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STRENGTH OF GEAR TEETH ^ g
STRENGTH OF GEAR TEETH.
The strength of the teeth and the horse-power that may be
transmitted by a gear depend upon so many variable and uncer-
tain factors that it is more a matter of practical experience and
judgment of the designer than a problem of theoretical calcula-
tion. Consequently, there are a great number of different for-
mulas and rules given by different authorities.
In the writer’s opinion, there are at the present time no form-
ulas or rules so well adapted to practical conditions as the
formulas constructed by Mr. Wilfred Lewis of Philadelphia.*
See American Machinist of May 4, 1893, page 3, and June
22, 1893, page 6.
Mr. Lewis in constructing his formulas assumes that the
gears are well made and mounted, so that the load is distributed
across the tooth and not concentrated on one corner only.
Mr. Lewis also assumes that the whole load is taken by one
tooth, and from a series of drawings of teeth of the involute,
cycloid and radial flank system, he determines the relative
strength of gear teeth of various forms and thereby obtains
the value of the factor y as given in the following table No. 54.
In his discussions upon the subject, Mr. Wilfred Lewis
obtains the general formula
:
W = s X pX/Xy.
W = Load in pounds transmitted by the gear.
s = Safe working stress of the material, taken as 8000 for
cast iron, when the working speed does not exceed
100 feet per minute. (See table No. 55)
p = Circular pitch of the gear.
/ = Face of gear in inches.
y = A factor depending on the form of the teeth. (See
table No. 54.)
* Mr. Wilfred Lewis kindly allowed the author to make use of his for
mulas in this book.
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