Torque measurements

Reduced torque versus time after steady shear of a 1 Pa s Newtonian oil at 10s with a 25 mm diameter, 0.1 rad cone, (a) Strong spring response of a torsion bar transducer with a spring constant, AT = 10 N ln/rad and 250 rad/s resonant frequency, simulated using eq. 8.2.1. (b) Force rebalance transducer with resonant frequency of 960 rad/s. From Mackay et al. (1992). 

Stiffer transducers have been made with strain gages (Macosko and Starita, 1971 Drislane et al., 1974). These devices can be very sensitive to temperature changes (Franck, 1985a). Very stiff and more thermally stable are piezoelectric transducers. Laun and Hirsch (1989) were able to measure stress overshoot and stress relaxation in less than 10 ms with a torque and normal force quartz load cell. However, it is difficult to get low torque levels with piezoelectric transducers. Furthermore, they are capacitance devices with a slowly decaying signal, making it difficult to accurately measure long relaxation processes. 

Force rebalance transducer. From Franck (1983a). 

Wall shear stress transducer mounted in a sliding plate rheometer. Shear between the sample and the wall causes die cantilever to deflect slighdy, which motion is detected by the capacitance probe. Adapted from Interlaken (1992). 

5-10 ms range. They are servo controlled similar to the rotary actuators. Hydraulic drivers can deliver much more force than a typical dc motor. The flat design of sliding plate rheometers can be an advantage for combining optical or scattering experiments with rheometry (Kannan and Komfield, 1992 Koppi et al., 1993 see also Chapter 9). 

Conveyor torque today is an essential part of decanter control. However, direct reading of conveyor torque is very difficult to achieve. Even direct reading of pinion torque is difficult, but could be done using strain gauges on the pinion shaft. However, the most usual method is to use a calibration of the braking device. The control device for the brake will give a read-out, on request, of the braking torque. 

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To determine the exact accelerating torque, measure the ordinates of torque as shown at different speeds and calculate the average torque as follows ... 

Figure 4-268. Sketch showing the theoretical position of strain gages for torque measurement.

A development of the moving die rheometer where the operation of the unit is fully computer controlled. The rate of oscillation, temperature and level of strain can all be run through a series of options. The torque measurements are also highly sophisticated. As a consequence, the unit can be set up to monitor processing parameters, then the cure behaviour and finally the finished dynamic properties of the cured material. It is manufactured by Alpha Technologies. 

Viscosity A 16 ml sample of the suspension removed after remixing was used for torque measurements using a Brookfield viscometer (Model LVTD) with a UL adapter. Initial torque readings were used for estimating viscosity in order to avoid problems arising from any settling of the kaolinite in the viscometer during the measurements. 

It is important to note that the small-scale agitator operations may be described in terms of impeller diameter and agitator speed, while manufacturing process equipment is more conveniently specified by horsepower and fluid velocity. For most standard turbine configurations, power number correlations are available to convert impeller diameter and agitator speed into a horsepower value for given fluid properties. Most laboratory bench equipment is designed to provide a torque measurement that can be readily converted to horsepower directly from the conditions of the pilot batches. 

With these instruments the torques measured at various rotational speeds of the cup or bob represent the original data. The shearing stress on the fluid at the wall of the cylindrical bob is given by... 

Reliable torque measurement is indispensable for protecting the extruder. Here, two speed ranges are particularly important. One of them is zero speed, when large torques can stress the screw shafts during start-up if the screw shafts are jammed by solidified product or product residue. The other is the operating situation when nominal extruder torque has been reached and the mechanical safety clutch should be kept from triggering. Case one is... 

When screw shafts are jammed, users often attempt to release the screw shaft by repeatedly switching the main motor on and off. Yet even with accurate torque measurement, torque limitation provides no guarantee against the screw shaft becoming twisted, since the torque limitation cannot absorb the motor s moment of inertia when the motor moves a bit. The same happens when a piece of metal falls into the extruder, abruptly jamming the screw shaft. 

The most precise torque measurement is achieved in the area of nominal speed. Here, accuracies are achieved to approx. 2.5% of nominal torque. The greater the field weakening, the more torque measurement accuracy decreases. 

Before a torsion shaft with strain gauges was employed to determine the torque on a rotating stirrer shaft, a swiveling motor had to be used and the torque measured by weighing. 

Direct torque measurement requires installation of strain gages on the impeller shaft or on the coupling between the motor and impeller shaft (Fig. 2). As the shaft is rotating, a device called slip ring is used to transmit the signal to the stationary data acquisition system. 

Planetary mixer instrumentation for direct torque measurement does not substantially differ from that of a high shear mixer. Engineering design should only take into account the planetary motion in addition to shaft rotation. ... 

For accuracy, in power number Np calculations, the power of the load on the impeller rather than the mixer motor should be used. Before attempting to use dimensional analysis, one has to measure/estimate power losses for empty bowl or dry stage mixing. Unlike power consumption of the impeller (based on torque measurements), the baseline for motor power consumption does not stay constant and changes significantly with load on the impeller, mixer condition, or motor efficiency. This may present inherent difficulties in using power meters instead of torque. Torque, of course, is directly proportional to power drawn by the impeller (the power number can be determined... 

In this study, the foregoing methodology of dimensional analysis was applied to a kneading process of drug-cyclodextrin complexation. Aoustin kneader with dual Z blades was instrumented for torque measurements and multiple runs were made at two scales (2.5 and 5L). 

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