Application of stress distribution improvement in the practice of making high-performance bridge Load Cell

[Summary]: By improving the distribution of the shear stress of the small-range bridge sensor in the patch area, the sensor product with a large difference from the theoretical calculation value is developed and manufactured.

[Key Word]: Stress distribution, Stress concentration, Bridge Load Cell

Introduction

Among the various structural sensors produced in the load cell manufacturing industry in my country, the steel bridge sensor has a unique structural form of support at both ends and a force in the middle, which enables the large tonnage sensor to enter the measurement of high accuracy and high reliability. field. At the same time, the force transmission component adopts a spherical indenter, which gives full play to the advantages of the steel ball that can be automatically reset and adjusted. It has good lateral force and impact resistance, easy installation, and good interchangeability. The key areas play a leading role. However, the range of sensors is mostly limited to more than 10 tons, and very few have a range below 10 tons. In order to meet the needs of users, we have carried out the development of a small range of 3 tons sensor.

1.Design of Bridge Type 3Ton Load Cell

In order to maintain the simplicity and versatility of installation and use, the 3t sensor adopts the installation size of the 10t sensor, and the other parts of the sensor except for the elastic body are required to be common. This requires us to only change the size of the strain zone during the sensor design process. Can not affect the sensor installation size.

1.1Structural design defects of small-range bridge Load Cell

The structure of the bridge load cell is fixed at both ends, and the shear force is applied to the I-shaped section (Figure 1)，

Figure 1                              Figure2

Therefore: the shear force on the patch part is: ½ F;

The shear stress is obtained from the stress analysis diagram of the section of the elastic body strain part (Figure 2):

When the known sensitivity is 2mV/V

ε=1200με, set ν=0.3, E=2.1×104kg/mm2;

Substituting the elastic body size of the 10t sensor (c × d × h=56mm × 52mm × 35mm) into the formula, obtain:

 t1=1.78mm, t2=-25.81mm (discard).

At the same time, ½(d-h)=½(52-35)=8.5mm 》1.78mm, the size of the upper and lower beams in the I-beam in Figure 2 is much larger than the size of the vertical beam (strain beam). In the process of loading the strain beam by the sensor, the strength of the upper and lower beams is much higher than that of the strain beam. The shear stress generated by the loading does not play its due role, so that the strain beam fails to produce similar deformations, so it cannot Sufficient micro-strain occurs so that the output sensitivity of the sensor is much lower than the 2mV/V required by the design.

1.2, Improvement Plan

Option One:

In order to make the shear stress play a sufficient role, adjust A, d and h so that the size of the upper and lower beams in Figure 2 is as close as possible to the size of the strain beam. Knowing from the design requirements, the dimensions c and A cannot be changed, and changing d is difficult to meet our requirements. Only by reducing h can we increase t. Due to the relationship between c, too small h will add a lot of difficulties to the production of sensors, so Use the scheme of reducing the local size of the strain area of  the elastomer, changing the size of d and h, but keeping the installation size unchanged (see Figure 3).

Figure 3

According to the elastic body size scheme of the scheme in Figure 3, we put 5 BM-LS-3 sensor tests to verify the sensitivity index of the sensor. One test index of the sensor is shown in Table 1:

Table 1

Judging from the test results of the five sensors in the table, the average sensitivity is 1.2718mV/V, and the consistency is good, but it is far from the sensitivity of 2mV/V required by the design calculation; and the linearity and hysteresis indicators have good consistency.

Option II:

On the basis of Scheme 1, referring to Figure 2 of the force on the strained part of the sensor, the shear force of the elastic body does not produce enough micro-strain on the strain beam is the main reason for the low sensitivity, so we can reduce the strain beam The size of t is used to improve the ability to generate micro-strain, and the distribution of stress components is the same as that of Scheme 1. Through our experiments, the effect of this scheme is not obvious.

Option II:

From the analysis in Figure (2), we know that where y is close to "0", the closer the shear stress τ is to its maximum value τmax, can we just generate the local area of the shear stress peak and remove the rest section. Taking into account the size required for the strain gauge patch and the ease of processing the elastomer, a through hole 2-ΦB was drilled along the load direction of the sensor (Figure 4). After a simple stress distribution analysis (Figure 4, the dotted line is the second solution) Stress curve), not only the maximum stress value τ2max than τ1max has also been greatly increased, but also more concentrated.

Figure 4

1.3 Test results

Take 3 sensors from the 5 sensors in Table 1 to drill the 2-ΦB through-hole test. The sensitivity test results are shown in Table 2. The linearity and hysteresis indicators have not changed much, and are close to the values in Table 1.

Table 2

Comparing Table 1 and Table 2, the sensitivity of the same sensor has changed greatly, all increased by about 1.2mV/V. On this basis, we continue to adjust the thickness of the strain beam to make the sensitivity closer to the target value of 2mV/V. The sensitivity results of the small batch sensor test after adjustment are shown in Table 3.

Table 3

The maximum sensor sensitivity is 2.251mV/V, the minimum is 2.00243 mV/V, and the dispersion is relatively large. The reason for this phenomenon is that when the sensor elastomer is processed to drill the through hole 2-ΦB, manual scoring is used to drill the two holes. The center position, symmetry and size consistency. As long as we use high-precision drilling tooling to ensure the processing accuracy of the two through holes in the subsequent processing and production of the elastomer, the sensor sensitivity consistency will be well guaranteed.

2. Conclusion

The test results of the 3t sensor produced this time all meet the requirements of C3 level in GB/T7551-1997 "Load Cell". Summarizing the design and production process this time, it is to make full use of the concentrated area of the shear stress distribution of the elastic body. Based on the theoretical design calculation, by adjusting the structural form of the elastic body strain beam, the shear stress of the elastic body is made as much as possible. The concentration in the patch area of the strain gauge plays a great role in adjusting the sensitivity of the sensor, which will help us to design and manufacture sensors with a smaller range in the future.