| 摘要 |
The horizontally rotatable upper section of a mobile crane is swingable in opposite directions by a speed reducer which is driven by a selectively operable reversible variable speed hydraulic motor. A fail-safe spring-loaded hydraulic brake located on the speed reducer operates, when needed, to slow or stop or prevent rotation of the upper section. A hydraulic control system which supplies pressurized hydraulic fluid to operate the motor and the brake includes a pedal-operated master brake cylinder for selectively applying the brake when the motor is in operation. The brake comprises a cylinder housing, an outer brake piston axially movable through a hole in the housing, and an inner brake piston axially movable in a bore in the outer brake piston. Each piston has an extended position and a retracted position. These pistons are movable alternately to extended (brake-apply) position to force a movable brake plate into frictional engagement with a brake disc which is mounted on and rotatable with a motor-driven input shaft of the speed reducer. A main biasing spring is provided to bias the outer brake piston into extended brake-apply position before the hydraulic motor is turned on and any time a selector valve is opened to dump pressurized fluid from a reservoir. An auxiliary spring is provided to bias the inner brake position into retracted position. The hydraulic control system is operable to start the motor and to supply fluid to shift the outer brake piston to retracted brake-release position. The pedal-operated master brake cylinder is then operable, if needed, to supply fluid to shift the inner brake piston to extended brake-apply position even though the outer brake piston is still retracted. |
