Page 240 - Cam Design Handbook

THB8 9/19/03 7:25 PM Page 228

228 CAM DESIGN HANDBOOK

2
) =-
F = - ( 675035lb.
i
386
The acceleration curves and inertia are shown in Figs. 8.7c and 8.7d. The external load
on the follower is +10lb in Fig. 8.7c. The weight of follower linkage is +2lb, Fig. 8.7f.
Now, we shall combine these forces. Fig. 8.7g shows the superimposed values to give a
combined force diagram. We see a fluctuating load from +116lb to -23lb, etc. It was stated
that the spring must exceed the net negative loads by a margin of 7lb. In Fig. 8.7g,
we shall plot the spring load curve. At the transiting point, the spring force must equal
23lb + 7lb = 30lb. At the lowest point, let us assume an initial spring preload of 14lb.
The spring has a spring index
-
D S 3014
k = + = =+51 2. lb in
s
Dd 5
16
where DS = spring force, lb
d = deflection, in
and the spring force at maximum rise
S - 14
2 = 51 2 .
1 1
4
S = 78 0 . lb.
2
Superimposing the spring force in Fig. 8.7g, we see that the spring force curve rises
appreciably as the cam rises. Note that the spring force is +(Ø) although it is now plotted
in reverse for better understanding of margin safety. Also, this spring may have to be
redesigned to fit the mechanism or to eliminate any spring surge. Last, it must be remem-
bered that the highest cam speed requires the largest spring since at any lower speed the
inertia forces are reduced.
Let us now combine all the forces. This is shown in Fig. 8.7h, in which we see a fluc-
tuating load curve. The critical load point is the transition point where the minimum dif-
ference of 7lb exists between the spring load and the external load. A slightly stronger
spring could have been chosen with a negligible effect on the system.
It should be noted that the foregoing problem serves primarily as a guide since recal-
culations are necessary. One of the factors for redesign would be to include the spring
weight of the follower. Note that the use of a spring-loaded system produces a load on the
cam regardless of the speed and accordingly affects the size and wear of the designed
parts.

8.11 TORQUE

8.11.1 Translating Roller-Follower Torque

An important part of the cam-follower system design is the determination of the torque,
which is continually changing through the performance of any machine. From the torque
we can determine the running performance of the machine, the size of the motor drive,
and the proportions of the drive shaft and related parts. The torque is especially signifi-
cant with: