Tribologytransactions

Design of Spur Gears Using Profile Modification 743
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lower in a 25 pressure angle compared to a 20 pressure angle.
This analysis holds for changing of s max from point C to point D
similar to point B and point C. The behavior of altered toothsum
gearing performs in the same way as discussed above for
changing of points from point C to point D.
Morphological Investigation
Generally, to ensure long-term transmission reliability in
automotive vehicles the gear designer must consider requirements
such as high static strength, bending fatigue, and rolling
contact fatigue (William and Richard (30)). In particular, rolling
contact fatigue cracks are one of the most important problems
for gears (Aslantas and Tasgetiren (16); Vera and Ivana (29)).
Under rolling contact, various surface damage (pitting, spalling,
and cracking) occurs and cracks develop in the gear tooth surface,
thus leading to loss of serviceability of the gear (Anders
and Soren (13); Singh, et al. (28)). Both macro- and microcracks
lead to surface damage, and subsurface cracks generate heavy
surface damage like pitting. In most material failures, tensile
stresses appear to be the common mode and the material surface
is stretched or pulled apart, thus leading to microscopic surface
cracking and macroscopic-scale pitting or spalling (Dimitrov,
et al. (17); Michele and Giuseppe (23)).
In a meshing gear pair LPSTC to the HPSTC only one pair of
gear tooth in contact and the normal force is higher for this
region. Below the LPSTC and above the HPSTC there is more
than one pair in contact (Marco, et al. (22)). Hence, in the present
investigation point B (LPSTC) and point D (HPSTC) on the
pinion are the critical points of contact that are considered in
gears for rolling type of failure. In this context, 96, 100, and 104
tooth-sum gears were selected for experimentation and are subjected
to fatigue loading. The surfaces at point B (LPSTC) and
point D (HPSTC) on the tested pinion tooth surface were
selected and specimens were prepared for morphological investigations.
SEM micrographs of these surfaces are shown in Fig. 3.
In the experimental results, it is observed that pitting failures
occur roughly between the initial point of a single-tooth contact
(point B) and pitch point C and from pitch point C to the final
point of the single-tooth contact (point D) because these are the
points of highest load when the contact ratio is greater than 1.2
and less than 2.
Figures 3a and 3b show SEM micrographs of the pinion tooth
surface at points B and D for a tooth-sum of 96. The sum of the
profile shift coefficient is 2.27 mm and in this case both gear and
pinion wer generated with equal amounts of profile shift and the
maximum contact stress at point B 1 is 189.48 MPa. This is the initial
point of the single-tooth contact. Microcracks are observed
at point B (Fig. 3a, higher magnification) and multiple macrocracks
are observed at point D (Fig. 3a, higher magnification). It
is found that the crack intensity and number of cracks are greater
at point D compared to point B. This is due to s max at D. From
these micrographs it can be seen that the surface deterioration at
the region of point D is greater compared to that at point B. In
driving and driven members the sliding direction and rolling are
opposite each other (as observed in all SEM micrographs) and
cracks grow opposite to the direction of sliding.
Figures 3c and 3d show SEM micrographs of 100 tooth-sum
pinion teeth surfaces at points B and D. From Figs. 3c and 3d it
is seen that the surface damage is more severe at point B (Fig. 3c,
lower magnification) compared to point D (Fig. 3d, higher magnification).
However, plastic deformation is observed on the surface
in both points of contacts. This is due to the kinematic
characteristics of the gear teeth contact and teeth lubricating
conditions (Joze and Gorazd (21)). It can be stated that the more
aggressive the contact conditions and a thin lubricating specific
film thickness lead to plastic deformation and a feather edge
forms due to plastic flow of material (Fig. 3c; Errichelo (18)).
Scuffing (scoring) is a form of gear tooth surface damage that
occurs due to the absence or breakdown of a lubricant film
between the contacting surfaces of the mating gears (Vera and
Ivana (29); Sheng and Ahmet (27)). Figures 3e and 3f show the
micrographs of 104 tooth-sum pinion teeth surfaces at points B
and D. It can be seen from Table 3 that the contact stress is at
point B and remains the same at point D because of the two-pair
mesh. Though the maximum contact stress value in this case is
lower than in the above two cases, the contact ratio in this case is
2.07, which is higher. This leads to a shorter path of contact and
the product of contact stress and sliding velocity is the highest
for this case of gearing compared to all other cases. Therefore,
the lubricant between meshing surfaces is subjected to squeezing
and breakdown of the lubricant film. This indicates a severe scoring
effect. The extreme pressure and high sliding velocity caused
ploughing of the material. In addition, dry sliding wear at the
point of contact generates high friction and material removal
from the surface acts as three-body wear and creates ploughing
of the material, as shown in Figs. 3e and 3f. However, the cases
of gearing with positive values of tooth number alterations are
marginally scored compared to standard gearing. Surface examinations
carried out by considering SEM microphotographs
revealed better surface integrity for the cases of negative number
of tooth alterations.
CONCLUSIONS
The analyses of maximum contact stress in altered tooth-sum
gearing for a tooth-sum of 100 when altered by §4% have been
studied. The magnitude of s max and morphological studies have
been done for point B and point D. From the computational,
experimental, and morphological analysis, the following conclusions
are drawn:
Better performance is obtained for negative alteration in
tooth-sum gears compared to standard gearing. This is
because the contact stress induced for negative alteration in
tooth-sums of 96, 97, 98, and 99 is lower compared to the
standard tooth-sum of 100. For positive alteration in toothsum
the contact stress at points B 1 and B 2 is higher; in turn,
standard gears perform better under these conditions for a
25 pressure angle. Similarly, the analysis holds for a 20
pressure angle.
It is proved that by a profile modification technique it is
possible to trace the point of switchover of s max . In addition,
it has design flexibility in gear design with respect to
the range of contact ratio.