Shear cell tester

The weaknesses associated with the flow through an orifice and angle of repose measurements limit their application for powder flow studies and hopper designs. Consequently, several powder shear testers and methods that permit a more thorough and precisely defined assessment of powder flow characteristics were developed. Shear testers that measure the frictional characteristics of a powder bed under load yield valuable information with regard to powder flow in high-speed tablet equipment. A number of types of shear cell testers are available, but the most common types used in the pharmaceutical industry are the Jenike shear cell and the Schulze ring shear tester.61,62... 

The Jenike shear cell tester is classified as a direct shear tester that is capable of providing information on a solids cohesive strength as well as its wall friction properties. The tester allows us to measure the strength of a powder blend as a function of pressure applied to it. These are two main considerations when design a bin or hopper to ensure reliable material flow. The tester consists of a base, a moveable shear ring resting on top of the base, and a top cover lid (Fig. 7.3).61 The base is fixed while the lid rotates at a constant low rate. Powder blend is placed in the ring and base and a... 

Flow Angle of repose tester Shear cell tester Flow time... 

In order to achieve unique numbers for various and different degrees of flowability Jenike calculated the failure function at a specific value of the unconfined stress. The specific value of the ratio of the major consolidation stress to the unconfined stress was taken as 6.5 Ibf/ft (3.11 kPa) in the initial, Jenike designed, 4 inch diameter shear cell, because at this value the relationship between fa and a showed only small deviations from linearity. Currently a value of 3 kPa is generally used with a standard shear cell tester. 

This tester is designed for cohesive materials so a limit is placed on the maximum particle size of powder used in the tester. Tests can be conducted at both ambient temperature and humidity or under conditions created in an environmental chamber. Tests conducted with the Jenike and Johanson Quality Control Tester (JJQC) have been compared with results obtained from a conventional Jenike shear cell tester using cohesive titanium dioxide, laundry detergent, baking soda and table salt. A plot of the failure pressure obtained from the JJQC tester and the Jenike shear cell tester strength showed that the JJQC test values are significantly lower at high consolidation pressures but were comparable to the Jenike values at low consolidation pressures (Ploof Carson 1994). 

Rajendran Nair et al. (1990, 1993) examined the cohesiveness of mixtures of a coal and 10% clay mix, which had a residual moisture eontent of 0.15% moisture, with mixes having various amounts of added water. The parameter of cohesiveness (Cm) was derived from the yield loei measured in a Jenike shear cell tester and the tensile strength measured was from... 

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... 

The ring or annular shear cell, was developed by Carr and Walker as early as 1968. In recent years this tester has undergone a number of modifications. Peschl has developed an annular shear cell in which the sample and shear cell consists of a full circle. This contrasts to the earlier cells that have a band of sample on the outer portion of the circle. This was done to eliminate wall friction. It is also rotated very slowly, since at low speed, velocity variability becomes more negligible in the shear measurement. In this way a full ring can be utilized and speed differences in the outside and inside of the ring become negligible. 

Behres, M. Klasen, C.-J. Schulze, D. Development of a shear cell for measuring the wah friction of bulk sohds with a ring shear tester. Powder Handhng Process 1998, 10 (4), 405 9. 

Generally, the annular shear cell is an acceptable alternative to the Jenike tester for engineering design. The advantages of the annular shear cell can be summarised as follows ... 

The failure function can be measured directly in a number of ways. Some are rather complex and still under development, like the new plane strain biaxial tester with flexible boundaries30, but the simplest method so far is the uniaxial compression test. Only the version developed by Williams et al,24 gives results close to those obtained indirectly with the Jenike shear cell, the other versions yield relative measurements only. 

As the name implies, the split cell testers use a cell, usually in the form of a ring similar to that used in the Jenike shear cell or the wall friction testers, but both the cell and supporting plate are split vertically, across the diameter. The sample is compacted vertically in the usual way (by application of normal loads via a lid) and the sample is then pulled apart by moving the split halves away. There have been some research devices designed on this principle, for example by Boden33, who suspended the cell supports on air bearings to minimize friction, but such devices are too expensive for general use. 

The Cohesion Tester has been used in industry, mainly as a quality control-type test. It can be useful as an aid to assessment of flow properties and of power requirements in mixers, bulk conveyors and feeders. The original developers of the tester, Warren Spring Laboratories35, have even tried to correlate the cohesion value with unconfined yield stress determined with the Jenike shear cell and found a good correlation for some powders (fc = 6 x C). The tester is, quite obviously, only useful with fine, cohesive powders because ... 

The limitations of the Jenike shear cell are that it is not very useful for measuring bulk solids with large shear deformations, e.g., plastic powders. The level of consolidation stresses required are inappropriate for pharmaceutical materials, and the quantity of material required is often beyond that available in the early stages of development. Alternative shear cells that have been used include annular shear cells (Nyquist and Brodin 1982 Irono and Pilpel 1982) and ring shear testers (Schulze 1996). 

Two aspects of texture were considered firmness of the intact segment and toughness of the carpellary membrane. Both were measured on stored segments with an Instron model 1011 Universal Tester (Instron Corp., Canton MA 02021) set on compression mode. Firmness was determined by crushing a pair of segments in an Ottowa Texture Measurement System to 3 mm final thickness. Crushed segments were then transferred to a Kramer Shear Cell and the force necessary to shear the membranes determined. 

Measurement of tensile stress cannot be measured directly with a Jenike shear cell or an annular shear tester, although one approach in the measurement of tensile strength of a powder is to determine the yield locus of the material and then to extrapolate part of this locus to zero normal stress. The negative intercept on the normal stress axis is the tensile strength of the material under investigation (Figure 1.18). 

Tensile strength is a fundamental failure property but tensile strength testing is dependent upon the direction of force necessary to cause separation of a bulk structure with respect to the direction of compaction or consolidation. Split cell testers pull the sample apart at 90° to the direction of compaction whilst the lifting lid or vertical shear testers pull in the same direction as the compaction/consolidation stress was applied. Results obtained from both methods differ greatly because tensile testing has a poor record of reproducibility, possibly due to the fact that consolidated powders in the tester cells may not be isotropic. 

Split cell testers Tensile strength measurements may also evaluate internal adhesion properties, independent of the mechanical interaction of shear plane, because T is measured at zero shear. Warren Springs Laboratories (WSL) designed two instruments one to measure the cohesive strength of powders at varied states of compaction and the other to measure tensile strength. The tensile strength measurement equipment, developed by WSL, was the WSL tensile tester, a diametrically divided split shallow circular cell. 

One way which relies upon the fundamental knowledge of the stress-strain-volume behaviour of bulk solids is dependent upon the development of testers such as the biaxial and triaxial shear testers as well as the now universally accepted Jenike shear cell, or the standard shear test tester. Other instruments, such as the annular shear cells and the cross-sectional Peschl and Colijn (1977) tester, use the same stress-strain-volume principle. These annular shear cells may also be used to evaluate a bulk powder flow function. The powder flow function, having been discussed previously, still requires a family of yield loci before cohesion can be evaluated. 

POSTEC - unaxial research tester In the POSTEC - uniaxial tester, discussed by Maltby and Enstad (1993), the sample is confined in a cylindrical die and wrapped in a flexible membrane which is stretched between the outer periphery of the piston and the inner perimeter of the lower part of the die. Since the membrane is stretched and in contact with the wall and powder, the sample is compacted homogeneously thus the wall friction between the specimen and the die is reduced. Comparison of the POSTEC uniaxial tester with a biaxial and Jenike-type shear eell testers, with the standardised CRM-116 limestone powder, indicated that the fc values obtained with the POSTEC are slightly less than those obtained with Jenike-type shear cells and a biaxial tester. Since the total time for... 

Figure 1.28 Comparison of the degree of flowability from two shear cells and a cylindrical mould tester (Bell etal. 1994).

Figure 6.8 Expanded view of an annular shear cell polymer to metal friction tester. Drawing by Richard Humpidge. 1975, Rapra Technology)...

The Peschl tester was the first (and for a long time, the only) automated shear cell available. The volume of the standard cell is relatively small, making it convenient for expensive bulk materials such as pharmaceuticals and agricultural chemicals. It is widely used for quality control and product development Testing limes are about 1/3 of that required with the Jenike cell. 

A similar concept with a number of engineering improvements has recently been developed by Schulze (Figure 5). The mechanism is much more sensitive than that employed by earlier ring shear testers, and the cell can be removed for cleaning and also for time consolidation testing. It is commercially available. Excellent correlation has been observed between the Schulze tester and the Jenike shear cell, as well as with more sophisticated research instruments. An automated version is available for situations where high productivity is required. As with the Peschl tester, testing times are about 1/3 of that required for comparable Jenike tests. 

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