Abstract: To improve reliability of a shaft sealing portion of a mechanical booster and simplify the structure, in a mechanical booster 1 having a motor 3 in tight contact with to a mechanical booster body 2 and rotating the shaft part 4, inserting a metal cylinder into a gap between a stator on a rotary shaft of the motor 3 and a fixed coil on the outside for partitioning stator area and a fixed coil area by the cylinder, supporting the both end of the rotary shaft of the motor 3 by bearings sealed with lubricant, and inserting a bearing fitting portion of a motor cover 30 into inside of the cylinder, the motor is sealed with a seal ring. The cylinder is made of stainless steel. A jacket water cooler is provided in the motor cover 30 around at area close to outside. The shaft part 4 of the mechanical booster body 2 is supported by bearings sealed with lubricant.

Abstract: The silencer 22 having an enclosed tank 23 includes an inlet port 24 positioned at one side of the tank 23, a discharge port 25 positioned at another side of the tank 23, and a partition plate 26 separating an inner space of the tank 23 into an inlet side chamber and an outlet side chamber. The partition plate 26 is formed with an orifice 27 communicating the separated chambers with each other. The partition plate 26 is also provided with a drain hole 28 communicating with both the chambers. The tank 23 is located at a position lower than the vacuum pump 12. A drain discharge line 33 may communicate with a lower portion of a casing of the vacuum pump 12. Furthermore, the tank may have a cooling water jacket 31 for cooling a wall of the tank 23.