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.
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 were the line-matrix printers made by Printroix and its licensees. In these, the hammers were arranged as a "hammerbank," a sort of combs that was 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. A coil wrapped around the pole-piece would neutralize the magnetic field. The leafspring 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 on the magnetic circuit, and eliminated unwanted resonances in the spring. The result was a near-doubling of speed. Recent designs have used electron discharge machining to produce complex, three-dimensional hammers that trade-off the magnetic circuit, resonances, and print speed.
The way the mechanism normally wears is that 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.