Gear as Power Transmission

Spur gears are more efficient. Helical gears are quieter. In older cars, reverse gear was generally straight cut -- thus the whine you would hear in reverse. It used to be that spur gears were cheaper to manufacture -- but now I'd guess the cost is very close.

Blink's reply is most accurate, but did not cover worm gearing which is less efficient than helical gearing due to friction, Hypoid gearing found in rear drive car rear ends suffers in the same way.

Bernie Katz

Good post, Ken. On the subject of efficiency, helical gears are more efficient in a sense because they can transmit more power than can a spur gear of the same face width. However, this additional power capacity comes at the price of more sliding friction and hence more heat.

The limitations of helical gears vs. spur gears? Mainly the side thrust from gears that have a steep helix angle. Also as I indicated above, the additional heat produced and the need for better lubricants. They are a bit harder to measure, but as far as manufacturing is concerned there should be little if any price difference. A little more time on the hobbing machine due to the "longer" teeth, but that's about it.

Ing. Robert Forbus

It is true, Robert, but you haven't said the principal problem of helical gears. They produce a big axial reaction in their shaft, increased with the helix angle. Normally, the biggest problems of this kind of gears are situated in their roller balls. The advantages of the minor width are less important because you are going to need bigger and more expensive roller balls.

I think you may be thinking about my reply, rather than Roberts. His "side thrust" is your "axial reaction". In either case, an important consideration.

Needle bearing thrust washers are also used to carry axial loading of helical gears, particularly in automotive transmissions.

In very large applications, herringbone gears are used like helical gears by have no axial thrust, The herringbone shape is essentially like two helical gears side by side, but hobbed or machined as one piece.

Bernie Katz

Sun Gear with following features is my favorite one:

• High speed ratio in small volume
• Input and output shafts are aligned
• Good for transmission of small and large magnitudes of power
• cylindrical shape of gear box suitable for flange mounting
• The axis of input and output shafts are coincident with the gear box axis

JFB

All the other posts here are very accurate and complete. AS a forum, I feel obliged to ask, even without knowing your application: have you considered belts? They usually are cheaper for the same efficiency of the transmission (gears must usually be grinded and polished and with forced lubrication to have a good efficiancy), but, of course, you must observe the work conditions, the load intensity, and good maintenance practices.