Page 204 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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cracks may grow and join, and small bits of metal are eventually separated leaving
surface pits (see Figure 2.9). Pitting is the result of slow surface destruction, leading
to the roughness of tooth surface, vibration and noise.
3) Wear
As gears mesh, both rolling and sliding occur. Pitting is primarily a consequence of
rolling contact while wear is the result of sliding contact [6]. In enclosed gearings
when lubrication is insufficient, or in open gearings the presence of foreign parti-
cles, together with sliding motion between meshing surfaces, will cause abrasive wear
on tooth surfaces. Wear may cause alteration of tooth profile and loss of conjugate
motion, resulting in impact or intolerable levels of vibration or noise.
4) Scuffing
Scuffing is a severe form of destructive adhesive wear. When a pair of gears carries
heavy loads at high speeds, high sliding motion between meshing teeth surface
generates frictional heat, leading to a high temperature, reducing the viscosity
of lubricant. Combined with the high pressure due to heavy loads on the mating
surfaces, gears confront inadequate lubrication or even lubrication failure, causing
direct metal-to-metal contact on tooth surfaces. As the gears rotate continuously,
local micro-welding and surface dragging and scoring happen eventually, especially
on the soft tooth surface. Scuffing significantly rough tooth profiles, resulting
improper meshing, erratic output angular velocity, dynamic loads and noise.
5) Plastic deformation
When soft teeth are subjected to heavy loads and particularly impact loads, the sur-
face material may flow plastically. Materials on the pinion tooth surface flow away

from the pitch line; while materials on the gear tooth surface flow towards the pitch
line. The deformed tooth profiles also lead to improper meshing, vibration or noise.
The failure modes mentioned before, although are described separately, two or
more of them may occur simultaneously in practice. In fact, sometimes one type
of failure may promote or even aggravate the other. It is designers’ responsibility to
prevent these failures at early design stage. This chapter mainly introduces the analysis
and design process for the prevention of tooth breakage and tooth surface pitting.
The design process for preventing other failures can be found in relevant design
handbooks [9, 10].

8.3 Strength Analysis for Spur Gears

Tooth surface pitting and tooth breakage are the two most prevailing failure modes in
gear drives. For proper power transmission, gears must be capable of running for an
expected life without significant pitting and gear teeth must be safe from breakage. These
are guaranteed by tooth surface contact strength analysis and tooth bending strength
analysis, respectively, starting with force analysis.

8.3.1 Forces on Spur Gear Teeth

When a pair of gears is in operation, the teeth of driving pinion push on the driven
teeth, exerting force on the gear. Thus, a torque is transmitted and, because the gear
rotates, power is also transmitted. Power transmission actually involves the application