Quick notes on couplings

1. Geometrical & Mechanical Engineering
Drawing – CAPE Unit II
Coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of
transmitting power.
Constant Velocity Joint
Constant-velocity joints (aka homokinetic or CV joints) allow a drive shaft to transmit power
through a variable angle, at constant rotational speed, without an appreciable increase in
friction or play. They are mainly used in front wheel drive, and many modern drive-
cars with independent rear suspension typically use CV joints at the ends of the rear axle half-
shafts, and increasingly use them on the prop shafts (drive shafts).
Types of Couplings
Rigid Coupling
Properties: used for more precise alignment, maximizes performance, increase expected life of
machine used to join two separate pieces of hardware or to repair a connection within a single
system, used to reduce shock and wear at point where shafts meet
Rigid: Sleeve Coupling
Properties:Alsoknownasbox coupling,consistsof a pipe whose bore isfinishedtothe required
tolerance basedonshaftsize,akeywayis made inthe bore to transmittorque,twothreadedholesare
providedtolockthe coupling,bothshaftendsare boundedbya muff/sleeve,made from castiron,
simple tomanufacture
Advantages: homokinetictransmission,accurate location
Disadvantages: misalignmentnotpermitted,vibrationtransferredthroughcoupling
Rigid: Flanged Coupling
Properties:Consistsof twoseparate castiron flanges,flangescoupled/heldtogetherbyboltsandnuts
(temperedsteel),mostaccurate andrigid,providesstrong connectionandhelpstobringshaftinto
alignment,resistanttoshifting,sealedusingagasket,made of steel, etc.

2. Application:pressurizedsystems
Advantages: can be disassembled
Disadvantages: doesn’tallow misalignment
Hydrodynamic Coupling (Fluid)
Properties:consistsof twoturbines(inputandoutput) anda housing,sealedviaoil-seals.Inputturbine
isconnectedto powersupply(e.g.engine),outputturbine connectedtodrive trainof system, stall
speedismax speedatwhichcouplingcanturn at full output powerwhenturbine islockedinplace,
designedwithminimumslip,canalsobe usedasa hydrodynamicbrakingsystem, low viscosityfluidis
sued.
Advantages: noshock loading,controlledstart-up
Disadvantages: losespercentage of energyinheatbyfriction
Uses:torque converterincar, industrial (cyclicloadingsystems),rail transportation,aviation
Flexible Coupling
Flexible couplings are used to transmit torque from one shaft to another when the two shafts
are slightly misaligned. Flexible couplings can accommodate varying degrees of misalignment
up to 3° and some parallel misalignment. In addition, they can also be used for vibration
damping or noise reduction. This coupling is used to protect the driving and driven shaft
members against harmful effects produce due to misalignment of the shafts, sudden shock
loads, shaft expansion or vibrations etc.

3. Universal Point Coupling
A universal joint, (universal coupling, U-joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint)
is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is
commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located
close together, oriented at 90° to each other, connected by a cross shaft. The Universal joint is
not a constant velocity joint. The Cardan joint suffers from one major problem: even when the
input drive shaft axle rotates at a constant speed, the output drive shaft axle rotates at a
variable speed, thus causing vibration and wear. To reduce the jerkiness of motion, a double
jointed shaft is used with universal joints at either end and both phased to cancel the change in
angular velocity.
Advantages: flexible, allows dynamic misalignment, cheap, easy to assemble, transmission
efficiency high
Disadvantages: fluctuating motion, no axial misalignment, requires regular maintenance, joint
may be critically damaged if not properly lubricated.
Uses: drive-shafts, automobile, centrifugal pumps, centrifugal fans
Oldham Coupling
An Oldham coupling has three discs, one coupled to the input, one coupled to the output, and a
middle disc that is joined (press-fit) to the first two by tongue and groove. The tongue and
groove on one side is perpendicular to the tongue and groove on the other. The middle disc
rotates around its centre at the same speed as the input and output shafts. Its centre traces a
circular orbit, twice per rotation, around the midpoint between input and output shafts. Often
springs are used to reduce backlash of the mechanism, allows large misalignment in short
length.
Advantages: protects rotary components from extreme shear force by using a replaceable
plastic coupler which breaks under extreme shear force, homokinetic transmission (CV),
economical, blind installation, low moment of inertia

4. Disadvantages: relatively small misalignment
Uses: pumps, valves, gearboxes

5. Beam
A beam coupling, also known as helical coupling, is a flexible coupling for transmitting torque
between two shafts while allowing for angular misalignment, parallel offset and even axial
motion, of one shaft relative to the other. This design utilizes a single piece of material and
becomes flexible by removal of material along a spiral path resulting in a curved flexible beam
of helical shape. Since it is made from a single piece of material, the BeamStyle coupling does
not exhibit the backlash found in some multi-piece couplings. Another advantage of being an all
machined coupling is the possibility to incorporate features into the final product while still
keep the single piece integrity.
Advantages: anti-backlash torque transmission, high angular misalignment allowances, long
fatigue life
Disadvantages: transmits shaft vibration
Uses: robotics, attaching encoders to shaft end
Double-Cardan (Double Universal-Point) Joint
Similar to double Cardan Shafts, except that the length of the intermediate shaft is shortened
leaving only yokes, allows two Hooke’s joints to be assembled back-to-back. These eliminate
the need to correctly phase the universal joints at the end of the intermediate shaft and are
used to replace Rzeppa CV joints. They require centring elements that will maintain equal
angles between the driven and driving shafts for true constant rotary motion.
Advantages: do not need to be phased, can be used for high articulation angles and torque
loads
Disadvantages: tends to generate additional vibration at higher speeds, require additional
torque to drive joint.