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Section I-C

Flexible Couplings

Page I-C

I-C. COUPLING INTERFACE CONNECTION

C. COUPLING INTERFACE CONNECTION I-C-1

1. Shrink Fit vs. Clearance Fit I-C-1

a. Clearance Fit (With Keys and Set Screw s) I-C-1

b. Shrink Fits I-C-1

2. Straight Shaft Installation I-C-2

a. Clearance Fits. I-C-2

b. Interference Fit. I-C-2c. Straight Shafts With Intermediate Bushings/Locking Rings. I-C-3

3. Taper Shaft Installation. I-C-4

a. General Discussion I-C-4

b. Taper Installation I-C-5

4. General Installation Guidelines. I-C-5

5. Rough and Semi-Finished bores. I-C-7

6. Key-Fit. I-C-8

7. Shrink f it effect on pilots. I-C-12


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Section I-C

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C. COUPLING INTERFACE CONNECTION

In this section w e w ill discuss coupling shaftconnections. This section w ill discuss keyed

and keyless shafts the next section w ill deal w ith

flange connections.

1. Shrink Fit vs. Clearance Fit

The question often arises as to w hen to use a

clearance fit vs. interference fit. Also how much

shrink should one use. It is very important to

ensure a proper fit of a hub on its shaft.

a. Clearance Fit (With Keys andSet Screw s)

Generally, clearance f its w ith set screw s are

used on shaf ts up to 4 inches. Torque is

transmitted through the keys. Set screw s are

usually provided over the key to minimize the play

and resist moments and forces.

All f lexible couplings resist misalignment w ith

reactionary moments and forces. The magnitude

depends on the type and size of the coupling.

These moments and forces are caused by

friction in lubricated couplings and the f lexing of

material in nonlubricated couplings. These

moments and forces can cause a loose hub to

rock on its shaft. Fretting can occur w hich can

cause failure of the shaft and/or the hub.

Therefore, clearance fit applications should be

limited to relatively small horsepow er applications

w here the reactionary loads are usually low .

b. Shrink FitsShrink f its that conform to AGMA 9002

recommendations are usually suf ficient to keep

the hub from rocking on the shaft. For standard

type couplings w ith keys the shrink should not

exceed 0.00075 in/in. Caution must be exercised

w hen specify ing heavy shrinks f or keyed hubs.

Remember that on keyed hubs the purpose of the

shrink is to keep the hub axially positioned on the

shaf t and resist the moments and forces

generated f rom unbalance and misalignment. If

one tries (or w ants) to drive totally through theshrink fit w hen a key is used this can lead to

hubs splitting over the keyw ays. If this is w hat

one w ants then take the key out and then go to a

heavier interference. Some standard couplings

have hubs made from material that can not

handle shrink fit or very little shrink

For high horsepow er and high-speed

applications (API 671 & ISO) keyless fits (straight

and tapered) are commonly used. The shrink for

keyless fits needs to be sufficient to handle the

expected normal and transient loads.

Interference rates range f rom 0.0015 in/in to

0.003 in/in.

Below is a chart (see Figure I-C-1) that show s

the maximum interference that generally can be

used w ith various hubs w ith and w ithout keys. It

is suggested you consult w ith the coupling

manufacturing to find out the maximum shrink

they recommend for their hubs.

Figure I-C-1 Rules of Thumb - Material

Hardness vs. Maximum Amount of Shrink

(for Steel)

Material

Hardness

(BHN)

Amount of

Shrink

Key

110 0.0003 in/in yes

160 0.0005 in/in yes

250 0.00075 in/in yes

160 0.001 in/in no

250 0.00175 in/in no300 0.0025 in/in no

330 0.003 in/in no


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Section I-C

Flexible Couplings

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2. Straight Shaft Installation

a. Clearance Fits.

This type of installation is relatively simpleRotate the shaft to position the keyw ay at 12

oclock. Install the key(s) in the shaf t keyw ay(s).

Coat the shaft and key w ith a thin layer of anti-

seize compound.

Make sure that any part that w ill not slide over

the coupling hub is placed back on the shaf t,

such as seals, carriers, and covers, and on gear

couplings, the sleeves.

Push the hub onto the shaft until the face of the

hub is f lush w ith the ends of the shaft. (Note:

Some coupling hubs are not mounted flush.

Check specific instructions.) If the hub does not

slide onto the shaft, check the clearances

betw een the bore and the shaf t. In addition,

check to insure that there is clearance between

the keys at the sides of the coupling keyw ay and

on top of the key (see section c-6).

Lock the hub in position (usually w ith

setscrew s). Make sure that setscrew s have a

locking feature such as a Nylok Insert, or use

locking compound. Some hubs use bolts, nuts, or

other means to secure the hub in place. See the

specific instructions.

b. Interference Fit.This type of installation is the same as that for

the straight shaft, w ith the exception that the

hubs must be heated before they slide on to the

shaf t. The coupling manufacturer usually

supplies information as to how to heat the hub

and to w hat temperature (Figure I-C-2). For a

steel hub, 1600F is required for every 0.001 in. of

interference per inch of hub diameter (0.001

in./in.). For example, a steel hub w ith a 4-in. bore

and an interference of 0.003 inch requires adifferential of 0.00075/0.001 x 1600F = 1200F.

Therefore, if the shaft temperature is 800F, the

hub temperature must be 2000F. This does not

account for human factors such as cooling due

to handling time, errors in measurements, and so

on. As a general rule, add 50 to 750F to the

calculated expansion temperatures to account

for these factors. The hub should be heated in

an oil bath or an oven; a torch or open f lame

should not be used. This could cause localized

distortion or softening of the hub material. It could

also cause an explosion in some atmospheres.

Oil bath heating is usually limited to approximately

3500F, or under the f lash point of the oil used.

Special handling devices are required such as

tongs, threaded rods placed in taped holes in the

hub, etc. Oven heating and induction heaters

offer some advantages over oil. Parts can be

heated to higher temperatures (usually not

exceeding 6000F) and the parts can be handled

w ith heat-resistant gloves. Regardless of the

method used, heat must be applied evenly to

avoid distortion. This is especially important

w hen using induction heaters. In any event,

extreme care must be exercised w hen handling

heated hubs to avoid Injury to personnel

It is also important w hen mounting interference

hubs to make sure that clearance exists over the

top of keys; otherw ise, w hen the hub cools, it

w ill rest on the key and produce high stresses in

the hub that could cause it to fail.


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Section I-C

Flexible Couplings

Page I- C-3

Figure I-C-2 Expansion Guide (Heat vs Bore)

c. Straight Shafts WithIntermediate

Bushings/Locking Rings.

(see Figure I-C-3)

Bushings come in two basic conf igurations:

internal or external.

Although installation varies w ith bushing design,

the net result is an interference f it betw een the

hub and the shaft. Tightening axial screw s to

draw up opposing tapered rings develops the

required interference.

Figure I-C-3A Intermediate Bushings


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Section I-C

Flexible Couplings

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Figure I-C-3B Intermediate Bushings

Figure I-C-3C Locking Ring

3. Taper Shaft Installation.

a. General Discussion

Tapered shaf ts have the advantage that theinterference betw een the hub and the shaft can

be accomplished by advancing the hub on the

shaf t. Depending on the amount of interference,

the hub may be draw n up w ith nuts or heating.

Removal of the hub is usually easier on tapered

shafts than on straight shafts.

Note: The shallow tapers used in coupling

applications are usually self-locking. This means

that the friction forces resisting the hub advance

are larger than the forces pushing the hub onto

the shaf t. Therefore, the only w ay to accomplish

the required advance is to heat the hub before

installation.

Applications using tapered bores require more

attention than those using straight shaf ts

because it is easier to machine tw o cylindrical

surfaces that match than tw o tapered surfaces.

The hub can be overstressed if it is advanced

too far on the shaft. Dirt and surface

imperfections can restrain the hub advance and

give the false impression that the desired

interference has been reached.

To determine the draw -up required for the

desired interference, use the follow ing equation

Draw up in = xi

T ( .) 12

Where:

dia.)on(in/ft.taper=T

Whenever poss ible, the area of contact between

the bore, shaft and respective plug or ring gage

should be checked w ith machinist bluing. A f inal

i = diametral interference (in.)


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Section I-C

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Page I- C-5

check of the contact area betw een the bore of

the hub and the shaf t should be done to verify

previous results. Recommended procedures canbe found in AGMA 9002-A86, Bores and

Keyw ays for Flexible Couplings and AGMA

9003-A91, Flexible Couplings-Keyless Fits. Fifty

to 80% contact area is an acceptable range,

although a minimum of 70% is the most desirable

(this is especially true for hydraulically fitted

hubs. If less than the required contact is

achieved, lapping the bore can increase the

contact area and/or the shaft w ith a lapping plug

or ring made from a master plug and ring gage. It

is not recommended that the master gages or

shaf t be used to lap the hub, as the gages could

end up w ith ridges. Ridges in the hub or shaft

w ill prevent proper hub installation and could

cause the hub or shaft to fail because of stress

concentrations.

b. Taper Installation(1) Light interference

(Under 0.0005 in./in.):

When the interference is less than 0.0005 in./ in.,

the hub can usually be advanced w ithout

heating. Although heating the hub is the most

common method, the hub can usually be

advanced by tightening the retaining nut or plate

on the shaf t. It is also common practice w hen

light interference is used w ith a combination of

keys and a retaining nut or plate to use a light

grease or antisieze compound betw een the hub,

shaftand threads on the shaft and nut. This

should help facilitate installation and future

removal and help prevent shaf t and/or bore

gauling.

(2) Medium interference

(Betw een 0.0005 and 0.0015 in./in.).

When the interference is over 0.0005 in./in., the

force required to advance the hub could become

too large for manual assembly. When this

occurs, the hub mustbe heat mounted or

hydraulically mounted. Heating hubs for mounting

is the most common method. Regardless of the

method used, the amount of draw -up must bemeasured.

(3) Heavy Interference

(Above 0.0015 in./in.)

When the interference is over 0.0015 in./in.,

hubs are usually heat mounted and removed

hydraulically. Some users prefer to both mount

and remove hydraulically.

4. Gene ral Installation Guidelines .

The follow ing is recommended as a general

guide w hen installing hubs on an equipment

shaft:

Install the hub on the shaft, insuring that the parts

mate properly and are burr-free and clean. Using

a depth gage or dial indicator (see Figure I-C-

4A) measure and record the initial reading.

Remove the hub and lubricate the bore or 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

stop, such as a clamp on the shaft, to insure

proper draw -up (see Figure I-C-4B). The reason

for this is that a hub advanced too far may not be

removable (too much force required or not

enough hydraulic pressure available to remove

the hub) and normally requires that the hubs be

cut of f. Even if the hub can be removed,

excessive advance may have caused the hub

material to yield and the coupling manufacturer

should be consulted before attempting to reuse

the hub.


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Flexible Couplings

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Figure I-C-4A Depth Gage

Figure I-C-4B Positive Stop

The hub is installed and advanced the required

amount.


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Section I-C

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The shaf t nut is then properly tightened and

locked in place. Locking is done w ith a tab

w asher or setscrew .

5. Rough and Semi-Finished bore s.

Most coupling manufacturers w ill supply

couplings w ith rough or semi-f inished bores.

While this bore condition helps to reduce

inventory requirements it is important that the

user properly bore and key these couplings;

otherw ise, the interface torque-transmission

capabilities can be reduced or the coupling

balance (or unbalance) can be upset.

Verifi cation of bore condition and

recommendati ons for fini shed bore

tolerances should be obta ined from the

specifi c coupli ng manufacturer. As a

general guide, the hub must be placed in a lathe

so that it is perpendicular and concentric to its

controlling diameters.

Rough Bore Example

On rigid hubs the pilot and face are usually

the controlling diameter and surface (see Figure

I-C-5A). On flex hubs (gear and grid) the gear

major diameter (OD) and hub face act as the

controlling diameter and surf ace. [Note:Some

manufacturers use the hub barrel as the

controlling diameter (see Figure I-C-5B).]

Semi-Finished Bore Example

(Straight Bore)

In this case, the f inished bore should be

machined using the semi-finished bore as the

controlling diameter. Indicate the bore for

concentricity and straightness.

Figure I-C-5 Setup for re-boring straight

shafts There are three types of errors that can occur

w hen hubs are bored:


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(1) The bore diameter is incorrect. Too

much interference w ill cause

installation problems and maydamage the hub or shaft w hile too

much clearance can produce

unbalance forces that may be

unacceptable to the system.

(2) The bore is eccentric, but parallel to

the hub axis. This can produce

unbalance forces that may be

unacceptable to the system (see

Figure I-C-6).

(3) The bore is at an angle to the OD of

the hub. In this case the

misalignment capacity of the coupling

is reduced (see Figure I-C-7).

Figure I-C-6 Eccentric Hub Bore

Figure I-C-7 Skewed Hub Bore

6. Key-Fit.

Most couplings must have one or tw o keyw ays cut in the hub. These should be cut generally per AGMA

9002-A86. Particular attention should be given to the follow ing:


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Section I-C

Flexible Couplings

Page I- C-9

FIGURE I-V-8 Keyway Offset FIGURE I-V-9 Keyway Parallelism

FIGURE I-V-10 Keyw ay Lead FIGURE I-V-11 Keyw ay Width and Height

The fitting of keys is important to insure the

proper capacity of the interface. Refer to the

AGMA standards on keyw ays and keys. As a

general rule, four f its must be checked:

1. The key should fit tightly in the shaf t

keyw ays.

2. The key should have a sliding fit (but not be

too loose) in the hub keyw ay.

3. The key should have a clearance w ith the top

of the keyw ay (see Figure I-C-12A).

4. The key should have chamfered corners.

Figure I-C-12 Fitted Keys


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Section I-C

Flexible Couplings

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Proper ly Fitted Key

Improperly Fitted Keys

The key should be chamfered so that it f its w ith-

out riding on the keyw ay radii (see Figure I- C-

12B). A sloppily f itted key can cause the keys to

roll or shear w hen Ioaded. The results of a

sloppy key fit are show n in Figure I-C-12C. The

forces generated by torque are at distance S,and this movement tends to roll the key and

can cause very high loading at the key edges.

On the other hand, too tight a f it w ill make

assembly very difficult and increase the residual

stresses,

w hich could cause premature failure of the hub

and/or shaft.

A key in the keyw ay that is too high could cause

the hub to split (see Figure I-C-12D). When there

is too much clearance at the top or sides of a

key, a path is provided for lubricant to squeezeout. For lubricated couplings, clearances

betw een keys and keyw ays should be sealed

w ith an appropriate sealant to prevent loss of

lubricant and thus starvation of the coupling.


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Section I-C

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Page I- C-12

7. Shrink fit effect on pilots.

When a coupling hub is pressed on a shaft

(shrink f itted), the bore expands to f it the largershaf t. Not only does the bore grow but so does

any diameter directly over the bore. Therefore if

the coupling has piloting f its that are directly over

the shaft they too w ill grow . As a general guide,

this grow th can be expressed as

G = xD

DiB

H

Where:

G - Grow th of pilot (in)

DB = Bore Diameter (in)

DH = Pilot OD (in)

i = Amount of shrink (in)

Compensation for this grow th must be

accounted for. For outside rabbets they can

grow so the assembly becomes diff icult or

impossible. Inside rabbet may loosen and cause

unbalance problems. In the case of gearcouplings the tooth tips may grow so they w ill no

longer s lide in the sleeve. This causes the

coupling to be mechanically locked. Coupling

manufacturers account for this in their designs

but if one elects to buy rough-bored couplings

and decides to put a heavy shrink the grow th of

the hub tooth might be greater than allow ed by

design and Lockup may occur.

.

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