Correcting machine misalignment and soft foot by laser
Is your rotating shaft alignment program making it difficult to achieve excellent tolerances? Are alignments taking longer than they should? Does your laser alignment system sometimes give unpredictable results? If you answered yes to any of these questions, the following information will be useful.
Shaft alignment awareness has dramatically increased in the last 15 yr. This interest is largely due to intensive education, training, and advertising by institutions and laser shaft alignment manufacturers. The payback on an investment in precision shaft alignment is well known and documented.
Laser alignment systems range from low-cost, low-performance, and bare-bones models to sophisticated machinery drive-train packages with graphics and prompts. Although most manufacturers offer a single system or variations on a single idea, some offer more -- up to three distinct systems -- with totally different operating interfaces and different measuring principles to achieve precision alignment.
Not all laser shaft alignment approaches are precision systems. Some systems have measurement performance similar to what is obtainable from inexpensive mechanical dial gauges. Others offer good raw sensor performance and a lot of features, but only the barest minimum implementation of each feature.
One system claims to compensate for thermal growth, but only allows input for one machine. Another system has a machine drive-train alignment feature (motor-gearbox-pump and two couplings), but the user cannot enter which feet are static or bolt bound, or enter thermal growth at the couplings or feet. A third system can only display short flexible couplings.
What laser systems measure
All shaft alignment systems must measure two factors: offset and angle. Offset is the distance between two rotating centerlines at a given point along the shaft centerlines, usually the coupling center. Offset is usually expressed in thousandths of an inch (commonly called thou or mil). Most laser systems measure offset correctly and with sufficient resolution. Measuring angle is a different story.
Angle is the change in offset/unit of length between shafts (Fig. 1). It is how much the offset between shafts changes when moving along the shaft 1 in. There is little standardization on units for measuring shaft alignment angles, commonly expressed in terms of mils/in. (same as a milliradian or mRad), in./in., mils/10 in., or the gap for a given coupling diameter.
Measuring the angle accurately is important because that figure is used to compute corrections at the feet (Fig. 2). The correction (shim or movement) at any foot is the same as the offset at that point along the shafts but of opposite sign.
Precision alignment defined
The principles of metrology (the science of measurement) can be applied to shaft alignment. For sound results, two rules apply: the 4:1 rule for measurement and the 10:1 rule for corrections.
According to the 4:1 rule, the alignment system must be able to measure four times better than the desired tolerance to give reasonable certainty of acceptable results. When a machine operates at 3600 rpm, a typical final alignment tolerance should be within 1-mil offset. An alignment system must measure within 0.25 mil of offset to indicate the correct offset alignment.
To correct alignment, use the 10:1 rule. The system should be able to measure 10 times better than the tolerance in order to produce accurate corrections. An alignment system must be able to measure to a resolution of 0.1 mil (0.0001 in.) to reliably produce alignment results typically desired for operation at 3600 rpm. For higher speeds, the minimum requirement is 0.05 mil (0.00005 in.) offset.
Typical alignment tolerances for angles require final angle values within 0.2 mil/in. of desired alignment. Applying the 4:1 and 10:1 rules to angular resolution requires the system to measure minimum angular resolutions of 0.05 and 0.02 mil/in. respectively for 3600-rpm operation. At higher speeds the requirements are 0.025 and 0.01 mil/in.
The definition of a precision alignment system is simple: The system must be able to align machines within alignment tolerances. Accordingly, it must measure repeatability within at least 0.1-mil offset and 0.02-mil/in. angle. Any system that cannot perform at this minimum threshold is not a precision alignment system.
Precision alignment systems are used to measure machine frame distortion, commonly and misleadingly called soft foot. Even though soft foot is a part of every aligner's vocabulary, there is often a lack of understanding about how a high-precision measurement system can help.
There are several symptoms of machine frame distortion. Even with precise alignment measurement systems, some rotating machines don't seem to respond to alignment corrections as they should. Other machines seem to be aligned excellently, but are difficult to turn by hand as though in a bind. Still others show running vibration signatures typical of misalignment even though precision aligned within tolerances to accurate thermal growth targets. Not surprisingly, some machines and machine trains exhibit some or all of these symptoms. Each one is usually a symptom of machine frame distortion.
The root problem is the internal misalignment caused when a machine frame is distorted. A distorted machine frame has the bearings misaligned with each other, internally misaligned within the ma- chine housing. The distortion usually comes from tightening the hold-down bolts, and the machine feet, onto pads that are not rigid, co-planar supports. It can also come from external stresses on the machine, such as pipe and coupling strains.
If a machine support is not rigid, the foot is essentially being mounted to a spring, which allows the foot to move when the hold-down bolt is tightened. Nonrigidity is usually caused by anything under the foot that is not solid when pressure is applied. The usual solution to this problem is to remove the deforming material from under the foot and re-shim the machine.
If a machine support is not in full co-planar contact with the foot, then the foot is essentially being forced to close an air gap when the hold-down bolt is tightened. The causes of the condition are:
- Bent foot
- Angled pads
- Missing shims
- One foot pad too high.
The solution is to correctly fill the gap with shims.
If the bearings of a machine are misaligned by tightening hold-down bolts, the position of the shaft changes because the bearings moved. Since bolt-clamping forces are mainly, if not entirely vertical, the motion of the improperly supported foot is the same. If one foot of a machine moves vertically, while the others remain bolted securely, the bearing at that end of the machine also moves vertically.
Machine frame distortion, or soft foot, always results in a change in vertical angularity. Detecting, measuring, and providing correction prompts for this change is one of the most valuable outputs of a laser shaft alignment system, provided the system has sufficient accuracy and software.
Angular precision required
To determine the angular precision necessary in the plant, analyze typical machines being maintained. Taking a medium industrial machine to be 40-in. between bearing centers with a 0.001-in. radial bearing clearance, and applying the 4:1 rule, would require an angular resolution of 0.006 mil/in. [(1 mil clearance/40 in.)/4 = 0.000006 in./in.]. Some laser systems have a minimum indication of machine frame distortion of 0.5 mil/in. or higher. This figure would require a bearing to move 20 mils before the system would even indicate it.
A smaller machine with 10-in. between the bearings and a 0.001-in. radial bearing clearance, would require an angular resolution of 0.025 mil/in. [(1 mil clearance/10 in.)/4 = 0.000025 in./in.]. Tighter ball bearing clearances require higher angular sensitivity.
From this analysis, a system must measure angles with better than 0.005-0.010 mil/in. resolution to be useful for soft foot analysis.
A better way
As important as high accuracy measurement of machine frame distortion is, it is not enough. With a system that only displays machine frame distortion, the mechanic must know how to analyze the patterns of bending and gaps under the feet. Traditionally this knowledge is gained from a long and slow learning process with much trial and error.
The latest generation of laser shaft alignment systems goes beyond analyzing alignment and merely measuring machine frame distortion. They incorporate an analysis tool, which is basically a sophisticated computer program built into the laser system that remembers each foot as it is measured for distortion and recognizes patterns.
It asks for feeler gauge readings under specific feet, if needed, to further distinguish possibilities. Then, the system recommends specific corrections to eliminate machine frame distortion. Such systems are a major step forward in machinery maintenance since they take the guesswork out of correcting for machine frame distortion.
-- Edited by Joseph Foszcz, Senior Editor, 630-320-7135, firstname.lastname@example.org
Laser shaft alignment systems vary from bare bones indicators to computerized analyzers.
Precision alignment must measure offset and angle 4-10 times better than the allowable tolerance.
Laser alignment systems should detect, measure, and prompt correction of soft foot.
Causes of nonrigid machine support
- Bent shims
- Plastic shims
- Burrs on shims or feet
- Cracked base plate
- Cracked grout
- Weak foundation
Advantages of alignment
- Seals and bearings last longer.
- Shafts and couplings don't fail as often.
- Vibration levels are low.
- Machines run cooler, using less energy in the process.
The author is available to answer questions on laser shaft alignment systems and applications. He can be reached at 305-591-8935.
See the "Fluid and mechanical power transmission" channel on www. plantengineering.com for more information related to this topic.
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