This document outlines a procedure for detecting misalignment between internal combustion engines and gas compressors using FFT (Fast Fourier Transform). The procedure models the engine and compressor shafts as points rotating around a central coupling circle. Vibration measurements at the shafts in the 1X and 2X rotational frequencies are analyzed to determine if they align, indicating proper alignment, or are misaligned. If misaligned, excessive wear on bearings can occur over the operating life. The procedure is applied using vibration data collected from an engine and compressor package in the field. Analysis of the 1X and 2X data plots a line showing the shaft alignment profile matches a later technical report finding actual misalignment between the engine and compressor.

1. Note: This procedure was developed by the author to help identify misalignment through a
practical procedure, however the author give thanks everyone that can use this and can
demonstrate their application.
PROCEDURE TO DETECT
MISALIGNMENT BETWEEN
INTERNAL COMBUSTION
ENGINES AND GAS
COMPRESSORS WITH FFT
JOSE LUIS RATTIA – Industrial Maintenance Engineer
Predictive Maintenance to Reciprocant Machinery
Email: JOSELRATTIA@GMAIL.COM
Short Description
[Practical procedure to detect misalignment conditions in packages to gas
compression Stations]

2. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
1
I will bring a procedure to diagnostic of misalignment between Engines and Gas
compressors this model has their basis in the hypothesis of a point that turns
around of a circle, how is showed below; in this figure a, there are two points 1 and
2, this model means that both shaft; engines and compressor should be collinear
within conditions of shaft alignment. This model consider the energy accumulated
in the points; 1, y 2, this energy can be sensed through a sensor that measure the
quantity of energy in both points.
Figure a. Elements to Model of diagnosis misalignment in coupled shafts
For that, we use the correct units to take amplitude of vibration due to forcing force
in the coupled shaft to engines and compressor, in this case is selected; Mils P-P,
to capture all energy in bearings shell depending what kind of machine be
evaluated. We need to draw a line that represent the position of both shaft under
conditions of alignment.
This model take; 1X and 2X order to plotting the line that say us, in what position
is running the shaft to engine and compressor, you can take until 6X to evaluate
wear in connrod and main bearings shell under correlation analysis, but at this time
we only treat 1X and 2X order to know about misalignment.
It is very important to take in account that all collected energy by sensor of
vibration permit us identify correct position of common shaft both of them; engine
and gas compressor, considering information in 1X and 2X orders.

3. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
2
After through an excel spreadsheet, you can graph data collection to see the profile
of misalignment. In the model arose you can consider the following condition to
have when the package is in functioning but with a forcing force inside of
crankshaft to rated speed, and this forcing force is presented with two face; angular
or parallel, and how a combination from both of them.
Figure b. Angular and Parallel Misalignment
If both points, 1 and 2, are considered to turns around of coupling circle, you could
have a factor called angular velocity that depend of; mass, turning radio, to obtain
a value of force in each unit to english or international system. This unit can be
related by the expression: F=m.r.w², when you increase radio, then the force
increase proportionally. An example of this you can see in the following figure c;
Figure c. Measurement of energy in main bearings with sensor using FFT.
In the figure c; this model present two differents conditions, the first with blue circle
fill line, and last one stroke line, the first have r1 how their turning radio, and the
Second, have r2 turning radio. The first measurement with r1 hope have less force
than second r2. By the way, you could have misalignment conditions generating

4. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
3
high rates of wearing in main and connrod bearings to engines and gas
compressor under cycle of life to r2.
Why this model suppose difference between; 1X and 2X?
This model use the differences of fundamental order 1X, in relation to 2X taking in
account two elements; inertial force and average changes in the speed due to
acceleration when reciprocating mass reach final stroke and both force are
opposite.
Figure d. Primary and Secondary force opposite at the ending stroke
The figure d; show us, how can be related both forces; primary and
secondary forces. In this case, primary force is opposite to secondary force, what
represent an arithmetic sum to both values considering their signs; positive and
negative.
What amplitude unit we can use to monitoring energy in main and connrod
bearings?
To measuring amplitude of vibrations in main and connrod bearings, you can use;
displacement: Mils P-P.

5. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
4
Given that you need plot a line that represents the coupled crankshaft to engine
and gas compressor, is required use same units in both case engines and gas
compressor measurements. This give us a reliable graph to see profile of
misalignment between engine and gas compressor. Now, is time to develop
procedure with a spreadsheet to look at complete profile. To do this procedure we
use Windrock 6320 PA to take data fft from main and connrod bearings shell in
engines and gas compressors. It is very important be clear in your mind what you
want to obtain, for that is necessary take data on reliable way to achieve a
successful data.
Figure e. Data of FFT main bearings on packages Engine Cat 3612.
Figure f. Data of FFT main bearings on Compressor Ariel JGC/4.

6. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
5
The datas before showed were collected in a package Caterpillar 3612 with Ariel
Gas compressor JGC/4.
Coming up next, proceed to create a data table to engine and gas compressor to
the seven (7) data collected from main bearings engines and four (4) data from
main bearings gas compressor. Here do not be show all data table to engine and
gas compressor for space reasons, however same procedure was followed to the
others data taken from engine and gas compressor.
Figure g. Data table to Engine Cat 3612
Figure h. Data table Ariel JGC/4
These data were taken directly from software windrock 6320 PA, later these were
proposed to organize how a differences between fundamental of run speed 1X,
and second order of run speed 2X, getting results up showed.

7. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
6
Then procedure to plot the graph that define profile to misalignment between
engines and gas compressor.
The graph below, show us; which is the profile followed to shafts coupled in normal
operation. The report delivered for technical support after inspection to check
alignment condition confirmed; misalignment between engine and gas compressor.
Figure i. Profile to alignment between engine and gas compressor.
Figure j. Technical report from Field service Caterpillar.

8. PROCEDURE TO DETECT MISALIGNMENT BETWEEN INTERNAL COMBUSTION ENGINES AND GAS
COMPRESSORS
By: JOSE LUIS RATTIA – PREDICTIVE MAINTENANCE ENGINEER
7
Conclusions
The procedure to detect misalignment in packages internal combustion engines
and gas compressors has been until now a practical procedure to identify
misalignment and take early corrective actions to guarantee reliability of the
package engine and gas compressor during normal operation.
Their application do not need stop the machine, besides it permit take control of
wearing in engines and gas compressor bearings.
The plot line can visualize the profile that could have the engines and gas
compressor crankshaft to full load conditions.
This procedure is been tested in differents packages units currently in Venezuela,
however their application have had successful until now in differents test
performed.