Why we have a clutch in the car?
It has been seen that the internal-combustion engine, unlike the steam engine, does not produce high power at low speeds; therefore the engine must be rotating at a speed at which sufficient power is developed, before the drive to the wheels is established.
This condition rules out the use of dog clutch since the connection of rotating engine to a stationary transmission shaft would damage the transmission and jolt the vehicle.
The clutch used must allow the drive to be taken up smoothly so that the vehicle can be gradually moved away from the stationary position.
Once moving, it will be necessary to change gear, and so a disengagement of the engine or transmission is required.
This is also is part of the clutch’s function.
These two duties can be performed by various mechanisms; the friction system is considered to be one of the most effective and efficient.
The friction clutch
The objectives of a friction clutch are:
- to connect a stationary machine part to a rotating part,
- to bring it up to speed,
- to transmit the required power with a minimum of slippage,
- serves as a safety device by slipping when the torque transmitted through it exceeds a safe value, thus preventing the breakage of parts in the transmission train.
The purpose of the automotive clutch:
- to be used in cars with transmission that are shifted by hand (manual transmission),
- it allows the driver to couple the engine to, or uncouple the engine from the transmission.
The clutch is located just behind the engine, between the engine and the transmission.
The simplest clutch is made up of two plates forced together by powerful springs forming, in effect, one part, linking the engine to the transmission system.
Types of Clutches according to the following:
* In modern automobile the common used clutch is:
a single or double dry friction disc with a diaphragm
spring, manually operated, and with hydraulic or electrical control.
Selection of Clutch Type:
The factors which must be taken into consideration in deciding what type of clutch is to be used are:
Torque (normal force, type of friction surfaces and number of surfaces)
Rotation speed (light, compact, internally balanced)
Available space (diameter, height)
Frequency of operation (small travel, a simple engaging mechanism, large cooling area, low inertia)
Parts of Automobile Friction Clutch
- Clutch Cover
- Pressure Plate
- Driven Plate (friction plate)
- Thrust Spring (diaphragm)
- Clutch Housing
- Release Lever (clutch fork)
- Ball Bearing (graphite block)
- Primary Shaft (gearbox input shaft)
- Pedal Linkage
The flywheel is the mounting surface for the clutch. The pressure plate bolts to the flywheel face. The clutch disc is clamped and held against the flywheel by the spring action of the pressure plate. The face of the flywheel is precision machined to a smooth surface. The face of the flywheel that touches the clutch disc is made of iron. Even if the flywheel were aluminum, the face is iron because it wears well and dissipates heat better.
The pilot bearing or bushing is pressed into the end of the crankshaft to support the end of the transmission input shaft. The pilot bearing is a solid bronze bushing, but it also may be a roller or ball bearing. The end of the transmission input shaft has a small journal machined on its end. This journal slides inside the pilot bearing. The pilot bearing prevents the transmission shaft and clutch disc from wobbling up and down when the clutch is released. It also assists the input shaft center the disc on the flywheel.
The flywheel is connected to the engine crankshaft via bolts, the flywheel surface facing the gearbox is used as a friction surface for the clutch driven plate, and the cultch cover is bolted on that surface and rotate with it. At the center of the flywheel is fitted a spigot bearings, which locates the front end of the gearbox primary shaft and allows for the difference in speed between the two members. This bearing (pilot bearing) can take the form of a ball race of plain bush (aligning the axis). Clutch pilot bearing is a part of the transmission system and is located at the end of the crankshaft and center of the flywheel. Transmission input shaft is connected to the flywheel center with the help of clutch pilot bearing. With the help of bearing the relative motion between the transmission input shaft and flywheel becomes smooth and power loss becomes zero.
Flywheels are often used to provide continuous energy in systems where the energy source is not continuous. In such cases, the flywheel stores energy when torque is applied by the energy source and it releases stored energy when the energy source is not applying torque to it. For example, a flywheel is used to maintain constant angular velocity of the crankshaft in a reciprocating engine. In this case, the flywheel—which is mounted on the crankshaft—stores energy when torque is exerted on it by a firing piston, and it releases energy to its mechanical loads when no piston is exerting torque on it. The hot gases expand, pushing the piston to the bottom of the cylinder. The piston is returned to the cylinder top (Top Dead Centre) by a flywheel, A flywheel is often used to ensure smooth rotation or to store energy to carry the engine through an un-powered part of the cycle.
A single cylinder 4-stroke engine shall require larger flywheel as compared to the multi-cylinder engine. For slow speed engine also the flywheel required is larger in size because of high value of I required, and vice versa.
The engine flywheel is fitted with a large gear ring around its rim. To start the engine the starter pinion gear is engaged with the flywheel ring gear to rotate the crank shaft.
Flywheel construction can differ in both materials and in design. Lighter, harder alloys are more likely to be used in high-performance cars, whereas the most common material in an average car is cast iron. Aluminum and forged steel are also used. Carbon fiber flywheels are currently being developed for race applications as well, but these are mega-expensive and are only likely to be seen in super-exotic factory cars.
Cast iron, of which it is usually made, is the best material for use with asbestos-base friction facings:
- Its graphite particles have a lubricating effect which prevents scoring and abnormal wear.
- Its great heat-absorbing capacity.
Dual mass flywheel (DMF):
The dual mass flywheel eliminates excessive transmission gear rattle, reduces gear change/shift effort, and increases fuel economy. The function of the dual mass flywheels is to isolate torsion crankshaft spikes created by diesel engines with high compression ratios.
Dual mass flywheels are designed to provide maximum isolation of the frequency below the engine's operating RPM, usually between 200-400 RPM. They are also most effective during engine startup and shutdown.
A DMF is built-up from two flywheels that are about the same diameter as a single flywheel would be, so each one will have about one-half the mass of a single flywheel The first flywheel is attached to the crankshaft and spigotted into the second flywheel in such a way that the two ‘wheels are able to oscillate with respect to each other
The cover is made by cold forming of 2.5-4 mm thick sheet steel. It is then aligned relatively to the flywheel axis with the aid of locating pins, collars or bolts.
* the clutch housing should ensure good airing:
- for cooling the friction surfaces
- to remove the wear products from them.
Openings are made in the housing subject to the condition of housing desired rigidity.
It also made of cast iron. The plate, as well as every other rotating part of the clutch has a flywheel effect. The plate made stiffer to distribute the pressure more evenly. Provide with more heat capacity and radiating surface.
The pressure plate must be driven from the flywheel by a number of tempered spring-steel straps, one end of each of which is riveted to the cover, while the other end is bolted to pressure plate. The straps are equally spaced and extended tangentially.
Pressure plate assemblies:
The two primary types of pressure plate assemblies are coil spring assembly and one with a diaphragm spring.
In a coil spring clutch the pressure plate is backed
by a number of coil springs and housed with them in a pressed-steel cover
bolted to the flywheel. The springs push against the cover.
Diaphragm spring pressure plate assemblies are widely used in most modern cars. The diaphragm spring is a single thin sheet of metal which yields when pressure is applied to it.
Diaphragm spring forms a convex or dished shape while in unloaded state. Clamping the clutch to the flywheel flattens the spring and provides the necessary thrust on the pressure plate.
The advantage of diaphragm spring clutches:
- lower pedal force required,
- lower weight,
- less effort to disengage,
- reduce rotational imbalance,
- uniform distribution of radial force,
- suitable for extra-high engine speeds, constant spring thrust and accurate balance are maintained,
- no separate release levers are required, giving improved release efficiency
- fewer parts required,
- spring load remains approximately constant as the facing wears,
- compact design, (decrease the clutch clearance limits and the clutch mass due to overlap of the function of the pressure spring and disengaging lever.
The disadvantages of the diaphragm springs:
- it is difficult to manufacture diaphragm spring for large axial forces.
Driven Plates (clutch disc):
Among the numerous criteria used to determine clutch size and clamp load configuration, maximum engine torque and the resulting friction energy are especially significant. The greater the clamp load, the smaller the friction radius can be. The diameter should be as small as possible, because it greatly affects clutch weight and cost. But the clutch disc also has to be large enough to handle thermal loads and facing wear.
The clutch disc, also called friction lining, consists of a splined hub and a round metal plate covered with friction material (lining). The splines in the center of the clutch disc mesh with the splines on the input shaft of the manual transmission. This makes the input shaft and disc turn together. However, the disc is free to slide back and forth on the shaft.
Clutch disc torsion springs, also termed damping springs, absorb some of the vibration and shock produced by clutch engagement. They are small coil springs located between the clutch disc splined hub and the friction disc assembly. When the clutch is engaged, the pressure plate jams the stationary disc against the spinning flywheel. The torsion springs compress and soften, as the disc first begins to turn with the flywheel.
Clutch disc facing springs, also called the cushioning springs, are flat metal springs located under the friction lining of the disc. These springs have a slight wave or curve, allowing the lining to flex inward slightly during initial engagement. This also allows for smooth engagement.
The clutch disc friction material, also called disc lining or facing, is made of heat-resistant asbestos, cotton fibers, and copper wires woven or molded together. Grooves are cut into the friction material to aid cooling and release of the clutch disc. Rivets are used to bond the friction material to both sides of the metal body of the disc.
Most clutch facing have a base of asbestos. The number of friction surfaces is equal to twice the number of driven discs.
Large driven plates have a tendency to spin (i.e. to continue rotating after the clutch pedal is depressed). To limit this trouble, the plate should be made as light as possible.)
A material suitable for use as a friction surface must meet the following conditions:
- It must have a high coefficient of friction
- It must not be affected by moisture and oil
- It must resist wear
- It must be capable of resisting high temperatures caused by slippages
- It must be capable of resisting high axial pressure
- should have the given rapture strength,
* There are groves in both sides of the friction-disc facings. These grooves prevent the facings from sticking to the flywheel face and pressure plate when the clutch is released. The groves break any vacuum that might form and cause the facing to stick to the flywheel or pressure plate.
** The facing or lining on the driven plate are secured by brass rivets, the heads being recessed into the lining to prevent scoring of the flywheel and pressure plate faces. As the lining wear down, the inner ends of the release leaver move away from the flywheel and after a predetermined a mount of wear has taken place; the lever will touch the cover.
Cushioning plate (center plate):
The cushioning plate, on which the friction facing are mounted, consists so series of cushion springs which is crimped radially.
During engagement, axial compression of the driven plate spreads the engagement over a great range of pedal travel and therefore makes it easier to make a smooth engagement.
During the disengagement, when the pedal is depressed, the clamping force will be released and the plate springs return back to its original position crimped (wavy) state, and that will get the driven plate to jump away from the flywheel to give ‘clear’ disengagement. While in this position, the linings will be held apart, and air will be pumped between the linings to take away the heat.
This plate is also slotted so that the heat generated does not cause distortion that would be liable to occur if it were a plain plate. This plate is of course thin to keep rotational inertia to a minimum.
Driven plate torsion springs:
The plate and its hub are entirely separate components, the drive being transmitted from one to the other through coil springs interposed between them. These springs are carried within rectangular holes or slots in the hub and plate and arranged with their axes aligned appropriately for transmitting the drive. These dampening springs are heavy coil springs set in a circle around the hub. The hub is driven through these springs. They help to smooth out the torsional vibration (the power pulses from the engine) so that the power flow to the transmission is smooth.
In a simple design all the springs may be identical, but in more sophisticated designs they are arranged in pairs located diametrically opposite, each pair having a different rate and different end clearances or using dual springs, where a smaller spring inside the original one.
The role of dual spring is progressively providing increasing spring rate to cater to wider torsional damping. Also, to avoid “plate float” which could happened when using identical springs with same rate (stiffness), plate float will happen if the vibration and torque variation became equal to the natural (resonance) frequency of the spring. Plate float will have negative effect on the transmission gears (gear rattling).
Driven plate torsional dampers:
It consists of friction plate and washer, to reduce gear rattling. The friction between the friction plate and the washer will damp the torsional vibration (as the roll of shock absorber in the car suspension).
Transition (gearbox) input shaft:
The clutch plate is assembled on a splined shaft that carries the rotary motion to the transmission. This shaft is called the clutch shaft, or transmission input shaft. This shaft is connected to the gear box or forms a part of the gear box.
Clutch operating system:
There are two common types used in passenger cars today. The mechanical cable operated system and the more modern and efficient the hydraulic clutch activation system.
Hydraulic clutch activation systems consist of a master and a slave cylinder. When pressure is applied to the clutch pedal (the pedal is depressed), the pushrod contacts the plunger and pushes it up the bore of the master cylinder. During the first 1/32 in. (0.8 mm) of movement, the center valve seal closes the port to the fluid reservoir tank and as the plunger continues to move up the bore of the cylinder, the fluid is forced through the outlet line to the slave cylinder mounted on the clutch housing. As fluid is pushed down the pipe from the master cylinder, this in turn forces the piston in the slave cylinder outward. A pushrod is connected to the slave cylinder and rides in the pocket of the clutch fork. As the slave cylinder piston moves rearward the pushrod forces the clutch fork and the release bearing to disengage the pressure plate from the clutch disc. On the return stroke (pedal released), the plunger moves back as a result of the return pressure of the clutch. Fluid returns to the master cylinder and the final movement of the plunger lifts the valve seal off the seat, allowing an unrestricted flow of fluid between the system and the reservoir.
The advantages of hydraulic clutch actuation system:
1) Self adjusting to a point.
2) Less effort compared to a mechanical clutch.
3) Self-lubricating whereas cables need to be lubricated time to time.
Clutch Release Mechanism
A clutch release mechanism allows the operator to operate the clutch. Generally, it consists of the clutch pedal assembly, either mechanical linkage, cable, or hydraulic or drive by wire.
If a vehicle has a mechanically operated linkage, it will incorporate either a shaft- and-lever linkage arrangement or a cable.
Systems that are made up of linkages, levers and pivot points are found primarily on older vehicles. These systems require regular lubrication and can only be designed to fit a limited range of configurations.
A cable operated clutch mechanism is relatively simple. A cable connects the clutch pedal directly to the clutch release fork. This simple design is flexible and compact. There is however, a tendency for cables to gradually stretch and eventually break due to age and wear.
Mechanical systems may use a system of levers but cable operation gives more flexibility and is more common.
A hydraulic clutch release mechanism (fig. 4-5) uses a simple
hydraulic circuit to transfer clutch pedal action to the clutch fork. It has
three basic parts- master cylinder, hydraulic lines, and a slave cylinder.
Movement of the clutch pedal creates hydraulic pressure in the master cylinder, which actuates the slave cylinder. The slave cylinder then moves the clutch fork.
Clutch-by-Wire, the unit replaces the mechanical link between clutch and pedal with an electrical clutch actuator, an electric clutch pedal and an electronic control unit (ECU). A pedal sensor measures the position of the clutch pedal and transmits this information to the ECU which also receives information about car behavior. The ECU in turn controls the clutch actuator and depending upon the driver’s wishes, the system can not only correct driver mis-operations but offer complete clutch automation. The system is designed to require lower stroke and effort to the pedal and improves pedal feel with “virtual” resistance to foot pressure. More compact than a conventional clutch actuation, the Clutch-by-Wire system improves driver crash protection since it enables an optimized, less intrusive, pedal box design
The clutch housing is also called the bell housing. It bolts to the rear of the engine, enclosing the clutch assembly, with the manual transmission bolted to the back of the housing. The lower front of the housing has a metal cover that can be removed for fly-wheel ring gear inspection or when the engine must be separated from the clutch assembly. A hole is provided in the side of the housing for the clutch fork. It can be made of aluminum, magnesium, or cast iron.
Every transmission needs to be equipped with a clutch housing in order to perform its function in the vehicle chassis. The clutch housing performs five roles for the vehicle. The first, or the primary role of the clutch housing is to act as mating and mounting device to fasten the transmission to the engine. The secondary of the clutch housing is to act as enclosure. It encloses the master clutch assembly and protects it from road dirt, grim, and exterior influences. The third function of the clutch housing is to provide mounting support. Depending on the clutch housing casting formation, it may be equipped with two machined exterior to attach mounting pads to the vehicle frame rail. These areas are identified as nodes and provide a “node mount” capability. Nod mounts not only provide support for the transmission case, but also provide stability for the engine. The forth role of the clutch housing is to provide a pivot point for the release of the master clutch assembly. It provides this through two cross-shaft bores which allow insertion of cross-shaft to provide a pivot for the master clutch release fork. The final roll of the clutch housing is to provide access to the clutch for adjustment.
The clutch fork, also called a clutch arm or release arm, transfers motion from the release mechanism to the release bearing and pressure plate. The clutch fork sticks through a square hole in the bell housing and mounts on a pivot. When the clutch fork is moved by the release mechanism, it PRIES on the release bearing to disengage the clutch.
A rubber boot fits over the clutch fork. This boot is designed to keep road dirt, rocks, oil, water, and other debris from entering the clutch housing.
The release bearing, also called the throw-out bearing, is a ball bearing and collar assembly. It reduces friction between the pressure plate levers and the release fork. The release bearing is a sealed unit pack with a lubricant. It slides on a hub sleeve extending out from the front of the manual transmission or transaxle.
The release bearing snaps over the end of the clutch fork. Small spring clips hold the bearing on the fork. Then fork movement in either direction slides the release bearing along the transmission hub sleeve.