Analysis of Common Faults of Loader Working Devices

Abstract: This article mainly introduces the common faults that occur in the working device of loaders during operation, and based on the author's working experience, proposes some solutions.

Key words: Fault analysis of loader working device

The working device of a loader is an important component of the loader. The quality of its working performance will directly affect the overall working efficiency of the loader. However, compared with other systems of the loader, the depth of research on it is insufficient. Especially in recent years, people have paid more and more attention to the appearance design of loaders, but there has been no improvement in their core internal components. Below, we will analyze one by one the faults that frequently occur in the working devices of loaders in recent years.

I. Bending of the pull rod of the working device:

As the name suggests, the pull rod is subjected to tension during operation. It will only be stretched or the hinge hole will lose its roundness, and there will be no bending. Therefore, in the design of the pull rod, we only consider the maximum tensile force it can withstand and do not take other forces into account. As a result, the pull rod becomes the weakest component in the working device. In actual work, if the design of the working device is unreasonable or the operation is improper, the pull rod will be subjected to other forces greater than the pulling force.

The first situation:

After the working device is unloaded at any position below the highest position, if the user's next action is not to lower the boom or retract the bucket, but to directly lift the boom, at this time, the force on the pull rod is not tensile force but compressive force. Illustrate with the following figure:

After unloading at the position shown in the diagram, if the boom is immediately lifted, due to the contact between the unloading limit block and the boom, the relative Angle between the bucket and the boom cannot be further reduced. At this time, the pull rod is pushed by the bucket, and this thrust acts on the tipping bucket cylinder through the rocker arm, pulling the piston rod outward. At this time, both the front and rear chambers of the tipping bucket cylinder are in a sealed state. The working device can only continue to move upward by draining oil from the front chamber and replenishing oil from the rear chamber of the tipping cylinder. If the maximum thrust generated by the pull rod cannot pull the piston rod of the tipping cylinder outward, the pull rod will eventually have to bend. Therefore, in this case, the maximum pressure on the pull rod is determined by the pressure of the relief valve in the front chamber of the tipping cylinder.

The second situation:

When the working device is performing excavation operations, the entire weight of the loader falls on the bucket and the rear wheels, while the front wheels do not bear any force. The cutting force of the bucket into the ground is provided by the thrust of the pull rod on the bucket. Therefore, at this time, the pull rod is subjected to pressure, and the magnitude of this pressure is determined by the weight of the entire machine and the pressure of the relief valve in the front chamber of the tipping bucket cylinder. The smaller of the two is taken. As shown in the following figure.

The third situation:

When the working device is in the unloading operation, users often operate the tipping bucket cylinder to unload thoroughly, and the bucket limit block collids violently with the boom. At this time, the rocker arm will exert an impact pressure on the pull rod. If the magnitude of the inertial force of the motion is not considered, the size of the pressure on the pull rod is also determined by the pressure of the relief valve in the front chamber of the tipping bucket cylinder, which is similar to the first situation. The difference is that in this working condition, the tipping cylinder is active, while in the first case, it is passive.

The above are three typical working conditions for the bending of pull rods. By using the method of torque balance in mechanical principles, we can identify the working condition under which the pull rod is subjected to the greatest pressure and conduct stability checks on the pull rod. For the third working condition, we can weld a limit block at the boom seat beam to prevent the tipping cylinder from transmitting too much impact pressure to the pull rod. As shown in the left picture. For the first and second working conditions, we can adjust the pressure of the relief valve in the front chamber of the tipping cylinder not to be too high to ensure that the pull rod is not bent. Based on experience, to determine whether the pressure in the front chamber of the tipping cylinder is too high, we have the simplest way. That is, under the first working condition, let the engine idle and lift the boom. If the boom cannot be lifted or you feel a significant drop in engine speed or even stall, it indicates that the pressure of the relief valve in the front chamber of the tipping cylinder is too high. You should test it with a pressure gauge and adjust it to the specified value.

However, the pressure of the valve should not be set too low to protect the pull rod. If the pressure is set too low, when the working device of the loader is at the high position of the full bucket, since the combined center of gravity of the bucket and the material is almost zero relative to the lever arm of the lower fulcrum of the bucket, as shown in the figure, the bucket is prone to reverse flip, and the material will fall off the edge of the bucket, which is likely to damage the cab and cause personal injury to the driver.

2. Inadequate bucket collection:

The phenomenon of "inadequate bucket retraction" refers to the situation where, after the working device of the loader has been in operation for a period of time, the bucket retraction limit block on the bucket fails to make contact with the boom, and the bucket retraction Angle does not reach the specified value.

The occurrence of this phenomenon is mainly due to the insufficient strength of structural components in the working device, resulting in plastic deformation. When designing the working device, the strength of the working device is often verified through force analysis when the loader is fully loaded and the material is inserted and lifted. We often believe that under such working conditions, the force on the working device is the greatest. In fact, this is not the case. Based on our work experience, we have analyzed and concluded that there are still several situations where plastic deformation occurs that we have overlooked. We hereby bring them up here so that everyone can take them into consideration when analyzing the structural strength of working devices in future design work.

The first situation:

When designing the working device, to prevent the loader from spilling materials during transportation, we adjusted the hinge point position of the connection between the tipping cylinder and the front frame. This made the working device move in an arc from the ground bucket collection position to the highest point of the specified transportation position. The bucket collection limit block of the bucket was always tightly against the boom. This could protect the working device from impact and prevent material from spilling. Moreover, it can simplify the driver's operation. There is no need to collect the bucket on the ground and then perform the bucket collection action when reaching the transportation position. This function is what we often call the "blocking function".

Through the movement trajectory, we carefully analyzed and found that if the working device with this function is not restricted by the bucket collection limit block, during the arc movement of the working device from the ground bucket collection position to the highest point of the specified transportation position, the relative Angle between the bucket and the boom gradually decreases. In actual work, due to the limitation of the bucket collection limit block, the bucket forces the pull rod to pull the rocker arm, pushing the piston rod of the tipping bucket cylinder back. The piston rod of the tipping cylinder is pushed back by the oil supply valve in the front chamber of the tipping cylinder and the oil return valve in the rear chamber. Generally speaking, the opening pressure of the rear chamber relief valve is 1.25 times the system pressure of the working hydraulic system. Therefore, during the arc movement of the working device from the ground bucket collection position to the highest point of the specified transportation position, there may be a slight pulling force on the pull rod and a bending moment on the rocker arm that are even greater than the corresponding force in the ground excavation condition. As shown in the left picture.

The second situation:

When the working device is unloading at the highest point, users often operate the tipping bucket cylinder to unload thoroughly, causing the bucket limit block to collide violently with the boom. Over time, a pit will appear on both the bucket limit block and the boom, and the unloading Angle of the bucket has far exceeded the design value. At this point, the connecting rod mechanism of the working device is approaching a dead point. That is, points A, B and C almost form a straight line. At this time, when the working device retracts the bucket, the pull rod and rocker arm will be subjected to an infinite pulling force. To prevent this phenomenon from happening, the most effective way is to weld the unloading limit block between the rocker arm and the boom to avoid excessive impact between the bucket and the boom.

There are also some other common faults of the working device, such as the bending of the piston rod of the oil cylinder, the asynchronous operation of the oil cylinder, and the cracking of the boom crossbeam, etc. Due to space limitations, they will not be elaborated here.