The characteristics and measurement difficulties of large-size gears are analyzed, which can be divided into the following aspects:
The size of the measured gear is very large, which makes the volume and weight of the measuring device very large, even to the extent that it is difficult to accept in Engineering (for example, the structural size of the desktop measuring instrument for gears with diameters of 5m and 8m will reach nearly 10m). In order to find a way to solve this problem, people adopt the idea of “leeches gnawing at bones”, and adopt upper and side measuring instruments, such as the currently applied measuring instruments es-430 and es-401, which significantly reduce the structural size of the measuring instrument. Even so, considering the high stiffness of the instrument structure, the structure of the top mounted and side mounted measuring instruments can not be very small.
If the size and parameters of the measured gear are too large, not only the supporting structure of the measuring device is too large, but also the structure of the corresponding moving pair (guide rail, bearing, etc.) and measuring element (such as grating, etc.) must be large, which brings great difficulties to its own manufacturing. For example, the high-precision manufacturing of the base disc and ruler on the tooth swing measuring instrument produced by Maag company in the early years is a difficult problem, which limits its structure
Due to the large structure and size of the measuring instrument, it is very difficult to install and adjust the measuring instrument. The use of crane solves the moving problem, but it is difficult to achieve accurate alignment and adjustment.
(2) Small dividing angle and large number of teeth
The number of teeth of large diameter gears is generally large, and the corresponding tooth splitting angle is small. For example, for gears with 200 teeth and module 20, the clamping angle of each tooth is 1.8 °, the involute tooth profile feature is close to a straight line, and the expansion angle section of each tooth from tooth root to tooth size is small. This is different from large module gears (the angle skin clamped by each tooth of large module gears is generally more than 30 °). Of course. The involute tooth profile from the tooth top to the tooth root is close to a straight line or circular arc, and has its positive aspect. For example, some measuring instruments are manufactured by using this feature. The negative side is that the measurement efficiency is low and the measurement accuracy is not high.
(3) Difficult selection of measurement datum
The theoretical datum of gear measurement is generally its design datum. For large module gears, because they are generally shaft gears, their center hole can be directly used as the measurement datum. For large-size gears, the design basis is the positioning hole or positioning stop. It is very difficult to use the positioning hole or positioning stop as the measurement basis. The positioning datum of the top mounted measuring instrument such as es-430 is the measured tooth surface, and the error of the measured tooth surface leads to the measurement datum error and affects the measurement accuracy. When using the rectangular coordinate method for measurement, it is required that the workpiece coordinate system has a strict alignment relationship with the instrument coordinate system. Due to the large structure of the measured gear, the datum conversion error from the workpiece coordinate system to the instrument coordinate system is also great.
(4) The problem of inertia in measurement is prominent
Because the freight of large-size gear is so large that it is unconventional, and in order to ensure the commercial degree of the measuring instrument, the size of its moving parts is also large, which makes the whole measuring system have great inertia and can not adapt to the frequent start and stop in the measurement. Even if it is reluctantly realized, it is difficult to achieve accurate positioning. This problem is particularly prominent in the pitch measurement, Under the influence of inertia, the large measured gear can not be accurately and frequently divided, and the tracking measurement of the probe can only be carried out under the condition of continuous rotation, which brings great difficulties to the control of the measurement system. In tooth direction measurement. The measuring motion shall be the combination of the rotation of the gear and the movement of the probe along the gear axis, that is, the rotation angle of the gear and the displacement of the probe shall meet the relationship of the gear helix angle. One disadvantage is that when the helix angle of the gear is small and the gear rotates a small angle, the probe will move a large displacement, which makes the tracking control of the probe very difficult. The more effective method is to reduce the speed of the measured gear, but it can cause the problem of low-speed crawling of the worktable.
(5) The problem of low measurement efficiency is prominent
The alignment and adjustment operation in large gear measurement should be said to be complex, coupled with high accuracy requirements, so the adjustment takes a long time, and improving the automation of the measurement process is an important content to be considered in the instrument design.
To sum up, the characteristics of large parameters, large size and too heavy weight of large gears lead to many difficulties in accuracy measurement. The purpose of this paper is to analyze the characteristics and difficulties of measuring the maximum of large gears, and to provide technologies and methods that can effectively solve the difficult problems of measuring.