivalent stress under working condition 4 are both bigger than that under the first 3 kinds of working conditions. Generally, maximum equivalent stress of improved frog under 4 kinds of working conditions, which is smaller than fatigue strength of material and, has a margin for strength reserve.

4. Conclusion

Through analysis of the above calculations, the following conclusions can be drawn:

(1) For existing MN frog, the maximum stress under ultimate load is slightly less than yield strength of material; the maximum stress under fatigue load is slightly bigger than fatigue strength of material; that shows its performance on Da-Qin heavy haul railway is weak. Especially when vertical and lateral wearing occurs, the inertia moment of frog section is lowered, the stress under ultimate and fatigue load easily goes beyond the limit, which leads to crack.

(2) Under ultimate load, the maximum stress of manganese frogs in 3 schemes is all smaller than fatigue strength of material, but the existing frog’s strength reserve is small; under the effect of fatigue load, the maximum fatigue stress of two improved frogs is smaller than fatigue strength of material. Generally speaking, strength reserve of improved frog in first scheme is better.

(3) The internal structure of frog is complicated with many connection parts. Stress concentration often occurs at sharp angle and in suddenly-changed transition area. These both lead to cracks. To resolve the problem, on the premise of enhancing internal health, grinding to chamfer at each part and especially at rail foot should be considered.

(4) The direct bearing capacity of flange way needs to meet the requirements of strength structure and safety and stability of train passing. Its contact with wheel flange is different from normal rail-wheel contact, which needs to be studied under dynamic situation to improve the structural design of manganese frog.

12号高锰钢整铸辙叉及其改进型受力分析比较