VACUUM SERVO BRAKES

When the vacuum is obtained from the manifold of the engine or a separately driven exhauster used to assist the braking effort the system is called vacuum servo-brakes.  The system consists of a vacuum reservoir connected through a non-return valve to the inlet mani­fold of the engine.  Two connections from vacuum reservoir, one on each side of the piston of the servo cylinder is provided; on left side the connection is through the control unit where as right side is connected directly.  The piston of the servo cylinder' is further connected to the piston of the boost cylinder or the brake linkage.  The control unit consists of a piston to which two valves are attached.  The lower valve controls the connection between the reservoir and the right side of the servo cylinder piston.  The upper valve controls the connection between the atmosphere and the left side of the servo cylinder piston.  The other side of the piston of the control unit is actuated by the pedal effort through a master cylinder.

When the brake pedal is at off position then the lower value is opened and the upper valve is closed.  Under this position the air from the atmosphere is disconnected and the vacuum from reservoir is created on both the sides of the piston of the servo cylinder.  When the brake pedal is depressed the brake fluid pushes the piston in the control unit.  This action closes the lower valve and opens the upper valve of the control unit, thereby -exposing the left side of the servo cylinder piston to atmospheric pressure and acting the vacuum on the right side.  This action of the vacuum in the right side of the servo piston moves it to the right thereby utilizing this movement through the mechanical or hydraulic means to the wheel cylinders and applying the brakes.  In this way the driver effort is utilized to control the positions of valves of the control unit and the vacuum effort is applied for braking through the booster unit.

MULTI PLATE CLUTCH

The clutch having more than three discs is referred as multi disc clutch or multi plate clutch. It is similar to single plate clutch but has more number of frictional and metallic plates. Due to the increase in the numbers of plates (friction) the frictional surface in contact is also increased which increases the capacity of the clutch to transmit the some torque the diameter of the plate clutch. Hence the clutches are mostly commonly in two wheelers and three wheelers due to compact in size. It is used in heavy duty transmission system for transmitting higher torque. (For example torque transmission in heavy earth moving equipments) and power take off (P. T. O.) transmission in tractors.

 

Construction: Construction of multi plate clutches is similar to single plate clutch except the arrangement of number of friction plates and metal plates. It consists of inner drum which is referred to clutch shaft and has a number of plates splined to the outer surface. Another drum is coupled to fly wheel and carries a number of plates splined to it inner surface. The plates are arranged in alternate manners. The plates can revolve with the drum as well as it can slide axially. A spring keeps the outer and inner plates pressed together, so that the driving members transmit the power to the driven member. The clutches can be disengaged by pulling the inner drum against the spring force.

      Multi plate clutch can be dry type or wet type. The clutch is partially filled with oil. The coefficient of friction in oil varies from 0.07 to 0.17 of asbestos based fabrics.

1. The oil acts as cushioning  medium to provide smooth engagement and disengagement
2. The oil also carries the heat dissipated by the clutch due to friction. This reduces operations temperature and increases the left of the clutch plates.
3. The oil acts as lubricant and reduces axial thrust lost due to bending on splines.

The major disadvantage is the reduction in coefficient of friction when immersed in oil. It can be compensated by using high operating pressure of different friction material. Generally cork inserted multi plate clutches are used in wet clutches. In wet clutches the fluid under pressure is fed along the shaft.

DIAPHRAGM CLUTCH

In this type of diagram type springs are used instead of coil / helical springs. This type of clutch does not require any release levers as the spring itself acts as the series of levers. This type of springs do not have constant rate characteristics as in the case of coil springs and the pressure on the diaphragm springs increases until it is in flat position, thereafter decreases after passing this position. Hence the driver does not have to exert heavy pedal pressure to hold the clutch out of engagement compared to coil spring type. In coil spring type the spring pressure increases when the pedal is depressed to disengage the clutch and high pressure is required to keep the clutch in disengaged position.

This clutch consists of conventional friction clutch, thrust plate, diaphragm type spring and release sleeve. The diaphragm is held between the inner end of the main bearing and its outer circumstance fits into the counter bore of the thrust plate. The central position of the diaphragm spring is divided into several segments by radial slots terminating into holes. These segments acts like spring providing the required thrust on the pressure plate. This simple arrangement eliminates the necessity for providing separate release levers.

Working:  In the engaged position the spring pivots on the inner pivot rings as it is held on the clutch cover so that its outer rings contacts with the pressure plate. Again in this conical position the spring exerts through pressure to keep the pressure plate in firm contact with the clutch plate and flywheel. When the pedal is depressed the linkage moves release bearing toward the flywheel. When the pedal is depressed the linkage moves release bearing towards the flywheel to disengage the clutch. As the bearing contacts with inner position of the conical springs it moves that position forward which cause the link to move backward. This removes the pressure on the pressure plate and release the clutch plate from contact with other driving members.

Another type of conical spring used is the crown spring. This type differs from the tapered finger type with its surface corrugated instead of flat and the centre section is continuous without any spring. The clutch spring fits between the pressure plate and clutch cover. The entire assembly is held together by six spring retainer located on the pressure plate. The actuation of this type of spring is similar to integral / split type diaphragm spring

SINGLE PLATE CLUTCH

Driving Members:  Driving members of single plate clutch are input shaft (crank shaft) fixed to the flywheel, pressure plate, and the clutch cover which is bolted to the flywheel. These fly components rotate along with the crank shaft in both engaged and disengaged condition. Fly wheel is attached to the crank shaft and has threaded bolts or holes or grooves for bolting the clutch covers. Machined surface of the flywheel contacts the clutch facing. The pressure plate applies the required force on the clutch plate which contacts with fly wheel.

To apply the required force pressure springs are attached between the pressure plate and clutch cover. Pressure plate can be withdrawn by releasing the spring with the help of release lever. Lugs are provided on the pressure plate for providing the release fingers. Pressure plate springs are provided inside, release finger, and anti-ratting springs are provided inside the clutch cover and the entire assembly is bolted to fly wheel.

Driven members: It is the clutch plate which is splined to the driven shaft (clutch shaft or input shaft of gear box) clutch plate is used with friction material on both the surfaces. It consists of a centre hub with internal splines which moves along the splined shaft during the transmission. The power is transmitted from the clutch to the shaft through these splines. It consists of torsional or cushioning springs which transmit the force applied to the facing to the central hub. The spring also reduces torsional vibrations and provides smooth engagement or disengagement of the clutch. The friction material is normally riveted to the projected portions of the clutch disc in CMVS and HMVS.

Actuating members: It consists of release fingers, withdrawal fork and release bearing. The outer end of the release finger is located on the pressure plate and inner end is projected towards the clutch shaft and are positioned with the help of anti-rattling springs. Withdrawal fork carrying the release bearing is pivoted in the clutch outer casing. The release bearing actuates the inner end of the release fingers.  

In fully engaged condition the driven plate is firmly clamped between the flywheel and pressure plate due to the force applied by springs. This forms a non-slip connection between the driving and driven plates. Hence when the flywheel rotates the clutch plate also rotates and this cause the transmission of power to the input shaft of gear box through splines. When clutch pedal is depressed the pressure on the driven plate is released by compressing the pressure springs through the release fingers. In this condition there is no force acting on the clutch plate and is free between the flywheel and pressure plate. This disengaged condition ensures easier shifting of gears.