Page 278 - Gear Technology Solutions

ing at the pinion and gear root, it is recommended to avoid profile shift to
achieve balanced conditions between both meshing members.

In order to further reduce the risk of scuffing and the risk of lubrication defi-
ciencies, the applied lubrication concept is most important. The optimal orien-
tation of a speed increasing gearbox is such that the gear has a horizontal axis
and is about 50% below the oil level, while the pinion just dips below the oil
level. This orientation will keep oil churning at the lowest possible level and
forced lubrication is not required. The basic rule is to keep the low-speed
member about 50% below the oil and ensure that the high-speed member just
contacts the oil, such that after long periods of not operating the gearbox, the
pinion is not dried off on all of its teeth.

Recommended oil types are high pressure fully synthetic oil, as used in hypoid
®
axles of premium class vehicles (for example Castrol SAF-XO). These syn-
thetic oils have low viscosity and provide high pressure and scuffing prevention
properties which are beneficial for speed increasing transmissions.

Figure 5: Scuffing in the root area of a hypoid pinion without profile shift

Figure 5 shows a severe case of scuffing in the high sliding area (dedendum)
of a hypoid gear. The gear in Figure 5 was mating with a pinion which had a
too large profile shift, which made the gear dedendum large. The large dis-
tance from the pitch line caused high sliding velocities along the flank-root
transition which initiated the scuffing. As mentioned in the last section, zero
profile shift in the driven pinion is advisable. The risk of scoring of hypoid gears
is always larger than in the case of spiral bevel gears, which comes from the
additional sliding in face width direction, in a magnitude that is nearly propor-
tional to the hypoid offset and larger than the profile sliding.

263