797F Off-Highway Truck: Hydraulic System: Hoist System


797F Schematic for the Hoist System (1) Gear Pump (Brake Cooling, Hoist) (2) Hoist Screens (3) Passage from the Control Valve and Mounting (4) Passage to the Brake Oil Coolers (5) Bypass Solenoids for the hoist pump (6) ON/OFF Solenoid for the hoist pilot (7) Control Valve (Brake Cooling, Hoist) (8) Hoist Control Solenoids(9) Hoist Cylinders

Location of Components


797F components for the steep brake cooling system (1) Gear Pump (Brake Cooling, Hoist) (2) Hoist Screens (7) Control Valve (Brake Cooling, Hoist) (9) Hoist Cylinders (10) Front Brake Oil Cooler (11) Rear Brake Oil Coolers (12) Hydraulic Tank (Brake, Hoist, Steering) (13) Control Valve and Mounting (14) Pump Drive(15) Piston Pump (Brake Cooling)

Note: The 797F XQ Off-Highway Truck uses the steep brake cooling system and not the shallow brake cooling system.

Operation


Hoist lever (A) RAISE position (B) HOLD position (C) FLOAT position(D) LOWER position

The hoist system is electronically controlled by the Chassis ECM. The hoist system is similar in operation to other Caterpillar off-highway trucks. The system consists of the following four operating positions:

RAISE position (A)
HOLD position (B)
FLOAT position (C)
LOWER position (D)

The truck should normally be operated with the hoist lever in the FLOAT position. Operating the truck with the hoist in the FLOAT position will ensure that the weight of the truck body is located on the frame and body pads.

Note: The weight of the truck body will be supported by the hoist cylinders if the truck is not operated with the hoist lever in the FLOAT position.

The Chassis ECM and Control Valve (Brake Cooling, Hoist) (7) also provide the following functions for the hoist control that are not controlled by the operator:

LOWER SNUB control
OVERCENTER control
RAISE SNUB control

LOWER SNUB control occurs while the truck body is being lowered to the DOWN position. Before the truck body contacts the truck frame, the Chassis ECM signals the hoist solenoids to move the hoist valve spools to the LOWER SNUB position. This will occur when the truck body is approximately located between 4 degrees to 15 degrees in relation to the truck frame. In the LOWER SNUB position, the float speed of the truck body is reduced in order to prevent the truck body from contacting the truck frame too hard.

OVERCENTER control prevents cavitation of the hoist cylinders by preventing the truck body from raising faster that is allowed by the system. This condition may occur if there is a sudden shift in the location of the load. While the truck body is being raised, the Chassis ECM signals the hoist solenoids to move the hoist valve spools to the OVERCENTER position when the truck body is approximately at a 40 degree angle.

RAISE SNUB control occurs when the truck body is being raised and the hoist cylinders are almost extended. The truck body will be approximately at a 55 degree angle. The Chassis ECM signals the hoist solenoids to move the hoist valve spools to the RAISE SNUB position. In the RAISE SNUB position, the raise speed of the truck body is reduced in order to prevent the hoist cylinder pistons from making hard contact with the cylinder heads.

Right inside frame rail
(1) Gear Pump (Brake Cooling, Hoist)
(2) Hoist Screens
(14) Pump Drive
(15) Piston Pump (Brake Cooling)(16) Bypass Switch (Hoist Screen)

Gear Pump (Brake Cooling, Hoist) (1) is attached to the top rear of Pump Drive (14) directly above Piston Pump (Brake Cooling) (15) for the steep brake cooling arrangement.

Note: The 797F XQ Off-Highway Truck uses the steep brake cooling system and not the shallow brake cooling system.

If the shallow brake cooling arrangement is used, then Gear Pump (Brake Cooling) (1) is attached to the top rear of Pump Drive (14) directly above Gear Pump (Brake Cooling). Gear Pump (Brake Cooling, Hoist) (1) pulls oil from Hydraulic Tank (Brake, Hoist, Steering) (12). Gear Pump (Brake Cooling, Hoist) (1) sends the oil through Hoist Screens (2) to Control Valve (Brake Cooling, Hoist) (7). Control Valve (Brake Cooling, Hoist) (7) is mounted to a bracket which is attached to the torque tube. Control Valve (Brake Cooling, Hoist) (7) uses reduced brake pressure oil from Control Valve and Mounting (13) as the pilot oil in order to shift the directional spools inside Control Valve (Brake Cooling, Hoist) (7) .

There is a bypass valve that is located inside the housing of each hoist screen. The bypass valves will open in order to protect the hoist system if the screens become restricted. Bypass Switch (Hoist Screen) (16) is also located on the housing of each hoist screen. The bypass switches provide input signals to the Chassis ECM. The Chassis ECM sends the signals to the Vital Information Management System (VIMS). VIMS will then inform the operator if the hoist screens are restricted.

Bypass Solenoids (5), ON/OFF Solenoid (6) for the hoist pilot, and Hoist Control Solenoids (8) are all located inside of Control Valve (Brake Cooling, Hoist) (7). When Hoist Cylinders (9) are raised and lowered, the Chassis ECM energizes ON/OFF Solenoid (6) for the hoist pilot, both Bypass Solenoids (5) for the hoist pump, and two of the four Hoist Control Solenoids (8). One Hoist Control Solenoid (8) is energized in order to provide supply oil to Hoist Cylinders (9). Another Hoist Control Solenoid (8) is energized in order to drain oil from Hoist Cylinders (9) .

When Control Valve (Brake Cooling, Hoist) (7) is in the HOLD, SNUB, or FLOAT position, the Chassis ECM de-energizes two Bypass Solenoids (5) for the hoist pump which causes all of the oil from Gear Pump (Brake Cooling, Hoist) (1) to flow through Front Brake Oil Cooler (10) and Rear Brake Oil Coolers (11) to the brakes.

Two pump relief valves and two line relief valves are located in Control Valve (Brake Cooling, Hoist) (7). The pump relief valves limit the hoist system pressure. The line relief valves also limit system pressure. The line relief valves are set higher than the pump relief valves. If the hoist valve is in the HOLD position and an external force moves the body, then the line relief valves will open.


Extended Hoist Cylinders (9)

Two Hoist Cylinders (9) are used in order to raise the body away from the frame of the truck. When the hoist lever is held in the RAISE position, supply oil flows to the head end of Hoist Cylinders (9) which causes the two-stage cylinders to extend to full length. Then, the oil from the rod end of the cylinders flows through Control Valve (Brake Cooling, Hoist) (7) to Hydraulic Tank (Brake, Hoist, Steering).

When the hoist lever is moved to the LOWER or FLOAT position and Hoist Cylinders (9) are extended, supply oil enters the rod end of the hoist cylinders which lowers the second stage of the cylinders. The oil from the head end of Hoist Cylinders (9) flows through Control Valve (Brake Cooling, Hoist) (7) to Hydraulic Tank (Brake, Hoist, Steering) (12).


Left inside frame rail (13) Control Valve and Mounting (17) Parking and Secondary Brake Accumulators(18) Brake Release Pump and Motor

In order to lower the truck body with an inoperable engine, hoist pilot pressure is required. Brake Release Pump and Motor (18) is located inside the left frame rail of the machine next to Parking and Secondary Brake Accumulators (17) and above Control Valve and Mounting (13). Use Brake Release Pump and Motor (18) in order to provide the pilot oil for the hoist. Use the following procedure to lower the truck body:

Turn the engine start switch to the ON position. The electric drive pump and the hoist cylinders will be energized.

Move the hoist lever to the RAISE position for 15 seconds. Then move the hoist lever to the FLOAT position.

Depress the brake release switch that is located on the dashboard of the operator's cab.

Note: For more information on lowering the truck body with an inoperable engine, refer to the following information: Operation and Maintenance Manual, SEBU8417, "Lowering the Body with the Engine Stopped", Braking System Systems Operation, "Towing" and Braking System Testing and Adjusting, "Towing the Machine - Preparation".

To raise the truck body with an inoperable engine, connect an Auxiliary Power Unit (APU) to the hoist cylinders. Use the oil ports that are located on the bottom of the left hoist cylinder. Repeat Step 1 through Step 3 in order to raise the truck body.

Note: During Step 2, do not move the hoist lever to the FLOAT position after 15 seconds. Keep the hoist lever in the RAISE position.

MARYGAR