24.6.
Comparison of Multi-coil and Diaphragm Spring Clutches
Clamping Characteristics.
Figure 24.9 illustrates the linear relationship between load and deflection of a multi-coil
clutch. As the springs are compressed (deflected) in a multi-coil clutch, and compressive clamping load increases proportionally. This provides a linear relationship between load and deflection (Fig. 24.9).
On the other hand, a non-linear relationship is exhibited between load and deflection of a diaphragm-spring clutch. The shape of the load-deflection curve mainly depends on the ratio hit, the dish height (h) in the free state to the thickness (t) of the diaphragm spring for a given spring size (Fig. 24.10A and B). In the figure W is diaphragm load and 0 is the cone-angle of the diaphragm.
The load-deflection characteristics, illustrated in Fig. 24. IOC, shows the variation of clamping load with the ratio dlt, the spring deflection (d) to diaphragm thickness (t) for five different ratios of hit. The spring load increases approximately linearly upto a deflection equal to the thickness of the spring (point A). Beyond this point, the characteristics differ for different ratios of hit. For hit = 1 the spring load increases at a greater rate and for hit = 3 the compressive load progressively drops with further deflection. Between these two extremes, for hit = 1.5, 2.0 and 2.5, the clamping load decreases with more spring distortion, but indicating a tendency to bottom out and even to increase again as it nears its full movement. Thus the load-deflection characteristic of the spring can be varied to suit the application, a typical value for clutches being 1.7.
Fig. 24.9. Deflection vs load for a multi-coil spring.
Fig. 24.10. Diaphragm spring characteristics.
Relationship between Driven-plate Wear and Clamping Thrust.
Coil springs provide maximum compressive clamping load when the driven-plate is new with maximum lining thickness (Fig. 24.11). As lining wears, the coil springs extend and accommodate the distance between the pressure-plate and the cover-pressing. This causes the spring to progressively loose its compressive thrust.
Fig. 24.11. Relationship between driven-plate thickness and clamping load for coil
and diaphragm springs.
In contrast, when the driven-plate is new the diaphragm spring is practically flattened, due to which it cannot exert its maximum clamping load on to the pressure-plate. As the lining wears the dishing of the diaphragm increases, so that the load acting on the pressure-plate increases effectively. However, this increased diaphragm thrust is counteracted by the gap caused between the pressure-plate and the cover-pressing, which gradually enlarges to such an extent that the compressive force of the spring becomes over-relaxed; so that the pressure-plate loading decreases with further driven-plate wear.
Merits of the Diaphragm Spring over Multi-coil Spring.
(a) The diaphragm spring is compact permitting the use of a shallow clutch bell-housing to enclose the clutch unit.
(b) Due to fewer moving parts squeaks, rattles, and wear are eliminated in diaphragm spring.
(c) This system does not require initial adjustment of the pressure-plate unit unlike the multi-coil spring clutch units, where a small clearance is necessary between the release-lever plate and the thrust bearing.
(d) In this design accurate balance of the clutch assembly is maintained under all operating conditions.
(e) The diaphragm acts as both clamping spring and release-finger.
(f) As the driven-plate wears, the spring axial load self-compensates in this clutch.
(g) Clamping load in diaphragm-spring is independent of the engine speed whereas coil springs tend to bend along their length and loose their thrust at high speeds.
(h) In this clutch disengagement pressures reduce with increase in pedal movement.
