Shear cells

Here, [L is the coefficient of internal friction, ( ) is the internal angle of friction, andc is the shear strength of the powder in the absence of any applied normal load. The yield locus of a powder may be determined from a shear cell, which typically consists of a cell composed of an upper and lower ring. The normal load is applied to the powder vertically while shear stresses are measured while the lower half of the cell is either translated or rotated [Carson Marinelli, loc. cit.]. Over-... 

A powder s strength increases significantly with increasing previous compaction. The relationship between the unconfined yield stress/, or a powder s strength, and compaction pressure is described by the powder s flow function FE The flow function is the paramount characterization of powder strength and flow properties, and it is calculated from the yield loci determined from shear cell measurements. [Jenike, Storage and Flow of Solids, Univ. of Utah, Eng. Exp. Station Bulletin, no. 123, November (1964). See also Sec. 21 on storage bins, silos, and hoppers.]... 

Texture measurements Texture of canned carrots was measured using Instron Universal Testing Machine (Model 1011) fitted with Kramer shear cell. Thirty grams of drained carrot cubes were evenly placed in the Kramer shear cell and were compressed, sheared and extruded using a crosshead speed of 100 mm/min. Each measurement was repeated 10 times and the mean was used to express the firmness of carrot cubes in Newton(N). 

Amidon and Houghton [48] completed a comparative study of several common methods of characterizing powder flow. Table 2 contains experimental results for a number of commonly used pharmaceutical excipients. Compressibility index, angle of repose, flow rate through an orifice, and shear cell data are presented. 

Angle of repose Flow rate through a 6-mm orifice Simplified shear cell ... 

Because the compressibility index can be thought of as a measure of the likelihood of arch formation and the ease with which arches will fail [44], and because it can be rapidly completed, it appears to be a useful measure of flow. Obviously, exceptions will occur, and caution should be exercised in its interpretation. As we will dicuss, the compressibility index values reported in Tables 2 and 3 and Fig. 4 are quite consistent with shear cell characterization. 

Fig. 6 Schematic diagram of the simplified shear cell of Hiestand. (Adapted from Ref. 48 with permission of the publisher.)...

In a slightly different form, Eq. (6) is commonly referred to as the Warren spring equation. Representative yield loci determined utilizing the simplified shear cell are shown in Fig. 7 for spray-dried lactose, bolted lactose, and sucrose. The yield locus for each material relates the shear strength to the applied load. 

The range of application of shear cell testing methodology is seen in Tables 2-6. Table 3 relates the flow properties of mixtures of spray-dried lactose and bolted lactose. These mixtures, in combination with the excipients tested, cover a broad range of flow. Tables 4 and 5, for example, show lot to lot variations in the flow properties of several materials, and Table 6 shows the variation in flow properties of bolted starch, sucrose, and phenacetin at different relative humidities (RH). Figure 8 presents the yield loci of sucrose at four different consolidation loads. Also shown in the figure are the shear indices determined at each consolidation load. 

Shear cell measurements offer several pieces of information that permit a better understanding of the material flow characteristics. Two parameters, the shear index, n, and the tensile strength, S, determined by fitting simplified shear cell data to Eq. (6), are reported in Table 2. Because of the experimental method, only a poor estimate of the tensile strength is obtained in many cases. The shear index estimate, however, is quite reliable based on the standard error of the estimate shown in parenthesis in Table 2. The shear index is a simple measure of the flowability of a material and is used here for comparison purposes because it is reasonably reliable [50] and easy to determine. The effective angle of internal... 

An analysis of the experimental errors associated with shear cell measurements indicates that, in general, neither the standard deviation nor the percent... 

Table 6 shows the usefulness of shear cell data in formulation development. Variations in relative humidity can profoundly influence flow this is a valuable piece of information for formulation development. Shear cell methodology thus provides useful data for optimizing the flow of formulations as well. 

A direct comparison of the results obtained by the simplified shear cell methodology and the Flowfactor Tester of Jenike and associates is difficult since the experimental procedures differ. Figures 10 and 11 show the yield loci obtained using both methods. For free flowing spray-dried lactose the yield loci are very similar (Fig. 10) and nearly linear, as is generally observed [45]. Bigger... 

Fig. 12 Plot of the shear index value, N, as determined using simplified shear cell methodology, versus Worst Case Bonding Index, BIW.

G. E. Amidon, D. P. Smith, E. N. Hiestand, and B. D. Tiffany, Flowability of Powders Using a Simplified Shear Cell, APhA National Meeting, November 1981. 

The resistance of a particulate mass to shear may be measured in a shear cell such as that described by Jenike et al. n x2) The powder is contained in a shallow cylindrical cell (with a vertical axis) which is split horizontally. The lower half of the cell is fixed and the upper half is subjected to a shear force which is applied slowly and continuously measured. The shearing is carried out for a range of normal loads, and the relationship between shear force and normal force is measured to give the shear strength at different degrees of compaction. 

Standard Shear Testing Method for Bulk Solids Using the Jenike Shear Cell. ASTM Standard D6128-00. ASTM International, 2000. 

The angle of repose of a powder blend, effective angle of internal friction (EAIF) from shear cell measurements, and the mean time to avalanche (MTA) in powder cohesivity tests are useful for assessing the flow of a tableting mixture at various scales (15 18). 

For example, the use of a shear cell for determination of flow properties (cohesion) has been conducted as shown in Figure 1. Despite some variability, the shear cell values obtained correlated well with the Carr Index. 

Figure 1 Shear cell index versus Carr Index.

The prediction power of the shear cell measurement for powder flow as judged by tablet weight uniformity is shown in Figure 2. Discrepancies, especially among powders with good flow, are observed. 

Figure 2 Shear cell index versus weight uniformity from a B3B tablet press.

Figure H2.2.3 A force/deformation curve illustrating a compression-extrusion test (10 mm/min deformation rate) for canned green peas using a Kramer shear cell (multiblade) at room temperature.

Place the specimen on the two metal parts of the lower portion of the Wamer-Bratzler fixture or the Kramer shear cell and fill the lower fixture to about half capacity. In the case of a wire cutting device, place the specimen on the lower plate. 

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