Reducing Shipping Vibration of Compressors in Roof Top Air-conditioning Units

Reducing Compressor Vibration during
Shipping of Rooftop
Air-Conditi ning Units

Abstract
Table of Contents
Market Trends/Challenges
Experiments Conducted
The Solution
Tolerance Stack-up Study for the manufacturability of the part
Validation
Key Benefits to the Customer
Best Practices
Common Issues
Conclusion
Reference
Author Information
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© 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.

Abstract
Reducing Compressor Vibration during Shipping of Roof Top Air-Conditioning Units | 3
A rooftop unit is a packaged air-conditioning unit which is used for both residential and commercial
purposes. This air-conditioning or heating system uses refrigerant as the heat exchange fluid and has
an evaporator, compressor and condenser as separate components. In most modern commercial
applications, the compressor and condenser are combined into a single piece of equipment called a
condensing unit. Refrigerant piping is custom-designed to meet the physical requirements of each
individual application which connect to various system components.
This unit is divided into 2 parts - the outdoor and the indoor unit. The outdoor unit mainly houses
the compressor, condenser, convenience outlet transformer, motor master etc. wwhhiillee tthhee iinnddoooorr
unit mainly houses the evaporator, re-heat coils, the blower section, etc. In the outdoor section, the
basepan is the main component that supports the compressor and the condenser coil.While the roof
top units are being shipped, the compressor vibrates which induces high stress on the base pan and
also leads to refrigeration circuit failure. Roof top units vary in weight anywhere from 3 to 6 tons.
One of the major issues faced in the HVAC (heating, ventilating, and air conditioning) industry is the
fluctuating response levels of these compressors at the time of transportation.The compressor
ccoonnttrriibbuutteess ttoo tthhee sseeccoonndd llaarrggeesstt mmaassss oonn tthhee uunniitt aafftteerr tthhee oouuttddoooorr ccoonnddeennsseerr ccooiill.. WWhheenn tthhee
response level of the compressor mounted on the basepan is high, it induces high stress on the base
pan. This stress is so high that it exceeds the ultimate strength of the material, thereby resulting in
the failure of the base pan, which is one of the most critical structural members of the unit. This leads
to a catastrophic failure of the unit before it even reaches the customer site. Subsequently, it would
lead to water leaking into the building through the duct work.
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performed, which predicts the mode shapes that are developed on the base pan and compressor at
different frequencies. Animated results can also be obtained to understand the relative movement of
the components, which is very helpful for design iterations. The natural frequency of the other
components can also be determined and it can also check if any of the components are going into
resonance.
TThhiiss wwhhiittee ppaappeerr iiss iinntteennddeedd pprriimmaarriillyy ffoorr DDeessiiggnneerrss oorr EEnnggiinneeeerrss wwhhoo aarree ddeeaalliinngg wwiitthh vviibbrraattiioonn
related issues during product design and who wish to understand how vibrations on the base pan of
a roof top unit can be minimized. This paper presents an overview of modal analysis, the response of
components within a roof top unit, various techniques used to minimize response of the compressor
on the basepan, the evaluation of design alternatives, the benefits of reducing the response level of a
compressor, and a case study showing how the design was evaluated to reduce the compressors’
response.

Market Trends/Challenges
It is a common phenomenon that compressors in roof top air-conditioning units tend to vibrate more
violently during transportation. Such vibrations can lead to failure in the piping system or the
compressor going into resonance when it is vibrating during shipping. Typical shipping frequencies are
in the range of 10-18 Hz. The natural frequency of any of the other components mounted on the base
pan should not lie in this range, as it might lead to the component going into resonance and thereby
resulting in a failure of the design.
Why does this problem arise?
It is a common phenomenon that compressors in roof top air-conditioning units tend to vibrate more
violently during transportation. Such vibrations can lead to failure in the piping system or the
compressor going into resonance when it is vibrating during shipping. Typical shipping frequencies are
in the range of 10-18 Hz. The natural frequency of any of the other components mounted on the base
pan should not lie in this range, as it might lead to the component going into resonance and thereby
resulting in a failure of the design.
Figure1. Simulation of base pan flexes at various frequencies
Experiments Conducted
Various methodologies were adopted to reduce the response of the compressor, which would lead to
reducing the stress level in the base pan. The base pan on which the compressor is mounted is made
of Draw-Quality material. The thickness is 16GA (1.50mm THK).

The material properties of this basepan are listed below.
The response level of the compressor was measured by placing tri-axial accelerometers on top of the
compressor and the compressor foot. The experimental setup is such that the unit will be placed on
concrete blocks and a shaker, that would induce a round 2 lb-force of energy into the unit,will be placed
under the unit’s right, at the center of the basepan.
A hole was drilled to have the shaker installed. The shaft of the shaker passes through this hole that has
been drilled and supported by fastening a nut onto it. Care was taken to see that the shaker was placed
on a plywood sheet and not directly on the ground. The experiment illustrated that there was a
tremendous amount of energy on the compressor and it was physically impossible ttoo ccoonnssttrraaiinn tthhee
compressor. This would cause the air-conditioning piping system to fail due to the stress developed in it
by the movement caused due to vibrations of the compressor. Later, various methods were adopted for
introducing a piece of plywood below the base pan to make it pre-stressed in the compressor area, to
stop the violent motion of the compressor.
Basepan Pre-stresses by
5.75” x 13.5” x 0.375” plywood
in the compressor area.
Figure 2. Plywood for experiment 1
Basepan Pre-stresses by
5.75” x 13.5” x 0.375” plywood
in the compressor area.
4” x 4” x 0.375”
pre-stress
at the center
of the Basepan.
Ultimate Tensile Strength – 51.2 KSI
Yield Strength – 42.6 KSI
% Elongation – 44%
Experiment-1
The dimensions of the plywood are 5.75” X 13.5” X 0.375”.
It was found that the response level increased at this size.
This might be due to the variability of the structure;
non-uniform or repeatable end conditions, etc. It is
desirable to select the size of the plywood so that it can
cover more than 90% of the surface below which the
ccoommpprreessssoorr iiss mmoouunntteedd..
Experiment-2
The dimensions of the plywood are 5.75” X 13.5” X 0.375”
at the compressor area, plus an additional 4” X 4” X 0.375”
plywood pre-stressed at the center of the base pan. This
was further enhanced by two pieces of 1” X 4” X 0.5”
plywood placed at the transformer mounting. It was
found that the response level increased. This might be
due to the variability of the structure; non-uniform or
rreeppeeaattaabbllee eenndd ccoonnddiittiioonnss,, eettcc..
1” x 4” x 0.5”
pre-stress at the
transformer mtg
Figure 3. Plywood for experiment 2

Slight Shift of the modes when putting OSB or Plywood under basepan
unit with bare base pan
Unit with shipping OSB plywood and a 5.75”x13.5” 0.375” plywood piece under compressor
Unit with shipping OSB plywood and a 5.75”x13.5” 0.375” plywood piece under compressor version 2
Table 1. Response level of the compressor
The RED color part
shown is the newly
designed
compressor plate to
provide additional
support to the
ccoommpprreessssoorr..
Y Axis
DOFS
Frequency
Foot 1
Base
g/lbf
Foot 1
Comp
g/lbf
Foot 2
Base
g/lbf
Foot 2
Comp
g/lbf
Foot 3
Base
g/lbf
Foot 3
Comp
g/lbf
Foot 4
Base
g/lbf
Foot 4
Comp
g/lbf
Comp Z
g/lbf
Comp Y
g/lbf
Comp X
g/lbf
8.00
12.75
37.75
0.0053
0.0197
0.0378
0.0058
0.0232
0.0433
0.0206
0.0191
0.0558
0.0172
0.01620
0.0331
0.0377
0.0159
0.0267
0.0341
0.0138
0.0904
0.0082
0.0227
0.0031
0.0138
0.0327
0.0558
0.0195
0.0175
0.0522
0.0255
0.0778
0.1168
0.0645
0.0569
0.0985
Y Axis
DOFS
Frequency
Foot 1
Base
g/lbf
Foot 1
Comp
g/lbf
Foot 2
Base
g/lbf
Foot 2
Comp
g/lbf
Foot 3
Base
g/lbf
Foot 3
Comp
g/lbf
Foot 4
Base
g/lbf
Foot 4
Comp
g/lbf
Comp Z
g/lbf
Comp Y
g/lbf
Comp X
g/lbf
8.25
12.50
35.50
0.0027
0.0145
0.0453
0.0034
0.0184
0.0453
0.0321
0.0297
0.0203
0.0181
0.0191
0.0144
0.0615
0.0104
0.0980
0.0564
0.0089
0.0022
0.0124
0.0189
0.0197
0.0199
0.0300
0.0240
0.0271
0.0105
0.0249
0.0626
0.0908
0.0856
0.1079
0.1210
0.0648
Y Axis
DOFS
Frequency
Foot 1
Base
g/lbf
Foot 1
Comp
g/lbf
Foot 2
Base
g/lbf
Foot 2
Comp
g/lbf
Foot 3
Base
g/lbf
Foot 3
Comp
g/lbf
Foot 4
Base
g/lbf
Foot 4
Comp
g/lbf
Comp Z
g/lbf
Comp Y
g/lbf
Comp X
g/lbf
8.25
13.00
36.25
0.0003
0.0021
0.0006
0.0045
0.0359
0.0279
0.0349
0.0142
0.0150
0.0021
0.0008
0.0011
0.0675
0.0183
0.0120
0.0610
0.0145
0.0213
0.0144
0.0307
0.0060
0.0228
0.0411
0.0277
0.0299
0.0256
0.0248
0.0725
0.0085
0.0096
0.1125
0.1017
0.0143
Table 1 shows the response level of the
compressor at various frequencies when a
2 lb-force is exerted on the basepan. A
compressor plate was designed so that we
could achieve double thickness at the
compressor foot locations.TThhee ccoommpprreessssoorr
plate was double riveted and we checked if
this lowered the response level of the
compressor.
Figure 4. Measurement positions on the compressor foot
Figure 5. Newly designed compressor plate
Compressor plate is
rivetted to the
basepan at 6 locations.
Basepan
Rivet
Part No: AT34AZ201
Newly Designed Compressor
Plate (Red color part)

Horizontal X-Plane (top of compressor)
Horizontal Y-Plane (top of compressor) Vertical Z-Plane (top of compressor)
Frequency Hz Frequency Hz Frequency Hz
Figure 6. Graphs showing experimental results
As we see from Figure 6, the response level of the compressor increased in the X, Y and Z directions
between 8 - 40 Hertz. Hence, double riveting of the sheet metal parts together was not an ideal solution.
The Solution
An idea was developed to check if the vibrations of
the compressor could be grounded and an
experimental set-up was developed. IItt wwaass nnootteedd
that the response of the compressor drastically
reduced. This was mainly due to the fact that the
vibrations were grounded through the 4” X 4” block
via the jacks. The challenge was to bring this into
engineering reality.
There were 3 challenges to overcome:
4” x 4” Block +
1/2” plywood
Jacks to act as
ground level as
the unit is
suspended over
concetrete
blocks.
Figure 7. Experimental set-up for grounding the vibrations
1. A decision had to be taken regarding the position of the 4” X 4” block between the OSB (oriented
strand board) and the ground.
2. The 4” X 4” block + ¾” plywood had to be placed in such a location that while the units are
stacked during shipping, the 4” X 4” block would lie on the top crate of the bottom unit and not
on the top cover itself.
3. Tolerance stack-up study had to be done to see that the OSB sheet along with the block could be
manufactured and the supplier’s acceptance received. Here the main challenge was the thickness
of the OSB sheet that the supplier could deliver.
Position of 4” X 4” Block + 3/4” Plywood
The proposed drawing in Figure 8 shows the position of the 4” X 4” block + ¾” plywood.

Fork Truck Opening
Fork Truck Opening
4” X 4” Block + 3/4” Plywood
Figure 8. Experimental set-up for grounding
the vibrations
Figure 9. Positioning of the block
The position of the block was selected such that while forking the unit the fork pocket ends should be
away from the block by at least 2 inches. Figure 9 shows the position and the offset distance of the block
from the fork truck openings.
Figure 10. Position of 4” X 4” block on the top crate in stacked-up condition
As seen in Figure 10, the block is supported on the top crate of the unit rather than the top cover. This is
6 inch pine wood.
Tolerance Stack-up study for manufacturability of the parts
A tolerance stack-up study was performed to understand the manufacturability of the parts and find out
the number of components that would be rejected/accepted using VisVSA software. 0.1683 - 0.3717 are
the values which are falling out of specification limits at a 95% confidence interval.

Test Results
Results at 95% Confidence Interval
Out of
Specification
Limits
Figure 11. Results of Tolerance stack-up study
Out of
Specification
Limits
The response level of the compressor was compared with various configurations as explained above.
The initial setup is the base pan with only the OSB. The modified base pan is the addition of the
compressor plate to double the thickness.
Horizontal Y-Plane Horizontal X-Plane (top of compressor) (top of compressor)
Frequency Hz Frequency Hz
Vertical Z-Plane (top of compressor)
Frequency Hz
Validation
Figure 12. Test results with different configurations.
This experiment was then verified with FEA using ANSYS workbench software. Von Misses Stress was
determined with and without wood. In Figure 13, we see that the stress was reduced by 58% with the
introduction of the 4” X 4” block.

Bottom View
Top View Bottom View
Figure 13. Stress graphs
Top View
Key Benefits to a Customer
Von Misses Stress (Mpa).
without wood
Von Misses Stress (Mpa).
with wood
The number of units which failed the transportation tests were significantly reduced
The OSB sheet provides a support to the base pan to avoid damages caused by the fork-lifts
The response level of the compressor reduced, increasing the durability of the piping system and
thus maintaining the life of the product
The stress induced on the base pan was reduced by 58%
Increase in the lifespan of the outdoor base pan
Reliability and durability of the unit due to the basepan hhaass iinnccrreeaasseedd
Reduction of warranty issues
Increase in the confidence level at design stage
Best Practices
After having designed a particular component, it is very important to ensure that it reaches the
customer site safely. Therefore the system’s design needs to account for all vibrations that the system
will undergo while it is being shipped. There are many best practices that are currently being
incorporated in the manufacturing industry. However we need to take into account the least costly one.

Some of the best practices would then include:
Mechanical springs
Elastomers, rubber, cork, etc.
Wire rope isolators
Thermocol
Common Issues
Some of the common issues if the shipping vibrations are not overcome:
Components going into resonance, thereby emitting lots of sound
Failure of system while it is delivered to the customer
Excessive stress developed in parts which might lead to failure before the warranty period
Conclusion
Roof top units can be transported either via road, rail or air. In most cases the units are mainly
transported via road to the distribution centers. It is also noted that units are mainly damaged while
being transported via road. These units are on the trailer for hours or days together, and experience
rugged road conditions.
For testing purposes, these long hours were constrained to 1 hour transportation tteessttss aass ppeerr AASSTTMM
(American Society for Testing and Materials) D4169. There are various assurance levels defined in this
test. However the most widely used is Assurance Level 1. Here the unit is placed on a hydraulic shake
table and shaken for an hour, the overall RMS value of ‘g’ experienced by the unit is 0.75. The unit is
then inspected after an hour to check all the failures. While units are being transported via road, they
are always stacked up depending on the load the trailer is carrying.
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input in the un-stacked configuration. While the unit is in the stacked configuration the amplification
factor on top of the compressor is double of what it is on the bottom unit. Since the compressor is one
of the heaviest components in the outdoor section and it rests on the base pan, it induces very high
stress which may lead to a catastrophic failure of the unit.
Various experimental methodologies were conducted to find out how the response level of the
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the OSB sheet and grounding the vibrations seemed to be the ideal solution for this problem.

1. http://www.kineticsystems.com/
2. http://www.newport.com/Vibration-Isolator-Selection-Guide/168531/1033/content.aspx
3. http://www.instructables.com/id/Types-of--Mechanical-Springs/
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Reference
Author Info
Mithun Srinivas Prabhakar
HCL Engineering and R&D Services
Vasu Raghuram
HCL Engineering and R&D Services

Reducing Shipping Vibration of Compressors in Roof Top Air-conditioning Units

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