Calculate Bearing Life

7/28/2019 Calculate Bearing Life

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Calculate Bearing Life

TimkenPosted 9-13-03

Basis for Calculation| Bearing Life Equation| Bearing Ratings |L10Life Calculation

Basis for calculation

Bearing life is defined as the length of time, or the number of revolutions, until a fatigue spall of a specific sizedevelops. This spall size, regardless of the size of the bearing, is defined by an area of 0.01 inch2 (6 mm2). This lifedepends on many different factors such as loading, speed, lubrication, fitting, setting, operating temperature,contamination, maintenance, plus many other environmental factors. Due to all these factors, the life of anindividual bearing is impossible to predict precisely. Also, bearings that may appear to be identical can exhibitconsiderable life scatter when tested under identical conditions. Remember also that statistically the li fe of multiplerows will always be less then the life of any given row in the system. For bearings where it is impossible to test alarge number of bearings, the long experience of The Timken Company will help you in your bearing lifecalculation.

L10 life

L10 life is the life that 90 percent of a group of apparently identical bearings will complete or exceed before thearea of spalling reaches the defined 0.01 inch2 (6 mm2) size criterion. If handled, mounted, maintained, lubricatedand used in the right way, the life of your tapered roller bearing will normally reach and even exceed thecalculated L10 life.

If a sample of apparently identical bearings is run under specific laboratory conditions, 90 percent of thesebearings can be expected to exhibit lives greater than the rated life. Then, only 10 percent of the bearings testedwould have lives less than this rated life. Figure 3-48 shows bearing life scatter following a Weibull distributionfunction with a dispersion parameter equal to 1.5.

Bearing life equation

As you will see it in the following, there is more than just one bearing life calculation method, but in all cases thebearing life equation is :
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L10 = (C / P)10/3 (B / n) a

L10 in hours

C = radial rating of the bearing in lbf or NP = radial load or dynamic equivalent radial load applied on the bearing in lbf or N. The calculation of P depends

on the method (ISO or Timken) with combined axial and radial loading

B = factor dependent on the method ; B = 1.5 106 for the Timken method (3000 hours at 500 rev/min) and106/60 for the ISO method

a = life adjustment factor ; a = 1, when environmental conditions are not considered ;n = rotational speed in rev/min.

This can be illustrated as follows :

Doubling load reduces life to one tenth. Reducing load by one half increases life by ten,

Doubling speed reduces life by one half. Reducing speed by one half doubles life.

In fact, the different life calculation methods applied (ISO 281, Timken method...) differ by the selection of theparameters used (i.e. the Timken formula is based on 90 million revolutions, whereas the others are based on 1million revolutions).

Bearing ratings

Depending on the life calculation method used, the bearing ratings have to be selected accordingly. The "Cr"rating, based on one million revolutions, is used for the ISO method, and the "C90" rating, based on 90 million

revolutions, is utilized for the Timken method.

The Timken rating is also published based on 1 million revolutions : C1 = C90 3.857

This will enable you to make a direct comparison between Timken bearings and those using ratings evaluated on abasis of 1 million revolutions. However, a direct comparison between ratings of various manufacturers can bemisleading due to differences in rating philosophy, material, manufacturing and design. In order to make a truegeometrical comparison between the ratings of different bearing suppliers, only the rating defined following theISO 281 equation should be used. However, by doing this, you do not take into account the different steel qualitiesfrom one supplier to another.

ISO 281 Dynamic Radial Load Rating Cr

This bearing rating equation is published by the International Organization for Standardization (ISO) and AFBMA.These ratings are not published by The Timken Company nor by any other bearing manufacturers. However, theycan be obtained by contacting our company.

The basic dynamic load rating is function of:

Cr = bm fc (i Lwe cos a)7/9 Z3/4

Dwe29/27

Cr = radial rating

bm = material constant (ISO 281 latest issuespecifies a factor of 1.1)

fc = geometry dependent factor

i = number of bearing rows within theassembly

Lwe = effective roller contact length

a = bearing half-included outer race angle

Z = number of rollers per bearing row

Dwe = mean roller diameter

Timken Dynamic Radial Load Rating C90

Even though the ISO method allows you to compare different bearing suppliers, the basic philosophy of The


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Timken Company is to provide you with the most practical bearing rating for your bearing selection process. Since1915 The Timken Company has developed and validated a specific rating method for its tapered roller bearings.The published Timken C90 ratings are based on a basic rated life of 90 million revolutions or 3000 hours at 500rev/min.

To assure consistent quality worldwide, we conduct extensive bearing fatigue life tests in our laboratories. These

audit tests result in a high level of confidence in our ratings. The basic dynamic load rating is used to estimate thelife of a rotating bearing and is a function of:

C90 = M H (i x Leff cos a)4/5 Z7/10

Dwe16/15

C90 = radial rating

M = material constant

H = geometry dependent factor

i = number of bearing rows within theassembly

Leff= effective roller contact length

a = bearing half-included outer race angle

Z = number of rollers per bearing row

Dwe = mean roller diameter

A rating based on 90 million revolutions is more realistic as most applications equal or exceed this duration. Fordouble row bearings in which both rows are loaded equally, the two-row rating considers the system life of theassembly as follows:

C90(2) = 24/5 C90 or C90(2) = 1.74 C90

The basic radial load rating of a four-row assembly is taken as two times the double row rating :

C90(4) = 2 C90(2)

and for a six-row assembly as three times the double row rating :

C90(6) = 3 C90(2)

The Timken Company also publishes K factors for its bearings. This factor is the ratio of basic dynamic radial loadrating to basic dynamic thrust load rating of a single row bearing:

The Timken Company also publishes K factors for its bearings. This factor is the ratio of basic dynamic radial load

rating to basic dynamic thrust load rating of a single row bearing:

K = C90 / Ca90

The smaller the K factor, the steeper the bearing cupangle (fig. 3-51). The relationship can also begeometrically expressed as:

K = 0.389 x cot a


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a = half included outer race angle

L10 life calculation

Single row bearing

Tapered roller bearings are ideally suited to carry all types ofloads : radial, axial or any combination. Due to the tapereddesign of the bearing, a radial load will induce an axial reaction

within the bearing which must be equally opposed to avoidseparation of the inner and outer rings. The ratio of the radialto the axial load (external axial load and induced load), thesetting and the bearing included cup angle determine the loadzone in a given bearing. This load zone is defined by an angle

which delimits the rollers carrying the load. If all the rollers arein contact and carry the load, the load zone is referred to asbeing 360 degrees.

In the case of combined loads, a dynamic equivalent radial loadmust be calculated to determine bearing life. The equationspresented below give close approximations of the dynamicequivalent radial loads. More exact calculations using computerprograms can be made that take into account such parametersas bearing spring rate, setting and supporting housing stiffness.

Combined radial and thrust load

ISO Method

Thrust Condition Thrust Condition

Net Bearing Thrust Load Net Bearing Thrust Load


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Dynamic Equivalent Radial LoadBearing A

Bearing B

PB = FrB

Dynamic Equivalent Radial LoadBearing APA = FrABearing B

L10 Life

Timken Method

Thrust Condition Thrust Condition

Net Bearing Thrust Load Net Bearing Thrust Load


PA = 0.4FrA + KAFaAif PA < FrA, PA = FrABearing BPB = FrB


PA = FrABearing BPB = 0.4FrB + KBFaBif PB < FrB, PB = FrB

L10 Life

ISO 281 Factors

e = 1.5 tan a Y = 0.4 cot a Y 1 = 0.45 cot a Y 2 = 0.67 cot a

Two-Row Bearing

Thrust Load Only


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ISO Method

Thrust ConditionFaA = FaeFaB = 0

Dynamic Equivalent LoadPA = YAFaAPB = 0

Thrust LoadFaA = FaeFaB = 0

L10 Life

Timken Method

Thrust ConditionFaA = FaeFaB = 0

Thrust LoadFaA = FaeFaB = 0

L10 Life

ISO MethodThrust Condition

Dynamic Equivalent Radial LoadPAB = FrAB + Y1ABFaePC = FrC

Thrust ConditionDynamic Equivalent Radial LoadPAB = 0.67FrAB + Y2ABFaePC = FrC


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L10 Life

Timken Method

Thrust Condition Dynamic Equivalent Radial LoadPA = 0.4FrAB + KAFaePB = 0PC = FrC

Thrust Condition Dynamic Equivalent Radial LoadPA = 0.5FrAB + 0.83KAFaePB = 0.5FrAB - 0.83KAFaPC = FrC

L10 Life

Keys for Effective Troubleshooting

By Warren RhudePosted 2-16-13

Equipment can malfunction for a variety of reasons. Mechanical contacts and parts can wear out; wires canoverheat and burn open or short out; parts can be damaged by impact or abrasion; etc. Equipment may operatein a manner far different than it was designed to, or not at all.

Typically, when equipment fails there is a sense of urgency to get it fixed and working again. If the defectiveequipment is part of an assembly line, the whole assembly line could be down causing unexpected time off and

lost revenue. If you are at a customer site to repair equipment, the customer may watch you, knowing that theyare paying for every minute you spend troubleshooting and repairing their equipment. Either one of thesescenarios and there are more, can put a lot of pressure on you to solve the problem quickly.

So What is troubleshooting?It is the process of analyzing the behavior or operation of a faulty circuit todetermine what is wrong with the circuit. It then involves identifying the defective component(s) and repairing

the circuit.Depending on the type of equipment, troubleshooting can be a very challenging task. Sometimes problems areeasily diagnosed and the problem component easily visible. Other times the symptoms as well as the faultycomponent can be difficult to diagnose. A defective relay with visual signs of burning should be easy to spot,whereas an intermittent problem caused by a high resistance connection can be much more difficult to find.

What makes an expert Troubleshooter?One trait of expert troubleshooters is that they are able to find

virtually any fault in a reasonable amount of time. Easy faults, complicated faults, they find them all. Anothertrait is that they typically replace only the components that are defective. They seem to have a knack for findingout exactly what is wrong. No trial and error here. So what is their secret?


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You might think that a person who has a very good understanding of how the equipment works, should be able totroubleshoot it effectively. Being a good at troubleshooting requires more than this.

Expert troubleshooters have a good understanding of the operation of electrical components that are used incircuits they are familiar with, and even ones they are not. They use a system or approach that allows them tologically and systematically analyze a circuit and determine exactly what is wrong. They also understand andeffectively use tools such as prints, diagrams and test instruments to identify defective components. Finally, theyhave had the opportunity to develop and refine their troubleshooting skills.

If you want to troubleshoot like the pros you wil l need to develop your skills in each of these areas. Lets look atthem in more detail.

1. Understand how the circuit works. This consists of understanding the operation of all thecomponents that are used in the circuit. This could include such components as: push buttons,contactors, various types of switches, relays, sensors, motors, etc.

Electrical circuits typically control or operate mechanical systems and components. You also need tounderstand how these mechanical aspects of the equipment operate to carry out the work.

You need to be able to determine how the circuit works under normal conditions and what effectchanging one of the circuit inputs has on the circuit operation. For example, what happens to the overallcircuit operation when a push button is pressed; which relays energize, which lights illuminate, does thepump start or stop, etc. You also need to be able to determine what effect a faulty component may

have on the circuit operation.

2. Use a logical, systematic approach to analyze the circuits behavior. This is critical. There areseveral approaches that troubleshooters use. They may have different steps or processes but they havethe following in common: They all approach problems systematically and logically thus minimizing thesteps and ruling out trial and error. One such approach used to teach troubleshooting is called the 5Step Approach. A summary of the key steps are:

o Observe Most faults provide clues as to their cause. There could be visual clues such as signs

of damage or improper operation. Dont forget to use your other senses; sounds and smellscan also provide valuable clues. Through careful observation and a some reasoning, mostfaults can be identified to the actual component with very little testing.

o Define Problem Area At this stage you apply logic and reasoning to your observations to

determine the problem area of the malfunctioning equipment.

o Identify Possible Causes Once you have the problem area(s) defined it is necessary to identify

all the possible causes of the malfunction.

o Determine Most Probable Cause Once the list of possible causes has been made it is necessary

to prioritize the items as to the possibility of them being the actual cause of the malfunction.o Test and Repair Once you have determined the most probable cause, you must test it to prove

it to be the problem or not.

3. Understand how to use tools such as prints, diagrams and test instruments to identifydefective components. Lets first look at prints and diagrams. Some of the key things you should beable to determine from these are:

o how the circuit should operate

o what kind of features the circuit has

o what voltages you should expect at various points on the circuit

o where components are physically located

o how the components are actually wired together

Various types of test instruments are available for testing electrical circuits. The ones you choosedepends on the type of circuit and its components. A common test instrument which is invaluable to atroubleshooter is a Multimeter. It is capable of measuring voltage and resistance with some meterscapable of other measurements such as current and capacitance.

You must be able to determine what type of test instrument to use, when and where to use it, and howto safely take readings with it.

4. Practice! Troubleshooting, like any skill, requires practice to become proficient. Practice can bedifficult to get. Depending on your job, you may not have the opportunity for enough troubleshooting


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practice. And even if you do, your employer may not want you to practice troubleshooting on equipmentthey depend on. Until you become reasonably competent, it is best to practice troubleshooting in acontrolled environment.

One option is to build or purchase equipment that can be used for troubleshooting. This equipment isdesigned with the ability to apply faults to it. Here you can practice your skills in a very realistic

environment without affecting equipment in use.

Another option which is gaining more popularity is the use ofcomputer simulations. These simulations can be extremelyrealistic and allow you to practice your troubleshooting skills in asafe environment. Some other advantages are portability, cost

effectiveness, and can be used in almost any location. Some eveninclude an assessment function that assesses the userstroubleshooting skill, provides feedback, and records theirtroubleshooting processes.

Review your own skills in each of these areas. Improving any one of them,along with a good dose of practice, will improve your troubleshooting skills.

Cavitation 1-3

The McNally Institute

Cavitation means that cavities are forming in the liquid that we arepumping. When these cavities form at the suction of the pump severalthings happen all at once.

We experience a loss in capacity.

We can no longer build the same head (pressure)

The efficiency drops.

The cavities or bubbles will collapse when they pass into the higher regions of pressure causing noise,

vibration, and damage to many of the components.

The cavities form for five basic reasons and it is common practice to lump all of them into the generalclassification of cavitation. This is an error because we will learn that to correct each of these conditions we mustunderstand why they occur and how to fix them. Here they are in no particular order:

Vaporization

Air ingestion

Internal recirculation

Flow turbulence

The Vane Passing Syndrome

Vaporization

A fluid vaporizes when its pressure gets too low, or its temperature too high. All centrifugal pumps have a

required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement issupplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degreesFahrenheit (20 degrees Centigrade) is the fluid being pumped.

Since there are losses in the piping leading from the source to the suction of the pump we must determine thehead after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required(N.P.S.H.R.) to prevent the fluid from vaporizing.

We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we arepumping, and this number must be equal to or greater than the Net Positive Suction Head Required.

To cure vaporization problems you must either increase the suction head, lower the fluid temperature, ordecrease the N.P.S.H. Required. We shall look at each possibility:
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Increase the suction head

Raise the liquid level in the tank

Raise the tank

Put the pump in a pit

Reduce the piping losses. These losses occur for a variety of reasons that include:

o The system was designed incorrectly. There are too many fittings and/or the piping is toosmall in diameter.

o A pipe liner has collapsed.

o Solids have built up on the inside of the pipe.

o The suction pipe collapsed when it was run over by a heavy vehicle.

o A suction strainer is clogged.

o Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather

o Something is stuck in the pipe, It either grew there or was left during the last time the system

was opened . Maybe a check valve is broken and the seat is stuck in the pipe.

o The inside of the pipe, or a fitting has corroded.

o A bigger pump has been installed and the existing system has too much loss for the increased

capacity.

o A globe valve was used to replace a gate valve.

o A heating jacket has frozen and collapsed the pipe.

o A gasket is protruding into the piping.o The pump speed has increased.

Install a booster pump

Pressurize the tank

Lower the fluid temperature

Injecting a small amount of cooler fluid at the suction is often practical.

Insulate the piping from the sun's rays.

Be careful of discharge recirculation lines, they can heat up the suction fluid.

Reduce the N.P.S.H. Required

Use a double suction pump. This can reduce the N.P.S.H.R. by as much as 27% or in some cases it will

allow you to raise the pump speed by 41%

Use a lower speed pump

Use a pump with a larger impeller eye opening.

If possible install an Inducer. These inducers can cut N.P.S.H.R. by almost 50%.

Use several smaller pumps. Three half capacity pumps can be cheaper than one large pump plus a

spare. This will also conserve energy at lighter loads.

It is a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal

cold water N.P.S.H. requirements, or 10 feet (3 meters), whichever is smaller. I would suggest you usethis as a safety margin rather than design for it.

Air ingestion

A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air gets into assystem in several ways that include:

Through the stuffing box. This occurs in any packed pump that li fts liquid, pumps from a condenser,

evaporator or any piece of equipment that runs in vacuum.

Valves above the water line.

Through leaking flanges

Vortexing fluid.

A bypass line has been installed too close to the suction.


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The suction inlet pipe is out of fluid. This can occur when the level gets too low or there is a false

reading on the gauge because the float is stuck on a corroded rod.

Both vaporization and air ingestion have an affect on the pump. The bubbles collapse as they pass from the eyeof the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller orcasing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization both occur they have separate solutions. Air ingestion is not as severe asvaporization and seldom causes damage, but it does lower the capacity of the pump.

Internal Recirculation

This condition is visible on the leading edge of the impeller, and will usually be found at the discharge tip workingits way back to the suction. It can also be found at the suction eye of the pump.

As the name implies the fluid recirculates increasing its velocity until it vaporizes and then collapses in thesurrounding higher pressure. This has always been a problem with low NPSH pumps and the term SPECIFICSUCTION SPEED was coined to give you a guide in determining how close you have to operate to the B.E.P. of apump to prevent the problem.

The higher the number the smaller the window in which you have to operate. The numbers range between 3,000and 20,000. Water pumps should stay between 3,000 and 12,000. Here is the formula to determine the suction

specific speed number of your pump:

rpm = Pump speedgpm = Gallons per minute or liters per second of the largest impeller at its BEPHead= Net positive suction head required at that rpm

For a double suction pump the flow is divided by 2 since there are 2 impeller eyes

Try to buy pumps lower than 8500.(5200 metric ) forget those over 12000 (8000 metric) except for

extreme circumstances.

Mixed hydrocarbons and hot water at 9000 to 12000 (5500 to 7300 metric) or higher, can probably

operate satisfactorily.

High specific speed indicates the impeller eye is larger than normal, and efficiency may be compromised

to obtain a low NPSH required.

Higher values of specific speed may require special designs, and operate with some cavitation.

Normally a pump operating 50% below its best efficiency point (B.E.P.) is less reliable.

With an open impeller pump you can usually correct the problem by adjusting the impeller clearance to themanufacturers specifications. Closed impeller pumps present a bigger problem and the most practical solution

seems to be to contact the manufacturer for an evaluation of the impeller design and a possible change in thedesign of the impeller or the wear ring clearances.

Turbulence

We would prefer to have liquid flowing through the piping at a constant velocity. Corrosion or obstructions canchange the velocity of the liquid and any time you change the velocity of a liquid you change its pressure. Goodpiping layouts would include:

Ten diameters of pipe between the pump suction and the first elbow.

In multiple pump arrangements we would prefer to have the suction bells in separate bays so that one

pump suction will not interfere with another. If this is not practical a number of units can be installed ina single large sump provided that :

The pumps are located in a line perpendicular to the approaching flow.

There must be a minimum spacing of at least two suction diameters between pump center lines.


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All pumps are running.

The upstream conditions should have a minimum straight run of ten pipe diameters to provide uniform

flow to the suction bells.

Each pump capacity must be less than 15,000 gpm..

Back wall clearance distance to the centerline of the pump must be at least 0.75 of the suction

diameter.

Bottom clearance should be approximately 0.30 (30%) of the suction diameter

The minimum submergence should be as follows:

FLOW MINIMUM SUBMERGENCE

20,000 GPM 4 FEET

100,000 GPM 8 FEET

180,000 GPM 10 FEET

200,000 GPM 11 FEET

250,000 GPM 12 FEET

The metric numbers are:

FLOW MINIMUM SUBMERGENCE

4,500 M3/HR 1.2 METERS

22,500 M3/HR 2.5 METERS

40,000 M3/HR 3.0 METERS

45,000 M3/HR 3.4 METERS

55,000 M3/HR 3.7 METERS

The Vane Passing Syndrome

You will notice damage to the tip of the impeller caused by its passing too close to the pump cutwater. Thevelocity of the liquid increases if the clearance is too small lowering the pressure and causing local vaporization.The bubbles collapse just beyond the cutwater and there is where you should look for volute damage. You willneed a flashlight and mirror to see the damage unless it has penetrated to the outside of the volute.

The damage is limited to the center of the impeller and does not extend into the shrouds. You can prevent thisproblem if you keep a minimum impeller tip to cutwater clearance of 4 % of the impeller diameter in the smallerimpeller sizes (less than 14' or 355 mm.) and 6% in the larger impeller sizes (greater than 14" or 355 mm.).

To prevent excessive shaft movement bulkhead rings can be installed in the suction eye. At the discharge ringscan be manufactured to extend from the walls to the impeller shrouds.

Pump and Driver Alignment 14-3

McNally InstitutePosted 07-10-03

In the pump business alignment means that the centerline of the pump is aligned with the centerline of thedriver. Although this alignment was always a consideration with packed pumps, it is critical with sealed pumpsespecially if you are using rotating seal designs where the springs or bellows rotates with the shaft.

A little misalignment at the power end of the pump is a lot of misalignment at the wet end, and unfortunatelythat is where the seal is located in most pump applications.

Misalignment will cause many problems:
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It can cause rotating mechanical seals to move back and forth axially two times per revolution. The

more the seals move the more opportunity for the lapped faces to open

Packing could support a misaligned shaft. A mechanical seal cannot.

Misalignment will cause severe shaft or sleeve fretting if you use spring loaded Teflon as a secondary

seal in your mechanical seal design.

The pump bearings can become overloaded.

The misalignment could be severe enough to cause contact between stationary and rotating sealcomponents:

The wear rings can contact.

The shaft can contact the restriction bushing often found at the end of the stuffing box.

The shaft or sleeve can contact the stationery face of the mechanical seal.

The shaft can contact the disaster bushing in an API (American Petroleum Institute) gland.

The impeller could contact the volute or back plate.

Regardless of the alignment method you select, you must start with a pump and driver in good repair. A perfectlyaligned piece of junk is still a piece of junk. You should also check the following:

A straight shaft that has been dynamically balanced.

Good wear rings with the proper clearance.

The correct impeller to volute, or backplate clearance.

The elimination of "soft foot".

Eliminate all pipe strain.

Good bearings installed on a shaft with the proper finish and tolerances.

A good mechanical seal set at the proper face load. The closer the seal is to the pump bearings the

better off you are going to be.

All pump to driver alignments consist of four parts:

You must level the pump and driver. If the pump is aligned without being level, the oil level will be

incorrect and you will develop bearing problems.

You then take a series of radial and axial measurements to see where the pump is located in respect to

its driver (motor).

You make calculations to see how far the driver must be moved to align the centerline of the pump to

the centerline of the driver. These calculations must consider that the pump and driver operatingtemperature will probably be very different than the ambient temperature when you are taking the

readings.

Most pump manufacturers should be able to supply you with the proper readings for a hot alignment.

They are the only people that know how their unit expands and contracts with a change in temperature.

You must now shim and move the driver to get the alignment. Most of the small pump designs are not

equipped with "jack bolts" so this will be the most difficult part of the alignment procedure. You cannotmove the pump because it is connected to the piping.

I see lots of pumps that have never been aligned properly. When you talk to the people that should beconcerned, you get the following comments:

Alignment is not important. I have been working with pumps for years and we never do it at this

facility.

And we do not do dynamic balancing of the rotating assembly either!

There is no time to do an alignment. Production wants the unit back on line, and they will not allow me

the time to do it properly.

We purchase good couplings. The coupling manufacturer states that their coupling can take a

reasonable amount of misalignment.

It turns out that there are at least three methods of getting a good pump to driver alignment, and a goodcoupling is not one of them. The coupling is used to transmit torque to the shaft and compensate for axial


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thermal growth, nothing else. You install a good coupling after you have made the pump to driver alignment, notinstead of making the alignment.

Here are some acceptable methods:

The reverse indicator method is an acceptable method, but it does take a great deal of time. There are plenty ofschools that teach this method if you are interested in learning how to do it:

Very accurate especially for small diameter flanges

Not affected by axial float.

Can be used with a flexible coupling in place.

You have to rotate both shafts

The laser is the latest method. It is also the most popular. There are lots of people that can teach you to use theequipment, once you have made the purchase.

The "C or D" frame adapter is probably the easiest method of all and available from most quality pumpmanufacturers It solves most of the problems with thermal expansion.

You use a machined, registered fit to insure the alignment.


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The shaft to coupling spool method:

The best method when there are big distances between the shaft ends.

A simple method to use.

Most people rotate both shafts

Face and rim method:

Use this method if one of the shafts cannot be rotated.

An excellent method for large shaft diameters (8 inches or 200mm or greater) or if the diameters are

equal to, or greater than the span from the bracket location to the face and rim location where thereadings are to be taken.

Not too good a method if there is axial float from sleeve or journal bearings.

Given a choice I would select the C or D frame every time.

The "C frame" is for inch sizes The "D frame" for metric sizes.

Automotive people use the same concept to align an automobile transmission to the engine. They call

the adapter a "bell housing". The concept was originally developed for the marine industry where it would be impossible to bolt the

motor and pump to the deck of the ship, and then do an alignment. The hull flexes making anyconventional alignment ineffective. The same logic applies to off shore drilling rigs.

The adapter does a better job of equalizing the heat transfer between the pump and the driver. It does

not all have to conduct through the shaft.

The adapter is available for all quality end suction centrifugal pumps. Check with your supplier for the

availability of one for your pump

When given a choice, select a ductile rather than a cast adapter.

Up to about thirty-horse power (22 KW) you hang the motor on the pump. Above thirty-horse power

(22 KW) you hang the pump on the motor.

The adapter solves the problem of "there is no time to do an alignment".

If your motor does not have a "C or D" end bell, one can be installed when the motor is rewound.

Some, but not all explosion proof motors are available with a C or D frame end bell. Check with your

supplier.

If you do not have a C or D frame adapter you will be involved in the last three steps of the four-step procedure.

Once you have made all the measurements, put in the recommended compensation for thermal expansion, andfigured out all the calculations for how much to move the driver, and in which direction; now comes the fun part;

moving the driver.

You can hit the motor with a big hammer, but small dimensions are hard to get with this method.


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Some people use an adjusting wheel that attaches to shims. This will give you a very precise movement that is

necessary for a proper alignment

Another method is to use an adjusting wheel that slips over the motor hold down bolts. Many mechanics makethere own tools and these units also work very well for precise motor movement.

How concerned should you be about alignment? You do it on your automobile when you notice uneven tire wearor the car drifts to one side of the road when you loosen your grip on the wheel, and have no problem justifyingthe cost and time involved. It is the same logic you use towards the added cost and time spent balancing thetires and wheels of your car.

We do not always apply the same logic to our very expensive rotating equipment in the shop, but we should. Amechanical seal should run trouble free until the carbon sacrificial face has worn away. When we inspect the sealswe remove from leaking pumps we find that in better than 85% of the cases there is plenty of carbon face left onthe seals. The seals are leaking prematurely and the seal movement caused by pump to motor misalignment is amajor contributing factor.

Understanding Shaft Alignment: Basics


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by Rich Henry, Ron Sullivan, John Walden and Dave Zdrojewski, VibrAlign, Inc.Posted 8-3-09

Part one of a four-part series that will cover alignment fundamentals and thermal growth,and highlight the importance of field measurements through two case studies.

Despite the best efforts to precisely align rotating machinery shafts, dynamic movement

(commonly believed to be due to the thermal growth of the machine casings) has resulted

in machines operating at less than optimum alignment conditions. This vexing problemhas plagued machine reliability professionals for decades.

What is shaft alignment?

Shaft alignment is the positioning of the rotational centers of two or more shafts such that

they are co-linear when the machines are under normal operating conditions.

Proper shaftalignment is not dictated by the total indicator reading (TIR) of the coupling hubs or the

shafts, but rather by the proper centers of rotation of the shaft supporting members (the

machine bearings).

There are two components of misalignmentangular and offset.

Offset misalignment, sometimes referred to as parallel misalignment, is the distance

between the shaft centers of rotation measured at the plane of power transmission. This is

typically measured at the coupling center. The units for this measurement are mils (where1 mil = 0.001 in.).

Angular misalignment, sometimes referred to as "gap" or "face," is the difference in the

slope of one shaft, usually the moveable machine, as compared to the slope of the shaft of

the other machine, usually the stationary machine. The units for this measurement arecomparable to the measurement of the slope of a roof (i.e., rise/run). In this case the rise

is measured in mils and the run (distance along the shaft) is measured in inches. The units

for angular misalignment are mils/1 in.

As stated, there are two separate alignment conditions that require correction. There arealso two planes of potential misalignmentthe horizontal plane (side to side) and the

vertical plane (up and down). Each alignment plane has offset and angular components,

so there are actually four alignment parameters to be measured and corrected. They arehorizontal angularity (HA), horizontal offset (HO), vertical angularity (VA), and vertical

offset (VO).

Shaft alignment tolerances

Historically, shaft alignment tolerances have been governed by the couplingmanufacturers design specifications. The original function of a flexible coupling was to

accommodate the small amounts of shaft misalignment remaining after the completion of


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a shaft alignment using a straight edge or feeler gauges. Some coupling manufacturers

have designed their couplings to withstand the forces resulting from as much as 3 degrees

of angular misalignment and 0.075 in. (75 mils) of offset misalignment, depending on themanufacturer and style of the coupling.

Another common tolerance from coupling manufacturers is the gap tolerance. Typicallythis value is given as an absolute value of coupling face TIR (as an example, a

specification migh read "face TIR not to exceed 0.005 in."). This number can bedeceiving depending on the swing diameter of the face dial indicator or the diameter of

the coupling being measured. In fairness, it should be noted that the tolerances offered by

coupling manufacturers are to ensure the life of the coupling with the expectation that theflexible element will fail rather than a critical machine component.

If this angular tolerance was applied to a 5 in. diam coupling, the angular alignment result

would be 1 mil/1 in. of coupling diameter or 1 mil of rise per 1 in. of distance axially

along the shaft centerline. If the coupling was 10 in. in diameter, the result of the

alignment would be twice as precise (0.5 mil/1 in.). This would lead one to conclude thatan angular alignment tolerance based on mils/1 in. would be something that could be

applied to all shafts regardless of the coupling diameter.

Harmonic forces are dangerous

When shafts are misaligned, forces are generated. These forces can produce great stresses

on the rotating and stationary components. While it is probably true that the coupling will

not fail when exposed to the large stresses as a result of this gross misalignment, thebearings and seals on the machines that are misaligned will most certainly fail under

these conditions. Typically, machine bearings and seals have small internal clearances

and are the recipient of these harmonic forces, not unlike constant hammering.

Excessive shaft misalignment, say greater than 2 mils for a 3600 rpm machine undernormal operating conditions, can generate large forces that are applied directly to the

machine bearings and cause excessive fatigue and wear of the shaft seals. In extreme

cases of shaft misalignment, the bending stresses applied to the shaft will cause the shaftto fracture and break.

Bearing life expectancy

The most prevalent bearings used in machinery, ball and roller bearings, all have a

calculated life expectancy, sometimes called the bearings L-10 life a rating of fatiguelife for a specific bearing. Statistical analysis of bearing life relative to forces applied to

the bearings has netted an equation (see "How Bearing Life is Affected by

Misalignment") describing how a bearings life is affected by increased forces due tomisalignment.

As the force applied to a given bearing increases, the life expectancy decreases by the

cube of that change. For instance, if the amount of force as a result of misalignment

increases by a factor of 3, the life expectancy of the machines bearings decreases by afactor of 27.


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Quite a bit of research in shaft alignment has been conducted over the past 20 years. The

results have led to a much different method of evaluating the quality of a shaft alignment

and to increasingly accurate methods of correcting misaligned conditions. Based on theresearch and actual industrial machine evaluations, shaft alignment tolerances are now

more commonly based on shaft rpm rather than shaft diameter or coupling

manufacturers specifications. There are presently no specific tolerance standardspublished by ISO or ANSI, but typical tolerances for alignment are shown in the table

"Typical Tolerances for Alignment."

Another common method of determining shaft alignment tolerances is to ensure the

machine feet are within a specified distance from what is considered "zero". This methodalso can be misleading. If a machine is considered to be aligned when the foot corrections

are less than 2 mils at both the front feet and back feet, serious misalignment can

sometimes be present. As a general rule, the smaller the machine footprint (distance fromfront feet to back feet), the worse the alignment condition based on these criteria for

alignment tolerance.

In Fig. 1, the motor foot distance front to back is 10 inches. The distance from the front

feet to the center of the coupling is 8 inches. If the front foot of the motor is left 2 milshigh and the back feet are left 2 mils low, the shaft alignment results will be as follows:

vertical angularity of 0.4 mil/1 in. open at the top of the coupling, and a vertical offset of

5.2 mils high at the plane of power transmission. If this machine operates at 1800 rpm, itwould be outside the acceptable shaft alignment tolerances. Again, this reinforces that a

set of shaft alignment tolerances based on shaft rpm would apply to all machines

regardless of their footprint. MT

How Bearing Life Is Affected By Misalignment

Formula notes: This formulation is credited to the work done by Lundberg and Palmgren

in the 1940s and 1950s through empirical research for benchmarking probable fatigue life

between bearing sizes and designs.

For ball bearings: L10 = (C/P)3 x 106; For roller bearings: L10 = (C/P)10/3 x 106

where:

L10 represents the rating fatigue life with a reliability of 90 percent


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C is the basic dynamic load ratingthe load which will give a life of 1 million

revolutions which can be found in bearing catalogs

P is the dynamic equivalent load applied to the bearing

Typical Tolerances For Alignment

Misalignment Using Machine Feet Distances

Fig. 1. Using machine feet distance to align a machine to acceptable tolerances can give misleading information.

Calculate Bearing Life

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