What materials are used in a clutch?

Description of a car clutch:

For a car to be operational at different speeds and be able to respond efficiently and reliably with reference to different driving styles; a variety of power outputs are required. This variety of power is made possible through the use of different gears. These gears range from very low ratios which are required to start the cars motion, to higher gears which is necessary to maintain or increase the velocity of the car (which requires less energy). A clutch is a mechanical device which is used to transmit power from a certain component to another, in this case changing from one gear ratio to another.  A clutch controls this transmission of power when it is engaged; the clutch can be engaged and disengaged at any time.

During the engagement period a certain amount of friction is produced; sue to this fact it is necessary that the materials used to produce clutches must be able to withstand a certain amount of friction and therefore be able to tolerate heat. The amount of friction may vary from vehicle to vehicle as more friction is produced by vehicles which are much larger and require movement of heavier loads; for this reason a number of materials are used to produce these clutches depending on the specific type of vehicle. The main materials used for clutches can be summed up to organic and ceramic materials, where organic materials are used for everyday use vehicles and ceramic materials are used for much larger vehicles and racing cars.

Materials used to manufacture car clutches:

There are a number of different materials that are used to manufacture car clutches including steel, aluminium, cotton fabrics, polymers, vegetable fibres, fibre glass and asbestos. Asbestos has been classified as unsafe and is generally not as common in modern day clutched.

The following table gives a brief overview of formulation trends for brake pads; this is relative to materials used in car clutches as they both require materials that can withstand friction. 

 

Fig. 2 Disk Brake

Ref: http://stmarysjin.org.uk/hsw/gif/disc-brake3.jpg

Here is a brief explanation of some of the materials used for car clutches:

–          Semi-metallic materials: A variety of metals including steel, iron and copper are used which only make up 30% to 65% of the final product. These clutches are harder, have a high heat resistance and are durable. The coefficient of friction is between 0.23 and 0.38 which is relatively low. These pads are reliable for heavy loads but not for high speed.

–          Ceramic materials: Contains organic and inorganic with other modifiers which include glass, rubber, carbon and Kevlar. The coefficient of friction is relatively high, between 0.33 and 0.4. This material is used in vehicles which are larger and require a more intense application; such as trucks, and in racing cars.

–          Organic materials is the most common type of material used, being capable for all round use in a variety of vehicles which have different specifications such as size. High copper content within these organic materials increases the heat transfer and effectively makes the clutch more reliable.

A number of paired combinations of these materials may be used for different parts of the clutch which best suits the type of vehicle to ensure the best strength and durability.

Manufacturing process of a car clutch:

This production process begins with the fly wheel; this component connects the clutch to the engine. Computerised cutting tools are used to shape this part; the materials used for the cutting tools have to be stronger than that of the material being cut for example diamond tips are usually used. Next the cover plate for the clutch is carved using a similar cutting tool. Details are very important when cutting these parts such as trimming the edges which reduces the clutches weight.

 The next step in this process is to level the surface of a friction disk (iron in this case), this is done by use an industrial diamond tipped bolt; this ensures that the contact with other parts of the clutch are enhanced. A steel insert (could be another material depending on the type of clutch produced) is then bolted to the flywheel; these bolts are tightened using a precise torque. A grinder is then used to flatten the surface of the steel attachment; this enhances the contact with the clutch disk.

Fig. 3 Basic clutch components

Ref: http://autorepair.about.com/library/a/1i/bl201i.htm

A number of stand bolts are placed through the flywheel and titanium cylinders (which are sculpted) are positioned on the bolts; titanium is light and very strong which is necessary due to the amount of energy the clutch must transfer. Then the clutch disk and another floater plate are placed on the fly wheel held in position by the stand bolts.

Next the pressure plate assembly is prepared. A number of cups are pressed into holes in the pressure plate cover. These cups hold heavy duty springs which are used to vary the pressure within the clutch pad. A number of levers are placed on the cover; these are used to compress and decompress the clutch pad. A pressure ring is then installed on the cover to keep all components in place. When the clutch is released the levers cause the clutch pad to compress and therefor the power is transferred from the engine to the transmission. The pressure assembly is then placed on the floater plate, held in place by the stand bolts.

The clutch is now complete, and a number of tests are done with the clutch to test if it is operating correctly and is up to standard. Throughout the manufacturing process a number of tools are used, these tools are designed very precisely and shaped accordingly to ensure its effectiveness of its operation. The materials of these tools are stronger than those of the materials it is operating on.

Car clutch improvements/modifications:

To start off, a common design change is to use a wet clutch instead of a dry clutch or vice versa. As the name implies a dry clutch is dry and a wet clutch has some sort of lubricant; usually engine oil. Both these clutch types has their advantages and disadvantages and usually are chosen to suite personal need. First we will look at the wet clutch:

–          This clutch has a very smooth performance and relies more on vicious effect more than specifically friction due to the lubricant. The main advantage of using such a clutch is to improve the lifespan and durability of the clutch itself. This clutch is easier to maintain but has its downfalls, such as the loss of energy due to the indirect contact (separated by lubricant); this problem can be easily solved by stacking multiple clutch plates together which will recover the initial coefficient of friction that is required. The cost of a wet clutch is slightly higher and this price will drastically increase if multiple clutch plates are used to recover the energy loss.

Now we look at the dry clutch:

–          The dry clutch is commonly used in manual transmission cars and relies on direct friction (pure friction) with the only medium being air. This type of clutch deals with much more energy due to the higher coefficient of friction between the plates; this makes the performance more effective and therefore is popular within racing cars. The cost is more affordable than a wet clutch considering short term use, but in long term wear and tear takes its toll and the lifespan of this clutch is shortened compared to a wet clutch.

 

Changing from one clutch type to the other does not have a drastic impact on the environment; the only possible issue would be water contamination from the lubricants used within the wet clutch.

 

Another simple but effective method of improving the performance of a clutch is to change from using the common rubber composite materials to Kevlar. Kevlar is known to withstand high temperatures and extreme stress; this makes kevlar a reliable material to use for car clutches.  Kevlar is already commonly used within luxurious cars because of its advantages over other materials and its longer life span. The only disadvantage is its cost, which is considerably higher initially compared to other materials but when taking in to consideration the lifespan, it seems to be an affordable option.

 

In general with any car clutches a number of detailed changes within the clutch system could be modified to improve the clutch performance. Some examples are increasing the amount of energy the springs can absorb within the clutch system; this can be done by changing to a more durable material. By changing the springs the clutch can be used at a faster rate. Another simple method is to have additional clutch plates; this will disperse the amount of frictional energy throughout the plates and therefore increase their lifespan.

 

The only drastic environmental effects would be the manufacturing of the products, when considering the raw material and the amount of electricity used when producing new parts that aren’t commonly used.

 

Bibliography:

–          Cars Buddy, n.d, How does a clutch work?, 2012/09/23, http://www.carsbuddy.com.au/how-does-the-clutch-work

 

–          Wikipedia, 18/09/12, Clutch, University of Bergen, 12/09/23, http://en.wikipedia.org/wiki/Clutch

 

–          Alberto, J, Edovirges, F, Schaeffer, L, 08/01/09, Analysis of wear in organic and sintered friction materials used in small wind energy converters, SciFlo brazil, 12/09/24, http://www.scielo.br/scielo.php?pid=S1516-14392008000300007&script=sci_arttext

 

–          Beardmore, R, 12/07/13, Friction Factors, Roymech, 12/09/24, http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm

 

–          How it’s made, 11/02/03, How it’s made drag racing clutches, youtube, 2012/09/25, http://www.youtube.com/watch?v=LJ-0gTqWMYA

 

–          Ryan, V, 2011, Kevlar and modern cars, Technology student, 2012/09/25 , http://www.technologystudent.com/joints/kevlar3.html

Mechanical device that connects and disconnects two rotating shafts or other moving parts

Friction disk for a dry clutch

A clutch is a mechanical device that allows the output shaft to be disconnected from the rotating input shaft.[1] The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does the work.

In a motor vehicle, the clutch acts as a mechanical linkage between the engine and transmission. By disengaging the clutch, the engine speed (RPM) is no longer determined by the speed of the driven wheels.

Another example of clutch usage is in electric drills.[2] The clutch's input shaft is driven by a motor and the output shaft is connected to the drill bit (via several intermediate components). The clutch allows the drill bit to either spin at the same speed as the motor (clutch engaged), spin at a lower speed as the motor (clutch slipping) or remain stationary while the motor is spinning (clutch disengaged).

Types

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Dry clutch

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Diagram of a dry clutch

A dry clutch uses dry friction to transfer power from the input shaft to the output shaft, for example a friction disk pressing on a car engine's flywheel. The majority of clutches are dry clutches, especially in vehicles with manual transmissions. Slippage of a friction clutch (where the clutch is partially engaged but the shafts are rotating at different speeds) is sometimes required, such as when a motor vehicle accelerates from a standstill; however the slippage should be minimised to avoid increased wear rates.

In a pull-type clutch, pressing the pedal pulls the release bearing to disengage the clutch. In a push-type clutch, pressing the pedal pushes the release bearing to disengage the clutch.

A multi-plate clutch consists of several friction plates arranged concentrically. In some cases, it is used instead of a larger diameter clutch. Drag racing cars use multi-plate clutches to control the rate of power transfer to the wheels as the vehicle accelerates from a standing start.

Some clutch disks include springs designed to change the natural frequency of the clutch disc, in order to reduce NVH within the vehicle. Also, some clutches for manual transmission cars use a clutch delay valve to avoid abrupt engagements of the clutch.

Wet clutch

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In a wet clutch, the friction material sits in an oil bath (or has flow-through oil) which cools and lubricates the clutch. This can provide smoother engagement and a longer lifespan of the clutch, however wet clutches can have a lower efficiency due to some energy being transferred to the oil. Since the surfaces of a wet clutch can be slippery (as with a motorcycle clutch bathed in engine oil), stacking multiple clutch discs can compensate for the lower coefficient of friction and so eliminate slippage under power when fully engaged.

Wet clutches often use a composite paper material.[citation needed]

Centrifugal clutch

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A centrifugal clutch automatically engages as the speed of the input shaft increases and disengages as the input shaft speed decreases. Applications include small motorcycles, motor scooters, chainsaws, and some older automobiles.

Cone clutch

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A cone clutch is similar to dry friction plate clutch, except the friction material is applied to the outside of a conical shaped object. A common application for cone clutches is the synchronizer ring in a manual transmission.

Dog clutch

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A dog clutch is a non-slip design of clutch which is used in non-synchronous transmissions.

Single-revolution clutch

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Cascaded-pawl single-revolution clutch in a teleprinter

The single-revolution clutch was developed in the 19th century to power machinery such as shears or presses where a single pull of the operating lever or (later) press of a button would trip the mechanism, engaging the clutch between the power source and the machine's crankshaft for exactly one revolution before disengaging the clutch. When the clutch is disengaged, the driven member is stationary. Early designs were typically dog clutches with a cam on the driven member used to disengage the dogs at the appropriate point.[3][4]

Greatly simplified single-revolution clutches were developed in the 20th century, requiring much smaller operating forces and in some variations, allowing for a fixed fraction of a revolution per operation.[5] Fast action friction clutches replaced dog clutches in some applications, eliminating the problem of impact loading on the dogs every time the clutch engaged.[6][7]

In addition to their use in heavy manufacturing equipment, single-revolution clutches were applied to numerous small machines. In tabulating machines, for example, pressing the operate key would trip a single revolution clutch to process the most recently entered number.[8] In typesetting machines, pressing any key selected a particular character and also engaged a single rotation clutch to cycle the mechanism to typeset that character.[9] Similarly, in teleprinters, the receipt of each character tripped a single-revolution clutch to operate one cycle of the print mechanism.[10]

In 1928, Frederick G. Creed developed a single-turn wrap spring clutch that was particularly well suited to the repetitive start-stop action required in teleprinters.[11] In 1942, two employees of Pitney Bowes Postage Meter Company developed an improved single turn spring clutch.[12] In these clutches, a coil spring is wrapped around the driven shaft and held in an expanded configuration by the trip lever. When tripped, the spring rapidly contracts around the power shaft engaging the clutch. At the end of one revolution, if the trip lever has been reset, it catches the end of the spring (or a pawl attached to it), and the angular momentum of the driven member releases the tension on the spring. These clutches have long operating lives—many have performed tens and perhaps hundreds of millions of cycles without the need of maintenance other than occasional lubrication.

Cascaded-pawl single-revolution clutches superseded wrap-spring single-revolution clutches in page printers, such as teleprinters, including the Teletype Model 28 and its successors, using the same design principles. IBM Selectric typewriters also used them. These are typically disc-shaped assemblies mounted on the driven shaft. Inside the hollow disc-shaped drive drum are two or three freely floating pawls arranged so that when the clutch is tripped, the pawls spring outward much like the shoes in a drum brake. When engaged, the load torque on each pawl transfers to the others to keep them engaged. These clutches do not slip once locked up, and they engage very quickly, on the order of milliseconds. A trip projection extends out from the assembly. If the trip lever engaged this projection, the clutch was disengaged. When the trip lever releases this projection, internal springs and friction engage the clutch. The clutch then rotates one or more turns, stopping when the trip lever again engages the trip projection.

Other designs

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• Kickback clutch-brakes: Found in some types of synchronous-motor-driven electric clocks built before the 1940s, to prevent the clock from running backwards. The clutch consisted of a wrap-spring clutch-brake that was coupled to the rotor by one or two stages of reduction gearing. The clutch-brake locked up when rotated backwards, but also had some spring action. The inertia of the rotor going backwards engaged the clutch and wound the spring. As it unwound, it restarted the motor in the correct direction.
• Belt clutch: used on agricultural equipment, lawnmowers, tillers, and snow blowers. Engine power is transmitted via a set of belts that are slack when the engine is idling, but an idler pulley can tighten the belts to increase friction between the belts and the pulleys.
• BMA clutch: Invented by Waldo J Kelleigh in 1949,[13] used for transmitting torque between two shafts consisting of a fixed driving member secured to one of said shafts, and a movable driving member, having a contacting surface with a plurality of indentations.
• Electromagnetic clutch: typically engaged by an electromagnet that is an integral part of the clutch assembly. Another type, the magnetic particle clutch, contains magnetically influenced particles in a chamber between driving and driven members—application of direct current makes the particles clump together and adhere to the operating surfaces. Engagement and slippage are notably smooth.
• Wrap-spring clutch: has a helical spring, typically wound with square-cross-section wire. These were developed in the late 19th and early 20th-century.[14][15] In simple form the spring is fastened at one end to the driven member; its other end is unattached. The spring fits closely around a cylindrical driving member. If the driving member rotates in the direction that would unwind the spring expands minutely and slips although with some drag. Because of this, spring clutches must typically be lubricated with light oil. Rotating the driving member the other way makes the spring wrap itself tightly around the driving surface and the clutch locks up very quickly. The torque required to make a spring clutch slip grows exponentially with the number of turns in the spring, obeying the capstan equation.

Usage in automobiles

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Manual transmissions

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Most cars and trucks with a manual transmission use a dry clutch, which is operated by the driver using the left-most pedal. The motion of the pedal is transferred to the clutch using hydraulics (master and slave cylinders) or a cable. The clutch is only disengaged at times when the driver is pressing on the clutch pedal, therefore the default state is for the transmission to be connected to the engine. A "neutral" gear position is provided, so that the clutch pedal can be released with the vehicle remaining stationary.

The clutch is required for standing starts and is usually (but not always) used to assist in synchronising the speeds of the engine and transmission during gear changes, i.e. while reducing the engine speed (RPM) during upshifts and increasing the engine speed during downshifts.

The clutch is usually mounted directly to the face of the engine's flywheel, as this already provides a convenient large-diameter steel disk that can act as one driving plate of the clutch. Some racing clutches use small multi-plate disk packs that are not part of the flywheel. Both clutch and flywheel are enclosed in a conical bellhousing for the gearbox. The friction material used for the clutch disk varies, with a common material being an organic compound resin with a copper wire facing or a ceramic material.[16]

Automatic transmissions

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In an automatic transmission, the role of the clutch is performed by a torque converter. However, the transmission itself often includes internal clutches, such as a lock-up clutch to prevent slippage of the torque converter, in order to reduce the energy loss through the transmission and therefore improve fuel economy.[17]

Fans and compressors

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Older belt-driven engine cooling fans often use a heat-activated clutch, in the form of a bimetallic strip. When the temperature is low, the spring winds and closes the valve, which lets the fan spin at about 20% to 30% of the crankshaft speed. As the temperature of the spring rises, it unwinds and opens the valve, allowing fluid past the valve, making the fan spin at about 60% to 90% of crankshaft speed.

A vehicle's air-conditioning compressor often uses magnetic clutches to engage the compressor as required.

Usage in motorcycles

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A basket clutch

Motorcycles typically employ a wet clutch with the clutch riding in the same oil as the transmission. These clutches are usually made up of a stack of alternating friction plates and steel plates. The friction plates have lugs on their outer diameters that lock them into a basket that is turned by the crankshaft. The steel plates have lugs on their inner diameters that lock them to the transmission input shaft. A set of coil springs or a diaphragm spring plate force the plates together when the clutch is engaged.

On motorcycles the clutch is operated by a hand lever on the left handlebar. No pressure on the lever means that the clutch plates are engaged (driving), while pulling the lever back towards the rider disengages the clutch plates through cable or hydraulic actuation, allowing the rider to shift gears or coast. Racing motorcycles often use slipper clutches to eliminate the effects of engine braking, which, being applied only to the rear wheel, can cause instability.

See also

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References

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