A safety mechanism to be incorporated with a hydraulically operated dockboard for preventing free descent of the ramp if a carrier pulls away from the dock when a load is on the ramp. The dockboard includes a ramp having its rear edge hinged to the dock and adapted to be raised upwardly to an inclined position by a hydraulic cylinder unit. The safety mechanism is located in the hydraulic supply line that supplies hydraulic fluid to the cylinder unit from a pump and the mechanism includes a pair of valves, one of which is biased to a normally closed position and the second valve is biased to a partially open position. To raise the ramp, the pump is operated and the pressurized fluid acts to move both valves to the fully open position. When the ramp is lowered in normal operation, the hydraulic fluid being displaced from the cylinder unit passes through the partially open second valve to provide a slow rate of descent from the ramp. If a truck or carrier pulls away from the dock when a load is on the ramp, the combined weight of the ramp and the load will increase the force of the hydraulic fluid flowing from the cylinder to the pump, thereby overcoming the spring force on the second valve and moving the second valve to a fully closed position. The closing of the second valve will prevent descent of the ramp and will retain the ramp in that position until the load is removed from the ramp.
A hydraulic hold down locking cylinder is disclosed by replacing mechanical lock arrangements commonly used on less expensive dock boards. This arrangement provides superior performance, reliability, and lower maintenance. In the preferred form all components are provided in a cylindrical casing which is mounted as a variable length locking link in the dock board.
In a vertically storing dock leveler having a platform which is stored in an upright position, a control circuit which enables retention of the platform in position in response to power failure, and which compensates for possible operator error or inattentiveness during power failure. A normally closed main solenoid valve can be opened only by pressing a lower switch to lower the platform. During normal operation, the valve will remain open to enable the platform to float, but when the platform is to be raised, or electrical power is removed, the valve is closed. As a result, after the platform is raised, the pressure in the main lift cylinder is maintained, and the platform stays at its upright position. Also, during an electrical power interruption, the deck is prevented from falling.
In a vertically storing dock leveler, more efficient, faster storage is enabled by providing a circuit which causes the lip of the leveler to maintain its position while the leveler platform is being raised. This position retention is in contrast to prior systems, in which raising the platform automatically caused pressure in the hydraulic cylinder controlling lip movement to drop, causing the lip to retract. By maintaining the lip position, lip extension can take less time.
A hydraulic hold down locking cylinder is disclosed by replacing mechanical lock arrangements commonly used on less expensive dock boards. This arrangement provides superior performance, reliability, and lower maintenance. In the preferred form all components are provided in a cylindrical casing which is mounted as a variable length locking link in the dock board.
A return-to-dock level mechanism for a power operated dockboard. The dockboard comprises a ramp which is hinged at its rear edge to the frame of the dockboard and a lip is hinged to the forward edge of the ramp. A ramp cylinder connects the frame and the ramp and acts to pivot the ramp upwardly between a downwardly extending below dock level position to an upwardly inclined position, while a lip cylinder interconnects the ramp and the lip and acts to pivot the lip from a downwardly handing pendant position to an extended position. The return-to-dock-level mechanism includes a pair of position actuated switches, such as mercury switches, which are connected in an electrical circuit with a timer and the pump of the hydraulic system. One of the switches is mounted on the ramp, while the other switch is mounted on the lip. When the ramp is below dock level with the lip extended, the ramp switch will be closed and the lip switch will be opened. As the lip falls from the extended position to the pendant position after a loading operation is completed, the lip switch will close, which energizes the timer and when the timer times out, the circuit to the pump is completed to operate the pump and supply fluid to the ramp cylinder to raise the ramp. When the ramp is elevated to a position slightly above dock level, the ramp switch will open to stop operation of the pump and the ramp will then descend by gravity to the horizontal cross traffic position.
