Gears are usually divided into vehicle gears and industrial gears. According to the Convention, gears with a diameter greater than 500 mm are called large gears; further subdivision, gears with a diameter between 500 and 3000 mm are called large gears, and gears with a diameter greater than 3000 mm are called super large gears. Large gears are usually industrial gears. The big gear is the key component of large-scale equipment, widely used in power generation, building materials, ships, mining machinery and other important fields, which is irreplaceable in the national economy and national defense construction. With the continuous improvement of the performance requirements of the key equipment for the large gear, the level of its processing equipment is also rapidly improved. At present, there are large hobbing machines with a machining diameter of 18 m and a module of 60 mm or more at home and abroad, and the gear processing accuracy has been greatly improved. Compared with the processing equipment, the large gear measuring equipment, especially the large gear measuring equipment, is relatively backward and even lack of necessary means. The reason is that with the increase of the parameters, size and weight of the big gear, the accuracy measurement technology has changed from quantity to quality. The main factors that cause big gear measurement to be difficult are as follows.
Gear size is large. For example, according to the conventional measurement method, the structure and volume of the measuring instrument will increase correspondingly, which brings great difficulties to the manufacture and assembly of the instrument. For example, to measure the tooth profile deviation of large gear with traditional generating method, the first problem is the manufacture of tangent long guide rail. For gears with module of 25 m m, number of teeth of 200, pressure angle of 20 ° and outer diameter of more than 5 m, the base circle is smaller than the root circle. In order to ensure that the measuring point is on the tangent line of the base circle, the probe should start to measure from the offset position. The offset is 775 mm calculated according to the method of entering the circle, plus the tooth profile development length of 148 mm. Considering the left and right tooth profiles, the travel of the tangent guide rail is greater than 1846 mm, It is very difficult to manufacture this kind of high precision long guide rail.
It is worth considering whether the turntable is needed in the measurement of the big gear. The use of turntable can reduce the measurement complexity; however, as for the large gear, its weight also brings great force to the shaft, resulting in the geometric deformation of the instrument; and the gear inertia is large, so it is difficult to accurately control the rotation and stop, and the measurement error caused by the small angle deviation θ will increase with the increase of the measured gear size R, that is to say, there is the so-called “error amplification” effect.
The measurement datum of gear should be consistent with its design and manufacturing datum. The design basis of gear is usually the positioning mandrel or inner hole. The size of large gear is large, so it is difficult to find the positioning hole with conventional measuring instruments. It is difficult to guarantee the measurement accuracy of the instrument because of the difference between the measurement datum and the design and manufacture datum.
The deformation of the gear itself is often ignored. In fact, the uneven temperature distribution inside and outside the big gear and its positioning and installation on the instrument will cause its own deformation, which will affect the measurement uncertainty, but it is difficult to estimate quantitatively.
At present, there is no corresponding sample plate in the measurement of large gear, so even if the measuring instrument is developed, its measurement value transmission is also a problem, which leads to the uncertainty of measurement, so it can be said that the measurement result is not credible. Therefore, the development of large gear template and instrument calibration technology is also one of the key technologies of large gear measurement.