A stored energy printer is a type of computer printer that uses the energy stored in a spring or magnetic field to push a hammer through a ribbon to print a dot. As compared to dot matrix printers that print a single column of dots at a time, these printers generally print an entire line of dots at a time and so are sometimes called line matrix printers.
This technology is used to produce premium impact printers that print for millions to billions of dots per hammer. The advantage of this technology is that it has the lowest known cost of ownership: ink is transferred by conventional typewriter-style ribbons, and the rest of the printer simply never wears out.
The most common printers to use this technology were the line-matrix printers made by Printronix and its licensees. In these, the hammers were arranged as a "hammerbank," a sort of comb that oscillated horizontally to produce a line of dots.
A character matrix printer has also been produced. In this printer, the hammers were machined from an oval of magnetically-permeable stainless steel, and the hammer-tips form a couple of vertical rows.
The original technology, patented by Printronix in 1974, was to have the top of a stiff leaf spring held back by a magnetic pole-piece. A tungsten carbide hammer would be brazed to the center-top of the leaf spring. When it was desired to produce a dot, a coil (electromagnet) wrapped around the pole-piece would neutralize the magnetic field. The leaf spring would snap the hammer away from the pole-piece, pushing the hammer out against a ribbon and push an image of a dot onto the paper.
Recent designs have performed complex optimizations in the magnetic circuit, and eliminated unwanted resonances in the spring. The result was a near-doubling of speed. Other improvements include the use of electrical discharge machining to produce complex, three-dimensional hammers that trade-off the magnetic circuit, mechanical resonances, and printing speed.
Normal wear usually occurs when the spring rubs against the pole-piece as it returns. This causes the pole-piece to wear, eventually requiring the pole pieces to be reground and recertified.
Hexavalent chrome plating on the pole-piece, combined with careful design, more than doubled speeds and improved life-span tenfold to something like a billion impressions per hammer.