Page 212 - The Tribology Handbook

B10 Cams and followers

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4 5 6

Where the cam is made up of geometric arcs and tangents
the appropriate values for the radii of curvature can be
Scuffing read from the drawing. Many cams are now generated
from lift ordinates computed from a mathematical law
incorporating the desired characteristics, so it is necessary
This is the local welding together of two heavily loaded to calculate the instantaneous radius of cam curvature
surfaces, particularly when a high degree of relative sliding around the profile. At any cam angle the instantaneous
occurs under poor lubrication conditions, followed by the radius of curvature at that angle is given by the following:
tearing apart of the welded material. It is particularly
likely to start from high spots, due to poor surface finish, For Jat followers (tappets)
during early running of new parts.
R, = Rb,, +y + 3282.81"
where = base circle radius in mm
y = cam lift at desired angle in mm
y" = cam acceleration at chosen angle in mm/
deg2
CHECKING THE TRIBOLOGICAL DESIGN R, = radius of curvature in mm
It is usual to assess cadtappet designs on the basis of the For curved followers
maximum contact stress between the contacting cam and
tappet, with some consideration of the relative sliding Rc= { [(Rb + RF +y)* + v2]3'2 ).
velocity. This requires the determination of the loads
acting between the cam and tappet throughout the lift (Rb RF +y)2 4- 2v2 - (Rb + RF +y)A
period (at various speeds if the mechanism operates over a
speed range), the instantaneous radius of curvature for the where Rb = cam base circle radius in mm
cam throughout the lift period, and the cam follower RF = follower radius in mm
radius of curvature. Figure 10.3 shows the relationship y = cam lift at chosen angle in mm
between these various quantities for a typical automotive V = follower velocity at chosen angle in mmhad
cam. In addition it is possible to assess the quality of = 57.29 X velocity in mm/deg
lubrication at the camhappet interface by calculating the A = follower acceleration mm/rad2
elastohydrodynamic (EHL) film thickness and relating = 3282.8 X acceleration (mm/deg2)
this to the surface roughness of the components. An
approximate method for the calculation is given later in The value for R, will be positive for a convex cam flank
this section. and negative for a concave (i.e. hollow) flank.

B10.2