Page 247 - Analysis and Design of Machine Elements

8.8 Lubrication and Efficiency Gear Drives 225
When a pair of gear teeth mesh with each other, both rolling and sliding occur between
contact surface of gear teeth, except at the pitch point where pure rolling occurs [6]. The
sliding may cause friction, wear, significant heat and power losses.
To reduce friction and wear, a continuous film of lubricant should be maintained
between the mating tooth surfaces to prevent direct metal-to-metal contact of mesh-
ing teeth. To keep operating temperature at an acceptable level, a sufficient flow rate
and adequate quantity of oil should be provided to the meshing surface to remove fric-
tional heat. Adequate lubrication and cooling capacity is critical for smooth operation,
power transmission and gear life.
Gears operate under a diversity of conditions and lubrication methods vary accord-
ingly. For light loads, low speeds, low power transmission, intermittent operation and
open gearings, oil can, drip oiler or periodically supplied grease may be applied to the
mating tooth surfaces. When gears operate in an enclosed housing at pitch line velocity
−1
less than 12 m s , large gears may dip into an oil supply sump at the bottom of gear case
and carry the oil to the mesh. For even high speeds and high capacity gearing systems,
positive oil circulation systems are often required, using a pump to draw oil from the
sump and deliver it at a controlled rate to the meshing teeth.
Power losses due to friction in spur gears typically range between 0.5–1.0% of the
transmitted power, and 1.0–2.0% in helical and bevel gears. The power transmission
efficiency depends on materials, surface characteristic, pitch line velocity and lubrica-
tion. In general, gear drives have high power transmission efficiency with long service
life and high reliability.

References

1 Budynas, R.G. and Nisbett, J.K. (2011). Shigley’s Mechanical Engineering Design,9e.
New York, USA: McGraw-Hill.
2 Juvinall, R.C. and Marshek, K.M. (2011). Fundamentals of Machine Component
Design, 5e. New York: Wiley.
3 Mott, R.L. (2003). Machine Elements in Mechanical Design, 4e. Prentice Hall.
4 Hindhede, U., Zimmerman, J.R., Hopkins, R.B. et al. (1983). Machine Design Funda-
mentals: A Practical Approach.New York:Wiley.
5 Pu, L.G. and Ji, M.G. (2006). Mechanical Design, 8e. Beijing: Higher Education Press.
6 Collins, J.A. (2002). Mechanical Design of Machine Elements and Machines: A Failure
Prevention Perspective, 1e. New York: Wiley.
7 American Gear Manufacturers Association. Standard ANSI/AGMA 2001-D04. Fun-
damental Rating Factors and Calculation Methods for Involute Spur and Helical
Gear Teeth. (Metric Edition). American Gear Manufacturers Association, Alexandria,
VA, 2004.
8 ISO 6336–1:2006 Calculation of load capacity of spur and helical gears. Part 1: Basic
principles, introduction and general influence factors. Switzerland: International
Organization for Standards, 2006
9 Wen, B.C. (2015). Machine Design Handbook, 5e, vol. 2. Beijing: China Machine
Press.