Figure 3-46.Camshaft and bushings.
The camshaft provides for the opening and closing
of the engine valves. The camshaft (fig. 3-46) is
enclosed in the engine block. It has eccentric lobes
(cams) ground on it for each valve in the engine. As the
camshaft rotates, the cam lobe moves up under the valve
tappet, exerting an upward thrust through the tappet
against the valve stem or the pushrod. This thrust
overcomes the valve spring pressure as well as the gas
pressure in the cylinder, causing the valve to open.
When the lobe moves from under the tappet, the valve
spring pressure reseats the valve.
On L-, F-, or I-head engines, the camshaft is located
to one side and above the crankshaft, while in V-type
engines, it is located directly above the crankshaft On
the overhead camshaft engine, the camshaft is located
above the cylinder head
The camshaft of a four-stroke-cycle engine turns at
one half of engine speed. It is driven off the crankshaft
through timing gears or a timing chain. (The system of
camshaft drive is dismissed later in this chapter.) In a
two-stroke-cycle engine, the camshaft must turn at the
same speed as the crankshaft, so each valve opens and
closes once in each revolution of the engine.
In most cases, the camshaft does more than operate
the valve mechanism. It may have external cams or
gears that operate the fuel pumps, the fuel injectors, the
ignition distributor, or the lubrication pump.
Camshafts are supported in the engine block by
journals in bearings. Camshaft bearing journals are the
largest machined surfaces on the shaft. The bearings are
made of bronze and are bushings, rather than split
bearings. The bushings are lubricated by oil circulating
through drilled passages from the crankcase. The
stresses on the camshaft are small; therefore, the
bushings are not adjustable and require little attention.
The camshaft bushings are replaced only when the
engine requires a complete overhaul.
Camshaft followers are part of the valve actuating
mechanism that contacts the camshaft. You will hear
them called valve tappets or valve lifters. The bottom
surface is hardened and machined to be compatible with
the surface of the camshaft lobe. There are two basic
type of followersmechanical and hydraulic.
MECHANICAL (or solid) tappets (fig. 3-47) are
simply barrel-shaped pieces of metal. When used in
flathead engines, they have an adjusting screw
mechanism to set the clearance between the tappets and
the valve stems. Mechanical tappets may also come
with a wider bottom surface. These are called
Figure 3-47.Mechanical tappets