| 摘要 |
<p>The holddown mechanism (14) includes a cylinder (15) connected to the ramp and a piston (16) is slidable within the cylinder and carries a tubular piston rod (17) which is connected to the frame (4). The piston is provided with a longitudinal passage (20) which is closed off by a valve member (21), and an actuating rod (22) is connected to the valve member and extends outwardly through the piston rod with the projecting end of the valve member being connected to a lever mechanism (28). The cylinder contains a hydraulic fluid such as oil, and the piston is normally locked in position in the cylinder so that the ramp is locked against movement relative to the frame. By manual operation of the lever mechanism, the actuating rod can be raised relative to the piston to open the valve member, thereby permitting fluid to flow from one end of the cylinder to the other to enable the ramp to be raised under the influence of the counterbalancing spring. Release of the lever mechanism will close the valve and hold the ramp in a given position. As an operator walks outwardly on the ramp, the added weight overcomes the counterbalancing force moving the ramp and cylinder downwardly relative to the actuating rod to open the valve member and permit the ramp to be freely lowered. A coil spring (40) interconnects the ramp and the cylinder to permit the cylinder to follow upward float of the ramp when the piston is in a locked condition.</p> |
