Glossary of Screening Terms

A / B / C / D / E / F / G / H / I / J / K / L / M / N / O / P / Q / R / S / T / U / V / W / X / Y / Z

Accuracy of Separation
The trueness of a separation as measured by the amount of off-size material within a screened grade or cut. Not to be confused with efficiency. Also called sharpness of separation.

Amplitude
The distance traveled by the screen surface during one cycle. The definition of distance varies from one manufacturer to another: e.g. for a circular motion some consider the radius as amplitude, others consider the diameter. See Stroke for a better description of motion.

Aperture
An individual opening in the screening surface.

Angle of Repose
That angle to the horizontal that material will assume naturally when in a pile.

Ball Deck
A deck or tray beneath the control mesh that retains balls. The machine's motion causes the balls to bounce and impact against the underside of the clothing to reduce screen blinding.

Bed Depth
The thickness of the material layer as it traverses the screen surface.

Blinding
A condition, such as coating or plugging, that reduces the open area of the screen surface.

Bulk Density
Screening is a volumetric process and the lighter the product is the greater the screening area needs to be.

Cable Suspension
A method of screener installation-hung from overhead-that isolates or reduces the transmission of out-of-balance forces.

Capacity
The amount of material that a screener can handle under specified conditions. Capacity statements are meaningless if not related to other parameters such as efficiency, accuracy of separation, and particle distribution.

Cloth
The wire screening surface, woven in square, rectangular, or slotted openings. Also called wire cloth or screen cloth.

Coating
A condition of blinding where moisture or static causes undersize particles to cling to the screen openings.

Deck
The screening surface. Also used to describe the number of surfaces within a machine, such as single deck or double deck (two deck).

Declogging
IIn order to promote feed capacity and screening efficiency it is important to keep the mesh clean. Therefore deluging balls can be used under the screen cloth. The gyratory motion of the screener will be very efficient for this deluging action. Cleaning balls can be made from different materials and in different sizes depending on the applications and the products. For fines separation the gyratory action can be combined with an ultrasonic device, which will give some additional deluging action on the mesh.

Degradation
Material breakage that is caused by handling, sometimes in the screener itself.

Efficiency
The ratio of the material amount passing through the screen surface to the undersize material amount in the feed that is available to pass through. Various formulas are sometimes used to factor in the presence of off-size material in the feed that passes through. Also called product yield.

Feed Rate
The amount of material presented to a screener for processing.

Fines
Particles smaller o size than the screen openings. Also called thrus or undersize.

Frequency
The number of times peak-to-peak amplitude or stroke takes place per unit of time, usually expressed in revolutions per minute or cycles per minute.

Gyratory Motion
The unique gyratory motion imparts a combination of actions to the material as it passes along the screen surface. It begins with a horizontal circular motion at the feed end, gradually diminishing along the length of the machine to an elliptical path, and finally to an approximate straight-line motion at the discharge end. There is no vertical component to this motion – thus keeping the material in constant contact with the screen surface.

Mesh
The number of openings (and fraction thereof) per linear inch of screen cloth, as measured from the center of a wire.

Near-Size
Material that is very close to the size of the screen cloth opening, generally considered within a ±25% range.

Open Area
The ratio of the screen cloth opening area to the total screen surface area. Also called percent open area.

Overs
Material that passes over the screen surface. Also called tailings.

Plugging
A condition of blinding where near-size particles wedge or jam into the screen openings, preventing the passage of undersize material.

Particle Size Analysis (PSA)
The main parameter for feed products.

Rate of Travel
The conveying speed of material over a screen surface, usually expressed in feet per minute.

Rejects
Unwanted material, either oversize or undersize.

Retention Time
The amount of time a particle spends on the screen surface.

Scalping
The removal of a small amount of oversize material (generally less than 5%) from feed that is predominantly fines (at least 50% half-size). Mesh aperture is big compare to product’s size. Screening is easier and high feed rate can be achieved. Typical example of scalping is sugar before packing in small 1kg bags.

Screening
The process, which classifies a feed material by its particles’ size. It is a volumetric process involving probability of a fine particle to go through the aperture of a wire mesh. Many parameters relating to the product (size, density, temperature, etc.) and to the applications (feed rate, air moisture, efficiency required, etc.) will affect the screening performance. It is important to know these parameters in order to select the correct type and size of screening equipment.

Sieve Analysis

The particle distribution of a representative sample of material. This is expressed in the percentages of a particle size group passing through and retained on standard testing sieves. (Tyler and U.S. Standard are the common standards n the U.S.)

Sizing
The feed product is classified into different fractions depending on the required applications. In this case mesh aperture is closed to product size. Screening technology is important and gyratory motion will allow high efficiency and separation precision. Feed capacity will depend on each product and mesh aperture. Typical application is sand screening into different fractions for abrasives applications.

Stratification
The process during screening where larger particles rise to the top of the bed of material and smaller particles sift through the voids to the bottom of the bed.

Stroke
The peak-to-peak distance traveled by the screen during one cycle; the diameter of a circular motion.

Testing Sieve
An instrument that uses stacked cylindrical screens with standardized apertures to determine the particle distribution of a representative sample of material.