Guide to rotating machinery alignment
What is misalignment? Rotating equipment will not be reliable if the alignment between the driving and driven shaft centrelines does not form a straight line....
November 1, 2005 | By Lloyd (Tex) Leugner
What is misalignment? Rotating equipment will not be reliable if the alignment between the driving and driven shaft centrelines does not form a straight line.
Misalignment causes up to 50% of machine vibrations and is the result of two common problems. Parallel offset misalignment is a condition where the centrelines of two shafts do not meet, whereas angular offset occurs where the centrelines of the shafts are not parallel to each other. Parallel and angular offset can also occur at the same time.
The alignment techniques described here will eliminate (or satisfactorily reduce) these conditions, providing smooth running of rotating equipment, extending equipment life, reducing maintenance costs and increasing productivity and reliability.
Symptoms of misalignment include:
* Process fluid and/or oil leaks
* Powdered rubber or lubricant directly below the drive coupling (depending upon the coupling type)
* Shaft fatigue or breakage
* Broken foot bolts (caused by severe vibration or stress on one or more bolts)
* ‘Shimmering’ of oil on the base plate or near the foot bolts
* Premature and/or frequent bearing or seal failure on one or both machines
* Internal heating of machines
* High operating temperatures, particularly at or near the coupling
* High vibration conditions, usually at both machines
* Cracked or broken foundations, particularly near or at the foot bolts
* Continuing or intermittent leaks at pipe joints caused by pipe strain (due to the vibrations)
* Increased energy consumption, by up to 10%.
Common methods to determine if a misalignment problem exists include the following.
Vibration analysis using phase measurements can find a problem. For example, coupling unbalance tends to be 180 degrees out of phase on the same shaft, while misalignment is often characterized by a 180-degree difference across the coupling.
Stroboscopic analysis will very often indicate a misaligned shaft or coupling condition if the reference mark is unsteady or rotates intermittently in both directions, but remember this condition might indicate unbalance, rather than misalignment. In either case, it suggests a problem that must be corrected.
Temperature monitoring is a very important inspection technique used to determine if misalignment is occurring. A misaligned coupling or shaft will cause dramatic temperature increases. Thermographic inspections have shown that very minor misalignment conditions will increase temperatures by more than 19C (65F).
Alignment methods include: 1. Reverse dial method, where both shafts can be rotated freely. Readings are taken when the shafts are rotated 180 degrees between positions 6:00-12:00 and positions 9:00-3:00. 2. Face-rim method, which requires the use of true couplings and readings are taken as in the reverse dial method. 3. Electro-mechanical method, which will calculate for bar and bracket sag. 4. Laser alignment method, which is the most precise, but airborne dust can affect accuracy. (Two popular systems are Combi-Laser and Optalign.)
1. Pre-alignment preparation includes the gathering of all of the necessary tools and doing a review of the machinery history files, including operator complaints, repair work orders and inspection reports. Review the files for operating temperature data and previous alignment data. Review vibration frequency data for coupling type, material composition, thermal growth and any alignment details.
A review of the history files and previous data for the machinery is critical for several reasons.
Thermal growth will dramatically affect the alignment, so it is very important to know what are the existing operating temperatures of machinery components, such as the coupling and bearings.
Thermal growth is affected by the type of material used for the machinery housings. Every type of material has a different coefficient of thermal expansion. For example, the coefficient of thermal expansion for aluminum is twice that of cast iron, so an aluminum motor could grow twice as much as a cast iron pump with the same temperature increase. Thermal expansion is approximately 0.01 mm (0.000394 in.) per metre (40 in.) for each degree Celsius increase in temperature.
There are many different types of couplings. Because they are constructed and/or mounted differently, they all have differing vibration frequencies and temperatures. These coupling types, their vibration frequencies and their operating temperatures should be known before the alignment is carried out.
2. Pre-alignment inspection includes the following steps:
a) Shutting down and locking out machinery and notifying the necessary plant personnel to ensure safety procedures are followed.
b) Visually inspecting of all component parts, such as the coupling, for loose bolts, rubber powder, leaking lubricant, etc.
c) Inspecting the shaft keyways for loose or worn keys, or worn keyways.
d) Inspecting the base plate for cracks, warpage, broken or loose foot bolts, or loose shims.
e) Cleaning the top and bottom of the base plate, and removing excess paint, burrs and any foreign material.
f) Inspecting the foundation and grouting for cracks and damage.
g) Inspecting all shims. Clean and remove burrs and discard rusty, misshapen or aluminum shims. Verify all shim thicknesses with an accurate micrometer; shims must be the same thickness throughout. Never use aluminum shims or copper or bronze shim stock; always use stainless steel shims. Record the number and thickness of shims at each mounting foot location (remember that foot stiffness decreases as the number of shims used increases). Always place thinner shims between thick shims.
h) Measuring the shaft and coupling runout and eccentricity with an accurate dial indicator.
3. Soft foot is a condition where there is a void between the foot of the machine, the shim pack and the machine base. This condition can cause distortion, reduced internal machine clearances, binding rotors and/or preloading of bearings or seals. Soft foot conditions must be corrected or eliminated before satisfactory alignment can be achieved.
4. Pipe strain conditions can cause distortions that will place unnecessary misalignment or stress on machine components or the machine itself. Pipe strain can cause resonant frequencies that can also excite the resonant frequencies of machines or machine components, which in turn can cause severe damage — sometimes catastrophic.
Pipe strain distortions can be determined using dial indicators and then tightening and loosening pipe flanges, while noting the changes in dial indicator readings. Pipe strain must be removed if the readings exceed 2 mils. Expansion joints may be used to eliminate very minor pipe thermal growth and very small pipe strain conditions.
5. Thermal growth is a condition whereby the machinery expands as it reaches operating temperature. Thermal growth must be compensated for when carrying out machinery alignment. It is therefore absolutely necessary to know and record the temperature measurements at each bearing location and at the housing near each foot. This thermal growth must be included (added) when shimming the feet.
Because alignment procedures differ, depending upon the tools and techniques used, there are no specific alignment procedures to follow. However certain recommended procedures are necessary if a proper alignment is to be carried out.
One such requirement is proper documentation of the task. This will vary, depending upon the alignment techniques used, but a simple documentation procedure s
1. Name of technician
2. Inspection results, such as soft foot and pipe strain corrections, if any
3. Thermal growth (at both machines)
4. Note machine orientation (left and right sides)
5. Machine dimensions (written on the alignment diagram)
6. Diagram of machines, feet configuration and size and thickness of shim packs
7. Distance from the centre of the hub to the dial locations
8. Distance from the dial location to the centre of the in-board feet and the distance between the feet
9. Note and compensate for bar and/or bracket sag when using face-rim or reverse dial methods.
All information should be noted on an appropriately designed report that is then filed with the machinery maintenance history.
Procedure notes for various alignment methods
Reverse dial method
a) Measure the misalignment by rotating the dial indicators 360 degrees. Dial gauge plungers should be pushed in at least 50%.
b) Make the calculations using the Data Validity Rule, which is: The readings on the two sides (3 and 9 o’clock) should, when added, total the bottom (6 o’clock) reading (see Fig. 1). Fig. 2 suggests that the measurement data has been affected by pipe strain, soft foot, loose couplings, or the dial indicator plungers have bottomed out.
c) Subtract bar or bracket sag from the bottom (6 o’clock) reading.
d) Calculate the machine using the Formula Method.
1/2 [(M1 + M2) LEG – M1] / Spacing
M1 = reading at the stationary machine
M2 = reading at the movable machine
Leg = the distance from the location of the stationary machine dial reading to the leg to be moved
Spacing = the centre-to-centre distance of the dial indicator bracket.
Note that the formula must be used four times — once for each leg (or foot). When any number is positive, the leg must be moved up.
e) Adjust the movable machine. Use a dial indicator at each foot to indicate how far each foot has moved when adjusting horizontally. Use a torque wrench to tighten the bolts and retake readings for confirmation.
a) Install the digital indicators (heads), which transfer the data to the alignment analyzer, from four 90-degree positions. The farther apart the heads are spaced, the more accurate the data.
b) The brackets which hold the heads should be mounted on the same plane. The electro-mechanical method is somewhat more accurate than the reverse dial method because it calculates for bar and bracket sag.
c) The distances between the feet and from the feet to the face of the heads are required for accuracy. The most critical distance to be noted is the distance between the faces of the heads. (It may be necessary to use plumb bobs to get accurate distance measurements from the foot to the face of the heads.)
d) Ensure that the foot bolts are properly torqued before making readings. (Take three readings with at least a 180-degree rotation and then use the Data Validity Rule).
e) Place heads on the ‘On’ position and zero them at the top position. Rotate the shaft, stopping at the left, right, top and bottom positions. View the data and make the recommended vertical adjustment first. The calculator provides various ‘moves’. Select the one that will be easiest to perform, then adjust the horizontal moves, torque all bolts and recheck all readings for confirmation.
Laser alignment method
This method is similar to the electro-mechanical method, but some preliminary work must be done before an accurate alignment can be achieved.
a) The machines must be ‘rough aligned’ so that the laser beams strike at, or near, their targets during the entire shaft rotation.
b) Define the alignment job using the analyzer. Enter the data necessary, such as machine type and component, direction of rotation, shaft turning speed, thermal growth data, laser orientations, machine dimensions, etc.
c) Identify soft foot problems using the analyzer. Correct as necessary.
d) Using the analyzer data solutions, select the easiest to perform and make the necessary vertical, then horizontal machine moves.
e) Torque all bolts, recheck data and recheck vibration data after the machinery has reached operating temperature.
Refer to Table 1 for the recommended acceptable tolerances for misalignment. Do not accept flexible coupling manufacturer’s specifications, or manufacturers’ recommendations for ‘acceptable’ levels of coupling misalignment, because flexible couplings will not compensate for most misalignment conditions.
This checklist contains items that should accompany a precision alignment. Numerous factors influence the specific checks required for a given alignment task. The objective is to perform a thorough examination of the condition of the machine as a part of a precision alignment. List Conditions and Comments after each item.
* Determine the difference between the machine’s operating and non-operating shaft positions. If the operating temperatures are required for thermal growth calculations, these need to be gathered while the machine is still operating. Methods available to accomplish this include taking temperature readings in the planes of the feet for thermal growth calculations, or using various measuring devices.
* If instrumentation is available and plant safety procedures permit, perform a running soft foot check and correction, prior to shutting down the machine.
* Before performing work, follow proper lockout, tagging out and isolation procedures. This may include closing suction, discharge and isolation valves, dampers, etc.
* Determine the alignment method to be used, gather all the necessary tools and equipment, and check these for proper operating condition.
* Prior to taking steps that may change ‘as found’ alignment conditions, assemble fixtures and take a complete set of readings.
* Inspect the machine base, foundation and feet for cracks, warped surfaces, corrosion, foreign materials or burrs, and repair as necessary.
* Inspect existing shim packs. Remove and replace any shims that are cracked, bent, rusted, hand-cut or made of improper materials, such as aluminum. Pre-cut stainless steel shims are recommended.
* Whenever possible, start with a large shim (e.g., 125 mils) under each foot to allow for vertical adjustment.
* Loosen vertical and horizontal jack bolts prior to moving the machine.
* Remove and replace any ‘cupped’ washers, bent bolts, etc.
* Remove dowel or taper pins from machine(s) to be moved and any that have been improperly installed, or that are unnecessary.
* Ensure all hold-down bolts are properly lubricated and torqued to the proper value and in the proper sequence.
* Measure and eliminate excessive pipe and electrical connection strain.
* Prior to rotating shafts, ensure bearings are properly lubricated.
* Rotate shafts slowly. If binding, rubbing or roughness are detected, determine the source and correct.
* Inspect the shafts of both machines for:
1. Excessive axial and radial movement.
2. Excessive runout (eccentricity or bend).
3. Smooth fixture mounting surfaces (no pipe wrench footprints).
* Inspect the coupling for the following and correct as necessary:
1. Looseness (grids, teeth, disks, elastomers, etc.)
2. Proper fit on shaft.
4. Worn teeth or grid members.
5. Correct type and amount of lubricant.
6. Correct set screw length and tightness.
7. Proper bolts and washers (note length, machining, weight, etc.)
8. Proper k
9. Match marks in correct position.
* Properly set the axial gap between coupling faces. For motors with sleeve bearings, ensure that the motor shaft is set at its magnetic centre before performing this step.
* Prior to performing precision soft foot checks or precision alignment, ensure adequate ‘rough’ alignment has been achieved.
* Inspect for and remove soft foot on all feet of both machines.
Lloyd (Tex) Leugner is the author of The Practical Handbook of Machinery Lubrication (3rd Ed.), and the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of lubrication and maintenance problems and provides training for industry. Leugner has written previously for Machinery and Equipment MRO and will be providing technical articles regularly in future issues. He can be reached at 403-932-7620 or firstname.lastname@example.org.