A Dynamometer Instrument for Measurement of Textural Characteristics of Foods

Peter W. Voisey Engineering Research Service

Research Branch Agrículture Canada

Ottawa, Ontario and

B. S. Kamel, G. Evans and J. l\f. de'lVIan Department of Food Science

University of Guelph Guelph, Ontarío

Abstraet An electronic dynamometer is described for measuring the

power input to food processing systems. It is a gearbox in which one of the U10ving components is held stationary by a strain gage force transducer. A texture measuring instrument was constructed in which a food can be forced through one or more perforated plates and given additional mechanical treatment by an impellor rotatin,g behind the plates. The forces on the two drive shafts are measured by two of the dynamometers. Texture measurement on dates is used as an example of the functioning of the apparatus.

Résumé Un dynamometre électronique est décrit pour mesurer la

consommation de courant eles systemes ele procéelés alimentaires. Il est constitué d'une boite a mouvements elans laquelle une des parties mobiles est bloquée par un transelucteur relié a un exten­sometre. Un texturometre construit par nous est conc;u pour forcer un aliment a travers une ou plusieurs plaques perforées et pour le soumettre a un traitement mécanique adelitionnel a l'aide d'une roue motríce tournant a l'arriere des plaques. Les forces exercées sur les eleux axes a traction sont mesurées par deux des dynamo­metres. A titre el'exemple du fonctionnement de l'appareiJ, la texture a été mesurée sur eles elattes.

Introduetion For the measurement of textural properties of

foods a variety of instruments have been developed (Szczesniak, 1973) based on application of tension, compression, shear, extrusion, flow or a combination of these. Since foods may be mixed, worked and pumped in some food processing operations and are chewed when eaten, it is often of interest to measure the reac­tion 'Of foods to intensive mechanical action involving such a combination. This is achieved in a number of rotary instruments by measuring the torque exerted by the bowl containing the product, e.g. in dough testing by the Brabender Farinograph (Brabender, 1965) 01'

by electronic recording mixers (Voisey et al.) 1970, 1971 ; Voisey, 1971) and for other products with record­ing food grinders (Voisey and del\fan, 1970a) or with food mixers (Voisey and deMan, 1970b). A food grinder was used by Vasic and del\fan (1968) to determine the effect of extrusion through perforated plates on the consistency of fats. After the development of the record­ing food grinder (Voisey and deMan, 1970a) it became of interest to build an instrument in which a food could be forced by an auger through perforated plates with simultaneous measurement of the power required on the shaft as wel1 as on the shaft of an additional pro­peller which can be made to rotate in the extruded producto This required the use of dynamometers to measure the power on two rotating shafts. The design

1 Gontribution No. 306 frem Englneering Researeh Serviee.

Can. Inst. Food Se!. Teehnol. J. Vol. 7, No. 4, 1974

and construction of this instrument and an example of its use for textural measurements on dates is given here. Experimental Deseription of Dynamometer

The dynamometer is a gearbox made up of gears and sprockets (Figure 1). Rotating input shaft A turns gear B. This rotates a second gear (B) which is on a common shaft with sprocket J which thus drives the output shaft F via a chain (G) and a second sprocket. The input shaft A and output shaft F rotate about a common axis but are independent of each other being mounted in separate bearings in the gearbox housing (not shown in illustration). The 'lower shaft carrying the second gear (B) and sprocket (J) is mounted in bearings in a block (e). This block is pivotted on bear­ings about the axis of the input and output shafts. Thus, the lower shaft can rotate about its own a.xis and the upper shaft axis. This can be visualized by holding the output shaft (F) stationary and turning the input shaft (A). It can then be observed that the block (e) would rotate with the input shaft.

Rotation of the block (e) is prevented by a beam (H) clamped at the top (D) to the gearbox casing amI to the bottom of the block. Thus, when the input shaft (A) is turned a force is exerted at the lower end of the beam which is proportional to the torque resisting rotation of the output shaft (F). This force is detected by 4 strain gages (E) bonded to the beam close to the upper clamp to form a force transducer (Perry and Lissner, 1955). The lower clamp is arranged so that the beam is not gripped but has some freedom of movement (0.05 mm). Thus, any misalignment between the upper and lower clamp s does not twist the beam about its axis 01' apply axial tension which would introduce errors. The beam is subjected only to force causing it to bend transversely.

The force transducer is connected to an eJectronic amplifiel' and recording apparatus so that continuous records of torque (on a strip-chart), maximum torque, 01' energy used (in analog 01' digital form) can be recorded (Vüisey, 1971). The transducer is calibrated by weights. A lever at the top of the block (e) has a cord (K) aHached at a radius of 10 cm from the upper shaft axis. The cord passes over a pul1ey (L) to hang vertical1y. Se]ected wehrhts suspended on the cord apply a precise torque about the upper axis. thus app]y­ing force to the transducer. The calibration can be performed with the gears and sprockets stationary or moving.

250

L~

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Fig. 1. Schematic diagram of dynamometer. A. inp~t shaft; B. gears;c. block; D. transducer clamp; E. stram gages; F. output shaft; G. chain; H. steel beam (transducer); J. sprocket; K. cord; L. pulley.

The components are assembled in a cast aluminum housing (Figure 2A) which is oil tight. The transducer is mounted on the cover (Figure 2B) and can be removed easi1y for servicing. The calibration at.tach­ments are permanently moullted on the case (FIgure 2C). The housing is filled with a light oil (SAE 10) to a depth sufficient to eontact the lower ~ear and sprocket so that aH moving parts are contmuously lubricatrd. AH the bearings are of the sealed type and do not require lubrication.

'rhe gears and sprockets can be selected so that there is a given ratio of speeds between the gearbox input and output shafts. In the unit described here the O'ears were identical (50 teeth), the upper sprocket had 17 teeth and the lower 30 teeth. Thus, the ratio between input and output shaft speeds was 30 to 17. The .load driven bv the dvnamometer can be connected to eIther shaft of 'tlle gea'~'box which can also be driven in either direction. Thus, in the configuration ehosen the gear­box can be used to either increase 01' deerease speed depelldin<r 011 which shaft is seleeted to drive the load. 'rile only l">change required if rotation is rever sed is to move the calibration pulley from one side of the gear­box to the other. Provision is made to do this.

The transducer is made of tool steel 1.91 x 0.64 x ]3.3 cm long. The sensitivity of the transducer is govel'ned by its width and thiekness, sinee the length is fixed by the gearbox geometry. Thus, the dynamo­meter sensitivity can be easily changed up 01' down by using a different width and/or thickness of metal to malee the transducer.

Fig.2. Details of dynamometer. A. view inside with cover re­moved, n. dynamometer and caver with transducer install~d; C. the dynamometer with calibration weight in position.

Fig. 3. A. test set-up used (C lever and D force transducer); B. calibrating the test force transducer (E).

Dynamometer Performance The dynamometer performance was tested by

simulating a load on the output shaft by a clutch used as a Prony brake following a technique similar to that of Kilborn and Tipples (1973). An eleetrically operated clutch (Model EC375 Warnel' Electrie Dralee and Cluteh Co., Beloit, Wisconsin) was mounted so that the moving component was attached to ~he gearl!ox output shaft (Figure 3A). A lever (C) ~av111g a radms of exactly 10 cm was attached to the flXed eomponent and connected to the base by a second force transdueer (D, Figure 3A). Thus, as current ~as s~pplied to the clutch bv its control (Model MCS10,,), frIetIon between the fixed and moving components applied tol'que to the dynamometer output shaft. The torque applied by the cÍutch was reacted by the force transducer whi.ch was connected to a recorder. This transducer was cahbrated by a spring scale (E, Figure 3B). The torque applied could be measured to an accuracy of ±0.25%. To make the clutch operate smoothly its plates were lubricated with powderecl graphite.

Two of the O'earbox clynamometers were tested to establish the rel~tionship between torques applied at the dvnamometer calibl'ation fixture, i.e. the indicated torqt;e ancl the torque load applied at the o~tpu~ shaft. This was performed with the gearbox runmng 111 both directions and with the load applied to either shaft of the gearbox. The teclmique used was to select a torque to represcllt full-scale load for the dynamometer (e.g., 10000 cm g) and hang an appropriate weight (1,000 g) on' the calibratioll fixture. The recording system was then adjusted so that the recordel' pen moved from zero to full-scale when this torque was applied to the calibration fixtlll'e. The force transducer holding the clutch was calibrated in a similar manner. '1'l1e applied torque was then raised in incrcments by increasing the current supplied to tlle clutch. At each increment the torque appliecl to the dynamometer and the ~orque illclicated by the dynamometer were noted. TlllS was repeated for several indicated dynamometer torque calibrations up to 80,000 cm g. Each test \Vas conducted with the dynamometer running continuously (63 and 153 rpm).

For eonvenience the following convelltion was acloptecl: a) the direction of rotation was noted :vhen facinO' the end of the output shaft; b) shaft F (FIgure 1) w:s considered the output and shaft A the input.

The results indicated that to determine the torque applied it was necessary to divide t~e in~icated tor~ue by a factor which depended on the dIrectIon of rotaüon and which shaft (A 01' F) carried the applied torque

J. Inst. Can. Sci. Technol. Aliment. Vol. 7, No 4, 1974