A pump shaft (10; 12) and a pump member (16; 18) mountable on the shaft for rotation with the shaft have respective contact surfaces that contact when the pump member is mounted on the shaft. At least one of the contact surfaces is formed such that corrosion products of the surface are substantially incompatible with the other contact surface so that joining of the shaft and pump member at the contact surfaces by corrosion is substantially prevented. The pump shaft and pump member may be parts of a dry pump.

A water lubrication screw type air compressor includes a compressor, an air-loop system and a water-loop system. The water-loop system comprises a water supply device, a first water-loop unit and a second water-loop unit. The water supply device comprises a first water-treatment device to modify the water quality and provide purified water. The first water-loop unit comprises a second water-treatment device to modify water quality and reduce the corrosion of metal components. The second water-loop unit comprises a water filter to remove the impurities in water and maintain the purity of cycling water. Therefore, the water-loop system in the present invention comprises multiple water-treatment devices and filters to maintain the water quality. The purified water ensures better cooling, lubrication and airproofing when injected into the compressor.

The filter device is applied to a small-size air compressor. The filter device includes a base member and a cover member. The base member has a back side and a front side. The back side is for sealing and connecting to a rear end of a driving motor of the air compressor. The front side is provided with a housing space surrounded with a side wall. The base member has multiple exhaust holes, allowing the housing space communicate with the driving motor's interior. At least one filtering material is placed inside the housing space. The cover member is joined to the base member to seal the housing space. The cover member is provided multiple air intake grooves. When the air compressor starts, external air is forcibly drawn through the air intake grooves, the at least one filtering material, and the exhaust holes into the driving motor.

The filter device is applied to a small-size air compressor. The filter device includes a base member and a cover member. The base member has a back side and a front side. The back side is for sealing and connecting to a rear end of a driving motor of the air compressor. The front side is provided with a housing space surrounded with a side wall. The base member has multiple exhaust holes, allowing the housing space communicate with the driving motor's interior. At least one filtering material is placed inside the housing space. The cover member is joined to the base member to seal the housing space. The cover member is provided multiple air intake grooves. When the air compressor starts, external air is forcibly drawn through the air intake grooves, the at least one filtering material, and the exhaust holes into the driving motor.