Aiming at the limitations of desktop measuring instrument, the upper measuring instrument has a great change in the measurement idea. The instrument is generally positioned on the tooth surface or tooth top of the measured gear and supported on the measured gear to complete the measurement of multiple errors of the gear. In terms of measurement principle, there can be base disc generation method, coordinate method, arc reference method or straight line reference method, etc.
For example, f-300 and f-360 top mounted tooth profile testers of Maag company in Switzerland adopt the base disc generation method, use the involute generation principle, take a sector plate installed concentrically with the measured gear and the radius is equal to the radius of the measured gear as the benchmark, and check the measured tooth profile error through the probe on the ruler with pure rolling: es-430 top mounted tooth profile tester of Maag company in Switzerland Harbin Measuring Tool and cutting tool factory has developed 32801921381 tooth profile measuring instrument, etc., which adopts rectangular coordinate method to measure the X and Y coordinate values of each point on the tooth surface, and obtains the tooth profile error through data processing; Goulder-milkron-a.e.i top mounted tooth profile gauge, etc. in Britain, adopts the arc reference method, and uses a certain arc curve to replace the involute for measurement. The arc center is generally selected as the curvature center of the tooth surface node, and the arc radius is the curvature radius of the node. This principle is suitable for measuring gears with low accuracy, small modulus and large number of teeth, and for measuring gears with slightly higher accuracy, The principle error of the measurement results can be compensated; The bridge type large gear measuring instrument of gubishev giant bulldozer factory of the Soviet Union.
The straight line datum method is used to replace the theoretical involute profile of the gear to be measured with a specific straight line to measure the actual profile. Because the principle error of this method is large, it is necessary to compensate the principle error of the measurement results. This principle is generally only used for the measurement of gears with low accuracy.
The upper measuring instrument is small in size and low in cost, which can ensure the measurement of certain accuracy. The disadvantage is that this kind of measuring instrument generally establishes the measurement datum on the gear to be measured with unknown error, so the positioning accuracy is low, which leads to the low accuracy of lobe measurement.