Ring shear cell

Bausch A. Hausmann R. Bongaitz C. Zinn T. Measurement of flowability with a ring shear cell, evaluation and adaptation of the method for use in pharmaceutical technology. In Proceedings of the 2nd World Meeting APGI/APV. Paris. May 1998 135-6. 

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. 

Nyquist, H., and A. Brodin. 1982. Ring shear cell measurements of granule flowability and the correlation to weight variations at tabletting. Acta Pharm. Suec. 19 81-90. 

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. 

We propose a way to obtain averaged macroscopic quantities like density, momentum flux, stress, and strain from "microscopic" numerical simulations of particles in a two-dimensional ring-shear-cell. In the steady-state, a shear zone is found, about six particle diameters wide, in the vicinity of the inner, moving wall. The velocity decays exponentially in the shear zone, particle rotations are observed, and the stress and strain-tensors are highly anisotropic and, even worse, not co-linear. From combinations of the tensorial quantities, one can obtain, for example, the bulk-stifftiess of the granulate and its shear modulus. 

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-... 

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... 

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. 

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 actual test a horizontal force is applied to the ring to shear the pre-sheared specimen along the horizontal plane but, this time, under a lower normal load. Most shear cells use a constant strain device to push the ring along and measure the applied force using a strain gauge. 

There is another type of shear cell, known as the ring cell or Peschl Shear Tester84. The cell is in the form of a full ring and is rotated like the annular shear cell. It is a very easy device for comparative measurements and another report from BMHB84,... 

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 shear cell is filled in a standard manner to produce a powder bed with a constant bulk density. A vertical (normal) force is applied to the powder bed and a horizontal force applied to the moveable ring. As the powder bed moves due to the horizontal shear stress, it will change volume, either expanding or contracting depending on the magnitude of the vertical force. A series of tests are performed to determine the vertical load under which the bed remains at constant volume when sheared, referred to as the critical state. Once the critical state has been determined, a series of identical specimens are prepared, and each is sheared under a different vertical load, with all loads being less than the critical state. 

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). 

A better method for determining the cohesive and frictional effects of particles is by using a shear cell (48,51,52). There are various cell configurations, the most popular proposed by Jenike (51). In the Jenike cell (Fig. 13), a powder is loaded and then compressed by twisting the lid of the cell. The number of twists required to load the powder to the point at which the resistance to shear (measured as stress applied to ring around the bed) is constant. This phase of the test is known as shear consolidation. The load is reduced and the resistance to shear is then recorded. A yield locus of this shear stress vs. the reduced load is obtained and used to calculate various flow-related parameters (47,48,51). Numerous parameters can be... 

---