30-06-2012, 01:35 PM
STUDY OF CLUTCHES
STUDY OF CLUTCHES.doc (Size: 1.9 MB / Downloads: 61)
INTRODUCTION :
A clutch is a mechanical device which provides for the transmission of power (and therefore usually motion) from one component (the driving member) to another (the driven member). The opposite component of the clutch is the brake.
Clutches are used whenever the ability to limit the transmission of power or motion needs to be controlled either in amount or over time (e.g. electric screwdrivers limit how much torque is transmitted through use of a clutch; clutches control whether automobiles transmit engine power to the wheels).
In the simplest application clutches are employed in devices which have two rotating shafts. In these devices one shaft is typically attached to a motor or other power unit (the driving member) while the other shaft (the driven member) provides output power for work to be done. In a drillfor instance, one shaft is driven by a motor and the other drives a drill chuck. The clutch connects the two shafts so that they may be locked together and spin at the same speed (engaged), locked together but spinning at different speeds (slipping), or unlocked and spinning at different speeds (disengaged).
CLASSIFICATION OF CLUTCHES:
POSITIVE CLUTCHES:
A Positive (Jaw) clutch consists of 2 segments one is permanently fastened to the driving shaft & second segment is free to slide axially on splined driven shaft. Facing members consists of square/spiral/gear shape teeth that locks in to mating recesses. Square jaws will transmit power when rotating in either direction Engagement & disengagement is done by sliding jaws.It provides positive lockup, but because it can’t slip, running engagement is limited to speed under 10 rpm.
Spiral jaw clutches couplings are made either right or left hand. A right hand clutch drives when turning clockwise as viewed from the plain end of the driving half. Spiral jaws permit shifting more readily, and are sometimes used where the clutch must be engaged or disengaged when in motion.
Advantages
Positive clutches doesn’t slips.
No heat is generated at clutch surface during engagement.
Disadvantages
Shock & noise generation when it’s engaged in motion.
They cannot be engaged at high speeds, sometimes cannot be engaged at rest unless the jaws are aligned.
Applications
Positive clutches are used where synchronize operations are required like in Power presses, Punches.
FRICTION CLUTCHES:
A friction clutch has its principle applications in the transmission of power of shaft and machines which must be started and stopped frequently. It’s application is also found in case in which a power is to be delivered to machines partially or fully loaded. The force of friction is used to start the driven shaft from restand gradually bring it up to the proper speed without excessive slipping of the friction surfaces. In automobiles, friction clutch is used to connect the engine to the driven shaft. In operating such a clutch, care should be taken so that friction surfaces engages easily and gradually brings the driven shaft up to proper speed. The proper alignment of the bearing must be maintained and it should be located as close to the clutch as possible.
1. The contact surfaces should develop a frictional force that may pick up and hold the load with reasonably low pressure between the contact surfaces.
2. The heat of the friction should be rapidly dissipated and tendenvy to grab should be at a minimum.
3. The surfaces should be packed by material stiff enough to ensure a raesonably uniform distribution of pressure.
The friction clutches are of following types:
1. Disc or plate clutches
2. Cone clutches
SINGLE PLATE CLUTCH:.
A disc clutch consist of a cluct plate attached to a splined hub which is free to slide axially on splines cut on the driven shaft. The clutch plate is made of steel and has a ring of friction lining on each side. The engine shaft supports a rigidly fixed flywheel.
A spring loaded pressure plate presses the clutch plate firmly against the flywheel when the clutch is engaged. When disengaged, the spring press against a cover attached to the flywheel. Thus, both the flywheel and pressure plate rotate with the input shaft. The movement of the clutch pedal is transferred to the pressure plate through a thrust bearing.
Fig shows the pressure plate pulled back by the release levers and the friction linings on the clutch plate are no longer in contact with the pressure plate or the flywheel. The flywheel rotates without driving the clutch plate and thus, the driven shaft.
When the foot is taken off the clutch pedal, the pressure on the thrust bearing is released. As a result of the spring become free to move the pressure plate to bring it in contact with the clutch plate. The clutch plate slides on splined hub and is tightly gripped bet the pressure plate and flywheel. The friction bet the linings on the clutch plate and flywheel on one side and the pressure plate on the other.