Analysis and Improvement of the Fracture and Failure of the Tilt Tie Rod of the Front Axle of PY160 Motor Grader

Front axle tilting mechanism

When the front axle needs to be tilted under special working conditions, the hydraulic circuit of the tilting hydraulic cylinder can be connected by operating the handle, causing the piston rod of the hydraulic cylinder to extend and retract. Through this, the front axle can be synchronously tilted to the left or right by connecting the pin shaft and the tilting pull rod. The maximum tilt Angle is mainly guaranteed by the stop blocks set on the left and right sides of the bridge frame. When the steering knuckle support collides with the stop block due to tilting, the pressure inside the tilting hydraulic cylinder rises. When the pressure value reaches the set value of the relief valve in the system (16MPa), the relief valve opens and the system is unloaded, thus ensuring the safe and reliable operation of the system.

Inspection and Analysis of the Accident of the inclined tie rod breaking

During the assembly and debugging of the entire machine, when conducting the maximum tilt Angle test of the front axle, the tilt tie rods broke several times. The fracture site is at the weld seam shown in Figure I, and it is when the front wheel is inclined to the right, that is, when the piston rod of the hydraulic cylinder is in an extended motion. However, the pressure gauge indicates that the system pressure is within the normal operating range, so the possibility of a malfunction in the hydraulic system has been ruled out.

The following problems were found during the inspection:

(1) The phenomenon of meat biting rarely occurs at the fracture of the weld seam, mainly manifested as the low welding quality of the weld seam. Due to the use of manual arc welding, the weld heights are inconsistent, with significant differences between high and low values. Meanwhile, the bevel at the end of the casing is very small, causing the weld seam to only adhere to the outer wall of the casing and the surface of the tie rod seat, which greatly reduces the load-bearing capacity of the weld seam.

(2) The sleeve and the pin shaft of the pull rod seat (left end) are in a clearance fit, and the fit clearance is too large (about 1.5mm), which causes it to lose its centering function and makes it difficult to ensure the coaxiality of the pull rod seats at both ends of the inclined pull rod welding assembly. As a result, it also accelerates the fracture due to the "special force" under the force.

(3) There are obvious squeezing marks at the joint surface between the left steering knuckle support and the stop block, indicating that during the right-leaning process of the front axle, the "overpositioning" phenomenon occurred due to the presence of the stop block. That is, during the right-leaning process of the front axle, the left steering knuckle support and the stop block come into contact first. At this time, the hydraulic cylinder is still in an extended motion, driving the steering knuckle support to tilt further. Meanwhile, the pressure in the hydraulic cylinder is also increasing further, thus reaching the ideal unloading protection state. However, before the pressure in the hydraulic cylinder is released, the inclined pull rod will be subjected to a certain horizontal pulling force under the action of the hydraulic cylinder, and this pulling force value will cause a sharp destructive effect on the weak links of the inclined pull rod, and eventually manifest as fracture failure.

(4) When the steering knuckle support on the left and right sides of the current bridge touches the stop block, the tilting tie rod will be subjected to a certain horizontal pressure at this time. Therefore, the possibility of its breakage when tilting in this direction is extremely small. Improvement of the front axle

To avoid the occurrence of similar faults, the following improvements can be made.

(1) Modify the design and process, enhance the welding strength, and improve the weld quality, that is:

① Increase the bevel size of the casing. As the casing itself is a seamless steel pipe with a diameter of 45x6, the original bevel size of 2.5x45° can be increased to 6x45°.

② Change the welding method. Change the original welding form to plug welding.

(2) Change the clearance fit between the sleeve and the pin shaft of the tie rod seat to an interference fit to ensure the coaxiality of the two tie rod seats after the welding of the inclined tie rod.

(3) To prevent the occurrence of "over-positioning" and change the tension state of the inclined pull rod when the hydraulic cylinder extends, the stop block can be moved to the bottom of the bridge frame. In this state, when the front bridge tilts to the right, the support of the right steering knuckle will first come into contact with the stop block at the bottom of the bridge frame. At this time, the extension of the hydraulic cylinder is blocked due to the contact between the two, causing its pressure to gradually increase. During this process, the rotation of the support of the right steering knuckle is hindered by the stop block, and the deformation after being blocked is very small. So at this time, the inclined pull rod will also bear a very small pulling force value, and the horizontal pulling force output by the hydraulic cylinder is converted into the pressure on the stop block by the steering knuckle support. When the current bridge tilts to the left, the tilting tie rods will still be subjected to horizontal pressure. Therefore, after such an improvement, regardless of whether the front axle tilts to the left or right, the tilting pull rod only bears pressure or a relatively small pulling force value after the tilting Angle reaches its maximum value until the hydraulic cylinder unloads the hydraulic system.

After such improvements, no more accidents of the front axle tilting tie rod breaking and failing have occurred.