The fundamental design for industrial hammermills is straightforward. Hammermill machinery consists of a rotating internal shaft to which steel hammers (or knives) are attached, all enclosed within a steel chamber. Plates line the interior, while bar grates or steel screens normally cover the outlet where materials are discharged. In most models, the interior plates are made so they can be easily replaced, as milling abrasive materials will cause greater wear on the hammermill's internal components.

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Material usually enters the hammermill from above, after which it's fed into the grinding chamber. Hammers attached to the rotating shaft strike the material repeatedly, breaking particles down, while contact with the chamber's sides reduces the material further. Unfinished product stays in the grinding chamber until particles are reduced to a point where they can pass through the screen or grate, covering the opening from which material is discharged. Understanding how the hammermill's machinery works help choose the best industrial reduction system for milling abrasives and other materials.

The Hammermill: Machinery for Handling Abrasive & Hard-to-Mill Materials

Also known as a type of impact mill, the hammermill is one of the pieces of industrial equipment most used for abrasive and hard-to-mill materials. Machinery for breaking down materials often requires exactness in its control of particle size as well, a task at which the hammermill excels. 

Hammermills are used for applications that include: 

• Dissolution: Used especially within the chemical, food processing, and pharmaceutical industries to break down material into smaller particles that can dissolve quickly into solutions.
• Energy: Used to reduce biomass, corn, coal, wood, or other solid material for producing fuel or biofuel sources.
• Food processing: Used for grinding down various kinds of ingredients for food, including varieties of legumes, soybeans, groundnuts, and beans; hammermills can even be used to remove hard shells from nuts like almonds and walnuts.
• General particle size reduction: Used generally to reduce the size of a material's particles, such as medicinal grains put into capsules in the pharmaceutical sector.
• Preconditioning: Used to precondition material so it can be ground even further.
• Powder production: Used to crush ingredients into intermediate powder grades with precise particle sizes for the chemical and pharmaceutical industries.
• Recycling: Used for breaking apart scrap when recycling various materials, including metal or plastics from old appliances and junked vehicles.

Hammermills are diverse in their design and capable of use in the primary, secondary, or tertiary stages of the reduction of various materials. They can be used for juicing fruits, and milling grains, among many other applications. Their versatility and robust construction are the key factors that make them one of the most widely used types of mills, especially for abrasive materials.

Types of Hammermills

There are five basic types of hammermills. Machinery will often differ even within the same type of hammermill, though the basic principles of operation are the same. Operating on the principle of impact, hammermills are industrial workhorses for any business needing to reduce material during production.

Gravity Discharge Hammermill

The most simply designed hammermill machinery internally features a shaft rotating at high speeds to which moving hammers are attached. Particle breakdown also occurs at the breaker plate within the grinding chamber. The simplicity and adaptability of this design make this hammermill a good fit for many industrial systems.

Applications for which gravity discharge hammermills are used include:

• aggregates such as concrete, gravel, or sand
• ceramics such as porcelain or stoneware
• coal and coke
• dry chemicals like lime or sodium bicarbonate
• glass and glasslike materials
• metals and alloys
• plastic and other resins

Replaceable steel plates line the interior of the hammermill to protect it when grinding abrasive materials.

Pneumatic Discharge Hammermill

Though the actual grinding mechanism for this mill is similar to that of the gravity discharge hammermill, the machinery in the interior differs. Pneumatic hammermills normally use thinner hammers and have a ribbed rather than a flat-lining plate. The ribs have a washboard effect on the product passing through the grinding chamber, working with the hammers to further reduce the material. 

Applications for which pneumatic discharge hammermills are used include:

• biomass such as forestry waste
• meat and bone meal
• paper products
• wood chips and scrap

Often with this type of hammermill, machine designs feature notched hammers that are specially designed to shred and tear. These types of hammermills tend to be used for lighter, lower-density materials.

Full Circle Screen Hammermill         

This hammermill's machinery features the same grinding mechanism, with thin hammers similar to pneumatic types. The main difference involves the extent to which the screen covers the evacuation area within the hammermill. Machines will have almost 300 degrees of screen coverage, which provides a much greater surface area from which material is discharged than other types. This results in more throughput per horsepower than the basic gravity discharge hammermill.

Applications for which full-circle screen hammermills are used include:

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• corn and other grains
• grasses like oat grass and wheatgrass
• planer shavings and sawdust
• spices such as cinnamon, cumin, oregano, and ground peppers

However, this type of hammermill requires a thinner screen that's at least somewhat pliable. As such, it's better suited for easier-to-grind materials that don't require a breaker plate. 

Horizontal In-Feed Hammer Mills

In this design of hammermill, machinery feeds the mill from the side rather than from above. This distinctive factor makes them perfect for breaking down longer bits of scrap and larger materials without the need to pre-grind. Horizontal hammermills also feature more robust screens over the discharge area.

Applications for which horizontal in-feed hammermills are used include:

• two-by-fours, scrapped pallets, and whole pallets
• various types of organic and inorganic scrap materials

The design of the hammers in these mills makes them capable of forcefully breaking down larger material, using feeding rolls to control the rate of material entering the grinding chamber.

Controlling Particle Size Reduction

The particle size needed for an application influences the exact type of milling equipment used, though this also depends on the properties of the raw material being processed. Particle size reduction serves an important role in the processing of bulk powders to enhance density, flowability, and reactivity. Depending upon the application, material with particles sized incorrectly will influence the properties and quality of the end product.

Some key factors that control particle size when utilizing hammermills are: 

• Screens or bar gates: About 70 percent of the control a manufacturer has over particle size is determined by the hammermill's screen or bar grate, as particles must be small enough to pass through it and into the discharge opening.
• Force: This accounts for the rest of the particle size of a finished product and is based on the impact force that occurs inside the hammermill; machinery within the mill determines the final particle size, which is based on factors involving the rotor and hammers.
• Rotor speed: The more quickly a rotor moves, the more force that's applied to the material, resulting in a material with smaller particles.
• Hammer size: The larger the hammer strikes the material within a hammermill, the more pieces it will break into.
• Number of hammers: Similarly, the more hammers that are used within reduction equipment, the smaller the material's particles will be.

To learn more about Prater's hammermills and other particle reduction equipment, contact the experts at Prater today.