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The invention of the motor vehicle marked an epochal leap in technological progress, and irreversibly altered the course of human history.

In fact, motorization is one of the greatest achievement of modern post-renaissance civilization.

Now to the more practical side, motor vehicles allow us to move easily from place to place and have shortened long walking distances into a matter of minutes-long driving distance.

Even so, what happens when the vehicle stops moving, while the engine is clearly running as attested to by the revving engine noise?

This can be inconveniencing, and to further complicate matters, the gears can be changed, but power is not transmitted. So, what can the problem be?

In this case, the most likely fault would be caused by the clutch unit. Basically, the clutch plate is not transferring power from the engine to the gear shaft.

This is time to consider acquiring a high-quality clutch, which can be obtained alongside its compatible counterparts if one acquires the Luk clutch kit. But first things first, what is a clutch?

What is the Clutch?

The clutch is basically a hardened disc/plate that can be pressed firmly against the flywheel so that it can transmit the flywheel rotational inertia and motion to the gear box.

Evidently, this mechanical device allows for engagement and disengagement of power transmission from the driving shaft (the flywheel-crankshaft-piston assembly) to the driven shaft (the gears-shaft assembly).

This allows for the driver to disengage the power transmission system so as to change gears (whose drive speeds determine the wheel speed) and then re-engage the power transmission with the new gears in place, and consequently the wheels will now travel at a different speed.

A Little Science – Why Clutch Disengagement Occurs when Changing Gears?

This mode of operation is defined by laws of mechanical physics which state that two separate, yet interlinked, rotating shafts can only rotate at the same speed if one shaft exclusively drives the other shaft.

Any attempts to cause the driven shaft to rotate at a different speed than the driving shaft would ultimately generate considerable wear and tear due to the friction generated by the force speeding up, or slowing down, the driven shaft.

Basically, this means that the driven shaft cannot rotate at a higher, or lower speed than the driving shaft if the two shafts are physically connected together.

Therefore, the flywheel that is bolted to the power unit (the engine) determines the drive speed of the input line shaft of the gear box.

This input shaft has gears of different sizes mounted along its length, and these gears engage with gears mounted on a different shaft (the transmission shaft of the gear box) so as to create a gear train that converts flywheel motion to wheel motion.

The front end of the input shaft is splined externally so as to mate with the internal splines of the clutch plate.

When the clutch is driven by the flywheel, it drives the input shaft, and in effect drives the input and transmission shafts at a synchronized speed, which is not necessarily the same shaft speed as gears determine the speed of the transmission shaft which extends into the rear differential unit as the propeller shaft.

what is a clutch

This gear train allow for gears to be disengaged when the interlocked teeth of the gears slide away from each other.

This means that the transmission shaft moves forward or backwards relative to the input shaft position. It is this disengagement that allows for a new gear set to be engaged.

As expected, if the input shaft is rotating when the gear train is being disengaged, the teeth that are sliding away from each other would end up hitting each other and breaking (as they are rotating at different speeds).

This would grind away the gear teeth and render the gears non-functional, and operationally, prevent the gear teeth on the two shafts from being re-engaged properly together which effectively disrupts power transmission from the input shaft to the power transmission shaft.

The result is that a fraction of the engine power is transferred inconsistently to the wheels.

To prevent this scenario, the clutch is disengaged from the wheel so that the input shaft and transmission shaft continue rotating at their current synchronized speed as the gear teeth are disengaged from each other, and a new set of gear teeth are interlocked together, that is, a new gear set is locked in place.

Thereafter, the clutch can be re-engaged to the flywheel so that power transmission is restored smoothly which is manifested by the smooth and seamless acceleration, or deceleration, of the vehicle.

It is therefore evident that the clutch serves one primary, and two secondary functions:
  1. Allows for the engine power unit to drive the wheels (Primary function).
  2. Allows the vehicle to accelerate and decelerate properly.
  3. Supports the disengagement of power transmission as it occurs when the neutral gear is engaged.

Why a High-Quality Clutch Kit improves Performance and Lifespan of the Power Transmission System?

The aforementioned functions make the clutch unit a critical device in the engine-power transmission system of the vehicle.

If it is pressed weakly onto the flywheel, the flywheel speed cannot be well synchronized with the clutch speed, and consequently the vehicle will move erratically as the wheels travel at a discordant, non-consistent speed.

This results in a bad driving experience.

As expected, there needs to be a mechanism for pressing the clutch onto the flywheel. The clutch and this mechanism make up the clutch kit.

The driver can regulate and modify the operation of this kit through a control system. This entire system makes up the clutch assembly.

Because the control system is prebuilt alongside the engine assembly and configured to sync with the overall driver operation unit; it cannot be easily replaced with a non-OEM (original equipment manufacturer) system.

Therefore, the best way to optimize the power transmission system at the engine-gear interface level is to install a high-quality and durable clutch kit.

The reason for using a high-quality clutch kit is that the pressure plate and diaphragm are made from high-quality materials that can absorb vibration and other mechanical shock without loosening.

While the clutch plate can resist wear and tear, and in effect remain in operation for a long time without suffering from regular slippage which makes power transmission inefficient.

One of the manufacturers of high-quality clutch kits is LuK Clutch Systems, LLC which was founded in Germany, with the American subsidiary being established in 1982. (For our in depth LuK clutch review article head here)

So, what makes the Luk clutch kit stand out from the other clutch kits in the market?

What Makes the Luk Clutch Kit Stand Out

This kit stands out against alternative kits in the market not only because of its high-quality design and durable components, but also because it comes with a compatible flywheel, alongside the standard clutch kit.

This is to ensure that the engine-to-gear power transmission system is made of fully compatible parts, which in turn maximizes efficiency, reduces frictional wear and tear, and stretches out the kit lifespan.

These parts are described below:

Parts of the Luk Clutch Kit

Luk clutch kit

The parts of any clutch kit are designed to operate as an integrated unit; and this explains why one cannot use parts made from different manufacturers to create a non-OEM, do-it-yourself kit, otherwise known as the Frankestein kit.

This review focuses on the Luk kit; and in order to understand how this kit operates as an integrated unit, it is important to describe the construction of the clutch kit which is made up of the following parts:

Clutch

In the broadest sense, clutches are divided into wet and dry clutch plates. Among the dry clutch plates is a special category of clutches known as friction plates.

They are described so because they use frictional forces to bind the clutch plate to the flywheel. The clutch plate of the Luk Kit belongs to this class of clutch plates.

Nonetheless, it is important to note that not the entire disc is in contact with the flywheel, but only the outer rim which makes about a third of the entire plate surface area.

This section (outer rim) is called the disc-friction facings (as the plate has 2 sides) and is made of composite friction-resilient materials, usually compound resin that is impregnated with copper wires – though some older models are made of asbestos.

In heavy-duty vehicles, hardened ceramic is used, but this causes the flywheel and pressure plates to wear much quicker as compared to use on non-ceramic clutches.

Construction-wise, the clutch plate is a composite of a plate and a surrounding ring. This ring is called a disc ring and it forms the disc-friction facings of the clutch plate.

On the other hand, the plate is called a drive plate as its center has internal splines that are mated to the external splines of the input shaft of the geared transmission system.

This drive plate features unique spring devices called dampers.

In Luk clutches, the disc ring and drive plate are welded together using the latest laser welding technology.

Likewise, the diaphragm springs that presses the disc ring on the flywheel are heat-treated so as to harden the springs while optimizing the tensile strength of the high spring steel material used.

As mentioned earlier, there are dampers. So, what are their main function in the clutch?

The process of clutch engagement and disengagement creates a lot of vibrations on the entire clutch disc, with the vibrations emanating from the centered spline and projecting outwards towards the disc-friction facing.

Such driveline vibrations and to a lesser extent gear rattles, can cause the disc-friction facing to wobble against the flywheel.

As expected, these radially-radiating vibrations need to be dampened before they reach the disc-friction facings, and this is done by fitting a series of damper springs in the drive plate.

There are 2 types of damper springs; the main damper springs, and a set of weaker damper springs.

The weaker damper springs serve to change the natural disc frequency in an idling engine, while the main spring does the same when the engine is running.

In both cases, the dampers are compressed by the radial vibrations, and this absorbs the vibrational energy and hence prevents such vibration from reaching the disc-friction facings.

Is it a Pull-Type or a Push-Type Clutch Plate?

The designation of clutch plates as either push-type or pull-type does not designate the process of how the clutch is disengaged from the flywheel, but rather describes the mechanism that causes the diaphragm to release its pressure on the clutch plate.

This is in turn determined by how the release bearings exert their action.

If the pressing of the clutch pedal causes the release bearings to be pulled towards the pressure plate, then this clutch is a pull-type; but if the same action causes the releases bearing to be pushed away from the pressure plate, then the clutch is a push-type.

The release bearings cause the diaphragm to release its pressure on the clutch plate hence allowing for disengagement of power transmission. The push-type release bearings are also known as thrust bearings.

The Luk kit is popular among light-duty vehicles, especially in the United States where it has dominated the light vehicle clutch market.

It provides excellent damping, and its friction plates are less liable to slippage. Moreover, its splined hub is wear-resistant which ensures that power is transmitted efficiently to the gear transmission system.

Diaphragm

This is a dished spring that applies clamping thrust onto the clutch plate. Its inclusion in the kit allows for its designation as a diaphragm-spring clutch kit.

This diaphragm is designed as a plate with a circular hole at the middle, and radial slots radiating from this hole and terminating into enlarged blunting holes that define the lower boundary of the solid outer rim/ring.

The radial slotting, as well, as the presence of the punched hub hole creates a series of radial fingers. the blunting holes ensure that spring forces are dampened around the circular hole edges which terminates force transmission and prevents such forces from causing the solid rim to crack.

As mentioned earlier, this is a dished disc which means that it has a convex (or a flattened conical) shape with the center of the convex being the punched hole, while the fingers fill in the solid portion of the convex, and the rim is a flat plate.

This shape gives the fingers their springing capacity, and for this reason they are called actuating fingers. These actuating fingers make up the release lever.

Release Bearing

This is mounted on the hub of the clutch plate, and it slides forwards and backwards on this hub. The release fingers of the diaphragm are attached to the external side of this bearing.

Thus, the bearing movement causes the actuating fingers to either clamp the diaphragm on the clutch plate or cause it to release its thrust from the plate.

Consequently, it is important that you ensure that you acquire a high-quality release bearing as this critical component determines whether a functional clutch plate will operate as required.

This bearing is operated by a lever mechanism that is ultimately controlled by the driver through the clutch pedal.

A pivoted fork is bolted to the rear end of the release bearing, and this fork is connected to the cable, or the more common hydraulic system that causes it to slide the bearing back and forth on the hub.

Pressure Plate

This is a large dish-like plate that encloses the clutch plate, diaphragm, and release bearing within a single enclosure.

It is bolted to the flywheel which ensures that it rotates at the same speed as the flywheel, while its center features a large hole that allows the splined hub of the clutch plate and the mounted release bearing to pass through.

Its primary function is to ensure that the clamping pressure of the diaphragm spring is not lost but is directed fully towards the disc-friction facings of the clutch plate.

A high-quality pressure plate ensures that the diaphragm is pressed firmly on the clutch plate when needed, and that the release bearing can slide properly on the clutch plate hub.

Flywheel

This is the primary accessory to the Luk Clutch kit model. The flywheel is bolted to the engine crankshaft hub, and it serves to transfer the crankshaft motion to the clutch plate.

Its large mass serves to amplify the rotational inertia of the crankshaft, and in effect transfer this amplified inertia to the clutch plate.

The pressure plate of the clutch kit is bolted directly onto the flywheel, which ensures that they rotate at the same speed, whether the clutch plate is engaged or not.

The pressure plate and flywheel are housed together in a bell housing that is bolted to the engine block.


How the Kit Operates as an Integrated Unit

luk-clutches-the clutch-kit

As mentioned earlier, the driver is the ultimate controller of the operation of the clutch plate. When the driver presses the clutch pedal, a lever mechanism causes a cable to pull the fork, and this causes the release bearing to be pulled away from the clutch disc drive.

This is possible if the fulcrum of the fork is placed between the bearing and cable. If the release bearing is placed between the fulcrum and the lever cable, then when the fork is pulled, the release bearing is pushed towards the disk drive.

This causes the actuating fingers to be compressed (or flattened out) under pressure, which results in the diaphragm releasing its pressure on the clutch plate, and the clutch plate is subsequently disengaged from the flywheel, and power transmission is interrupted.

When the clutch pedal is released, the diaphragm immediately exerts a clamping thrust on the clutch plate, while the pressure plate ensures that the diaphragm ring is pressed tightly onto the disc-friction facing of the clutch plate.

It is therefore evident that at its default mode, the clutch is engaged; and therefore, the intervention by the driver causes disengagement.

Now that the operation and design of clutch kit assembly has been explained; head over to our top 5 luk clutch kit review review and buyer’s guide to find out what to look for when buying one.