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Valve seats are ground with a special seat grinder so that they are perfectly concentric with the
valve guide. Worn valve seats can be removed from the cylinder head and replaced with new seats.
valve springs (reciprocating engine components). Helically wound steel-wire springs used to close the
poppet valves in the cylinders of an aircraft reciprocating engine. Almost all aircraft engines use more than
one spring to close each of the valves.
valve spring tester (reciprocating engine overhaul equipment). A tester used to determine the amount of
force needed to compress a valve spring to a specified height. Weak valve springs do not close the valves
tightly enough, and they must be discarded when the engine is overhauled.
valve stem (reciprocating engine component). The portion of a poppet valve that rides in the valve guide
in the cylinder head of a reciprocating engine.
The rocker arm presses on the end of the valve stem to open the valve.
valve stretch gage (reciprocating engine overhaul tool). A special gage that measures the radius between
the valve head and the valve stem. If this radius is greater than that of the gage, the valve has been stretched
and it must be discarded.
valve timing (reciprocating engine specification). The relationship between crankshaft rotation and the
opening and closing of the intake and exhaust valves in the cylinders of a reciprocating engine.
When the valves are properly timed, the intake valve opens a specified number of degrees of
crankshaft rotation before the piston reaches the top of the exhaust stroke. The intake valve closes a
specified number of degrees after the piston starts moving upward on the compression stroke. The exhaust
valve opens a specified number of degrees before the piston reaches the bottom of the power stroke, and the
exhaust valve closes at the correct position of the crankshaft after the piston has started down on its intake
stroke.
valve-timing clearance (reciprocating engine specification). A specified clearance between the end of
the valve stem and the rocker arm of an air-cooled aircraft engine equipped with solid valve lifters. The
valves in one cylinder, usually cylinder number one, are adjusted to the valve-timing clearance, and the
valve operating cam mechanism is adjusted so the valves in this cylinder open and close when the
crankshaft is in the correct position.
After the cam mechanism is adjusted, the valves are reset to the correct cold clearance.
Valve-timing clearance is also called the hot, or operating, valve clearance of the engine.
vanadium. A silvery-white, ductile, metallic chemical element. Vanadium’s symbol is V, its atomic
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 611
number is 23, and its atomic weight is 50.942. Vanadium is used as an alloying element for steel to increase
its hardness and toughness. Vanadium steel alloyed with chromium is used to make mechanics’ hand tools.
vane-type pump (fluid pump). A constant-displacement fluid pump that uses sliding vanes to move fluid
through the pump. An eccentrically mounted shaft is turned inside a circular housing that has an intake port
on one side and an outlet port on the other. Flat vanes are free to slide back and forth through slots in the
shaft as it turns.
As the shaft turns, the volume of the chamber connected to the intake port increases and pulls fluid
into the pump. This fluid is carried around the inside of the pump until it reaches a point connected to the
outlet port. At this point, the vanes sliding through the shaft decrease the volume of the chamber, and fluid
is forced out of the pump. Vane-type pumps move a comparatively large volume of fluid, but produce a
relatively low pressure and are used as fuel pumps and low-pressure air pumps.
vanishing point (mechanical drawing). The point in a linear perspective drawing at which the lines
forming parallel sides of an object meet. Drawings made in linear perspective cause the object to appear to
have depth.
vapor. The gaseous state of a material. Water is normally a liquid, but when it evaporates, it turns into
invisible water vapor. Certain metals can be turned into a vapor in the intense heat of an electric arc and the
low pressure of a vacuum.
vapor-cycle air cooling system. A sealed mechanical refrigeration system used to lower the temperature of
the air in an aircraft cabin. A liquid refrigerant, such as R-12 or R-134a, is sprayed into the evaporator coils,
and as it evaporates, it absorbs heat from the cabin air blown across the evaporator.
The refrigerant, now in its vapor form, flows from the evaporator into a mechanical compressor
where it is compressed, increasing both its temperature and pressure. The hot, high-pressure vapor flows
　
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