Theory of Machine
The two parallel and coplanar shafts are connected by gears having teeth parallel to the axis of the shaft. This arrangement is known as

Helical gearing
Bevel gearing
Spur gearing
Spiral gearing

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Theory of Machine
Open pairs are those which have

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

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