The measurement basis of universal testing machine and its accuracy evaluation parameters

Universal testing machineTheir own fault may cause the accuracy of the test machine to not meet the requirements. Therefore, during the actual measurement of the test machine, the factors that may affect its accuracy must be eliminated and the error of the test machine must be controlled within a reasonable range.
There are two common test machines, one is the traditional hydraulic pendulum test machine, and the other isElectronic testing machine.
There are many types of electronic universal testing machines, but whether it is a microcomputer-controlled electronic testing machine or a simplified test machine to reduce costs, its basic principle of force measurement is the load measurement unit of its closed-loop measurement and control system, the sensor collects signals, and reacts them through the instrument through a series of changes such as instrument amplifiers. Therefore, the accuracy of the electronic test machine can generally be corrected through the software's own calibration procedure.
1. Structure and working principle of hydraulic pendulum tester
The structure and working principle of the hydraulic universal material testing machine are the prerequisites for correctly analyzing the factors influencing its accuracy. The main unit and the dynamometer are the main structures of the hydraulic pendulum test machine.

The dynamometer part of the universal testing machine uses a hydraulic tangent pendulum force measuring mechanism. Through the Pascal principle, a large inconvenient force is turned into a smaller measurable force, and then this force is indicated through certain means (dial).
During loading, the oil pump motor is turned on, the oil supply valve 17 is opened, and the oil pump sends the oil into the working oil cylinder 4 and pushes the working piston 5 to load the sample; at the same time, the oil enters the force measuring cylinder 22 through the return oil pipe 19 and the force measuring oil pipe 21 (at this time the return oil valve 20 is closed, and the oil cannot flow back to the oil tank), and enters the force measuring oil cylinder 22, pressing the force measuring piston 23, causing it to drive the pull rod 24 to move downward, thereby forcing the swing rod 26 and the pendulum 25 to deflect about the fulcrum point with the push rod 27.
When the push rod is deflected, the toothed rod 28 is pushed horizontally, and the pointer gear of the indices disc is driven to rotate the indices pointer 29 about the center of the indices disc 30 . The angle of rotation of the force indicator is proportional to the total pressure on the force cylinder piston (ie, the tension force exerted by the tie rod 24). Because the oil pressure in the force measuring cylinder and the working cylinder is the same, the total pressure on the pistons of the two cylinders is proportional (the ratio of piston area). In this way, the angle of the force pointer is proportional to the total pressure on the working cylinder piston, that is, the load on the sample. After calibration, the pointer can directly indicate the load magnitude on the indicator dial.

Universal material testing machines are generally equipped with pendulums of different weights and are available. For pendulums with different weights, the hydraulic pressure required is not the same, that is, the load is not the same. Therefore, the force range indicated by the scale on the display dial should match the weight of the pendulum. Taking the universal material testing machine as an example, it is equipped with three pendulums: A, B, and C. The force measurement range corresponding to the pendulum A is 0~60kN, A+B corresponds to 0~150kN, and A+B+C corresponds to 0~300kN.
When starting the oil pump motor, the size of the oil supply valve can adjust the speed of the oil entering the working oil cylinder, so it can be used to control the speed of increasing load. Opening the oil return valve 20 can cause the oil in the working oil cylinder to return to the oil tank 37 through the oil return pipe 19, thereby removing the load on the sample.
Before the test begins, in order to eliminate the influence of the weight of the movable frame, the oil should be started to pump oil and the movable platform should be slightly raised. Then adjust the balance thallium 31 of the force measuring part to keep the swing rod vertically, and make the force pointer point at zero point.

2. Evaluation parameters of measurement basis of universal material testing machine and its accuracy
The calibration regulations are the basis for the measurement department to carry out calibration work. The calibration procedures are also the main reference basis when carrying out calibration work.
The calibration of hydraulic universal material testing machines is generally referred to the "JJG139-1999 Tension, Pressure and Universal Material Test Machine Calibration Regulations", and appropriate adjustments are made according to the on-site situation. See the appendix for details.
Commonly used indicators representing the measurement results of universal material testing machines include indicator relative errors and indicator repeatability errors. In addition, there are uncertainties. This part will be studied in the next chapter.
If the reading methods of the measurement universal material test machine are different, the calculation methods of the display value error will also be different, and can be divided into the following two reading methods.
This reading method is based on the universal material testing machine indicator device and read the number on the standard force measuring instrument.
The second reading method is to read the data on the universal material testing machine indicator based on the standard value of the standard force measuring instrument.

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