Shear cells Jenike

Fig. 10 Yield loci of spray-dried lactose determined using the Jenike shear cell and the simplified shear cell. Open circles Jenike shear cell open squares simplified shear cell. (Adapted from Ref. 48 with permission of the publisher.)...

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

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... 

Bilgili E, Yepes J, Stephenson L, Johanson K, Scarlett B. 2004. Stress inhomogeneity in powder specimens tested in the Jenike shear cell Myth or fact Part. Part. System Charact. 21(4) 293-302. 

In a shear cell, such as the Jenike shear cell,22 the test powder is consolidated in a shallow cylindrical chamber which is split horizontally. The lower half of the cell is fixed and a shear force is applied to the upper moveable part at a constant low rate (see example E5, section 7.5.5, where experiments were obtained at a constant rate of 0.03 rpm, corresponding to a linear velocity of 1 mm min-1). Shearing can be carried out for each of a series of normal loads on pre-consolidated samples, so that at the end of the test the relationships between the shear stress and normal stress at various bulk densities are obtained. 

There are several different methods available for determining the flow properties of powders. Shear cell methods provide an assessment of powder flow properties as a function of consolidation load and time. There are a number of types of shear cells available, the most common being the Jenike shear cell [21]. 

Fig. 7 The Jenike shear cell. (From York, P. Int. J. Pharm. 1980, d, 101.)...

Cohard, C. Gonthier, Y. Chuha, D. Verain, A. Comparative rheological study of different materials with the jenike shear cell. J. Pharm. Belg. 1984, 39 (4), 209-216. 

A precise definition of the flowability of a powder is only possible with several numbers and curves, derived from a family of yield loci of the powder (measured with a shear cell) - see section 4 for further detail. Jenike23 proposed a simpler classification, according to the position of one point of the failure function (at a fixed value of the unconfined yield strength, say 5 lbf (22.3 N) with the Jenike shear cell, i.e. 3112 Pa or 65 lbf/ft2) with respect to the flow factor line (straight line through the origin, at a slope l///where//is the flow factor) - see Fig. 8 for a schematic representation of this. 

The so-called shear cells are used for direct shear tests, where the powder specimen is consolidated in the vertical direction and then sheared in a horizontal plane. There are basically two types of shear cells in use today the Jenike shear cell (sometimes referred to more generally as the translational shear box) and the annular (or ring) shear cell (the rotational shear box). As the equipment needed is highly specialized (and hence outside the scope of this Guide) and as manufacturers instructions are usually adequate, the following contains only an outline description of both the hardware and the test procedures. 

Fig. 17 shows the Jenike shear cell in a schematic diagram a circular (internal diameter 95 mm), open-ended shear box is split horizontally, the base is immobile and the ring can slide freely in the horizontal direction. The normal stress, which is applied via the lid, is first used to consolidate the specimen and then to load it during test. 

In the Jenike shear cell, the failure is forced to be in a lenticular space around the horizontal shear plane rather than throughout the whole sample and shear strains cannot therefore be determined. This disadvantage is overcome to some extent in the simple shear apparatus which confines the powder specimen in a rubber membrane or a hinged box, thereby allowing the failure to occur in the whole of the sample. As the whole thing distorts during test, shear strains can be evaluated. The shear stresses are not, however, uniformly distributed and the method has not been widely used other than in research. 

In annular shear cells which represent a commercial alternative to the Jenike shear cell), the shear stress is applied by rotating the top portion of annular shear box (see Fig. 18). These devices allow much larger shear distances to be covered both in sample... 

Lower values of yield loci and failure functions measured than with the Jenike shear cell. 

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. 

The results obtained for the failure function of the Ti02 used in the study agreed closely with that obtained using a Jenike shear cell. 

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. 

A mould in the form of a ring, like the Jenike shear cell ring for example, is used and a lid which just fits inside it. The base of the cell and the lower face of the lid are covered with sellotape on which a glue is spread. The cell is filled with the powder to be tested and it is scraped level with top of the cell the lid is placed in position, on top of the sample. 

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). 

American Society for Testing and Materials 1997. Standard shear testing method for bulk solids using the Jenike shear cell. American Society for Testing and Materials, Baltimore, MD. 

More reproducible results are obtained if the shear strength of a well defined, often specially prepared agglomerate is measured (Fig. 5.24b). This method was adapted from the well known shear cell for the evaluation of cohesive particulate solids (Jenike shear cell and derivatives [5.5]). 

Shear strength of a bulk material in differing states of dilation is a key property of interest for flow considerations. The conventional hopper design method for mass flow is based upon critical state theory, and a Jenike shear cell is used to secure yield locus values upon which a design procedure is based. This technique is universally accepted, but not widely used for small hoppers for various reasons. Significant cost and expertise is required to obtain accurate values, compared with full-scale trials and... 

There are basically three types of shear cells available for powder testing (a) the Jenike shear cell, also known as the translational shear box (b) the annular or ring shear cell, also called the rotational shear box and (c) the rotational shear cell, which is a fixture of a powder rheometer. 

The Jenike shear cell has been considered for long time the testing cell for establishing standard procedures in industrial applications and research. It has been recognized as one of the standards for testing bulk solids in the United States and in Europe, being especially focused on cohesive powders. The complexity of this method is such that errors due to poor technique can easily arise. A reference material has therefore been produced with which laboratories can verify both their equipment and experimental technique. The reference material consists of 3 kg of limestone powder packed in a polyethylene jar. It is accompanied by a certificate giving shear stress as a function of normal applied stress for four different powder compaction stresses. 

The angle of internal friction can be measured directly by the "grooved plate" method. The base of the Jenike shear cell is replaced by a metal plate in which a number of saw-toothed grooves are cut (Figure 2.15). These... 

The technique of shear testing has traditionally been used to determine the shear behavior of consolidated powders. As discussed previously the Jenike shear cell testing is generally accepted as an established method for measuring powder flowability and standards based on it have been issued, both in Europe and the United States. Some disadvantages of the Jenike shear cell have also been mentioned. The process is extremely time consuming and tedious and correct consolidation of sample is difficult to achieve. It can be hardly used for all types of bulk solids due to its limited shear... 

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