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Assignment on Clutch

Aug 10, 2014

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Automotive

salim-azad

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.

  • Topic: Clutch 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. Clutch for a drive shaft: The clutch disc (center) spins with the flywheel (left). To disengage, the lever is pulled (black arrow), causing a white pressure plate (right) to disengage the green clutch disc from turning the drive shaft, which turns within the thrust-bearing ring of the lever. Never will all 3 rings connect, with no gaps. 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 drill for 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). The rest of this article is dedicated to discussions surrounding types of clutches, their applications, and similarities and differences of such.
  • Fig: Rear side of a Ford V6 engine, looking at the clutch Fig: Single, dry, clutch friction disc. Friction Clutches Friction clutches are by far the most well-known type of clutches. A clutch is a device used to transmit the rotary motion of one shaft to another when desired. The axes of the two shafts are coincident. In friction clutches, the connection of the engine shaft to the gear box shaft is affected by friction between two or more rotating concentric surfaces. The surfaces can be pressed firmly against one another when engaged and the clutch tends to rotate as a single unit. Materials Various materials have been used for the disc friction facings, including asbestos in the past. Modern clutches typically use a compound organic resin with copper wire facing or a ceramic material. A typical coefficient of friction used on a friction disc surface is 0.35 for organic and 0.25 for ceramic. Ceramic materials are typically used in heavy applications such as trucks carrying large loads or racing, though the harder ceramic materials increase flywheel and pressure plate wear. Push/Pull Friction disk clutches generally are classified as "Push Type" or "Pull Type" depending on the location of the pressure plate fulcrum points. In a pull type clutch, the action of pressing the pedal pulls the release bearing, pulling on the diaphragm spring and disengaging the vehicle drive. The opposite is true with a push type, the release bearing is pushed into the clutch disengaging the vehicle drive. In this instance, the release bearing can be known as a thrust bearing (as per the image above).
  • Pads Clutch pads are attached to the frictional pads, part of the clutch. They are most commonly made of rubber but have been known to be made of asbestos. Clutch pads usually last about 100,000 miles (160,000 km) depending on how vigorously the car is driven. Dampers In addition to the damped disc centres which reduce driveline vibration, pre-dampers may be used to reduce gear rattle at idle by changing the natural frequency of the disc. These weaker springs are compressed solely by the radial vibrations from an idling engine. They are fully compressed and no longer in use once drive is taken up by the main damper springs. Load Mercedes truck examples: A clamp load of 33KN (33,000N) is normal for a single plate 430. The 400 Twin application offers a clamp load of a mere 23KN (23,000N). Bursts speeds are typically around 5,000rpm with the weakest point being the facing rivet. Manufacturing Modern clutch development focuses its attention on the simplification of the overall assembly and/or manufacturing method. For example drive straps are now commonly employed to transfer torque as well as lift the pressure plate upon disengagement of vehicle drive. With regards to the manufacture of diaphragm springs, heat treatment is crucial. Laser welding is becoming more common as a method of attaching the drive plate to the disc ring with the laser typically being between 2-3KW and a feed rate 1m/minute. A frictional clutch has its principal application in the transmission of power shafts and machines which must be started and stopped frequently. Its application is also found in cases in which power is to be delivered to machines partially or fully loaded. The force of friction is used to start the friction surfaces in automobiles, friction clutches are used to connect the engine to the driven shaft. In operating such a clutch, care should be taken so that the friction surfaces engage easily and gradually brings the driven shaft up to the proper speed. The proper alignment of the bearing must be maintained and it should be located as close to the clutch as possible. It may be noted that 1. The contact surfaces should develop a frictional force that may pick uop and hold the load with reasonably low pressure between the contact surfaces. 2. The heat of friction should be rapidly dissipated and tendency to grab should be at a minimum. 3. The surfaces should be backed by a material stiff enough to ensure a reasonably uniform distribution of pressure.
  • The friction clutches of the following types are important from the subject point of view: 1. Disc or plate clutches (single disc or multiple disc clutch) 2. Cone clutches 3. Centrifugal clutches Disc or plate clutches Single-plate clutches A single-plate clutch unit consists of a friction type disc, a pressure plate assembly, and a release bearing and operating fork. Fig: Single plate clutch Most light vehicles use a single-plate clutch to transmit torque from the engine to the transmission input shaft. The flywheel is the clutch driving member. The clutch unit is mounted on the flywheels machined rear face, so that the unit rotates with the flywheel. The clutch unit consists of - a friction-type disc, with 2 friction facings and a central splined hub - a pressure plate assembly, consisting of a pressed steel cover, a pressure plate with a machined flat face, and a segmented diaphragm spring. And a release bearing and operating fork. The friction disc is sandwiched between the machined surfaces of the flywheel and the pressure plate when the pressure plate is bolted to the outer edge of the flywheel face. The clamping force on the friction facings is provided by the diaphragm spring. Unloaded, it is a dished shape. As the pressure plate cover tightens, it pivots on its fulcrum rings, and flattens out to exert a force on the pressure plate, and the facings.
  • The transmission input shaft passes through the center of the pressure plate. Its parallel splines engage with the internal splines of the central hub, on the friction disc. With engine rotation, torque can now be transmitted from the flywheel, through the friction disc, to the central hub, and to the transmission. Fig: Single disc or plate clutch A disc clutch consists of a clutch plate attached to a splined hub which is free to slide axially on splines cut on the drive 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 springs press against a cover attached to the flywheel. Thus, both the flywheel and the pressure plate rotate with the input shaft. The movement of the clutch pedal is transferred to the pressure plate through a thrust bearing. Figure 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, the springs become free to move the pressure plate to bring it in contact with the clutch plate. The clutch plate slides on the splined hub and is tightly gripped between the pressure plate and the flywheel.
  • The friction between the linings on the clutch plate, and the flywheel on one side and the pressure plate on the other, cause the clutch plate and hence, the driven shaft to rotate. In case the resisting torque on the driven shaft exceeds the torque at the clutch, clutch slip will occur. Fig: Forces on a single disc or plate clutch Multiple plate clutches This type of clutch has several driving members interleaved or "stacked" with several driven members. It is used in race cars including F1, IndyCar, World Rally and even most club racing, motorcycles, automatic transmissions and in some diesel locomotives with mechanical transmissions. It is also used in some electronically controlled all-wheel drive systems. Fig: Multiple plate clutches
  • In a multi-plate clutch, the number of frictional linings and the metal plates is increased which increases the capacity of the clutch to transmit torque. Figure above shows a simplified diagram of a multi-plate clutch. The friction rings are splined on their outer circumference and engage with corresponding splines on the flywheel. They are free to slide axially. The friction material thus, rotates with the flywheel and the engine shaft. The number of friction rings depends upon the torque to be transmitted. Th