Presentation Presentation onon

CouplingsCouplingsPresented byPresented by

H.L.GadiyarH.L.Gadiyar2.10.20072.10.2007

CouplingsCouplings• A coupling is a device used to connect two shafts

together at their ends for the purpose of transmitting power.

Shaft couplings are used in machinery for several purposes, the most common of which are the following:

• To provide for the connection of shafts of units that are manufactured separately such as a motor and generator and to provide for disconnection for repairs or alternations.

• To provide for misalignment of the shafts or to introduce mechanical flexibility.

• To reduce the transmission of shock loads from one shaft to another.

• To introduce protection against overloads. • To alter the vibration characteristics of rotating units.

Major ManufacturersMajor Manufacturers• John Crane-(formerly known as

Flexibox)All pumps• Love-Joy-Small size pumps, AOP,MOP• Thomas-Sundyne• Koplex-Compressor• RENK• Eagle• Bendix

Glossary related to Glossary related to couplingscouplings

• Backlash: is the circumferential clearance in the flexible element (s) that is necessary to provide angular misalignment capability and assembly.

• Coupling Rating: Coupling rating is a torque capacity at rated misalignment, axial displacement and speed

• Distance Between Shaft Ends (DBSE): The distance between shaft ends is defined as the distance from the face of one shaft (including any thread end) to the face of the next shaft. For integral flanges, this measurement shall be from the mating face.

• Hub: A hub is that part of a flexible coupling which is machined for mounting on a shaft.

• Hub Bores: Bores are cylindrical or conical holes in hubs of couplings with axes coincident with the rotational axis of the coupling. Hub bores may be cylindrical or tapered.

• Nominal bore: The nominal bore is a commonly used term to identify the basic bore size without tolerance.

Contd….Contd….

• Rated Speed: Rated speed is the maximum speed at which the coupling is capable of transmitting the coupling continuous rated torque while simultaneously subjected to the rated angular misalignment and the coupling rated axial displacement

• Endurance limit: is the fatigue limit of a coupling component subjected to combined constant and alternating stresses. Beyond this limit the material can be expected to fail after some finite number of cyclic loads. Below this limit the material can be expected to have infinite life (or a factor of safety greater than 1)

• Factor of safety (F.S.): factor of safety is used to cover uncertainties in a coupling design; analytical assumptions in stress analysis, material unknowns. manufacturing tolerances, etc.. Under given design conditions the F.S. is the ratio of strength (or stress capacity) to actual predicted stress; where the stress is a function of torque, speed, misalignment and axial displacement.

• Service factor: Service factor or Application Factor is a factor to account for unusual and occasional loading due to the operating conditions /application of the coupling.

• Misaligned Shaft: Misaligned shafts can be parallel offset, angular, axial, or any combination thereof.

Type of couplingsType of couplings

• Couplings can be broadly classified in to two main classes: 1. Rigid couplings 2. Flexible Couplings• Rigid Couplings : The rigid coupling consists of simple

hollow cylinders having internal diameters of the proper size to fit over the shaft ends being joined. The coupling length depends on the desired shaft gap between ends. For light applications, the shafts are attached to the coupling by setscrews. Rigid coupling do not accommodate for misalignment. Any misalignment will affect the life and the performance of the coupling.

• Flexible Couplings: are designed to transmit torque while permitting some radial and axial and angular misalignment. Flexible couplings can accommodate angular misalignment up to a few degrees and some parallel misalignment.

Rigid CouplingsRigid Couplings• Rigid couplings are basically of 3 types:

1. Sleeve or muff coupling 2. Clamp or split-muff or compression coupling 3. Flanged Coupling

Flanged CouplingsFlanged Couplings 

Flexible couplingsFlexible couplings Flexible couplings have been classified into two

categories:1. General purpose couplings2. Special purpose couplings

General purpose couplingsGeneral purpose couplings• These couplings operated to a maximum speed of 4000 rpm and a

power rating of 750KW.• They are connected to machines that operate below their first lateral

critical speedFollowing Criteria need to be defined:Maximum continuous power (HP horsepower)

Speed (rpm) Peak Torque Distance between shaft ends Driver shaft diameter Driven shaft diameter Maximum axial displacement Maximum parallel shaft offset/angular alignment• These couplings are covered by following standards:1. API 610: Centrifugal pumps for Petroleum, heavy duty chemical and

gas industry,2. ISO 14691: Petroleum and Natural Gas Industries-general purpose

couplings

Special purpose couplingsSpecial purpose couplings• These are engineered couplings used in applications generally not

by general purpose couplings• Criteria such as:

Environmental conditions of temperature, Corrosive/hazardous atmosphere, Special balance requirements, Expected misalignment conditions, Potential excitations of critical frequencies, Type of bearings and lube oil supply have to be taken into consideration while selecting a special purpose couplings.

• These are covered by followings standards:API 671-Special purpose coupling for Petroleum, chemical and gas industry servicesISO 10441-petroleum and natural gas industries-flexible couplings for mechanical power transmission-special purpose applications

Types of General Types of General purpose couplingspurpose couplings

• The following types are included, as general purpose couplings

1. Mechanical element couplings a. gear-straight/crowned tooth b. chain-steel/nylon

c. universal joints2. Elastomeric element couplings:

Compression type: jaw, block, pin-bush, do-nutShear type: Urethane type, chorded type, Donut

3. Metallic Element Couplings:a. Disc type- Circular, Scalloped, Straight, Link

Types of Special purpose Types of Special purpose couplingscouplings

• The following types are included as special purpose couplings

1. Mechanical element Couplingsa. High performance gear type

2. Elastomeric element Couplinga. Block type

3. Metallic Element Couplingsa. Disc typeb. Diaphragm type: tapered, one piece, multiple

straight, multiple convoluted.

Selection ProcedureSelection Procedure1. For proper selection the following data should be obtained: Maximum continuous power (HP horsepower) Speed (rpm) Peak Torque Distance between shaft ends Driver shaft diameter Driven shaft diameter Maximum axial displacement Maximum parallel shaft offset/angular alignment2. Calculate minimum coupling rating using appropriate application

factor. (min 1.5) Minimum coupling rating = ( KW*F/ N) 3. Select a coupling from vendors catalogue with same /higher

rating. 4. Check that the hub bore capacity is suitable and speed capability. 5. Specify DBSE.

ServiceService factor ( F ) factor ( F )• For smooth power driving machines ( steam/gas turbines, motors): - Constant Torque 1.00 (Centrifugal Compressors, Feed Pumps, etc) - Moderate torque Fluctuations

(Fans, Screw Compressors, etc) 1.50 - Substantial Fluctuations (Reciprocating Pumps, Heavy duty fans and blowers 2.00

For power driving machines like ( Steam engine/ water turbines): - Constant Torque 1.50 (Centrifugal Compressors, Feed Pumps, etc) - Moderate torque Fluctuations

(Fans, Screw Compressors, etc) 2.00 - Substantial Fluctuations (Reciprocating Pumps, Heavy duty fans and blowers 2.50

ExampleExample• 150 KW electric motor is to be coupled to a centrifugal

pump at 2960 rpm. Calculate the coupling rating –Suggested coupling M/s John crane make.

• R= 1000 x 1.3 x 150/ 2960 = 65.87 KW per 1000 rpm

Therefore selection can be TSKS-0075 –refer standard

table.standard hub bore = 65mmlarge hub bore up to = 90 mm

Note : While Capacity up-gradation care should be taken to ensure correct rate of coupling is used.

Coupling designationCoupling designation• All vendors have a unique system of designating couplings

providing a unique code for ordering purposes. Thus coupling designation is highly proprietary.

• For example let us consider the 16 digit coding system of JOHN CRANE couplings.

MODO 0125 0177 0000

DBSE

Coupling rating in KW/1000 rpm

Basic Coupling type

Hub type and condition

Hub type and condition

Balance Correction

Material of Construction

Coupling Designation Coupling Designation (contd.)(contd.)

• Basic coupling type:LS-4 Tangential link membraneTSK-6 6 Tangential link membraneTLP-8 8 Tangential link membraneTTH-10 10 Tangential link membraneMHS- Spoke form membrane MODO Spoke Form Membrane

• Materials of construction:O- Standard Materials for coupling RangeX- Spark Resistant MaterialsW-Special materialsD- Materials for low temperatureE- Materials for low temperature and spark resistant

Coupling Designation Coupling Designation (Contd.)(Contd.)

• Balance Correction:0- No Correction.1- Trans. Unit/Membrane Unit Corrected to Commercial Limits2- Transmission Unit Balance Spacer (M series).3- Transmission Unit and Hubs Corrected to Commercial Limits.5- Special Balance Requirements6- Balance to API 610 8th Edition.

• Hub Types and Condition:0- No Hub supplied3- Special Design Hub4- Adapter5- Standard Hub Unbored6- Large Hub Unbored7- Standard Hub Machined8- Large Hub Machined

Coupling Evaluation Coupling Evaluation

Disc type couplingDisc type coupling

Exploded view-disc type Exploded view-disc type couplingcoupling

Types of MembranesTypes of Membranes

Applications of Disc Type Applications of Disc Type CouplingsCouplings

• The disk coupling is used in a variety of applications.

• The most common is on:

1. Medium-horsepower pumps. 2. Marine drives, 3. Cooling tower drives, 4. Generators, 5. compressors, 6. fans and 7. machine tools.

Grid CouplingGrid Coupling

Working phenomenaWorking phenomena• The basic metallic grid coupling (Fig. 4) consists of two hubs that

contain slots, which may be straight, tapered, or contoured. The two hubs are connected by a “serpent” style metallic grid (spring).

• Metallic Grid couplings have been used successfully for more than 60 years.

• Grid couplings can accommodate 3 types of misalignments:• 1. Parallel misalignment: Movement of the grid in the lubricated

grooves accommodates for parallel misalignment.• 2. Angular misalignment: The grid-groove design permits a rocking

and sliding action of the lubricated grid and hubs.

( Contd. )( Contd. )• End float: Unrestrained end float for both driving and driven

members is permitted because the grid slides in the lubricated grooves.

• The amount of axial movement can also be limited.

Applications of grid couplingApplications of grid coupling• The metallic grid coupling has been applied to nearly all types of

industrial applications, from centrifugal pumps to steel mill applications.

• Some typical applications are:• Agitators • Machine Tools• Blower • Mixers• Compressors • Paper mills• Cranes • Pumps• Elevators • Fans• Generators • Textile industry

Limited end float couplingsLimited end float couplings

Limited End float CouplingsLimited End float Couplings• The limited end float couplings

are a modification of disc type couplings, however these couplings usually consist of a standard hub and a larger hub.

• A limited-end float coupling is designed to limit the movement of the coupled shaft ends with respect to each other where one shaft has no thrust bearing for centering. A limited end float design is commonly used in couplings for sleeve-bearing motors.

Fluid couplingsFluid couplings

Fluid couplings-working Fluid couplings-working phenomenaphenomena

• A fluid coupling is a sealed chamber containing two similarly shaped toroids facing components, impeller and turbine, immersed in fluid (usually oil ).

• The driving impeller, often referred to as the pump or driving torus is rotated by the prime mover, which is typically an electric motor which churns the fluid.

• The moving fluid reaches the casing and the enclosed shape forces the flow in the direction of the driven impeller, referred to as the turbine or driven torus. Here, the Coriolis force reaction transfers the angular fluid momentum outward and across, applying torque to the turbine, thus causing it to rotate in the same direction as the pump. The fluid leaving the centre of the turbine returns to the pump, where the cycle repeats.

• Fluid couplings are widely used in industrial applications, especially in machine drives that involve high inertia starts or constant cyclic loading.-Application examples are Conveyor train.

Diaphragm couplingsDiaphragm couplings

Multiple convoluted Multiple convoluted diaphragm couplingdiaphragm coupling

DiaphragmDiaphragm couplingcoupling• This coupling obtains its flexibility from the free span between the

diaphragm OD and ID. Torque for these couplings is transmitted between the OD and ID. The diaphragm elements can be of constant or variable thickness, usually with maximum thickness at the smaller diameter.

• All three shapes commonly used have some shape modification that helps to reduce their size and increase their flexibility.

• 1. Tapered contoured diaphragm : This diaphragm shape is designed for constant shear stress from ID to OD. The tapered shape greatly increases the coupling’s flexibility capabilities.

(Contd.)(Contd.)• Multiple straight diaphragm: By using multiple thin plates rather

than one thick plate the flexibility of this coupling is greatly increased compared to that of a single diaphragm. With thin diaphragms in parallel, the stresses are usually lower.

• Multiple convoluted diaphragm : This coupling incorporates all the flexibility of the earlier one, but also has a convoluted shape, which helps to increase its flexibility in the axial direction and provided linear axial stiffness.

The diaphragm coupling is typically used in high-performance applications where a failure or downtime can be very costly.

GearedGeared CouplingCoupling

Gear coupling-working Gear coupling-working phenomenaphenomena

• Gear coupling is a mechanical device for transmitting torque between two shafts that are not coaxial. The two joints are connected by a third shaft, called the spindle.

• Each joint consists of a 1:1 gear ratio internal/external gear pair. • Gear couplings have higher torque densities than universal joints

designed to fit a given space while universal joints induce lower vibrations.

• The limit on torque density in universal joints is due to the limited cross sections of the cross and yoke.

• The gear teeth in a gear coupling have high backlash to allow for angular misalignment. The excess backlash can contribute to vibration.

• Single joint gear couplings are also used to connected two nominally coaxial shafts. In this application the device is called a gear-type flexible, or flexible coupling. The single joint allows for minor misalignments such as installation errors and changes in shaft alignment due to operating conditions.

Applications of GearApplications of Gear CouplingCoupling::

Gear couplings are used on the following types of equipment:

Centrifugal pumps, Conveyors, Exciters, Fans, Generators, Blowers, Mixers,Hydraulic pumps, Compressors, Steel mills and auxiliary equipment, Cranes, Hoist,Mining machinery, and others, particularly in high torque applications.

Installation of couplingsInstallation of couplings• A. Preparation.• Obtain the drawing of the coupling. • Review and read the information• Inspect the components to assure that all parts are in order.• Prior to assembling the coupling , clean and disassemble the components.• Check for burrs and nicks on the mating surfaces. If they exist, remove (stone,• file, sand).• Measure and inspect Bore dimensions, Keyways, Coupling lengths, Diameter of• bolt circle (DBC), bolts, and holes of the mating flanges.

B. Hub Installation• 1. Straight shafts• Install the key(s) in the shaft keyway(s)• Make sure that any part that will slide over the coupling hub is placed back on the

shaft, such as seals, carriers, and covers, and on the gear couplings, the sleeves.• Push the hub onto the shaft until the face of the hub is flush with the ends of the

shaft. Check to assure that there is clearance between the keys at the sides of coupling keyway and on top of the key

• Lock the hub in position ( usually with setscrew ).

Installation (contd.)Installation (contd.)• In case of interference fit, the hubs must be heated before they slide onto

the shaft. The hub should be heated in an oil bath or an oven; a torch or open flame should not be used. This could cause localized distortion or softening of the hub material.

• It is also important when mounting interference hubs to make sure that clearance exists over the top of keys; otherwise, when the hub cools, it will rest on the key and produce high stresses in the hub that could cause it to fail.

• 2. Straight shafts with Intermediate Bushings.Intermediate bushings come in two basic configuration: with and without flanges. To assemble, insert the bushings into the hub without tightening the screw or bolts; then slide the hub and bushing onto the shaft. Since the bushing is tapered, tighten the screws on the shaft. Once the hub is at the correct position, the screw should be tightened gradually in a crisscross pattern to the specific torque.

Installation (contd.)Installation (contd.)3. Taper bore couplings:• Tapered shafts have the advantage that the interference between the hub

and the shaft can be accomplished by advancing the hub on the shaft.• Removal of the hub is usually easier on tapered shafts than on straight

shafts.• Applications using tapered bores requires more attention that those using

straight shafts because it is easier to machine two cylindrical surfaces that match than two tapered surfaces.

The following is recommended as a general procedure:• 1. Install the hub on the shaft, assuring that the parts mate

properly and are burr-free and clean. Using a depth gauge or dial indicator measure the initial reading.

• 2. Remove the hub and lubricate the bore of shaft if hydraulic assist is to be used; if not, heat in oil or an oven. When using a heating method for mounting hubs, it is best to provide a positive step, such as a clamp on the shaft, to assure proper draw-up. The reason for this is that a hub advanced too far may not be removable and normally requires that the hubs be cut off.

• 3. The hub is installed and advanced the required amount.

4. Hydraulic bore installations:4. Hydraulic bore installations:

• After the axial pull-up values and blue contact of the taper have been verified, the hub maybe installed onto the shaft without the O-rings and backup rings. Reference measurement should be done to ensure correct axial pull up.

• The O-rings and backup rings shall then be lubricated with hydraulic oil and installed onto the hub in the specified arrangement..

• The hydraulic mounting devices are fitted and pressurized to push the hub to its desired axial position.

• After a sufficient waiting period (typically 1-2 hours) the hydraulic oil is allowed to escape from the coupling interface. The hydraulic mounting device may then be removed. After the axial pull-up values and the blue-contact of the taper have been verified, the hub may be installed onto the shaft without the O-rings

• The shaft nut maybe installed and locked in place. It is recommended for some time to elapse before applying torque to the coupling to draining of any trapped oil.

Hub bore tolerance Vs. Hub bore tolerance Vs. Shaft toleranceShaft tolerance

A shaft-Hub bore assembly usually has ‘interference fit’.

Shaft Hub bore h6 P7 k6 M7 n6 J7 j6 N7 m6 K7 p6 H7

Key/keyway fitsKey/keyway fits• The keyways shall be machined as per the tolerances

specified in AGMA 9002-A86.• The important checks for the keys shall be:

1. The key is tight in shaft keyways2. Key has slide fit (not loose) in hub keyway.3. Clearance radial fir between top of key and hub keyway.4. Key edges to be chamfered to fit radiuses in keyways.5. Single keys should fill the voids in the hub and shaft exactly for dynamic balance, with no excess material.6. Double keys should be of same weight.

AlignmentAlignment• Misalignment is the deviation of relative shaft position from a

collinear axis of rotation measured at the point of power transmission when the equipment is running at normal operating conditions.

• For general purpose equipment, alignment of equipment should be within 0.001 inch/inch of separation between flex points.

• For high speed applications (generally over 3600 rpm), alignment should be within 0.0005 inch/inch of separation between flex points.

• Excessive misalignment causes the following symptoms in rotating equipment:

(1) Premature bearing and seal failures

(2) Excessive radial and axial vibration.

(3) High casing temperatures at or near the bearings or high discharge oil temperatures.

Alignment (Contd.)Alignment (Contd.)

(4) Excessive amount of oil leakage at the bearing seals.

(5) Loose foundation bolts.(6) Loose or broken coupling bolts.(7) Unusually high number of coupling failures.(8) The shaft are breaking (or cracking) at or close to the inboard

bearings or coupling hubs.

Maintenance of couplingsMaintenance of couplings• The following points should be taken care of in routine

maintenance of couplings:1. Sufficient lubricant supply: Inspect grease/oil quality and

quantity. Inadequacy of lubricant can lead to cold material flow, cold welding, thinning of the gear teeth & gear lock-up (in case of geared couplings).

2. Regular cleaning of sludge (in continuous lubricated oil couplings): Sludge build up can cause gear locking up. Sludge also collects corrosive residue, which can corrode coupling parts and act as a source of crack initiation for fatigue propagated failure of part. Hence coupling should be cleaned of sludge every 8000 hrs.

3. Proper alignment: Misalignment can lead to cracking or breaking of the gear teeth.

4. Observations for symptoms of overloading and rectification: Overloading can lead to tooth shearing or pitting of the gears, diaphragm distortion or breakage, distortion/breaking of flexible element of disc couplings

Maintenance of couplings Maintenance of couplings (Contd.)(Contd.)

5. Inspect for loose fasteners, keys hubs, flange adapter and pilot joints

6. Inspect tightening torque of fasteners.7. Rotate the equipment and listen for abnormal sounds, broken

parts may give out clicking sounds.8. Visually inspect for any nicks, cracks or breakages of flexible

element in case of disc/diaphragm coupling.

Failure of gear couplingsFailure of gear couplings

Tooth thinning due to poor lubrication

Tooth breakage due to misalignment

Gear coupling failures Gear coupling failures ( Contd.( Contd.))

Tooth shearing due to overload Contamination due to sludge

Diaphragm coupling failureDiaphragm coupling failure

Stress corrosion fatigue failure of diaphragm

Diaphragm coupling failure Diaphragm coupling failure (contd.)(contd.)

Failure of tapered contour diaphragm

Failure due to overloading

Coupling GuardsCoupling Guards• Coupling guards should be made of a spark proof material

as per zone/area classification. Non-sparking guards shall be used in Zone 0,1 and 2

• Dry couplings generate a lot more windage. This leads to churning of air that generates heat and pressure differential pockets in the coupling guard.

• Coupling guards shall be designed to minimize the impact of these phenomena.

• The coupling enclosure of dry type coupling shall be designed so as not to exceed 80 C. The above limit may be lowered in case the couplings are equipped with electronic instrumentation.

• Coupling guards are usually made of brass to avoid sparking.

Applicable StandardsApplicable StandardsAGMA COUPLING STANDARDS

1. AGMA 9000-C90 Flexible Couplings - Potential Unbalance Classification

2. AGMA 9001-B97 Flexible Coupling - Lubrication3. AGMA 9002-A86 Bores and Key ways for Flexible Couplings (inch

Series) 4. AGMA 9003-A91 Flexible Couplings - Keyless Fits5. AGMA 9004-xxx Flexible Coupling - Mass Elastic Properties and Other

Characteristics 6. AGMA 516.01 Metric Dimensions for Gear Couplings 7. AGMA 510.3 Nomenclature for Flexible Couplings 8. AGMA 922-A98 (Information Sheet) Load Classification and Service

Factors for Flexible Couplings

API Coupling Standards: API 671-Special Purpose Couplings For Service in the Petroleum, Chemical and Gas Industries

Typical failuresTypical failures• Frequent failure of Membranes:-

Analysis found that key way was loose on shaft slot.

• Axial Movement of coupling of a charge pump after motor change.

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