| 摘要 |
<p>A hydraulic pressure control valve (10) is employed to control brake pressure in a transit car (90). The valve uses a stepped valve spool (50) having a differential area (54, 56) exposed to supply pressure to provide a small resultant force on the spool (50). A linear electric solenoid (60) provides a linearly variable force on the valve spool (50) to variably position the spool. In one embodiment, the linear solenoid force opposes the resultant supply pressure force and a spring force to position the spool and control hydraulic brake pressure. Car load weigh adjustment and emergency braking are electrically controlled via the solenoid (60). In a second embodiment, the resultant hydraulic pressure force is opposed by the linear solenoid force to position the spool (50) during normal service braking. Brake pressure is proportioned to car weight by varying current to the linear solenoid. When the transit car emergency brake pipe (122) is dumped, load weigh air pressure and spring forces position the spool in an emergency brake pressure position proportional to car weight. These forces are normally neutralized by emergency brake pipe pressure (96) during service braking. In a third embodiment, the selectively variable linear solenoid force supplements the resultant pressure force to position the valve spool (50) against the force of a spring (112) and load weigh air pressure (148). This embodiment provides emergency brake pressure proportional to car weight when electric current is cut off to the solenoid (60).</p> |
