Shafts, rotating, sealing application

Disk Filters A disk filter is a vacuum filter consisting of a number of vertical disks attached at intervals on a continuously rotating horizontal hollow central shaft (Fig. 18-127). Rotation is by a gear drive. Each disk consists of 10 to 30 sectors of metal, plastic, or wood, ribbed on both sides to support a filter cloth and provide drainage via an outlet nipple into the central shaft. Each sector may be replaced individually. The filter medium is usually a cloth bag slipped over the sectors and sealed to the discharge nipple. For some heavy-duty applications on ores, stainless-steel screens may be used. 

With close-clearance seals it is important that the shaft be closely maintained in its rotating position flexible shaft design (operation above the first lateral critical) is usually not acceptable. Bearings that maintain the closest alignment of the shaft are obviously the best for such applications and, for this purpose, close-clearance journal-type bearings are used. 

The coefficient of thermal expansion must also be accounted for in design calculations. The normal operating temperature for transmission applications is 120°C. A material must be able to operate at this temperature and not expand to a greater volume than the mating parts allow. The coefficient of friction (p) of the material should be considered as well when designing a dynamic seal. Rotating shafts require a lower value for p in order to reduce wear. 

Where conventional lubricants cannot be applied for reasons of incompatibility with the working fluid at operational conditions of temperature and pressure, it has been found necessary to develop alternative methods, and dry lubrication" using solid lubricants has been adopted for cryogenic applications. Components that lend themselves to.this treatment are bearings and seals for rotating shafts and pistons and cylinders in reciprocating systems. 

Packing is used to seal shafts in a variety of applications. In equipment where the shaft is not continuously rotating (e.g., valves), packed boxes can be used successfully without any leakage around the shaft. In rotating applications, such as pump shafts, the application must be able to tolerate... 

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