JLG T350 Service Manual - Manual (Page 38)

SECTION 3 - CHASSIS & TURNTABLE
3-2 – JLG Lift – 3121198
pressure of the linings against the drum produces a
slight drag.
5. Remount the wheel and brake drum.
6. Repeat the above procedure on all brakes.
7. Lower the trailer to the ground.
3.3 ELECTRIC BRAKES
The electric brakes on a trailer are similar to the drum
brakes on an automobile. The basic difference is that the
automotive brakes are actuated by hydraulic pressure
while electric trailer brakes are actuated by an electro-
magnet. With all of the brake components connected into
the system, the brake will operate as follows:
When the electrical current is fed into the system by the
controller, it flows through the electromagnets in the
brakes. The high capacity electromagnets are energized
and are attracted to the rotating armature surface of the
drums, which moves the actuating levers in the direction
that the drums are turning.
The resulting force causes the actuating block at the shoe
end of the lever to push the primary shoe out against the
inside surface of the brake drum. The force generated by
the primary shoe acting through the adjuster assembly
then moves the secondary shoe out into contact with the
brake drum.
Increasing the current flow to the electromagnet causes
the magnet to grip the armature surface of the brake drum
more firmly. This results in increasing the pressure against
the shoes and brake drums until the desired stop is
accomplished.
Brake Controller
A tow vehicle brake controller is required. These control-
lers have gain control to vary the amount of current to the
brakes, and a level control which sets the controller's iner-
tia sensor to sense deceleration. The level adjustment
also can be used to vary when the trailer braking is felt.
The gain or output control adjustment usually controls the
maximum amount of amperage available to the brakes.
This can be adjusted for varying trailer loads.
The electric brakes that operate in conjunction with the
tow vehicle brakes must be “synchronized” so that brak-
ing is properly distributed to the tow vehicle brakes and
the trailer brakes. For proper operation and synchroniza-
tion, read and follow the axle/brake and the brake control-
ler manufacturers’ instructions
Brake Cleaning and Inspection
Your trailer brakes must be inspected and serviced at
yearly or 12,000 miles intervals or more often as use and
performance requires. Magnets and shoes must be
changed when they become worn or scored thereby pre-
venting adequate vehicle braking. Clean the backing
plate, electromagnet, and brake shoes. Make certain that
all the parts removed are replaced in the same brake and
drum assembly. Inspect the magnet arm for any loose or
worn parts. Check shoe retractor springs, hold down
springs, and adjuster springs for stretch or deformation
and replace if required.
Shoes and Linings
A simple visual inspection of your brake linings will tell if
they are usable. Replacement is necessary if the lining is
worn (to within 1/16” or less), contaminated with grease or
oil, or abnormally scored or gouged. It is important to
replace both shoes on each brake and both brakes of the
same axle. This is necessary to retain the “balance” of
your brakes.
3.4 HYDRAULIC (SURGE) BRAKES
In the hydraulic brake system, hydraulic fluid from the
master cylinder is used to actuate the hydraulic wheel cyl-
inder, which, in turn, applies force against the brake shoes
and drum. The main differences between automotive
hydraulic brakes and hydraulic trailer brakes are the actu-
ation systems that transfer the braking signal from the tow
vehicle to the brake. A surge tongue actuator is required
for a trailer with hydraulic brakes.
The hydraulic brake uses a single acting cylinder. Upon
actuation, the primary shoe is pressed against the brake
drum, which causes the shoe to move in the direction of
rotation. This movement in turn actuates the secondary
shoe through the adjuster link assembly. Braking in
1. Primary Shoe
2. Retractor Spring
3. Secondary Shoe
4. Hold Down Spring 5. Adjuster Assembly
6. Adjuster Spring
7. Magnet
8. Actuating Lever
Figure 3-1. Electric Brake Assembly