169 The percussion mechanism it self can be divided into the rotary portion of the mechanism and the oscillating  portion, with a Scotch yoke performing the conversion from rotational motion to translation.  See Figure 3  for a mechanism cross -section with the rotary and oscillating portions clearly delineated.  

Rotary Portion of the Mechanism – actuator, gears , and bearings   The r otating components of the mechanism consist of a gear motor which drives a gear train in the  mechanism with the final element being the crank shafts that push the diver plate up and down. The actuator  output has an attached pinion spur gear that turns a mating gear . The mating gear is screwed to the end of  a part that combines a hollow shaft and a bevel gear and this shaft is supported in the mechanism housing  by an angular contact bearing pair, arranged back -to-back, with spacers . The bevel gear meshes with three  synchroniz ed bevel pinions which are attached to the crank shafts . Each bevel pinion / crank s haft assembly  is also supported by an angular contact bearing pair, arranged back -to-back.  

Transition from Rotary to Oscillatory -  three Scotch yokes   The t ransition between the rotary and oscillating parts of the mechanism is accomplished by the three  Scotch yokes  – see Figure 4 for details . A spherical roller, which is  a 3/8-in (9.5-mm) diameter Grade 10  440C ball  with a precision hole added, is placed on each crank shaft and is free to spin on the crank shaft  pin in a plain bearing arrangement . The crank pin to roller ball interface is grease filled with Castrol Braycote  Micronic 600 EF . As the  crank turns, the spherical roller spins on the crank pin and also rolls back and forth  in a cylindrical groove on the sid e of the driver plate while pushing the driver plate up and down . Three  cranks arranged 120° apart from one another fully define the height and attitude of the driver plate, except  for rotation about the axis of hammering motion, so no overturning moment i s carried by the roller and the  driver plate needs no additional linear guidance. This arrangement balances loads and minimizes sources  of friction and drag. Anti-rotation rollers consist of solid Vespel crowned rollers that ride in a plain- bearing  arrange ment directly on a stub shaft that is integrally machined into the sides of the driver plate in between  each groove . These provide very loose guidance by preventing the driver plate from rotating in- plane, which  would be an incidental motion.  

Figure 4. (a) A conventional Scotch yoke which is the means of converting rotation into translation in the  percussion mechanism (b) Vertical section view through bevel pinion, crank shaft, roller ball, and driver  plate (c) Horizontal section through divider plate sh owing three cranks and anti -rotation rollers in guides  

Oscillating Portion of the Mechanism –  driver plate / springs / hammer   The oscillating portion of the mechanism consists of the driver plate, springs, hammer shaft, hammer,  hammer bushings and the guide shaft  – see again Figure 3. The driver plate only pushes against the springs  as it oscillates, but it has a large clearance hole in the center to allow it to move up and down along the axis  of hammer motion without contacting the hammer  shaft . Two springs are arranged one on top of and the  other below the driver plate and are captured and preloaded between the hammer and hammer shaft, so