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Chapter 4.0: Description of Operation
4.1: The brake is spring applied and actuator released. A heavy-duty compression spring within the
actuator pulls down on the brake lever and forces the shoes against the wheel.
4.2: The actuator piston rod extends when power is applied, the brake-shoe pressure removed and
running shoe clearance established.
4.3: Subject to the effect of brake geometry, braking torque depends upon three major factors:
4.3.(a): The diameter of the wheel.
4.3.(b): The coefficient of friction of the lining material.
4.3.(c): The force with which the linings are applied to the wheel.
4.4: Power applied to the actuator cancels the brakes ability to apply torque.
4.5: The force with which the linings are applied to the wheel depends on the spring length and
characteristics. Spring length increases as linings wear.
4.6: When the brake is released, Automatic Equalization of shoe clearance ensures equal clearance
between each shoe and the wheel.
4.7: Because the applied brake shoe force is proportional to the compressed length of the torque
spring, a slight decrease in brake torque occurs as linings wear (unless automatic adjustment is
used).
4.8: Adjusting the reserve stroke to its specified setting, restores the required spring force.
NEMA states: “The torque ratings apply at a worn lining condition defined as the point where re-
adjustment is required as recommended by the manufacturer”.
4.9: Automatic Equalization (AE) is available on all brakes of this class.
4.10: To understand how the automatic equalization works, it is important to recognize that the total
available shoe clearance is directly related to the length of active stroke and that the actuators
“Fully Released” position is the only consistent value for actuator stroke. The “Brake Applied”
position varies as the linings wear, resulting in a reduced “Reserve Stroke”. The “Brake Applied”
position also varies due to wheel expansion caused by temperature increase; this results in an
increased “Reserve Stroke”.
4.11: On brakes without Automatic Adjustment, after some significant lining wear and the brake wheel
close to ambient temperature, a reduction in the “Reserve Stroke” and a corresponding increase
in the “Active Stroke”, will be noticed. Corrective action will be necessary to ensure the minimum
reserve stroke. (See Chapter 6: “Actuator Stroke Adjustment / Brakes without Automatic
Adjustment).
4.12: On brakes equipped with the Automatic Adjustment (AA) option, the reserve stroke is maintained
by a one-way clutch mechanism arranged to shorten the effective brake rod length – but only
when adjustment is required – as follows:
4.12.(a): A retractable pin mounted on the brake lever, oscillates within a slot in the (AA) clutch
ring. (See Fig. 2A).
8/17/2006 Page 11 of 33 MST/E Electric Shoe Brakes Manual
560022-R6