Laser Alignment is a Real Investment

	Laser Alignment is a Real Investment

2/6/2008	By: Jesus R. Sifonte, P.E.
	There are several ways to reduce costs for your company, one of the most important ones being the aplication of proper corrective maintenance practices. Find out how to turn in a profit by employing the precision of laser-based instruments in shaft alignment.

By: Jesus r. Sifonte, P.E.

“F

ive mils” -That was the answer I recieved some years ago, the first time I saw 2 mechanics performing alignment with dial indicators to a Cooling Tower pump and asked about alignment tolerances. Were they right? Furthermore, why is alignment necessary? What is the criterion for shaft alignment tolerance? Rotating machinery alignment has become a critical aspect today, since studies demonstrate that about 50% of all mechanical failures are related to misalignment. The life of mechanical seals, oil seals, bearings, couplings, shafts, belts, sheaves and, gears are compromised when machinery is operated while misaligned.

What is Shaft Misalignment?

Misalignment occurs when shafts tied by a common coupling element do not turn around the same centerline. This condition also occurs when pulleys or sprockets driving the same set of belts or chains are not placed in the same vertical and horizontal planes (Pulley misalignment will be the subject of an upcoming technical article). Misalignment is often overlooked, but it is the culprit of many production delays. Technically, misalignment has two components, and they are referred to as parallel and angular misalignment respectively. Each one is measured differently as the parallel or offset misalignment is measured in mils (or millimeters) and the angular misalignment is measured in mils/in (millimeters/centimeter). Normally, Precision Alignment requires correction of both components according to recommended tolerances.

-Measured in Mils or Millimeters -Measured in mils/in oR mm/cm

Figure 3:Commonly Found in practice

Figure 2: The Centerline of the two shafts is not parallel

Figure 1: The Centerline of Two shafts are not concetrical, but parallel

What are the symptoms of misalignment?

It requires expertise to diagnose misalignment while the machine is running. Spectral Vibration Analysis can identify a misalignment condition, but it is required to stop the machine to measure it and account for the amount of angular and parallel misalignment present in both vertical and horizontal planes. A lot of extra force is applied to the bearings when shafts are misaligned causing the following symptoms:

1. Premature bearing, oil seal and, mechanical seal failure.

2. High radial and axial vibrations.

3. Loose foundation bolts.

4. Broken coupling bolts.

5. Coupling wear and / or consecutive coupling failures.

6. Broken or leaky bearing oil seals.

7. High bearing casing temperatures.

8. Similar machinery are less noisy and shaky and have longer operating life.

9. Shaft failure.

Benefits of Precision Alignment

The purpose of shaft alignment is to increase the operating lifespan of rotating machinery. Assuming the the components have been designed and installed properly, precision alignment will ensure they remain in service for years. Modern laser alignment technologies provide engineers and industrial mechanics with very precise tools for achieving fast and accurate alignment for a wide range of applications including: coupling, pulley, vertical shafts, cardan, etc. More advanced tools provide geometrical measurement capabilities and are suitable for specialized applications like: straightness, flatness, squareness, and diaphragm center for turbine internal alignment. These extra features enable users to further evaluate and correct mechanical defects.

There are many benefits associated with Laser Alignment. Some of them are as follows:

1) Increased Mechanical Components Life

2) Increased packing and oil seals life

3) Decreased Electrical Consumption

4) Decreased Machinery Vibration

5) Increased Machinery Uptime

6) Avoid Premature Failure of Critical Components

7) Significant Reduced Machinery Maintenance Costs

8) Improved Worker Morale

What about Precision Alignment Tolerances?

Generally speaking, alignment tolerances are driven by the speed of the machine. Thus, the higher the machine’s RPM, the tighter the tolerance. Both angular and parallel maximum deviations are recommended for the given RPM ranges. The desired coupling alignment condition is attained when both components are within tolerances.

Offset	Excellent		Acceptable
rpm	mils	mm	mils	mm
0000-1000	3.0	0.07	5.0	0.13
1000-2000	2.0	0.05	4.0	0.1
2000-3000	1.5	0.03	3.0	0.07
3000-4000	1.0	0.02	2.0	0.05
4000-5000	0.5	0.01	1.5	0.03
5000-6000	<0,5	<0,01	<1,5	<0,03

Angular Error
rpm	mils/”	mm/100	mils/”	mm/100
0000-1000	0.6	0.06	1	0.1
1000-2000	0.5	0.05	0.8	0.08
2000-3000	0.4	0.04	0.7	0.07
3000-4000	0.3	0.03	0.6	0.06
4000-5000	0.2	0.02	0.5	0.05
5000-6000	0.1	0.01	0.4	0.04

Table 1: General Shaft Alignment Tolerances Guide Table

Other considerations may be taken into account besides the machine speed such as the manufacturer’s specifications and/or rises in temperature. Modern alignment systems carry built in tolerance criteria like the one shown in the above table, plus the ability to accept other criteria like manufacturer or operator recommended values. Rises in temperature are also taken into consideration by modern alignment tools.

Interpreting the Alignment Results

Vertical and Horizontal Alignment Condition Results

Once alignment readings are taken and displayed like the following image, the values are compared to the appropriate set of tolerances for angular and parallel deviation. Assuming 1800 RPM for the machine speed, we obtain 2-4 mils and 0.5-0.8 mils/in as the tolerance for such machines. The results are interpreted as follows: