Introduction to the Flywheel Clutch
The following information discusses the components and operation of the flywheel clutch.
Couplings
 | |
| Illustration 1 | g01084543 |
One of the first challenges facing automotive engineering pioneers was devising couplings that would smoothly transfer the turning power or torque that is produced by the engine to the drive wheels. An ideal coupling would allow the engine to start and initially run without any load. The coupling should also enable the driving wheels to be engaged gradually so the vehicle can be controlled and maneuvered comfortably at low speeds.
Over the years, many devices have been developed in order to achieve these results. Engines in heavy industrial machines are frequently exposed to additional stresses, even under normal operating conditions. Specialized couplings have been engineered in order to meet the special needs of this equipment.
 | |
| Illustration 2 | g01084544 |
A flywheel clutch connects and disconnects power from the engine flywheel to the transmission.
A flywheel clutch is used mostly with manual shift transmissions. The flywheel clutch is sometimes referred to as a friction clutch. The friction between driving and driven members actually absorbs some of the shock. This permits a more gradual engagement. Therefore, with the use of a friction clutch, engaging power causes less strain and wear on the power train components than a direct connection.
Illustration 2 shows the basic parts of a friction type flywheel clutch. There are three essential parts to the flywheel clutch. A flywheel plate or a disc (1) is known as the driving plate. The flywheel plate is mounted to the engine flywheel. The driven plate (2) is splined to the shaft. When the driven plate turns, the driven plate will also turn the shaft. The small collar (3) is an actuating collar. The actuating collar is used to push the driving plate and the driven plate together. The clutch plate assembly is aligned with the engine by inserting the end of the shaft into a pilot hole in the center of the flywheel.
 | |
| Illustration 3 | g01084547 |
In Illustration 3, a simple hand lever and rod (1) has been added to the back of the actuating collar (2). By pushing forward on the hand lever, the actuating collar moves against the driven plate (3). The driven plate then slides forward and engages with the driving plate (4). The driving plate will be rotating at engine speed. The moment the plates begin to touch, the driven plate and the shaft begin to turn. Complete engagement occurs when the two plates are forced together as tightly as possible. The shaft will then rotate at engine speed.
The two main types of flywheel clutches that are discussed in this section include the dry clutch and the wet clutch. Dry clutches are air cooled and are generally suitable for lower horsepower machines where there is less initial torque during engagement. Dry clutches are used primarily in small tractors and automobiles. The advantage of a dry clutch is that the dry clutch offers a larger contact area. Dry clutches are not recommended for applications where frequent disengaging or slipping is required, because the dry clutch material is more prone to heat build up. Therefore, dry clutches are not used in most heavy machine applications.
Wet Clutch
 | |
| Illustration 4 | g01084548 |
Wet clutches (Illustration 4) are so named because they contain fluid (oil). Wet clutches are used on high horsepower machines, particularly in applications where there is frequent engagement and disengagement in the course of operation. As in a dry clutch, sliding friction between the disc and plates causes heat to build up. The oil in a wet clutch carries the heat away. Oil also reduces the shock load that is produced when the clutch is engaged.
Dry Flywheel Clutch
 | |
| Illustration 5 | g01084550 |
A basic dry flywheel clutch assembly is shown in Illustration 5. The dry flywheel clutch is quite similar to the basic friction-type clutch previously discussed. The dry flywheel clutch is rather simple when broken down into the basic parts.
Clutch Shaft
 | |
| Illustration 6 | g01084552 |
The clutch shaft (Illustration 6) is the backbone of the clutch because all of the clutch components are assembled upon or around the clutch shaft. The large round surface on the left end is the brake drum. The small piece extending from the left of the brake drum is the mount for the universal joint, which connects the clutch shaft to the input shaft on the transmission.
 | |
| Illustration 7 | g01084555 |
The other end of the clutch shaft fits into the pilot hole and bearing in the flywheel (Illustration 7).
Clutch Components
 | |
| Illustration 8 | g01084557 |
The pressure plates and driving disc assembly are mounted on the clutch shaft in Illustration 8. These three parts provide the friction contact surfaces of the dry clutch.
 | |
| Illustration 9 | g01084559 |
Illustration 9 shows the components individually. The rear pressure plate is on the left. The driving disc is in the center. The driving disc includes heat grooves on both surfaces of the disc in order to help dissipate heat and reduce wear. The front pressure plate, on the right, provides the mounting base for the rear pressure plate. The gear teeth on the front pressure plate mesh with the teeth in the center of the rear pressure plate. The pressure plates are splined to the clutch shaft. The driving disc is driven by the flywheel.
 | |
| Illustration 10 | g01084560 |
From the left, the parts arranged on the shaft (Illustration 10) are the rear pressure plate, the driving disc, the front pressure plate, and the flywheel on the right. The driving disc and the flywheel are driving parts. The driving disc and the flywheel are connected to the engine. The driving disc and the flywheel revolve freely when the clutch is disengaged.
 | |
| Illustration 11 | g01084562 |
Illustration 11 shows the relationship of the driving disc to the flywheel. The outer teeth of the driving disc mesh with the inner teeth on the flywheel. The front pressure plate is splined to the clutch shaft. The inset circle shows how the disc teeth mesh with the flywheel inner teeth.
The driving disc rotates with the flywheel whenever the engine is running. The plates and the shaft remain idle. The plates and the shaft do not turn until pressure is exerted on the rear pressure plate. To engage the plates and the disc, the rear pressure plate is pressed against the driving disc until the driving disc is firmly clutched between the rear and front pressure plates. Power is transferred from the driving disc to the pressure plates.
Cam Link and Sliding Collar Assembly
 | |
| Illustration 12 | g01084564 |
Illustration 12 shows the cam link and sliding collar mechanism. The cam link and sliding gear apply pressure to the rear pressure plate when the clutch is engaged.
Clutch Engaged
 | |
| Illustration 13 | g01084566 |
In Illustration 13, the front of the sliding collar mechanism (1) screws onto the base of the front pressure plate (2). The sliding collar is free to slide back and forth on the shaft. The raised area on the back of the rear pressure plate (3) is in contact with the rounded cam surface of the cam link. The sliding collar is now all the way forward, in this case to the right, and the clutch plates and disc are engaged. The position of the large arrow shows that the cam and the sliding collar are holding the pressure plates and the driving disc in an engaged position. In this position, all components would rotate except the sliding collar that is connected to the linkage.
Clutch Released
 | |
| Illustration 14 | g01084568 |
Illustration 14 shows the sliding collar and the cam link mechanism in the released position. In the released position, the sliding collar has been pulled away and moved toward the rear of the shaft. The cam collar base is still mounted securely to the base of the front pressure plate, but the cam links have been pulled backward. This rearward movement of the cam links has also released the pressure of the round cam surfaces from the rear pressure plate. The springs that are mounted between the front and rear pressure plates also assist to disengage the clutch.
Clutch and Yoke Arm
 | |
| Illustration 15 | g01084578 |
The steel yoke arm (1) shown in Illustration 15 has two fingers that engage a block-type trunnion on the side of the collar. Another yoke is mounted on the other side of the sliding collar in the same way. The yokes pivot together on a control shaft that is mounted in the base of the clutch housing.
Control Lever and Clutch Brake Assembly
 | |
| Illustration 16 | g01084580 |
The control lever and clutch brake assembly in Illustration 16 shows how the yoke and collar are linked to the control lever. Illustration 16 also shows the brake drum, the brake shoe, and the brake shoe arm. When the hand lever is pulled back, the lever will lift the rod on the end of the control shaft of the yoke assembly. The yoke will then move forward and engage the plates and disc (not shown). When the hand lever is pushed forward by the operator, the yoke arm goes down, and the collar slides back and disengages the clutch. When the clutch is disengaged, the brake arm will pivot downward and force the brake shoe against the round brake drum. This will stop the rotation of the clutch shaft when the clutch is disengaged. Stopping the rotation of the shaft eases gear shifting.
Wet clutches were designed for heavier engines and more powerful engines. By adding a thin film of oil between the clutch plates, the clutch can be engaged and disengaged more smoothly and the clutch would carry a heavier load. Another method of increasing the load capacity of clutches was to add more pressure plates and driving discs. This would increase the overall friction surface area of the clutch without increasing the overall size of the clutch.
Wet Clutch
 | |
| Illustration 17 | g01084581 |
The shaft for a wet clutch is shown in Illustration 17. The brake drum is on the left end. The shaft on a wet clutch is shorter than the shaft on a dry clutch.
 | |
| Illustration 18 | g01084582 |
A universal joint is bolted onto the left end of the shaft in Illustration 18. The universal joint is bolted to the input shaft of the transmission. A large hub is mounted on the end of the shaft. The outer teeth of the hub will hold two friction discs.
 | |
| Illustration 19 | g01084585 |
In Illustration 19, two pressure plates (1) are splined to the flywheel and two discs (2) are splined to the clutch hub. The two pressure plates will rotate with the flywheel when the engine is running. The two discs will not rotate until the discs are forced against the pressure plates. In the wet clutch, the plates drive and the discs are driven. Compressing the clutch causes the power to flow through the clutch. The power flows through the clutch components in the following order:
- Flywheel
- Plates
- Discs
- Hub
- Shaft
Clutch Plate and Disc
 | |
| Illustration 20 | g01084586 |
The clutch disc (Illustration 20) on the left, is made of sintered bronze and has small grooves. The tabs that are cut into the disc help to disengage the discs and plates because the tabs are bent outward slightly. The teeth on the inner diameter of the disc mesh with the outer teeth on the hub.
The pressure plate (Illustration 20) on the right, is heavy steel and the outer teeth mesh with the corresponding teeth in the inner ring of the flywheel.
Wet Clutch Basic Assembly
 | |
| Illustration 21 | g01084588 |
The three basic assemblies of the wet clutch are listed below:
- Driving parts (1)
- Driven parts (2)
- Actuating mechanism (3)
Illustration 21 shows the actuating mechanism of a typical wet clutch. The operating principle is similar to the dry clutch. A sliding collar actuates a cam link mechanism. The sliding collar and the linkage compresses the clutch and the loading plate applies force to the clutch pack. The sliding collar controls the actuating mechanism.
Actuating Mechanisms
 | |
| Illustration 22 | g01084590 |
Illustration 22 shows two types of actuating mechanisms. A cam link actuating mechanism is on the right. In the left view, steel rollers perform the same function as the cam link mechanism.
In a clutch with the cam link mechanism, the top link is connected to an adjustment ring. A roller is connected to the top link and the bottom link. The bottom link is connected to the sliding collar. As the collar moves forward, the bottom link pushes the roller toward the loading plate and the roller pivots on the top link. The roller rolls up the loading plate until the collar is far enough forward that the bottom link snaps over the vertical position. This over center position holds the clutch engaged.
In a clutch with the steel roller mechanism, the top roller is connected to a large roller. The large roller is connected to an adjusting ring. The bottom roller is connected to a sliding collar. As the collar moves forward, the bottom roller pushes the top roller toward the loading plate. The top roller pivots on the larger roller. The top roller rolls up the loading plate until the collar is far enough forward so that the center of the bottom roller snaps past the center of the top roller. This over center position holds the clutch engaged.
 | |
| Illustration 23 | g01084595 |
Illustration 23 shows a yoke that is mounted to the sliding collar, which controls most wet clutch actuating assemblies. The yoke arm slides on the collar. The control linkage moves the yoke arm in order to slide on the collar.
Wet Clutch and Oil Pump
 | |
| Illustration 24 | g01084596 |
The exterior of a wet clutch is shown in Illustration 24. The brake drum and the attached universal joint are also visible. Directly below the brake drum is the oil pump housing which houses a simple gear-type oil pump. The pump is driven by the flywheel gear. The sump that contains the oil supply is below the oil pump and at the bottom of the clutch housing.
 | |
| Illustration 25 | g01084598 |
Illustration 25 is a cutaway view of the bottom of the clutch housing. The oil is filtered through a screen (1). The oil can be drained by removing the plug at the bottom of the clutch housing. The oil is drawn through the passage on the right and moves through a passage in the housing up to the oil pump.
 | |
| Illustration 26 | g01084599 |
Illustration 26 shows a cutaway view of a wet clutch with the oil pump at the lower left. The oil flow moves from the pump up through the rear of the housing. The oil flow enters the shaft in the area of the rear bearing and lubricates the bearing. The oil then flows through the shaft into the area where the hub is splined to the shaft. Oil passes through oil ports drilled in the base of the hub, moves up to the pressure plates and driven discs, and flows between them. The oil then flows out of the small ports in the flywheel, shown at the top and bottom of the flywheel, and falls back into the sump at the bottom of the housing.
 | |
| Illustration 27 | g01084601 |
The cutaway shown in Illustration 27 shows the entire oil flow circuit. Oil that is stored in the bottom of the housing is filtered through a screen and drawn up to the oil pump. Oil moves from the pump to the rear shaft bearing, through the shaft to the hub, and into the pressure plate and disc assemblies. The oil splashes around the hub, the plate, and the disc area and forms a thin film between the plates and discs. Oil then flows through ports in the flywheel and back down into the sump in the base of the housing.
Clutch Principles
The following statements provide a review of the clutch principles:
- Friction transfers power between the driving discs and the driven plates.
- Sliding friction causes heat. This heat is not excessive in some small machines so the small machines are able to use a dry clutch. Oil carries away the heat in wet clutches. Wet clutches are used in most machines.
- Different disc materials are used for different applications.
- In wet clutches, oil acts as a buffer between the discs and plates and oil movement actually causes the discs to start moving with the plates before the plates completely engage.
Clutch Benefits
The following statements provide a review of the clutch benefits.
- Direct power transfer from engine to transmission
- The clutch is required for operation of manual transmissions.
- A manual transmission system is less expensive to build and maintain.