Transmission 2

Powershift Transmission

Powershift transmissions shift under full power from the engine. In a manual shift transmission, engine torque must be disconnected before a gear selection is completed. A powershift transmission—by means of clutch packs and planetary gearing or countershaft gears in constant mesh—shifts without interrupting drive torque or synchronizing gears. Most manufacturers use a fluid coupling with powershift transmissions. A fluid coupling, such as a torque converter, can absorb variations in a torque load during shifting, but in some instances, a clutch is also used to input engine torque to a powershift transmission. When a driveline clutch is used, it is limited to starting the vehicle from a stationary position. Depending on the application of the vehicle, many manufacturers have eliminated the use of both the torque converter and/or the clutch assembly. Instead, slow and smooth oil modulation to the clutch packs allows engine torque to be transmitted directly to the first gear clutch pack and planetary gear set. Powershift transmissions do not need to break torque from the engine during shifts while the vehicle is in motion.

Fig 1. Transmission in Construction equipment

Hydrostatic Drive Train

The hydrostatic drive is an automatic fluid drive that transmits engine power to the drive wheels or tracks by using fluid under pressure. Mechanical power from the engine is converted to the hydraulic power by a pump -motor team. This power is then again converted back to mechanical power for the drive wheels or tracks.

The pump-motor team is the core of the hydrostatic drive system. Generally, the pump and motor are joined in a closed hydraulic loop; the return line from the motor is connected directly to the intake of the pump, rather than to the reservoir (Figure 2). A charge pump maintains the system pressure, using supply oil from the reservoir.

Fig 2. Pump and motor in Hydrostatic drive

The hydrostatic drive functions as both a clutch and transmission. The final gear train then can be simplified with the hydrostatic unit supplying infinite speed and torque ranges as well as reverses speeds. To understand hydrostatic drive, we must understand two principles of hydraulics:

• Liquids have no shape of their own.
• Liquids are incompressible.

The basic hydrostatic principle is as follows

• Two cylinders connected by a line are both filled with oil. Each cylinder has a piston.
• When a force is applied to any one of the pistons, the piston moves against the oil. Since the oil is not compressible, it acts as a solid connection and moves the other piston in the same direction.

In a hydrostatic drive, numerous pistons are used to transmit power, one group in the PUMP transferring power to another group in the MOTOR. The pistons reside in a cylinder block and revolve around a shaft. The pistons also move in and out of the block remaining parallel to the shaft.

Fig 3. Squash plates in Hydrostatic drive

To provide a pumping action for the pistons, a plate called a swash plate is present in both the pump and motor (Figure 3). The pistons move against the swash plates. The angle of the swash plates can be changed, so the volume and pressure of oil pumped by the pistons can be varied or the direction of the oil reversed. A pump or motor with a movable swash plate is called a variable -displacement unit. A pump or motor having a fixed swash plate is called a fixed displacement unit. Basically, there are four pump-motor combinations:

• Variable displacement pump driving a fixed displacement motor.
• Fixed displacement pump driving a fixed displacement motor.
• Variable displacement pump driving a variable displacement motor.
• Fixed displacement pump driving a variable displacement motor.

Remember: The three factors that control the operation of a hydrostatic drive are:

• Rate of oil flow —gives the speed.
• Pressure of the oil —gives the power.
• Direction of oil flow—gives the direction.

The pump is driven by the engine of the vehicle and is linked to the speed that is set by the operator. It pumps a fixed stream of high -pressure oil to the motor. Since the motor is linked to the tracks or drive wheels of the machine, it gives the machine its travel speed.

Differential

The term differential is derived from the term to differentiate —to show a difference. The actual speed needs of each wheel can be split by the differential without interrupting the power sent to the wheels.

Construction

The crown gear is bolted to the differential case halves, as shown in Figure 4. The breakdown of the parts in the differential assembly is shown in the exploded view. The four legs of the spider are bolted into corresponding holes in the differential case halves. Four pinion gears are mounted to the spider legs, which are in mesh with the teeth of the two side gears.

Fig 4. Exploded view of Differential system

The differential case is held in place with tapered roller bearings on each end that are adjusted with a preload when the running pattern is set up. The flow of power is sent to the carrier assembly from the crown gear where the flow of power is divided and sent to the axles. The side gears have internal splines that mesh with the internal splines of the axles. The carrier assembly is flange-mounted to the banjo housing. The axles are inserted into the ends of the banjo housing and mesh internally with the side gears.

Differential Operation

If a U-turn is performed with a loader, you would see the differential action that takes place inside a conventional differential. If the inside wheel slows down 10%, the outside wheel will speed up 10%.

As an example, a loader with a 10-foot width makes an 180-degree turn. The inside wheel is traveling on a 10-foot (3.05 m) radius and the outside wheel is traveling on a 20-foot (6.1 m) radius. If you perform the calculations, you will see that the inside wheel actually travels a distance of 31.5 feet (9.6 m), while the outside wheel travels over twice that distance, 63 feet (19.2 m). This shows why a differential is necessary on off-road equipment.

Without differential action, the tires would take a real beating—not to mention the strain on the drive axle components. The inside wheel has more resistance when making the turn; this causes unequal torque to be applied to the side gears in the differential. This change in resistance forces one axle to slow down thus forcing the differential pinions to walk around the side gear. This movement, in turn, causes the opposite side gear to speed up an equal amount. There is a downside to this design. If one wheel loses traction, the differential will operate the same as if in a turn. The same amount of torque is sent to both wheels, but it can only be equal to the amount that is required to turn the wheel with the least amount of resistance.

Fig 5. Transmission system

Final Drive

The term final drive, as the name suggests, is the final stage in a series of reductions that begin at the engine flywheel, flow through the torque converter and transmission, and end at the final drive (wheels or tracks). Figure 6 shows a typical final drive assembly used on a motor grader. The term can actually be applied to four types of drive systems: chain drives, pinion drives, planetary drives, and straight axle drives. Final drives receive torque from the transmission, reducing its speed and increasing the torque through the use of various gear and chain drive arrangements.

Fig 6. Final Drive

The main purpose of a final drive is to increase torque and decrease speed; however, some manufacturers take advantage of this arrangement to drop the power flow lower to the ground, such as in a bulldozer or grader. The advantages of this include reducing the drive speed and stresses and increasing the available torque at the drive wheels to move large loads.

General maintenance of Power Train

• Demonstrate knowledge of hydrostatic power train
• Service and repair final drives
• Service power shifts transmissions
• Service and inspect drive lines
• Service and maintain mechanical transmissions