BENDIX SD-20-6608 User Manual

Bendix® A2LS™ Hydraulic Drum Brakes w/Parking

DIRT SHIELD

SD-20-6608

CROSS-OVER PIPE

RETAINING SPRING

WHEEL CYLINDER END SHOE RETURN SPRING

EXPANDER END SHOE

RETURN SPRING

WHEEL CYLINDER

EXPANDER

FIGURE 1 - A2LS™ AUTOMATIC ADJUSTING TWO LEADING SHOE DRUM BRAKE

DESCRIPTION

The A2LS™ drum brake is a hydraulically operated duo-duplex brake with an automatic adjustment and parking brake feature. The A2LS™ drum brake (automatic adjust, two leading shoes) provides two leading shoes in either direction of drum rotation and an automatic adjustment of the brake shoes to compensate for lining wear.

Both the wheel cylinder unit and expander unit have cast integral brake mounting lugs, each taking the brake torque load. The wheel cylinder unit contains two single acting hydraulic cylinders with automatic adjusters. The expander unit contains a double acting hydraulic cylinder with a park brake wedge expander. Wheel cylinder and expander units have either a 1-5/8 inch (41,3mm) or 1-3/4 inch (44,5mm) diameter bore depending upon vehicle brake requirements. The units are hydraulically connected together with a bridge pipe or crossover tube.

GASKET

SHOE HOLD-DOWN CLIP (NOT

FITTED TO EARLY BRAKES)

The single web brake shoes are located by slots in the wheel cylinder tappet head, the expander input tappet/ piston and abutment tappet, and are retained by the shoe return springs. As with all Bendix automatic adjusting brakes, provision is made for manual override of the automatic adjuster. This should only be needed for initial adjustment of the shoes to a speciﬁ c drum clearance, or to back off shoes to enable a lipped drum to be removed.

The brake assembly incorporates an integral backing plate (dirtshield) to protect the brake from excessive water and dirt intrusion. The backing plate (dirtshield) contains a steady post for each brake shoe to insure that lining material is kept parallel to the drum braking surface. Both brake shoes are free to slide on their abutment faces against the backing plate (dirtshield) and are therefore able to centralize themselves to the drum. Lining material is 3/4 inch (19,11 mm) thick in the center of the shoe tapering to approximately 5/8 inch (15,9 mm) at each end. Lining

CROSS-OVER

PIPE

CAP SCREWS

GROMMETS

inspection holes and manual override access holes are covered by rubber grommets.

INPUT/

TAPPET

WHEEL

CYLINDER

PISTONS

PISTON

(ARROW INDICATES DIRECTION OF DRUM ROTATION)

TORQUE

LOAD

TORQUE

LOAD

WHEEL

CYLINDER

PISTONS

FIGURE 2 - SERVICE BRAKE APPLICATION

ROLLER/TAPPET

PISTON

ABUTMENT

TAPPET

OPERATION

On brake pedal application, hydraulic ﬂ uid pressure is delivered to the wheel cylinder and expander unit. This hydraulic pressure forces the wheel cylinder pistons outwards, Figure 2, moving the brake shoes against the drum. At the same time hydraulic pressure also forces the input tappet piston and abutment tappet in the expander unit outward, moving the opposite end of the brake shoes toward the drum. The expander input tappet/piston pushes against one shoe, and the roller tappet/piston pushes on the expander abutment tappet and through this to the other shoe.

(ARROW INDICATES DIRECTION OF DRUM ROTATION)

FIGURE 4 - REVERSE DRUM ROTATION

When the brake shoe lining contacts the drum the shoes are forced around, sliding on the abutments, in the direction Of drum rotation. In the forward direction, Figure 3, the torque load from one shoe is taken by the wheel cylinder piston bottoming in its bore in the wheel cylinder, and from the opposite shoe by the expander abutment tappet resting against the expander unit body. In the reverse direction, Figure 4, the torque load from one shoe is taken by the other wheel cylinder piston bottoming in its bore in the wheel cylinder, and from the opposite shoe by the expander input tappet/piston head resting against the expander body . In this way , regardless of forward or reverse drum rotation, the brake will always be a two leading shoe brake.

PARKING BRAKE APPLICATION

On application of the parking brake, the spring actuator screwed into the back of the expander unit housing pushes the wedge assembly forward. As the wedge assembly

TORQUE

LOAD

FIGURE 3 - FORWARD DRUM ROTATION

TORQUE

LOAD

WEDGE

ASSEMBLY

FIGURE 5 - PARKING BRAKE APPLICATION

PISTON

DRIVE RING

SEAL

SPRING ADJUSTER

ADJUSTER

SHAFT HELIX

DRIVE RING

FIGURE 6 - WHEEL CYLINDER ASSEMBLY CUT-AWAY

moves forward, the wedge rollers force the expander abutment tappet and roller tappet/piston apart, Figure 5, and directly, or through the expander input tappet/piston the brake shoes are forced against the drum. In parking brake operation the brake assembly is a one leading and one trailing shoe brake, in either forward or reverse drum rotation.

AUTOMATIC ADJUSTMENT OF BRAKE SHOES

In the A2LS™ drum brakes, automatic adjustment is controlled by the wheel cylinder units. Adjuster assemblies within the wheel cylinder units control the clearance between the drum and brake shoe by the back-lash built into each adjuster mechanism. During braking application, when no adjustment is required, brake shoe travel takes place within the back-lash available. As the linings wear and the distance between the lining and drum exceeds the predetermined clearance, travel of the piston exceeds the back-lash available and adjustment of the brake shoe is automatically made. Each wheel cylinder piston has an adjuster mechanism screwed into it (Figure 6). The rotation of this adjuster relative to the piston adjusts the brake shoes according to the amount of lining wear. The adjuster shaft has a fast helix thread; onto this is mounted a drive ring having a corresponding thread with an axial clearance (back-lash) built in.

MANUAL OVERRIDE

WHEEL

The adjuster drive ring is held in light contact with its seat in the cylinder body by the drive ring spring. The brake shoe tip engaging in the piston tappet head, and a spring between the adjuster shaft assembly and the face of the adjuster bore, insure that a back-lash is maintained on the mating threads of the adjuster shaft and piston.

When the brake pedal is applied, hydraulic ﬂ uid pressure causes the piston to move outwards, lifting the adjuster

AT REST

SHOE RETURN

SPRING LOAD

DRIVE

RING

SEATED

DRIVE

RING

ADJUSTER

SCREW

PISTON AND ADJUSTER

(SCREW MOVE OUT)

ADJUSTER

SCREW SEATED

FIGURE 7 - ADJUSTER OPERATION

NORMAL APPLICATION

(NO ADJUSTMENT REQUIRED)

DRIVE

RING

SEATED

ADJUSTER

SCREW

DRIVE

RING

SPRING LOAD

BRAKE APPLICATION

(ADJUSTMENT REQUIRED)

DRIVE RING-LIFTED OFF

SEATING-ROTATES RIDES

DOWN HELIX AND RE-SEATS

FIGURE 8 - ADJUSTER OPERATION

BRAKES HELD ON (DRIVE RING

ALREADY SETTLED IN NEW

POSITION)

DRIVE RING RE-SEATED

shaft off its seat in the cylinder body. If the movement of the adjuster shaft helix through the adjuster drive ring is less than the axial clearance (back-lash), no rotation of the adjuster shaft takes place. (Figure 7)

When the movement of the adjuster shaft helix through the adjuster drive ring is greater than the axial clearance, rotation of the drive ring relative to the adjuster shaft will occur. (Figure 8) As the adjuster shaft helix passes through the drive ring, the drive ring, lightly loaded by the drive ring spring, will rotate in its seat in the cylinder body. As the brake pedal is released, the brake shoe return springs cause the brake shoe to push the piston and adjuster shaft assembly back through the cylinder.

As the axial tolerance between the adjuster shaft helix and adjuster drive ring is taken up, the load on the drive ring helix forces it onto its seat. The resultant friction keeps it from further rotation. As the piston and adjuster continue through the cylinder, the adjuster shaft helix is forced to rotate through the drive ring. (Figure 9) This rotation of the adjuster shaft, relative to the piston, unscrews the adjuster shaft from the piston, increasing the overall length of the adjuster and piston assembly. This in turn reduces the brake shoe lining to drum clearance.

The automatic adjustment rate is variable; each application of the brake takes up only a percentage of the excess shoe to drum clearance. This avoids over adjustment when the drums get hot and expand.

Note: Due to manufacturing tolerances, it is unlikely that

all brake shoes will run at the same clearances. No detrimental effect will result and no attempt should be made to standardize these clearances.

MANUAL OVERRIDE MECHANISM

The manual override mechanism is used to override the automatic adjuster to set the initial shoe to drum clearance. This initial adjustment is made after new brake shoes have been installed, or other maintenance work has been performed requiring the removal of the brake drum. This adjustment ‘is necessary to avoid excessive movement of the wheel cylinder pistons and to achieve a correct and stabilized running clearance.

The manual override is also used to un-adjust the brakes when removing the drum when a lipped drum condition exists.

IMPORTANT! PLEASE READ:

BRAKES HELD ON (DRIVE RING ALREADY

SETTLED IN NEW POSITION)

SHOE RETURN

SPRING LOAD

DRIVE RING RE-SEATED

FIGURE 9 - ADJUSTER OPERATION

DRIVE RING

SEATED

RETURN STROKE

ADJUSTMENT COMPLETED

The manual override should not be used as an alternative to the auto-adjuster. It is essential that the auto-adjust mechanism be kept in good operating condition to insure safe and reliable brake operation.

PREVENTIVE MAINTENANCE

The A2LS™ drum brakes normally do not require any service except when new linings are being installed. The automatic adjusters eliminate the need for any periodic adjustment of the brakes. To insure safe operation of the brakes, the following service checks should be performed.

The following is a recommended time table; the actual times may vary depending upon the use of the vehicle and the type of operation and condition the vehicle is exposed to.

LINE HAUL VEHICLES

Every 50,000 miles (80,000 km), six months, or 900 operating hours, whichever occurs ﬁ rst, perform the service checks outlined in this manual.

CITY DELIVERY

Every 15,000 miles (24,000 km), two months, or 300 operating hours, whichever occurs ﬁ rst, perform the service checks outlined in this manual.

SERVICE CHECKS

With the vehicle on a level surface, block the wheels to prevent movement of the vehicle.

1. Remove one or more of the lining inspection plugs. Check the thickness of the brake linings.

2. Activate the brake pedal. Observe the brake shoes through the inspection holes and make sure that the brake shoes move against the drums smoothly and promptly . Release the brake pedal and make sure the brake shoes retract promptly and smoothly.

3. Apply the parking brake. Observe the brake shoes through the inspection hole and make sure the brake shoes engage the drum.

4. Check all hydraulic ﬁ ttings and the surface of the backing plate (dirtshield) for any evidence of hydraulic ﬂ uid leakage.

5. Refer to the vehicle manual for service checks of other brake related items.

WARNING! PLEASE READ AND FOLLOW THESE INSTRUCTIONS TO A VOID PERSONAL INJURY OR DEATH:

When working on or around brake systems and components, the following precautions must be observed at all times:

1. Park the vehicle on a level surface, apply the

parking brakes, and always block the wheels. When working around or under the vehicle, stop the engine and remove the key from the ignition. Always keep hands away from chambers as they may apply as system pressure drops. Always wear safety glasses.

2. When working in the engine compartment, the

engine should be shut off and the ignition key should be removed. Where circumstances require that the engine be in operation, extreme caution should be used to prevent personal injury resulting from contact with moving, rotating, leaking, heated or electrically charged components.

3. Do not attempt to install, remove, disassemble

or assemble a component until you have read and thoroughly understand the recommended procedures. Use only the proper tools and observe all precautions pertaining to the use of those tools.

4. Following the vehicle manufacturer’s recommended procedures, deactivate the electrical system in a manner that safely removes all electrical power from the vehicle.

5. If the vehicle is equipped with an air over hydraulic brake system or any auxiliary pressurized air system, make certain to drain the air pressure from all reservoirs before beginning ANY work on the vehicle. If the vehicle is equipped with an AD-IS® air dryer system or a dryer reservoir module, be sure to drain the purge reservoir.

6. Never connect or disconnect a hose or line containing pressure; it may whip. Never remove a component or pipe plug unless you are certain all system pressure has been depleted.

7. Never exceed manufacturer’s recommended pressure.

8. Never attempt to disassemble a component until you have read and understand all recommended procedures. Some components contain powerful springs and injury can result if not properly disassembled. Use only proper tools and observe all precautions pertaining to use of those tools.

9. Use only genuine Bendix® replacement parts, components and kits.

A. Use only components, devices and mounting

and attaching hardware speciﬁ cally designed for use in hydraulic brake systems.

B. All replacement hardware, tubing, hose,

ﬁ ttings, etc. must be of equivalent size, type and strength as the original equipment.

10. Components with stripped threads or damaged parts should be replaced rather than repaired. Do not attempt repairs requiring machining or welding unless speciﬁ cally stated and approved by the vehicle and component manufacturer.

11. Prior to returning the vehicle to service, make certain all components and systems are restored to their proper operating condition.

ADJUSTMENT

OPENING

FIGURE 10 - REAR VIEW OF THE BACKING PLATE (DIRTSHIELD)

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