head 21 is also given a vertical rectilinear motion as the stud 40 revolves owing to the rotation of the gear 41 caused by the servo motor 47 through the gear train the vertical rectilinear motion of the cross head 21 is indicated in fig1 and 4 by the reference character y from what has been thus far described it will be understood that the cross head 21 can be prevented from any tendency to rotate about an axis normal to the plane in which lies the frame 22 and hence the bed 13 this is apparently attributed to the fact that the gears 29 and 30 are of identical pitch diameter and both studs 27 and 28 are apart the same distance from the axes of their revolution the symmetric relationship between the studs 27 and 28 at every point of their rotation also contributes to the eliminating of the rotation of the cross head 21 however the symmetric relationship is not always required since one of the pins 31 and 32 can be displaced relative to the other along the horizontal direction designated by the character y in fig1 without failure of the eliminating of the rotational tendency of the cross head 21 the mechanical movements of the above mentioned mechanism is based on the same general principles as those underlying a known 34 crank and slotted cross head 34 or the so called 34 scotch yoke 34 the distance which the studs 27.