Balance, vibration, and vibration analysis (FA/108-00)

February 26th, 2001

There are many factors that can have a substantial effect on the vibration level of air movement equipment; such as wheel balance, rotational speed, drive components, motor operation and wiring. Providing equipment that operates well within acceptable vibration levels requires consistent quality in production and attention to details. Only quality-driven companies spend the extra time and money required to make this kind of commitment. In this article, we will discuss wheel balancing, define the parameters and process of conducting vibration tests, and review some typical vibration analysis techniques. First let's look at wheel balancing.

Balancing

Many fan manufacturers, including Greenheck, produce a full line of products ranging from small ceiling fans to large industrial centrifugal fans. There are many application standards for different sizes and types of fans.

Fan Application Categories
Application Examples Driver Power kW (HP) Limits Fan Application Category. BV
Residential Ceiling fans, attic fans, window AC
<= .15 (0.2)
BV-1
> .15 (0.2)
BV-2
HVAC & Agriculture Building ventalation and air conditioning; commercial systems
<= 3.7 (5.0)
BV-2
> 3.7 (5.0)
BV-3
Industrial Process and Power Generation, etc Baghouse, scrubber, mine, conveying, boilers, combustion air, pollution control, wind tunnels
<= 298 (400)
BV-3
> 298 (400)
BV-4
Transportation and Marine Locomotives, trucks, automobiles
<= 15 (20)
BV-3
> 15 (20)
BV-4
Transit/Tunnel Subway emergency ventilation, tunnel fans, garage ventalation, Tunnel Jet Fans
<= 75 (100)
BV-3
> 75 (100)
BV-4
ANY
BV-4
Petrochemical Process Hazardous gases, process fans
<= 37 (50)
BV-3
> 37 (50)
BV-4
Computer Chip Mfg Clean room
ANY
BV-5

Based upon its particular operation and performance, every fan belongs to a Fan Application Category. (See Fan Application Categories chart.)

Responsible fan manufacturers balance the fan impeller assemblies to allowable residual unbalance prior to assembly of the unit. These limits are based on ANSI S 2.19 "Balance Quality of Rigid Bodies." This standard establishes allowable residual unbalance based on the balance quality grade, impeller weight and impeller rotational speed. Refer to ANSI S2.19 and AMCA Standard 204-05 for further information on balancing standards.

Fan Application Category
Balance Quality Grade for Rigid Rotors/Impeller
BV-1
G 16
BV-2
G 16
BV-3
G 6.3
BV-4
G 2.5
BV-5
G 1.0

Balancing is defined as "the process of adding (or removing) mass in a plane or planes on a rotor in order to move the center of gravity towards the axis of rotation." As the definition of balancing implies, material is either added to or removed from the rotating element to attain an acceptable balance level. In most cases adding weight is preferable, and depending on the type of fan and the fan design duty, different methods of adding weight are employed. In some cases weight is added by means of a metal clip placed on the blade or wheel tip. Typically this method is used for lighter duty fans operating at low speeds. As the speed and size of the fan increases, stainless steel bolts, nuts, or washers on steel products or aluminum bolts, nuts or washers on aluminum products are used as balancing weights instead of clips. These methods are employed on impellers that are balanced after they have been painted, because the added weight of the paint could have an effect on the overall balance of the assembly. In other cases where the weight of the paint is negligible compared to the weight of the impeller, metal plates of varying sizes are welded onto the impeller in the appropriate location to attain adequate balance and then the impeller is painted.

Welding balance plates to a centrifugal wheel prior to painting.(Shown on dynamic balancing stand)

 

 

 

 

 

 

 

 

 

 

 

In some cases, notably those units with very small unpainted fan rotors weighing less than eight ounces, residual unbalance can be difficult to determine accurately. Therefore, the fabrication process alone of these products must ensure that the weight is distributed equally about the axis of rotation because these smaller fan rotors do not undergo the same balancing process.

It is important to understand the difference between static and dynamic balancing.

Static balancing is usually used on a very thin rotor, like a flywheel, where most of the mass lies in a single plane. Static balancing does not require that the rotor rotates. An example of static balancing is a "bubble balance" on an automobile tire. Here the wheel assembly is placed horizontally on a pivot point and weight is added to the front of the rim until the wheel is level.

Dynamic balancing is used on most rotors where more than one correction plane is required to balance the rotor. The rotor must be rotated to detect and correct "couple unbalance" where two equal unbalance masses are spaced 180 degrees apart at opposite ends of the rotor. An example of dynamic balancing is a "spin balance" on an automobile tire. Here the wheel assembly is rotated and weights are attached to both the front and back of the wheel rim.

A rotor that is dynamically balanced is also statically balanced. For this reason, a meaningful specification only requires dynamic balancing. Static balancing would be redundant.

Vibration

Vibration analysis is a cost-effective and useful diagnostic tool to ensure smooth running fans. Smaller units may not be vibration tested after assembly because the vibratory energy is much lower and has little impact on the installation or the life of the fan. Vibration is defined as "the alternating mechanical motion of an elastic system, components of which are amplitude, frequency and phase." The two components of vibration that fan manufacturers are most concerned with are amplitude and frequency. Amplitude defines how far the rotating body moves from the center rotating axis, and the frequency is the number of cycles, or revolutions that occur within a specified time period.

Permanently secured stainless steel bolts, nuts and washers used as balancing weights after paint

 

 

 

 

 

 

 

 

 

 

Frequency is most often measured in either Hz (cycles per second) or CPM (cycles per minute). Amplitude can be measured in displacement (mils), velocity (in/sec), or acceleration (g's). Of these three, velocity is typically used to describe the vibration of a fan because it represents a fairly constant level of vibration severity independent of the fan rpm. A velocity measurement gives a vibration severity description that can be compared at any rotational speed. In contrast, displacement measures the maximum distance of a vibrating body from its neutral position. This gives a vibration severity description at only one specific speed and cannot be compared without measuring across a range of different speeds. Acceleration measures amplitude as the time rate of change of velocity, which again, is not a particularly useful description of vibration severity except for some special analysis applications.

Vibration Testing

When a fan is ready to undergo vibration testing, it is first mounted on the test bed as either a rigidly supported or a flexibly supported installation. A rigidly supported system should have a natural frequency above the running speed. (An example of a rigidly mounted fan is one that is mounted directly to a heavy concrete foundation.) A flexibly supported system should have a natural frequency below the running speed. (An example of a flexibly mounted fan is one mounted on spring isolators.)

A tri-axial accelerometer measures the velocity amplitude in the horizontal, vertical, and axial directions.

 

 

 

 

 

 

 

 

 

 A tri-axial accelerometer is placed on each bearing, which measures the vibration in the horizontal, vertical and axial directions. The vibration is measured and recorded as either filter-in or filter-out. A filter-in vibration reading is taken only at one frequency, typically the fan rpm. A filter-out vibration reading measures the vibration over a wide frequency range and is calculated as the square root of the sum of the squares of the filter-in readings over that particular frequency range.

Once the unit has been prepared for vibration testing, it is operated at the design speed and tested to ensure the unit falls below the maximum allowable vibration. The chart to the right shows maximum allowable vibration corresponding to each appropriate fan application category.

At Greenheck, a unit that undergoes vibration testing does not ship from our facility until the vibration level is within acceptable limits. If the maximum allowable vibration level is exceeded, various balancing and vibration elimination techniques are used to correct the unit.

Vibration Limits for Tests Conducts in the Factory

Values shown are peak velocity, inches/s, Filter-In, at the factory test speed
Fan Application Category
Rigidly Mounted in./s
Flexibly Mounted in./s
BV-1
0.50
0.60
BV-2
0.20
0.30
BV-3
0.15
0.20
BV-4
0.10
0.15
BV-5
0.08
0.10

Analysis

Examination of a fan's vibration signature can reveal possible sources of excessive vibration or vibration peaks. Identifying the specific frequency at which the vibration occurs is one way to begin an analysis. For example, a vibration peak occurring at the fan speed is most likely a sign of wheel unbalance and can most often be remedied with minor trim balancing. A vibration spike that occurs at the motor RPM could be an indication of motor pulley unbalance. A spike at two times the fan RPM could indicate looseness, bearing misalignment or a bent shaft. Other possible sources of vibration, which are not always as easy to distinguish, are those due to external factors such as electrical vibrations (torque pulses that occur at two times the line frequency). Another example would be a vibration spike caused by a bearing fault in the inner ring. This type of spike would occur at a frequency that is a function of the bearing geometry and fan speed.

All Greenheck centrifugal, vane axial and industrial fans are vibration-tested prior to shipment. The vibration signiture of each fan (shown above) becomes a permanent record with the fan serial number, available to the customer upon request.

Specifications

Vibration specifications should be reviewed carefully to identify any special requirements. Special requirements can add significant labor hours and cost to the vibration testing process. It is important to write a meaningful vibration specification that will ensure your system will operate without excessive vibration, but also one that is not so stringent that it is difficult to meet without adding significant cost and time to the project. Following are some recommendations to consider in reviewing or composing a meaningful vibration specification:

- Select the appropriate "Balance and Vibration Grade." This will not only ensure a smooth-running unit, but will also help to avoid additional time and costs involved with meeting unnecessarily stringent vibration levels. For example, it would not be practical or beneficial to expect a light duty fan to meet the same vibration requirements as a large, heavy-duty industrial unit.

- Specify that the vibration testing be conducted at the fan manufacturer's shop and not at the job site. Conducting a vibration test at the jobsite introduces additional variables, which are outside the fan manufacturer's control. Guaranteeing jobsite vibration levels can be very involved and expensive.

- Specify whether the unit is to be rigidly mounted or flexibly mounted to the vibration test stand. A rigidly mounted unit is the standard configuration, and is the only option if the fan is manufactured less motor and drives. A unit can be vibration tested while flexibly mounted upon request if the fan manufacturer is supplying the entire unit, including motor, drives, isolation or isolation and base.

- Specify that the readings be measured in velocity amplitude (inches per second, peak) at the design RPM (filter-in). Filter-in readings guarantee a good level of fan construction and reasonable residual fan unbalance. Filter-out readings become more involved and costly due to additional components that have to be controlled to attain the desired vibration level.

- Specify that the vibration readings be taken in the horizontal, vertical and axial directions on each fan bearing. Measuring the vibration levels in all three planes and at each of the bearings is important in getting an accurate picture of a heavy duty fan's actual vibration levels.

Sample Specification

Following is a typical specification example for a belt-driven centrifugal fan in Application Category BV-3: "Each fan shall be vibration tested before shipping, as an assembly, in accordance with AMCA 204-05. Each assembled fan shall be test run at the factory at the specified fan RPM. Vibration signatures shall be taken on each fan bearing in the horizontal, vertical, and axial directions. The maximum allowable fan vibration level shall be 0.15 in/sec peak velocity, filter in, at the fan RPM when the fan is rigidly mounted.

Summary

We have discussed many factors that affect the vibration levels of operating air movement equipment. Clearly it is a complex subject. It should be apparent to the reader that there are numerous factors contributing to the overall vibration level of an assembled fan. One of the main reasons to purchase the "complete package from Greenheck is that the entire assembled unit (including the fan, motor, drives, base, bearings, accessories, etc.) is vibration-tested as a whole and verified as running within allowable vibration limits.

 

 

One less worry

With a complete package there are only a few additional external factors that can adversely affect the vibration level of the fan once it is installed at the jobsite. Purchasing a complete package gives you one less thing to worry about.

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