valve clearance (reciprocating engine specification). The clearance between the end of the valve stem and the rocker arm when the valve is seated and the cam follower is off the cam lobe.
valve core (aircraft wheel component). A spring-loaded air valve screwed into a metal tube installed in an aircraft wheel. When the tire mounted on the wheel is being inflated, the stem of the valve core is depressed by the air chuck, and air flows into the tire. When the air chuck is removed, the stem seats in the valve core, and the air is trapped in the tire.
To deflate the tire, the valve stem is depressed, allowing air to flow out through the core.
valve duration (reciprocating engine specification). The amount of time, usually measured in degrees of crankshaft rotation, an intake or exhaust valve remains off its seat.
valve face (reciprocating engine component). The portion of a poppet valve that forms a seal with the valve seat in an aircraft engine cylinder head. The valve face is ground so it is perfectly smooth and concentric with the valve stem.
valve float (reciprocating engine operation). The condition of engine operation in which an intake or exhaust valve does not seat when the rocker arm is off the valve stem. It is possible for the engine to operate at a speed at which valves are being opened at the natural, or resonant, frequency of the valve springs. If this happens, the valve springs lose their effectiveness and will not close the valves, and the valves will float.
To prevent floating, most aircraft engine valves are closed with more than one spring. These springs are wound with a different pitch and of different size wire, giving them different resonant frequencies.
valve grinding (reciprocating engine overhaul procedure). The step in the overhaul of a reciprocating engine in which the valve faces are ground to remove all pits and the worn surface. The ground surface is smooth and perfectly concentric with the valve stem.
valve guide (reciprocating engine component). Bronze or cast-iron sleeves installed in the cylinder head of an aircraft reciprocating engine. The stems of the intake and the exhaust valves ride up and down in the valve guides which hold them in the exact center of the valve seats.
valve lag (reciprocating engine specification). The number of degrees of crankshaft rotation after the piston passes the top or the bottom center of its stroke at which the intake or exhaust valves open or close. For example, if the exhaust valve closes 15° of crankshaft rotation after the piston passes over top center and starts down on its intake stroke, the exhaust valve lag is 15°.
valve lapping (reciprocating engine overhaul procedure). The procedure in which the intake and exhaust valves are given a final processing so they form an airtight seal with the valve seat.
A small amount of a very fine abrasive paste, valve-grinding compound, is spread on the valve face, and the valve is placed in the valve guide. The valve is rotated by hand to lap the valve face and seat together to remove any material that would prevent an airtight seal.
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