Page 216 - The Tribology Handbook

BIO Cams and followers

[: SURFACE FINISH
u = w +: - Rc]
Extremely good surface finishes are desirable for successful
o = cam speed in radls operation, as the EHL lubrication film is usually very thin.
Rb = base circle radius (mm) Typical achievable values are 0.4 ,urn R, for the cam,
y = cam lift (mm) and 0.15 pm R, for the tappets.
R, = cam radius at point of contact (mm)
This applies for flat tappets, and for curved tappets with
a radius much larger than the cam radius it can be used as
a reasonable approximation.
Ideally the values for u and R, should be calculated for
all points on the cycle, but as a minimum they should be SURFACE TREATMENTS
calculated for the base circle and the maximum lift
position. Some surface treatment and heat rea nent processes
For cams with curved sliding contact followers the which can be used with cams and tappets are given below:
equation for u is very complex. However, to check the
value of u at the maximum lift position only, the following Phosphating Running-in aid. Retains lubricant.
approximate formula can be used. ‘Tufftride’ Running-in aid. Scuff resistant
‘Noscuff Greater depth than Tufftride. Less
hard.
3280y“ RF + (Rb
u=o[ 2 ‘Sulf. B.T.’ Low distortion. Anti-scuffing
(RF d- Rb +Y) Flame hardening, Can give distortion
Induction hardening
(curved follower, max lift position only) Laser hardening Low distortion. 0.25 to 1 mm case
depth
where Carburising 0.5 mm case depth typical
Nitriding Depth 0.3 mm. Hardening and
y = rnax cam lift (mm) scuff resistance.
y“ = rnax cam acceleration at nose (mm/deg*) Plasma Nitriding As nitriding, but low distortion
which is a negative value Sulfinuz Good scuff resistance
RF = follower radius (mm) Boriding Good wear resistance
Rb = base circle radius (mm).
Evaluation of mode of lubrication

Once a value for the film thickness has been calculated,
the mode of lubrication can be determined by comparing it
with the effective surface roughness of the components. OIL AND ADDITIVES
The effective surface roughness is generally taken as the
combined surface roughness R,,, defined as The oil type is frequently constrained by requirements of
other parts of the machine. However, for best lubrication
R,, = (RqI2 + R,,2)0.5 of the cam and tappet (Le.: thickest EHL film), the
viscosity of the lubricant at the working temperature
R,, and R, are the RMS roughnesses of the cam and should be as high as possible. Often the best way of
tappet respectively, typically 1.3 times the R, (or CLA) achieving this is to provide good cooling at the cams, by
roughness values. means of a copious supply of oil.
If the EHL film thickness h is greater than R,, then Trends in vehicle engine design such as overhead
lubrication will be satisfactory. camshafts, and higher underbonnet temperatures, have
If the EHL film thickness is less than about 0.5 Rpl then led to high camshaft temperatures and low lubricant
there will be some solid contact and boundary lubrication viscosities. Some cam wear problems may be partially
conditions apply. Under these circumstances, surface attributed to this.
treatments and surface coatings to promote good running- Oil additives, principally ZDDP (zinc-dialkyldithio-
in will be desirable, and anti-wear additives in the oil may phosphate) and similar, are used in vehicle engine oils,
be necessary. and are beneficial to cam and tappet wear. There is
Alternatively, it may be appropriate to improve surface evidence that these additives can promote pitting at high
finishes, or to change the design to an improved profile temperatures, due to their chemical effects. Additives
giving better EHL films, or to use roller followers which should therefore be used with care, and are certainly not
are inherently easier to lubricate. an appropriate alternative to good design.

B10.6