Page 293 - Tribology in Machine Design

278 Tribology in machine design

In practice the specified exponent values may be quite different depending
on the load and service life required, expressed by the number of load cycles N.
Besides, depending on the microstructure of the material, the surface
finish, the character of the oil additives and other similar factors, the slope
of the S-N curve in a given lubrication regime may change. Thus, in the
boundary lubrication regime the slope may vary from an exponent of as low
as 2 to as high as 5. A mixed lubrication regime may vary in slope from 4 to 7.
The thick film lubrication regime is usually characterized by an exponent
12
7
in the range of 8 to 16, particularly in the range of 10 cycles to 10 cycles.
Figure 8.3 should be considered as representing the average data and the
real application conditions may vary considerably from that shown in the
figure. In the case of heavy pitting some action should be taken in order to
stop or at least slow down the damaging process. Usually an oil with higher
viscosity provides the remedy by slowing down the pitting and creating the
conditions for the pitted surfaces to recover. Pitting is not particularly
dangerous in the case of low-hardness gears and a moderate amount of
pitting is usually tolerated in medium-hardness gears. The opposite is true
for hard gears where virtually no pitting can be tolerated. Work-hardening
of the surface material is taking place during pitting, due to that, the surface
is toughened and becomes more resistant to pitting. It is quite often the case
that if the lubrication of the gears is efficient, pitting is a transient problem
ceasing completely after some time.

8.4. Gear failure due Scuffing is usually defined as excessive damage characterized by the
to scuffing formation of local welds between sliding surfaces. For metallic surfaces to
weld together the intervening films on at least one of them must become
disrupted and subsequently metal-metal contact must take place through
the disrupted film.
When two spheres, modelling the asperities on two flat surfaces, are
loaded while in contact, they will at first deform elastically. The region of
contact is a circle of radius a, given by the Hertz theory discussed in Chapter 3.
When the load is increased, plasticity is first reached at a point beneath
the surface, at about 0.5a below the centre of the circle of contact. The value
of the shear stress depends slightly on the Poisson ratio but for most metals
2
has a value of about 0.47P m, where P m = (W/na ) is the mean pressure over
the circle of contact. At this stage F m takes the value 1.17, where Y is the
yield stress of the softer metal.
As the load is increased, the amount of plastic deformation increases and
the mean pressure rises. Eventually the whole of the material in the contact
zone is in the plastic state and at this point the mean pressure P m acquires its
maximum value of about 3Y. The load corresponding to full plasticity is
about 150 times that at the onset of plasticity.
There is, therefore, an appreciable range of loads over which plastic flow
takes place beneath the surface without it extending to the surface layers
themselves. In these conditions, welding does not occur and this possibility
of changing the surface profile by plastic flow of the material beneath, gives