284Chapter 7 Shuttle Frame AssemblyShuttle Drive MotorThe shuttle drive motor is built into the shuttle assembly casting and spins acrankshaft, to which are attached the big ends of two connecting rods.(Figure 56.) The small end of one connecting rod attaches to the hammerbank; the small end of the other connecting rod attaches to a counterweightframe surrounding the hammer bank. (The hammer bank and thecounterweight constitute the shuttle assembly.) The shuttle drive motor,acting through the crankshaft and connecting rods, converts rotary motion tolinear and opposing motion of the hammer bank and counterweight, in anarrangement similar to that of a horizontally-opposed gasoline engine.Mechanically, this design achieves the same benefits as that type of engine:perfect primary balance, low vibration, and durability.Each hammer spring is a stiff leaf spring with a cylindrical tungsten carbide tipattached to the free end. (Figure 57.) A powerful permanent magnet isimbedded along the length of the hammer bank and acts on the hammersprings through individual pole pieces. The pole pieces magnetically attractand hold the free end of the hammer spring under tension. This is called theretracted state.Figure 57. Hammer Springs and Hammer Bank (Detail)Two electromagnetic coils are mounted behind each hammer and woundaround each pole piece. The coils are normally de-energized. When hammerdriver logic determines that a hammer must print a dot, a current pulseenergizes the coils behind the hammer. The polarity of the resulting magneticfield opposes the field of the permanent magnet, canceling the magneticattraction and releasing the hammer, which springs forward, striking theribbon and paper and leaving an impression of its tip (a “dot”) on the paper.184106aNOTE: 6615 hammer bank shown.Hammer Bank Assembly:Coils, Magnet, Pole Pieces,Hammer SpringAssembliesAlignment PinHammer SpringAssemblyTungstenCarbideHammer Tip
