Two separate brake systems are used on the 777F. The two brake systems are the service/retarder brake system and the parking/secondary brake system. The parking/secondary brakes are spring engaged and hydraulically released. The service/retarder brakes are hydraulically engaged and spring released.
The braking system is also equipped with a Brake ECM that controls the braking system functions, including the Automatic Retarder Control (ARC) and the Traction Control System (TCS).
The air system on the previous model trucks has been completely removed.
The main components in the braking system are:
- Brake charging pump (1)
- Brake cooling pump (standard oil cooled front brakes) (2)
- Accumulator charging valve (3)
- Brake accumulators (4)
- Cab brake manifold (5)
- Service brake valve (6)
- Brake oil filter (7)
- Front slack adjuster (8)
- Brake accumulator check valve (9)
The rear brakes on the 777F Trucks are oil cooled. Shown is a cutaway illustration of an oil cooled brake assembly. The brakes are environmentally sealed and adjustment free. Oil continually flows through the brake discs for cooling. Duo-Cone seals prevent the cooling oil from leaking to the ground or transferring into the axle housing. The wheel bearing adjustment must be maintained to keep the Duo-Cone seals from leaking.
The smaller piston (yellow) is used to engage the secondary and parking brakes. The parking brakes are spring engaged and hydraulically released. The larger piston (purple) is used to engage the service and retarder brakes. The service and retarder brakes are engaged hydraulically and released by spring force.
The standard oil cooled front brakes are also environmentally sealed and adjustment free. The piston (yellow) is used to ENGAGE the service/retarder brakes. The front brakes do not have a second piston for the parking/secondary brakes.
When the wheel is removed for service, the small plug at the lower left must be removed (the brake assembly is equipped with two similar plugs). Two 3/8 inch bolts must be installed at the plug locations to hold the brake discs and plates in position during wheel removal. The bolts ensure proper alignment of the teeth on the discs and plates during installation.
With the optional disc and caliper design brakes, the brake caliper assemblies are fastened to the spindle and do not rotate. The brake disc is fastened to the wheel and rotates with the wheel. Air can be bled from the front brakes through the bleed valves. During brake application, hydraulic oil from the brake cylinders forces the brake pistons against the brake carrier linings (brake pads). The brake linings are forced against the disc to stop the rotation of the wheel
The brake charging pump (1), the brake oil cooling pump (2), and the hoist pump (3) are mounted to the pump drive gear on the left rear side of the engine. The 777F brake system accumulators are charged by the brake charging pump, which supplies oil to the accumulator charging valve. The oil cooling pump sends oil to the oil coolers before the oil flows to the front and rear brakes for brake cooling.
The brake system filter (1) is located on the left outer rear frame next to the left rear strut mount. The brake filter includes a filter bypass switch (2), which sends a signal to the Brake ECM if the filter is restricted. The Brake ECM sends a signal to the monitoring system, which illuminates the brake system-check indicator lamp.
The accumulator charging valve (1) is located on the left side of the frame near the brake accumulators. The accumulator charging valve directs oil to the brake accumulators, brake oil coolers, and the tank. Once the accumulators are charged, the excess oil flow is sent to cool the brakes before returning to the tank.
The Brake ECM monitors the pressure in the service brake accumulators with the brake accumulator pressure switch (2). If the pressure in the service brake accumulators is low, the Brake ECM will signal the monitoring system to turn on the brake system-check indicator lamp. A relief (3) valve limits the pressure in the brake charging circuit.
A pressure tap (4) on the line between the brake charging pump and the accumulator charging valve is used to check the charge oil pressure from the pump. The pressure tap (5) on the charging valve is used to check the oil pressure in the service brake accumulators.
The accumulator charging valve maintains the pressure in the accumulators at a constant rate while the engine is running. If the machine has lost power or the hydraulic pump has failed, the pressure in the accumulators will permit several applications of the service brakes. This illustration shows the accumulator charging valve in the CUT-IN position. When the accumulator oil pressure decreases below a certain point, the accumulator charging valve reaches the cut-in pressure setting. The pressure decrease allows spring force to move the cut-in/cut-out spool to the left and oil flows to the right end of the unloading valve. The orifice in the unloading valve restricts the pump flow to the brake cooling system. Oil flow to the brake accumulators increases and the accumulators are charged.
The accumulator oil pressure switch sends a signal to the Brake ECM to alert the operator when the brake oil pressure drops below the minimum operating pressure.
This illustration shows the accumulator charging valve in the CUT-OUT position. When the accumulator oil pressure increases to the cut-out pressure setting, the increased pressure causes the cut-in/cut-out spool to move right against spring force. Oil at the right end of the unloading valve flows to the tank. Oil pressure on the left end of the unloading spool overcomes the decreased oil pressure on the right end of the spool and spring force. Most of the brake charging pump oil now flows to the brake cooling system.
The check valve prevents high accumulator oil pressure from flowing to the brake cooling system.
The accumulator charging valve remains in the CUT-OUT position until the pressure in the accumulators decreases to the cut-in pressure setting.
The pressure relief valve regulates the oil pressure in the brake circuit. Any excess oil that is not required by the brake cooling system or the brake circuit is diverted back to the hydraulic oil tank.
There are the three brake accumulators for the 777F located on the left side of the frame. The service brake accumulators (1) and parking brake accumulator (2) are charged by the brake charging pump and supply the required oil flow to engage the front and rear service brakes and release the rear parking brakes.
A check valve in the circuit between the parking brake accumulator and the service brake accumulators allows only the parking brake accumulator to be charged when using the electric brake release pump.
The cab brake manifold (1) is mounted below the cab on the left upper frame. The cab brake manifold contains the ARC control solenoid (2) and the front service brake solenoid (3). The ARC control solenoid is part of the ARC system. The ARC system uses the rear service brakes and the front oil cooled brakes to automatically control the speed of the truck.
The service brake pressure switch (4) is located near the cab brake manifold toward the front of the machine. The service brake pressure switch sends a signal to the Brake ECM when the service brakes are engaged. The Brake ECM will use the signal from the pressure switch to energize the stop lamp relay (located in cab) and turn on the brake lights. In a low pressure situation, the Brake ECM will signal the monitoring system to activate the brake system-check indicator.
The service brake valve (1) is mounted below the floor of the operator’s cab. When the service brake pedal (2) is depressed, an internal spool directs oil flow from the service brake accumulators to the rear service brakes.
The amount of oil flow to the front service brakes is determined by the Brake ECM based on a signal from the service brake pedal position sensor (3). The Brake ECM allows some oil from the brake accumulators to flow to the front brakes by controlling the position of the front brake solenoid located in the cab brake manifold.
When the manual retarder lever (1) is activated, a PWM signal is sent to the Brake ECM. The Brake ECM sends a signal to the ARC solenoid and the front brake solenoid. The solenoids control the amount of oil flow to the service brakes based on the position of the retarder lever.
If the ARC switch (2) is activated, the Brake ECM sends a signal to the ARC solenoid and the front brake solenoid. The solenoids control the amount of oil flow to the service brakes based upon the input signals that the Brake ECM receives from the engine speed sensor.
NOTE: If the truck is equipped with the optional front caliper type brakes, the Brake ECM will command all oil to flow to the rear brakes when the retarder lever is moved or the ARC switch is activated.
The optional engine brake switch (3) is also an input to the Brake ECM. The Brake ECM communicates the status of the brake switch to the Engine ECM via the Cat Data Link. The Engine ECM controls the compression brake application (if equipped).
The front brake switch (4) is installed on machines with caliper type front brakes. When activated, the front brake switch sends a signal to the Brake ECM which allows the front brakes to be engaged when the brake pedal is depressed. When the front brake switch is in the OFF position, only the rear brakes will be engaged when the brake pedal is depressed.
The 777F has two slack adjusters. The top illustration shows the rear slack adjuster (1). The rear slack adjuster is located above the rear differential. The bottom illustration shows the front slack adjuster (2). The front slack adjuster is located on the left strut frame support. The slack adjusters compensate for brake disc wear by allowing a small volume of oil to flow through the slack adjuster and remain between the slack adjuster and the brake piston under low pressure. The slack adjusters maintain a slight pressure on the brake piston at all times. Brake cooling oil pressure maintains a small clearance between the brake discs. The service brake oil pressure can be tested at the taps (3) located on top of the slack adjusters.
This illustration shows sectional views of the slack adjuster when the brakes are RELEASED and ENGAGED. When the brakes are ENGAGED, oil from the brake cylinder enters the slack adjuster and the two large pistons move outward. Each large piston supplies oil to one wheel brake. The large pistons pressurize the oil to the service brake pistons and ENGAGE the brakes.
Normally, the service brakes are FULLY ENGAGED before the large pistons in the slack adjuster reach the end of their stroke. As the brake discs wear, the service brake piston will travel farther to FULLY ENGAGE the brakes. When the service brake piston travels farther, the large piston in the slack adjuster moves farther out and contacts the end cover. The pressure in the slack adjuster increases until the small piston moves and allows makeup oil from the brake cylinder to flow to the service brake piston.
When the brakes are RELEASED, the springs in the service brakes push the service brake pistons away from the brake discs. The oil from the service brake pistons pushes the large pistons in the slack adjuster to the center of the slack adjuster. Makeup oil that was used to ENGAGE the brakes is replenished at the brake cylinder from the makeup tank.
The spring behind the large piston causes some oil pressure to be felt on the service brake piston when the brakes are RELEASED. Keeping some pressure on the brake piston provides rapid brake engagement with a minimum amount of brake cylinder piston travel.
The slack adjusters can be checked for correct operation by opening the service brake bleed screw with the brakes RELEASED. A small amount of oil should flow from the bleed screw when the screw is opened. The small flow of oil verifies that the spring behind the large piston in the slack adjuster is maintaining some pressure on the service brake piston.
The service brake bleed screw (1) is identified by an "S" on the brake anchor plate casting next to the screw. The parking brake bleed screw (2) is identified by a "P" on the casting. Another check to verify correct slack adjuster operation is to connect a gauge to the pressure tap on top of the slack adjuster and another gauge at the service brake bleed screw location on the brake anchor plate casting.
With the service brake pedal depressed, the pressure reading on both gauges should be approximately the same. When the brakes are RELEASED, the pressure at the slack adjuster should return to zero. The pressure at the service brake bleed screw location should return to the residual pressure held on the brakes by the slack adjuster piston.
If the slack adjuster residual pressure is too low, it could indicate a failed slack adjuster. High residual pressure may indicate a failed slack adjuster or warped brake discs. To check for warped brake discs, rotate the wheel to see if the pressure fluctuates. If the pressure fluctuates while rotating the wheel, the brake discs are probably warped and should be replaced.
To check for brake cooling oil leakage, block the brake cooling ports and pressurize each brake assembly to a maximum of 138 kPa (20 psi). Close off the air supply source and observe the pressure trapped in the brake assembly for five minutes. The trapped pressure should not decrease.
The parking brake valve (1) is located on the inside left frame rail behind the center cross member. The parking valve receives oil flow from the parking brake accumulator. Contained within the valve is a parking brake solenoid valve (2) and a purge solenoid valve (3).
When the parking brake solenoid is energized by the Brake ECM, the parking brake valve directs oil flow through the TCS valve to release the rear parking brakes. There are no parking brakes on the front wheels. When the transmission shift lever is moved to PARK a signal is sent to the Brake ECM to engage the parking brakes. There is not a separate parking brake control switch. The secondary brake pressure switch (4) sends a signal to inform the Transmission/Chassis ECM that the secondary/parking brake is engaged.
When the machine is shut down, the purge solenoid is energized by the Transmission/Chassis ECM and the purge valve drains the brake accumulators to tank.
The parking brake pressure can be checked at the left parking brake tap (1) and at the right parking brake tap (2).
The secondary brake pedal position sensor (arrow) is located on the back of the secondary brake pedal. The position sensor sends a signal to the Brake ECM indicating the position of the secondary brake pedal. The Brake ECM sends a signal to the parking brake solenoid which controls the secondary brake application at the rear brakes.
The secondary steering/brake release/QuickEvac pump and motor are located on the front of the front crossmember as previously shown. The brake retract pump section (arrow) provides oil to release the parking brakes and hoist pilot oil for lowering the body on trucks with a dead engine.
The diverter (towing) valve (arrow) is located on the left hoist cylinder frame support. The diverter valve is used to unlock the brakes for towing and must be manually shifted before towing. Once the valve is shifted, oil flow from the electric secondary steering/brake retract pump is directed to the parking brake valve to release the parking brake.
To release the parking brakes for service work or towing, the electric motor on the pump is energized by the brake release switch located in the cab. When the key start switch is turned ON, the secondary steering system is energized for three seconds to check the system. Since the towing pump is driven by the same electric motor as the secondary steering pump, the diverter valve allows the towing pump oil to flow directly to the hydraulic tank during the secondary steering test.
To shift the diverter valve, loosen the two diverter valve clamp bolts and slide the plate and the spool to the left. After the spool is shifted, tighten the diverter valve clamp bolts. When the electric motor is energized, supply oil can flow from the towing pump, through the diverter valve, to the parking brake valve. The brake release pump is also used to provide pilot oil to lower the body when the engine is off.
This schematic shows the oil flow through the brake cooling system on the 777F Trucks with standard oil cooled front brakes. The brake cooling pump supplies oil to the brake coolers and the front and rear brakes. The brake cooling system also receives oil from the following components:
- Hoist valve (in the HOLD, FLOAT, and SNUB positions)
- Accumulator charging valve
- Torque converter lockup clutch relief valve
- Torque converter outlet relief valve
The pressure in the brake cooling system is limited by a relief valve located in the hoist valve. The relief valve is usually needed only when the brake cooling oil is cold. When brake cooling oil is at operating temperature, the brake cooling oil pressure is usually much lower than the setting of the oil cooling relief valve.
The brake cooling pump (1) is a gear type pump that is attached to the drive gear at the rear of the engine. The brake cooling pump is located between the hoist pump (2) and the brake charging pump (3). Oil flows from the brake cooling pump to the brake oil coolers.
The brake oil coolers (arrows) are located on the right side of the engine. Engine coolant from the water pump flows around the brake oil coolers and back to the cylinder block. The engine coolant transfers the heat from the brake oil system to the engine coolant. Oil from the brake cooling pump flows through screens (not shown) before flowing through the brake oil coolers.
Shown is the left rear brake housing. Brake cooling oil pressure can be tested at the two taps (arrows) located in the brake cooling oil tubes. One tap is located on the brake cooling inlet tube and another tap is located on the brake cooling outlet tube. The pressure measured at the brake inlet tube (from the oil coolers) will always be higher than the pressure measured at the brake outlet tube.
A brake oil temperature sensor is located in a brake oil cooling tube on the truck. The brake oil temperature sensor sends a signal to the Brake ECM indicating brake oil temperature. The Brake ECM will send a signal over the Cat Data Link, which informs the monitoring system to display the temperature on the brake temperature gauge.
The most common cause of high brake cooling oil temperature is operating the truck in a gear range which is too high for the grade and not maintaining a high enough engine speed. The engine speed should be maintained at approximately 1900 rpm during long downhill hauls. Make sure the oil cooling relief valve is not stuck open. Also, make sure the pistons in the slack adjuster are not stuck and holding too much pressure on the brakes.
See You Soon!!!
MARYGAR
Thanks,
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ReplyDeleteOP, can you perhaps indicate the following on the 777 F:
System Operating Brake Pressure
Accumulator sizes and pre-charge pressure (service brakes)
Brake system relief pressure
Brake charging pump size
Accumulator cut-in and cut-out pressures
Your assistance would be highly appreciated!
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