Analysis of force rotation phenomenon of load cell used in Truck Scale

[Summary] In the process of frequent heavy-load weighing in electronic truck scales using column load cells, the rotation of column load cells often leads to entanglement of cables, which causes certain problems in the use of truck scales. In response to this problem, this article lists the current common anti-rotation measures for column sensors, simply analyzes their structural characteristics, and combines the mechanical model of the truck scale, and analyzes the force transmission process of the load cell on the load cell based on the surface contact stress theory. Rotation is simplified as simply rotating around a fixed axis, and it is not difficult to understand the rotation of the sensor.

[Key words] Truck scale, Load cell, Contact stress of rotating surface, Rotate around a fixed axis

I:Introduction

Electronic truck scale has simple structure and high weighing accuracy. It is widely used in highways, ports, metallurgy, mines, factories and enterprises. It is one of the most widely used weighing instruments in today's social, economic and trade weighing control. The load cell is an important component arranged between the carrier and the foundation, which plays a role of force-electricity conversion. The main function of the sensor is to present the weight parameter value of the goods in the form of electrical signals. At present, the weighing sensors used in large scale weighing instruments at home and abroad roughly include: column type load cell, bridge type load cell, axisymmetric torsion ring type load cell, spoke type load cell,

Figure 1.1 Schematic diagram of column load cell

The two ends of the elastic body of the column type load cell are generally a classic tumbler structure, which has the advantages of compact structure, stable signal, strong anti-overload ability, high response frequency and wide range. It is more common in the use of truck scales. However, the column-type load cell has poor resistance to lateral and eccentric load, and there will be a bad situation in use, that is, it will continue to rotate slightly during use, and the phenomenon of cable entanglement of the load cell will occur. The rotation of the heavy sensor continues to accumulate, and even the cable is broken.

There have been related accidents during the use of the truck scale: the tensioned cable directly pulled the load cell down under the action of a strong instantaneous impact, and the scale body was tilted, which brought huge safety hazards to the use of the truck scale. Economic losses. If the cable is wound too tightly, the output signal will be unstable, the reading will be unstable, and the accuracy will be reduced. In addition, during the use of the truck scale, because the space where the load cell is located is relatively small and relatively airtight, it is difficult to find the cable winding problem in time. Even if a problem is found, the staff needs to use jacks to lift up the scale body and restore the load cells wrapped around the cable to normal one by one, which wastes time and energy and affects the reputation of the company. Therefore, the requirements of the column load cell for installation and application are indeed higher, and the anti-rotation structure design is essential.

2. List of existing methods to solve the load cell rotation problem

As shown in Figure 2.1, this anti-rotation structure uses an anti-rotation pin to prevent the rotation of the load cell. The sensor itself is designed with an anti-rotation gear ring, which belongs to the current conventional anti-rotation structure. For column type load cells, the limit distance has strict requirements. Generally, the front and rear driving directions are 3mm~5mm, and the left and right width directions are 2mm~4mm. However, due to the influence of thermal expansion and contraction in winter and summer, and even morning and evening, users have not adjusted the limit in time. Distance, the limit distance of the load carrier may even exceed 20mm, too large limit distance will cause the load carrier to swing very large, and the strong instantaneous impact force can easily make the anti-rotation pin fall off [1].

Figure 2.1 Anti-rotation structure 1 of load cell

Ningbo SAINTBOND column-type load cell's lower indenter and the end of the elastic body mill the plane at the same time. The installation method is shown in Figure 2.2. After installation, ensure that the milled plane is consistent with the driving direction. The anti-rotation structure is simple to process and has obvious anti-rotation effects. Figure 2.3 shows the lower indenter of the anti-rotation load cell. A positioning pin is inserted into the indenter to prevent the load cell from rotating. The bolts fix the gear ring, and the gear ring presses the indenter, but when the force is relatively large, the strong momentary force may cause the load cell to rotate with the lower indenter. Figure 2.4 shows the anti-rotation structure. The sensor and the base adopt multi-edge surface contact, and the rotating force is broken down on multiple edge surfaces to avoid excessive concentrated force and damage the load cell and base.

Figure 2.2 Anti-rotation structure of load cell 2

Figure 2.3 Anti-rotation structure of load cell 3     2.4 Anti-rotation structure of load cell

3:Analysis of the reasons for the rotation of the load cell

Why does the column type load cell installed on the electronic truck scale rotate? Column-type load cells have a long history of use in automatic rail scales, and there is little rotation at the installation site. The difference is that: the carrier of an automatic rail scale is generally only 3.7m~4m in length, and its limit adopts a tensioned structure, so that the carrier does not move in the longitudinal and lateral directions, while ensuring that the vertical force is not affected [2 ]. The length of the load-bearing device of a truck scale is relatively large (usually about 18m), and the limit device adopted is to allow the load-bearing device to have a certain displacement in the longitudinal and lateral directions, so the load cell may also shake accordingly. How does the shaking of the load carrier cause the weighing load cell to rotate?

3.1 Analysis based on contact pressure theory        Figure3 Schematic diagram of truck scale weighing

The vehicle belt speed upper scale produces instantaneous impact on the load carrier, including longitudinal impact and vertical downward impact. When designing and selecting models, various extreme situations should be fully considered. Taking the maximum axle load of 50t as an example, the following is the calculation method of impact force in the horizontal direction. (Refer to "General Code for Design of Highway Bridges and Culverts" (JTG D60-2004))

The impact coefficient can be calculated as follows:

In the formula,

E—— elastic modulus, 206000MPa;

I——The cross-sectional inertia of the scale body, 75.71×109mm4;

M——weight of scale, 5t;

l——length of scale body, 6m;

p——The unit beam length mass, 5t/6m=0.83t/m;

By calculating the fundamental frequency of different scales of our company，

  The values are all less than 2Hz, take the maximum value 2 and substitute it into the formula for calculating the impact coefficient:

Considering the actual application, the impact coefficient should be appropriately amplified, and the amplified safety factor 1.1 is obtained

F=Car load×impact factor×magnification factor=50×0.1×1.1=5.5t

The column-type load cell is in contact with the upper indenter. Point E is called the initial contact point. The normal line of the curved surface at point E is taken as the z-axis. When the elastic bodies of the two curved surfaces are in contact with each other under pressure, contact stress will be generated. , The mechanical model is shown in Figure 3.2 and Figure 3.3. The plane curves AEB and CED are plane curves generated by the intersection of a cutting plane perpendicular to the Z axis and a curved surface. Each plane curve has a radius of curvature at point E. The plane where the plane curve lies is the yz plane, from which the positions of the coordinate axes x and y are obtained. Different plane curves correspond to the X axis and Y axis at different positions, and the z axis coincides with each other, and the angle between x1 and x2 and between y1 and y2 is represented by Φ. The two curved surfaces are in contact and pressed together. The pressure P acts along the z-axis. Near the initial contact point, the material deforms locally, and a small elliptical plane is formed by the contact point. The semi-major axis a of the ellipse is on the x-axis, and the short The half axis b is on the y axis. The unit pressure of each point on the elliptical contact surface is related to the deformation of the material. The deformation on the z-axis is large, and the maximum unit pressure P0 will be generated along the z-axis. The unit pressure P of the remaining points is distributed according to the law of the ellipse.

Figure 3.2 Curved body coordinates and contact ellipse        Figure 3.3 Internal contact between two spheres

The equation is

Unit pressure:

Radius is  R1、R2 The two spheres are in contact with each other, and under the action of pressure P, a circular contact area with radius a is formed, namely When the elastic modulus of the two sphere materials is E1=E2=E and Poisson's ratio μ1=μ2=0.3, the Hertz formula can be obtained:

Maximum contact stress:

Contact relative displacement When the horizontal impact force is small, the rolling friction is less than the maximum rolling friction, and the weighing platform and the sensor do not move relative to each other. Taking the contact position of the sensor and the lower pressure head as the fulcrum, the weighing platform drives the sensor to move in the direction of horizontal force, resulting in angular displacement, and the load cell does not rotate. As the horizontal force increases, the inclination angle of the load cell is greater than the critical angle, the weighing platform and the load cell slide relative to each other, and the motion states of the two are no longer consistent, but the direction of the friction force experienced by the load cell is determined by the direction The direction of translation is determined. When the load cell has a certain angular displacement, the pressure P is no longer transmitted along the center axis of the load cell, and an angle is generated, as shown in Figure 3.4. The force P on the contact surface interacts with the friction force f to produce a component force perpendicular to the center axis of the load cell on the contact surface. Since the component force is a certain distance from the center axis of the elastic body, f'will make the scale The heavy sensor rotates around its own central axis, as shown in Figure 3.5, and the rotation is generated.

Figure 3.4 Schematic diagram of angular displacement of load cell                

Figure 3.5 Sketch of rotation around a fixed axis

According to the field application, we know that the loading direction of the truck is determined by the site installation position, not completely parallel to the longitudinal centerline of the load carrier, the force F generated in the horizontal direction will also change accordingly, if a certain weighing If the installation height of the sensor is lower than the overall level, the impact will be more obvious, as shown in F'in Figure 3.1. The direction and magnitude of the horizontal force are constantly changing intricately when the load carrier is subjected to constant impact and rebound of the horizontal and longitudinal limits. It is these two reasons that cause the load cell to rotate. However, we can see from the analysis process how to reduce the possibility of rotation: one is to avoid large axle loads, so that the weighing platform is stressed to reduce deformation; the other is to control the loading speed of the truck to reduce the impact inertia in the horizontal direction; It is to ensure the verticality of the load cell installation and avoid the large inclination angle of the load cell installation. After force is applied, the load cell tilt is increased and the angular displacement of the load cell is reduced. The above are some of my superficial opinions, please correct me for any improprieties.