Misalignment is the second most common cause of machine vibration and premature bearing failure, after imbalance. Two shafts connected by a coupling can be misaligned in two ways: offset (parallel) misalignment, where the shaft centerlines are parallel but displaced, and angular misalignment, where the centerlines intersect at an angle. Most real-world conditions involve a combination of both.
Flexible couplings are designed to accommodate some misalignment, but "accommodate" does not mean "tolerate indefinitely." A jaw coupling rated for 1 degree of angular misalignment can physically operate at that angle, but the cyclic loading on the coupling elements, bearings, and seals at that condition dramatically shortens their life. Good alignment practice targets 10% to 25% of the coupling's rated capacity, not the maximum. This guide covers alignment measurement methods, tolerance standards, coupling type differences, and the thermal growth compensation that makes precision alignment worth the effort.
Offset and Angular Misalignment Defined
Offset misalignment (also called parallel or radial misalignment) exists when the two shaft centerlines are parallel but not collinear. The offset is measured as the distance between the centerlines at the coupling, usually in mils (thousandths of an inch) or millimeters. A machine with 10 mils of offset has shaft centerlines that are 0.010 inches apart at the coupling faces.
Angular misalignment exists when the two shaft centerlines meet at an angle. It is measured in mils per inch of coupling diameter (mils/inch) or in degrees. A coupling with 1.0 mil/inch of angularity on a 10-inch coupling diameter has shaft centerlines that diverge by 10 mils across the coupling face. In degrees, 1.0 mil/inch equals about 0.057 degrees, or roughly 3.4 minutes of arc.
Both types of misalignment must be corrected in two planes: the vertical plane (shimming the machine feet) and the horizontal plane (moving the machine sideways). This means a complete alignment job involves four corrections: vertical offset, vertical angularity, horizontal offset, and horizontal angularity. Modern laser alignment systems measure all four simultaneously and calculate the exact shim and foot moves needed.
The effects of offset and angular misalignment are different. Offset misalignment generates radial forces on the bearings that cycle twice per revolution. It shows up as a 2× RPM vibration component, predominantly in the radial direction. Angular misalignment generates axial forces that push and pull the shafts along their length, showing up as 1× and 2× RPM vibration in the axial direction. High axial vibration is the signature of angular misalignment.
Alignment Tolerances by Coupling Type
Different coupling designs have very different tolerance for misalignment. The alignment target should be based on the coupling type and the operating speed, not just a generic "good alignment" number. Higher speeds demand tighter tolerances because the dynamic forces from misalignment increase with the square of speed.
Typical alignment tolerances (at operating temperature):
- Rigid couplings (sleeve, clamp, flange): Offset < 1 mil, angularity < 0.5 mil/inch. These couplings have zero tolerance for misalignment by design. They transmit all misalignment forces directly to the bearings.
- Gear couplings: Offset < 3 mils, angularity < 1.0 mil/inch at speeds below 3,600 RPM. Gear couplings can accommodate moderate misalignment through tooth sliding but generate significant heat and wear when misaligned.
- Disc/diaphragm couplings: Offset < 5 mils, angularity < 0.5 mil/inch. These handle offset well through flexure but are sensitive to angular misalignment.
- Jaw/spider couplings: Offset < 5 mils, angularity < 1.0 mil/inch. The elastomeric element absorbs misalignment but degrades faster when continuously loaded.
- Tire/rubber couplings: Offset < 10 mils, angularity < 1.5 mil/inch. Most forgiving but the rubber element has a finite life that shortens with misalignment.
These tolerances represent good practice for machines running at 1,800 to 3,600 RPM. For machines above 3,600 RPM, tighten all tolerances by 50%. For machines below 1,200 RPM, you can relax tolerances by about 50%, but tighter is always better. The cost of good alignment is the same regardless of the tolerance target, so there is rarely a reason to aim for the relaxed standard.
Alignment Methods: Straightedge to Laser
The three practical alignment methods, in order of increasing accuracy:
- Straightedge and feeler gauge: Place a straightedge across the coupling halves to check offset. Use a feeler gauge or taper gauge at the coupling faces to check angularity. Accuracy is about 2 to 5 mils, limited by the straightedge flatness and the operator's eye. Adequate for slow-speed machines (below 900 RPM) with flexible couplings. Not suitable for precision alignment.
- Dial indicator (rim and face): Mount dial indicators on one coupling half and rotate to read runout on the rim (offset) and face (angularity) of the other half. Accuracy is about 0.5 to 1.0 mil with good technique. This has been the standard method for decades. The limitation is bracket sag: the indicator mounting bracket deflects under its own weight, introducing a systematic error. Sag must be measured and corrected, which adds time and a source of error.
- Laser alignment: Two laser/detector units mounted on the coupling halves. The system measures offset and angularity simultaneously in both planes, compensates for bracket sag automatically, and calculates exact shim and foot move values. Accuracy is 0.1 to 0.5 mil. Laser alignment cuts alignment time by 50% or more compared to dial indicators and eliminates the most common sources of human error.
The reverse indicator method (two dial indicators, one on each coupling half, reading the opposite rim) is more accurate than rim-and-face because it eliminates axial float errors. It is the preferred dial indicator method for high-speed or critical machines. Laser systems essentially automate the reverse indicator geometry with higher resolution sensors.
Regardless of method, soft foot must be corrected before alignment begins. Soft foot is a condition where one or more machine feet do not sit flat on the baseplate. It acts like a spring: tightening the hold-down bolt distorts the machine frame and changes the bearing alignment. Check for soft foot by loosening each foot bolt one at a time and measuring the gap with a feeler gauge. Any foot that lifts more than 2 mils has soft foot and needs a shim correction.
Thermal Growth: Why Cold Alignment Must Be Wrong
Machines that operate at temperatures significantly different from ambient experience thermal growth: the housings, shafts, and support structures expand as they heat up. A motor bearing housing at 160°F on a 70°F day has grown vertically by an amount determined by the coefficient of thermal expansion (CTE), the temperature rise, and the distance from the anchor point to the shaft centerline.
For steel: ΔL = L × CTE × ΔT, where CTE = 6.5 × 10⁻&sup6; in/in/°F. A motor with 12 inches from the base to the shaft centerline, experiencing a 90°F temperature rise, grows vertically by 12 × 6.5 × 10⁻&sup6; × 90 = 0.007 inches (7 mils). If the driven equipment (a pump handling cool fluid) only grows 2 mils, the net thermal offset is 5 mils. If you align the shafts perfectly when cold, they will be 5 mils out of offset when hot.
The solution is to intentionally offset the cold alignment by the predicted thermal growth. In the example above, set the motor shaft 5 mils low when cold, so it grows up to the correct position when hot. This is called "thermal growth compensation" or "hot alignment targeting." Most laser alignment systems have a thermal growth calculator built in. You enter the shaft heights, expected operating temperatures, and material, and the system adjusts the alignment targets automatically.
For machines where the thermal growth is uncertain, the gold standard is a hot alignment check. Run the machine until it reaches steady-state operating temperature, then take alignment readings. The difference between the hot alignment readings and perfect alignment is the thermal growth vector. Use that vector as the cold alignment offset for the next time the machine is assembled. Most critical machines in refineries and power plants have thermal growth vectors documented from hot alignment checks and stored as alignment targets.
Documenting and Verifying Alignment
A precision alignment is only as good as its documentation. Every alignment job should produce a record that includes: the as-found condition (before any corrections), the as-left condition (final alignment), the coupling type and gap, the thermal growth offset applied (if any), and the soft foot corrections made. This record is the baseline for future comparison.
After alignment and coupling assembly, always run the machine and take a vibration reading before signing off. A good alignment on paper that produces elevated vibration means something is wrong: a soft foot was missed, a coupling element is defective, a shim stack is compressing, or the baseplate has a crack. The post-alignment vibration reading is the final quality check.
Laser alignment systems store the alignment data digitally, making trending possible. If you align the same pump every annual turnaround, you can see whether the baseplate is settling, the foundation is shifting, or the thermal growth targets need adjustment. This trending data is valuable for long-term reliability planning.
Common alignment verification mistakes:
- Not rechecking after tightening the last bolt: The act of tightening hold-down bolts can shift the machine. Always re-verify alignment after all bolts are torqued.
- Ignoring pipe strain: Connecting piping to a pump exerts forces that can shift the pump's position by several mils. If possible, take a final alignment check after piping is connected. If the alignment shifts more than 2 mils, the piping stress is excessive and needs to be corrected at the pipe supports, not by re-aligning the pump.
- Using too many shims: A shim stack should never exceed 4 to 5 shims. Each additional shim adds compliance (springiness) to the support. If you need more than 5 shims, replace the stack with fewer, thicker shims or machine the base to the correct height.