Page 22 - Tribology in Machine Design

Introduction to the concept of tribodesign 9

very corrosive due to the presence of sulphur and other harmful elements
present in the fuel and oil. Corrosion can be particularly harmful before an
engine has warmed up and the cylinder walls are below the 'dew-point' of
the acid solution.
The normal running-in process can be completed during the period of the
works trial, after which the wear rate tends to fall as time goes on. High
alkaline oil is more apt to cause abnormal wear and this is attributed to a
lack of spreadability at high temperatures. Machined finishes are regarded
as having more resistance to scuffing than ground finishes because of the
oil-retaining characteristics of the roughened surfaces. The use of taper face
rings is effective in preventing scuffing by relieving the edge load in the
earliest stages of the process. A high phosphorous lining is better than a
vanadium lining in preventing scuffing. The idea of using a rotating piston
mechanism to enhance resistance to scuffing is an attractive option.

1.2.4. Cam and cam followers
Although elastohydrodynamic lubrication theory can now help us to
understand how cam-follower contact behaves, from the point of view of its
lubrication, it has not yet provided an effective design criterion.
Cam-follower systems are extensively employed in engineering but do
not have an extensive literature of their own. One important exception to
this is the automotive valve train, a system that contains all the
complications possible in a cam-follower contact. The automotive cam and
tappet can, therefore, be regarded as a model representing this class of
contacts. In automotive cams and tappets the maximum Hertz stress
usually lies between 650 and 1300 MPa and the maximum sliding speed
1
may exceed 10ms~ . The values of oil film thickness to be expected are
comparable with the best surface finish that can be produced by normal
engineering processes and, consequently, surface roughness has an import-
ant effect on performance.
In a cam and tappet contact, friction is a relatively unimportant factor
influencing the performance and its main effect is to generate unwanted
heat. Therefore, the minimum attainable value is desired. The important
design requirement as far as the contact is concerned is, however, that the
working surfaces should support the imposed loads without serious wear or
other form of surface failure. Thus it can be said that the development of
cams and tappets is dominated by the need to avoid surface failure.
The main design problem is to secure a film of appropriate thickness. It is
known that a reduction in nose radius of a cam, which in turn increases
Hertzian stress, also increases the relative velocity and thus the oil film
thickness. The cam with the thicker film operates satisfactorily in service
whereas the cam with the thinner film fails prematurely. Temperature
limitations are likely to be important in the case of cams required to operate
under intense conditions and scuffing is the most probable mode of failure.
The loading conditions of cams are never steady and this fact should also be
considered at the design stage.