sectioni b

7/29/2019 Sectioni b

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Section I-B

Flexible Couplings

Page I-B

I-B. GENERAL SELECTION CONSIDERATIONS

B. GENERAL SELECTION CONSIDERATIONS I-B-1

1. Selection Factors I-B-1

a. Selection Steps I-B-1

b. Types of Information Required I-B-2

c. Interaction of a Flexible Coupling I-B-3

d. Final Check I-B-3

2. Couplings Ratings I-B-3

a. Torque Ratings: I-B-3

3. Design and Selection Criteria Terms I-B-4

a. Factor of Safety I-B-4b. Service Factor I-B-4

c. Endurance Limit I-B-5

d. Yield Limit (Y.L.) I-B-5

e. Coupling Rating I-B-5

f. Maximum Continuous Rating (M.C.R.) I-B-5

g. Peak Rating (P.R.) I-B-5

h. Maximum Momentary Rating (M.M.R.) I-B-5

4. Continuos Operating Conditions and Fatigue Factor of Safety I-B-5

a. Analysis of a Flexible-element I-B-5

b. Summary of Stresses I-B-7

5. Peak and Maximum Momentary Conditions I-B-86. Recommended Factors of Safety I-B-8

7. Service Factors I-B-9

a. General Service Factors I-B-9

b. API Recommended Service Factor I-B-9

8. Speed Ratings I-B-9

a. Maximum Speed Based on Centrifugal Stresses. I-B-9

b. Other Considerations for Maximum Speed. I-B-10

9. Ratings in Summary I-B-10

10. Moments and Forces Transmitted by Couplings I-B-10

a. Bending Moment I-B-10

b. Axial Force I-B-10


2/11

Section I-B

Flexible Couplings

Page I- B-1

B. GENERAL SELECTION CONSIDERATIONS

1. Selection Factors

Flexible couplings are a vital part of a me-chanical power transmission system. Unfortu-

nately, many system designers treat f lexible

couplings as if they w ere a piece of hardw are.

The amount of time spent in selecting a cou-

pling and determining how it interacts w ith a

system should be a function not only of the cost

of the equipment but also how much dow ntime it

w ill take to replace the coupling or repair a fail-

ure. In some cases the process may take only a

little time and be based on past experience;

how ever, on sophisticated systems this may

take complex calculations, computer modeling,

Finite Element Analysis (FEA) and possibly even

testing. A system designer or coupling user just

cannot put any f lexible coupling into a system

w ith the hope that It w ill w ork. It is the system

designer or the user's responsibility to select a

coupling that w ill be compatible w ith the system.

Flexible coupling manufacturers are not usually

system designers; rather, they s ize, design, and

manufacture couplings to fulfill the requirements

supplied to them. Some coupling manufacturers

can offer some assistance based on past expo-

sure and experience. They are the experts in the

design of f lexible couplings, and they use their

know-how in design, materials, and manufactur-

ing to supply a standard coupling or a custom-

built one if requirements so dictate. A f lexible

coupling is usually the least expensive major

component of a rotating system. It usually costs

less than 1%, and probably never exceeds 10%.

of the total system cost. Flexible couplings are

called upon to accommodate for the calculated

loads and forces imposed on them by the equip-

ment and their support structures. Some of these

are motions due to thermal grow th deflection of

structures due to temperature, torque-load varia-

tions, and many other conditions. In operation,

flexible couplings are sometimes called on to ac-

commodate for the unexpected. Sometimes these

may be the forgotten conditions or the conditions

unleashed w hen the coupling interacts w ith the

system, and this may precipitate coupling or

system problems. It is the coupling selector's re-sponsibility to review the interaction of the cou-

pling w ith that system. When the coupling's inter-

action w ith the sys tem is forgotten in the selec-

tion process, coupling life could be shortened

during operation or a costly failure of coupling or

equipment may occur.

a. Selection StepsThere are usually four steps that a coupling se-

lector should take to assure proper selection of a

flexible coupling for critical or vital equipment.

1.The coupling selector should review the initial

requirements f or a flexible coupling and select

the type of coupling that best suits the sys-

tem.

2.The coupling selector should supply the cou-

pling manufacturer w ith all the pertinent infor-

mation on the application so that the coupling

may be properly s ized, designed1 and manu-

factured to f it the requirements.

3.The coupling selector should obtain the flexi-

ble couplings characteristic information so the

interactions of the coupling w ith the system

can be checked to assure compatibility such

that no determinal forces and moments are

unleashed.

4.The coupling selector should review the inter-

actions and if the system conditions do

change, the coupling manufacturer should be

contacted so that a review of the new condi-

tions and their effect on the coupling selected

can be made. This process should continue

until the system and the coupling are compati-

ble. The complexity and depth of the selection

process for a flexible coupling depend on

how critical and how costly downtime w ould

be to the ultimate user. The coupling selector

has to determine to w hat depth the analysis

must go. As an example, a detailed, complex

selection process w ould be abnormal for a

15-hp motor-pump application, but w ould not


3/11

Section I-B

Flexible Couplings

Page I- B-2

be for a multimillion-dollar 20,000-hp motor,

gear, and compressor train.

b. Types of Inform ation RequiredCoupling manufacturers w ill know only w hat

the coupling selector tells them about an applica-

tion. Missing information required to select and

size couplings can lead to an improperly s ized

coupling and possible failure. A minimum of three

items are needed to size a coupling: horse-

pow er, speed, and interface information. This is

sometimes the extent of the information supplied

or available. For coupling manufacturers to do

their best job, the coupling selector should supply

as much information as is felt to be important

about the equipment and the system. See Figure

I-B-1 for the types of information required. Fig-

ures I-B-2 and I-B-3 list specific types of infor-

mation that should be supplied for the most com-

mon types of interface connections: the flange

connection and the cylindrical bore.

Figure I-B-1 Types of Information Required

1. Horsepow er

2. Operating speed

3. Interface connection information4. Torques

5. Angular misalignment

6. Offset misalignment

7. Axial travel

8. Ambient temperature

9. Potential excitation or critical frequencies

a. Torsional

b. Axial

c. Lateral

10. Space limitations11. Limitation on coupling generated forces

a. Axial

b. Moments

c. Unbalance

12. Any other unusual condition or require-

ments or coupling characteristics

Note: Information supplied should include all op-

erating or characteristic values of equipment for

minimum, normal, steady state, transient, peak

and the f requency of their occurrence.

Figure I-B-2 Information Required for Cylin-

drical Bores

1. Size of bore including tolerance or size of

shaft and amount of clearance or interfer-

ence required

2. Length

3. Taper shaft

a. Amount of taperb. Position and size of o-ring grooves if re-

quired

c. Size and location of oil distribution grooves

d. Size and location of oil distribution grooves

e. Max. pressure available for mounting

f. Amount of hub draw -up required

g. Hub OD requirements

h. Torque capacity required

4. Minimum strength of hub material or its

hardness

5. If keyw ays in shaf t

a. How many

b. Size and tolerance

c. Radius required in keyway

d. Location tolerance of keyw ay respective

to bore and other keyw ays

Figure I-B-3 Types of Interface Inform ation

Required for Bolted Joints

1. Diameter of bolt circle and true location2. Number and size of bolt holes

3. Size, grade and types of bolts required

4. Thickness of w eb and f langes

5. Pilot dimensions

6. Other


4/11

Section I-B

Flexible Couplings

Page I- B-3

c. Interaction of a Flexible Cou-pling

A very important but simple fact often over-

looked by the coupling selector is that couplingsare connected to the system. Even if they are

selected, sized, and designed properly, this does

not assure trouble-free operation. Flexible cou-

plings generate their own forces and can also

amplify system forces. This may change the

systems original characteristics or operating

conditions. The forces and moments generated

by a coupling can produce loads on equipment

that can change misalignment, decrease the life

of a bearing, or unleash peak loads that can

damage or cause failure of the coupling or con-

nected equipment.

Listed below are some of the coupling char-

acteristics that may interact w ith the system. The

coupling selector should obtain from the coupling

manufacturer the values for these characteris-

tics and analyze their affect on the system.

1.Torsional stiffness

2.Torsional damping

3.Amount of backlash

4.Weights

5.Coupling f lyw heel eff ect*

6.Center of gravity

7.Amount of unbalance

8.Axial force

9.Bending moment

10.Lateral stiff ness

11.Coupling axial cr itical **

12.Coupling lateral critical **

*The flyw heel effect of a coupling (WR2) is the

product of the coupling w eight times the square

of the radius of gyration. The radius of gyration

is that radius at w hich the mass of the coupling

can be considered to be concentrated.

**Coupling manufactures w ill usually calculate

coupling axial and lateral critical values w ith the

assumption that the equipment is infinitely rigid.

A couplings axial and lateral critical values com-

prise a couplings vibrational natural frequency in

terms of rotational speed (rpm)

d. Final CheckThe coupling selector should use the coupling

characteristics to analyze the system axially, lat-

erally, thermally,and torsionally. Once the analy-

sis has been completed, if the system's operating

conditions change, the coupling selector should

supply this information to the coupling manufac-

turer to assure that the coupling selection has

not been sacrif iced and that a new coupling size

or type is not required. This process of supplying

information between the coupling manufacturer

and the coupling selector should continue until

the system and coupling are compatible. This

process is the only w ay to assure that a system

w ill operate successfully.

2. Couplings Ratings

a. Torque Ratings:It has become more and more confusing as

to w hat Factor of Safety (F.S.) and Service

Factor (S.F.) (also Application or Experience

Factor) really mean. Many people use Service

Factors and Factor of Safety interchangeably.

There is an important distinction, how ever, and

understanding the difference is essential to

ensure a proper coupling selection for a

particular application.

1) Factors of Safety.

Factor of Safety are used in the design of a

coupling. Coupling designers use Factors of

Safety because there are uncertainties in the

design. The designer's method of analysis uses

approximations to model the loading and there-

fore the calculated stresses may not be exact.

Likew ise, the material properties such as

modulus, ultimate strength, and fatigue strength

have associated tolerances that must be consid-

ered. Today, w ith the use of such computa-

tional tools as FEA, stress analysis is generally

capable of more accurate results than in the

past. In addition, the properties of the materials


5/11

Section I-B

Flexible Couplings

Page I- B-4

used in most coupling components are more

controlled and better know n. Therefore, cou-

plings designed today vs those designed tw enty

years ago can indeed operate safely w ith low ercalculated factors of safety. Also, the design

factor of some couplings such as Flexible Ele-

ment couplings can be lower than other types of

couplings simply because the "safeness" is more

accurately predicted.

Some couplings of today have s tress loading that

is more easily determined. The stresses f rom

misalignment, axial displacement, and torque are

generally more accurately know n than is the

case w ith a other coupling. Some couplings

(such as gear couplings) have a number of vari-

ables that affect their design (such as tooth

form, surface f inish, materials, temperature, and

lubrication), "life" and "safeness" are difficult to

evaluate for many mechanically flexible and

elastomeric type couplings. Generally, torque is

the most s ignificant load contributor to the overall

stress picture in most couplings. The fatigue

factor of safety of a Flexible-Element couplings

are generally not as affected by torque, because

the failure mode in dry couplings is not very sen-

sitive to torque during continuous operating con-

ditions.

2) Service Factors

On the other hand service factors are used

to account for the higher operating torque condi-

tions of the equipment to w hich the coupling is

connected. A service (or experience) factor

should be applied to the normal operating torque

of, for instance, a turbine or compressor. This

factor accounts for torque loads w hich are not

normal but w hich may be encountered continu-

ously such as low temperature driver output,

compressor fouling, or possible vibratory tor-

ques. Also service factors are some times used

to account for the real operating conditions,

w hich may be 5-20% above the equipment rat-

ing. Different Service Factors are used or rec-

ommended depending on the severity of the ap-

plication. Is it a smooth running gas turbine driven

compressor application or w ill the coupling be

installed on a reciprocating pump application?

Also remember that Service Factors should beapplied to continuous operating conditions rather

being used to account for starting torques, short

circuit conditions, rotor rubs, etc.

3. Design and Selection Criteria Terms

a. Factor of SafetyFactor of Safety (F.S) is used to cover un-

certainties in a coupling design; analytical as-

sumptions in stress analysis, material unknow ns,

manufacturing tolerances, etc. Under given de-

sign conditions the F.S. is the ratio of strength (or

stress capacity) to actual predicted stress;

w here the stress is a function of torque, speed,

misalignment, and axial displacement. A Design

Factor of Safety (D.F.S.) is the Factor of Safety

at the catalog rated conditions of torque, speed,

misalignment, and axial displacement. It is used

by the manufacturer to establish the coupling

rating, because it is the maximum loading that the

manufacturer says his coupling can safely w ith-

stand. The Factor of Safety that most w ould be

interested in however, is the Factor of Safety at

the particular set of application loads that the

coupling is continuously subjected to. We have

defined this to be an Application Factor of Safety

(A.F.S.). In fact , the Application Factor of Safety

is the measure of safety w hich w ould allow the

coupling selector to evaluate the coupling the

"safeness" under actual operation and allow the

coupling selector to compare various couplings.

b. Service FactorService Factor (Application Factor or Expe-

rience Factor) (S.F.) is normally specified by the

purchaser (although assistance is sometimes

given by the manufacturer). It is a torque multi-

plier. It is applied to the operating torque (called,

the normal operating point in API 671) of the con-

nected equipment. The Service Factor torque

multiplier is used to account for torque loads that


6/11

Section I-B

Flexible Couplings

Page I- B-5

are beyond the normal conditions and are of a

recurring nature. Couplings are generally se-

lected by comparing the selection torque (S.F. x

Normal Operating Torque) to the coupling's maxi-mum continuous rating. Service Factors account

for conditions such as a compressor fouling,

changes of the pumped fluid (molecular w eight,

temperature or pressure), or any other repetitive

loading conditions that may occur over 106 revo-

lutions of the coupling. And sometimes Service

Factor are used to account for the real operating

conditions of the equipment, w hich may be 5-

20% above the equipment rating. Service Fac-

tors should not be applied to account for starting

torques, or short circuit torques, although these

conditions are sometimes stated as being a multi-

ple of normal torque.

c. Endurance LimitThe Endurance Limit is the failure strength

limit of a coupling component subjected to

combined constant and alternating stresses.

Beyond this limit the material can be

expected to f ail after some f inite number of

cyclic loads. Below this limit the material can

be expected to have infinite life (or a Factor

of Safety of greater than 1.0).

d. Yield Limit (Y.L.)The Yield Limit (Y.L.)- is determined by the

manufacturer to be the failure strength limit

of a coupling component that w ill cause

detrimental damage. If this limit is exceeded,

the coupling should be replaced.

e. Coupling RatingThe Coupling Rating is a torque capacity at

rated misalignment, ax ial displacement, and

speed. This applies to the ratings given

below .

f. Maximum Continuous Rating(M.C.R.)

The maximum continuous rating (M.C.R.) - is

determined by the manufacturer to be thetorque capacity that a coupling can safely

run continuously and has an acceptable

Design Factor of Safety.

g. Peak Rating (P.R.)The Peak Rating (P.R.) is determined by the

manufacturer to be the torque capacity that

a coupling can experience w ithout having

localized yielding of any of its components.

Additionally, a coupling can handle this

torque condition for 5,000-10,000 cycles

w ith out failing.

h. Maximum Mom entary Rating(M.M.R.)

The Maximum Momentary Rating (M.M.R.) is

determined by the manufacturer to be the

torque capacity that a coupling can

experience w ithout ultimate failure, where

localized yielding (damage) of one of its

components may occur. A coupling can

w ithstand this occurrence for one brief

duration. Af ter that, the coupling should be

inspected and possibly replaced. (This is

also sometimes called the Short Circuit

Torque Rating.)

4. Continuos Operating Conditions and

Fatigue Factor of Safety

We w ill use the f lexible element of a special

purpose coupling as a example of how an

endurance factor of safety is calculated. This

type of analysis is generally used on special

purpose application and can be used on any

metallic torque transmitting component of any

coupling (such as bolts, spacers, hubs, etc.)

a. Analysis of a Flexible-e lementThe diaphragm, diaphragm pack, or disc

pack is the heart of a flexible-element coupling


7/11

Section I-B

Flexible Couplings

Page I- B-6

and in general is the most highly stressed

component during continuous operation. It must

accommodate the constant (steady state, or

mean) stresses from axial displacement, torque,and centrifugal effects w hile also w ithstanding

the alternating (cyclic) stresses from angular

misalignment and possible alternating torques.

Note that normally other components of the

coupling such as flanges, tubes, and bolts may

not be subjected to the same magnitudes and

types of stresses.

To analyze a Flexible Element and determine

it's (and generally the coupling's) Application

Factor of Safety at different loading conditions,

it's Endurance Limit must be determined. The

problem here, though, is w hat failure criteria

should be used to determine this limit. What

assumptions are made in combining the

stresses? Once a criteria is selected, how is the

Factor of Safety determined? What is anappropriate Factor of Safety for a particular type

of coupling? There are many "correct" answ ers

to these questions, and generally the choices are

left up to the coupling manufacturers.

Lets consider the follow ing load conditions

and stresses (see Figure I-B-4 ) for, let's say, a

diaphragm coupling in a turbine driven

compressor application. (Note that the stresses

represented below are for illustration purposes):

Figure I-B-4 Load conditions and Stresses for a Diaphragm Coupling

Condition Amount Stress (psi)

Torque 400.000 in-lb 42,000 psi constant, shear

Speed 13,000 12,000 psi constant, bi-axial

Axial displacement +/- 0/120 35,000 psi constant, bi-axial

Angular Misalignment +/- 0.25O 17,000 psi alternating, bi-axial

In order to calculate the fatigue Factor of

Safety, there are four basic avenues that must

be taken. They are:

1.Determine the basic, normal stresses that

result f rom the stated operating conditions.

2.Apply an appropriate failure theory to

represent the combined state of stress.

3.Apply an appropriate fatigue failure criteria in

order to establish an equivalent mean, and an

equivalent cyclic stress from w hich to

compare the material fatigue strength.

4.Calculate the Factor of Safety

First, the w ay in w hich the above stresses

in this example w ere determined are subject to

evaluation. Various methods may be employed

to determine the normal stresses show n above.

These methods include classical solutions,

empirical f ormulas, numerical methods, and FEA.

The accuracy of each of these methods are

largely dependent on the loading assumptions

made in the analysis.

Second, after calculating the f luctuating normal

stresses, they must be combined to provide an

accurate representation of the bi-axial state of

stress by applying an appropriate failure theory.

Many theories may be employed. The most

accurate choice is generally a function of

material characteristics and the type of loading.

Among the failure theories that might be

employed are: Maximum Princ iple Stress,

Maximum Shear Stress, and Maximum Distortion

Energy (von Mises). Third, after an appropriate

failure theory has been applied, an equivalent

constant, and an equivalent alternating stress

must be determined by applying an appropriate

fatigue failure criteria. The possible choices here

include: Soderberg Criteria, Goodman Criteria,

Modified Goodman Criteria, and Constant Life

Fatigue diagrams Lastly,

a fatigue Factor of Safety can be determined by

comparing the equivalent stress to the fatigue


8/11

Section I-B

Flexible Couplings

Page I- B-7

failure strength. In order to compare the fatigue

strength to the equivalent stress, an assumption

must be made as to how the stress increase is

most likely to occur. The most common approachis a combination of constant and cyclic (a

proportional increase of all stresses and loads)

nature.

b. Summary of StressesIn this example, w e've combined the

stresses using the Distortion Energy Failure

Theory, and applied the Modified Goodman

Fatigue Failure Criteria to obtain combined mean

(constant) stress (sm) of 87,500 psi and a cyclic

(alternating) stress (sa) of 17,000 psi. The

endurance strength (Se) is 88,000psi, the yield

strength (Sty) is 165,000psi, while the ultimate

diaphragm material strength (Stu) is 175,000 psi.Using a Modified Goodman Diagram (Figure I-B5 ,

the constant and alternating stresses are plotted

and using the proportional increase method a

safety factor of 1.44 is found.

Figure I-B-5 Modified Goodman Diagram


9/11

Section I-B

Flexible Couplings

Page I- B-8

5. Peak and Maximum Momentary Condi-

tions

Just like continuous torque ratings, there aredifferent w ays to rate a coupling's capability to

handle non-continuous peak torques or low fre-

quency high cyclic torques. These can be

caused by such things as motor start-ups, short

circuit conditions, compressor surges, or other

transient conditions. Some questions to ask are:

What is the nature of the load? Is it due to a

synchronous motor start-up w ith hundreds of

high torque reversals during a daily start-up? Is

it a single unidirectional torque induction motor

driver application? As for the coupling, how

much capacity above the maximum continuous

rating is there before serious damage to the cou-

pling occurs? Some couplings have a catalogue

peak rating in the range of 1.33-2 times the

maximum continuous catalog torque rating, Most

of theses couplings can handle torques 1.75-2.5

times there normal rating before detrimental dam-

age occurs.

For the Factor of Safety for MaximumContinuous Rating, w e recommend that the

Factor of Safety be determined using the

Modified Goodman Criteria ( the most w idely

accepted fatigue failure criteria for steel

components). We further recommend that these

Factors of Safety be calculated by proportional

increase in stress assumptions (see Figure I-B-

5). For Peak Rating and Maximum Momentary

Rating the Factor of Safety is determined by the

ratio of Y ield Strength to the stresses calculated

at these ratings. Note that the factors listed

below are factors for rated conditions only and

that the factors for a particular application (w ith

or w ithout Service Factors applied) can be

expected to generally be higher.

6. Recomm ended Factors of Safety

The follow ing has been adopted by API 671 (see Figure I-B-6) w ith these Factors of Safety one

can compare couplings for a particular application. Below is the recommended minimum Design factors of

Safety for the various ratings Factor Basis of Safety

Finally, the values in the table are recommended as a guide and do not ref lect how good a job that

w as done in determining and combining the stresses used to obtain them. A certain level of trust is

required w ith each coupling manufacturer based on experience w ith the product and organization.

Figure I-B-6 Minimum Factors of Safety

Coupling Capacity Design Factor Basis of Factor of SafetyMax. Continuos Rating 1.25-1.35 Min* Endurance

Peak Rating 1.15 Min Yield

Max Momentary Rating 1.0 Min Yield

* 1.25 w hen limit is based on Modified Goodman

* 1.35 w hen limit is based on Constant Life Fatigue Diagrams


10/11

Section I-B

Flexible Couplings

Page I- B-9

7. Service Factors

a. Gene ral Service FactorsService Factors. Service f actors have

evolved f rom experience and based on past fail-ures. That is, af ter a coupling failed, it w as de-

termined that by multiplying the normal operating

torque by a factor and then sizing the coupling,

the coupling w ould not fail. Since coupling manu-

facturers use different design criteria, many dif-

ferent service factor charts are in use for the

same types of couplings.

Therefore, it becomes important when

sizing a coupling to fol low and use the ra t-

ings and service factors recommended in

each coupling manufacturer's catalog and

not to intermi x them with other manufac-

turers' procedures and factors.

If a coupling manufacturer is told the load

conditions-normal, peak, and their duration-a

more detailed and probably more accurate sizing

could be developed. Service factors become

less significant w hen the load and duty cycle

are known. It is only w hen a system is not ana-

lyzed in depth that service factors must be used.

The more that is know n about the operating con-

ditions, the closer to unity the service factor can

be

b. API Recommended Service FactorAPI 671 defaults to a 1.5 Service Factor for

Metallic Element couplings and 1.75 for gear

couplings. These service factors are to be ap-

plied to the normal operating torque. Note: API

cautions that if reasonable attempts to achieve

the specified experience factor fail to result in a

coupling w eight and subsequent overhung mo-

ment commensurate w ith the requirement for

rotor dynamics of the connected machines, a

low er factor may be selected by mutual agree-

ment of the purchaser and the vendor. The se-

lected value shall not be less than 1.2.

8. Speed Ratings

The true maximum speed limit at w hich a

coupling can operate cannot be divorced from

the connected equipment. This makes it very dif

f icult to rate a coupling because the same cou-

pling can be used on different types of equip-

ment. Lets forget the system for a moment andconsider only the coupling.

a. Maximum Speed Based onCentrifugal Stresses .

The simplest method of establishing a cou-

pling maximum speed rating is to base it on cen-

trifugal stress (St):

= density of material (lb/in3)

V = velocity (in./sec)

g = acceleration (386 in./sec2 )

Where Do is the outside diameter (in.) and

Material ( ( ( (lb/in3)

Steel 0.283Aluminum 0.100

S

t

=V2

g


11/11

Section I-B

Flexible Couplings

Page I- B-10

b. Other Considerations forMaximum Speed.

1) Lateral critical speed

2) Heat generated from w indbag3) limitations of lubricants used (e .g.,

separation of grease .

4) Forces generated from unbalance

5) Other system considerations.

9. Ratings in Summary

the important thing to remember is that the

inverse of a service factor or safety factor is an

ignorance factor. What this means is that w hen

little is known about the operating spectrum, a

large service factor should be applied. When the

operating spectrum is know n in detail, the serv-

ice factor can be reduced. Similarly, if the prop-

erties of a coupling material are not exactly

know n or the method of calculating the stresses

are not precise, a large safety factor should be

used. How ever, w hen the material properties

and the method of calculating the stresses are

know n precisely, a small safety factor can be

used.

Sometimes the application of service and

safety factors to applications w here the operat-

ing spectrum, material properties, and stresses

are know n precisely can cause the use of un-

necessary large and heavy couplings. Similarly,

not know ing the operating spectrum, material

properties, or stresses and not applying the ap-

propriate service factor or safety factor can

lead to undersized couplings. Ultimately, this can

result in coupling failure or even equipment fail-

ure.

10. Moments and Forces Transmitted by

Couplings

a. Bending MomentMost coupling exhibit a bending stiffness. If

you f lex them, they produce a reaction. Bending

stiff ness (KB) is usually expressed as

KB = in.-lb/deg

Therefore, the moment reaction (M) per flex

element is

M = KB x = in.-lb

Note: Some couplings (e.g., gear couplings) ex-

hibit bending moments that are functions of man

factors (torque, misalignment, load distribution,

and coeff icient of f riction).

b. Axial ForceTw o types exist: force from the sliding of

mating parts and force from the f lexing of mate-

rial

Sliding force (F) (e.g., gear couplings)

Where

T = torque (in-lb)

= coeff icient of friction

R = radius at w hich sliding occurs (in.)

Flexing force (F) (e.g., rubber and metallic

element couplings):

Where

KA = axial stiffness (lb/in. per mesh)

S = axial def lection (in.)

Note: For tw o meshes, KA = KA/2

F =T

R

F = K x S = lbA

sectioni b

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