Handbook Of Shaft Alignment Episode 1 Part 7 doc

Difficult to calculate reaction forces and moments of turbomachinery rotors when usingthese couplings since the values for the coefficient of friction between the gear teeth varyconsider

. Requires lubrication

. Temperature limitation due to lubricant

. Difficult to calculate reaction forces and moments of turbomachinery rotors when usingthese couplings since the values for the coefficient of friction between the gear teeth varyconsiderably

Coupling hubs and sleeves on shafts

Coupling hubs and sleeves on spool

FIGURE 4.8 Two different variations of gear couplings employing a spool piece

Centerline of coupling hub (with gear teeth)

FIGURE 4.9 Tilted and pivoted positions of the gear teeth in its sleeve

4.4.1.3 Metal Ribbon Couplings

The metal ribbon coupling was introduced around 1919 by Bibby Co Metal ribbon couplingsconsist of two hubs with axial ‘‘grooves’’ on the outer diameter of the hub where a continuousS-shaped grid meshes into the grooves Misalignment and axial movement is achieved byflexing and sliding of the grid member in specially tapered hub ‘‘teeth.’’

. Capacity: up to 70,000 hp=100 rpm

. Maximum recommended speed: to 6000 rpm

. Shaft bores: to 20 in

. Shaft spacing: to 12 in

Tilted position

Pivoted position

Pivoted position Tilted position Pivoted position

FIGURE 4.10 Gear tooth tracking pattern when subjected to misalignment conditions

. Special designs and considerations: Grid fabricated from hardened, high-strength steel.Close coupled hubs with removable spacer available.

Advantages:

. Easy to assemble and disassemble

. Long history of successful applications

4.4.1.4 Universal Joint Couplings

Perhaps the oldest flexible coupling in existence is the universal joint coupling This coupling

is also known as the Cardan or Hooke joint The basic design consists of U-shaped shaft endswith a hole drilled through each ‘‘U’’ to accept a ‘‘þ’’ shaped cross.

If one universal joint is used to connect two shafts together, then only pure angularmisalignment can exist where the centerlines of rotation intersect at the center of the ‘‘þ’’

shaped cross For a flexible coupling to accept both parallel and angular misalignment, thereFIGURE 4.11 Metal ribbon type coupling (Courtesy of Falk Corporation, Milwaukee, WI Withpermission.)

must be two flexing points Therefore most universal joint couplings have two cross=yolkassemblies as illustrated in Figure 4.12 and Figure 4.13.

When one universal joint is used it is important to recognize that variations in angularvelocity will occur between the two connected shafts often referred to as the ‘‘Cardan error.’’Sinusoidal motion will occur in the axial and torsional directions, producing axial vibrationand torsional (i.e., twisting) vibration particularly if the torque is transmitted and therotational speed is high

When two universal joints are used it is important to recognize that sinusoidal motion willalso occur in the axial and torsional directions if the ‘‘entrance’’ and ‘‘exit’’ angles are not thesame as shown in Figure 4.13 When these angles are the same in both planes, perfectkinematic balance exists across the coupling, canceling the torsional and thrust variance.4.4.1.5 Flexible Link

The flexible link coupling utilizes a series of cross laced, metallic links with one end of eachlink attached to a disc mounted on the driven shaft and the other end of each link attached to

a disc mounted on the driver shaft The links are matched in pairs so that when one is intension, the other is in compression Misalignment and axial displacement is accomplished by

a flexing action in the series of cross links

FIGURE 4.12 Universal joint (Courtesy of Zurn Industries, Erie, PA With permission.)

Yoke ears not in proper orientation on connector shaft

Angle not the same

as other end

With a single universal joint, if the input and output shafts are not in line, a variation of the output shaft speed (w) will result called Cardan error When

two universal joints are used where the entrance and exits angles are the same

with the yokes aligned properly, the system is kinematically balanced producing synchronous shaft rotation at the input and output ends.

FIGURE 4.13 Universal joint basics

. Capacity: up to 1100 hp=100 rpm.

. Maximum recommended speed: to 1800 rpm

. Shaft bores: up to 20 in

. Shaft spacing: close coupled or 100 mm spacer with certain designs

FIGURE 4.14 Flexible link coupling (Courtesy of Eaton Corporation, Airflex Division, Cleveland, OHunder license from Dr Ing Geislinger & Company, Salzburg, Austria With permission.)

FIGURE 4.15 Leaf spring coupling (Courtesy of Eaton Corporation, Airflex Division, Cleveland, OH.With permission.)

. Special designs and considerations: An axial ‘‘fixation’’ device can be installed to preventany axial movement if desired Different designs can accommodate unidirectional orbidirectional rotation.

Advantages:

. No lubrication required

Disadvantages:

. Limited axial movement

. Limited misalignment capabilities (can accept pure angular misalignment only)

4.4.1.6 Leaf Spring

This coupling employs a series of radially positioned sets of leaf springs attached to an outerdrive member and indexed into axial grooves in the inner drive member The chamber aroundeach spring set is filled with oil When the spring pack is deflected, damping occurs as the oilflows from one side of the spring pack to the other

. Capacity: up to 15,000 hp=100 rpm

. Maximum recommended speed: 3600 rpm

. Shaft bores: up to 12 in

. Shaft spacing: up to 40 in

. Special designs and considerations: Designed primarily for diesel and reciprocatingmachines Capable of transmitting shock torque values substantially higher than othercouplings until springs reach their maximum allowable angular movement where theradial stiffness increases substantially Various spring stiffnesses can be installed in eachsize coupling to properly match the torsional requirements to the drive system

Advantages:

. Torsionally soft with good damping characteristics

. Freedom of axial shaft movement

FIGURE 4.16 Pin drive type coupling (Courtesy of David Brown Gear Industries, Agincourt, Ontario,Canada With permission.)

. Requires lubricant for damping

. Temperature limitations due to lubricant

. Torsional characteristics change drastically with loss of oil

4.4.1.7 Pin Drive

A series of metal pins with leaf springs are placed near the outer diameter where they engage into

a series of holes bored into both shaft hubs Some pin designs consist of a pack of flat springs withcylindrical keepers at each end that act as the flexing element in the coupling design The springsets can swivel in the pin connection to allow movement across the width of the spring set.. Capacity: up to 3800 hp at 100 rpm

. Maximum recommended speed: to 4000 rpm

. Shaft bores: to 13 in

. Shaft spacing: close coupled (1=8 to 1=2 in.)

. Special designs and considerations: Drive pins can be fabricated to accommodate varioustorsional flexibility requirements and are indexed into oil impregnated bronze bushings inthe coupling hubs

. Capacity: up to 67,000 hp=100 rpm but varies widely with design

. Maximum recommended speed: approximately 5000 rpm (varies widely with design). Shaft bores: up to 30 in

. Shaft spacing: up to 100 in (varies widely)

. Special designs and considerations: A considerable amount of inventiveness and ingenuityhas been applied to this type of coupling design through the years as evidenced by the largearray of design variations The elastomeric medium is generally natural or syntheticrubber, urethane, nylon, teflon, or oil-impregnated bronze As the elastomer is markedlysofter than the hubs and solid-driving elements (wedges, pins, jaws, etc.), wear is minimaland replacement of the elastomer itself is all that is usually needed for periodic servicing.Advantages:

. Minimal wear in coupling

. Acts as vibration damper and isolator

FIGURE 4.17 Elastomeric couplings [Courtesy of (a and b) Lovejoy Corporation, Downers Grove, IL;(c) Holset Engineering Company, Cincinnati, OH; (d) T.B Wood’s Sons, Chambersburg, PA.With permission.]

(continued )

FIGURE 4.17 (continued)

FIGURE 4.18 Contoured diaphragm coupling: (a) cutaway view of the flexible disk at one end of thecoupling; (b) entire coupling (Courtesy of Kopflex Corporation, Baltimore, MD With permission.)

. Acts as electrical shaft current insulator in some designs

. Potential safety hazard if elastomeric member releases from drive elements

. Some designs may cause undesirable axial forces

. Heat generated from cyclic flexing of elastomer

4.4.2 METALLICMEMBRANE=D ISK-TYPECOUPLINGDESIGNS

4.4.2.1 Diaphragm Couplings

Transmission of power occurs through two flexible metal diaphragms, each bolted to theouter rim of the shaft hubs and connected via a spacer tube Misalignment and axialdisplacement is accomplished by flexing of the diaphragm members

. Capacity: up to 30,000 hp

. Maximum recommended speed: up to 30,000 rpm

. Shaft bores: up to 7 in

. Shaft spacing: 2 to 200 in

. Special designs and considerations: Metal diaphragm couplings are a highly reliabledrive component when operated within their rated conditions Exceeding the maximumallowable angular or parallel misalignment values or axial spacing will eventuallyresult in disc failure As the diaphragm is, in effect a spring, considerations must begiven to the axial spring rate and vibration characteristics to insure that the diaphragmcoupling natural frequency does not match rotating speeds or harmonics in the drivesystem

. Limited axial displacement and oscillation

. Proper shaft spacing requirements are generally more stringent than other coupling types. Excessive misalignment will transmit high loads to shafting

OD bolts Spacer bolts Diaphragm pack

Rigid hub Shipping

screw

Spacer (a)

Bolted Connections

FIGURE 4.19 Flexible disk couplings [Courtesy of (a) Zurn Industries, Erie, PA; (b) Thomas-Rexnord,Warren, PA; (c) Schmidt Couplings, Cincinnati, OH With permission.]

4.4.2.2 Flexible Disc Couplings

The flexible disc coupling is very similar in design principles to the diaphragm coupling withthe exception that multiple, thinner discs or a noncircular flexing member is used as theflexing element instead of circular, contoured diaphragm elements

FIGURE 4.20 Rigid coupling (Courtesy of Browning Mfg., Maysville, KY With permission.)

FIGURE 4.21 Rigid coupling on vertical pump

. Capacity: up to 65,000 hp=100 rpm

. Maximum recommended speed: up to 30,000 rpm

. Shaft bores: to 12 in

. Shaft spacing: to 200 in

. Special designs and considerations: It is important to note that two disc packs (ordiaphragms) are needed to accommodate parallel misalignment whereas a single disccan only handle pure angular misalignment Convolutions in the discs provide a linearstiffness vs deflection characteristics as opposed to flat disc profiles Once again, coup-ling axial resonance information must be known to prevent problems where a match mayoccur with machinery running speeds, higher order harmonics, or subsynchronous for-cing mechanisms (oil whirl, looseness of bearing housings, clearance induced whirls, etc.).Advantages and disadvantages:

. Same as diaphragm couplings

4.5 RIGID COUPLING DESIGN

Well before flexible couplings came into existence, rigid couplings were used to connect two(or more) shafts together Although flexible couplings are used on the vast majority ofrotating machinery drive systems today, rigid couplings still have their place and are fre-quently used on systems where very little misalignment occurs and in situations, where highhorsepowers are transmitted from shaft to shaft, or in vertical pump applications where one

of the drive motor bearings is carrying the weight (thrust) of the armature and pump rotors

It is important to recognize that when two shafts are connected together with rigidcouplings, the two separate shafts have effectively become one continuous shaft Thereforethe ‘‘misalignment’’ tolerances for rigid couplings are the same tolerances that apply foracceptable runout conditions on a single shaft as discussed in Chapter 6 There are two classicrigid coupling alignment techniques—the 16-point and 20-point methods and they are dis-cussed in Chapter 7 but other techniques have also been successfully employed on rigidcouplings It is highly recommended that rigid couplings be disconnected when takingalignment measurements between the two shafts as explained in Chapter 1 and Chapter 2

4.6 FLEXIBLE COUPLING LUBRICATION

The majority of mechanically flexible couplings require lubrication There are basically twomethods used to lubricate couplings: single charge or continuous feed Greases are generallyused in single charge lubricated couplings and the type is generally specified by the couplingmanufacturer

Problems that can occur in greased packed couplings are:

1 Loss of lubricant from leakage at: lube seals, shaft keyways, mating flange faces, orlubricant filler plugs

2 Excessive heat generated in the coupling from an insufficient amount of lubricant,excessive misalignment, or poor heat dissipation inside the coupling shroud, whichreduces viscosity and accelerates oxidation

3 Improper lubricant

4 The centrifugal forces generated in the coupling can be high enough to separate greasesinto oils and soaps

As soaps have a higher specific gravity than oil, it will eventually collect where the force isthe highest (namely where the gear teeth are located), causing a buildup of sludge.

Periodically inspect the inside of the coupling guard and directly under the coupling to see ifany leakage is occurring If so, do not continue to add more grease because the oil usuallyleaks out and the soaps continue to build up Thoroughly clean the coupling, replace the sealsand gaskets, and replenish with the correct kind and amount of lubricant, preferably a

‘‘coupling grease’’ in which the oil and soap have the same specific gravity so they do notseparate under centrifugal force

Continuous feed lubrication systems generally use the same lube oil as the bearingsand spray tubes are positioned to inject a directed stream of oil into the coupling as shown

in Figure 4.22

In addition to supplying lubricant to the coupling, a continuous supply of oil acts as anexcellent heat transfer agent, maintaining a relatively stable temperature in the coupling.However contaminants in the oil, particularly water (which often condenses in lube oil tanks)

or corrosive process gases carried over from the inboard oil seals on compressors, can damagethe coupling in time Stainless steel lube oil piping, condensate and particulate matter removalwith lube oil centrifuges, 5–10 mm filters, and entrained gas venting systems will alleviatemany of these problems

4.7 COUPLING INSTALLATION

Once a flexible coupling has been selected for a specific service, the next important step isproper installation It is quite easy to destroy an expensive coupling assembly in short orderdue to sloppy shaft fits, incorrect key dimensions, improperly measured shaft diameters,storage in a corrosive environment, and so on After the coupling has been uncrated, thefollowing steps should be performed before installation is even attempted

1 Insure that the correct type of coupling was ordered and all the parts are with it (bolts,spacer spool, hubs, cover, gaskets, etc.) Clean any and all protective coatings from thesurfaces Thoroughly inspect each component for damage

2 Physically measure all the dimensions against the coupling drawing and partslistings paying particular attention to coupling hub bores, keyway dimensions, andspool length

Oil return Coupling sleeve

3 Measure the shafts where the coupling is going to be installed Measure the outsidediameters of each shaft, the keyway widths and depths, and the distance between theshafts, insuring that each shaft has been placed in its normal axial position duringoperation as shown in Figure 4.24 through Figure 4.26.

4 If possible, assemble the entire coupling before it is placed on the shaft checkingfor proper gear tooth clearances, elastomeric member fits, bolt hole diameter fits,and clearances

4.8 COUPLING HUB ATTACHMENT METHODS

There are a variety of methods employed to attach the coupling hubs to a shaft, each onehaving its advantages and disadvantages Recommended guidelines for installing these vari-ous shafts to coupling hub fits are outlined ahead and should be followed to insure a properfit to prevent slippage or unwanted shaft fretting Shaft fretting occurs when a coupling hub isFIGURE 4.23 Pitting across entire surface of gear teeth from operating with no lubricant