The general working device is composed of boom, stick, bucket, rocker arm, connecting rod, boom cylinder, stick cylinder, bucket cylinder, various pin shafts and corresponding steel pipes and high-pressure hoses. In order to prevent the boom cylinder from sinking too quickly when the bucket is fully loaded and the lifting stops, and to prevent the bucket from crashing to the ground when the boom cylinder hose suddenly bursts, the boom cylinder is also equipped with a boom holding valve, in which the boom is generally a welded structure. One end is hinged with the platform, the other end is connected with the stick, and the middle is hinged with the boom cylinder. This kind of integral boom has the advantages of simple manufacture, good stress and low cost, but the position of its front and rear intersection can not be changed. In the process of excavation, we often encounter different degrees of soil soft and hard, and different excavation resistance. In order to improve the production efficiency, people want to choose the larger bucket capacity and larger excavation range when digging soft soil. When digging hard soil, the bucket capacity is also larger, but the digging range can be adjusted smaller, but this kind of integral boom is difficult to meet the requirements. Therefore, people have developed a combined boom, that is, the boom is divided into upper boom and lower boom.
The upper boom and the lower boom can be inserted into different holes of the lower boom with two pins, so as to obtain six connection positions of the upper boom with different elongation and different bending angles, so as to adapt to different excavation conditions. Transposition work can be carried out on site through oil cylinder and walking. It has the following characteristics
1. The combined arm is adjustable, which is more suitable for different working conditions and working ranges and where there is a lack of machines and tools. But the structure is complex, the self weight is large, and the strength and rigidity are relatively poor.
2. The upper boom of the combined arm can be refitted with different replacement devices to meet the requirements of various working conditions.
3. The running speed of the tire excavator is high, the vibration and turning radius can be reduced after the combined arm is retracted, and the transportation length can be reduced by removing the upper arm.
For medium-sized excavators, the boom cylinder generally uses two, which are separated from the left and right sides under the boom. The hinged shaft of the piston rod end of the boom cylinder adopts the structure of double supporting short shaft, which is separated from the left and right, and the upper and lower supports of the cylinder also adopt spherical plain bearings. In this way, the boring error and welding deformation of the boom and platform do not affect the expansion of the oil cylinder and the lifting of the boom, but the structure is more complex than the general boom structure, and the cost is higher. The end hinge structure of the piston rod of the boom cylinder adopts the cantilever supporting through shaft, and the ball hinge bearing is also simplified as the bearing bush. The form and position tolerance between the axes is mainly guaranteed by the processing technology, and the end face gasket is used as the compensation. Practice has proved that this structure is more economical and practical.
The hinge of boom and stick, the joint of upper and lower boom and the fork opening at the upper end of lower boom are the weakest parts. Under the action of side and eccentric digging force or inertia force of rotary brake, the root of fork opening and lower arm opening is subject to bending, torsion, tension and shear compound stress, which is easy to crack or deform. Therefore, the selection of plate thickness and weld section of this structure should be careful.
Although the combined arm can be adjusted, it needs to be disassembled on site, which is not convenient enough. Sometimes it even needs the help of lifting equipment.
