TIMING GEARS (GEAR TRAINS)
Because the
crankshaft must rotate twice as fast as the camshaft, the drive member on the crankshaft
must be exactly one half as large as the driven member on the camshaft So for the camshaft
and crankshaft to work together, they must be in time with each other. This initial
position between the two shafts is designated by marks that are called timing marks.
To obtain the correct initial relationship of the components, align the corresponding
marks at the time of assembly. Timing gears keep the crankshaft and the camshaft turning
in proper relation to one another, so the valves open and close at the proper time. This
is accomplished by gear-drive, chain-drive, or belt-drive gear trains (fig. 3-66).
In a gear
drive setup (fig. 3-66), the timing gear
on the crankshaft meshes directly with the gear on the camshaft. Timing gears are commonly
used on heavy-duty applications due to their dependability; however, they are noisier than
a chain or belt drive. Since they are keyed to their respective shafts, they can be
replaced if they become worn. With directly driven gears, one gear usually has a mark on
two adjacent teeth and the other mark on only one tooth. To time the valve properly, mesh
the gears so the two marked teeth of one gear straddle the single marked tooth of the
other.
A timing chain
and sprockets can also be used to turn the camshaft (fig. 3-66). This is the most common type of gear
train. There are two types of timing chains. One is a silent link type that is used in
standard and light-duty applications. The other is the roller-link chain, which is used in
heavy-duty applications. Like timing gears, the chain sprockets have timing marks. The
correct timing may be obtained by hating a certain number of chain-link teeth between the
marks or by lining up the marks with a straightedge, as shown in figure 3-67.
In a belt
drive gear train, the sprockets on the crankshaft and the camshaft are linked by a
continuous neoprene belt (fig. 3-66). The belt has square-shaped internal
teeth that mesh with the teeth on the sprockets. The timing belt is reinforced with nylon
or fiber glass to give it strength and prevent it from stretching. This drive
configuration is limited to overhead camshaft engines.
Most engines
with a chain drive and all belt-driven engines use a tensioner. The tensioner pushes
against the belt or chain to keep it tight. This serves to keep it from slipping on the
sprockets. This provides more precise valve timing and compensates for component wear and
stretch. Engines with a belt drive usually use a spring-loaded idler wheel. Engines with a
chain drive use a fiber-rubbing block that is either spring loaded or hydraulic.
NOTE
Always check the
manufacturer’s service manual when you are in doubt about the method of timing used
for the engine you are overhauling. |
