Gear Lubricants
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Background & Properties
July 2017
Types of Gears
Hypoid Gears
A modification of the spiral bevel gear, hypoid gears transmit motion between non-intersecting shafts that cross at right
angles. Hypoid gears have inherently strong teeth, excellent load-carrying ability and offer noiseless operation. The
sliding motion of the hypoid gear requires lubricants with EP additives to maintain film strength. Hypoid gears can be
used for the most severe applications.
Worm Gears
When shafts are intersecting at right angles and the driving gear is much smaller in diameter than the driven gear, this
is called a worm gear. The two most common types of worm gears are throated and non-throated. The throated
worm gear has angled teeth that increase the number of teeth in mesh at any given time. As with spiral bevel gears
and helical gears, this feature increases load-carrying ability and quiets operation. On the contrary, the non-throated
worm gear has only one tooth at a time in mesh, thus decreasing the gear's ability to carry loads. Due to the sliding
nature of the worm gear, lubricants employing special additives are necessary. Worm gears are typically found where
great velocity reductions are required.
Rack and Pinion Gears
A rack and pinion gear set is commonly found in automotive steering applications. Motion is transferred from the
circular motion of the pinion gear to a linear gear alignment called the rack, or bar. Gear teeth are more tolerable,
giving drivers a better feel while steering.
Gear System Lubrication Problems
The sensitive gears described above can experience problems for a number of reasons. Below are some of the more
common ones:
• Inadequate lubrication or low gear oil level;
• Incorrect type of gear lubricant;
• Wrong lubricant viscosity;
• Poor-performing lubricant;
• Dirty/contaminated gear lubricant; and
• Misalignment of gear sets.
Any or several of the above factors can contribute to gear wear. The most common types of gear wear are micro-
pitting, spalling, scoring and ridging. Basically, these different types of wear cause small pieces of metal to be removed
from the system, which eventually causes tooth failure and severe system contamination.
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