Abstract: Abstract: The gear pumps are used for pumping the working fluid, and its key component is a pair of gear pairs. In the power transmission, the backlash of the gear pair is used to form the lubricating oil film to avoid sticking due to the friction and heat expansion of the gear teeth, but it also affects the stability of the oil film. The choice of backlash in the gear pump is also limited. The backlash of the gear has an influence on the trapped oil performance and volumetric efficiency of the gear pump, while the definition whether there is a backlash existed and the definition what is large backlash and what is small backlash is vague. Based on the common requirements of different backlash values to transmission performance and trapped oil performance, the special trapped-oil circulation and trapped-oil process were analyzed in this study, and the emphasis was on the double teeth meshing range and the single tooth meshing range. From the connection aspect of two different trapped-oil volumes in double teeth meshed range and the improvement aspect of trapped oil performance in single tooth meshed range, we used to separately calculate dynamic trapped-oil flow rate and its maximum value under the two different ranges of double teeth meshing and single tooth meshing, as well as the different formulas to separately calculate dynamic backlash values, its mean value, its lower limiting value used for the connection in double teeth meshed range, and the relief in single tooth meshed range. From such exercises, we derived the definition what was large backlash and what was small backlash. The backlash therefore was defined as, a small backlash was when the backlash value was less than the maximum peak of the backlash for connection, and when the backlash value was greater than the maximum peak of the backlash for connection and less than the maximum peak of the backlash for trapped oil relief, it was a large backlash, and when the backlash value was greater than the maximum peak of the backlash for trapped oil relief, it belonged to the large backlash. An instance of an external gear pump which backlash was 30, 50, 200 μm, was operated and its operation results were analyzed by the theory we developed. The results showed that when the trapped oil flow peak ratio of the unloading area and the connected area was 3, the unloading burden of the former was large. In fact, the really communicating to the communication area required up 2.41 mm backlash, which did not actually exist. So the gear pairs used in gear pumps was the gear pairs with backlash forever. The absolute connection of each other of two different trapped-oil volumes in double teeth meshed range and the absolute trapped-oil relief in single tooth meshed range were nonexistent, but only relatively existed under a certain permission pressure difference. As long as the trapped-oil relief requirement in single tooth meshed range was satisfied by an adopted backlash value, then the connection of two different trapped-oil volumes each other would naturally be met by the adopted backlash value. Backlash for trapped oil relief was larger than the backlash for connection, which can be used in definition what was large backlash and what was small backlash, and the backlash for trapped oil relief can be used as the upper limit. The error in the calculation and experiment was 7.5%, which was reasonable, and the upper limit of the safety margin was 20%, which was reliable. This research provided a reference for distinguishing large backlash between small backlash by Pump backlash defining, and which also provided a theoretical basis for the subsequent studies.