17) A disturbing mass m1 attached to the rotating shaft may be balanced by a single mass m₂ attached in the same plane of rotation as that of m₁, such that (where r₁ and r₂ are the radii o

18) In railway axle boxes, the bearing used is

19) When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of the other, the pair formed is known as a

20) The two elements of a pair are said to form a __________ when they permit relative motion between them.

21) When the axes of the shafts, over which the gears are mounted, move relative to a fixed axis, then the train is known as reverted gear train.