Page 233 - Cam Design Handbook

THB8 9/19/03 7:25 PM Page 221

CAM MECHANISM FORCES 221

The friction force between two bodies is defined as the force at their surface that resists
their relative movement. The coefficient of friction
F
m = (8.3)
F n
where F = the friction force, lb
F n = normal reaction of one body on another, lb.
To initiate the relative motion, the static coefficient of friction is
F
m = s
s
F
n
where F s is the friction force between bodies not sliding on each other. To maintain the
motion the kinetic coefficient of friction is
F
m = k
k
F
n
where F k is the friction force between bodies sliding on each other.
8.6.1 Flat-Faced and Spherical-Faced Followers
These kinds of followers are selected from materials compatible with the cam surface
materials. The following factors affect the coefficient of friction between the cam and fol-
lower surfaces: (a) cleanliness of the surfaces; (b) wear of the parts; and (c) surrounding
environmental contaminants such as particles of ink, paper, dust, and textiles which may
result in lubrication starvation, (d) kind of lubrication system, (e) cavitation of lubricant,
(f) elastohydrodynamic lubricant film on surfaces, and (g) windage due to moving parts.
Also, multiple startups and shutdowns cause metal-to-metal contact, resulting in wear
debris and affecting friction. Sliding followers have a coefficient of friction m k = 0.12 to
0.20. The starting coefficient of friction m s is about 40 percent more than the kinetic m k.

8.6.2 Rolling Element Cam Followers

Rolling-element cam followers are made up of either ball, roller, or needle bearings
encased in a steel ring and supported by a steel stem (See Fig. 8.2). They are most popular
because of their small size and low friction. In addition to the factors affecting the previ-

FIGURE 8.2. Commercial roller follower (needle
bearing) (Courtesy of McGill Manufacturing Co.,
South Bend, Indiana).