Page 196 - The Tribology Handbook

B7 Friction clutches

Lining design for oil-immersed clutches

For working in oil, multi-plate clutches are suitable. Oil
acts as a cushion and energy released by heat is carried
away by oil. The main disadvantage is a reduction in
friction, but this can be counteracted by higher operating
pressures. As long as there is an oil film on the plates, the
friction characteristic and engagement torque remain low,
but as soon as the film breaks the engagement torque rises
rapidly and may lead to rapid acceleration. The friction
surface pressure should usually not exceed 1 MN/m2 with
a sliding speed maximum of 20 m/s, steel on steel. With
oil-immersed clutches having steel and sintered plates the
relationship between the static and dynamic coefficient of Figure 7.15 Anti-distortion slot in friction plate
friction is more favourable. Friction surface pressure and
sliding speed may be up to 3 MN/m2 and 30 m/s.

Facing grooves and anti-distortion slots

The most common facing groove is a single- or multi-lead
spiral. This helps to prevent the formation of an oil film,
which, if formed, would lower the coefficient of friction. It
also provides space for the oil to be dispersed during clutch
engagement. Spiral grooves are between 0.6 and 1.5 mm
wide and 0.2-1 mm deep, depending on diameter and face
thickness. The pitch is between 1.5 and 6.0 mm, depend-
ing on the size of the disc.
Figure 7.15 shows an anti-distortion slot in a sintered
metal facing disc. Two widths of slots are frequently used,
4 or 5 mm wide. The slots minimise distortion and warp-
ing. Thermal expansion of the sintered metal and backing
plates from absorption of heat causes expansion and
contraction with temperature variations. The slots permit
dimensional changes without buckling or dishing the
backing plate. External and internal slots are used as in
Figure 7.16. They number from 4 to 8 of each.
On moulded and woven fabrics the slots are normally
2.5-3.5 mm wide and 1-1.5 mm deep. Eighteen to twenty-
four equi-distant slots or sets of three are usual. Thermal
expansion slots are not required for these materials.
Clutch plates (Figure 7.17) are also available, which
present a sine wave cross-section along a periphery, i.e. a
corrugation, which generates a spring action. During
engagement the pressure on the friction surface increases
until the sine wave becomes a flat surface. The spring Figure 7.16 Slots to counteract expansion and
action ensures positive declutching and only line contact contraction of plates
results in the disengaged position, thus ensuring minimum
torque and heating when idling.

Figure 7.17 Friction plates with sine wave cross-
section
B7.6