Strength Analysis and Experiment of High Speed Railway Gearbox Bracket

Light-weighting of high speed railway equipments has become a major trend, which leads the upgrades of equipments. A new C-shaped bracket has been produced to connect the driving gearbox with the bogie. This paper built a three-dimensions model of a C-shaped bracket, got the maximum and distribution of the stress and deformation under two different working conditions form finite element analysis (FEA). Then it presents a method to perform a corresponding experiment. It is observed that the computed values form FEA are in very good agreement with the experimental values, which both verified the structural strength of this C-shaped bracket.


INTRODUCTION
With the rapid and continuous development of China's high speed railway, transmission technology turns decisive [1].There are several methods, most of which use a suspender to connect the driving gearbox to the bogie.The C-shaped bracket is one of the latest solutions for such connections while light-weighting the equipments in high speed railway [2].Consequently, the structural strength of the bracket has become a major concern.
Finite-element analysis (FEA) is wildly used in stress analysis [3], but usually, due to the complexity of the loads and fixation, there is a scarcity of the corresponding experimental values to compare with for the computed values from FEA.
This paper built a finite-element model of a C-shaped bracket, compared the values from both FEA and the corresponding experiment which is performed with customized holding devices, to provide the basis and direction for the light-weighting of high speed railway equipments

Finite-Element Model of C-Shaped Bracket
There are three methods of modeling in a FEA software [4]: (1) Model directly in the FEA software.
In this paper, SolidWorks is used to built the threedimensions model of the C-shaped bracket which shows in Fig. (1).The model was saved as a IGES file, then imported to Ansys, a FEA software.

Material Parameters
Apply the material of the bracket, which is grade E cast steel, to the model.Table 1 lists the it's Young's modulus, yield strength and Poisson's ratio.

Boundary Conditions
In Fig.
( 2) is a C-shaped bracket installed to a CRH3A driving gearbox which is able to operate at 250 km per hour.It also shows the loads during positive and reversed rotation.While operating, the bracket is fixed to the bogie by 4 bolts.There is a bumper block on each side where the bracket connect to the gearbox to absorb the shock.The vertical stiffness of the block is 6.6 kN/mm, and the working stroke is 4~5 mm.
To set up the boundary conditions, add fixed supports on the surfaces where the bracket contacts to the bogie by bolts, and 2 spring elements to simulate the bumper blocks.

Loading Definition
Obviously, the severest working conditions of the Cshaped bracket is when under the short circuit torque.The load that the bracket sustained is where, F ---the force to the bracket; M ---the short circuit torque; i ---the gear ratio; L ---the distance between the center of load and the wheel shaft.
The direction of this load is straight up during reversed rotation, and down if it's positive rotation.The gravity of the gearbox shared to the bracket is far less than this load, so can be ignored.
Table 2 lists the gear ratio, the distance between the center of load and the wheel shaft, and the short circuit torque of this transmission gear.Therefore, the load under the short circuit torque is ±112 kN, which is also listed in Table 2.

Results
The stress nephogram of the bracket in during positive and reversed rotation are in Fig.

Holding Devices and Experiment Method
This paper presents a set of holding devices for the stress experiment as Fig. (5) shows.The hydraulic jack generate the extrusion force, which can be adjusted by the force sensors, to load the bracket fixed to the welded frame.A continuous data acquisition system (CDAS) [7] is used to record the signals from the 45 o strain rosettes attached to the danger points which determined by the FEA results.Fig (6) shows the danger points of the bracket.
The holding devices are welded structures made of Q235 steel plates.To reach the 112 kN load for both bracket, the hydraulic jack has to generate more than 224 kN extrusion force.Fig. (7) shows the stress nephogram with a max stress of 178.4 MPa, which is less then the yield strength of Q235 steel (235 MPa), the safety factor is 1.32, satisfies the strength requirement.This method can simulate two rotating modes under real conditions such as fixation and loads.Two bracket can be tested at the same time, and it's easy to perform.

Experiment Procedures
To perform the test, the following steps are taken:
According to Hooke's law, the maximum principle stress is: where, σ max ---maximum principle stress; E modulus.

SUMMARY OF FEA AND EXPERIMENTAL RESULTS
Table 3 and
(3), shows the max stress of 281.7 MPa and 277.0 Mpa, both at the conner next to the loading point.Fig. (4) shows the total deformation.

•
Install the brackets to the frame as design requires; • Zero the sensors before being placed into position; • Attach the 45 o strain rosettes to the danger points, connect them to the CDAS, start recording; • Supercharge the hydraulic jack till the readings of the sensors reach 112 kN; • Keep the load for 5 minutes, then unload; • Repeat step 4 and 5 for three times to get the mean value; • Finish.

Fig. ( 9
Fig. (9) is a diagram of a 45 o strain rosette.The principle strain is calculated as follows:
Fig. (10) show the FEA and experimental stress of each measured point during positive rotation; Table 4 and Fig. (11) show the stress during reversed rotation.