The current loaders usually adopt the design of bending waist steering. The front and rear frames are connected by hinge pins. During the working process, the hinge pins are subjected to multiple forces. Their state is very similar to the waist, the center of human movement, so they are also commonly called waist axis. The design of the hinge part has a key impact on the stability of the whole machine. Don't ask why, just try to make your waist unable to move heavy objects and you will know.
Press the blackboard to highlight the key points when discussing the advantages and disadvantages of mechanical structures and their applications
Discussing design without demand and discussing structure without stress state are all hooliganism. The frame of the loader with bending waist steering presents the characteristics of thick ends and thin middle. The hinge pin part is thin and needs to bear forces in multiple directions at the same time. The most common structure is sandwich type, that is, there is a single double hinge plate on the upper and lower front/rear frames, and the upper and lower hinge pins pass through the hinge plate along a concentric line to connect the front and rear frames. Although the fixing method of the pin shaft and the hinge plate, the bearing type, and the bearing positioning method of each model are different, the force characteristics of this part are the same.
These forces have different magnitudes and directions, and the strength of wear on the parts related to the hinge pin is also different. This post/attempts to show whether several hinged structures can maintain stable operation after being subjected to external forces, and the corresponding wear characteristics, so as to compare the advantages and disadvantages of several types of hinged structures. The structure is determined, and the force is added to observe.
The first function of the hinge pin is to transmit the driving force. Most loaders use 4-wheel drive, and the front and rear axles each contribute half of the driving force. This force will cause the two frames to have a tendency to squeeze each other (academic name stress), and the installation state of the pin shaft is that both ends are supported in the seat hole of the double hinge plate, and the middle is the force transmission point of the single hinge plate. This stress state is academically defined as shear stress.
The second function is to ensure the reliability of the connection between the front and rear bodies when the bucket is digging/transporting materials. These two working conditions have two lever relationships as a whole: with the front wheel as the fulcrum, the weight and length of the body after the front wheel touches the ground balance the digging resistance of the bucket before the front wheel touches the ground plus the weight of the working device; the other is to use the hinge pin shaft as the fulcrum, and the front body plus the material weight balance the weight of the rear body. Both lever connections have a tendency to bend the body from the hinge pin position. In detail, the upper hinge pin is separated to both ends while the lower hinge pin is squeezed against each other. But the similarities are that they both have a tendency to bend all the hinge plates.
Since the maximum digging force of most current models can lift the rear body off the ground, the force on the hinge pin is the greatest at this time, which is approximately equal to the deadweight plus the rated weight (this conclusion comes from a public paper published by a member of Liugong Research Institute). At the same time, there is friction resistance when the bearing rotates, but it is very small compared to other forces. During the operation of the loader, the hinge pin is subjected to these forces at the same time. Regardless of the specific hinge installation form used for the same model, the force it bears is constant.
Although the hinge plate welded from steel is very hard, it will actually deform when it is subjected to a force that exceeds its strength. Among the forces on the hinge plate, the bending moment caused by the weight of the material or the digging resistance will cause it to bend slightly, and the upper and lower pin holes/upper and lower hinge pins on the double hinge plate have different deformations due to the difference in force: the upper end plate is larger than the lower plate, and the upper hinge is larger than the lower hinge. It should be pointed out that the higher the distance between the lower hinge point and the closer the distance between the two hinge points, the greater the deformation. Designers need to calculate the thickness of the steel plate when designing to ensure that the hinge plate can recover after deformation. Because the deformation of the lower hinge point is small, the positioning function is generally placed at the lower hinge point to extend the life of the parts.
From a wide range of examples, articulated bearings used spherical bearings/floating pins in the early days, and now heavy-duty vehicles use tapered roller bearings. The life of the former is obviously not as long as the latter. The fundamental reason is that the spherical bearing is limited by its own performance and can withstand axial forces worse than tapered roller bearings. As shown in Figure 2, when a single hinge plate is overloaded, the pin will be subjected to an upward force due to the upward offset on the plane. The spherical bearing bears this force with a thinner edge, while the tapered roller bearing bears this force with a thicker edge. In addition, the material difference between the two makes the life difference obvious.