Hammer Mills

In 1830 a patent was issued in United States for a machine

…which comprised a wooden box containing a cylindrical drum, apparently made of wood, on which iron knobs or hammers were fastened. The expectation was that this drum, when revolved at 350 rpm, would shatter the rock fed into the box. (McGrew 1950)

This machine never went into commercial production, but it was the forerunner of the hammer mill, which is the most widely used crusher in which high-velocity impacts are used to break rocks. The distinctive features of the hammer mill are the high shaft speed, which may be in the 700-1,500-rpm range. The high velocities of the tips of the hammers—up to 3,000-4,000 m/min—gives them sufficient kinetic energy to shatter rocks at the instant of impact. These velocities could be achieved with electrical motors more easily than with steam engines, so hammer mills had to wait for electricity in the 20th century to be widely used for breaking hard rocks even though they were used in the 19th century. Figure 5.19 gives a sectional view of the operation of a hammer mill.

The material is fed uniformly to the breakage zone by way of an inclined plate. The hammers are attached to the shaft so that they are free to move at right angles to the shaft through 180 degrees. They deliver heavy blows, shattering the rock and throwing it against the breaker plate. Broken pieces rebound, and the procedure is repeated until the fragments leave the mill through a grate in the bottom of the machine. The capacity of a hammer mill is more dependent on the nature of the material than is the case for most other crushers. It also relies on the velocity of the hammers and the aperture of the grate. The energy in the hammers is maintained by the kinetic energy in the rotor shaft, often supplemented by heavy flywheels attached to the ends of the shaft. Hammer mills tend to produce a cubical product because the impact action is well suited for breaking flat or elongated rocks that do not escape easily through the grate bars in the bottom of the crusher. Consequently, they are used at times for producing aggregate with a cubical shape even though high wear may occur because of hard rock.

The reduction ratio in a hammer mill is very high, up to 20:1 in open circuit and much more in closed circuit. The potentially high wear rates of the hammers and side plates generally limit the use of hammer mills to the crushing of soft, nonabrasive rocks such as limestone, coal or gypsum, or for the grinding of hay and grains for chopped feed. A rule of thumb has been that hammer mills are suitable for materials containing not more than 5% silica, doubtful for a silica content of 10%-15%, dangerous if 20%- 25%, and prohibitive if it exceeds 30%.

Updated: 24.03.2016 — 12:06