Handbook Of Shaft Alignment Episode 1 Part 8 pps

If the shaft is supported in sliding-type bearings, the amount of lift on the shaft should bewithin the acceptable radial bearing clearance range.. Measure with Plastigage up to 8 mils a

5.2 DIAL INDICATOR BASICS

Since the use of dial indicators will be discussed frequently in this chapter, Figure 5.1 showsthe basic operating principle of this versatile measurement tool It is highly recommended toget familiar with this device since it will be used for a wide variety of tasks in the overallprocess of machinery installation, troubleshooting, problem solving, and shaft alignment

5.3 DAMAGED, WORN, OR IMPROPERLY INSTALLED MACHINERY

COMPONENT CHECKS

Every once in a while, you may have the pleasure of installing brand new rotating machinery

If you are in the construction industry that is primarily what you will be doing However, in amaintenance organization, you will very likely be working with equipment that has been inservice for sometime and invariably it is required to find and correct a problem with the

Dial indicator basics

Stem moves outward

Needle rotates counter clockwise

Stem moves inward Needle moves clockwise

equipment Chapter 1 discussed the four different maintenance philosophies The ultimategoal of a quality maintenance group is to achieve proactive or prevention maintenance status.The capacity to detect ensuing problems with machinery, stop the damage before it becomes afinancial loss to the company, have the capability to quickly detect the problems with theequipment, and engineer the corrective measures to prevent the malady from occurring again

is the ultimate goal Very few people have been able to attain this level of performance Thischapter discusses many of the tasks that make the difference between run-to-failure mainten-ance and proactive or preventive maintenance

If machinery has been operating for sometime, the bearings that support the rotor mayhave sustained some damage and it is suggested that some checks should be made to insurethat the bearings are in good working order One of the simplest tests that can be performed is

a shaft ‘‘lift check’’ as shown in Figure 5.2 and Figure 5.3

Positioning a dial indicator on top of the shaft as close as to get it to the inboard bearing, it

is essential to anchor the indicator to a stationary object with a magnetic base or a clamp.Then lifting the shaft upward enough to detect if any motion occurs, but not with so muchforce as to permanently deform the shaft, can easily happen by using a hydraulic piston, chainhoist, or overhead crane

If the shaft is supported in rolling element-type bearings as shown in Figure 5.4, the amount

of lift on the shaft should be negligible (i.e., 0 to maybe 1 mil) If there is an excess amount ofshaft lift with a rolling element bearing, four possible reasons for this is as follows:

1 The inner race of the bearing is loose on the shaft

2 There is too much clearance between the rolling elements and the inner and outerraceways

3 The outer race is loose in its housing

4 A combination of two or more of the items above

Shaft lift check

Lift upward on each shaft and note the dial indicator readings

Place the indicators on top of the shaft or coupling hub and hold the dial indicators steady

FIGURE 5.2 How to perform a shaft lift check

If the inner race is loose on the shaft, the inner race will begin ‘‘skidding’’ on the shaft,eventually damaging the shaft (if it has not already done so) If this condition exists, themachine’s running is stopped immediately and the bearing is removed to make a thoroughinspection of the shaft, bearing, and bearing housing The shaft and the bearing have to bereplaced.

If there is too much clearance between the rolling elements and the inner and outerraceways, the rollers will begin skidding on the raceways, eventually damaging the bearing(if it has not already done so) If this condition exists, the machine’s running is stoppedimmediately and the bearing is removed to make a thorough inspection of the shaft, bearing,and bearing housing The shaft and the bearing have to be replaced

FIGURE 5.3 Performing a lift check on a pump shaft

Rolling element bearings began to appear in the

early 1900' s and are also referred to as antifriction or

ball bearings The bearing consists of an inner race,

rolling elements, and an outer race Sometimes the

rolling elements are held in place with a cage assembly As the shaft turns, a film of lubricant forms between the rolling elements and

the raceways The oil film thickness can range

between1 and 3 µ m (4 to 12 millionths of an inch)

and the oil pressures at the minimum oil film

thickness are very high (approximately 40 kpsi)

If the oil film breaks down, metal to metal contact

between the rolling elements and the raceways can

occur causing damage to the bearing Damage to the

rolling elements, raceways, or cage assembly can be

detected through vibration analysis.

Inner race Outer race

Rolling elements

FIGURE 5.4 Rolling element bearing design

If there is too much clearance between the outer race and the housing, the outer racewaywill begin skidding on inside the housing and eventually damaging the housing (if it has notalready done so) If this condition exists, the machine’s running is stopped immediately andthe bearing is removed to make a thorough inspection of the shaft, bearing, and bearinghousing The bearing housing and machine case have to be replaced.

There are other types of ‘‘fixes’’ possible for items 1 and 3 (i.e., loose inner or outer race)but they are usually not recommended for long-term satisfactory performance It may betempting to use epoxy-based adhesives between the raceway and shaft or raceway andhousing but that is not the best corrective measure Sounds like a good idea until you have

to remove the bearing at some later date It may be tempting to sleeve the shaft or the housingbut that is also not the best corrective measure The ability to make a sleeve to achieve thecorrect interference fits enables to fix the sleeve correctly in the first place It is recommended

to consult the equipment manufacturer for the correct procedure for installing new bearingsand the proper type and amount of lubricant to use for that bearing

If the shaft is supported in sliding-type bearings, the amount of lift on the shaft should bewithin the acceptable radial bearing clearance range Figure 5.5 shows the basic operatingprinciple of sliding-type bearings As noted in Figure 5.5, the ‘‘rule of thumb’’ for radialbearing clearance should be from 3=4 to 2 mils=in of shaft diameter for oil-lubricated babbitbearings If the amount of lift is greater than the maximum clearance for that shaft diameter,the bearing should be removed and inspected With cylindrical sliding-type bearings, another

Radial (aka diametral) bearing clearance should range from 3/4

to 2 mils/in of shaft diameter (e.g., a 4 in diameter shaft should have

a clearance range of 0.003 to 0.008 in.)

Measure with Plastigage up to

8 mils and soft solder above 8 mils

Radial (aka diametral) bearing clearance

These are the oldest bearings known to man

dating back thousands of years As the

shaft rotates, a wedge of oil forms between

the shaft and the bearing surfaces lifting the

shaft upwards Once the oil wedge is formed,

the shaft moves slightly to one side and does

not run in the exact center of the bearing

The minimum oil film thickness occurs at a

line drawn through the shaft and bearing

centerlines called the shaft attitude angle.

The minimum oil film thickness can range

from 0.3 to 2 mils and acts as a damping medium

for small amounts of shaft motion (vibration)

The lubricant used in rotating machinery is typically

oil but the lubricant could really be any fluid

(compressible or incompressible,

e.g., water or nitrogen) under varying circumstances

or, for environmental reasons.These bearings

are also known as :

recommended method for checking bearing clearance is given in Figure 5.6 Plastigage or softsolder can be used for the clearance check In addition to the clearance check, a ‘‘tilt andtwist’’ check should be made as shown in Figure 5.7 The tilt and twist checks are performed

to determine if the centerline of the bore of the bearing is parallel to the centerline of rotation

of the shaft in the up and down (tilt) and side-to-side (twist) direction An alternative check is

a ‘‘blue check’’ where a thin coat of Prussian bluing is applied to the lower half of the bearing.The bearing is then installed into its lower hemisphere, and the shaft is lowered onto thebearing and then lifted to allow the removal of the lower half of the bearing The bearing isthen examined to determine how much of the bluing is transferred to the shaft to insure thatthere is at least 80% shaft to bearing contact Figure 5.8 shows a bearing in the process of bluechecked Bearing in mind that blue checking will determine if there is a tilt problem but notnecessarily a twist problem

Some bearings are spherically seated in their housing to hopefully compensate for any tiltand twist conditions Figure 5.9 shows an arrangement for a large steam turbine bearing

Sliding bearing clearance checks

Radial (aka diametral) bearing clearance should range from 3/4 to 2 mils/in of shaft diameter (e.g., a 4 in diameter shaft should have a clearance range of 0.003 to 0.008 in.)

Remove the upper bearing

half and place some

Plastigage or soft solder on

the top of the shaft

Upper bearing housing

Lower bearing half Upper bearing half

Bearing pedestal or machine case

Radial (aka diametral) bearing clearance

Shaft

Install the upper bearing half and tighten the bolts to the appropriate value

Remove the upper bearing half and measure the width of the Plastigage or thickness

of the soft solder

FIGURE 5.6 Sliding bearing clearance checks

Tilt and twist in a sliding bearing

Remove the upper

bearing half and

place some Plastigage

or soft solder on

the top of the shaft

Install the upper

bearing half and

tighten the bolts to

the appropriate value

Remove the upper

bearing half and measure the

width of the Plastigage

or thickness of the soft

solder at both ends If the

thickness is not the same,

a tilt condition exists

Bearing is in a tilted position Bearing is in a twisted position

Remove the upper bearing half and measure the gaps on both sides

of the shaft at the front and back of the bearings with feeler gauges

If all four gaps are not the same amount and equal to half of the total radial bearing clearance, a twist condition exists

FIGURE 5.7 Finding a tilt and twist problem in a sliding bearings

FIGURE 5.8 Checking contact on a sliding bearing with bluing

where the bearing assembly is held in position with three support blocks Shims can be added

or removed from each support block to position the bearing in the vertical and lateraldirections and to allow for a small amount of clearance to enable the bearing and pads topivot in the spherically shaped housing

A shaft supported in sliding-type radial bearings can float axially and therefore requiressome device (or force) to maintain its correct axial position In electric motors supported insliding-type radial bearings, the electromagnetic force centers the armature in the housing.This is often referred to as magnetic center or ‘‘mag center.’’ To find mag center, it isnecessary to disconnect the coupling between the motor and what it is driving, and to startthe motor up and run it ‘‘solo.’’ When the motor has attained its normal operating speed, it isadvised to scribe a line with a felt tip pen or soap stone onto the rotating shaft (care should betaken while doing this) near the inboard bearing using the seal housing or another stationaryobject on the motor as a reference point The motor is de-energized (i.e., shut down) and tostop the armature from rotating After properly safety tagging the breaker, the armature isrotated by hand and as it is rotating, the armature in the axial direction is pushed or pulleduntil the scribed line that was made on the shaft aligns with the selected stationary reference.This is where the armature wants to run under normal operating conditions This will becomeimportant later on during the alignment process to get the correct axial position between theshafts

Other rotating machines supported in sliding-type radial bearings do not have an magnetic force to center the shaft like motors do So a thrust bearing is used There are threemajor components to a thrust bearing:

electro-45˚

Shims Lower bearing half

Machine housing

Shaft Upper bearing half Upper bearing retainer

FIGURE 5.9 Spherically seated sliding bearing on adjustable support blocks

1 The thrust runner or thrust disk: This is a disk permanently attached to the shaft.

2 An active thrust bearing: This is the bearing that the thrust runner typically seats againstwhile it is operating A film of lubricant prevents the thrust runner from wearing thethrust bearing out

3 An inactive thrust bearing: It looks the same as the active thrust bearing and under normaloperation the thrust runner never seats against it, since most machinery wants to thrust inone direction only However, if the shaft wants to move in the opposite direction, thisbearing will stop the shaft before it contacts something stationary in the machine

Some of the most catastrophic failures of machinery have occurred due to a thrust-bearingfailure or due to improperly installing and setting the correct thrust-bearing clearance Tocheck this clearance, it is essential to position a dial indicator on the end of the shaft (orcoupling hub) and anchor the indicator to a stationary object with a magnetic base or aclamp The shaft toward the operator is pulled until it seats against one of the thrust bearingsand zero the indicator as shown in Figure 5.10 The shaft away from the operator is pusheduntil it seats against the other thrust bearing as shown in Figure 5.11 This is repeated for two

or three times and the amount of indicator travel each time is observed Typically, the bearing clearance is somewhere between 15 and 40 mils but it is recommended to consult theequipment manufacturer for the correct thrust-bearing clearance and the procedure to correct

thrust-it if thrust-it is not wthrust-ithin the recommended range

Figure 5.12 shows a lower half of a tilt pad-type sliding bearing Notice that there is someevidence of wear in the pads With tilt pad-type sliding bearings, a mandrel (a cylindrical barmachined to the same outside diameter as the shaft) is used in concert with a dial indicator forthe clearance check This can be done on a table and the procedure is the same as the shaft liftcheck except that the mandrel is placed in a vertical position, the assembled bearing is slid

FIGURE 5.10 Performing a thrust-bearing clearance check, step 1

over the mandrel, and a dial indicator is positioned against the bearing and then anchored tothe table The bearing is then moved toward and away from the dial indicator to measure theclearance.

The radial bearing clearances mentioned above are not for all types of sliding-type ings Water-lubricated ‘‘cutlass’’-type bearings have greater clearances New cutlass bearingstypically have clearance between 15 and 20 mils and maximum clearances typically should notexceed 80 mils With these types of bearings, clearance checks can be made with feeler gauges

bear-at four points around the circumference of the bearing A cutlass bearing with excessiveclearance on a dredge drive shaft is shown in Figure 5.13 Figure 5.14 shows the feeler gaugereadings on that bearing, indicating an excessive amount of clearance Notice that there seems

to be a twist problem with this bearing The condition and fit of bearings is extremely

FIGURE 5.11 Performing a thrust-bearing clearance check, step 2

FIGURE 5.12 Lower half of a tilt pad-type sliding bearing

important in rotating machinery and should be one of the first items that should be checkedbefore alignment but there are other components that need to be examined for mechanicalintegrity.

In a large majority of rotating machinery, some type of fluid or gas is present inside themachine case and unless it is sealed properly, the fluid or gas will leak out The lubricant in thebearings can also leak out if proper sealing is not achieved Sensory clues are the first sign oftrouble with seals If one can notice the seeping out of oil from the machine case under theshaft or oil on the base plate, it is a sign that leakage is occurring

Air or steam leaks frequently can be audibly detected (sound) Frequently high-pressureleaks can be outside the range of human detection and may require leak detection sensors and

FIGURE 5.13 Cutlass-type water bearing with excessive clearance

0.145 ⬙

0 ⬙ 0.037 ⬙ 0.098 ⬙

0.198 ⬙

0 ⬙ 0.125 ⬙ 0 ⬙

FIGURE 5.14 Measured clearances on above bearing

equipment to be located The typical range for hearing for humans is from 20 to 20,000 Hz(1 Hz ¼ 1 cycle per second).

To contain compressible or incompressible fluids inside a machine case, there are four mostcommonly used types of shaft seals: labyrinth, lip, mechanical, and packing Figure 5.15shows the basic design of each of these seals Although these seals are shown with anoverhung centrifugal pump, they are used on a wide variety of rotating machinery Figure5.15 illustrates how to prevent the fluid that is pumped from leaking out along the shaft byemploying the two most common sealing methods of either mechanical packing or mechan-ical seals

Mechanical packing consists of flexible rings that look like braided rope with a square crosssection The packing rings are inserted into a cylindrical cavity surrounding the shaft calledthe ‘‘stuffing box’’ as shown on the top part of the shaft in Figure 5.15 Frequently threepacking rings are inserted A device called the ‘‘lantern ring’’ contains three more packingrings The packing gland is then bolted to the pump housing to compress the packing rings toprovide the seal The mechanical packing was never meant to provide a perfect seal and someleakage should occur just to keep a film of fluid between the packing rings and the shaft,otherwise the packing will eventually wear away the shaft Although it is not shown in thediagram, a sacrificial shaft sleeve is often installed in this area so that the packing does notdamage the shaft itself A hole is bored through to the stuffing box area where the lantern ring

is located This is used to inject a fluid (frequently the process fluid itself) into the stuffing box

to provide the lubricating film between the packing and the shaft or shaft sleeve

Mechanical seals consist of a stationary seal ring and a rotating seal ring as shown on thebottom part of the shaft in Figure 5.15 There are a variety of designs incorporating one ortwo sealing ring sets The mating faces of the sealing rings are ground flat and have a very

Packing Lantern ring

Packing gland

“Stuffing box” area Rotating seal

Stationary seal

Spring O-rings

Lip seal

Labyrinth seal

FIGURE 5.15 (See color insert following page 322.) Four commonly used seals in rotating machinery

smooth surface finish (frequently 4 rms or better) To keep the faces together a spring (orsprings) is employed Again, this needs to be a film of fluid between the rotating andstationary seal faces The design premise of a mechanical seal is that as the process fluidattempts to traverse across the mating seal faces, by the time it gets to the outside world, thefluid has vaporized For processes where the vapors could be harmful, double seals aretypically used and a nonvolatile ‘‘barrier fluid’’ is injected into the stuffing box area.

To insure successful sealing capabilities, it is important to insure that the shaft is centered inthe stuffing box or seal housing For the pump shown in Figure 5.15, assuming the bolt-holepatterns for the bearing housing and the stuffing box housing were machined concentric tothe bearing bores and that the shaft is not bent, the clearance between the outside of the shaftand the bore of the stuffing box should be the same all the way around Usually this is true,but not always In other pump designs, however, the bearing housing is not part of concen-trically machined housings and may not ‘‘automatically’’ be centered to the bore of thestuffing box In either case, it may be wise to check the concentricity of the stuffing box

To determine if the shaft is centered, the distance from the outside surface of the shaft tothe inside bore of the seal housing is measured at four points 908 apart These measurementscan be taken in a number of different ways Feeler gauges, snap gauges, and in some cases, amagnetic base and a dial indicator can be used for the measurements Figure 5.16 shows asnap gauge that is used to measure the distance between the outside surface of a pump shaftand the bore of the stuffing box where packing is used to seal the water inside the pump.Figure 5.17 shows the initial measurements taken on this pump The pump shaft is loweredtoward the east On this particular pump, the bearing housing has to be positioned to centerthe shaft in the stuffing box before aligning the pump to its driver

In addition to keeping the process fluid from leaking out along the shaft, the bearinglubricant needs to be present in the bearing housing with seals Figure 5.15 shows two of the

FIGURE 5.16 Snap gauge used to measure stuffing box clearance

more commonly used lubricant seals: lip seals and labyrinth seals Lip seals are frequentlymade of rubber and can easily be installed backwards if care is not taken While performingthe preliminary checks on a motor and pump drive, it was observed that an oil seal wasinstalled backward on the electric motor as shown in Figure 5.18.

Since rotating machinery is likely to exhibit some vibration during operation, there is apossibility that the integrity of the housing can degrade over moderate to long periods of time.High-stress concentration areas will begin to cyclically fatigue and cracks may begin to formand then propagate A complete visual inspection of the machine casing and housing mayuncover problem areas Figure 5.19 shows where a crack was found on a machine housingduring a visual inspection Cracks can be very difficult to visually detect without help and diepenetrant checks may be warranted

Stuffing box

Pump shaft 1.118 ⬙

0.985 ⬙

FIGURE 5.17 Stuffing box clearance measured on pump in Figure 5.16

FIGURE 5.18 Oil seal installed backwards on a motor

For machinery that has been operating for some period of time, it is also suggested that avisual inspection be made of the coupling (flexible or rigid) for wear or problem areas Figure5.20 shows excessively worn teeth on a gear coupling Elastomeric couplings will degraderapidly under moderate to severe misalignment conditions Figure 5.21 shows extreme wear

on a new elastomeric coupling which had been subjected to 20þ mils=in of misalignment thatoccurred over a period of just 30 weeks of intermittent operation

FIGURE 5.19 Cracked weld on machine housing

FIGURE 5.20 Excessively worn gear coupling

5.4 RUNOUT

The term ‘‘runout’’ describes a condition where a rotating object is not concentric orperpendicular with its centerline of rotation Runout is also referred to as an ‘‘off center’’

or ‘‘eccentric’’ condition and should be one of the first things to check on the machinery that

is attempted to be aligned All rotating machinery shafts, or any device that is attached to ashaft such as coupling hubs, shaft sleeves, impellers, fan blades, armature windings, gears,blades, shrouds, or other types of components rigidly attached to shafts, will exhibit somerunout condition There is runout in just about anything and everything that rotates and it isjust a matter of how much runout is present Some runout can be as small as 10 millionths of

an inch or as high as 100 mils or greater

Runout checks are standard operating procedures when assembling components onto arotating shaft First the shaft itself is checked and then each component that is rigidlyattached to the shaft is checked for excessive runout conditions Moderate to excessive runoutwill cause moderate to excessive vibration in the machinery where the greatest amount ofvibration will appear at the running speed of the machine making it appear as an ‘‘out-of-balance’’ condition In many cases, it is not wise to attach balance correction weights toreduce the vibration The correction weights may reduce the dynamic forces that are causingthe vibration but they will not reduce or remove the eccentricity condition If an internalrotating part such as a pump impeller has excessive runout, the balance correction weightswill reduce the dynamic forces but the eccentric impeller may contact a stationary objectinside the pump case, potentially causing a catastrophic failure when the unit is started orlater when it reaches operating conditions

Rotating equipment manufacturers understand this and are very careful to insure thatexcessive runout does not cause rub conditions However, the majority of rotating machinery

is shipped to its final destination without the power transmission device (e.g., coupling hub orV-belt sheave) attached to the end of its shaft This is most often installed on the machine atthe factory where the equipment will be used in production All too often, runout checks arenot made when these parts are placed at the end of a shaft The runout problems that occurwith the power transmission device (e.g., coupling hub or V-belt sheave) will be discussed indetail

There are two basic types of runout conditions, radial and face runout Radial runoutquantifies the eccentricity of the outer surface of a shaft or a component rigidly attached to ashaft with respect to the shaft’s centerline of rotation Face runout quantifies the amount ofnonperpendicularity that may exist at the end of a shaft or on surfaces of components rigidlyattached to a shaft Runout conditions are typically measured with dial indicators as illus-trated in Figure 5.22 Runout checks should also be made at several points along the length of

FIGURE 5.21 Excessively worn elastomeric coupling