Machinery's Handbook, 31st Edition
CAMS AND CAM DESIGN
2381
of the weight of the return spring ( 1 ∕ 3 ), and the weight of the members of the external mechanism against which the follower pushes, for example, the weight of a piston: (15) where W = equivalent single weight; W f = follower weight; W s = spring weight; and W e = external weight, all in pounds. Spring Forces: The return spring, K s , shown in Fig. 21a must be strong enough to hold the follower against the cam at all times. At high cam speeds the main force attempting to separate the follower from the cam surface is the acceleration force R at the point of maxi mum negative acceleration. Thus, at that point the spring must exert a force F s , (16) where F e = external force resisting motion of follower, and F f = friction force from fol lower guide bushings and other sources. When the follower is at its lowest position ( R min in Fig. 21a), it is usual practice to have the spring provide some estimated preload to account for “set” that takes place in a spring after repeated use and to prevent roller sliding at the start of movement. The required spring constant K s , in pounds per inch of spring deflection, is: (17) where y a = rise of cam from R min to height at which maximum negative acceleration takes place. The force, F y , that the spring exerts at any height y above R min is: (18) F e s a F yK preload y s = + W W W W 3 1 f s e = + + F R W F F s f e f = − − − K y F preload − s =
W f
Guide Bearings
d
P
µ F 2
Return Spring K s
l 2
F 2
l 2
µ F 1
d
F 1
l 1
(b)
Follower Roller
l 1
v
Cam Surface
F n sin α
α
Pitch Curve
F n
(a)
F n cos α
ω 75° 100°
Cam Surface
R min
Fig. 21. (a) Radial Translating Follower and Cam System; (b) Force Acting on a Translating Follower Pressure Angle and Friction Forces: As shown in Fig. 21b, the pressure angle of the cam causes a sideways component F n sin α which produces friction forces μ F 1 and μ F 2 in the guide bushing. If the follower rod is too flexible, bending of the follower will increase
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