797 Off-Highway Truck Power Train Transmission Caterpillar


Transmission
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1.1. Transmission Planetary Groups
2.1. Transmission Operation
3.1. Transmission Clutch Engagement
4.1. Transmission Clutch Latching



Illustration 1g00934651

View of transmission

(1) Transmission lubrication valve

(2) Transmission filter

Power flows from the torque converter to the transmission through a drive shaft. The transmission is mounted to the rear axle housing. The output shaft of the transmission engages the pinion gear of the differential. Power from the transmission flows through the differential. The power divides equally to the final drives that are in the rear wheels.

The transmission is electronically controlled and hydraulically operated. The Transmission ECM and the Chassis ECM analyze inputs that are received from various sensors and switches. The Transmission ECM will then energize the solenoids that control the flow of oil to the transmission clutches.

The power shift transmission is a planetary design that contains seven hydraulically engaged clutches. The transmission provides seven forward speeds and one reverse speed.

Transmission filter (2) is mounted on the front transmission case. Transmission lubrication valve (1) is mounted on the rear transmission case.

Transmission Planetary Groups




Illustration 2g00934664

Front and rear transmission planetary groups

(3) Input shaft

(4) Front transmission planetary group

(5) Rear transmission planetary group

(6) Output shaft

The transmission contains two planetary groups. Power flows from the torque converter through input shaft (3) to front transmission planetary group (4). Clutches one, two, and three are in front transmission planetary group (4) .

Rear transmission planetary group (5) sits behind front transmission planetary group (4). Clutches four, five, six, and seven are in rear transmission planetary group (5). Power flows from front transmission planetary group (4) to rear transmission planetary group (5) and to output shaft (6). Output shaft (6) drives the pinion gear of the differential.

Transmission Operation




Illustration 3g01962166

View of transmission

(1) Transmission lubrication valve

(2) Transmission filter

(7) Port

(8) Port

(9) Port

(10) Housing

Oil from the transmission section of the pump flows through port (7) to the transmission filter (2). Oil flows through the transmission filter (2) to the transmission control valves. When the solenoids for the control valves are energized by the Transmission ECM, oil flows to the transmission clutches. Oil also flows from the transmission filter (2) to the modulating valve for the torque converter lockup clutch. Oil from the transmission charging section of the pump is used in order to engage the lockup clutch which provides direct drive.

After the lockup clutches are engaged, most of the transmission charging oil flows out of a port on the right front side of the transmission case and to the torque converter inlet relief valve. Oil from the transmission section of the pump joins with oil from the torque converter section of the pump and is used in by the torque converter in order to provide a fluid coupling.

Oil from the transmission lubrication section of the pump flows through port (8) to the transmission lubrication valve (1). Oil flows through transmission lubrication valve (1) to the rear transmission planetary group and through port (9) to the front transmission planetary group. Lubrication oil is used in order to cool and lubricate the gears, bearings, and clutches in the transmission planetary groups. Oil from the transmission section of the pump and transmission lubrication oil that falls to the bottom of the transmission case is scavenged from the case through two magnetic screens located in housing (10). The scavenged oil returns to the torque converter housing through a diffuser.

Transmission Clutch Engagement




Illustration 4g01962167

(11) Latching solenoid

(12) Transmission main relief valve

(13) Clutch modulating valves

(14) Latching valve

(15) Pilot drain port

(16) Orifice

(17) Shuttle spool

(18) Accumulator

(19) Latching pressure sensor

Latching solenoid (11) is energized by the Transmission ECM, which allows charging pressure to flow to the left side of shuttle spool (17) in latching valve (14). Charging pressure, in combination with the bias spring, prevents shuttle spool (17) from blocking pilot drain port (15) of clutch modulating valve (13). When the solenoid for clutch modulating valve (13) is energized, transmission charging oil flows to the clutch and through orifice (16) to accumulator (18). Accumulator (18) becomes fully stroked when clutch pressure reaches 1550 kPa (225 psi). Clutch pressure builds on the right side of shuttle spool (17), but will not move shuttle spool (17) since clutch pressure is unable to overcome the combination of charging pressure and bias spring.

Transmission Clutch Latching




Illustration 5g01962198

(11) Latching solenoid

(12) Transmission main relief valve

(13) Clutch modulating valves

(14) Latching valve

(15) Pilot drain port

(16) Orifice

(17) Shuttle spool

(18) Accumulator

(19) Latching pressure sensor

If the Transmission ECM detects a fault in the solenoid for clutch modulating valve (13) or in latching solenoid (11), latching solenoid (11) is de-energized. When latching solenoid (11) is de-energized, the transmission is immediately latched in the current gear. If electrical power is lost to both latching solenoid (11) and clutch modulating valve (13), control port pressure to the clutch pistons will decay. Accumulator (18) and orifice (16) are provided for each clutch in order to slow the accumulator decay of the clutch. This will allow shuttle spool (17) in latching valve (14) to shift to the latched position before an engaged clutch is released.

If electrical current to clutch modulating valve (13) is lost due to an electrical system failure, the charging pressure in the spring cavity of shuttle spool (17) is vented to tank through latching solenoid (11). Accumulator pressure moves shuttle spool (17) against the bias spring and will block pilot drain port (15) of clutch modulating valve (13). Charging pressure will then flow past shuttle spool (17) for latching valve (14) and enter the pilot chamber of clutch modulating valve (13). The spool for clutch modulating valve (13) will shift to the right, causing clutch pressure and charging pressure to be the same.

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