Maintenance Engineering -- Couplings

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Couplings are designed to provide two functions: (1) to transmit torsional power between a power source and driven unit, and (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.


The sections that follow provide overviews of the more common coupling types, rigid and flexible. Also discussed are couplings used for special applications, floating-shaft (spacer) and fluid (hydraulic).

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 pump). 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's not possible to use keys and keyways.

Flanged Couplings A flanged rigid coupling is composed 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 Diagrm X.1.

Split Couplings A split rigid coupling, also referred to as a clamp coupling, is basically a sleeve that's 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 Diagrm X.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.

Compression Coupling A rigid compression coupling is composed of three pieces: a compressible core and two encompassing coupling halves that apply force to the core. The core is composed 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 Diagrm X.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 in. 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.

Diagrm X.1 Typical flanged rigid coupling.

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.

Diagrm X.2 Typical split rigid coupling.

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 Diagrm X.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 Diagrm X.5.

Diagrm X.3 Typical compression rigid coupling.

Diagrm X.4 Typical chain coupling. Roller-chain Coupling. Coupling Cover (½ Shown) (Optional) Roller Chain 1 Required to Join Couplers Coupling Body(s) 1 Required for Each Shaft

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 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 don't have a well-defined fatigue limit. Their service life is determined primarily by conditions of installation and operation.

Diagrm X.5 Typical gear-tooth coupling.

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. Diagrm X.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 thermal expansion of the equipment components. Diagrm X.7 illustrates a typical bellows coupling.

Flexible Shaft or Spring: Flexible shaft or spring couplings are generally used in small equipment applications that don't experience high torque loads. Diagrm X.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 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. Atypical diaphragm coupling is illustrated in Diagrm X.9.

Diagrm X.6 Typical laminated disk-ring couplings.

Diagrm X.7 Typical bellows coupling.

Diagrm X.8 Typical flexible shaft coupling.

Diagrm X.9 Typical diaphragm coupling.

Elastomeric: 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 its 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's 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 Diagrm X.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 illustrated in Diagrm X.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 spacer coupling) is required for this type of maintenance. Examples of couplings for this type of application are shown in Diagrm X.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 Diagrm X.13.

Diagrm X.10 Typical elastomeric couplings.

Diagrm X.11 Typical metallic-grid couplings.

Diagrm X.12 Typical floating-shaft or spacer couplings.

Diagrm X.13 Typical floating-shaft or spacer couplings for high-temperature applications.

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, which permits the transmission of power between 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). Diagrm X.14 is an illustration of a typical hydraulic coupling.

Diagrm X.14 Typical hydraulic coupling.


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's crucial to identify the correct type and size of coupling needed. Even if 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's important to consider the type of equipment that it connects. For example, demanding applications such as variable, high-torque machine-trains require couplings that are specifically designed to absorb radical changes in speed and torque (e.g., metallic-grid). Less demanding applications such as run-out table rolls can generally get by with elastomeric couplings. Tbl. X.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.


Tbl. X.1 Coupling Application Overview

Application Coupling* Selection Recommendation Limited Misalignment Compensation

[[ Variable, high-torque machine-trains operating at moderate speeds Run-out table rolls Vertical pump shafting Keys and keyways not appropriate (e.g., brass shafts) Transmission of proportionately high torque at low speeds Transmission of proportionately high torque at both high and low speeds Allowance for axial movement and reduction of heat and axial vibration Moderate rotational torque and shaft speed Small equipment that does not experience high torque loads Torsional stiffness while allowing flexibility in axial movement Light- or medium-duty applications running at moderate speeds Gradual acceleration and limited maximum torque for fixed operating speeds (e.g., compressors).

Variable or high torque and /or speed transmission Maintenance requiring considerable distance between the driver and driven shaft ends Misalignment results from expansion due to high process temperatures



Metallic-grid combination couplings

Elastomeric flexible couplings

Flanged rigid couplings, split rigid or clamp couplings Rigid compression couplings Chain couplings (mechanical-flexing) Gear couplings (mechanical-flexing) Laminated disk-ring couplings (material-flexing) Bellows couplings (material-flexing) Flexible shaft or spring couplings (material-flexing) Diaphragm material-flexing couplings Elastomeric couplings (material-flexing) Hydraulic or fluid couplings Flexible couplings rated for the maximum torque requirement

Floating-shaft or spacer couplings


[*See Tbl. X.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.

Cost: Coupling cost should not be the deciding factor in the selection process, although it will certainly play a part in it. Although higher-performance couplings may be more expensive, they actually may be the cost-effective solution in a particular application. Selecting the most appropriate coupling for an application not only extends coupling life but also improves the overall performance of the machine train and its reliability.


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; don't 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's necessary to heat the hubs to achieve the proper interference fit, an oil or water bath should be used. Spot heating with a flame or torch should be avoided because it causes distortion and may adversely affect the hubs.

Care must be exercised during installation of a new coupling or the reassembly of an existing unit. Keys and keyways should be coated with a sealing compound that's resistant to the lubricant used in the coupling. Seals should be inspected to ensure that they are pliable and in good condition. They must be installed properly in the sleeve with the lip in good contact with the hub. Sleeve flange gaskets must be whole, in good condition, clean, and free of nicks or cracks. Lubrication plugs must be cleaned before being installed and must fit tightly.

The specific installation procedure is dependent on the type and mounting configuration of the coupling. However, common elements of all coupling installations include: spacing, bolting, lubrication, and the use of matching parts. The sections to follow discuss these installation elements.

Spacing: Spacing between the mating parts of the coupling must be within manufacturer's tolerances. For example, an elastomeric coupling must have a specific distance between the coupling faces. This distance determines the position of the rubber boot that provides transmission of power from the driver to the driven machine component. If this distance is not exact, the elastomer will attempt to return to its relaxed position, inducing excessive axial movement in both shafts.

Bolting Couplings are designed to use a specific type of bolt. Coupling bolts have a hardened cylindrical body sized to match the assembled coupling width. Hardened bolts are required because standard bolts don't have the tensile strength to absorb the torsional and shearing loads in coupling applications and may fail, resulting in coupling failure and machine-train damage.

Lubrication Most couplings require lubrication, and care must be taken to ensure that the proper type and quantity is used during the installation process. Inadequate or improper lubrication reduces coupling reliability and reduces its useful life. In addition, improper lubrication can cause serious damage to the machine-train.

For example, when a gear-type coupling is over filled with grease, the coupling will lock. In most cases, its locked position will increase the vibration level and induce an abnormal loading on the bearings of both the driver and driven unit, resulting in bearing failure.

Matching Parts Couplings are designed for a specific range of applications, and proper performance depends on the total design of the coupling system. As a result, it's generally not a good practice to mix coupling types. Note, however, that it's common practice in some steel industry applications to use coupling halves from two different types of couplings. For example, a rigid coupling half is sometimes mated to a flexible coupling half, creating a hybrid. While this approach may provide short-term power transmission, it can result in an increase in the number, frequency, and severity of machine-train problems.

Coupling Alignment The last step in the installation process is verifying coupling and shaft alignment.

With the exception of special application couplings such as spindles and jackshafts, all couplings must be aligned within relatively close tolerances (i.e., 0.001-0.002 in.).


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.

Determining Cause of Failure When a coupling failure occurs, it's important to determine the cause of failure.

Failure may result from a coupling defect, an external condition, or workman ship during installation.

Most faults are attributed to poorly machined surfaces causing out-of specification tolerances, although defective material failures also occur.

Inadequate material hardness and poor strength factors contribute to many pre mature failures. Other common causes are improper coupling selection, improper installation, and /or excessive misalignment.

Lubrication Requirements Lubrication requirements vary depending on application and coupling type.

Because rigid couplings don't require lubrication, this section discusses lubrication requirements for mechanical-flexing, material-flexing, and combination flexible couplings only.

Mechanical-Flexing Couplings It is important to follow the manufacturer's instructions for lubricating mechanical-flexing couplings, which must be lubricated internally. Lubricant seals must be in good condition and properly fitted into place. Coupling covers contain the lubricant and prevent contaminants from entering the coupling interior. The covers are designed in two configurations, split either horizontally or vertically.

Holes are provided in the covers to allow lubricant to be added without coupling disassembly.

Gear couplings are one type of mechanical-flexing coupling, and there are several ways to lubricate them: grease pack, oil fill, oil collect, and continuous oil flow.

Either grease or oil can be used at speeds of 3,600 rpm to 6,000 rpm. Oil is normally used as the lubricant in couplings operating over 6,000 rpm. Grease and oil-lubricated units have end gaskets and seals, which are used to contain the lubricant and seal out the entry of contaminants. The sleeves have lubrication holes, which permit flushing and re-lubrication without disturbing the sleeve gasket or seals.

Material-Flexing Couplings Material-flexing couplings are designed to be lubrication free.

Combination Couplings Combination (metallic-grid) couplings are lubricated in the same manner as mechanical-flexing couplings.

Periodic Inspections It is important to perform periodic inspections of all mechanical equipment and systems that incorporate rotating parts, including couplings and clutches.

Mechanical-Flexing Couplings To maintain coupling reliability, mechanical-flexing couplings require periodic inspections on a time- or condition-based frequency established by the history of the equipment's coupling life or a schedule established by the predictive maintenance engineer. Items to be included in an inspection are listed below. If any of these items or conditions is discovered, the coupling should be evaluated to determine its remaining operational life or be repaired/replaced.

_ Inspect lubricant for traces of metal (indicating component wear).

_ Visually inspect coupling mechanical components (roller chains and gear teeth, and grid members) for wear and /or fatigue.

_ Inspect seals to ensure they are pliable and in good condition. They must be installed properly in the sleeve with the lip in good contact with the hub.

_ Sleeve flange gaskets must be whole, in good condition, clean, and free of nicks or cracks.

_ Lubrication plugs must be clean (to prevent the introduction of contaminants to the lubricant and machine surfaces) before being installed and must be torqued to the manufacturer's specifications.

_ Setscrews and retainers must be in place and tightened to manufacturer's specifications.

_ Inspect shaft hubs, keyways, and keys for cracks, breaks, and physical damage.

_ Under operating conditions, perform thermographic scans to deter mine temperature differences on the coupling (indicates misalignment and /or uneven mechanical forces).

Material-Flexing Couplings Although designed to be lubrication-free, material-flexing couplings also require periodic inspection and maintenance. This is necessary to ensure that the coupling components are within acceptable specification limits. Periodic inspections for the following conditions are required to maintain coupling reliability. If any of these conditions are found, the coupling should be evaluated to determine its remaining operational life or be repaired/replaced.

_ Inspect flexing element for signs of wear or fatigue (cracks, element dust or particles).

_ Setscrews and retainers must be in place and tightened to manufacturer's specifications.

_ Inspect shaft hubs, keyways, and keys for cracks, breaks, and physical damage.

_ Under operating conditions, perform thermographic scans for temperature differences on the coupling, which indicates misalignment and /or uneven mechanical forces.

Combination Couplings Mechanical components (e.g., grid members) should be visually inspected for wear and /or fatigue. In addition to the items for mechanical-flexing couplings presented in Section 2, the grid members on metallic-grid couplings should be replaced if any signs of wear are observed.

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, physical damage, wear, and /or fatigue. Any component having any of these conditions should be replaced.


A key is a piece of material, usually metal, placed in machined slots or grooves cut into two axially oriented parts to mechanically lock them together. For example, keys are used in making the coupling connection between the shaft of a driver and a hub or flange on that shaft. Any rotating element whose shaft incorporates such a keyed connection is referred to as a keyed-shaft rotor. Keys provide a positive means for transmitting torque between the shaft and coupling hub when a key is properly fitted in the axial groove.

The groove into which a key is fitted is referred to as a key seat when referring to shafts and a keyway when referring 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.

Diagrm X.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 instead of torsional load.

The effective key length, ''L'' is that portion of the key having full bearing on hub and shaft. Note that the curved portion of the key seat made with a rotary cutter does not provide full key bearing, so ''L'' does not include this distance.

The use of an end mill cutter results in a square-ended key seat.

Diagrm X.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 rectangular (flat) key is used.

The ends are either square, rounded or gibe-head. The gibe-head is usually used with taper keys. If special considerations dictate the use of a keyway in the hub shallower than the preferred square key, it's recommended that the standard rectangular (flat) key be used.

Hub bores are usually straight, although for some special applications, taper bores are sometimes specified. For smaller diameters, bores are designed for clearance fits, and a setscrew is used over the key. The major advantage of a clearance fit is that hubs can be easily assembled and disassembled. For larger diameters, the bores are designed for interference fits without setscrews. For rapid-reversing applications, interference fits are required.

The sections to follow discuss determining keyway depth and width, keyway manufacturing tolerances, key stress calculations, and shaft stress calculations.


The formula given below and Diagrm X.17, Tbl. X.1 (square keys), and Tbl. X.2 (flat keys) illustrate how the depth and width of standard square and flat keys and keyways for shafts and hubs are determined.

Diagrm X.15 Keyed shaft. Key size versus shaft diameter.

Diagrm X.16 Key shapes. TOP VIEWS: Square Ends, Square and Round, Rounded Ends; SIDE VIEWS: Gibe Head Taper, Plain Taper 2 Diagrm X.17 Shaft and hub dimensions.

Tbl. X.2 Standard Square Keys and Keyways (inches)* where:

C = Allowance or clearance for key, inches D = Nominal shaft or bore diameter, inch H = Nominal key height, inches W = Nominal key width, inches Y = Chordal height, inches Note: Tables shown below are prepared for manufacturing use. Dimensions given are for standard shafts and keyways.

Diagrm X.18 Manufacturing tolerances.


Keyway manufacturing tolerances (illustrated in Diagrm X.18) are referred to as offset (centrality) and lead (cross axis). Offset or centrality is referred to as Dimension ''N''; lead or cross axis is referred to as Dimension ''J.'' Both must be kept within permissible tolerances, usually 0.002 in.


Calculations for shear and compressive key stresses are based on the following assumptions:

1. The force acts at the radius of the shaft.

2. The force is uniformly distributed along the key length.

3. None of the tangential load is carried by the frictional fit between shaft and bore.

The shear and compressive stresses in a key are calculated using the following equations (see Diagrm X.19):

Ss = 2T / ((d) _ (w) _ (L) ) Sc = 2T/(d) _ (h1) _ (L) where:

d = Shaft diameter, inches (use average diameter for taper shafts) h1 = Height of key in the shaft or hub that bears against the keyway, inches. Should equal h2 for square keys. For designs where unequal portions of the key are in the hub or shaft, h1 is the minimum portion.

Hp = Power, horsepower L = Effective length of key, inches RPM = Revolutions per minute Ss =Shear stress, psi Sc = Compressive stress, psi T = Shaft torque, lb-in. or Hpx63000/RPM w = Key width, inches Diagrm X.19 Measurements used in calculating shear and compressive key stress.

Key material is usually AISI 1018 or AISI 1045. Tbl. X.4 provides the allow able stresses for these materials.

Example: Select a key for the following conditions: 300 Hp at 600 RPM; 3-inch diameter shaft, 3/4 -inch _ 3/4 -inch key, 4-inch key engagement length.

The AISI 1018 key can be used since it's within allowable stresses listed in Tbl. X.4 (allowable Ss = 7,500, allowable Sc = 5,000).

Note: If shaft had been 2 3/4 -in. diameter (4-in. hub), the key would be 5/8 -in. _ 5/8 -in., Ss = 9,200 psi, Sc = 18,400 psi, and a heat-treated key of AISI 1045 would have been required (allowable Ss = 15,000, allowable Sc = 30,000).


Torsional stresses are developed when power is transmitted through shafts. In addition, the tooth loads of gears mounted on shafts create bending stresses.

Shaft design, therefore, is based on safe limits of torsion and bending.

To determine minimum shaft diameter in inches:

Minimum Shaft Diameter = Example:

Hp = 300 RPM = 30 Material = 225 Brinell From Diagrm X.20 at 225 Brinell, Allowable Torsion = 8000 psi Minimum Shaft Diameter =

Tbl. X.4 Allowable Stresses for AISI 1018 and AISI 1045 Diagrm X.20 Allowable stress as a function of Brinell hardness.

Tbl. X.5 Shaft Diameters (Inches) and Their Cubes (Cubic Inches)

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