Page 299 - Tribology in Machine Design

284 Tribology in machine design

pose a problem for the designer. For example, the material may have hidden
metallurgical defects or its hardness may not be uniform but it still meets
the required contact stress value. Similar problems are encountered when
tooth accuracy and lubrication efficiency are estimated in quantitative
terms. One should then rely on past experience or on similar successful
designs already in service.

8.5.3. Subsurface originated pitting
Subsurface originated cracks are quite usual in the case of surface-hardened
teeth. Cracks usually start at the case/core interface or entirely within the
case, then coalesce, and as a result of that, rather large areas of the tooth
may be damaged. The continuous operation of gears promotes the progress
of tooth damage and this cannot be tolerated for long periods of time. A
standard engineering practice in fighting against subsurface originated
pitting has been to make the case deeper. Other changes in the operating
conditions of gears, for instance, better surface finish, use of different types
of oil, alterations in pitch line velocity, proved to be rather ineffective. The
only factors which have any real influence on this type of tooth failure are
the intensity of the load, the case depth and the tooth geometry. It is known
from engineering practice that the final drives in slow moving vehicles are
exposed to a considerable danger of subsurface originated pitting. The
gears used in the final drives are spur, helical, bevel or hypoid. They are
usually case-carburized and almost always heavily loaded with contact
stresses in the region of l.SGPa.

8.5.4. Evaluation of subsurface pitting risk
Subsurface originated pitting is a serious form of tooth wear and under no
circumstances can it be tolerated. Therefore it is vitally important to ensure
during the design stage that the gear unit will operate with a minimum risk
of subsurface pitting. There are a number of procedures to estimate the risk
of subsurface pitting recommended by specialist books on gears and each of
them is based on experimental data. Usually the first step in estimation
procedure is to determine the half-width of the Hertzian contact zone, a.
The next step is to determine the maximum shear stress at a depth la, 2a, 3a
and so on, below the surface, assuming that the friction between the
contacting teeth is about 0.1. Proceeding in the way described above, a plot
can be produced of maximum shear stress as a function of the depth below
the surface of a tooth. Additional curves can be plotted to illustrate the
relationship between the shear stress capacity of the material and the case-
hardness expected at different depths. In order to minimize the risk of
subsurface pitting this second curve should be above the maximum shear
stress curve. If we know at which depth the shear strength of the material is
equal to or higher than, the maximum shear stress it is possible to evaluate,
in an approximate way, the case depth required. For example, if the load on
the tooth is below 0.2 GPa then a case of hardness 50 HRC should extend to