The details of Hammer mills major components
A hammer mill is a machine whose purpose is to shred or crush aggregate material into smaller pieces. These machines have many sorts of applications in many industries.
The hammer mill can be used as a primary, secondary, or tertiary crusher. A major disadvantage of the hammer mill is the tremendous amount of wear (linings must constantly be replaced).
In this article, we will share some details about hammer mill structure, they are as follows:
The major components of these hammer mills include:
1. a delivery device is used to introduce the material to be ground into the path of the crusher hammers. A rotor comprised of a series of machined disks mounted on the horizontal shaft performs this task.
2. free-swinging hammers that are suspended from rods running parallel to the shaft and through the rotor disks. The hammers carry out the function of smashing the ingredients in order to reduce their particle size.
3. a perforated screen and either gravity- or air-assisted removal of ground product. Acts to screen the particle size of the hammer mill to ensure particles meet a specified maximum mesh size.
In hammer mill, there is the feeder device :Materials are introduced into the paths of the hammers by a variable speed vein feeder. This type of feeder can have its motor slaved by a programmable controller to the main drive motor of the hammer mill. The operational speed of the feeder is controlled to maintain optimum amperage loading of the main motor.
The amount of open area in a hammer mill screen determines the particle size and grinding efficiency. The screen must be designed to maintain its integrity and provide the greatest amount of open area. Screen openings (holes) that are aligned in a 60-degree staggered pattern optimize open area while maintaining screen strength. This method will result in a 40 percent open area using 3.2 mm (1/8 inch) holes aligned on 4.8 mm (3/16 inch) centers.
Feed producers need to pay particular attention to the ratio of open screen area to horsepower. Recommended ratio for grains would be 55 cm2 (~ 8-9 inches square) per horsepower (Bliss, 1990). Not enough open area per horsepower results in the generation of heat. When the heat generated exceeds 44C to 46C (120-125F), capacity may be decreased as much as 50 percent.
Besides these, Impact is the primary force used in a hammer mill. Anything which increases the chance of a collision between a hammer and a target; increases the magnitude of the collision; or improves material take-away provides an advantage in particle size reduction. The magnitude of the collisions can be escalated by increasing the speed of the hammers.
Hammer mill hammers design and configuration:
The design and placement of hammers is determined by operating parameters such as rotor speed, motor horsepower, and open area in the screen. Optimal hammer design and placement will provide maximum contact with the feed ingredient.
The number of hammers used for a hammer mill of 1,800 rpm, should be 1 for every 2.5 to 3.5 horsepower, and for 3,600 rpm, one for every 1 to 2 horsepower. Hammers should be balanced and arranged on the rods so that they do not trail one another. The distance between hammer and screen should be 12 to 14 mm (~ 1/2 inch) for size reduction of cereal grains.
The velocity or tip speed of the hammers is critical for proper size reduction. Tip speed is the speed of the hammer at it’s tip or edge furthest away from the rotor, and is calculated by multiplying the rotational speed of the drive source (shaft rpm) by the circumference of the hammer tip arc.
The removal of sized material from a hammer mill is a critical design feature. Proper output of material affects not only the efficiency of operation, but also particle size. When the correct ratio of screen area to horsepower is used and proper distance between hammers and screen face is maintained, most of the correctly sized particles will exit the screen in a timely manner. Anderson (1994) stated the particles that do not pass through the screen holes become part of a fluidized bed of material swept along the face of the screen by the high-speed rotation of the hammers. As these particles rub against the screen and each other their size is continually reduced by attrition. This excessive size reduction is counterproductive. Energy is wasted in the production of heat, throughput is restricted, and particles become too small.
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