Shrink fits

Table 10.2 Shaft and bearing housing diameters and their tolerances, to provide a tight or shrink fit to the bearings...

Appears suddenly at or above rotor critical speed when critical is below one-half operating speed. Increasing speed increases vibration amplitude, blit whirl frequency remains constant. When speed IS decreased, vibration disappears below where it first appeared. Fnction-indnced rotor whirl Encountered m bnilt-np rotors or rotors with shrink fits or rotor disassembly to inspect fits, increase shrink fits Coupling friction has been known to induce whirl... 

Assembly features - torqued eonneetions, shrink fits, snap fits, shear pins and... 

An effective way of assembling a machine element such as a gear or pulley to a shaft is by shrink fitting. This involves setting up a radial pressure between a shaft with a slightly larger diameter than the inside diameter of the machine element, termed an interference fit. The part is heated to allow assembly and then on cooling the pressure on the inside diameter is established which maintains its ability to transmit torque from the shaft. 

Hysteretic whirl. This type of whirl occurs in flexible rotors and results from shrink fits. When a radial deflection is imposed on a shaft, a neutralstrain axis is induced normal to the direction of flexure. From first-order considerations, the neutral-stress axis is coincident with the neutral-strain axis, and a restoring force is developed perpendicular to the neutral-stress axis. The restoring force is then parallel to and opposing the induced force. In actuality, internal friction exists in the shaft, which causes a phase shift in the stress. The result is that the neutral-strain axis and neutral-stress axis are displaced so that the resultant force is not parallel to the deflection. The... 

Some initial impulse unbalance is often required to start the whirl motion. Newkirk has suggested that the effect is caused by interfaces of joints in a rotor (shrink fits) rather than defects in rotor material. This type of whirl phenomenon occurs only at rotational speeds above the first critical. The phenomenon may disappear and then reappear at a higher speed. Some success has been achieved in reducing this type of whirl by reducing the number of separate parts, restricting the shrink fits, and providing some lockup of assembled elements. 

Self-Excited Vibration Hyslerelic whirl N> Nt Nf A equipment misaligned Shrink fits and... 

Loose assembly of bearing liner, bearing casing, or casing and support Loose rotor shrink fits Friction-induced whirl Thrust bearing damage... 

Broken hub, keys sheared Too much shrink fit on shaft... 

On slow-speed equipment, where the hub does not have to be removed for maintenance and integral hubs are not available from the supplier, a straight fit can be used with a reasonably heavy shrink—1.25 mil/ in. of diameter. Tolerances may allow this value to vary from 1.0 to 1.5 mil/in. of diameter however, 1.0 mil/in. may be loo loose for some applications, while 1.5 mil/in. over-stresses hubs at maximum bore sizes. In light of this, each application should be carefully evaluated if there is no experience at hand to use as a guide. Of course, field removal at the higher shrink fit is mnrv difficult. Keys should not be used. 

Where removal is necessary, keyed hubs on straight shafts should use 0.5 mil/in. to 0.75 mil/in. shrink fit. Caution should be exercised in these applications. 

If the hub removal is necessary, such as required on compressor with non-split seals, a tapered hub fit on the shaft should be used. The removable hubs should have tapped puller holes. The shaft should be keyless with the preferred method of installation and removal by use of hydraulic dilation. Two injection ports 180° apart should be used whether injection is through the shaft or through the hub. Shrink fits should be 2 to 2.5 mil/in. of diameter. API 671 rcL ommends 1,5 mil/in. minimum, but experience indicates the heavier shrink may be required. For the juncture rating calculation, a f ra lion value of. 12 is recommended. 

If keyed tapered shafts are furnished, as may be true with some-installations, particularly for machines under 1,000 hp with small shafts, two keys are recommended. A shrink fit of 1 to 1.5 mil/in... 

Shop performance test, 416 Shrink fitted hubs, 335 Shrouds, stator blades, 248 Sidestream arrangements, 143 Sidestream compres.sors,... 

In Section 4.2.2 the central role of atomic diffusion in many aspects of materials science was underlined. This is equally true for polymers, but the nature of diffusion is quite different in these materials, because polymer chains get mutually entangled and one chain cannot cross another. An important aspect of viscoelastic behavior of polymer melts is memory such a material can be deformed by hundreds of per cent and still recover its original shape almost completely if the stress is removed after a short time (Ferry 1980). This underlies the use of shrink-fit cling-film in supermarkets. On the other hand, because of diffusion, if the original stress is maintained for a long time, the memory of the original shape fades. 

Individual fins may be preformed and inserted over the tube, after which the mechanical bond may be obtained by either shrink fitting the fins onto the tube or by expanding the tube radially outward to make pressure contact with the fin material. The means to expand the tube may be hydraulic by pressurizing the tube beyond its yield point or it may be of a mechanical nature, in which an oversized ball or rod is pushed through the length of the tube, forcing the tube material outward against the fin. 

Long-life special alloy valves have chromium-plated stems, hardened shrink-fit valve seats, and replaceable guides. 

Figure 13.45. Stress distribution in a shrink-fitted compound cylinder (a) Due to shrinkage (h) Due to pressure...

Shrink-fitted compound cylinders are used for small-diameter vessels, such as compressor cylinder barrels. The design of shrink-fitted compound cylinders is discussed by Manning (1947) and Jawad and Farr (1989). 

The impellers can be an open-faced (stationary shroud) or closed-face (rotating shroud) design. As long as the tip velocities are below 1000 ft/s, closed-face impellers can be used. The standards allow the impellers to be welded, riveted, milled, or cast. Riveted impellers are unacceptable, especially if the impeller loading is high. Impellers are to be assembled on the shaft with a shrink fit with or without key. Shrink fits should be carefully done because excessive shrink fits can cause a problem known as hysteresis whirl. In compressors where the impellers require their thrust to be balanced, a balance drum is acceptable and preferred. 

If specified, rotors with shrink fit impellers shall have mechanical means to limit movement in the direction opposite to normal hydraulic thrust to 0,75mm (0,030 in) or less. 

The stress distribution within the thick-walled structure can be turned to better eveness by introducing appropriate benefitial pre-stresses with multilayer shrink fitting or autofrettage. 

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