RU2222808C2 - Device to examine structural-mechanical properties of food materials - Google Patents

Abstract

FIELD: food industry. SUBSTANCE: device has case put on unadjustable support and adjustable supports and is fitted with level indicator. Device includes horizontal plate with bushing attached to it, stepped post moves in bushing, stepped groove in post contacts key. Beam of rectangular section of dynamometer with two strain- gauge transducers cemented above and beneath and number of holes drilled in side of beam opposite to strain-gauge transducers is anchored on free end of upper step of post. Two holders of indenters are anchored from beneath on free end of beam, rods of separable indenters are fixed in holder and forced into examined product placed in thermostatted vessel. Lower step of post enters nut installed in bearing and set in rotation through gear wheel of gear-box by reversible electric motor. Pickup of linear displacement of indenter comes in the form of cup-shaped disc uniformly perforated over circumference. Illuminator is placed opposite perforations in disc, photodetector is positioned on the other side in opposition to illuminator. Displacement of stepped post is limited by limit switches which contact its protrusion. Device is controlled by microprocessor of recording unit. EFFECT: enhanced accuracy and widened measurement range of device. 6 dwg

Description

 The invention relates to devices for research in a wide range of changes in the structural and mechanical characteristics of elastic-plastic-viscous materials, mainly food products (hard and soft cheeses, cottage cheese, curd cheese, margarines, butter, yoghurts, etc.).

 A device for studying the structural and mechanical properties of products, containing a base, a rack, a plate for placing the test material, an indent holder with replaceable indenters, a movement mechanism, force and displacement sensors, recording equipment [1].

 Its disadvantages are as follows: the impossibility of using poorly structured systems with low strength for research; low accuracy of measurements.

 The closest technical solution that we have chosen as a prototype is a device for studying the structural and mechanical properties of low-strength food products, containing a body, a rack, a plate for placing a vessel with a test sample, a movement mechanism, an indent holder with replaceable indenters, a force meter, an indenter linear displacement sensor recording equipment [2].

 In this device, the force meter is made in the form of a dynamometric spiral with strain gauges glued on it, having along the entire length of the hole for the connecting screws and thereby forming a strain gauge ring of variable diameter. The disadvantage of the design of the load meter is the different bending stiffnesses of the left and right parts of the strain gauge ring due to the presence of an unequal number of holes weakening the cross section of the ring. This will lead to a violation of its strictly ellipsoidal shape during loading, which in turn will contribute to the deviation of its lower part from the vertical axis, the emergence of additional lateral forces on the introduced indenter and, ultimately, to a decrease in the measurement accuracy. It should also be noted that from the theory of direct bending (see N.M. Belyaev. Resistance of materials. M: Nauka, 1976. - S.217-222, formula 11.9) it is known that within the elasticity of the ring material, tensile strain and the compressions in the surface layer will be the greater, the further this layer will be located from its neutral (middle) layer. Due to the fact that for measuring small efforts the thickness of the strip from which the ring is made is small, then, based on the foregoing, the strain of the strain gauge glued to the ring will be insufficient to obtain high accuracy of the force measurement results. In addition, it should be noted a narrow range of operation of the load meter, as well as a certain complexity of changing the stiffness of the strain gauge ring and the need for repeated calibration.

 The purpose of the invention is improving accuracy and expanding the range of measurements.

 This goal is achieved by the fact that in the device for studying the structural and mechanical properties of products, containing a body, a rack, a plate for placing a vessel with a test sample, a movement mechanism, an indent holder with replaceable indenters, a force meter, an indenter linear displacement sensor and recording equipment, the force meter is made in in the form of a beam of rectangular cross-section cantileverly mounted on a rack with ohmic resistance sensors (hereinafter referred to as strain gauges) glued on it from above and below blasts adjacent to the rack, and the side opposite the load cells and strictly along the longitudinal axis of the ravine drilled a number of holes. At its free end, two indenters with fixing screws are installed from below and perpendicular to the longitudinal axis. At the same time, a stand with a longitudinal groove in the central part is installed in a sleeve fixed perpendicularly to the plate and equipped with a key in contact with the stand groove; a thread is made at the lower end of the stand, which enters the nut fixed in the thrust bearings from vertical movement and rotated from the electric drive. The indenter linear displacement sensor is made in the form of a disk mounted coaxially on the nut and having radial slotted slots located with equal circumferential pitch, opposite which a illuminator is installed on one side of the disk and a photocell (pulse counter) is located on the other side of the disk opposite the illuminator.

 The device allows you to perform the following studies at specified indenter speeds: determining the ultimate shear stress; study of adhesive properties; determination of strength and deformation characteristics during compression of the sample; determination of the strength properties of samples during cutting by a string and during bending; study of relaxation processes.

 In FIG. 1 shows a diagram of the device; figure 2 is a view of the load meter in arrow A; figure 3 is a section of the rack in section BB; figure 4 - indenter disk; figure 5 - string knife; figure 6 - a device for testing samples on bending.

 The device has a housing 1 mounted on one fixed 2 and two adjustable supports 3 and 4 and provided with a level 5. A sleeve 7 is fixed on the plate 6, in which the stand 8 is moved, the longitudinal groove 9 of which is in contact with the adjustable key 10, rigidly attached to the plate 6 screws 11, and the longitudinal groove and dowel are triangular in shape, which allows to exclude its angular displacements during translational movement of the rack (Fig. 3).

 At the upper end of the strut 8, on a sleeve 12 fixed by a screw 13, a beam of a force transducer 14 of rectangular cross section is cantilevered, onto which identical load cells 15 are glued on top and bottom in the area of its fastening, and in the area of their location in the beam 14 they are drilled laterally and strictly longitudinal its axis is somewhat the same diameter of the holes 16.

 The readings from the strain gauges 15 enter the recording unit (not shown conventionally in the diagram) equipped with a microprocessor, where they are summed by the absolute value, which additionally increases the sensitivity and accuracy of the force measurements, amplified and issued on a digital indicator. In addition, on the recording unit there are setters for the force F, the indenter Н displacements, the system stand-by time under load “Pause”, the rack 8 displacement switch “up” or “down”, as well as the toggle switch for setting the measuring range “10F” or “F”. In addition, connectors are provided for connecting a recorder and a computer.

 At the free end of the beam 14 below and perpendicular to its longitudinal axis are fixed two indenters 17 and 18, equipped with screws 19 for fixing the rods 20 of the replaceable indenters 21.

 In the lower part of the rack 8, a thread 22 is made, which enters the nut 23, which is fixed from vertical movement in the bearing support 24 and is driven into rotation through the gear 25 of the gearbox 26 by a reversible electric motor 27.

 The sensor of the indenter linear displacement meter is coaxially mounted on the nut 23 and is made in the form of a disk 28 with radial slotted slots 29 evenly spaced around the circumference, opposite which a constant light source 30 is installed below, and a photodetector (pulse counter) 31 from above, the signals from which are transmitted to the recording a block where, after processing, they are displayed on a digital indicator in absolute units of movement of the indenter 21 or transmitted to the recorder (or to a computer).

 The movement of the rack down is limited by the limit switch 32, and up - by the limit switch 33 in contact with its protrusion 34. The signals from the pulse counter 31, the limit switches 32 and 33 are also received in the recording unit.

 The test sample 35, placed in a thermostatic container 36, having an annular protrusion 37 at the bottom, is fixed on the plate 6 for this protrusion with two pairs of diametrically located latches 38, equipped with screws 39, coaxial with the indent holder 17 or 18.

At the same time, the distances X ₁ and X ₂ (Fig. 1), which determine the position of the indent holders 17 and 18 with respect to the load cells 15, are designed so that the measured forces are, for example, 10: 1. This will allow, when researching high-strength products using an indenter 17, to obtain strength values by multiplying the measurement result by a factor of 10, i.e. "comma transfer", performed in the recording unit by simply switching the toggle switch, which does not require reconfiguration (calibration) of the electronic force measuring circuit.

The presence in the beam 14 of the force meter holes 16 in the area of the sticker of the load cells 15 reduces the rigidity of the beam for bending, compared with the rest of its length, which will cause a local increase in the deformation of its surface layers, and consequently, the load cells 15. In addition, the implementation of the beam 14 is significantly larger (thickness), compared with the thickness of the strip of the dynamometer ring of the prototype force meter [2], it will cause the appearance of significantly higher tensile and compression stresses in the surface layers of the beam in the area of the load cell sticker 15, and therefore, on the basis of Hooke's law — within the elasticity of the material, the stresses σ cause strains directly proportional to them (Δl):
σ = E • ε = E • Δl / l,
where E is the modulus of elasticity of the material of the beam 14;
Δl is the absolute linear deformation of the surface layers of the beam (i.e., strain gauges 15);
l is the initial length of the plot on which Δl is measured (i.e., the base of strain gauges 15).

 All this will improve the accuracy of force measurement. And the use of two indenters 17 and 18 will allow changing the range of force measurements within the required limits.

 The device operates as follows. Before taking measurements, the plate 6 of the device using the adjusting supports 3, 4 is set at level 5 in a horizontal plane. Set the maximum speed in the gearbox 26 and turn the up switch. At the same time, the rack 8 accelerates upward, focuses 34 on the tip 33 and stops (a short beep is heard).

 The ultimate shear stress is measured in the following sequence. A vessel 36 with the test product 35 is fixed on the plate 6 with clamps 38 and screws 39 strictly along the axis of the holder 17, if the product has high strength (or along the axis of the holder 18, if the product is weak) and thermostatted. Lower the load meter 14 down, without bringing the cone 21 to contact with the surface of the sample 35, and fix it with a screw 13.

Depending on the strength of the structure of the test product, the initial value of the force F ₀ , the immersion depth H of the cone 21 in the sample 35 are set on the recording unit. The required cone penetration rate is set on the gearbox 26. Turn on the toggle switch "down". In this case, the value of F ₀ is zeroed, and the cone 21 begins to penetrate the sample. After reaching the force F ₀ , its value is again zeroed and at the same time, the reading of the loading force F and the immersion depth H begins; voltage is supplied to the illuminator 30, and the photodetector 31 begins to transmit pulses to the recording unit, where they are converted into a signal proportional to the movement N. Upon reaching a given H, the actuator 27 stops and the values of the reached load F and the given immersion depth N appear on the digital indicator.

The ultimate shear stress is obtained using the formula of P. A. Rebinder
τ = K _α • F / H ² ,
where K _α is the coefficient depending on the angle at the apex of the cone 21.

 After that, the maximum speed is switched on on gearbox 26, the toggle switch is moved up and the strut returns to its upper starting position. Measurement is over.

Measurement of the strength characteristics of the products in compression is as follows. Pasteous materials are placed in the vessel 36, and cubic or cylindrical samples are cut out of the solid materials and mounted on the plate 6 coaxially with the required indent holder 17 (18), in which the rod with the disk 40 is fixed (Fig. 4). The load meter 14 is lowered, not bringing the disk 40 to touch with the surface of the sample. Set F ₀ and turn the toggle switch down. The value of F ₀ is reset, the disk 40 moves down at a given speed. When the load on the sample F _{0 is} reached, its value is reset again and the force F and deformations of the sample H begin to be counted with the chart recorder writing F = f (Н). When the maximum load is reached and its decrease (when the sample is destroyed), the drive 27 is turned off and the achieved F and N are displayed on the indicator.

 The yield strength F is used to calculate the yield strength for plastic products or the tensile strength for brittle products.

To determine the elastic and plastic strains, the values F ₀ , F, and the loading rate are set. Set the appropriate diameter of the disk 40 in the indenter 17 (18), and on the contrary - the test sample 35. Turn on the toggle switch "down". In this case, the disk 40 begins to move downward at a given speed, the values of F ₀ and H are reset. When the disk 40 contacts the sample 35, the countdown of F and H starts. Upon reaching the set value of F, the displacement value H ₁ is fixed on the indicator, and the motor 27 is reversed and the disk 40 starts to move up. Upon reaching the value of F ₀ the indicator displays the value of the displacement H ₂ . According to the obtained H ₁ and H ₂ judge the elastic and plastic properties of the material.

To study adhesion, discs of a suitable diameter are used 40, the speed of the disc is set, the initial force F ₀ , the force F, to which the sample will be additionally loaded; and set the switch “Pause” the exposure time of the sample T at a given force F, turn the toggle switch “down” and lower the disk to contact with the sample.

Upon reaching F ₀ , it is reset to zero and starts counting F and H, which are displayed on the indicator and on the recorder. When F is reached, the disk is stopped and pause T is held, during which the control circuit 27 maintains the specified F. After pause T has ended, the disk 40 starts to move upward and at the moment of its separation from the sample 35 fixed to the plate 6, the corresponding value is fixed on the indicator F _max and H, and on the recorder is a diagram F = f (H).

To study the cutting process, a rod 20 is inserted into the indent holder, on which a rigid bracket 41 is welded with a tensioned string 42 (Fig. 5), and a product sample in the form of a parallelepiped is laid on the plate 6. Set F ₀ , the depth H and the cutting speed, the range of force. The toggle switch is moved to the down position. In this case, the string 42 is embedded in the sample. When F ₀ is reached, the result is reset to zero and the counting of the cutting force F and the cutting depth H starts, recording the chart on the recorder. Upon reaching the specified H, the values F and H will be displayed on the indicator. Knowing the length L of the string 42 included in the product, the specific cutting force F _y = F / L (n / m) is calculated.

 To study the samples for bending, a stand is used, consisting of a base 43 with a longitudinal groove 44, along which two supports 45 are mounted with the possibility of movement, fixed with screws 46, on which a flat sample 47 is laid. In this case, the sample is loaded by a plate 48 welded to the rod 20 (6), fixed in the indenter 17 (or 18).

The sample stand is mounted on the plate 6 so that the plate 48 divides the distance between the supports 45 exactly in half. After that, F _{0 is set} , the force range and the loading speed are selected. After pressing the toggle switch “down”, the plate will begin to bend sample 47, the load will reach the value F ₀ , zero and then the force F and deformation of the sample to bend N. will begin. If the sample is destroyed, the values F and H will be displayed on the indicator, the electric drive 27 will turn off.

The relaxation study is carried out at a given force F. First, the values of F ₀ and F are set, the loading speed is selected, and a sample 35 with a vessel 36 is mounted on the plate 6. A cone 21 (for studying the relaxation of tangential stresses) is fixed to the indent holder 17 (or 18) or the disk 40 (to study relaxation of normal stresses). Then turn on the toggle switch "down". The values of F ₀ and H are reset to zero. The cone (or disk) is in contact with the sample, and when F _{0 is} reached, the count of displacements H starts. The current values of F and H are recorded on the recorder.

 Upon reaching the set value of F, the load is suspended and the relaxation time begins to count down with the timer built into the recording unit until a constant value of F. is established. The indicator displays the value of the displacement H, at which the force is equal to the set F, as well as the relaxation time (in seconds).

 When a computer is connected and the appropriate software is available, the results of all types of tests can be processed and displayed or printed, and can also be used as commands for controlling technological equipment.

Sources of information
1. USSR Copyright Certificate 506804, cl. G 01 N 33/02. 1979.

 2. Copyright certificate of the USSR 807175, cl. G 01 N 33/02, 1981.

Claims (1)

A device for studying the structural and mechanical properties, comprising a housing, a rack, a plate for accommodating a vessel with a test sample, a movement mechanism, an indent holder with replaceable indenters, a force meter, an indenter linear displacement sensor, a recording unit, characterized in that the force meter is made as a console mounted on the rack of the beam of rectangular cross section with two load cells glued on it opposite from above and below in the area adjacent to the rack, and from the side opposite the load cells and three, along the longitudinal axis, several holes were drilled in the beam, and two indenters are fixed at the free end of the beam from below and perpendicular to the longitudinal axis, while a stand having a longitudinal groove is installed in a sleeve equipped with a key that contacts the groove of the rack and is fixed perpendicular to the plate, and at the lower end of the rack, a thread is made that enters the nut fixed in the thrust bearings from vertical movement and rotated by an electric drive, and the indenter linear displacement sensor It is made in the form of a disk mounted on a nut and coaxially with it, in which radial spaced slotted slots are made with equal pitch around the circumference, opposite which a constant light source is installed on one side of the disk, and a light receiver is located on the other side and opposite this source.

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