CHAPTER 2 POWER TRAIN

The heart of the power train is the internal combustion engine that provides the power required to move a vehicle. However, this task is made much more efficient with the aid of the transmission and the other drive-line components that make up the power train (fig. 2-1). This chapter covers the basic principles of manual and automatic transmissions, propeller shaft assemblies, and final drives.

TRANSMISSIONS

Power from the engine provides the torque required for the transmission to overcome inertia. Inertia is a property of matter by which it remains at rest or in uniform motion in a straight line unless acted upon by some external force. In this case, the inertia of the vehicle at rest is overcome by an external force-the engine power in the form of torque. Once the vehicle is moving, acceleration begins and increases and very little torque is then required. The bigger the load on the engine, the bigger and more efficient the transmission must be. Once a vehicle gains the desired speed, it moves along with very little effort until something is encountered, such as a grade in the road, that increases the resistance to its movement. Now torque is required again and the operator has to select a lower gear.

The transmission (fig. 2-2) provides the mechanical advantage that enables the engine to move the vehicle. It allows the operator to control the power and speed of the vehicle and allows disengaging and reversing the flow of power from the engine to the wheels by means of a clutch.

CLUTCH

The clutch engages and disengages the engine crankshaft to or from the transmission and the rest of the power train. Engine power to the load must be applied slowly to allow a smooth engagement and to lessen shock on the driving and driven parts. After engagement, the clutch must transmit the engine power to the transmission without slipping. Additionally, the engine must be disconnected from the power train in order to shift gears.

Figure 2-1.-Typical power train.

Figure 2-5 shows the proper free travel and linkage. Several clutch troubles may occur during vehicle operation that should be documented and turned in before too much damage occurs. These troubles include incorrect free travel, slipping, chattering, or grabbing

Figure 2-2.-Typical manual shift transmission.

Clutches transmit power from _{the clutch driving} member to the driven member by friction. In the DISC CLUTCH (fig. 2-3), the driving plate secured to the engine flywheel gradually contacts the driven member (disc) attached to the transmission input shaft. The contact is made and held by strong spring pressure controlled by the operator with the clutch pedal (fig. 2-4). With only light spring pressure, there is little friction between the two members, and the clutch can slip; therefore, do not use the clutch pedal as a footrest. As the spring pressure increases, friction also increases, and less slippage occurs. When the operator's foot is removed from the clutch pedal and the full spring pressure is applied the speed of the driving plate and driven disc is the same and all slipping stops. The flywheel and the transmission input shaft are then connected.

Improper adjustment can damage or ruin a clutch. when engaging; spinning or dragging when engaged; and clutch noises.