Laser Shaft Alignment -Reliability Engineering

There are a number of different methods whereby acceptable rotating machine alignment can be achieved. These range from an inexpensive straight edge to the more sophisticated and inevitably more expensive laser systems.  We can condense these methods into three basic categories

• Eyesight – straightedge and feeler gauges

• Dial indicators – mechanical displacement gauges

• Laser optic alignment systems.

Within each category there are a number of variations and options. It is not the intention here to evaluate all of these options.

We will concentrate on the most widely used methods in each category

Alignment methods :

1. Eyesight

The straightedge This method of shaft alignment was common practice in many plants, provided a flexible coupling was used.

It was considered good enough to eyeball the alignment and bolt the machine down. The system is certainly cheap and equipment is readily available.

The corrective values for the machine feet were usually estimated according to the experience of the engineer carrying out the alignment. Most often corrections at machine feet need to be repeated on a trial and error basis before the “eyeball” alignment condition was completed. Even then there is no certainty that the completed alignment was correct.

Since the resolution of the human eye is limited to 0.1mm, alignment accuracy is correspondingly limited. Additionally without having carried out extensive checks on the fitting accuracy of the coupling on the shaft, no direct correlation between the completed alignment and the actual alignment of the machine shafts can be made.

At best this alignment method can be described as coupling alignment not shaft alignment as defined earlier.

2. The feeler gauge

Although classified here as an “eyesight” method of shaft alignment the feeler gauge method under certain circumstances and for some machines can be perfectly acceptable. In the installation and alignment of turbine sets where the coupling half is an integral part of the rotor shaft and has no flexible elements, it is possible for a skilled turbine engineer to align the two coupling halves very accurately. (As noted in the section on alignment tolerances, no allowance for offset or gap is permissible on these “solid” type of couplings).

Using the feeler gauge or a vernier caliper the engineer accurately measures any gap between the coupling halves. Jacking oil is then used to rotate the shafts together through 180 degrees and the “gap” is then checked again (with the jacking oil off). This procedure is then carried out for the horizontal alignment measurements.

Readings are usually graphically plotted to establish alignment condition and any necessary corrections that are required. In some cases engineers will rotate one shaft through 180 degrees and take additional readings.

These readings are then averaged to eliminate any possible shaft machining errors. The averaged readings form the basis for the alignment graph. On machines that employ flexible elements in the coupling design, the use of feeler gauges is beset with the same limitations as the straightedge method and can only be described as coupling alignment.

3. Dial indicators

The use of dial indicators for the vast majority of shaft alignment tasks where a flexible coupling element is used represents a substantial step forward in accurate shaft alignment methods.

There are a number of dial setups that can be used to effect the alignment of machines, this section will review some of these. There are a number of factors that the engineer should take into account before embarking on a dial indicator alignment task.

4. Laser shaft alignment

Access to precision shaft alignment and the benefits that this brings (see following section) are readily available when laser shaft alignment is used on plant.

A summary of the advantages offered by laser systems are shown here:

• Precision alignment with no manual input of data and no graphical or numeric calculations to perform

• Graphic display of alignment results at the power transmission planes of the coupling, and shim and adjustment corrections at the machine feet

• No mechanical fixtures - no bracket sag

• No need to disassemble the coupling to effect an alignment

• No need to take readings at predetermined locations such as 12;00, 3;00, 6;00 and 9;00 o’clock positions; results can be obtained with less than 90 degrees of shaft rotation

• Data storage and print out of results for report generation of alignment condition

• Certified calibrated accuracy of the laser system to comply with ISO 9000 requirements

• Universal bracket systems which cover all types of alignment applications; no need for special “Christmas tree” brackets for long spacer shaft measurement

• Menu driven operator system allows use by a wide range of engineering skills and disciplines

• Live dynamic display of vertical and horizontal corrections during alignment corrections

• Inbuilt go/no go alignment standards for analysis of alignment accuracy

Having identified some of the benefits and advantages that can be obtained by using a laser alignment system to carry out shaft alignment, it is important to establish the functionality of the alignment system that will suit the users requirements.

There are a number of systems available and a number of manufacturers who offer laser alignment systems.

Causes of machine breakdown

Experience and coupling manufacturers maximum misalignment recommendations would suggest otherwise. Anecdotal evidence suggests that as much as 50% of machine breakdowns can be directly attributed to incorrect shaft alignment.

It is true that flexible couplings are designed to take misalignment, typically up to 10 mm or more radial offset of the shafts. But the load imposed on shafts, and thus the bearings and seals increase dramatically due to the reaction forces created within the coupling when misaligned.

1. Anti-friction bearings

Bearings are precision manufactured components designed to operate with clean lubrication and constant but restricted operating temperatures.

Components manufactured within 0.005 mm accuracy are:

• Not able to withstand operating for long periods at elevated temperatures caused by misalignment.

• Not able to withstand contamination caused by mechanical seal failure which has allowed ingress of dirt, grit, metallic elements or other objects.

• Not manufactured to operate for long periods with misalignment imposing axial shock loads on the carefully machined and honed components.

In addition to the damage imposed on the bearings through the misalignment itself, when mechanical seals fail, bearings have to be removed from the shaft assembly, sometimes re-fitted or in most cases replaced. Removal and re-fitting in itself can cause bearing damage. Most pump manufacturers and repairers recommend that when repairing damaged pumps, bearings should always be replaced irrespective of apparent condition, since it is easy to miss minor damage to the bearing that will progressively worsen after re-fitting.

2. Mechanical seals

Seal wear increases due to shaft loading when shafts are misaligned. Pump seals are a high cost item often costing up to a third of the total pump cost. Poor installation and excessive shaft misalignment will substantially reduce seal life. Manufacturers have addressed the problem of poor installation practice by the introduction of cartridge type seals which can be installed with little or no site assembly. Seals however have precision ground and honed components with finished accuracy of 2 microns (0.002 mm). They do not tolerate operation in a poorly aligned condition, face rubbing, elevated temperatures, and ingress of contaminants quickly damage expensive components. Seal failure is often catastrophic, giving little or no pre-warning. The resultant plant downtime, seal replacement costs, pump repair costs and bearing replacements makes seal failure due to misalignment an expensive and unnecessary problem.

3. Machine vibration

Machine vibration increases with misalignment. High vibration leads to fatigue of machine components and consequently to premature machine failure.

The benefits that accrue from adopting good shaft alignment practice begin with improved machine operating life thus ensuring plant availability when production requires it.

Accurately aligned machinery will achieve the following results.

• Improve plant operating life and reliability

• Reduce costs of consumed spare parts such as seals and bearings

• Reduce maintenance labor costs

• Improve production plant availability

• Reduce production loss caused by plant failure

• Reduce the need for standby plant

• Improve plant operating safety

• Reduce costs of power consumption on the plant

• “Push” plant operation limits in times of production need

• Obtain better plant insurance rates through better operating practice and results