Breaking by Impact: The Swing Hammer and the Grate That Sets the Size

Take two similar pieces of stone. Squeeze one slowly in the vise until it fails. Strike the other once, hard, with a ball peen hammer on an anvil or a steel plate. Spread both sets of fragments out and look at them. The squeezed stone tends to split along its weakest plane and give a few large, often slabby pieces. The struck stone shatters from the point of impact outward, and gives more pieces, more of them roughly equidimensional, and noticeably more dust. That difference is the whole reason both kinds of machine exist. Concrete aggregate wants cubes, so impact crushers are used to shape it. A hard abrasive ore wants compression, because impact hammers would be eaten alive. Soft, friable, damp or sticky feeds — coal, limestone, gypsum, clay, bark, grain — suit impact, because they break easily and they clog a squeezing machine. Eye protection, and mind where the fragments go. Struck stone travels.

You do not need a working mill to understand the rotor. Cut two discs from steel plate, drill matching holes on a bolt circle, and hang flat hammers between them on pins through those holes. Mount the pair on a shaft in bearings. Then **balance it**, and do it properly, because this is the part that hurts people. Weigh every hammer on the digital scale. Any hammer that differs from its opposite number leaves an out-of-balance mass turning at speed, and the force goes as the square of the speed exactly as it did in step 2. Match hammers in pairs and fit matched pairs diametrically opposite. Write the weights on them. Static-balance the assembly on two level knife edges: it should come to rest in any position rather than always settling with the same point down. Correct by removing metal, never by adding it in a way that can come off. **When hammers are replaced, they are replaced in matched sets, never singly.** A worn hammer is lighter than a new one, and one new hammer among nineteen worn ones is a bomb. This is the single most important sentence in the rung.

A hammer mill tells you how it is doing, audibly, and it is worth learning the sounds. **Starved** — a clear ringing, the motor barely loaded. Hammers are hitting the odd particle and mostly hitting air. The machine is wearing without producing, and the wear pattern it develops running empty is not the one it develops running loaded. **Loaded correctly** — a steady roar, the note constant, the motor's current steady. **Overfed** — the note drops and drags, the motor current climbs, and the product coarsens, because material is being pushed through the grate before it has been hit enough times. An overfed hammer mill will eventually choke and stall, and restarting a mill full of material is how shafts get bent. Feed it steadily. This is exactly what the last rung of this batch is about: a steady feed does not come from a hopper with a hole in it, it comes from a feeder. And never open one until the rotor has stopped. A hammer rotor carries an enormous amount of stored energy and coasts for minutes after the power is off, silently.

**US 1,041,495, 'Pulverizer', William K. Liggett and Walter J. Armstrong of Columbus, Ohio, application filed 23 January 1909, granted 15 October 1912.** The specification's own opening line is exact about what it is: *'This invention relates to pulverizers of the swing hammer type, and has as its object the provision of an improved hammer mechanism which will be efficient to grind the material and possess great wearing qualities.'* The claim is about the hammer mechanism itself — keyed end plates and a series of perforated disk plates carrying the hammer pins — and about keeping the dust inside, which the patent mentions in the same breath. Both concerns date the machine precisely: by 1909 the swing hammer was established, and what was being competed on was how long the hammers lasted and whether the room filled with dust. **Honest limits.** Impact is wrong for hard abrasive rock: the hammers wear out faster than they pay for themselves, and a quartzite feed will destroy a set in hours. It makes fines whether you want them or not, and the finer the grate the more of them. It is the dustiest machine in this batch by a wide margin, and the dust from many feeds is an explosion hazard as well as a health one — grain, coal, sugar and aluminium dusts have all detonated in mills. It is loud in a way that damages hearing in minutes, not hours. And it is unforgiving of tramp metal in a way the swinging hammers only partly mitigate: fit a magnet upstream and use it.