4.1: The brake is spring applied and actuator released. A
4.2: The actuator piston rod extends when power is applied, the
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 readjustment 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
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 |
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