Bendix Commercial Vehicle Systems SD-05-1200 User Manual

Bendix Manual Slack Adjusters

WORM GEAR

LOCK SLEEVE

ADJUSTING SCREW

(WORM SHAFT)

ADJUSTING GEAR

GREASE HOLE

SPLINE

FIGURE 1 - POSITIVE LOCK TYPE SLACK ADJUSTER

SD-05-1200

WORM SHAFT (LOCK

SCREW)

WORM GEAR

ADJUSTING

GEAR

ADJUSTING SCREW

(WORM SHAFT)

SPLINE

FIGURE 2 - BALL INDENT TYPE SLACK ADJUSTER

DESCRIPTION

In an s-cam type foundation brake, the final link between
the pneumatic system and the foundation brake is the slack
adjuster. The arm of the slack adjuster is fastened to the
push rod of the chamber with a yoke, and the slack adjuster
spline is installed on the brake cam shaft.

Primarily , the slack adjuster is a lever , converting the linear
force of the air chamber push rod into a torque which turns
the brake cam shaft and applies the brakes.

Standard slack adjusters contain four basic components;
the body, worm, gear and adjusting screw. The adjusting
screw is provided to adjust the slack caused by the wear of
the brake lining.

All slack adjusters utilize the worm and gear principle and,
fundamentally, differ only in torque limit specifications; A
type 20 slack adjuster has a limit of 20,000 inch pounds
torque, a type 30 slack adjuster has a limit of 30,000 inch
pounds torque, etc.

Slack adjusters are manufactured with various arm lengths
and various configurations; straight, offset, etc. to satisfy
various installation requirements. Splines are available in
several different types and sizes.

OPERA TION

When the brakes are applied, air pressure forces the air
chamber diaphragm and push rod to move; this rotates the
slack adjuster, which in turn rotates the cam shaft. This
causes the “S” cam to spread the brake shoes which contact
the brake drum.

When the brakes are released, air pressure is exhausted
from the air chamber. The chamber return spring and the
brake shoe return springs return the brake cam, cam shaft,
slack adjuster and chamber push rod to the released position.

ADJUSTING MECHANISM

The adjusting mechanism of a slack adjuster consists of an
adjusting screw (worm shaft), worm and slack adjuster worm
gear. T urning the adjusting screw nut on the end of the worm
shaft rotates the worm shaft and worm. The worm meshes
with and rotates the slack adjuster worm gear which is
connected to the brake cam by a splined cam shaft. The
turning of the slack adjuster worm gear rotates the cam
shaft and brake cam, spreading the brake shoes,
compensating for brake lining wear.

There are two types of adjusting mechanisms used on Bendix
slack adjusters currently manufactured. The light to medium
torque rated slack adjusters (Fig. 1) use a positive lock
mechanism consisting of a spring loaded lock sleeve, which
when positioned properly , engages the adjusting screw nut,
preventing the adjusting screw (worm shaft) from rotating.

The heavier torque rated slack adjusters (Fig. 2) utilize the
lock ball or plunger and worm shaft indent principle
adjustment lock. The lock ball or plunger must engage the
indent on the worm shaft after the adjustment is completed.
An audible metallic click can be heard when engagement is
made.

1

Using the Positive Lock Slack Adjuster Mechanism: (Fig. 1)

Wipe the adjusting screw nut and locking sleeve area clean.
Position wrench or socket over the adjusting screw and
disengage the locking sleeve by depressing the lock sleeve.
Make the necessary adjustment by turning the adjusting
screw with the locking sleeve depressed.

When adjustment is completed, the adjusting screw nut
should be positioned so the locking sleeve engages the
adjusting screw nut, thus locking the adjusting screw in
place. DO NOT ATTEMPT TO TURN THE ADJUSTING
SCREW WITHOUT FULLY DEPRESSING THE LOCK
SLEEVE.

Using the Ball Indent Slack Adjuster Mechanism: (Fig. 2)

Before proceeding with adjustment, measure distance from
top of lock screw head to slack adjuster body.

T o adjust, back off lock screw (counter clockwise) and make
necessary adjustment by turning the adjusting screw. After
adjustment is complete, retighten lock screw, making certain
that lock ball is engaged on the plunger shaft. (Proper
engagement can be confirmed by checking the measurement
from the top of the lock screw head to the slack adjuster
body. It should be the same before and af ter the adjustment.)

PROCEDURE

Vehicle brakes should normally be adjusted using the vehicle
or brake manufacturer’s recommendations. If they are not
available, the following can be used:

1. Bring the vehicle to rest on a level surface and chock the
wheels.

2. Mechanically release (“cage”) the spring brakes.

BRAKE ADJUSTMENT CHECK

A. PREFERRED METHOD
Determine the brake chamber size. Make a 100 psi

application to the service brakes and measure the push rod
stroke. Using the chart (Fig. 3) compare the actual chamber
stroke to the recommended maximum stroke to determine
if brake adjustment is required.

CLAMP RING TYPE CHAMBER DATA

(Dimensions in inches)

Effective * With Brakes

Type (Sq. In.) Diameter Stroke Adjusted Readjusted

12 12 5-11/16 1-3/4 short as 1-3/8
16 16 6-3/8 2-1/4 possible 1-3/4
20 20 6-25/32 2-1/4 without 1-3/4
24 24 7-7/32 2-1/4 brakes 1-3/4
30 30 8-3/32 2-1/2 dragging 2
36 36 9 3 2-1/4

*Dimensions listed do not include capscrew head projections
for rotochambers and bolt clamp projections for clamp type brake
chambers.

Area Outside Max. Brakes Should Be

6 6 4-1/2 1-5/8 Should 1-1/4
9 9 5-1/4 1-3/4 be as 1-3/8

Max. Max. Stroke

Stroke at Which

Slack Adj. Arm Length Push Rod Movement (stroke)

“A” Dim. (in.) “B” Dim. (in.)

5 1-1/2

5-1/2 1-1/4

61

6-1/2 3/4

B

A

FIGURE 4

B. ALTERNA TE METHOD
Determine the slack adjuster arm length (Dim. A). Measure

push rod movement (stroke) by manually extending push
rod until brake shoes contact drum. Refer to chart (Fig. 4). If
Dim. “B” is greater than allowable stroke indicated brakes
need to be adjusted.

BRAKE ADJUSTMENT

A. PREFERRED METHOD
Raise the vehicle wheel that is to be adjusted off the ground

so that it turns freely. Turn the slack adjuster adjusting
mechanism until the brakes begin to drag. Adjustment is
then backed off until the wheel turns freely . This adjustment
method will result in the shortest possible actuator stroke
without the brakes dragging.

B. ALTERNA TE METHOD
Regardless of the brake chamber size or the slack arm length

adjust the slack adjuster so that there is 3/8" travel of the
push rod when manually extended to contact the brake shoes
to the brake drum. After adjustment, check for brake drag
by gently striking the brake drum with a hammer. When the
brake shoes are away from the drum, a ringing sound will be
heard. A dull sound indicates brake drag and requires
readjustment until the drag is eliminated.

NOTE: If the brakes cannot be adjusted per the above

instructions inspect the foundation brake and drum
for worn or damaged components.

PREVENTIVE MAINTENANCE

Every month, 8000 miles or 300 operating hours; check push
rod travel. Stroke should be short as possible without brakes
dragging or chamber rod binding. Adjust if necessary.

Every six months, 500,000 miles or 1800 operating hours;
lubricate slack adjuster with chassis lube N.L.G.I. grade 1
or 2. Apply a sufficient amount of grease to completely fill
body cavity.

FIGURE 3

2