Discussion on the design and selection of connectors in the use of Load Cell

【Summary】The load cell is called the heart component of electronic weighing system and electronic weighing instrument. The accuracy and stability of the weighing sensor are directly related to the accuracy of the weighing system. In practical use, the load cell must be combined with the corresponding connector in the weighing system to ensure the stable and reliable work of the system. The function of connector can not be underestimated in the whole weighing system. Following certain principles in design and selection has become a consensus in the industry. Scientific design and reasonable selection of connectors will achieve twice the result with half the effort.

【Key Word】The weighing Load Cell, Weighing system, Connector of load cell, Design of load cell

1:Preface

The load cell is called the heart component of electronic weighing system and electronic weighing instrument. With the rapid development of science and technology, electronic weighing system and electronic weighing instrument made by weighing sensor have been widely used in all walks of life, realizing the rapid and accurate weighing of materials. In particular, with the emergence of electronic micro processing technology and the continuous improvement of industrial production process automation, the weighing sensor has become a necessary device in process control. From the commonly used weighing instruments such as electronic pricing scale and electronic truck scale, to the batching system for mixing and distributing various raw materials, the automatic detection in the production process and the control of powder feeding quantity, etc., the weighing sensor has been applied.The load cell is installed between the carrier and the foundation mounting surface. If only do a simple connection to put into use, not only can not achieve the expected weighing accuracy, but also easy to cause damage to the weighing sensor and safety accidents. As the key component of weighing value measurement, the connection of weighing sensor directly affects the accuracy of weighing measurement results. In practical application, connectors play a more important role in resisting various interference forces.

1. When the material is added to the carrier, the bearing beam of the carrier will inevitably generate disturbance, which will cause a corner of the fulcrum. If the connection between the bearing beam of the carrier and the load cell is not flexible, it will inevitably produce "false signal", and may also cause damage to the sensor.

2. When the weighed vehicle enters the platform of the carrier, braking will produce considerable horizontal impact force. Although the limit device is installed, it is also necessary to have a suitable weighing connector to ensure quick reset. As far as possible to reduce the impact force transmitted to the load cell.

3. The coefficient of linear expansion between the carrier and the foundation installation surface is inconsistent in most cases, and the lateral force caused by the change of ambient temperature will make the load cell output its independent weighing value.

4. When large electronic weighing instrument is installed, there must be a certain deviation between the installation position of the carrier and the installation foundation surface. Therefore, the connector must ensure a certain swing angle and displacement.

5. The carrier is usually connected by welding. As time goes on, irregular distortion will be produced due to stress release, which also requires proper changes in the shape and position of the connector.

2:Problems in design and processing of connectors

How to solve the inevitable mechanical interference of load cell in use mainly depends on reasonable connector to reduce or eliminate. The concentricity, size, surface roughness and material hardness of the connector structure will affect the accuracy of the load cell. The smoothness of the connector should be high, otherwise it will deform after being stressed. Table 1 shows the deformation values of steel joints after grinding and planing. Table 2 shows the deformation of three different materials under the same roughness and 5MPa pressure. When the material of the connector is softer and the finish is poor, it will produce a great deformation under a very small load. The deformation of the connector will change the resultant point and cause the performance of the load cell to change.

Table1:deformation value of steel joint after grinding and planing

The design principle of the load cell connector is to ensure that the weighing value of the load cell can be transmitted through the load axis of the load cell, and the connector can rotate, slide and swing flexibly during the design of the connection; and it also has certain rigidity, hardness and roughness to meet the requirements of eliminating or weakening the import of non measuring value. The function and processing quality requirements of the load cell connector can be summarized as follows:

1. the weight value to be transmitted shall be accurate, and the load of the load cell shall be in line with its axis or controlled within the allowable error range.

2. when weighing and measuring, the load carrier will produce horizontal displacement or impact. The load carrier will deform irregularly due to stress release, and the load carrier will expand and expand under different temperature due to the difference of linear expansion coefficient. At this time, the connection piece shall have flexible rotation and swing, so the weighing value obtained by the weighing sensor is real and effective.

3. in the direction of force, the connection shall have certain rigidity. When loading, the smaller the deformation requirements of the connector shall be, the better.

4. the connector shall have good automatic reset function.

5. ensure certain roughness and corresponding hardness can ensure that the contact stress under large load is less than the allowable contact stress and no depression occurs. The friction force and friction moment do not affect the flexibility.

3:Design of common connectors and reference size for selection

The common weighing connectors are divided into pressing direction and pulling direction.

Most of the load cell pressure connection parts adopt the structure of column or spherical surface to realize the force value transmission, that is, the gravity generated by all the heavy objects on the carrier is transmitted to the load cell by the point contact formed by the spherical surface, so that the gravity line coincides with the ideal stress axis of the load cell. In addition, the pressing connector also has the following functions: 1. It has good automatic reset function. In the weighing system, there will always be slight lateral impact and movement when the carrier receives the heavy object (the serious influence caused by excessive impact force needs special limiting device to overcome), so that the action point of gravity on the load cell deviates from the axis, resulting in weighing error. 2. It has safety protection and fatigue resistance. The connector is located between the carrier and the load cell, and works in the state of movable connection. If there is no restraint device, the carrier will get rid of the load cell and damage the weighing system under the strong external impact. Generally, the steel ball and concave spherical surface as shown in the figure are used for loading. The design and selection principles are recommended in Table 3.

Table 3 design and selection principles of steel ball and concave spherical surface

Universal joint is commonly used in pull direction, which has excellent reset performance. It can always make the gravity line of the weight coincide with the ideal mechanical axis of the load cell, and can resist the interference of lateral force in any direction. If a rotatable device is configured at the same time, any interference torque on the gravity line can be eliminated before the load acts on the load cell. In order to ensure the safety, the thread size and length generally refer to table 4 below. Figure 2 is a schematic diagram of a commonly used pull connector.

Table 4 reference table of thread size and length

4:Conclusion

The stable and reliable work of the load cell is inseparable from the reasonable connection. According to the author's years of experience, combined with many years of market feedback information collected. Most of the problems in the field application are caused by the unreasonable selection and installation of connectors in the installation process. No matter how good the load cell is, if it does not match with the reasonable connector, the accuracy of the whole weighing system may be seriously out of tolerance or cannot be used. The industry pays more attention to the design of load cell, but often ignores the connector. The purpose of this paper is to introduce the connector, hoping to attract the attention of designers and users.