On May 18, 2011, a dump truck user had the failure of parts damage after refitting the gearbox. He asked me about the cause of the failure. His vehicle is a dump truck type of incomplete vehicle. Previously, under the condition of insufficient overall traction in the working environment, he refitted the 9js119 gearbox to increase the speed ratio of the auxiliary gearbox, and replaced the relevant parts of the existing gearbox with the auxiliary gearbox deceleration wheel and welding shaft of 8js118 series gearbox produced by fast.
After one week of use, the user has accidents such as the fracture of the high-grade cone ring of the synchronizer, the bending and looseness of the lock pin of the lock pin synchronizer, and the fracture of the fatigue internal bonding teeth of the reduction wheel. The user suspected that it was caused by quality problems. Through the macro observation and analysis of the damage of all parts and components, this accident is not the full responsibility accident of the parts themselves, but mainly the use accident caused by improper operation. The causes of the accident are described below through more detailed illustrations and instructions:
In order to refit the gearbox to adapt to the operation of the vehicle, the key is that the user needs to understand the differences after refitting in detail, because the refitting of the auxiliary gearbox reduction system will increase the traction of the whole vehicle in the low-grade area, but there is no change in the high-grade area, and the change of speed ratio will also change the shift habit, Especially in the process of high-speed to low-speed transmission of main and auxiliary box structure.
It can be clearly seen from the gearbox ratio table of fast that the auxiliary gearbox reduction ratio of 8js118 series gearbox is 4.06, while that of 9js119 gearbox is 3.36. Before modification, It is assumed that the shift speed is different from high gear to low gear (as shown in the figure above) 1800-1300 = 500 rpm, but the gear shift speed difference after modification reaches 2100-1300 = 800 rpm. For many drivers, if they are used to driving 9js119 series gearbox, if the two foot clutch and oil coax are used to increase the engine speed, the speed can be controlled above 500 and below 600 for normal gear shift, and if the improved If the gearbox is operated similarly, the sub gearbox cannot get the synchronization effect because the speed difference does not reach the ideal state, and the shift noise is caused by the strong impact of the meshing sliding sleeve of the synchronizer and the combined teeth in the reduction wheel.
在和驾驶员进行沟通的过程中我们还发现一个普遍而严重的问题，该驾驶员在对变速箱的驾驶理解上存在严重问题，他的概念里面是法士特的变速箱不需要踩离合器来进行换档（起步除外），也就是说高低档操作都不使用离合器，那么在因为对换档转速差存在习惯性误解后会出现副箱在高低档换档的过程中出现严重的非正常工作（副箱采用气动换档，因为有时间滞后问题，此类操作极易引起同步器的锥环以及高低档齿轮的啮合齿出现损坏问题）。
We found an interesting phenomenon in the process of observing the parts. The internal joint tooth root of the reduction gear of the auxiliary gearbox basically has no excessive fillet. In the structural design of the gear, such design is also possible if normal and sufficient strength is guaranteed, but it is based on the above abnormal modification and abnormal operation, The instantaneous impact load becomes a hidden killer.
Make a simple description of the characteristic fatigue strength of local parts with SW. The middle and upper part of the figure below is a rough sample of a simple single tooth of the gear. There is no transition fillet at the root of the gear, and there is a transition fillet in the lower picture.
Through a material stress analysis software of SW system, a stress analysis video can be obtained through simple setting. It can be clearly seen from the video (the color change on the right is from blue to red, which means the stress, and the red area is the dangerous fracture area). The type without transition fillet is easy to cause instantaneous fracture when the structural strength decreases due to excessive instantaneous stress.
After knowing what causes the damage of the gearbox, the user is relatively satisfied to buy new parts for maintenance, but this accident occurs every day in the actual operation! For refitting the gearbox, be sure to communicate with the maintenance personnel who are relatively familiar with the use before refitting, and change all kinds of bad operation behaviors in habitual operation. At the same time, under the condition that most of the parts are processed automatically at present, the damage of normal parts caused by the post market modification of parts can be realized by increasing the processing process and sufficient post market training to guide users to use reasonably.
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