Page 294 - Tribology in Machine Design

Lubrication and efficiency of involute gears 279

a means of smoothing out surface irregularities without causing excessive
damage. This is one of the mechanisms utilized during running-in.
When sliding is introduced, however, a tangential stress field due to
friction is added to the normal load. As the friction increases the region of
maximum shear stress moves from 0.5a beneath the surface upwards whilst,
simultaneously, a second region of high yield stress develops on the surface
behind the circle of contact. The shear stress at the surface is sufficient to
cause flow when the coefficient of friction reaches about 0.27. With plastic
deformation in the surface layer itself, welding becomes possible. For a
normal load which just suffices to cause shear at 0.5a beneath the surface, an
increase in the friction to 0.5 causes shear over the whole area of contact in
considerable depth. Also, as the load increases, the coefficient of friction
necessary to cause flow in the surface, decreases. Experiments strongly
suggest that scuffing originates primarily with an increase in the coefficient
of friction. Scuffing is usually associated with poor lubrication.
As scuffing starts, the damage is not great when the oxide films are
disrupted and the metals first come into contact. Usually the damage builds
up as sliding proceeds. At first it is localized near the individual surface
asperities where it is initiated. During further motion the regions of damage
grow, and eventually coalesce with a great increase in the scale of the
deformation. This could imply that the tendency to scuff depends upon the
amount of sliding. In spur gears, for example, the motion is one of rolling at
the pitch line and the proportion of sliding increases as the zone of contact
moves away from it. It was observed that scuffing occurs away from the
pitch line.
Another significant factor to consider is the speed of sliding as it directly
influences the surface temperature. The temperature rise is sensitive to the
load but varies as the square root of the speed. The rise is usually greater for
hard metals than for soft but it is most sensitive of all to the coefficient of
friction. Therefore, the maintenance of low friction, through efficient
lubrication, is of prime importance in reducing the risk of scuffing.
The risk of scuffing could also be significantly reduced by the proper
selection of gear materials. The first rule is that identical materials should
not rub together. If for some reason the pair of metals must be chemically
similar, their hardness should be made different so that the protective
surface film on at least one of them remains intact, preventing strong
adhesion. Metallic pairs which exhibit negligible solid solubility, are more
resistant to welding and subsequently to scuffing, then those which form a
continuous series of alloys. The role of the natural surface films is to prevent
welding. If they are hard and brittle and the metal beneath is soft, the
likelihood of them being broken increases significantly. The best films are
ductile but hard enough to compete with the underlying metal.
There are many factors which may initiate gear scuffing but only two of
them are really important. The first is the critical temperature in the contact
zone and the other is the critical thickness of the film separating the two
contacting surfaces.