Gears with helical teeth can be used instead of spur gears to connect parallel shafts. Helical gears will withstand higher loading than an equivalent size spur gear; the reason is that when the mid-section of teeth are in contact, the line of contact runs from the outside diameter on one end of the tooth to the root on the other. In consequence the bending moment on the tooth is only a little over half as much as it would be for the entire load on the tip as in spur gears. Thus the load is transferred by sliding action rather than chopping which gives much more silent running than with spur gears and a better wear rate. End thrust has to be considered when using helical gears as the angle of the teeth produces an axial component; this end thrust will increase with helix angle. Helical gears are not interchangeable - a right hand helix will only run with a left hand helix. Double helical gears are sometimes used to eliminate end thrust. They consist of right and left hand helical teeth cut on the same gear (forming a herringbone pattern). The resulting teeth formation produces opposing axial components thus eliminating end thrust. These gears are very expensive to produce and are not used unless absolutely necessary.
Mechanical Engineering Design Topics Page - Gearing Selection Page
Pages created by David Price - 9/08/95
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