Mechanical Flexing Mechanical-flexing couplings provide a flexible connection by permitting the coupling components to move or slide relative to each other. To permit such movement, clearance must be provided within specified limits. It is import- ant to keep cross loading on the connected shafts at a minimum. This is accomplished by providing adequate lubrication to reduce wear on the coupling components. The most popular of the mechanical-flexing type are the chain and gear couplings. Chain Chain couplings provide a good means of transmitting proportionately high torque at low speeds. Minor shaft misalignment is compensated for by means of clearances between the chain and sprocket teeth and the clearance that exists within the chain itself. Figure 10.2 Typical split rigid coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 174 174 Maintenance Fundamentals The design consists of two hubs with sprocket teeth connected by a chain of the single-roller, double-roller, or silent type. A typical example of a chain coupling is illustrated in Figure 10.4. Special-purpose components may be specified when enhanced flexibility and reduced wear are required. Hardened sprocket teeth, special tooth design, and barrel-shaped rollers are available for special needs. Light-duty drives are some- times supplied with non-metallic chains on which no lubrication should be used. Gear Gear couplings are capable of transmitting proportionately high torque at both high and low speeds. The most common type of gear coupling consists of two identical hubs with external gear teeth and a sleeve, or cover, with matching internal gear teeth. Torque is transmitted through the gear teeth, whereas the necessary sliding action and ability for slight adjustments in position comes from a certain freedom of action provided between the two sets of teeth. Slight shaft misalignment is compensated for by the clearance between the matching gear teeth. However, any degree of misalignment decreases the useful life of the coupling and may cause damage to other machine-train components such as bearings. A typical example of a gear-tooth coupling is illustrated in Figure 10.5. Figure 10.3 Typical compression rigid coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 175 Couplings 175 Material Flexing Material-flexing couplings incorporate elements that accommodate a certain amount of bending or flexing. The material-flexing group includes laminated disk-ring, bellows, flexible shaft, diaphragm, and elastomeric couplings. Various materials such as metal, plastic, or rubber are used to make the flexing elements in these couplings. The use of the couplings is governed by the oper- ational fatigue limits of these materials. Practically all metals have fatigue limits that are predictable; therefore, they permit definite boundaries of operation to be established. Elastomers such as plastic or rubber, however, usually do not have a well-defined fatigue limit. Their service life is determined primarily by conditions of installation and operation. Roller-chain Coupling Coupling Cover (½ Shown) (Optional) Roller Chain 1 Required to Join Couplers Coupling Body(s) 1 Required for Each Shaft Figure 10.4 Typical chain coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 176 176 Maintenance Fundamentals Laminated Disk-Ring The laminated disk-ring coupling consists of shaft hubs connected to a single flexible disk, or a series of disks, that allows axial move- ment. The laminated disk-ring coupling also reduces heat and axial vibration that can transmit between the driver and driven unit. Figure 10.6 illustrates some typical laminated disk-ring couplings. Bellows Bellows couplings consist of two shaft hubs connected to a flexible bellows. This design, which compensates for minor misalignment, is used at moderate rotational torque and shaft speed. This type of coupling provides flexibility to compensate for axial movement and misalignment caused by ther- mal expansion of the equipment components. Figure 10.7 illustrates a typical bellows coupling. Flexible Shaft or Spring Flexible shaft or spring couplings are generally used in small equipment applications that do not experience high torque loads. Figure 10.8 illustrates a typical flexible shaft coupling. Diaphragm Diaphragm couplings provide torsional stiffness while allowing flexibility in axial movement. Typical construction consists of shaft hub flanges Figure 10.5 Typical gear-tooth coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 177 Couplings 177 Figure 10.6 Typical laminated disk-ring couplings. Figure 10.7 Typical bellows coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 178 178 Maintenance Fundamentals and a diaphragm spool, which provides the connection between the driver and driven unit. The diaphragm spool normally consists of a center shaft fastened to the inner diameter of a diaphragm on each end of the spool shaft. The shaft hub flanges are fastened to the outer diameter of the diaphragms to complete the mechanical connection. A typical diaphragm coupling is illustrated in Figure 10.9. Elastomeric Elastomeric couplings consist of two hubs connected by an elasto- meric element. The couplings fall into two basic categories, one with the element placed in shear and the other with its element placed in compression. The coupling compensates for minor misalignments because of the flexing capability Figure 10.8 Typical flexible shaft coupling. Figure 10.9 Typical diaphragm coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 179 Couplings 179 of the elastomer. These couplings are usually applied in light- or medium-duty applications running at moderate speeds. With the shear-type coupling, the elastomeric element may be clamped or bonded in place, or fitted securely to the hubs. The compression-type couplings may be fitted with projecting pins, bolts, or lugs to connect the components. Polyurethane, rubber, neoprene, or cloth and fiber materials are used in the manufacture of these elements. Although elastomeric couplings are practically maintenance free, it is good practice to periodically inspect the condition of the elastomer and the alignment of the equipment. If the element shows signs of defects or wear, it should be replaced and the equipment realigned to the manufacturer’s specifications. Typ- ical elastomeric couplings are illustrated in Figure 10.10. Combination (Metallic-Grid) The metallic-grid coupling is an example of a combination of mechanical-flexing and material-flexing type couplings. Typical metallic-grid couplings are illus- trated in Figure 10.11. The metallic-grid coupling is a compact unit capable of transmitting high torque at moderate speeds. The construction of the coupling consists of two flanged hubs, each with specially grooved slots cut axially on the outer edges of the hub flanges. The flanges are connected by means of a serpentine-shaped spring grid that fits into the grooved slots. The flexibility of this grid provides torsional resilience. Special Application Couplings Two special application couplings are discussed in this section: (1) the floating- shaft or spacer coupling and (2) the hydraulic or fluid coupling. Floating-Shaft or Spacer Coupling Regular flexible couplings connect the driver and driven shafts with relatively close ends and are suitable for limited misalignment. However, allowances sometimes have to be made to accommodate greater misalignment or when the ends of the driver and driven shafts have to be separated by a considerable distance. Such is the case, for example, with end-suction pump designs in which the power unit of the pump assembly is removed for maintenance by being axially moved toward the driver. If neither the pump nor the driver can be readily removed, they should be separated sufficiently to permit withdrawal of the pump’s power unit. An easily removable flexible coupling of sufficient length (i.e., floating-shaft or Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 180 180 Maintenance Fundamentals spacer coupling) is required for this type of maintenance. Examples of couplings for this type of application are shown in Figure 10.12. In addition to the maintenance application described above, this coupling (also referred to as extension or spacer sleeve coupling) is commonly used where equipment is subject to thermal expansion and possible misalignment because of high process temperatures. The purpose of this type of coupling is to prevent harmful misalignment with minimum separation of the driver and driven shaft ends. An example of a typical floating-shaft coupling for this application is shown in Figure 10.13. Figure 10.10 Typical elastomeric couplings. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 181 Couplings 181 The floating-shaft coupling consists of two support elements connected by a shaft. Manufacturers use various approaches in their designs for these couplings. For example, each of the two support elements may be of the single-engagement type, may consist of a flexible half-coupling on one end and a rigid half-coupling on the other end, or may be completely flexible with some piloting or guiding supports. Floating-shaft gear couplings usually consist of a standard coupling with a two- piece sleeve. The sleeve halves are bolted to rigid flanges to form two single-flex Figure 10.11 Typical metallic-grid couplings. Figure 10.12 Typical floating-shaft or spacer couplings. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 182 182 Maintenance Fundamentals couplings. An intermediate shaft, which permits the transmission of power be- tween widely separated drive components, connects these. Hydraulic or Fluid Hydraulic couplings provide a soft start with gradual acceleration and limited maximum torque for fixed operating speeds. Hydraulic couplings are typically used in applications that undergo torsional shock from sudden changes in equipment loads (e.g., compressors). Figure 10.14 is an illustration of a typical hydraulic coupling. COUPLING SELECTION Periodically, worn or broken couplings must be replaced. One of the most important steps in performing this maintenance procedure is to ensure that the correct replacement parts are used. After having determined the cause of failure, it is crucial to identify the correct type and size of coupling needed. Even if Figure 10.13 Typical floating-shaft or spacer couplings for high-temperature applica- tions. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 7:35pm page 183 Couplings 183 [...]... calculations DETERMINING KEYWAY DEPTH AND WIDTH The formula given below and Figure 10 .17 , Table 10 .1 (square keys), and Table 10 .2 (flat keys) illustrate how the depth and width of standard square and flat keys and keyways for shafts and hubs are determined TOP VIEWS SIDE VIEWS Square Ends Plain Taper Square and Round Rounded Ends Figure 10 .16 Key shapes Gibe Head Taper ... maximum torque requirement (continues) 18 6 Maintenance Fundamentals Table 10 .1 (continued) Application Coupling* Selection Recommendation Greater Misalignment Compensation Maintenance requiring considerable distance between the driver and driven shaft ends Floating-shaft or spacer couplings Misalignment results from expansion due to high process temperatures *See Table 10 .6 for an application overview for... L L Rotary Cutter End Mill Cutter Figure 10 .15 Keyed shaft Key size versus shaft diameter Couplings 19 3 Figure 10 .16 shows various key shapes: square ends, one square end and one round end, rounded ends, plain taper, and gibe head taper The majority of keys are square in cross-section, which are preferred through 4 -1 2 -in diameter shafts For bores over 4 -1 2 in and thin wall section of hubs, the.. .18 4 Maintenance Fundamentals Figure 10 .14 Typical hydraulic coupling practically identical in appearance to the original, a part still may not be an adequate replacement The manufacturer’s specification number usually provides the information needed for part selection If the part is not in stock, a cross-reference guide will provide the... run-out table rolls can generally get by with elastomeric couplings Table 10 .1 lists the coupling type commonly used in a particular application Mode of Operation Coupling selection is highly dependent on the mode of operation, which includes torsional characteristics, speed, and the operating envelope Couplings 18 5 Table 10 .1 Coupling Application Overview Application Coupling* Selection Recommendation... mating parts of the coupling can cause interference between the bore and shaft, preventing proper operation of the coupling Coupling Installation Once the inspection shows the coupling parts to be free of defects, the hubs can be mounted on their respective shafts If it is necessary to heat the hubs to achieve the proper interference fit, an oil or water bath should be used Spot 18 8 Maintenance Fundamentals. .. application couplings such as spindles and jackshafts, all couplings must be aligned within relatively close tolerances (i.e., 0.0 01 0.002 in.) LUBRICATION AND MAINTENANCE Couplings require regular lubrication and maintenance to ensure optimum trouble-free service life When proper maintenance is not conducted, premature coupling failure and/or damage to machine-train components such as bearings can be expected... coupling type Because rigid couplings do not require lubrication, this section discusses lubrication requirements for mechanical-flexing, material-flexing, and combination flexible couplings only 19 0 Maintenance Fundamentals Mechanical-Flexing Couplings It is important to follow the manufacturer’s instructions for lubricating mechanical-flexing couplings, which must be lubricated internally Lubricant seals... and/or fatigue In addition to the items for mechanical-flexing couplings presented in Chapter 2, the grid members on metallic-grid couplings should be replaced if any signs of wear are observed 19 2 Maintenance Fundamentals Rigid Couplings The mechanical components of rigid couplings (e.g., hubs, bolts, compression sleeves and halves, keyways, and keys) should be visually inspected for cracks, breaks,... to hubs Key seating is the actual machine operation of producing key seats Keyways are normally made on a key eater or by a broach Key seats are normally made with a rotary or end mill cutter Figure 10 .15 is an example of a keyed shaft that shows the key size versus the shaft diameter Because of standardization and interchangeability, keys are generally proportioned with relation to shaft diameter . application is shown in Figure 10 .13 . Figure 10 .10 Typical elastomeric couplings. Keith Mobley /Maintenance Fundamentals Final Proof 15 .6.2004 7:35pm page 18 1 Couplings 18 1 The floating-shaft coupling. single-flex Figure 10 .11 Typical metallic-grid couplings. Figure 10 .12 Typical floating-shaft or spacer couplings. Keith Mobley /Maintenance Fundamentals Final Proof 15 .6.2004 7:35pm page 18 2 18 2 Maintenance Fundamentals couplings operating envelope. Figure 10 .14 Typical hydraulic coupling. Keith Mobley /Maintenance Fundamentals Final Proof 15 .6.2004 7:35pm page 18 4 18 4 Maintenance Fundamentals Table 10 .1 Coupling Application