Page 23 - Tribology in Machine Design

10 Tribology in machine design

1.2.5. Friction drives
Friction drives, which are being used increasingly in infinitely variable
gears, are the converse of hypoid gears in so far as it is the intention that two
smooth machine elements should roll together without sliding, whilst being
able to transmit a peripheral force from one to the other. Friction drives
normally work in the elastohydrodynamic lubrication regime. If frictional
traction is plotted against sliding speed, three principal modes may be
identified. First, there is the linear mode in which traction is proportional to
the relative velocity of sliding. Then, there is the transition mode during
which a maximum is reached and, finally, a third zone with a falling
characteristic. The initial region can be shown to relate to the rheological
properties of the oil and viscosity is the predominant parameter. However,
the fact that a maximum value is observed in the second zone is somewhat
surprising. It is now believed that under appropriate circumstances a
lubricant within a film, under the high pressure of the Hertzian contact,
becomes a glass-like solid which, in common with other solids, has a
limiting strength corresponding to the maximum value of traction.
Regarding the third zone, the falling-off in traction is usually attributed to
the fall in its viscosity associated with an increase in temperature of the
lubricant.
Friction drives have received comparatively little attention and the
papers available are mainly concerned with operating principles and
kinematics. In rolling contact friction drives, the maximum Hertz stress
may be in excess of 2600 MPa, but under normal conditions of operation
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the sliding speed will be of the order of 1 m s~ and will be only a small
proportion of the rolling speed. The friction drive depends for its
effectiveness on the frictional traction transmitted through the lubricated
contact and the maximum effective coefficient of friction is required.
Because the sliding velocities are relatively low, it is possible to select
materials for the working surfaces that are highly resistant to pitting failure
and optimization of the frictional behaviour becomes of over-riding
importance.

1.2.6. Involute gears

At the instant where the line of contact crosses the common tangent to the
pitch circle, involute gear teeth roll one over the other without sliding.
During the remaining period of interaction, i.e. when the contact zone lies in
the addendum and dedendum, a certain amount of relative sliding occurs.
Therefore the surface failure called pitting is most likely to be found on the
pitch line, whereas scuffing is found in the addendum and dedendum
regions.
There is evidence that with good quality hardened gears, scuffing occurs
at the point where deceleration and overload combine to produce the
greatest disturbance. However, before reaching the scuffing stage, another
type of damage is obtained which is located in the vicinity of the tip of both