| TABLE 1.4 LOAD-CARRYING UNIT VS. LUBRICATION OF SPUR GEARS FOR ACETAL COPOLYMER (CELCON), ACEIAL RESIN (DELRIN) AND NYLON RESIN (NYLON ETC.). | ||||
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Like metal gearing, plastic spur gears are the most prevalent type and also
the easiest to design in
regard to stress level. The design procedure for helical and bevel gears is
analagous to those for spur, differing mainly in the values of the various
equation modification parameters. We first consider the design for bending
fatigue strength and follow this with a discussion of design for
surface durability (contact stress), For a given spur gear Table 1.48
determines which of these should be used.
In regard to standards it should be noted that the AGMA standards have been generated around metal gears. An exception is AGMA 141.01 which is devoted to plastic gearing.
20.5.2 Bending Stress -Spur Gears—The basic Lewis Formula, the use of which for metal gears has been described in Section 13, is used also for plastic gears with suitable modifying factors. It can be expressed in the form:
Wt = SFY KLKvKt (70)
where
KL = lubrication factor
Kv = velocity-dependent dynamic factor
Kt = temperature factor
As already explained in Section 13.1, the remaining notation is as follows:
Wt
= tangentially transmitted Load lbs
S =
maximum tooth bending stress , lbs/in2
F =
face width of gears, in.
ln the event torque or horsepower (H.P.) are specified, these are related to the tangent.ially transmitted load as follows:
Torque (in.-lbs.) = (1/2)WtD
(71)
and H.P. = WtDn
(72)
126050
where