Eccentric machines

Figure 1 shows tablets (300 mg) of native dextran obtained from Leuconostoc mesenteroides B-512F (Sigma) with a 10% water (w/w) punch in an eccentric machine (Manesty) at 15 kN with tablet side (flat-flace diameter) 10mm. Axial displacement of water was observed according to the change in color. 

FIGURE 1 Native dextran tablet press in eccentric machine shows variability of color due to different pressure between upper and lower punches. 

Binary mixtures were prepared with varying drug contents (60, 70, 80, 90, and 95%) keeping constant the drug and excipient particle size. Table 24 gives the composition of the studied batches as well as the tablet thicknesses. The mixtures were compressed on an eccentric machine (Bonals A-300) without any further excipient. Cylindrical tablets with a mean dosage of 500 mg and a diameter of 12 mm were prepared at the maximum compression force accepted by the formulations. 

The most often measured force is the upper punch force. For the eccentric machine it is the force which controls densification for rotary tableting machines upper and lower punch forces have ideally the same values. Schmidt et al. [67] measured force with a single punch of a rotary tableting machine. Ejection force is visible as a small lower punch signal which occurs shortly after the end of one compaction cycle. It is measured by lower punch instrumentation but needs more resolution. A review of force measurement is given by Bauer-Brandl [68]. 

Some basic parameters can be directly read from the curves. For the force values upper and lower punch forces and ejection forces should be mentioned, and for the time values contact time should be mentioned. Deduced parameters such as pressure and normalized contact time can be calculated and further statistical data are often used for characterization (Table 2). Due to the different shapes of force-time curves from eccentric tableting machines compared with those from rotary tableting machines, some parameters can only be calculated from eccentric machine data and some can only be calculated from rotary machine data. 

Only eccentric machine data allow us to calculate the R value (maximum upper punch force/maximum lower punch force), which is an indication of friction. They also allow us to calculate the time difference between the maximum upper punch force and the maximum lower punch force. Only dwell time and the minimum force during the dwell time can be calculated for rotary tableting machine data. The rise time of rotary machines is defined as the time during the compression phase, and peak offset time is defined as the time difference between maximum pressure and vertical alignment of the punches. Further the inflection points during the compression and decompression phases are mostly only calculated for rotary machine data. 

One possibility to analyze the tableting process is to describe the areas under the curve during compression and decompression and to draw conclusions on plastic and elastic parts of deformation. Emschermann and Muller [91] applied this method to data from eccentric machines (Figure 14). Similar area comparisons were performed by the research group of Schmidt [92-94] for rotary machines (Figure 15). They tried to gain information on elasticity by calculating differences between the area under the plot in the compression phase and the area under the plot in the decompression phase. A sophisticated technique to interpret area data under one... 

Krumme, M., Schwabe, L., and Fromming, K. H. (1998), Development of computerised procedures for the characterization of the tableting properties with eccentric machines. High precision displacement instrumentation for eccentric tablet machines, Acta Pharm. Hung., 68,322-331. 

Many of the machines apply the stress or strain based on a circular drive mechanism and so they are called eccentric machines. One such machine for tensile and compressive testing is shown in Figs. 1.10 and 1.11. This machine may compress and extend a test specimen repeatedly. 

The stress and strain in eccentric machines vary in a sinusoidal manner as depicted in Fig. 1.12. This shows the change in stress or strain versus time. There are several descriptive parameters noted on this figure that are useful in specifying or describing the test conditions. 

Figure 1.10 Diagram of an eccentric machine for tensile and compressive oscillation fatigue tests.

The overhead eccentric jaw crusher Nordberg, Telsmith Jnc., and Cedarapids) falls into the second type. These are single-toggle machines. The lower end of the jaw is pulled back against the toggle by a tension rod and spring. 

Balance machines rate their sensitivity in eccentricity, e, where e is the apparent center of gravity shift due to the unbalance. To obtain e, simply divide the inch-ounce of unbalance by the rotor weight in ounces, in this case. 

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

Rotary, positive displacement machines in which axial vanes slide radially in a rotor mounted eccentrically within a cylindrical casing. Available in lubricated and non-lubricated construction the discharge air is normally free from pulsation. 

Lubricating-film instability is the dominant failure mode for sleeve bearings. This instability is typically caused by eccentric, or off-center, rotation of the machine shaft resulting from imbalance, misalignment, or other machine or process-related problems. Figure 44.48 shows a Babbitt bearing that exhibits instability. 

This correlation was obtained as a result of extensive measurements with shake flasks of volume = 100-2000 ml and corresponding inner diameter dg= 6.1-16 cm, a filled volume ofVL= (0.04-0.2)V, and eccentricity of shaking machine of 25 and 50 mm. 

With modern balancing machines, it is feasible to balance components mounted on their arbors to U = 4 Nh (nominally equivalent to ISO grade G1), or even lower depending upon the weight of the assembly, and to verify the unbalance of the assembly with a residual unbalance check. However, the mass eccentricity, e, associated with unbalance less than U = 8W/n (nominally equivalent to ISO grade G2.5) is so small (e.g. U = 4Wh gives e = 0,000 070 in for an assembly intended to run at 3600 r/min) that it cannot be maintained if the assembly is dismantled and remade. Balance grades below G2.5 (8W/n) are, therefore, not repeatable for components. 

The thickness of the calendered product must be uniform in both the machine and cross-machine directions. Any variation in gap size due to roll dimensions, setting, thermal effects, and roll distortion due to high pressures developing in the gap, will result in product nonuniformity in the cross-machine direction. Eccentricity of the roll with respect to the roll shaft, as well as roll vibration and feed uniformity, must be tightly controlled to avoid nonuniformity in the machine direction. A uniform empty gap size will be distorted in operation because of hydrodynamic forces, developed in the nip, which deflect the rolls. The resulting product from such a condition will be thick in the middle and thin at the edges, as shown in Fig. 15.2. 

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