Maintenance Fundamentals Episode 1 part 10 pdf

The material-flexing group includes laminated disk-ring, bellows, flexible shaft, diaphragm, and elastomeric couplings.. Laminated Disk-Ring The laminated disk-ring coupling consists of

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

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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

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

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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

Figure 10.6 Typical laminated disk-ring couplings.

Figure 10.7 Typical bellows coupling

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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

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

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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

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

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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

COUPLINGSELECTION

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

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 information needed to verify ratings and to identify a coupling that meets the same requirements as the original

Criteria that must be considered in part selection include equipment type, mode of operation, and cost Each of these criteria is discussed in the sections

to follow

Equipment Type

Coupling selection should be application specific, and therefore it is important to consider the type of equipment that it connects For example, demanding appli-cations such as variable, high-torque machine-trains require couplings that are specifically designed to absorb radical changes in speed and torque (e.g., metal-lic-grid) Less demanding applications such as 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

Figure 10.14 Typical hydraulic coupling

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Run-out table rolls Elastomeric flexible couplings

Vertical pump shafting Flanged rigid couplings, split rigid or

clamp couplings Keys and keyways not appropriate (e.g.,

brass shafts)

Rigid compression couplings

Transmission of proportionately high

torque at low speeds

Chain couplings (mechanical-flexing)

Transmission of proportionately high

torque at both high and low speeds

Gear couplings (mechanical-flexing)

Allowance for axial movement and

reduction of heat and axial vibration

Laminated disk-ring couplings (material-flexing)

Moderate rotational torque and shaft

speed

Bellows couplings (material-flexing)

Small equipment that does not experience

high torque loads

Flexible shaft or spring couplings (material-flexing)

Torsional stiffness while allowing flexibility

in axial movement

Diaphragm material-flexing couplings

Light- or medium-duty applications

running at moderate speeds

Elastomeric couplings (material-flexing)

Gradual acceleration and limited

maximum torque for fixed operating

speeds (e.g., compressors).

Hydraulic or fluid couplings

Variable or high torque and/or speed

transmission

Flexible couplings rated for the maximum torque requirement

(continues)

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 clutches.

Note: Rigid couplings are not designed to absorb variations in torque and speed and should not be used in such applications Maximum in-service coupling speed should be at least 15% below the maximum coupling speed rating.

Torsional Characteristics

Torque requirements are a primary concern during the selection process In all applications in which variable or high torque is transmitted from the driver to the driven unit, a flexible coupling rated for the maximum torque requirement must be used Rigid couplings are not designed to absorb variations in torque and should not be used

Speed

Two speed-related factors should be considered as part of the selection process: maximum speed and speed variation

Maximum Speed When selecting coupling type and size, the maximum speed rating must be considered, which can be determined from the vendor’s catalog The maximum in-service speed of a coupling should be well below (at least 15%) the maximum speed rating The 15% margin provides a service factor that should

be sufficient to prevent coupling damage or catastrophic failure

Speed Variation Variation in speed equates to a corresponding variation in torque Most variable-speed applications require some type of flexible coupling capable of absorbing these torsional variations

Operating Envelope

The operating envelope defines the physical requirements, dimensions, and type

of coupling needed in a specific application The envelope information should include shaft sizes, orientation of shafts, required horsepower, full range of operating torque, speed ramp rates, and any other data that would directly or indirectly affect the coupling

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Couplings must be installed properly if they are to operate satisfactorily This section discusses shaft and coupling preparation, coupling installation, and alignment

Shaft Preparation

A careful inspection of both shaft ends must be made to ensure that no burrs, nicks, or scratches are present that will damage the hubs Potentially damaging conditions must be corrected before coupling installation Emery cloth should be used to remove any burrs, scratches, or oxidation that may be present A light film of oil should be applied to the shafts prior to installation

Keys and keyways also should be checked for similar defects and to ensure that the keys fit properly Properly sized key stock must be used with all keyways; do not use bar stock or other material

Coupling Preparation

The coupling must be disassembled and inspected prior to installation The location and position of each component should be noted so that it can be reinstalled in the correct order When old couplings are removed for inspection, bolts and bolt holes should be numbered so that they can be installed in the same location when the coupling is returned to service

Any defects, such as burrs, should be corrected before the coupling is installed Defects on the 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