Chapter 8. Coupled Shaft Alingment

Chain couplings

3 types: double roller, silent and plastic Used in unsophisticated applications such as agricultural equipment and machinery. Used for low to moderate speed and torque applications on med and small equipment applications. All metal, rugged, lightweight, and economical method for connecting two shafts. Consists of two sprockets connect with a double roller chain. Opened and lubricated by brushing grease on the chain. Do not transmit as much power as gear but are less costly.

Fail-Safe

A coupling that will continue to operate for a period of time after the torque-transmitting element has failed.

Reversing Torque

A cyclic torque that passes through zero and becomes negative "reverses" to the opposite direction.

Parallel Misalignment (Radial)

A measure of offset distance between the centerlines of driving and driven shafts. Accommodating this requires either a full flex coupling (gear, grid, and dual element disc or diaphragm) or two single flex couplings in series.

Angular Misalignment

A measure of the angle between the centerlines of driving and driven shafts. Flex rigid or half couplings (single element disc or diaphragm) provide this because there is only one flex plane.

Axial Misalignment

A projection or movement along the line of the axis of rotation. ex: sliding the hub in either direction may change the position of a coupling hub. Gear couplings exhibit the best capability to handle this.

Torsional Vibration

A secondary load transferred through the power transmission system and the equipment connected to the system. Assoc. with: Internal combustion engines, reciprocating piston type compressors, vane passing frequencies of some centrifugal pumps, grinding mill drives, kiln drives, rolling mill drives, variable speed motors, and the start up of synchronous motors.

Transiet Torque

A torque of short duration, not necessarily expected, not happening on a regular basis but occurring when a system is upset. May or may not be equal to or greater than peak torque.

Axial Freedom

Allows for variation in coupling position on the shaft at time of installation.

Cyclic Torque

Any torque requirement of the system that varies with time. Can be of a smooth, periodic variation like a sine wave or could be an erratic variation. The vibratory torque that occurs at the operating speed in a torsional vibrating system.

In Shear Spider

Axially twice as wide as standard spiders for straight sided jaw hubs. Push hubs of coupling apart so that the jaws rotate in separate planes. Radially removable elastomer transmits torque through shear and compression. Should only be used for electric motor driven applications: centrifugal pumps, fans, mixers, gear boxes, and plastic extruding machines.

Disc Couplings

Became popular as high-speed machinery created demand for non-lubricated, low maintenance, long life couplings. Machinery includes boiler feed pumps, gas and steam turbines, compressors, high-speed test stands and marine propulsion systems. Consists of two flanged hubs and one or more disc packs and a center member.

Elastomer Block Compression Couplings

Bonded Elastomer In-Shear Couplings Torsionally Stiff (Flywheel) Couplings Similar to jaw coupling in that torque is transferred from one hub to the other by compressing captured rubber blocks. Hubs consists of an external claw hub matched to an internal pocket hub that contains the elastomer. Ideal shape for the elastomer is a cylinder loaded radially. Coupling is used for both shaft-to-shaft connections as well as shaft-to-flywheel connections. Popular application (high hp): diesel driven generator, synchronous motor driven compressor Popular Application (low hp); electric motor driven reciprocating compressor.

Jaw Coupling

Both hubs have jaws formed around their circumferences, pointing towards the opposing hub. Jaws mesh loosely with each other, gaps between them, and sometimes the central inner space are filled with a "spider".

Emergency Braking Torque

Brakes are applied to stop the equipment in a very short time. Torque will exceed the normal braking torque by the inverse ratio of the time required to stop in each case.

Clamped Elastomer in Shear Couplings (Corded Tire)

Came about in 1950 to deal with transient torque peaks and shock loads in diesel driven pumps. Looks like an auto tire and consists of two flanged hubs equipped with clamping plates Transmits torque through the friction of the clamp applied to the inner rims of the tire and a shearing element. Slippage occurs at about 4x the rated torque. Torsionally soft and can dampen vibration. High radial softness Accomodates up to 4 deg of angular misalignment and allows for axial shaft movement.

Two Basic Failure Modes of Elastomeric Couplings

Can break down due to fatigue from cyclic loading when hysteresis (internal heat build up on elastomer) exceeds its limits. Can break down from enviromental factors such as high ambient temperatures, ultraviolet light or chemical contamination.

Setscrews

Clearance fit hubs with keys commonly use these, tightened radially against the key to hold both key and hub in place. In clearance fit hubs used without keys, they are tightened against a flat seat machined onto the shaft circumference.

Compression Type vs. Shear Type

Compression types can transmit higher torque and tolerate greater overload. Compression types offer a greater degree of torsional stiffness. Shear types accommodate more parallel and angular offset while inducing less reactionary load to the bearing. Shear types offer a greater degree of torsional softness.

Purpose of Couplings

Connect two shafts end to end in the same line. To transmit power from one shaft to another allowing them to rotate at the same RPM. Compensate for minor amount of misalignment and random movement between the shafts. Act as a sacrificial element. Dampen torsional (rotational) vibration in the driven and driving equipment.

Adaptations of Grid Couplings for special applications

Controlled Torque Disc Brake Brake Wheel Engine Flywheel Piloted High speed designs

Using Driver Torque

Coupling can be selected based on driver capabilities using nameplate values or start-up torque. When coupling is chosen by driver hp, make sure there is no gear reducer between the driver and driven loads.

Using System Torque with Little or No Service Factor

Coupling can be selected based on the exact requirements of the system. Requirements must include all the torque values to be transmitted through the coupling.

Using Driven Equipment Torque and a Service Factor

Coupling can be selected using the normal operating torque of the driven equipment, adjusted for coupling location, multiplied by a service factor.

Using Torque Information in Coupling Catalogs

Coupling manufacturers have several methods of listing the coupling capabilities in their catalogs. Normal torque capability is the most common value shown in the catalog that the coupling can transmit over its design life.

Infinite Life Couplings

Couplings that do not wear in normal operation and by design the acceptable loads do not exceed the fatigue life of the parts. Transmit torque and compensate for misalignment through distortion within their flexing elements rather than by sliding or rubbing movement of loosely fitted parts. Tire, disc, diaphragm, some donut types, wrapped-spring, flex-link, and most motion control types. Only used on maintenance free systems where maximum torque requirements (transient, cyclic and start-up) are known.

Grid Spring Couplings

Design has two flanged hubs with slots cut axially into the perimeter of their flanges, forming a ring of narrow, closely set teeth around each flange. Single serpentine grid of spring steel is wrapped around both flanges. Lubrication is required, so a collar type cover with seals and gaskets is used to hold the lubricating grease in place. Torque is transmitted from one hub to another through the grid spring. Used in med and small equipment applications (pumps and high capacity bulk conveyors) and req. lubrication.

Benefits of Shear - Donut Type

Design is double-engagement which makes it radially soft and eliminates reactionary load on bearings and shafts when misaligned. Dampens out most peak overloads and prevents vibratory torque from going back to the driver. Sleeve has large open center which allows for close positioning of the shafts. Torque overload capacity is only 3 or 4 times the rated torque and provides a "fusible link" protection for the in-shear couplings.

Continuous running torque

Design torque for the system. The driver horsepower and operating speed.

Donut Shaped Elastomeric Couplings

Developed in 1970 for use in diesel engines. Consists of a rubber donut fastened with cap screws to hubs. Can come in a square, rectangular, octagonal, etc cross-sectional design. Can handle a load in either direction as the load shifts to alternate legs still in compression. Can handle alternating and cyclic loads without backlash.

Bonded Elastomer in Shear Couplings

Developed in 1980 for diesel flywheel applications. Very low torsional stifness ratio (1.5 -12). Torque capability of these couplings range from 900 inch-pounds to more than 1,000,00 inch-pounds depending on size and type. Used with the largest diesel engines.

Which couplings are non-lubricated, low maintenance, infinite life couplings?

Disc and Diaphragm

Types of Metallic Flexible Element Couplings

Disc: Used in general purpose applications and high energy applications and motion control applications. Diaphragm: High Energy Applications Bellows: Motion Control Applications Link: General purpose applications. Spiral Wound Springs: General Purpose Applications

Start-up Torque

Driver torque capability at start-up available to accelerate the driven equipment to operating speed.

Advantages of Chain Couplings

Easy to install Easy to maintain Easy to rough align. Allows for quick shaft disconnection by removal of the chain.

Advantages of Curve Jaw Coupling Designs

Encapsulated spider legs permit higher speed ratings. Extends angular misalignment capacity to 1.3 deg. Have four jaws pushed further out toward the perimeter allowing for a large open center. Allows larger max bores and accommodates close "BSE" dimensions.

Limitations of Curved Jaw Coupling Designs

Encapsulation of spiders prevents the use of radially removable spiders (wrap around, jaw). Damping capacity is lessened under greater loads. Sintered iron used translates to heavier couplings. Length of coupling is greater than straight sided jaw coupling. Spacer couplings can only be achieved by using extended hub lengths which adds a lot of weight and still does not allow for a true drop-out section.

Limitations of Metallic Couplings

Fatigue or wear plays a major role in failure. May need lubrication. Often many parts to assemble. Most need very careful alignment. Usually cannot damp vibration or absorb shock. High electrical conductivity, unless modified with insulators.

Lubricated Coupling Types

Gear Couplings Grid Spring Couplings Chain Couplings

Application Categories

General-Purpose Industrial, Specific-Purpose Industrial, High-Speed, Motion Control, and Torsional.

Metallic Couplings

Has no elastomeric element to transmit the torque. Flexibility is gained through either loose fitting parts which roll or slide against one another (gear, grid, chain). Primary cause of failure is wear for those with moving parts. Primary cause of failure is fatigue with those of membrane types.

Coupling characteristics most important for motion control

High torsional stiffness/low radial stiffness. Low inertia Constant Velocity No wind-up Zero Backlash in Coupling components Shaft Interface Corrosion Resistance Withstand peak torque loads that are high relative to the nominal torque and size of the coupling.

Coupling Selection Factors

Horsepower Torque Speed Shaft Sizes Environment Conditions Type of Prime Mover Load Characteristics Space Limitations Maintenance and installation requirements.

Elastomeric Couplings

Include all designs that use a non-metallic element (rubber or plastic) within the coupling. Classified into compression or shear.

Keyways

Industrial applications use keys to transmit torque from shaft to coupling hub. Single square keyway is most popular on industrial coupling applications up to 6.5 inches diameter. Rectangular is used on larger bores.

Jaw with Elastomer In-Shear

Instead of the jaw hubs interlocking, the use of an in-shear spider pushes the hubs apart and aligns the jaws of each hub along the same axial plane. In-shear spider is twice the axial width of an standard spider. Urethane is the most common elastomer material available. Has a combination of durability, chemical resistance, and torque/load carrying strengths, Should be selected only for electric motor driven applications (centrifugal pumps, fans, mixers, gearboxes, and plastic extruding machines).

Compression type Elastomeric Couplings

Jaw Donut Pin & Bushing

Torsionally Stiff Couplings (Flywheel)

Jaw coupling, donut shaped compression loaded, and unclamped donut in shear are sometimes supplied with Hytrel or other stiff elastomers such as Zytel or urethane. Stiff elastomer is used for greater torque capability without going to a larger size. Stiff elastomers have less resilience and restrict the angular misalignment capability.

Hub-to-Shaft Mounting Attachments

Keyways Setscrews Splines Friction Interference (shrink) fits Tapered Shafts

Gear Couplings

King of coupling types (can do things that many other couplings cannot do). Usually made from carbon steel. Power Intensive. Coupling uses the meshing of gear teeth to transmit torque and to provide for misalignment. Achieve misalignment capability through backlash (looseness-of-fit). Misalignment reduces the load rating and life, but increases the wear of this coupling. Used for for med and large equipment applications. Commonly found in metal industry applications: steel or wire mills, paper mills where the high torque to size ratio and speed capabilities provide value.

Limitations of Bonded Urethan Tire

Large number of fasteners required for installation and removal of the elastomer. High Fatigue of the element and the bond between steel and elastomer under torsional vibrations.

Bore Limited Couplings

Limited by the hub bore size because the flex element is capable of transmitting all the torque that shaft size will normally deliver.

Torque Limited Couplings

Limited by the torque capability of the flexible element. Elastomeric coupling fall under this category due to the flexing element running out of torque carrying capacity before the connecting shafts reach their full torque potential.

Peak Torque

Max torque required by the system. Normally a one time event or limited to a specified number of occasions.

Non-Lubricated Coupling Types

Metallic Flexible Element Couplings Disc Couplings Diaphragm coupling Triple Wound Spring Coupling

Applications of Grid Spring Couplings

Most common type of driver is an electric motor. Ability of coupling to dampen vibrations allows it to be used with reciprocating engines. Excels in applications where an all metal coupling is desired but moderate vibration damping is required.

Service Factor Rated Horsepower (Torque)

Nameplate capabilities are multiplied by the service factor. Service factor is also shown on the nameplate.

Diaphragm Coupling

O.g. developed as an infinite life, non lubricated, low maintenance design for aircraft applications. Introduced into industrial applications to meet the demands of high-speed high horsepower service in petrochemical processing. Flexible element is a metal plate loaded in shear ny introducing torque at the outside diameter and transferring it to the inside diameter. Used for extreme applications of torque and speed and where reliability is paramount. (helicopter drives to high horsepower high-speed gas turbine drivers for generators and compressors.)

Advantages of Gear Couplings

Offer more modifications. A wider size torque bore range than any other type. Capable of transmitting high torque at high speed. Have axial slide capability, low speed high torque capability, shifter capability, and spindle capability.

Shear Donut Sleeve Designs

One Piece Solid - Identified by material as EPDM, neoprene, and Hytrel. Least expensive of the rubber sleeves, available in various sizes. One Piece Split - Identified by material as EPDM and neoprene. Used where the shafts are positioned closely together and sleeve must be "peeled away" for replacement. Two Piece Split - Made up of two completely separated halves. For the EDPM and neoprene styles, a retaining ring is used to prevent the sleeve from bowing outward or being flung off under speed. Hytrel does not require a ring. Design provides the greatest ease of installation and replacement.

Axial Displacement

One type of misalignment that must be handled by coupling. The change in axial position of the shaft and part of the coupling in a direction parallel to the axial centerline. Caused by thermal growth or a floating rotor, some couplings limit this displacement and are called limited end float couplings.

Curved Jaw Couplings

Product available in bore sizes up to 5/32" - 5 11/16" and torque from 35 in lbs - 66,375 in lbs. Faces of jaws have a concave or cupped shape provides a built in encapsulation of the spider legs by the hubs. Corresponding spider legs are crowned or curved both axially and radially to follow the jaw face shape.

Features of Jaw with Elastomer In-Shear

Radial removable spider. In-shear design for non-failsafe operation No metal-to-metal contact should the elastomer fail. No need for tools to install or replace the elastomer. Non-lubrication benefit of an elastomeric coupling. 2 deg angular misalignment.

Advantages of Bonded Urethane Tire

Radial removal of the element halves. High angular misalignment capability (4 deg) Shock Load Cushioning. In its standard close coupled configuration, it can span greater BSE length than most compression couplings. Has large opening in the center of the tire to allow complete flexibility in positioning shaft ends. Outside diameter of design is smaller than the corded tire type for similar shaft and torque capacities.

Limitations of Elastomeric Couplings

Sensitive to chemical and high temperatures. Usually not torsionally stiff enough for positive displacement. Larger in outside diameter than metallic coupling with the same torque capacity. Difficult to balance as an assembly. Some types do not have a good overload torque capacity. Speed is a problem for elastomeric couplings. The deflection of an elastomeric coupling is large for the load applied.

Compression Block Type

Serve heavy duty applications that require shaft size and/or torque ratings beyond the capability of standard jaw couplings. Transmit torque through independent blocks of elastomeric material, in cube, oval, or wedge shapes. Advantage of easily changing its torsional stiffness by varying the hardness and design of the blocks. Used in compressors, large fans, blowers, mixers, and municipal or irrigation pumps.

Shear - Type Donut

Shear Loaded Design. 2 flanges and 1 sleeve. Sleeve is a spool shaped tubular element with serrations molded around the perimeter which mate with corresponding serrations molded into both hub flanges.

Benefits of In Shear Spider

Simplified Maintenance. Non-failsafe operation. Greater angular misalignment capacity of 2 deg. Greater Torsional Softness.

Advantages of Grid Spring Couplings

Slots are tapered to allow space for grid to flex providing for torsional flexibility under different loads, vibration dampening, and cushioning of shock loads. As load increases more of the tooth comes in contact with the grid, allowing the coupling to handle shock loads. High torque-to-outside diameter ratio. Less expensive alternative where torque levels start to require comparably related elastomeric couplings to be much larger. Resilience of the grid gives this design a damping capability that is not available with an all-metal coupling.

Damping

Some couplings greatly reduce the amount of vibration transmitted between driver and driven shafts because of the damping capacity of an elastomer in the coupling.

Limitations to the corded tire type coupling

Speed and Space. As speed increases the coupling exerts axial forces on the shafts due to the centrifugal forces working on the elastomer. Geometry of the tire makes for a large outside diameter for its torque capability. Design variation can be made which includes an inverted tier coupling in which the tire element arcs inward toward the axis, thus overcoming the centrifugal forces at speed.

Advantages of Diaphragm Coupling

Stronger than disc coupling. Made in larger diameters than disc coupling. Has more axial displacement than disc coupling. Disconnects upon failure, causes it to become a fusible link.

Bonded Urethan Tire

Success in the process pump industry because of several features that the corded tire lacks. Design utilizes a urethane material that is bonded to two half circle metal rings which are then bolted two to hubs. Torque is transmitted from the hubs through the shoes and then the shear plane of the split urethane tire. Bushings are commonly used on this style of coupling. Ideal in applications such as pumps, screw compressors, blowers, mixers, crushers, and general power tramsmission drives.

Limitations of Grid Spring Couplings

Tends to be higher maintenance than an elastomeric type coupling due to its lubrication requirement, lesser misalignment capacity and installation time.

Backlash

The amount of free movement between two rotating, mating parts.

Bore

The central hole that becomes the mounting surface for the coupling on the shaft.

Distance between shafts

The distance between the faces (or ends) of driving and driven shafts, usually expressed as the "BE" (between ends) dimension or "BSE" (between shaft ends) dimension.

Axial Forces

The driver or driven equipment can generate thrust forces in which case the coupling will pass those forces to the next available bearing with thrust capability.

Stall or Lockup Torque

The torque that passes through the system when the system stalls or otherwise comes to a stop because of some activity within the driven system.

Finite Life Coupling

Those that wear in normal operation, because of using sliding or rubbing parts to transmit torque and compensate for misalignment. Jaw, gear, grid, sleeve (shear), nylon sleeve gear, chain, offset and pin & bushing types. Usually have lower purchase costs than infinite life couplings. Periodic maintenance is required.

Shear Type Elastomeric Couplings

Tire Sleve Molded Elements

Starting Torque

Torque needed when the system starts its operation. Torque can be greater or less than the normal operating torque.

Shutdown Torque

Torque required to bring the equipment from operating conditions to a shut down condition. Can be the normal braking torque or could be a result of friction or load in a system that is coasting to a stop.

Torque to accelerate or to decelerate

Torque required to increase or decrease the equipment operating speed. In the case of acceleration, the available torque for acceleration is the difference between the driver capability and the system requirements at the current speed. Decelerating torque comes from braking devices or from frictional drag or other energy drains within the system that cannot be overcome by the driver.

Normal Breaking Torque

Torque used to decelerate or reduce the speed of equipment when the brakes are applied in a normal manner. Torque is time dependent, and moves through the system.

Advantages of Elastomeric Type Couplings

Torsionally Soft. No lubrication or maintenance. Good vibration damping and shock absorbing qualities. Field replaceable elastomers Usually less expensive than metallic couplings that have the same bore capacity. Lower reactionary loads on bearings. More misalignment allowable than most metallic types.

Advantages of Metallic Couplings

Torsionally stiff Good high temperature capability Good chemical resistance with proper materials selction. High torque in a small package. High speed and large shaft capability. Available in stainless steel. Zero backlash in may types. Relatively low cost per unit torque transmitted.

Coupling Characteristics

Two Categories of flexible: "Finite life" or "infinite life"

Motion Control

Two shafts to be synchronized, moving at exactly the same speed to exactly the same location. Found in applications: shaft encoders, revolvers, servo devices, linear and ball screw actuators, robots, step motors, light duty pumps, and metering devices, plotters, medical equipment, positioning tables, computers, and radar. Designed for low inertia couplings.

Plastic Chain Couplings

Use nylon chain which requires no lubrication. Suitable for light loads in applications where lubricants are undesirable because of possible contamination. Also suitable for corrosive atmospheres. Quieter in operation than metal chain couplings and require no protective cover. Applications include: textile and food industry

Metallic Flexible Element Couplings

Use various metal alloys to achieve high power density. Uses the flexibility of thin metal elements to handle misalignment requirements while strength of metal is used to transmit torque. Flexibility is gained through either loose fitting parts which roll or slide against one another (gear,grid, chain) - sometimes referred to as "mechanical flexing" or through flexing/bending of a membrane (disc, flex link, diaprhagm, beam, bellows). Those with moving parts are less expensive but need to be lubricated and maintained. Primary cause of failure is wear (overloads shorten their life through increased wear rather than sudden failure). Membrane types are more expensive but do not need lubrication and require little maintenance Primary cause of failure is fatigue (can fail quickly when overloaded).

Silent Chain Couplings

Used for heavy duty drives requiring high torque. Operate at speeds up to 5,000 RPM depending on their size.

Straight Side Jaw Coupling

Used for light to medium duty applications with a torque capacity of 6,228 in-lbs. Angular and Parallel misalignment present. 1/2-1 deg, 0.010" - 0.015"

Cylindrical Hub

Used in donut couplings when one hub is smaller than the other. Fits inside the donut.

Torsional Couplings

Uses a resilient elastomer as the flexing medium. Elastomer shape used in this coupling is important for damping and damping is an important attribute for the torsional coupling. Most successful shapes are radially loaded cylinders, toruses, and sphere.

Triple-Wound Spring Couplings

Uses a tension spring fitted with rounded cap like hubs on both ends to facilitate shaft mounting. Middle coil is wound in a direction opposite the inner and outer coils to transmit torque effectively in either direction. Flexing the spring accommodates angular misalignment. Each spring can flex independently creating two flex planes that accommodate parallel misalignment. Has both backlash and windup. Sometimes used for indexing, positioning, or robotic applications. Must not be operated in excess of the max capability or the failure is likely to be sudden and dramatic.

Nameplate Rated Horsepower (Torque)

Value is derived from the driver capability shown on the nameplate as a horsepower and a speed. Based on specific inputs to the driver such as voltage, and amps or kVA(electric motor).

Applications of the Shear Type Donut Style

Where system alignment is hard to maintain over a long period of time and the coupling needs to tolerate the drift. Non-piloted pump application - Motor and pump are on a common base plate but there is no pump mounting bracket. Where shafts are closely coupled (minimal "BE" dimensions). Where shafts are relatively small for the torque loads, or the bearings are light duty. Works well on electric motor driven applications with uniform loads (centrifugal pumps, blowers and fans, screw compressors, some conveyors, line shafts, and vacuum pumps.

Which coupling has the largest outside diameter of the three: disc, gear, or diaphragm?

diaphragm

Normal Operating Torque

steady state torque required by the system when operating the system at normal design conditions. The level at which the equipment designer certifies the equipment performance.