A modified pressure wheel for the instrumentation of rotary tabletting machines

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Abstract
IN any research into tabletting processes, it is necessary to know the force applied at the punch so that the tabletting pressure may be calculated. Much work has been done using single punch machines having strain gauges bonded to the shank of the punch (Higuchi, Nelson & Busse, 1954; Shotton & Ganderton, 1960). High-speed rotary machines require additional sophistication to enable useful results to be obtained, and there are two methods known. The first is that used by Shotton, Deer & Ganderton (1963). The resistance of strain gauges near the punch tips was used to control the frequency of an oscillator, and the signal was transmitted from the rotary part of the machine to a stationary aerial by radio emission. Thus the difficulty of providing a slip ring connection to the strain gauges was circumvented. However, because the radio transmitter took up space normally occupied by punches, it was difficult to obtain readings from more than one pair of punches, and impossible to obtain readings from all. In the second method (Knoechel, Sperry & others, 1967), strain gauges were placed on the compression screws interposed between the arm holding the movable axis of the pressure wheel and the spring used to adjust the compression force applied. Deflection, measured by the strain gauges, was proportional to the force applied to the punch. Since the gauges were stationary, their output could easily be displayed on a cathode ray oscilloscope, and the pressure to each punch recorded as its head passed under the pressure wheel.

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