Shaft Alignment

‫‪Shaft Alignment‬‬

‫‪SHAFT ALIGNMENT‬‬ ‫‪Maintenance Report‬‬

‫تقي الدين احمد عبد هللا الشميري ‪2012\314‬‬

‫ميكانيكا ‪ -‬صيانة نظم ميكانيكية‬ ‫المهندس غالب البعداني‬

‫هيثم عبداالله الصلوي‬

‫‪2012\427‬‬

‫احمد محمد الوصابي‬

‫‪2012\435‬‬

‫يحيى احمد السماوي‬

‫‪2012\303‬‬

‫خالد عبد العزيز الحجازي‬

‫‪2012\364‬‬

‫مصطفى صالح عتيق‬ ‫‪1‬‬

‫‪2012\2343‬‬

Shaft Alignment

Shaft Alignment

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Shaft Alignment

Contents                    

Abstract Introduction Alignment terms Misalignment causes Misalignment detection Vibration due to shaft misalignment Misalignment effects on bearing and seals Misalignment and power consumption Effects of base conditions and thermal variations Alignment preparation Shaft alignment techniques Laser versus rim and face Hot alignment checks Alignment tolerances Harmonic forces Bearing life expectancy Thermal growth and shaft alignment The benefits of alignment The importance of motor shaft alignment References

Abstract Proper alignment of rotating equipment has a long been recognized as a prerequisite to safe, reliable operation. Achieving proper alignment, however, involves much more than merely adjustment the machines so that the shafts are coaxial. It involves the design and implementation of the equipment and of the system into which the equipment is integrated, Important issues in design and implementation of the entire system are highlighted, along with details of the final alignment process.

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Shaft Alignment

Introduction with requirements of organizations to cut costs and manage operation and maintenance budgets better, minimizing equipment downtime has become increasingly important. An aspect of trouble-free operation is the alignment of rotating equipment. Alignment should be completed on any equipment that has a driver and driven equipment. It should not be standard practice to only align “high energy” or high-speed pumps. ALL pumps should be considered for alignment. To provide an overview of alignment, the discussion in this article applies primarily to horizontal centrifugal pumps that are operating in a general service and driven by a separate driver through a flexible coupling on a common baseplate. Many of the terms and techniques described can be used for other types of pumps and rotating equipment, but the installation and operation instructions should be followed as required by the manufacturer. This article is meant to be used as a primer only. Extensive training and practice should be completed prior to aligning equipment that is in service. IN GENERAL

The lack of alignment, or misalignment, is a condition where the rotating centerlines of two or more machinery shafts are not in line with each other. This is not easy to detect when machinery is running. The rotating equipment is aligned following a series of detailed steps of a few alignment techniques. It can be said that the goal of alignment is to position the equipment so that any deviations of the shaft centerlines are below specified or required criterion. Further, the objective is to minimize equipment downtime and maximize the operating life of the rotating equipment. Misalignment of rotating equipment can have many effects on the equipment and the entire system. As can be seen in Figure 1, the greater the misalignment, the greater the reduction in time the equipment can continuously run until failure; the lower the misalignment, the greater the length of time between failure due to the effects of misalignment.

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Shaft Alignment

Alignment Terms There are a few basic terms used in alignment that should be reviewed. The specific terms dis- cussed in this section are important to a successful installation.

 ANGULAR MISALIGNMENT Angular misalignment is the term used when two shaft centerlines are at an angle to each other, as shown in Figure 4. Angular misalignment can occur in the horizontal or vertical plane of the shafts. A typical objective is to obtain angular misalignment to less than 1°.

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Shaft Alignment

 PARALLEL MISALIGNMENT When the equipment shaft centerlines are parallel, but are offset from each other, as shown in Figure 5, it is called parallel misalignment. This type of misalignment can also occur in the horizontal or vertical plane. It is important to note that equipment shafts can have angular and parallel misalignment simultaneously. Both must be considered for a successful installation.

 BOLT BOUND The term “bolt bound” is used to describe a condition that occurs during the alignment process, where the driver or pump cannot be moved any further. The best method to resolve this problem is to fill-weld the existing bolt hole, re-drill in the proper location, and re-tap. This problem can also be resolved by removing the problem equipment and increasing the diameter of the mounting holes in the feet.

 SHIMS Shim stock or precut shims are used to raise the driver during the alignment process. The proper shimming of the driver should occur only after the installation of the baseplate has been finalized and the connection of the piping to the pump is completed. Typical of the baseplate manufacturers, a mini- mum of 0.125” shim under the driver feet will allow for proper alignment. Although a mini- mum is recommended, it must be noted that too many shims may make the installation “springy”. A stack of the same size of shims, as shown in Figure 6, should have five or fewer shims of the same size. The precut shims should be sized to fit the bolt of the equipment feet. The stack should also have the thinner pieces of shim sandwiched between the thicker pieces. The use of homemade carbon steel shim is not recommended. In addition 6

Shaft Alignment to the possibility of rusting, the cut carbon steel may have inconsistency in its thickness, thereby causing a soft foot. Precut shim of 304 Stainless Steel material is available. These precut shims usually have the thickness measured and the exact thickness is etched on the shim. Although the thickness may be marked on the shim, the thickness should be double checked before installation.

These precuts shims may come in a convenient box that contains many sizes, as seen in Figure 7. Besides 304 stainless steel, precut shim is also available in Mylar for pumps that are in chlorine or hydrochloric acid services. For proper use and installation of the shim, the mounting pad and the equipment feet must have a smooth surface, free of burrs, and milled flat. The shim is inserted until contact is made with the bolt, and then moved back away from bolt to insure proper clearance.

 SOFT FOOT Soft foot is a condition that occurs when one or more equipment feet are not firmly on the sole- plate or baseplate. With a soft foot condition, the equipment will have a tendency to move during operation. Soft foot should be corrected before final alignment and operation. There are a number of different types of soft foot. It is important to identify the type of soft foot condition that is present. The method to correct the soft foot depends upon the cause of the soft foot. Using the 7

Shaft Alignment wrong method to correct the soft foot may intensify the problem and make it worse. Soft foot can initially be found using feeler gauges. Prior to tightening the equipment feet down or shimming, feeler gauges should be used to measure the gap under each corner of the

same foot. The readings at each location under each foot will provide information about the type of soft foot condition.  Parallel Air Gap or Short Foot The most common soft foot condition is a parallel air gap, or short foot. This condition is identified when there is an equal gap at all four corners of a foot. A short foot can occur when one leg of the driver is too short, when the foot mounting pad is not in the same plane as the other(s), or the foot has an inadequate amount of shim un- der it. If the alignment process has been started, and an accurate or consistent alignment cannot be obtained, it is likely that there is a soft foot condition present. To check for the parallel air gap, set up the dial indicator on a foot and loosen mounting bolt. If the foot moves more than 0.002”, or the soft foot standard defined by the user, the foot is considered soft and needs to be corrected. This should be done for all feet and documented. The following example is typical for correcting parallel air gap or short foot. Figure 8 shows a view of a motor with various parallel air gap 8

Shaft Alignment measurements at each foot. It should be assumed that the gap shown is the same under each corner of the foot.

 DIAL INDICATORS Dial indicators are used to measure the amount of misalignment between the equipment. A dial indicator (Figure 9) can assist in determining alignment within ±0.001”. The dial indicators come from the manufacturer pre-calibrated, and must be rechecked on a regular schedule. Shaft alignment kits are available starting at approximately $600.00. These kits will contain two

(2) alignment frames, roller chain, two (2) dial indicator assemblies with swivel joints and mounting rods, tubing of varying lengths, instruction manuals, alignment worksheets, and a sturdy carrying case with handle. This type of kit will handle the vast majority of shaft alignment situations. When initially getting ready to use a dial indicator, make sure that it works properly and can travel the full range of the indicator. Upon installation of the indicators and at the beginning of the process, note the direction that the indicator travels. The indicator should be adjusted to start at zero (0.000”). The indicator needle traveling clockwise is considered positive, while counterclockwise is considered negative travel. During the alignment process and a full 360° sweep, the summation of the left and right side readings should equal the summation of the top and bottom readings. In addition, the indicators should have read the same reading (0.000”) when it reaches the original position. 9

Shaft Alignment

 BAR SAG Bar sag is a deflection that occurs in the over- hung indicator bar due to the bar’s weight and the weight of the indicator. Because of the precision that is required for alignment, the bar sag must be compensated for. To determine the amount of bar sag, mount the indicator on a pipe in same approach, distance, etc. as it will be mounted on equipment shaft. Next, zero the indicator on top. Then roll the dial indicator 180°, so that it is located at the bottom. Read the indicator. During the alignment process, this bar sag amount must be included in your alignment calculations. Instead of zero on the top of the dial indicator, dial in the positive value of the bar sag reading. During the alignment process, the dial indicator should read zero when rolled to bottom.

Misalignment Causes Many causes exist for shaft misalignment. To ensure an alignment is successful, all of the possible causes of misalignment must be addressed. The following are the most prevalent causes: 1. Relative Movement – Thermal growth, or expansion, can cause one piece of equipment to move proportionately to another, causing relative movement misalignment. Different materials expand at different rates when heated. Thermal growth must be accounted for when equipment normally operates above ambient temperature. 2. Strain – Strain induced by attached piping runs can force equipment out of alignment. Misalignment caused by strain can reoccur after a successful alignment due to the continuous action of forces caused by strained equipment. 3. Torsional Movement – The initial high torque caused during startup can force shafts out of alignment, causing torsional movement misalignment. 4. Settling – Over time, foundations or base-plates can settle to lower positions, causing settling misalignment. The equipment can be realigned, but without addressing the cause of the misalignment, the problem can reoccur. 5. Human Error – A mistake in an alignment procedure or not completing an alignment procedure can cause human error misalignment. 10

Shaft Alignment 6. Misbored Couplings – Manufacturing defects, creating couplings that are misbored or otherwise distorted, can cause misbored coupling misalignment. This form of misalignment is mostly found on new equipment; however, damaging a coupling during an alignment will cause similar problems.

Misalignment Detection Misaligned rotating machinery causes high cost to the industry as it causes premature damages to the machinery, loss in production and excessive energy is consumed. Misalignment is the most common cause of machinery malfunction. A poorly aligned machine could cost 20% to 30% of machine down time, replacement parts, inventory and energy costs. Large returns are usually seen by regularly aligning the machine. The total operation life is extended and process conditions are optimized for efficiency. Hence it becomes extremely important for the maintenance and engineering professionals to understand machine malfunctions caused by misalignment. Vibration signatures are widely promoted for studying machine malfunctions. However, most literature will not be able to provide a clear picture of signature characteristics uniquely and directly attributable to misalignment. Every author will report different signatures. There are still no reports of systematic, controlled experiments with varying parameters. However, we can carry out various experiments to elucidate any consistent features of vibration signatures for misaligned machinery. To begin with let’s consider a simulator which is fault free and is able generate controlled faults, it should possess three machine operating parameters, coupling types, amount of misalignment and the motor speed which will be systematically varied while all the other parameters will be held constant. The machine should be fault free with the exception of deliberate misalignment which is varied systematically. Therefore, baseline vibration data is recorded for each of the test conditions. Vibrations should be monitored via sensors which should be placed in strategic locations to get accurate data. The X, Y, Z coordinate system is used to show direction. For this experiment we can use three different stiffness couplings, four levels of offset should be used on the left bearing housing to simulate a combination of angular and parallel misalignment. Equivalent offset on the right bearing housing to get parallel misalignment. The experiment must contain four speeds of rotation and 11

Shaft Alignment the goal here must be to determine the effects of coupling stiffness, level and type of misalignment and finally the speed of rotation on vibration spectra. Data can be acquired from the specially designed hardware and software for the simulator. The purpose of this experiment should be to examine the spectra due to misalignment between the motor and the rotor shafts. Spectral comparisons should be made across coupling measurement points on left bearing housing and the motor. The data should be compared in both vertical and axial direction. If the results at 960 and 2100 RPM do not show significant vibration, the study can be limited to a higher speed of 2900 and 3800 RPM. A correlation between misalignment and vibration signature could not be discerned. The data for all cases contained several harmonics. Both axial and lateral vibration was present in all cases. The dominant harmonic varied from condition to condition. As a general rule, as expected, increased misalignment yielded increased vibration peaks. As another general rule, peak vibrations in the misaligned machine were in the axial direction. For predictive maintenance applications where the goal is machinery health monitoring, it is sufficient to realize that the problem is complex. One can routinely trend the vibration spectra until it becomes severe. But for root cause analysis, one must exercise caution and perform a detailed analysis. Obviously, the rules provided in training courses and wall charts are doubtful at best. The observed changes that occurred with shifts of speed and misalignment do not show a typical signature for misalignment vibration spectra. Hence, it can be concluded that misalignment vibration is a strong function of machine speed and coupling stiffness. A single point vibration spectrum does not provide good and reliable indication of machinery misalignment. Observations of spectra in axial and radial directions at various speeds and several points are needed to diagnose misalignment effects. Orbital plots of vertical and horizontal measurements in the time domain might also be needed. Non-linear dynamic modelling must be performed to gain full understanding of misalignment effects. Finally, more work in this field is simply needed in order to develop simple rules for diagnosing machinery shaft misalignment.

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Shaft Alignment There are several symptoms indicating misalignment. By keeping your eyes open you can find them without any special equipment.          

Premature bearing, seal, shaft and coupling failure Excessive radial and axial vibrations High casing temperature at or near the bearings or high discharge oil temperature Excessive amount of oil leakage at the bearing seals The coupling is hot while it is running and immediately after unit shutdown. Look for rubber powder inside the coupling shroud Loose foundation bolts Loose or broken coupling bolts Excessive amount of grease on the inside of coupling guard Similar pieces of equipment are vibrating less or seem to have longer operating life The shafts are breaking (or cracking) at or close to the inboard bearings or coupling hubs

Vibration Due to Shaft Misalignment Unlike imbalance, misalignment does not produce forces that are similar in the radius of the machine. Vibration in one radial direction may be very different in amplitude when compared to vibration in another radial direction. This is one instance where amplitude readings are not only helpful in detecting a problem but also helpful in diagnosing a problem. Misalignment can result in a machine having high vertical vibration on one end and high horizontal vibration on the other with other radial readings remaining low. Imbalance is not likely to cause this amplitude pattern. Vibration amplitude directly correlates with the amount of imbalance and equipment speed. If machine speed remains constant and the imbalance is doubled, then vibration amplitude should also double. If the speed of the equipment doubles and the imbalance remains the same, then the centrifugal forces will increase by a factor of four and the vibration amplitudes would increase accordingly. However, correlating the effects of misalignment with vibration amplitude can be difficult. Misalignment effects on vibration are much more complex than with unbalance. In certain cases, improving alignment 13

Shaft Alignment may actually cause vibration radial amplitudes to increase. The reason for this is that misalignment may restrain movement in one or more directions. In one instance, the author was called to a plywood plant to help balance a knife hammer mill. Before his arrival, the site vibration analyst collected vibration data and determined that one of the problems with the hammer mill was rotor imbalance. When the machine was opened, it was observed that several balance weights were attached at what appeared to be random locations around the rotor. This is a red flag indicating that problems other than imbalance may be present. Since balancing problems should be corrected only after all other problems have been addressed, the team decided to determine if other problems existed and correct them first. A review of the vibration data clearly indicated that rotor imbalance was a problem. However, the data also indicated shaft misalignment and looseness. A PRÜFTECHNIK laser alignment system was on-site, so the alignment problem was addressed first. A shaft angular misalignment greater than 7 mils per inch with over 30 mils of offset between the motor and hammer mill rotor was found and corrected. After the machine was started and new vibration data was collected, the vibration amplitudes were found to be much higher. The vibration analyst was perplexed and asked if we had done the wrong thing by aligning the machine. The author’s answer was, “No, the precision alignment helped us to know that the looseness is most likely in the bearings.” The gross misalignment existing previously had the shafts in such a bind that shaft movement was restricted. In all likelihood, this misalignment had greatly contributed to the severe bearing wear, and if it had been corrected sooner, bearing life would have been extended significantly. When the shafts were put into alignment, the binding no longer prevented shaft movement in the badly worn bearings, permitting vibration amplitudes to increase. The bearings were replaced, the shafts were re-aligned yet again, and finally the rotor was balanced. New vibration data revealed the machine to be within plant vibration standards. Phase Indications Angular misalignment is indicated by an approximate phase difference of 180 degrees across the coupling when measured in the axial direction. 14

Shaft Alignment Be aware of mistakes made in measuring axial phase by not allowing for transducer orientation. Offset misalignment produces approximately a 180-degree phase difference when comparing a radial phase on one machine to the same radial direction on the other machine across the coupling. Phase differences may be anywhere from 150 to 210 degrees. The more severe the misalignment, the closer the phase differences will be to 180 degrees. When the vibration is at a frequency equal to shaft speed (1×), phase is the best tool for distinguishing between misalignment and unbalance. Take several phase readings at each point to ensure accurate phase data that may vary due to speed changes. Vibration Indications When axial vibration at 1× or 2× is 50 percent or more of radial vibration, suspect misalignment. Be careful here because shaft misalignment isn’t the only problem that can cause high axial vibration. Other causes of high axial vibration: A. Bent shaft. B. Resonant whirl. C. Bearing cocked on shaft (bearing misalignment). D. Resonance in the axial direction. E. Worn thrust bearings. F. Worn helical or bevel gears. G. Sleeve bearing motor hunting for magnetic center. H. Couple component of unbalance. Suspect misalignment when radial vibration at 2× shaft speed is 50 percent or more of shaft speed vibration. Misalignment can also produce several harmonics of running speed. These harmonics are produced when shaft movement is restricted and the waveform is clipped. Unlike the harmonics produced by looseness, the harmonic spikes in the spectrum should not have a raised noise floor. Time waveform plots of misalignment should be somewhat similar during each shaft revolution. Looseness will appear much more complicated in the time waveform. 15

Shaft Alignment Coupling problems may look like alignment problems. 1× is usually most affected. However, different coupling types produce different vibration indications. Too long or too short coupling spacers may cause high 3× which may be confused with misalignment. The author’s experience has shown this problem to produce a high 1× vibration also. A bent shaft produces high axial vibration. Phase shift will be 180 degrees when comparing top to bottom of bearing and 180 degrees from one side of bearing to the other. In the past, the author performed a test known as the running soft foot test when vibration tests indicated an alignment problem or a possible resonance due to soft foot. This test involves loosening machine mounting bolts one at a time while the machine is running. If done improperly, this test can be dangerous. Modern shaft alignment tools have made it so easy to check soft foot that there is no longer a need for performing the risky running soft foot test. When vibration testing indicates shaft misalignment, simply use a laser tool to check the alignment.

Misalignment Effects on Bearings and Seals The bearings operating life span is directly affected by the forces it is exposed to. The slightest misalignment can generate excessive forces to the bearings and seals. A misaligned machine causes stress to both bearings and shafts. As an effect of this the seals open up, allowing lubrication leakage and contamination to enter. All together the bearings lifetime is dramatically shortened.

Misalignment and Power Consumption Misalignment has a direct impact on the power consumption. Documented cases have shown savings ranging from 2 to 17%. The thermogram below shows two different couplings and the amount of heat generated due to misalignment.

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Shaft Alignment

Effects of Base Conditions and Thermal Variations Even if you align your machines properly there is a risk that they break due to misalignment. If the foundation where the machine is mounted is warped you may have a soft foot condition, i.e. one of the machine feet has a poor contact with the floor. It is of major importance that this is corrected before performing the alignment. If the alignment is performed when the machines are cold, and no compensation for thermal growth is considered, the machines will probably be running severely misaligned at running temperature.

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Shaft Alignment

Differences in temperature can cause severe misalignment due to the fact that machine casings and pipes expand with raised or lowered temperature.

Alignment Preparation Before starting the alignment process, follow these steps: 

Make sure that the mounting faces of the bedplate and the rotating machine are free from paint, rust, weld spatters, burrs or other debris.



Throw away any shims used during previous alignments that are rusty, painted or damaged.



Check to see if the holes for the holding-down bolts are large enough to allow adequate movement for alignment. This helps avoid unnecessary delays later in the process.



Check the distance between the shaft ends and align the machines approximately. You can do this by laying a straight edge against the machines and ensuring that the hub faces are parallel. One issue that end users often overlook during alignment is the presence of a soft foot, where the machine is not sitting evenly on the bedplate. This can twist the machine frame, putting an unexpected load on the bearings and, ultimately, causing the frame to fracture. End users can detect a soft foot by mounting a bracket carrying a dial indicator on one shaft and placing the plunger on the second shaft, with all the holding-down bolts lightly tightened. Next, work around the machine, fully tightening the bolts and then slackening them one at a time. Any excessive movement on the indicator signifies the presence of a soft foot, which will require that the end user fit shims to the machine 18

Shaft Alignment before starting the alignment. Modern laser alignment equipment also often contains a program that can check for soft foot without undergoing this manual procedure.

Preparation Checklists The following checklists can help end users prepare thoroughly for alignment. The lists can be adjusted as needed to suit specific machinery and the chosen alignment method. 

Off-site Checklist Safety regulations



Working permits



Time limits for stopping production



Alignment tolerances



Thermal offsets



Available space



Shaft rotation



Shim sizes



Alignment system



Batteries, if required for laser equipment



Specifications for the machine setup



On-site Checklist Machine serial numbers



Safety conditions



Available space



Foundation condition



Bedplate condition



Bolt condition



Adjustment capacity



Shim condition



Presence of leaks Pre-alignment Checklist 19

Shaft Alignment 

Machine temperature



Gross soft foot check



Replacement of old shims



Coupling assembly



Adjustment of any mechanical looseness



Run-out conditions



Pipe strain



Final soft foot checks to adjust small errors after removing gross soft foot



Repeatability test to ensure that tightening and loosening the feet bolts gives consistent results; this test also checks the condition of any shims or binding of bolts

Shaft Alignment Techniques After preparing for alignment, end users must choose an alignment method appropriate for their machinery. Many alignment methods require brackets and arms for mounting the measuring equipment. As the length of the arms increases, the arms become more susceptible to sagging, which can cause significant measuring errors if end users do not take the sagging into account. Figure 1 illustrates the typical method to check for sagging arms. End users should add the values they record to the readings they take during alignment. The major methods for checking alignment are discussed in this section.

Method 1 — Face and Periphery 20

Shaft Alignment The face and periphery method is the oldest and most widely used dial indicator method. Figure 2 illustrates the correct setup for this method, which is good for large-diameter hubs with a short distance between shaft ends.



Advantages: The end user only needs to rotate one shaft.



Visualizing shaft positions is easier compared to other methods.



Disadvantages: It is difficult to obtain face readings if there is any axial float.



An end user typically has to remove the coupling.



It is more complicated to make the graphical calculations compared with other methods.



Any out-of-roundness in the hub periphery or out-of-squareness in the hub face—meaning the hub flange is not round or flat and square relative to the shaft—will affect the readings.

Method 2 — Reverse Periphery (Indicator) Method The reverse periphery (indicator) method is becoming increasingly popular and is the optimal method for most alignments. Figure 3 illustrates the correct setup for this method. Advantages: 21

Shaft Alignment 

It is generally more accurate than the face and periphery methods.



Axial float, or out-of-roundness or out-of-squareness in the hub, does not affect the readings.



It is easier to plot graphically compared with other methods.



It is possible to make measurements with the coupling in place.



Disadvantages: End users must rotate both shafts.



It is not accurate for close-coupled shafts.

Method 3 — The Laser System The modern laser system (see Figure 4) consists of either two laser/detector units or one laser and one mirror/prism reflector unit. One unit is attached to each shaft, and the laser/detectors are connected to a keyboard/display unit by a cable or via a wireless Bluetooth system. The system operates using the principles of reverse periphery, detecting the movement of the laser beams as a measure of the misalignment.

22

Shaft Alignment



Advantages: The system fits a variety of shafts.



End users do not have to remove the coupling.



End users can measure long spans with no sagging issues.



It is not affected by axial float.



The system detects the soft foot condition easily.



Disadvantages: The system is not suitable for couplings with backlash.



Heat or steam can affect its accuracy.



The system is expensive.

23

Shaft Alignment

Other Alignment Methods Other alignment methods include shaft-to-coupling spacer, optical systems and electronic indicators. For equipment with long distances between the shaft ends, it is possible to fit the coupling and check the alignment of one shaft to the spacer and then check the spacer’s alignment with the second shaft.

Lasers Versus Rim and Face Techniques With most laser systems, you are encouraged to rotate both shafts during the measurement process. This is because we want to use the rotational axis of one shaft as the datum (reference line) and then compare the rotational axis of the second shaft to the datum. If only one shaft is rotated, the datum is dependent on the accuracy of the coupling surface. You can improve the results with indicator methods by rotating both shafts while taking alignment measurements. The face indicator is used to measure angular misalignment. The face indicator measures the actual gap difference at the diameter that the face indicator travels. With larger diameters you will see larger face indicator readings. Angularity should be expressed as a slope, not a displacement. Therefore, you must divide the face reading by the diameter to calculate the true angular relationship of the two shafts. The rim indicator is used to measure offset misalignment. When the rim indicator is set to zero and subsequently rotated 180deg, the total indicator reading is divided by two to determine the actual offset misalignment. Laser systems report angular misalignment as a slope and

24

Shaft Alignment the actual offset misalignment at a user-specified axial shaft location. For close coupled machines, the user will typically specify the coupling center as the axial location for offset misalignment because the destructive misalignment forces are generated there.

Hot Alignment Checks Another way to gather this data is to perform a hot alignment check of the affected piece of equipment. The procedure for this is relatively simple. The machine is aligned off line and the results of the alignment are documented (horizontal angularity, horizontal offset, vertical angularity, and vertical offset). The machine then is placed on line and allowed to reach normal operating conditions. At this point, the machine is shut down and allowed to stop rotating. The alignment system is remounted on the machine and the shaft alignment is re-measured and documented. Now the machine may be aligned hot by re-shimming and repositioning the moveable machine as quickly as possible. One drawback of this method is that the machine will begin to cool as soon as it is shut down, adversely affecting the accuracy of the hot alignment check. If the two sets of alignment readings were documented, a set of cold alignment targets can be calculated. Alignment results (hot) – alignment results (cold) represents the change in the alignment condition of the machine from cold to hot. The alignment targets for this machine will be the opposite of the changes in the alignment parameters. While this is currently a widely used method of hot-aligning machines, it will measure only the changes in the shaft alignment due solely to the changes in the machine’s temperatures. Discharge pressure, shaft torque, multiple machines operating in parallel, electrical loading of a generator, etc., also can play a large role in the change in the alignment condition from off line to running. These changes most often will be seen in the horizontal plane, but could affect the vertical alignment as well Yet another factor to consider is the location of the machine. If a machine is located indoors in a controlled environment, the operating characteristics should be relatively constant throughout the year. A machine that operates outdoors and is exposed to large changes in temperature also could exhibit extreme changes in its shaft alignment as the temperature changes (as in the change of seasons).

25

Shaft Alignment

Alignment Tolerances No industry standard on alignment exists. Proper shaft alignment is especially critical when the motor is operated at high speeds. Standard industry norms for alignment tolerances are cited in Table 1.

In practice, proper alignment is difficult to achieve without using alignment equipment such as dial indicators or laser alignment tools. The proper shaft alignment procedure is to secure the driven equipment first because moving a pump, for example, would stress the connecting piping. Next, install the coupling to the driven equipment. The motor should then be moved into proper alignment and joined to the coupling. After the equipment has operated long enough to become temperature stabilized, shut it down and immediately recheck alignment. Due to thermal growth, machines that are aligned in the “cold” preoperating condition are almost always out of alignment when operating temperatures are attained. Many equipment manufacturers publish thermal offset values so the alignment technician can correct for thermal growth during the initial alignment process.

26

Shaft Alignment

Harmonic Forces 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, the bearings 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 under normal 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 shaft to 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 bearing’s L-10 life— a rating of fatigue life for a specific bearing. Statistical analysis of bearing life relative to forces applied to the bearings has netted an equation describing how a bearing’s life is affected by increased forces due to 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 C is the basic dynamic load rating—the load which will give a life of 1 million revolutions which can be found in bearing catalogs P is the dynamic 27

Shaft Alignment equivalent load applied to the bearing 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 machine’s bearings decreases by a factor of 27. 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 the research 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 standards published by ISO or ANSI, but typical tolerances for alignment are shown in the table.

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 method also 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 from front 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 mils high 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 28

Shaft Alignment power transmission. If this machine operates at 1800 rpm, it would 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.

Shaft Alignment and Thermal Growth Machine conditions change from the time the machine is off line to when it is running under normal operating conditions. Some of these changes are due to process forces (e.g., fluid pressures, airflow, etc.). The most notable of these changes is the change in the temperature of the machine bearings and supports. This is called the machine’s thermal growth. Thermal growth is the change in the length of a particular metal as a result of the change in temperature of that metal. Typically, when a metal bar is heated, it will get longer. These changes can be very small (0.0005 in.) or they can be very large, depending on the length of the piece of metal and its coefficient of linear expansion. Temperature changes unequally That was a fairly simple example and does not accurately reflect what will happen to an actual machine. In reality, the temperatures of all the 29

Shaft Alignment machine supports will change; however, they will almost never change equally. Using the above machine example, consider the change in shaft alignment if the outboard end (OE) bearing temperature changed by 20 F and the drive end (DE) bearing temperature changed by 50 F. The drive end bearing would grow by 4.4 mils; however, the outboard bearing would grow only by 1.8 mils. The result will be a change in both the offset and angular alignment. If the motor feet are 20 in. apart, the change in the angular alignment will be 0.13 mil/in. [(4.4 – 1.8)/20 = 0.13] open at the top of the coupling. Changes in the temperature of machines from off line to running can have a significant impact on the shaft alignment. These changes in the shaft alignment can be accommodated in a few different ways. One way is to align the machines to zero and then remove or add the amount of shim under the machine feet as determined by the temperature data. Another way is to gather the alignment data, graph the results, and predetermine the actual shim corrections based on the graph. With today’s modern laser alignment technology, accounting for thermal changes at the machine feet is actually a simple evolution. Most alignment systems on the market today have within them a function that allows the user to program the foot targets of the machine being aligned. For the previous example, the front foot target would be –4.4 mils and the back foot target would be –1.8 mils. After programming the determined foot target values at the machine feet, the user aligns the machines to zero on the display unit. The shaft alignment system will automatically calculate the required foot corrections to leave the feet at the prescribed positions. As the machine heats up, the shaft centerlines will grow into a properly aligned condition.

The Benefits of Alignment Having correctly aligned machines reduces the risk of breakdowns. Typical problems arising from poor machine alignment are: • LOST PRODUCTION • LEAKING SEALS • INCREASED VIBRATION • HIGHER ENERGY CONSUMPTION • BEARING FAILURE • SHAFT BREAKAGE • COUPLING WEAR • QUALITY PROBLEMS 30

Shaft Alignment There is much to be gained in terms of time and money by adjusting the machines within the prescribed tolerances. Investing in a laser-based measuring system such as Easy-Laser® will benefit the company by reducing the cost of spare parts and lost production. And the working environment is improved at the same time.

The Importance of Motor Shaft Alignment The objective of optimized shaft alignment is to increase the operating life span of rotating machinery. To achieve this goal, components that are the most likely to fail must be made to operate within their acceptable design limits. While misalignment has no measurable effect on motor efficiency, correct shaft alignment ensures the smooth, efficient transmission of power from the motor to the driven equipment. Incorrect alignment occurs when the centerlines of the motor and the driven equipment shafts are not in line with each other. Misalignment produces excessive vibration, noise, coupling, and bearing temperature increases, and premature bearing, coupling, or shaft failure.

There are three types of motor misalignment: Angular misalignment occurs when the motor is set at an angle to the driven equipment. If the centerlines of the motor and the driven 31

Shaft Alignment equipment shafts were to be extended, they would cross each other, rather than superimpose or run along a common centerline. The “gap” or difference in slope of the motor shaft when compared with the slope of the stationary machine shaft can have horizontal misalignment, vertical misalignment, or both. Angular misalignment, in particular can cause severe damage to the driven equipment and the motor. Parallel misalignment occurs when the two shaft centerlines are parallel, but not in the same line. There are two planes of parallel misalignment as shafts may be offset horizontally (displaced to the left or right), vertically (positioned at different elevations), or both. Combination misalignment occurs when the motor shaft suffers from angular misalignment in addition to parallel misalignment.

Couplings Larger motors are usually directly coupled to their loads with rigid or flexible couplings. Rigid couplings do not compensate for any motor-todriven-equipment misalignment, while flexible couplings tolerate small amounts of misalignment. Flexible couplings also can reduce vibration transmitted from one piece of equipment to another, and some can insulate the driven equipment shaft against stray electrical currents. Even flexible couplings have alignment requirements, defined in the instruction sheet for the coupling. However, it is a mistake to rely on coupling flexibility for excessive misalignment, because flexing of the coupling and of the shaft will exert forces on the motor and driven-equipment bearings. These forces may result in premature bearing, seal, or coupling failures, shaft breaking or cracking, and excessive radial and axial vibrations. Secondary effects include loosening of foundation bolts, and loose or broken coupling bolts. Operating life is shortened when shafts are misaligned.

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Shaft Alignment

References 1. National Electrical Manufacturers Association (NEMA), 1300 North 17th Street, Suite 1847, Rosslyn, VA 22209; http://www.nema.org. 2. https://www.johncrane.com/the-john-cranedifference/thought-leadership/easy-shaft-alignment 3. Shaft Alignment by Jack Essinger – consulting engineering – Acculing, Inc. – Willis, Texas 4. Troubleshooting Alignment Data David Zdrojewski, VibrAlign, Inc. 5. http://www.impomag.com/article/2009/10/vibratio n-due-shaft- misalignment 6. http://www.fixturlaser.com/ShaftAlignment/Benefits -of-shaft- alignment/Why-shaft-alignment/

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