a shaft 25 extends between the legs 23 of the brackets the shaft 25 defines an axis 26 and pivotally supports a pivot plate 27 pivoting of the pivot plate 27 about the shaft 25 is achieved by an adjustment mechanism 29 in the preferred embodiment the adjustment mechanism 29 is in the form of a crank 31 with a handle 33 the crank 31 receives a stud 35 that passes through a slot 72 in the frame top wall 11 the stud 35 has right hand threads at one end 37 and left hand threads at the other end 39 the threads at end 37 engage mating threads of a trunion 41 the trunion 41 is supported on the frame top wall by a pair of bearings 43 the stud end 39 engages a clevis 45 the clevis 45 is attached to the pivot plate by a swivel pin 47 it is thus seen that oppositely turning the crank 31 causes the pivot plate to pivot in the directions of arrows 44 and 46 about the shaft 25 secured to the pivot plate 27 close to the adjustment mechanism 29 is an electric motor 47 for example a flange of the motor 47 may be secured to the pivot plate by fasteners not shown the motor armature shaft 51 extends through the pivot plate a.