Theory of Machine
To obviate axial thrust, following gear drive is used

Double helical gears having opposite teeth
Single helical gear in which one of the teeth of helix angle a is more
Mutter gears
Double helical gears having identical teeth

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Theory of Machine
Higher pairs are those which have

Elements of pairs not held together mechanically
Surface contact between the two elements when in motion
Two elements that permit relative motion
Point or line contact between the two elements when in motion

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