DESIGN and
INSTALLATION. These two simple perspectives of rocker arm function must be
understood separately. Because you can have one and not the other. If the "design geometry"
isn't correct, then your "installed geometry" can only be set for the
lesser of two evils, either setting the pivot points for a MID-LIFT
relationship on the "valve" side of the rocker arm, or the "push rod"
side of the rocker. Not both. On a stud mounted system, this is done by
changing the pushrod length. On a stand (shaft) system, this is done by
adjusting the stand's height.
From the above two
perspectives we establish that there are TWO kinds of geometry:
The
first is: "Design Geometry™."
The second is: "Installed
Geometry™."
Understanding the difference between
DESIGN and INSTALLED geometry is one good example; and it is
not only critical in choosing the right parts
for your engine, but also installing them correctly.
All Over-Head Valve (OHV) engines have pushrods that link
the cam and tappet information to the rocker arm. The rocker arm pivots
about an axis upon the cylinder head, being pushed up by the pushrod, so
that the opposite end of the rocker arm pushes down upon the tip of the
valve. This is really "engines 101" -- but it's worth placing in
perspective for our novice readers. As stated elsewhere, there are two
other perspectives that need to be understood, in addition to what we
call "design geometry" and "installed geometry." These other two
perspectives refer to the differences in operating characteristics of
the components we just spoke of. The "cam" is a rotating device, with an
eccentric lobe to it, that allows anything sitting upon it to be pushed
up as it rotates. This is an over simplification that is really very
precise and exact in its operating characteristics. But this basic fact
is true. The component that rides upon it is known as a "tappet" or
"lifter" or "cam follower," depending on who's talking. Each term have
their roots in the American language of engine talk, but the part is the
same. Riding upon the cam follower, is the pushrod. Then the rocker arm,
and finally the "valve" of our "valve train." Everything, except the cam
and the rocker arm, are linear in their operating characteristics. They
basically follow an in-line path. But the cam and the rocker arm are
"radial" and rotate about an axis. Actually, the rocker arm doesn't
rotate, it reciprocates (stops and reverses itself); but none-the-less
it follows around an axis, and this has mechanical characteristics that
must be understood to design and install it properly on the engine with
these linear components.
What really makes a difference is the driving force of
the rocker arm, the PUSH-ROD. All engines have an inherent angle with
the valve, which is on the other side of the rocker arm's motion. On
some engines, this angle leans away (and we call this a negative attack
angle), but on most American made engines, this angle leans IN toward
the valve centerline. So in keeping with a simple concept of minimum
"in-and-out" motion for the pushrod, we place the axis for the top of
the pushrod at a 90 degree angle, to the axis of the rocker arm itself.
But to make the rocker arm truly MID-LIFT in "Design Geometry," it has
to do this same thing on the opposite side, where the VALVE is.
If either side of the rocker arm does not do this, then
one side or both (depending on the "Installed Geometry"), will move out
of its radial path more than it needs, and the consequences will be
wasted "linear" motion from the cam to the valve; excessive harmonics at
high speed; extra contact heat from extra friction within the socket,
and finally extra side loads being applied in the direction where the
linear deviation occurs. In example, if the pushrod is too high in the
rocker (a common trait of most companies for many years), then as the
cam lifts the tappet and pushrod up in the block, the opposite tip of
the pushrod, which is "floating" with the rocker arm's motion, will push
upward and in toward the stud. As it leaves this straight up path, and
begins going around and in toward the stud, it will "slow down" the
rocker's motion. Because it is not following the cam and tappet where
maximum velocity occurs, by being in-line with the straight line motion
the tappet wants to follow. This is ONLY the pushrod side. The valve tip
side has it's own chaos too, if this isn't established right, in
relation to the valve tip. The height of the roller tip ABOVE the
TRUNNION (or shaft, for stand mount rockers), determines it's
over-arcing upon the valve. Understand though, that the best it can do,
is rotate the same amount of degrees that the cam and pushrod have
dictated above. So if the above hypothetical condition exists, the
roller tip end of the valve will also slow down as the rocker's pushrod
tip does, by following up and around the rocker axis, from being mounted
too high.
^
(REV:
090613)
