Traversing cutters

With large tonnages, samples taken from conveyors can represent large quantities of material that need to be further reduced. Often, a traversing cutter is used as a primary sampler, and the extracted sample is further cut into a convenient quantity by a secondary sampling device. It must be borne in mind that the secondary sampler must also conform to the golden rules of sampling. This equipment is satisfactory for many applications but it has limitations, which restrict its use. These are  

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Example 5 Solids Sampling by Linear Traversing Trajectory Cutter lucremeut weight S hy a linear traversing cutter from hulk material flow of fine powder B expressed in unit weight per unit time is calculated hy... 

Commercial samplers are available that combine a traversing-type sampler and an unacceptable table sampler. An alternative design is the radial cutter or Vezin sampler. These samplers vary in size from a 15-cm laboratory unit to a 152-cm commercial unit. 

An alternative method is sampling directlv from a moving or stationary conveyor with cutter traverse througk the complete material bed carried on the conveyor. The alternative method cannot assure executing complete extractions, or through-stream sampling, because in many applications residual fines from the material stream remain on the conveyor surface. 

The cutter moves at constant speed (or constant rotation rate in the case of a rotary-motion sample cutter) such that the entire flow of material is traversed by the cutter, with the further requirement that the stopped position of the cutter at either limit of traverse (out of stream) is at sufficient distance from the stream so that no material from the stream enters the cutter while it is held stationaiy between traversing operations. 

Cutter blade length extends beyond the material stream width on either side of the stream and volume of the cutter is sufficient to ensure all material taken into sample can be contained in the cutter body. Cutter blades are parallel, and are beveled to a sharp edge in the case of linear-motion traverse. For rotary-motion sample cutters, sharp edges of the cutter blades are radial to the center point of rotation. 

Example 6 Slurry Sampling by Rotary Traverse of Gravity Flow Increment volume, quantity of slurry extracted hy one cutter rotation, is S from hulk slurry flow B expressed in voliime-per-imit time. R is cutter rotation per minute. D is cutter angle opening, with D/360 extraction ratio for continuous cutter rotation. 

Conveyor-belt speeds above approximately 300 ft per minute (1.5 meters per second) impart sufficient momentum to material discharging at its head pulley to cause lifting of material streams in a trajectory from the head pulley. A trajectory is illustrated in Fig. 19-7. Blades of the sample cutter are positioned to intersect the trajectoiy. See Fig. 19-7 for an example of a linear-traversing bottom-dump cutter installation. Calculation of trajectory profiles are described in the Conveyor Equipment Manufacturers Association publications and similar references. 

An electric-drive linear-traversing sampler of minimum standard manufactured size with cutter ana controls will range upwards of 8,000. 

Example 5 Solids Sampling by Linear Traversing Trajectory Cutter. . . 19-9... 

The bucket-type cutters have the advantage of being able to collect and transport the sample laterally, without the loss of headroom they collect and hold the sample, however, and thus allow material build-up within the bucket if the powder is a little sticky. Another disadvantage is that, whilst the mass of the diverter-type cutter remains the same during its traverse across the falling stream, the mass of the bucket cutter increases rapidly and the drive systems must be powerful enough to maintain its speed. 

As to the different designs available in each of the two groups, the reader is referred to two excellent recent publications by Plowman2 and Merks5. One design worthy of a special mention is the cross-belt type cutter (or rotating hammer sampler) which swings in plane perpendicular to the movement of a conveyor belt and scoops a well-cut sample off the belt. Unlike the other, linear cutters, this one is not limited in its speed but it should traverse the bed of powder on the belt in the shortest possible time. 

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