Hydraulic Four-Way Poppet Valve
Figure 5-23, shows a typical four-way, poppet-type, directional-control valve. It is a manually operated valve and consists of a group of conventional spring-loaded poppets. The poppets are enclosed in a common housing and are interconnected by ducts so as to direct the fluid flow in the desired direction.
The poppets are actuated by cams on the camshaft. They are arranged so that the shaft, which is rotated by its controlling lever, will open the correct poppet combination to direct the fluid flow through the desired line to the actuating unit. At the same time, fluid will be directed from the opposite line of the actuating unit through the valve and back to the reservoir or exhausted to the atmosphere.
Springs hold the poppets to their seats. A camshaft unseats them to allow fluid flow through the valve. The camshaft is controlled by moving the handle. The valve is operated by moving the handle manually or by connecting the handle, by mechanical linkage, to a control handle. On the camshaft are three O-ring packings to prevent internal and external leakage. The camshaft has two lobes (raised portions). The contour (shape) of these lobes is such that when the shaft is placed in the neutral position, the lobes will not touch any of the poppets.
One cam lobe operates the two pressure poppets; the other lobe operates the two return/exhaust poppets. To stop the rotating camshaft at the exact position, a stop pin is secured to the body and extended through a cutout section of the camshaft flange. This stop pin prevents over travel by ensuring that the cam lobes stop rotating when the poppets have unseated as high as they can go.
Figure 5-23 shows a working view of a poppet-type, four-way valve. The camshaft rotates by moving the control handle in either direction from neutral. The lobes rotate, unseating one pressure poppet and one return/exhaust poppet. The valve is now in a working position. Pressure fluid, entering the pressure port, travels through the vertical fluid passages in both pressure poppet seats. Since only one pressure poppet is unseated by the cam lobe, the fluid flows past the open poppet to the inside of the poppet seat. It then flows out one working port and to the actuating unit. Return fluid from the actuating unit enters the other working port. It then flows through the diagonal fluid passages, past the unseated return poppet, through the vertical fluid passages, and out the return/exhaust port. By rotating the camshaft in the opposite direction until the stop pin hits, the opposite pressure and return poppets are unseated, and the fluid flow is reversed. This causes the actuating unit to move in the opposite direction.
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