Greenheck Fan FA-103-00 User Manual

A technical bulletin for engineers, contractors and students in the air movement and control industry.

Fan Application

FA/103-00

P.O. Box 410 • Schofield, WI 54476 • 715.359.6171 • Fax 715.355.2399

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 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 B

or L10and B50or

. The terms B10and L10had the same meaning

and the terms B

and L50also had the same

meaning. It’s time to end the confusion! In today’s terminology the preferred term is L

. However, L

is sometimes used, therefore both meanings must be understood.

life: The preferred term in specifying bearing life.

The American Bearing Manufacturers Association (ABMA), formerly the AFBMA defines the Basic Rating Life, L10as 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. L

life is also referred to

by manufacturers as the `minimum expected life’.

life: Or average life.

Although the L

life is the proper method of specifying fatigue life per the ABMA, another term is often used in the industry. The L

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. L

life equals five

times the L

life.

In other words, to get a L

life equal to a L

80,000-hour life, you must specify the L50life to be 400,000 hours. The following chart shows a comparison of L

to L50equivalents.

Required L

Equivalent L

(avg)

Life Hours Life Hours

20,000 100,000 40,000 200,000

80,000 400,000 100,000 500,000 200,000 1,000,000

product application guide

Greenheck Product Application Guide

Fan Application No. FA/103-00

Writing the Bearing Specification

Basic Rating Life, L10is a useful tool when specifying a given level of bearing construction. When required to provide a given L

life, all equipment manufacturers must supply the same capacity bearing for a given RPM and shaft diameter. Also, an 80,000 hour L

bearing will have

a theoretical life twice as long as a 40,000 hour L

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, (L

) 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 & Vent catalog, you can specify L

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 L

80,000 hours and your fans run an average of 8 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 L10200,000 hour life are optional for most centrifugal and vane axial products. However, in most cases, it is not practical to specify L

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 (L

or L50) terms in front of the required hours of life. If your specification reads 200,000 hour bearing life, your chance of getting what you want is minimal. Some suppliers will assume L

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 L

200,000 hour life and add unnecessary cost. The best suppliers will ask for a confirmation of the L

life required.

No Guarantee

Bearing Basic Rating Life is theoretical and is based on a collection of statistics. Specifying a L

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 the 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

L10Life Average running hours per day

81624

80,000 27.5 yrs 13.7 yrs 9.2 yrs

100,000 34.3 yrs 17.2 yrs 11.4 yrs

200,000* 68.6 yrs 34.3 yrs 22.8 yrs

*Years of bearing life for fans running seven days a week, fifty two weeks per year.

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