The performance curve of vacuum pump unit is used to express the conversion characteristics of water flow energy under different working conditions, which is the external manifestation of the internal flow law of the pump. Like ordinary centrifugal pump, the constant speed characteristic curve of engine cooling water pump of vacuum pump unit is the relationship curve of flow and head, flow and efficiency, and flow and power at a certain speed, As shown in Figure 3a, it can directly describe the operation performance of the engine cooling water pump at constant speed. However, because the speed of the engine cooling water pump is constantly changing when it works, the variable speed characteristic curve of the pump must be given, as shown in Figure 3B. The variable speed characteristic curve of the engine cooling water pump mainly maps the flow, head and power curves corresponding to different speeds, In order to reflect the comprehensive performance of engine cooling water pump more intuitively, it is necessary to draw the two performance curves on the same graph to show the performance parameters, which increases the hydraulic loss and reduces the efficiency. At the same time, the cavitation phenomenon of engine cooling water pump is more likely to occur under high temperature, high speed and large flow conditions.At the same time, the flow in the top clearance of the semi open impeller causes the main flow to collapse and the flow velocity in the impeller passage to increase, which worsens the cavitation characteristics of the impeller and seriously affects the normal operation of the cooling system, The special working environment and matching condition decide that the design of engine cooling water pump is different from ordinary centrifugal pump. As the "heart" of cooling system, engine cooling water pump works in harsh environment with extremely limited space, The working life of the pump should be equal to or more than that of the engine overhaul period. Therefore, the reliability requirements of the engine cooling water pump and its components, such as water seal, bearing, pump shaft and impeller, are very high, so it is necessary to realize the same service life of the pump. However, due to the compact assembly structure, the shaft bearing is widely used in the engine cooling water pump to replace the common shaft and bearing combination in the centrifugal pump, The axial force produced by cooling fan and pump impeller will also change with the constant change of rotating speed. The clearance between pump shaft and support will increase noise and vibration, which will have an adverse impact on the operation performance and water seal work of the pump.