Page 385 - Cam Design Handbook

THB12 9/19/03 7:34 PM Page 373

CAM SYSTEM DYNAMICS—ANALYSIS 373

• dissymmetry such as core shifts in castings and rough surfaces on forgings
• nonhomogeneous material; blowholes in castings, slag inclusions or variations in crys-
talline structure
• distortions at design speed in builtup design
• eccentricity in which the journals are not concentric or matching holes are not circular
• misalignment of bearings
• improperly installed parts or misalignment
Balancing problems can be minimized by careful design in which imbalance is con-
trolled. Large amounts of imbalance require large corrections. If such corrections are made
by removal of material, additional cost is involved and part strength may be affected. If
corrections are made by addition of material, cost is again a factor and space requirements
for the added material may be a problem.
Manufacturing processes are a major source of conditions that create imbalance. Unma-
chined portions of castings or forgings that cannot be made concentric and symmetrical
with respect to the shaft axis introduce substantial imbalance. Manufacturing tolerances
and processes that permit any eccentricity or lack of squareness with respect to the shaft
axis are sources of imbalance.
Almost all machinery contains a flywheel as an energy reservoir. The flywheel is
generally installed on the camshaft close to the cam body. The balance of the flywheel is also
a factor to be considered. Flywheels are composed of steel, cast iron, aluminum graphite,
and other materials, depending on the application; see Shigley and Mischke (1996).

12.3.5.2 Crossover Shock (Backlash). Let us now consider a phenomenon called
crossover shock. Crossover shock exists with roller-follower closed-track cams when the
contact of the follower shifts from one side to the other. Thus, the clearance or backlash
between the follower and the cam are taken up with resulting impact. We know that
crossover shock exists in all roller cam followers. The greater the clearance, the greater is
the shock. The cumulative backlash is the sum of all the clearance, play, or slack in both
the input and output transmissions, adjusted if necessary by gearing ratios, and in the cam
track and cam follower. Typical examples are slack chain drives, gear tooth clearances,
oversized enclosed cam tracks, and worn follower roller bearings; there are many others.
Backlash occurs at a point where the acceleration of the follower changes from positive
to negative or vice versa. This is also the point of maximum follower velocity. The
crossover shock and the maximum velocity of the mechanism become less as the follower
is made stiffer. Crossover shock can be kept to a minimum by using a rigid follower system
with a high natural frequency or by maintaining high manufacturing clearance tolerances,
which is expensive. Preloading of the follower by utilizing dual rollers, Chap. 10, is a
practical means of alleviating this detrimental condition in which backlash is removed
from the system. Preloading is a design possibility only for lightly loaded mechanisms. In
heavily loaded machines, however, it is usually not practical for preloading to avoid
crossover shock. Van der Hoek (1966) and Koster (1970) have considered the problem of
backlash traversal based on the model of nonlinear film squeeze effect, a damping phe-
nomenon. The reduction of backlash is essential to decrease undesirable roller sliding
and skidding action, vibrations, wear, and noise. It cannot be totally eliminated, however.
Proper choice of lubricant and reduction of clearance between the roller and its groove
will aid considerably in this problem.
Backlash with open-track systems can often be eliminated by applying sufficient exter-
nal force with a spring or the payload weight, or even a friction force, to ensure that there