An introduction to Shaft coupling types

An introduction to Shaft coupling types 

When two shafts are joined directly together, the joint is called a coupling.Shaft couplings are used to transfer drive between two (normally co-axial) shafts. They allow rigid, or slightly flexible
Couplings are designed to provide two functions:
 (1) to transmit torsional power between a power source and driven unit
 (2) to absorb torsional variations in the drive train.
They are not designed to correct misalignment between two shafts. While certain types of couplings provide some correction for slight misalignment, reliance on these devices to obtain alignment is not recommended
Rigid Couplings
A rigid coupling permits neither axial nor radial relative motion between the shafts of the driver and driven unit. When the two shafts are connected solidly and properly, they operate as a single shaft. A rigid coupling is primarily used for vertical applications, e.g., vertical pumps. Types of rigid couplings discussed in this section are flanged, split, and compression.
Flanged couplings are used where there is free access to both shafts. Split couplings are used where access is limited on one side. Both flanged and split couplings require the use of keys and keyways. Compression couplings are used when it is not possible to use keys and keyways
1- Flanged Couplings
A flanged rigid coupling is comprised of two halves, one located on the end of the driver shaft and the other on the end of the driven shaft. These halves are bolted together to form a solid connection. To positively transmit torque, the coupling incorporates axially fitted keys and split circular key
rings or dowels, which eliminate frictional dependency for transmission. The use of flanged couplings is restricted primarily to vertical pump shafts. A typical flanged rigid coupling is illustrated in Figure 1

FIG. 1

2- Split Couplings
A split rigid coupling, also referred to as a clamp coupling, is basically a sleeve that is split horizontally along the shaft and held together with bolts. It is clamped over the adjoining ends of the driver and driven shafts, forming a solid connection. Clamp couplings are used primarily on vertical pump shafting. A typical split rigid coupling is illustrated in Figure .2. As with the flanged coupling, the split rigid coupling incorporates axially fitted keys and split circular key rings to eliminate frictional dependency in the transmission of torque.

FIG. 2

3- Compression Coupling
A rigid compression coupling is comprised of three pieces: a compressible core and two encompassing coupling halves that apply force to the core. The core is comprised of a slotted bushing that has been machine bored to fit both ends of the shafts. It also has been machined with a taper on
its external diameter from the center outward to both ends.
The coupling halves are finish bored to fit this taper. When the coupling halves are bolted together, the core is compressed down on the shaft by the two halves, and the resulting frictional grip transmits the torque without the use of keys. A typical compression coupling is illustrated in Figure .3.

FIG 3

Flexible Couplings
Flexible couplings, which are classified as mechanical flexing, material flexing, or combination, allow the coupled shafts to slide or move relative to each other. Although clearances are provided to permit movement within specified tolerance limits, flexible couplings are not designed to compensate for major misalignments. (Shafts must be aligned to less than 0.002 inches for proper operation.) Significant misalignment creates a whipping movement of the shaft, adds thrust to the shaft and bearings, causes axial vibrations, and leads to premature wear or failure of equipment.
1-Mechanical Flexing
Mechanical-flexing couplings provide a flexible connection by permitting the coupling components to move or slide relative to each other. In order to permit such movement, clearance must be provided within specified limits. It is important 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.
1-1 Chain couplings
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. 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 .4.
Special-purpose components may be specified when enhanced flexibility and reduced wear is required. Hardened sprocket teeth, special tooth design, and barrel-shaped rollers are available for special needs. Light duty drives are sometimes supplied with nonmetallic chains on which no
lubrication should be used.

FIG 4

1-2 Gear couplings
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 .5

FIG 5

2- 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, plastics, or rubber, are used to make the flexing elements in these couplings. The use of the couplings is governed by the operational 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
2-1 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 movement. The laminated disk-ring coupling also reduces heat and axial vibration that can transmit between the driver and driven unit. Figure .6 illustrates some typical
laminated disk-ring couplings

FIG 6

2-2 Bellows couplings
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 thermal expansion of the equipment components. Figure.7 illustrates a typical bellows coupling.

FIG 7

2-3 Flexible Shaft or Spring
Flexible shaft or spring couplings are generally used in small equipment applications that do not experience high torque loads. Figure .8 illustrates a typical flexible shaft coupling

FIG 8

2-4 Diaphragm couplings
Diaphragm couplings provide torsional stiffness while allowing flexibility in axial movement. Typical construction consists of shaft hub flanges 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 9.

FIG 9

2-5 Elastomeric couplings
Elastomeric couplings consist of two hubs connected by an elastomeric element. The couplings fall into two basic categories, one with the element placed in shear and the other with the element placed in compression. The coupling compensates for minor misalignments because of the flexing
capability 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. Typical elastomeric couplings are illustrated in Figure .10.

FIG 10

3- 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 illustrated in Figure .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 serpentineshaped spring grid that fits into the grooved slots. The flexibility of this grid provides torsional resilience.

FIG 11

Special Application Couplings
Two special application couplings are discussed in this section:
(1) floating shaft or spacer coupling 
(2) 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 spacer coupling) is required for this type of maintenance. Examples of couplings for this type of application are shown in Figure.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 .12
. 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 couplings. An intermediate shaft that permits the transmission of power between widely separated drive components, in turn, connects these.

FIG 12

Hydraulic or Fluid  couplings
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 13  is an illustration of a typical hydraulic coupling.

FIG 13

coupling depending on the application. Figure 14  shows a typical ‘application chart’ for several common types.

FIG 14

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