Page 547 - Cam Design Handbook

THB16 9/19/03 8:04 PM Page 535

AUTOMOTIVE CAMSHAFT DYNAMICS 535

which is a result of the tensile strength of the cam material (see Chap. 9). Steel has the
highest tensile strengths (depending on the hardness) of all the alternative materials.

16.3.2.2 Ductile Iron. Ductile (or nodular) iron is a strong candidate for replacing steel
roller cams today. Ductile iron is a form of cast iron in which the graphite (carbon) in the
metal is contained in spheres (nodules) rather than the usual flakes found in normal gray
iron castings.
Both austempering and postgrind induction hardening have been utilized as alternative
processing methods to reduce processing costs and improve performance of ductile iron
for camshafts. Austempering is a hardening and controlled quench process that makes
the ductile iron stronger to absorb more stress, yet the hardness of austempered iron is less
than that of iron treated with conventional heat and quench hardening. The resultant iron
is not susceptible to the phenomenon of grinding-induced cracking.
Postgrind induction hardening allows the manufacturer to finish machining the
camshaft completely before induction hardening the cam lobes. The unhardened casting
machines and grinds rapidly without damage. Engine manufacturers using postgrind indu-
ction hardening normally allow base circle runout tolerances to increase to 0.0020 inch,
and they may also allow the surface of the lobe to crown 0.0040 inch in the center of the
lobe.
16.3.2.3 Miscellaneous Materials. The same comments are valid for sliding follower
composite camshafts except that materials may be steel, powdered metal steel, powdered
metal tool steel, or other powdered metal materials that show durability for roller follow-
ers. Since there are some very hard and durable powdered metal alloys, sometimes the
lobe material is very difficult to grind.
16.3.2.4 Roller Follower. Roller followers are usually needle bearings with the roller
in the follower chosen to be 52100 bearing steel on a hardened 52100pin. Roller follow-
ers usually operate with rolling and some sliding at high speeds. The action occasionally
presents a problem in that in the event of bouncing on the valve seat a flat spot may prevent
the rolling action and thus significantly promote wear on the cam. This in turn affects the
performance of the engine.

16.4 CAMSHAFT CURVES

Let us investigate the automobile camshaft curves applied to the system. Characteristic
curves of displacement (lift), velocity, acceleration, and jerk will be studied and manipu-
lated to yield the best performance of the follower. The total cam curve is generally com-
posed of a ramp and the main event. The purpose of the ramp is to minimize the backlash
and to control the proper valve seating velocity and seal. The ramp events that in turn are
blended with sine segments (with no discontinuity) are:
• No ramp, main event only (not suggested)
• Constant velocity ramp
• Constant acceleration ramp
• Constant jerk ramp

Usually a constant-velocity ramp is chosen. The main event designs (flank of cam),
which blend with a ramp, are: