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Understanding Bearings for the Fan Industry
Bearings
are one of the most critical components in the operation of a fan and careful
consideration must be given to the selection of the appropriate bearing for
each application. Fan manufacturers
that have a comprehensive product line ranging from small light duty commercial
fans to heavy duty, high-speed industrial fans, inherently acquire more
experience in bearing application.
Through the years Greenheck has worked closely with our bearing supplier
partners to establish the quality features required for our full range of
products, and to provide the bearing life expectancy required by our industry.
The
following information is intended as a guide to understand fan-bearing life,
components and some of the typical applications for which they are used. We will begin our discussion by defining
some of the industry terms used to describe bearing life expectancy.
Bearing Life
Bearing
life is usually expressed as the number of hours an individual bearing will operate
before the first evidence of metal fatigue develops in the rings or rolling
elements. In past years, four different
terms were used when referring to bearing life. The terms commonly used were B10 or L10 and
B50 or L50. The terms B10 and L10
had the same meaning and the terms B50 and L50 also had
the same meaning. It’s time to end the
confusion! In today’s terminology the
preferred term is L10.
However, L50 is sometimes used, therefore both meanings must
be understood.
L10 life: The preferred term in
specifying bearing life. The
American Bearing Manufacturers Association (ABMA), formerly the AFBMA defines
the Basic Rating Life, L10 as the bearing life associated with a 90%
reliability when operating under conventional conditions, i.e., after a stated
amount of time, 90% of a group of identical bearings will not yet have
developed metal fatigue. L10
life is also referred to by manufacturers as the “minimum expected life.”
L50 life: Or average life. Although the L10 life is the
proper method of specifying fatigue life per the ABMA, another term is often
used in the industry. The L50,
or average life, is accepted as the bearing life associated with a 50%
reliability, i.e., after a stated amount of time, only 50% of a group of
identical bearings will not yet have developed metal fatigue. L50 life equals five time the L10
life.
In
other words, to get a L50 life equal to a L10 80,000-hour
life, you must specify the L50 life to be 400,000 hours. The following chart shows a comparison of L10
to L50 equivalents.
Writing the Bearing Specification
Basic
Rating Life, L10, is a useful tool when specifying a given level of
bearing construction. When required to
provide a given L10 life, all equipment manufacturers must supply
the same capacity bearing for a given RPM and shaft diameter. Also, an 80,000 hour L10 bearing
will have a theoretical life twice as long as a 40,000 hour L10
bearing and hence will last longer in the field.
Here’s
Greenheck’s recommendation for a typical bearing specification:
Bearings shall be air handling quality, heavy duty
grease lubricated, ball or roller type.
Bearings shall be selected for a Basic Rating Life (L10) of
80,000 hours at maximum operating speed and horsepower for each construction
level. (Air handling quality means the bearings meet the requirements for use
in air handling applications: high
speeds, long life and quiet operation.
All bearings are 100% tested for excessive noise levels and bore
dimensions are verified to be within tolerances.)
Note: If all the fan products you are specifying
are from the Greenheck Fan and Vent catalog, you can specify L10
100,000 hour life bearings at no extra charge.
The
chart below provides another way to look at the expected bearing life. Assuming you specified Greenheck’s standard
bearing life of L10 80,000 hours and your fans run an average of
eight hours per day, you can expect 27.5 years of life on 90 percent of the
bearings.
In
most cases, the Basic Rating Life will be much greater than shown because the
bearings are selected for the maximum RPM and horsepower for each size and fan
class. Most fans are selected
significantly below their maximum fan rpm.
*Bearings with a L10
200,000 hour life are optional for most centrifugal and vane axial products. However, in most cases, it is not practical
to specify L10 200,000 because of the associated cost. (And, do you really need the bearings to
last for 68 years?) It is more
practical for your customer to spend the additional money on maintenance.
Avoid
writing bearings specifications without having the correct Basic Rating Life (L10
or L50) terms in front of the required hours of life. If your specificaiton reads 200,000 hour
bearing life, your chance of getting what you want is minimal. Some supplier will assume L50
200,000 life is all that’s required and you will end up with an inferior
bearing system. Other suppliers will
assume you are specifying the optional L10 200,000 hour life and add
unnecessary cost. The best supplier
will ask for a confirmation of the L10 life required.
No Guarantee
Bearing Basic Rating Life is theoretical and is based on a collection of statistics. Specifying a L10 life does not guarantee that the fan bearings will have a 90% reliability when installed on a fan in the real world. The calculation for Basic Rating Life assumes proper lubrication is provided, no shock or vibration exists, alignment is virtually perfect, no debris enters the bearings and ambient temperatures are not extreme. In the real world, none of these conditions are realistic and the “installed life” of a bearing will depend on the application and maintenance.
To get as close as possible to the specified life, the installer and end user must follow the recommendations in the manufacturer’s installation and maintenance instructions. Once the bearing life expectancy is clearly defined, that information can then be combined with other bearing requirements to select the most appropriate bearing for each application.
Bearing Selection
Most manufacturers use some type of bearing selection program that calculates equivalent bearing loads and bearing life. Some of the main selection criteria include shaft diameter and weight; lubricant viscosity; motor horsepower; fan sheave pitch diameter, weight and location on the shaft; fan speed; fan wheel weight; bearing and wheel locations on the shaft; thrust loads; and the direction of belt pull. Most often the selection program used is one developed by a bearing manufacturer. Greenheck engineers use Dodge bearing selection software. Based on the output results of this data, Greenheck engineers are able to select the bearings that best meet each individual application.
Light Duty Bearings
Ball bearings with stamped steel housings are well suited for applications with very light loads and lower speeds. The main design characteristic that defines stamped bearings is the rubber sleeve around the bearing insert, which snaps into the stamped house. The use of these bearings is limited to fan products with 3/4 inch and smaller diameter shafts, and one horsepower and smaller motors.
Air Handling Quality
When the operating level of the ball bearings with stamped steel housing is exceeded, air handling quality ball, spherical or tapered roller pillow block or flange-mount bearings are primarily specified.
These bearings have been engineered specifically for air handling applications and feature special construction and tighter quality control. In order for a bearing to be of “air handling quality,” it must meet the following specifications:
1. Reduced Swivel Torque. Proper alignment of the bearing to the shaft is essential in preventing vibration, noise and damage to the bearing components. To allow a bearing to self align under light loading, it must have a low swivel torque. A low swivel torque enables the outer race of the bearing to pivot or swivel within the cast pillow block, allowing the bearing to align to the shaft much easier. It should be noted that this feature is special only to air handling ball bearings and tapered roller bearings. Spherical designs inherently allow for easy self-aligning because the race has a radius that allows the rollers to pivot somewhat within the race, so the race doesn’t need to pivot within the pillow block.
2. Noise and Vibration Testing. All “air handling quality” bearings are 100% vibration tested. These high test standards are designed to discover microscopic manufacturing defects that may either cause higher noise levels or lead to premature failure of the bearing.
3. Bore Size Test. All “air handling quality” bearings are 100% inspected to insure that the inner race diameter is within tolerance. A bore size larger than tolerance would allow the bearing to shift which could cause misalignment.
The benefits of “air handling quality” bearings to the life of ventilation equipment are being seen throughout the HVAC industry. More and more manufacturers are providing air handling quality bearings as their standard.
Ball Bearings
The Dodge D-Lok pillow block bearing incorporates a “heavy-clamp-style” locking collar as an alternative to the typical set screw locking system. This design allows the bearing race to be tightened concentrically about the shaft. This type of locking mechanism gives two distinct advantages over the set screw method. First, it maintains one common center axis of the shaft and bearing, whereas tightening of the set screw forces the shaft off to one side, offsetting its rotating axis. Secondly, the shaft and bearing race come into surface-to-surface contact upon tightening. This increase in contact surface area not only improves the holding strength of the bearing to the shaft, but it also eliminates problems with burring on the shaft caused by the tightening of set screws.
Application
Greenheck uses Dodge D-Lok ball bearings for all centrifugal, vane axial and industrial units within the ball bearing application range. In our fan and ventilation line, Dodge D-Lok ball bearings are used on Greenheck’s extended pressure CUBE exhaust fans (Model Cube-XP).
The balance of the fan and ventilator products that exceed the limitations of the stamped steel bearings use the air handling ball bearing with the set screw locking method. This includes fans with a shaft diameter of one inch and larger, and a motor of 1 1/2 horsepower and larger. In applications with lower speeds the D-Lok mechanism is available, however, the D-Lok advantages are negligble and not cost justified.
When the application load and speed exceed the capacity of ball bearings, roller pillow block bearings are typically specified. Like ball bearings, roller bearings can carry a combination of radial and thrust loads; however, roller bearings, in order to operate properly, require a radial load at least equal to the thrust load. Insufficient radial loading allows the rollers to skid within the race, which could cause premature failure of the bearing components. Tapered roller bearings that are similar to the spherical rollers, are commonly specified for vertical applications where high thrust loads are encountered.
Currently, roller bearings are available with either the standard set screw-locking method, or the Dodge Imperial patented adapter-mounting system (spherical roller bearings only).
Thus far all of the bearing options we have discussed have been in a solid housing, either stamped or pillow block. Once the application exceeds the speed limit for the contact seal and grease sump design of the solid housing, a split pillow block bearing is typically specified. The split pillow block design incorporates a friction-free labyrinth seal and a larger grease cavity. Higher speeds can then be attained and the rollers become the limiting factor instead of the seal.
Split pillow block housings incorporate a tapered adapter sleeve-mount locking system. Because of the geometry of the sleeve-mount design, a larger bearing is required for the same shaft size as would be used in a solid pillow block design. Consequently, the capacity of the rollers increases, thus requiring a higher minimal radial load for proper operation of the rollers.
In some applications where contaminants need to be kept out of the bearing, special seals other than the labyrinth-type are used in the split pillow block housing. In these cases, the seal may be a speed-limiting factor and must be accounted for in the selection of the bearing.
In certain products, such as some vane axial belt drive fans, the installation of a pillow block bearing can be difficult simply because of physical space constraints within the belt tube. In such a case, a flange-mount bearing can be used. The bearing components are essentially the same as the pillow block version, but the housing allows for face mounting around the shaft instead of pedestal mounting under the shaft.
Summary
The information in this article is a guideline to help communicate a better overall understanding of bearings and good bearing application.
Don’t let your air handling equipment suffer a short life span because of premature bearing failure. Applications are often times unique, and your selection will be based upon the bearing options best suited to your job specifications.
Greenheck engineers have over 40 years of experience in bearing selection and work closely with our vendors to ensure customers receive the highest quality products. Determine what bearing life your application requires and let Greenheck’s experienced engineering team specify the right bearing for the job.
Fan
Application No. FA/103-00