| 摘要 |
<p>A hydraulic actuator (26) raises and lowers an implement in response to a height sensor acting through a spring (44) against one end of the spool of a four position spool valve (48). The other end of the spool is acted upon by a spring (78). At the correct height the spool assumes neutral position (A). Low and high deviations in height shift the spool to a slow raise position (B) and a slow lower position (C) respectively which provide for raising and lowering at a rate which is slow by virtue of a flow restriction (118). When the implement is raised to a transport position and the spring (44) is untensioned, the valve assumes a position (D) in which the flow restriction (118) is bypassed for rapid lowering of the implement. Lowering is prevented by a check valve (96) until a pressure line (54) to the valve is pressurized from a selector valve (50). Pressure in a branch line (107) then opens the check valve. The selector valve (50) has a fast raise position in which it reverses the connections to the spool valve. The pressure in what is normally the return line (56) is communicated by a branch line (126) to the same end of the spool as that acted upon by the spring (78) whereby the spool is forced into position (D) in which the slow restriction (118) is bypassed and the reversed connection raises the implement rapidly.</p> |
