A door operator that is generally used for a vertical door, such as a vertically lifted roll-up door or a garage door, has at least one braking equipment used to brake the door operator and maintain the rolled-up door or door panel at a specific position or a stationary position. A braking equipment of this type, such as Applicant granted U.S. Pat. No. 6,055,885, is usually designed to retract a braking pad pushed by a spring to release a braking state by means of an electromagnet without the braking pad resting on a brake lining of a braking component during opening or closing a door through actuating the door operator. When the door operator stops running, and the electromagnet is switched off, the door operator is maintained in a braking state.
In another patent of Applicant, U.S. Pat. No. 7,055,283, a braking equipment for a door operator with a small load is introduced. As the load applied on the door operator is always not heavy, it is possible to use a compression spring with a lower spring force to brake such a door operator. Due to the lower spring force, an electromagnet is not required to retract the braking pad. As long as the door operator is actuated, the force generated by the door operator is sufficient to overcome the braking force generated by the compression spring, and thus a roll-up door or door panel is drawn. Braking equipment of this type is only suitable for the door operator which is adapted for a small roll-up door or door panel. In addition, due to the absence of the electromagnet, the cost of the braking equipment may be reduced, and a more compact one may be developed.
The well-known braking equipments used for a door operator, such as the two braking equipments described above, provide the braking force only by a single braking pad being pushed against the braking component for the door operator, and the braking component is unmovable along an axle of the door operator and unrotatable with respect to the axle. In other words, a well-known fastener or a commonly-known fixed-connection means is used to fix the braking component onto the axle. At this moment, the braking component and axle must in substance be seen as the same rigid body in order to convert the braking force applied on the braking component into an effective torque for the axle of the door operator and brake the door operator. Nevertheless, using a single-sided braking equipment provides insufficient braking force to brake a door operator with relatively higher loads, such as large roll-up doors or large door panels. This could result in problems related to safety.
On the other hand, while the spring exerts a pushing force on the braking component, a counteracting force opposite to the pushing force is applied on the connection of the braking component to the axle. The counteracting force acts on the connection continually and/or periodically, and thus the connection is possible to be damaged. In addition, in order to increase the braking force of the single-sided braking equipment, the pushing force from the spring is increased. Consequently the wear rate of the brake lining of the braking component and the counteracting force applied on the connection are also increased. The aforesaid will have negative effects to the braking equipment.
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