codeproject vb.net barcode generator FIGURE 21.6 Cross section of a face seal for severe operating environments. in Software

Printer EAN-13 in Software FIGURE 21.6 Cross section of a face seal for severe operating environments.

FIGURE 21.6 Cross section of a face seal for severe operating environments.
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SEALS 21.8
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BEARINGS AND LUBRICATION
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FIGURE 21.7 Metal seal ring application on a rotating shaft.
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FIGURE 21.8 Stuffing box for a rotating shaft.
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Packing material is usually obtained in straight lengths of square or rectangular cross section. Pieces are cut off and formed into rings that fit the stuffing box. The choice of packing material depends on the fluid to be sealed. Available packing materials include artificial fibers, asbestos, cotton, graphite, jute, leather, and metals. The metal packings are used for temperature conditions where the other materials are inadequate. The metal packings are formed from foil which is compressed into the proper packing shape. In the design of stuffing boxes, small clearances are provided between the shaft and surrounding parts. The small clearances minimize extrusion of the packing into the clearance spaces. Valve stems undergo a helical motion rather than a rotary motion when the valve is opened or closed. Investigations into the prevention of valve leakage and wear of valve stems resulted in a procedure for establishing packing dimensions for valve stems [21.3].
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SEALS 21.9
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21.2.6 Noncontacting Seals Frictional losses occur with sealing methods that utilize physical contact between a rotating and a stationary part. With high rubbing velocities, friction losses may be a significant factor. Those losses can be eliminated by using a seal that does not require physical contact. A noncontacting seal, however, cannot prevent leakage completely, although it does reduce it to a tolerable level. One method of achieving a low leakage rate is to provide a very small clearance between the shaft and the surrounding housing or bushing. The longer the lowclearance passage, the greater the reduction in leakage. A type of noncontacting seal called the labyrinth seal (Fig. 21.9) is used on such machines as large blowers and steam turbines. It can be used to retain lubricant in the bearings or to seal the working fluid in the machine. Effectiveness of the seal depends on small clearances between the seal and the shaft. In sealing the working fluid, the small clearances create a series of pressure drops between the working fluid and the atmosphere. Labyrinth seals are usually made from a relatively soft metal such as aluminum or bronze so that the shaft is not damaged if contact between shaft and seal occurs. The simplest type is shown in Fig. 21.9, but other types are also used.
21.3 SEALS FOR RECIPROCATING MOTION
Some of the sealing methods used for rotary motion are also satisfactory for reciprocating motion. O-rings, compression packings, metal sealing rings, and some additional types are used to seal reciprocating rods, shafts, and pistons.
21.3.1 O-Rings The O-ring is used extensively because of the low installed cost and effectiveness as a seal. It is well adapted to sealing reciprocating motion as well as for use as a static seal. Figure 21.10 shows applications on a piston and piston rod as well as a static seal application. Many such applications are for hydraulic cylinders in which the hydraulic oil acts as the lubricant for the O-rings.
FIGURE 21.9 Labyrinth type of noncontacting seal.
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SEALS 21.10
BEARINGS AND LUBRICATION
FIGURE 21.10 Applications of O-rings as a static seal and as seals for reciprocating motion.
Rectangular-section rings are not suited for reciprocating motion and are used only in static applications. The shape of the groove for circular-section O-rings for sealing reciprocating motion is the same as that for static applications (Fig. 21.2). The recommended groove depth E, however, is slightly different for reciprocating motion. Recommended dimensions are available in SAE J120a and in the manufacturers literature. (a) An O-ring must seal the clearance space between the reciprocating and stationary parts, for example, between the piston and the cylinder in Fig. 21.10. The amount of clearance that can be permitted depends on the pressure differential across the O-ring and the ring hardness. If the clearance is too great, the O-ring is extruded into the clearance space (Fig. 21.11a). The reciprocating motion then tears away small pieces of the O-ring, which results in a leaking seal and contamination of the working fluid by the (b) O-ring particles. FIGURE 21.11 (a) Extrusion of O-ring into The pressure limitations of O-rings clearance space due to pressure. (b) Use of can be overcome by the use of backup backup ring to prevent extrusion. rings (Fig. 21.11b) or other devices that prevent O-ring extrusion. Backup rings are made from leather, plastics, or metal. The metal rings are split like a piston ring for radial compression during assembly into the cylinder.
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