Instruction manual

6–6

Principles of Operation

Each hammer spring is a stiff leaf spring with a cylindrical, tungsten carbide

tip on the free end. (See Figure 6–6.) A permanent magnet runs the length of

the hammer bank and acts on the hammer springs through individual pole

pieces. The pole pieces magnetically attract and hold the free end of the

hammer spring under tension. This is called the retracted state.

Hammer Bank Assembly:

Fret Alignment

Hammer Spring

Tungsten Carbide

Hammer Tip

Assembly (“Fret”)

Coils, Magnet, Pole Pieces

Hammer Spring Assemblies

NOTE: LG04 hammer bank

shown.

Figure 6–6. Hammer Springs and Hammer Bank (Detail)

Two electromagnetic coils are mounted behind each hammer and wound

around each pole piece. The coils are normally de–energized. When hammer

driver logic determines that the hammer must print a dot, a current pulse

energizes the coils. The polarity of the resulting magnetic field opposes the

field of the permanent magnet, canceling its effect and releasing the hammer.

The hammer springs forward, strikes the ribbon and paper, and leaves a dot

impression of the hammer tip on the paper.

While the hammer is in flight the coil is de–energized and its magnetic field

collapses. After striking the ribbon and paper, the hammer rebounds and the

permanent magnet recaptures it. When the shuttle reaches the end of a sweep,

it reverses direction, the paper is moved up one dot row, and the hammer

springs print the next row of dots as the shuttle sweeps in the opposite

direction.