The Hammer Mill is used either as a one-step primary crusher for reducing run-of-quarry material to as small as <1-in. size, or as a secondary crusher taking 4~8-in. primary-crusher product down to <¾-in. or finer. Its use as a rock crusher is almost wholly confined to the softer, easily crushable materials such as phosphates, gypsum, barite, asbestos rock, cement rock, and the like, medium-hard limestone being the hardest rock commonly crushed; as a secondary it is used for more abrasive material, especially if this is brittle, and scattered instances are reported of its use on siliceous gravels; in general, however, high maintenance is to be expected, if the siliceous content of a feed is in excess of 5%. The mill is particularly useful for clayey material that would clog reciprocating-type primary crushers. It also has wide use in crushing bituminous coal at coke-oven and power plants, and in disintegration, by shredding, of various fibrous organic materials such as plant stems (wood and straws), bones, and hoofs.

The Hammer Mill of which a number of different forms are shown below, comprises essentially a plurality of flailing hammers a which strike rock particles either when they are falling freely through air or as they rest on a stationary metal surface g inclined more or less in the direction of the hammer travel, and the struck particles, or fragments thereof, are thereupon, thrown with great force against other fixed surfaces k surrounding the flailing hammers, or are pinched at an angle between the moving hammers and fixed surfaces i usually perforate.

Machines of the general hammer mill type vary widely in details of construction, particularly as regards the conformation and material of the hammers, the placing and conformation of the breaker plates, the presence or absence of and the type of exit grating, and the position of the feed inlet. A typical medium-duty grate type machine is shown below, item A. The hammers a are suspended by pins b between heavy steel disks c, which are spaced along shaft d by suitable spacers and keyed thereto. The shaft d is carried in heavy bearings in the ends e of the main frame. A heavy flywheel is mounted on one end of the shaft; the other end is fitted with a drive pulley or is attached, through flexible coupling, directly to the driving motor. Rotation of the machine illustrated is counter-clockwise. The bottom of the feed hopper f carries heavy breaker plates, which may be moved forward, to compensate for wear, by suitable adjusting screws. A grid or screen for determining product size is formed by the longitudinal grate bars i. The top of the crushing zone is enclosed by an imperforate cover j. As the hammers wear beyond the limits of adjustment of the breaker plate g, they may be rehung further from the center of rotation, in other holes (l). In most forms of the hammer mill, the grid frame is also hinged (Items E and F) or otherwise arranged for gradual adjustment toward the center of rotation, as well as for dropping away for quick discharge of the load in case of a clog-up or sudden shutdown.

In some hammer mills (Type I) entering coarse material is first struck while partly supported against a stationary plate (Items A, E, G) and the hammers tend to drive broken fragments toward the grid; in others (Type II) entering material is first struck by rising hammers (Items B, D, F) and fragments are thrown against breaking elates along the top and down-coming sides, from which they bounce back into the hammer path for further blows before they reach the grid; and in yet others (Type III) the first blow is substantially horizontal (Items C, H, J, and, to a certain extent, in G), with some opportunity for reflection into the coarse-crushing zone before falling into the fine-crushing zone on the grid. The form I has heavy anvil bars carried on adjustable plates, so that the bars may be spaced at the most favorable distance from the hammers. On the grid the crushing action is, in part, simple impact against unbacked-up particles, and in part shear of pieces wedged between or lying upon the bars. Type I machines are medium-to light-duty; Types II and III heavy-duty.

Grid is sometimes omitted, particularly in top-feed machines, in order to save excessive wear with abrasive feeds or to escape clogging with sticky materials.

The rotor is usually speeds up for such service, but, even so, there is no positive guard against discharge of oversize, and if a definite limiting upper size is important, the machine must be put in closed circuit with a screen. In secondary crushing and in pulverizing service a separate circuit-closing screen should always be used whether the machine contains a grate or not.

Tramp iron in feed is a source of grate-bar and hammer breakage, with possible resultant wreckage of the entire machine. Where possible, it is best removed by a magnet on the feed line. Some forms (Items G and H) provide a catch pocket a into which it is hoped the iron will be somewhat preferentially flung by centrifugal force and held for cleanout.

Main frame in large heavy-duty primary machines is made of deeply ribbed cast-steel sections with the main bearings and the bearings for the swing shaft for breaker plates and grid frame cast integral in a one-piece base. The upper housing is either cast or built up of plate and structural shapes. In either case flanged joints, carefully machined, are provided for bolting the housing to the frame. The hopper is made of rolled steel shapes and plates. In machines for lighter duty only the ends of the base are heavy iron or steel castings, with cast seats for the main bearings; rolled structural shapes and heavy plate are used for the balance of the frame and housing.

Housing should be kept down in size as much as is consistent with sufficient spacing of anvils or breaker plates. These latter are necessarily tough as well as hard; they consequently flow when worn unless properly backed. If the necessary backing is made an integral part of them, the discard is relatively enormous; hence either they, must be backed against the housing, or a special backing frame must be provided inside the housing.

The housing should provide ready access for changing hammers; this is especially important with abrasive material, where hammers may require reversal as often as once per 24 hr.

Shaft is made very heavy, of forged high-carbon or alloy steel. In one machine the shaft is 22-in. diameter through the crushing zone and turned down to somewhat smaller diameter at the bearings.

Bearings are of extra heavy ring-oiling dynamo type or, in the best machines, of roller type. Every endeavor should be made to dust-proof them efficiently.

Disks (Item A) are made of cast steel, with heavy hubs, bored and key-seated for the main shaft, and carefully bored in register with each other for the hammer-spending pins. They should be made without projections from face or edge essential to their functioning, as these wear excessively and thus shorten life. Designs that can be adapted to either stirrup or slugger hammers are useful.

Hammers are made of chilled iron, forged high-carbon steel, cast manganese steel, or special tough hard alloy steels, and in a variety of shapes according to service. They weigh from a few pounds to 250 lb. each.

Forms A, B, and C are BAR TYPE for light duty; A and C for relatively coarse product, B for a finer product produced by more attrition grinding on the grid. Forms D, E, and F are of the so-called SLUGGER TYPE, for heavy duty; in each of these forms some provision is made for saving discard metal.

Hammer Mill Types of Hammers

Such forms are usually heat-treated to produce hard heads and tough shanks. Form E is cast with a cored-out head to permit compensation for wear by means of additions of lead in the cored cavity, with the idea of thereby decreasing troublesome and possibly destructive vibration due to uneven wear. In some forms, e.g., form F, the hole for the rotor pin is cored out for bushings of various eccentricities to permit maintenance of a relatively constant hammer circle, as a remedy to vibration. Form G has a replaceable head, designed to be pinned to the shank; this is better than similar forms with riveted heads, but the pins bend in service and are hard to remove; heads held on by lugs are better, if they are so designed as to insure against loss while running. Forms H and I are light and heavy STIRRUP TYPES respectively; they strike with greater impact than the slugger types, and are more effective in attrition grinding on the grid, but when they are forced back, more of the effective hammer circle is lost, and with them the rotor gets out of balance more frequently owing to uneven wear. A later form, designed with deeper sides, a bridge at the center, and with the face troughed, gave definitely longer life and lower metal costs per ton; the deeper sides increased the area of striking surface, which reduced both circulating load and wear on hammer arms; the bridge prevented deformation and thus made removal of worn heads a simpler and quicker job. The average new weight of heads was 28.6 lb. and the discard weight, 18 lb. If sides are made too deep for complete penetration, rejection weight increases owing to lack of wear at the inner portion. The trough reduced discard weight without reducing life. This change in design reduced metal cost for manganese-steel hammers and for chrome-steel. The essence of hammer design is to so apportion the metal that the head will maintain a face as large and as nearly in a radial plane as possible until wear has reached the point that breakage is imminent. With renewable tips, one shank will usually last as long as 3 or 4 tips.

Capacity of open circuits is decreased, and circulating loads in closed circuits tend to increase rapidly after the hammers are about half-worn, but the net reduction per hp-hr. is not greatly affected by hammer wear except near the end, provided penetration is complete.