Hammer Mill

A hammer mill is a type of crusher, which can be used for grinding rock, forage, or waste materials. It is designed to convert larger pieces of material into smaller particle sizes.

Hammer mills are available with or without screens, which are used to separate particle sizes. Screenless models utilize air flow to separate particles. They were invented because replacing the screen, which is considered the most fragile component of the hammer mill, is costly and inconvenient. This is especially true for third world nations with little access to spare parts.

[edit] History

A patent was issued in 1830 for a machine with a wooden box containing a cylindrical drum, revolving at 350 revolutions per minute, on which hammers were attached. The machine was designed to shatter any rock fed into the box.^[1] This machine never went into commercial production, but is considered as the forerunner to the hammer mill.

The hammer mill has gone on to become the most widely used crusher utilizing high-velocity impacts to break rocks.^[2]

However, while initially designed to crush rocks, the hammer mill was adapted for grinding grain for livestock feed. It was discovered that many types of forage benefit in nutritive value after being broken down.

[edit] Gehl's Grain Grinder

The Gehl company produced the first grain grinding hammer mill in the 1920s. It dominated the market for 30 years, during which time it also developed a portable truck mounted mill.^[3]

[edit] Developing a Screenless Hammer Mill

In 1990, Carl Bielenberg of Appropriate Technology International (ATI) began developing a screenless hammer mill. His prototype separated flour from larger particles through an opening in the circumference of the grinding chamber. Flour passed through the opposite side of the rotating blades while the larger pieces continued inside the chamber.

Initial tests produced a larger, courser material than conventional hammer mills. He developed a series of improvements, but was incapable of designing a machine that could perform to the standards of conventional mills.

He presented his machine to a series of MIT students in hopes that they could produce a more successful machine.

An MIT student named Amy Smith headed the project. Her redesigned mill was capable of running continuously without clogging. It was also capable of being manufactured in a small village workshop. This second aspect was extremely important to Smith. After discovering the failings of conventional hammer mills, she was determined to develop a machine that would be useful for third world countries.

Many African women used the hammer mill to grind grain, but its screen was prone to breaking. Screens cannot be produced locally and are expensive to replace.

Smith dedicated her research to “the African women who cannot afford the 5 cents it takes to mill a kilogram of flour and thus spend hours performing the back-breaking labor necessary to prepare food for their families.”^[4]

[edit] Features/How it Works

[edit] Rock Crushing Hammer Mill

These mills have high shaft speed, anywhere from 700 to 1,500 revolutions per minute.^[5] This enables the hammers to shatter rocks at the moment of impact.

All material is fed into the mill by an inclined plate. The hammers are attached at right angles to the shaft and deliver heavy blows, shattering the rock and throwing it against a breaker plate. The pieces rebound off the plate and are continually hit by the hammers until they are small enough to leave the mill through a grate at the bottom of the machine.

[edit] Grain Grinding Hammer Mill

This hammer mill consists of a cylinder or rotor made up of several plates connected to the main shaft or axle. Hammers are attached to the plates by a series of pins. The outside of the rotating cylinder is a perforated screen, which separates the particles. The size of the holes in the screen can vary from 1/32 inch to 2 inches (0.076 to 5.1 cm) or more.^[6]

These mills can be single, double or triple reduction models with either rigid or swinging hammers. Double and triple reduction models are equipped with knives or blunt discs on one side of the rotor to chop longer stemmed materials like maize fodder or alfalfa before coming in contact with the hammers.

[edit] Screenless Hammer Mills

Instead of utilizing screens, these hammer mills use air pressure to separate the particles. Initial designs used a lower pressure gradient within the mill.

MIT students in charge of advancements increased the pressure of the airflow by taking advantage of the radial pressure gradient in addition to the axial gradient. They had two sources of airflow moving through the mill, moving the grain faster throughout increasing production.

[edit] Common Manufacturers

• Aadvanced Machinery Inc.
• Andritz Sprout
• Blue Tech Inc.
• California Pellet Mill
• C.S. Bell Co.
• Glen Mills Inc.
• Hazemag
  
• Sandvik
• Schutte-Buffalo Hammer Mill
• Stedman

[edit] References

1. ↑ Lynch, Alban J.; Rowland, Chester A. The History of Grinding 2005. Society of Mining Metallurgy and Exploration: 2005.
2. ↑ Lynch, Alban J.; Rowland, Chester A. The History of Grinding. Society of Mining Metallurgy and Exploration: 2005.
3. ↑ Ghel Company. Funding Universe. 2008-09-24.
4. ↑ Screenless Hammer Mill. D-Lab. 2008-09-24.
5. ↑ Lynch, Alban J.; Rowland, Chester A. The History of Grinding. Society of Mining Metallurgy and Exploration: 2005.
6. ↑ Feeds and Feeding of Dairy Animals. 2008-09-24.