This invention relates to devices for controlling the position of movable valve elements for positioning such elements when the valve is not operating and, more particularly, to a positioning apparatus especially useful with spool valves for positioning the spool upon installation of the valve, during shutdown, or at any other time the valve is not operating.
A major concern in industry using machinery and other devices controlled by valves, and especially pneumatic, hydraulic, and other fluid valves, is the accurate determination of the position of the movable valve element within the valve. Such position determination is important upon initial installation, loss of fluid pressure when in service, or during replacement of the valve so that operation of the controlled machinery can be accurately predicted. Without such determination, a press, punch, or ram device controlled by a valve can be unexpectedly and inadvertently operated upon start-up or return of fluid pressure resulting in serious injury to operators, maintenance personnel, or the like. The problem is especially serious when using fluid-type spool valves including a shiftable spool element which can be moved to any one of a plurality of positions to control the passage of pressurized fluid through various fluid conduits.
One well-known method for positioning the spool of a fluid-operated spool valve is to use a spring acting on either one or both of the ends of the spool valve such that the spring or springs automatically return the spool to a determined or neutral position. It has been discovered, however, that such method has several drawbacks because of the necessity of providing sufficient spring force to shift the spool to the predetermined position. Namely, the constantly contacting spring increases the minimum permissible operating pressure of the valve and causes an inbalance of forces acting in one direction of spool movement. Further, the spring itself is cycled at the same rate as the valve subjecting it to fatigue and wear, necessitating frequent maintenance, and reducing the reliability of the positioning apparatus. Thus, the normal operating function of a spool valve is severely altered with such prior known positioners.
A second method widely used for positioning spool valves of the double solenoid, two-position type are detents designed to hold the spool in one or both of its two extreme positions. Such detents normally include holding members biased into contact with recesses or the like on the spool itself. Such detents also are disadvantageous because they raise the valve's minimum operating pressure thereby requiring an additional force to unlock the spool and start its motion. If the fluid system somehow fails during the movement of the spool, the spool may coast to a stop before reaching one of the locking detents thereby preventing accurate determination of its position as mentioned above.
It has now been discovered that the present invention eliminates the above problems by providing a positioner which is actuated only if the system is shut down, depressurized or otherwise not operating. The device remains ready for actuation at all times but is unused until called upon thereby prolonging its life to an extent greater than for any known positioning apparatus. Further, the device avoids the necessity of an additional force to move the spool, causes no imbalance of forces acting upon the spool and is cycled only at a fraction of the repetitions to which the spool is ordinarily subjected. The present invention, therefore, allows a designer to accurately determine the position of the valve at all times during start-up thereby increasing the safety and realibility of systems controlled by the valve.