Goal of the work: learn to experimentally determine the elastic modulus (Young's modulus) of rubber.

Means of education:

· equipment: tripod, set of weights, rubber cord, ruler, dynamometer.

· guidelines for performing laboratory work, calculator.

Progress of laboratory work

Permission to perform laboratory work

Run the test:

1. Deformation - change...

A. shapes and positions in space; B. body shape and size;

IN. Volume and position in space; G. there is no right answer.

2. Deformation in which the layers of the body shift relative to each other is called deformation….

A. shift; B. sprains; IN. bending; G. there is no right answer.

3. Deformation that completely disappears after the cessation of external forces is called....

A. elastic; B. inelastic; IN. plastic; G. there is no right answer.

4. The dependence of physical properties on the direction inside the crystal is called...

A. anisotropy; B. entropy; IN. isotropy; G. there is no right answer.

1. The figure shows the tension diagram of the material. Specify the yield area.

A. 0-A; B. A-B; G. B-C; D. C-D.

Enter your answers in the table:

Theoretical part

Let us derive a formula for calculating Young's modulus: Hooke's law σ=E·|ε|, where E is Young's modulus. From here (1). Knowing that (2) and (3) and substituting formulas (2) and (3) into formula (1) we obtain: ( 4), where: E – Young’s modulus, Pa; F – load weight, N;

x 0 – length between marks on an undeformed cord, m;

S – cross-sectional area of ​​the cord in the stretched state, m 2 ;

Δх – absolute elongation of the cord, m.

Calculations and measurements

1. Attach the rubber cord to the tripod and mark two marks A and B on the cord. Without stretching the cord, measure the distance between the marks.

2. Hang the load from the lower end of the rubber cord, having previously determined its weight. Measure the distance between the marks on the cord and the cross-sectional dimensions of the cord when stretched.

3. Perform the same measurements by hanging two and three weights.

4. Calculate Young's modulus using formula (4) for each experiment.

5. Enter the results of measurements and calculations in the reporting table 1

E 1 = =___________Pa,

E 2 = =___________Pa,

E 3 = =___________Pa,

E av = =___________Pa.

5. Analyze the obtained result E avg, comparing it with the table value of the Young's modulus of rubber E table. =7MPa. Summarize the results of your work. Draw a conclusion on the work done.

Conclusion: _______________________________________________________________________

____________________________________________________________________________________________________________________________________________________________

Control questions

1. What is deformation? What types of deformation do you know?

2. Does the modulus of elasticity depend on the cross-section of the rubber cord and its length?

3. What quantity is measured in this work with the smallest error?

4. How does a change in the temperature of a rubber cord affect the elastic modulus?

Answers:

Municipal educational institution

"Yagodninskaya secondary school"

Methodological development of laboratory work

Physics teacher:

Open lesson in 10th grade on the topic: laboratory work "Measuring the elastic modulus of rubber"

Lesson objectives: ensuring a more complete assimilation of the material, forming an idea scientific knowledge, development of logical thinking, experimental skills, research skills; skills in determining errors when measuring physical quantities, the ability to draw correct conclusions based on the results of work.

Equipment: installation for measuring Young's modulus of rubber, dynamometer, weights.

Lesson plan:

I. Org. moment.

II. Repetition of material knowledge of which is necessary to complete laboratory work.

III. Performing laboratory work.

1. The order of the work (as described in the textbook).
2. Determination of errors.
3. Carrying out the practical part and calculations.
4. Conclusion.

IV. Lesson summary.

V. Homework.

DURING THE CLASSES

Teacher: In the last lesson you learned about the deformations of bodies and their characteristics. Let's remember what deformation is?

Students: Deformation is a change in the shape and size of bodies under the influence of external forces.

Teacher: The bodies around us and we are subject to various deformations. What types of deformations do you know?

Student: Deformations: tension, compression, torsion, bending, shear, shear.

Teacher: What else?

Elastic and plastic deformations.

Teacher: Describe them.

Student: Elastic deformations disappear after the cessation of external forces, while plastic deformations remain.

Teacher: Name elastic materials.

Student: Steel, rubber, bones, tendons, the entire human body.

Teacher: Plastic.

Student: Lead, aluminum, wax, plasticine, putty, chewing gum.

Teacher: What happens in a deformed body?

Student: Elastic force and mechanical stress appear in a deformed body.

Teacher: What physical quantities can characterize deformations, for example, tensile deformation?

Student:

1. Absolute elongation

2. Mechanical stress?

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Teacher: What does it show?

Student: How many times is the absolute elongation less than the original length of the sample?

Teacher: What's happened E?

Student: E– coefficient of proportionality or modulus of elasticity of the substance (Young’s modulus).

Teacher: What do you know about Young's modulus?

Student: Young's modulus is the same for samples of any shape and size made from a given material.

Teacher: What characterizes Young's modulus?

Student: The elastic modulus characterizes the mechanical properties of the material and does not depend on the design of the parts made from it.

Teacher: What mechanical properties are inherent in substances?

Student: They can be brittle, plastic, elastic, durable.

Teacher: What characteristics of a substance must be taken into account when using it in practice?

Student: Young's modulus, mechanical stress and absolute elongation.

Teacher: What about when creating new substances?

Student: Young's modulus.

Teacher: Today you will do a lab to determine the Young's modulus of rubber. What is your goal?

Using rubber as an example, learn to determine the modulus of elasticity of any substance.

Knowing the elastic modulus of a substance, we can talk about its mechanical properties and practical applications. Rubber is widely used in various aspects of our lives. Where is rubber used?

Student: In everyday life: rubber boots, gloves, rugs, elastic bands, plugs, hoses, heating pads, etc.

Student: In medicine: tourniquets, elastic bandages, tubes, gloves, some parts of devices.

Student: In transport and industry: tires and wheel tires, gear belts, electrical tape, inflatable boats, ladders, O-rings and much more.

Student: In sports: balls, fins, wetsuits, expanders, etc.

Teacher: There is a lot that can be said about the use of rubber. In each specific case, rubber must have certain mechanical properties.

Let's move on to doing the work.

You have already noticed that each row received its own task. The first row works with an elastic band. The second row contains fragments of a hemostatic tourniquet. The third row is with fragments of the expander. Thus, the class is divided into three groups. All of you will determine the elastic modulus of rubber, but each group is invited to conduct their own small research.

1st group. Having determined the elastic modulus of rubber, you will receive results, after discussing which, draw a conclusion about the properties of the rubber used to make underwear elastic.

2nd group. Working with different fragments of the same hemostatic tourniquet and determining the elastic modulus, draw a conclusion about the dependence of Young’s modulus on the shape and size of the samples.

3rd group. Study the device of the expander. After completing laboratory work, compare the absolute elongation of one rubber string, several strings and the entire expander harness. Draw a conclusion from this and, perhaps, come up with some of your own proposals for the manufacture of expanders.

When measuring physical quantities, errors are inevitable.

What is error?

Student: Inaccuracy in the measurement of a physical quantity.

Teacher: What will guide you when measuring the error?

Student: Data from Table 1 p.205 of the textbook (the work is performed according to the description given in the textbook)

After completing the work, a representative of each group makes a report on its results.

Representative of the first group:

When performing laboratory work, we obtained the values ​​of the elastic modulus of the elastic band:

E1 = 2.24 105 Pa
E2 = 5 107 Pa
E3 = 7.5 105 Pa

The modulus of elasticity of a linen elastic band depends on the mechanical properties of the rubber and the threads braiding it, as well as on the method of weaving the threads.

Conclusion: underwear elastic is very widely used in underwear, children's clothing, sportswear and outerwear. Therefore, for its production, various types of rubber, threads and various methods of weaving are used.

Representative of the second group:

Our results:

E1 = 7.5 106 Pa
E1 = 7.5 106 Pa
E1 = 7.5 106 Pa

Young's modulus is the same for all bodies of any shape and size made from a given material

Representative of the third group:

Our results:

E1 = 7.9 107 Pa
E2 = 7.53 107 Pa
E3 = 7.81 107 Pa

To make expanders, you can use different types of rubber. The expander harness is assembled from individual strings. We've looked into this. The more strings, the larger the cross-sectional area of ​​the bundle, the lower its absolute elongation. Knowing the dependence of the properties of the tourniquet on its size and material, it is possible to make expanders for various physical education groups.

Lesson summary.

Teacher: To create and use various materials, you need to know their mechanical properties. The mechanical properties of a material are characterized by its elastic modulus. Today you have practically defined it for rubber and drawn your conclusions. What are they?

Student: I learned to determine the elastic modulus of a substance, evaluate errors in my work, made scientific assumptions about the mechanical properties of materials (in particular, rubber) and the practical application of this knowledge.

Students hand in control sheets.

For home: repeat § 20-22.

Laboratory work

"Measurement of the elastic modulus of rubber"

Discipline Physics

Teacher A.B. Vinogradov

Nizhny Novgorod

2014

Goal of the work: experimentally determine the elastic modulus of rubber.

Equipment: a rubber band with a loop at one end and a knot at the other, a dynamometer (or two sets of laboratory weights), a tripod, a ruler with millimeter graduations, tangent calipers.

Brief theoretical information.

Young's modulus characterizes the elastic properties of a material. This is a constant value that depends only on the material and its physical state. Since Young's modulus is included in Hooke's law, which is valid only for elastic deformations, then Young's modulus characterizes the properties of a substance only under elastic deformations.

Young's modulus can be determined from Hooke's law:

F/S= E Dl/l 0 , hence E= F l 0 /S Dl, Where Dl= l-l 0 , S=a·b, F=mg.

Exercise:

2.Prepare answers to test questions.

3.Prepare a report form.

Work order:

1.Measure the width and thickness of the tape using a caliper and calculate its cross-sectional area S0.

3. Fasten the end of the tape with a knot in the leg of the tripod and insert the hook of the dynamometer (or weight) into the loop so as to stretch the tape by 1-2 cm.

4.Remove the load and measure its initial length (from the attachment point to the loop).

5.Stretch the tape 2-3 cm and measure the deforming force.

6. Repeat the experiment with extensions of 4 and 6 cm.

7. Based on the results of each experiment, calculate Young’s modulus.

8.Find the average value of Young's modulus in three dimensions.

9.Evaluate the accuracy of the measurements taken. d = D E/E= D F/F +2 Dl / l +2Da / a

10. Explain for what purpose it was necessary to carry out the operation described in paragraph 3.

11. Enter the results of measurements and calculations into the table:

experience

Initial tape length l 0 , m

Tape width

A, m

Tape thickness

b, m

Transverse area

th tape section

S, m2

Defor

worldly force

F, N

Elongation

Δ l, m

Young's modulus

E, Pa

Average value of Young's modulus

E avg, pa

Error

d, %

Contents of the report.

The report must contain:

1. Title of the work.

2. Purpose of the work.

3.List of necessary equipment.

4. Formulas for the required quantities and their errors.

5. Table with the results of measurements and calculations.

6.Answers to security questions.

7. Conclusions about the work done.

Control questions.

1.What is Young's modulus?

2.What is called the elastic limit?

3. A load weighing 200 g is suspended from a steel thread with a diameter of 2 mm and a length of 1 m. How much will the thread elongate if Young's modulus for steel is 2.2 * 1011 Pa? What is the relative elongation of the thread?

4.What is mechanical tension and how is it measured?

Bibliography.

1. Zhdanov L. S., Zhdanov G. L. Physics (textbook for secondary special educational institutions- M. Higher School 1995) § 13.1-8 (2).

2. Dmitrieva V.F. Physics (Textbook for secondary specialized educational institutions - M. Higher School 2001) § 42-49 (2).

Purpose of work: learn to find the elastic modulus of rubber. The setup for measuring the Young's modulus of rubber is shown in Figure a.

Young's modulus is calculated using the formula obtained from the law

Guka: where E is Young's modulus; P - elastic force,

Arising in a stretched cord and equal to the weight of the weights attached to the cord; § - cross-sectional area of ​​the deformed cord; 10 - the distance between marks A and B on the stretched cord (Fig. b); I - the distance between the same marks on the stretched cord (Fig. c). If the cross section has the shape of a circle, then the cross-sectional area is expressed through the diameter

Cord:

The final formula for determining Young's modulus is

View:

Execution example:

The weight of the loads is determined by a dynamometer, the diameter of the cord by a caliper, and the distance between marks A and B by a ruler. To fill out the table, we will perform the following calculations: 1) AI1- absolute instrumental error AI1= 0.001 A0/ - absolute reading error A01= 0,0005 A1- maximum absolute error A1 = A and I+ A 01 = 0,0015 2) A&O= 0,00005 A0O= 0,00005 JSC= A and B + A 0 B = 0,0001 3) AAndR= 0,05 A0P= 0.05 AR = A and P + A 0 P = 0,05 + 0,05 = 0,1

Conclusion:The obtained result of the elastic modulus of rubber coincides with the table.

In the cereal industry, non-metallic materials (rubber, abrasive, etc.) are widely used for the manufacture of working parts of hulling and grinding machines.

Rubber. Rubber differs from other technical materials in a unique set of properties, the most important of which is high elasticity. This property inherent in rubber, the main component of rubber, makes it an indispensable structural material in modern technology.

Unlike metals, plastics, abrasives, wood, leather and other materials, rubber is capable of very large (20..30 times more than steel), almost completely reversible deformations under the influence of relatively small loads.

The elastic properties of rubber are maintained over a wide range of temperatures and deformation frequencies, and deformation is established in relatively short periods of time.

The elastic modulus of rubber at room temperature is in the range of (10... 100) 105 Pa (the elastic modulus of steel is 2000000 10 5 Pa).

An important feature of rubber is also the relaxation nature of deformation (a decrease in stress over time to an equilibrium value). Rubber lends itself well to mechanical cutting and grinds well.

The elasticity, strength and other properties of rubber depend on temperature. The elastic modulus and shear modulus of most types of rubber remain approximately constant when the temperature rises to 150 C; with a further increase in temperature, they decrease and the rubber softens. At approximately 230°C, rubber (almost all types) becomes sticky, and at 240°C it completely loses its elastic properties.

Rubber is distinguished by extremely low volumetric compressibility and a high Poisson's ratio of 0.4...0.5 (for steel 0.25). The exceptional ability for highly elastic deformation and high fatigue strength of certain types of rubber are combined with a number of other valuable technical properties: significant wear resistance, high coefficient of friction (from 0.5 and above), tensile and impact strength, good resistance to cuts and their growth, gas , air and water resistance, petrol and oil resistance, low density (from 0.95 to 1.6), high chemical resistance, dielectric properties, etc. Thanks to the unique set of technical properties, rubber has become one of the most important structural materials for various types of transport, Agriculture, mechanical engineering, as well as for the production of sanitary and hygiene products, consumer goods.

The efficient operation of machinery and equipment in many industries largely depends on the durability and reliability of rubber products.

Rubber hardness. Rubber hardness refers to its ability to resist being pressed into it by an indenter (a steel needle with a blunt end or a steel ball). Knowing the hardness of rubber is necessary for a comparative assessment of the hardness of rubber parts. Of great practical importance is the fact that the hardness of rubber can be used to approximately determine many of its other properties, in particular the elastic modulus of rubber.

The most common method is to determine the hardness of rubber with a hardness tester: TIR-1 according to GOST 263 - 75. The deviation of the hardness value from its average value is usually no more than ±4% for soft rubber, and ±15% for the hardest grades.

The measurement of rubber hardness occurs in the region of its elastic deformations, as a result of which the hardness of rubber is a characteristic of its elastic, not plastic properties. This makes the hardness of rubber different from the hardness of metals, which is characterized by plastic deformations. Therefore, the hardness value of a rubber can be used to determine its elastic properties, such as elastic modulus or shear modulus.

IN technical conditions elastic and shear moduli are usually not given, but rubber hardness is almost always given. Therefore, knowledge of the dependence of moduli on hardness is very important, especially for preliminary calculations of the elasticity characteristics of rubber products.

It should also be taken into account that rubber hardness can be measured on almost any rubber product, but special samples are needed to determine the elasticity and shear moduli.

Numerous studies have established that the elastic modulus E and the shear modulus G are related to each other by the ratio E = 3 G and almost do not depend on the brand or composition of rubber, in particular on the type of rubber on the basis of which the rubber is made, but depend only on the hardness of the rubber. For rubber of different compositions of equal hardness, the elastic moduli and shear moduli differ by no more than 10%.

The value of permissible compressive and shear stresses for rubber products. The permissible compressive stresses are several times higher than the permissible tensile stresses, which is explained by the sensitivity of stretched rubber to local defects and surface damage.

The permissible stresses in parallel shear and torsion are lower than the permissible tensile stresses, especially under long-term dynamic loading. The possibility of a short-term impact load in most cases does not lead to a reduction in permissible stresses if the rubber operates at normal temperatures. With long-term dynamic loads, the permissible stresses are significantly reduced.

In the domestic literature, for rubber parts, the permissible compressive stress is recommended to be 11 10 5 Pa. It refers to rubber general purpose medium hard. However, in many cases, rubber products work well for a long time at significantly higher stresses. This indicates that for some brands of rubber the permissible stress values ​​are underestimated.

When assessing the strength of rubber-metal products, the permissible stresses must be selected taking into account not only the tensile strength of the rubber, but also the strength of the fastening of the rubber to the metal.

The tear strength of attaching rubber to metal using an ebonite layer is usually determined by the strength of the rubber and is in the range (40...60) * 10 3 N/m.

Heat resistance of rubber. This indicator characterizes the performance of rubber at elevated temperatures. Heat resistance is determined by the change with temperature of those indicators of material properties that are most important for the specific conditions of use of the tested rubber. Heat resistance is characterized by a heat resistance coefficient, which is the ratio of rubber property indicators selected as a comparison criterion at elevated and room (23 ± 2 C) temperatures. Typical property indicators used to evaluate the heat resistance of rubber often use the results of measurements of tensile strength, elongation at break, or any other characteristics that are important for the specific conditions of use of the material.

Wear resistance of rubber. Rubbers and products made from them are often used in conditions of prolonged friction that occurs under significant loads.

Therefore, it is important to know how a product wears out due to friction. Since it is difficult to reproduce all possible friction conditions, the assessment of rubber wear resistance is based on determining its behavior under two extreme conditions - friction on a smooth surface or friction on a very rough surface, for which abrasive paper is used.

When testing rubber samples for abrasion under rolling conditions with slipping, the operation of various products, but primarily tires, is simulated. Therefore, this test method is used to evaluate the properties of rubber used for the manufacture of wheel treads.

A quantitative characteristic of abrasion is the ratio of the loss of material due to its intense abrasion to the work of friction forces expended in this case. Abrasion is expressed in m3/MJ. Sometimes the inverse value is also measured - abrasion resistance. It represents the amount of work of friction forces that must be done in order for a sample to be abraded in a volume of 1 cm 3; abrasion resistance is expressed in MJ/m 3.

Fatigue endurance of rubber. Under operating conditions, rubber products very often experience repeated periodic loads. In this case, the destruction of the sample (product) does not occur immediately, but after a certain, sometimes very large number of loading cycles. This is due to the gradual accumulation of microscopic damage in the sample, which ultimately, adding to each other, leads to a catastrophic phenomenon - destruction. An indicator of fatigue endurance is the number of cycles of repetitive loading that a rubber sample can withstand before failure. The fatigue endurance test of rubber is carried out under strictly fixed conditions with repeated stretching of the samples, carried out at a frequency of 250 or 500 cycles per minute with relatively small deformations.

Frost resistance of rubber. This indicator characterizes the ability of the material to work at low temperatures. As the temperature decreases, any rubber gradually “hardens”, becomes stiffer and loses its main quality used for making products from it - easy deformability under relatively small loads and the ability to undergo large reversible deformations.

Behavior of rubber low temperatures characterized by frost resistance coefficient and brittleness temperature.

The tensile frost resistance coefficient is understood as the ratio of elongation at a certain low temperature to elongation at room temperature under the same load, and the load is selected in such a way that the relative elongation of the sample at room temperature is 100%. Rubber is considered frost-resistant at the selected test temperature if the frost resistance coefficient does not decrease below 0.1, i.e. the rubber can still stretch by 10% without breaking.

The brittleness temperature is determined as follows. The sample is fixed in a cantilever and a load is sharply (impacted) applied. The brittleness temperature is understood as the maximum temperature (up to 0°C) at which a sample is destroyed under the influence of an impact or a crack appears in it.

Rubberized rollers. Rubber-coated rollers used in A1-ZRD type machines are the main working parts. A rubberized roller consists of metal reinforcement and a rubber coating, which are connected to each other with glue during the vulcanization process. The roll reinforcement is a steel pipe (sleeve) 400 mm long with an outer diameter of 159 mm and an inner diameter of 150 mm.

At the ends of the reinforcement, grooves measuring 12 x 12 mm are milled, which are used for installing a rubber roller on the axle shafts of the device for fastening the rolls.

A layer of rubber coating 20 mm thick is applied to the surface of the reinforcement using injection molding followed by vulcanization. The rubber mixture intended for the manufacture of rolls is made according to recipe No. 2-605.

Rubber-fabric plates. Rubber-fabric plates RTD-2 are used for the manufacture of decks of rolling deck machines 2DSHS-ZA. Decks are made directly at the millet mill by assembling and fastening rubber-fabric plates in a deck holder. The plates are made by vulcanization from rubber compound type 4E-1014-1 and rubberized fabric. The plate contains eight layers of rubber and seven layers of rubberized fabric.

Rubber-fabric plates RTD-2 are produced according to TU 38 of the Ukrainian SSR 20574-76.

For the manufacture of brake strips in RC-125 grinding units, rubber plates are used that are approved for contact with food products (GOST 17133 - 83). Plates are produced of low (M), medium (C) and high (P) hardness with a thickness from 1 to 25 mm and square side dimensions from 250 to 750 mm.

According to physical and mechanical indicators, this rubber is characterized by the following data: conditional tensile strength from 3.9 to 8.8 MPa (based on natural rubbers); relative elongation after rupture from 200 to 350%; hardness according to TIR 35...55; 50...70 and 65...90 arb. units (three ranges).

Abrasive materials. Any mineral of natural or artificial origin, the grains of which have sufficient hardness and cutting (scratching) ability, is called an abrasive material.

Abrasive materials used for the manufacture of abrasive wheels are divided into natural and artificial.

Natural abrasive materials of industrial importance are minerals: diamond, corundum, emery, garnet, flint, quartz, etc. The most common are diamond, corundum and emery.

Corundum is a mineral consisting of aluminum oxide (70...95%) and impurities of iron oxide, mica, quartz, etc. Depending on the content of impurities, corundum has different properties and color.

Emery is a fine-grained rock consisting mainly of corundum, magnetite, hematite, quartz, gypsum and other minerals (corundum content reaches 30%). Compared to ordinary corundum, emery is more fragile and has less hardness. The color of emery is black, reddish-black, gray-black.

Artificial abrasive materials include diamond, CBN, Slavutich, boron carbide, silicon carbide, electrocorundum, etc.

Artificial abrasive materials have limited the use of natural ones, and in some cases have replaced the latter.

Silicon carbide is an abrasive material that is chemical compound silicon and carbon produced in electric furnaces at a temperature of 2100...2200 °C from quartz sand and coke.

For abrasive processing, the industry produces two types of silicon carbide: green and black. By chemical composition And physical properties they differ slightly, but green silicon carbide contains fewer impurities, has slightly increased fragility and greater abrasive ability.

Electrocorundum is an abrasive material produced by electrical surfacing of materials rich in aluminum oxide (for example, bauxite and alumina).

The grain size (grain size of abrasive materials) is determined by the size of the sides of the cells of two sieves through which the selected abrasive grains are sifted. The grain size is taken to be the nominal size of the side of the cell in the light of the mesh, on which: the grain is retained. The grain size of abrasive materials is designated by numbers.

The bond serves to bind individual abrasive grains into one body. The type of bond of an abrasive tool significantly affects its strength and operating modes.

Ligaments are divided into two groups: inorganic and organic.

Inorganic binders include ceramic, magnesia and silicate.

The ceramic bond is a glassy or porcelain-like mass, the constituent parts of which are refractory clay, feldspar, quartz and other materials. A mixture of binder and abrasive grain is pressed into a mold or cast. Cast wheels are more fragile and porous than pressed ones. The ceramic bond is the most common, since its use in abrasive tools is rational for the greatest number of operations.

The magnesium binder is a mixture of caustic magnesite and a solution of magnesium chloride. The process of making a tool using Loy bond is the simplest - making a mixture of emery and magnesium bond in a given ratio, compacting the mass in a mold and drying.

The silicate binder consists of liquid glass mixed with zinc oxide, chalk and other fillers. It does not provide strong fixation of grains in the circle, since liquid glass weakly adheres to abrasive grains.

Organic binders include bakelite, glyphthalic and vulcanite.

Bakelite bond is bakelite resin in the form of powder or bakelite varnish. This is the most common of the organic ligaments.

Glyphthalic binder is obtained by the interaction of glycerin and phthalic anhydride. An instrument made with a glypthal bond is approximately the same as with a bakelite bond.

Vulcanite bond is based on synthetic rubber. To make wheels, abrasive material is mixed with rubber, as well as sulfur and other components in small quantities.

The following are accepted for connections: symbols: ceramic - K, magnesia - M, silicate - C, bakelite - B, glyphthalic - GF, vulcanite - V.

The hardness of an abrasive wheel refers to the resistance of the bond to the tearing of grinding grains from the surface of the wheel under the influence of external forces. It is practically independent of the hardness of the abrasive grain. The harder the wheel, the more force must be applied to tear the grain out of the bunch. An indicator of the hardness of an abrasive tool is the depth of the hole on the surface of the circle (when using the sandblasting method for measuring hardness) or the reading of the Rockwell instrument scale (when using the ball indentation method). Abrasive wheels are made in a variety of shapes and sizes.

Static imbalance of the abrasive wheel. In accordance with GOST 3060 - 75, static imbalance of the grinding wheel characterizes the imbalance of the grinding wheel caused by the mismatch of its center of gravity with the axis of rotation.

The measure of static imbalance is the mass of the load, which, being concentrated at a point on the periphery of the circle, opposite to its center of gravity, moves the latter to the axis of rotation of the circle,

Depending on the number of imbalance units and the height of the circle, four imbalance classes are established. As the imbalance class increases, a larger amount of unbalanced mass is allowed.

Abrasive wheels are the main working parts of a number of machines used for grinding grain during the production of cereals. Such machines include A1-ZShN-Z, A1-BShM-2.5, ZShN, RC-125, etc.

The abrasive wheels used in the A1-ZShN-Z and ZShN machines are prefabricated structures consisting of a grinding wheel mounted in two steel bushings. The bushings act as hubs through which the abrasive wheels are attached to the machine shaft. On the lower bushing there are 12 holes symmetrically located for installing a balancing weight and three spacer rods, ensuring the placement of circles on the shaft at intervals.

In this case, two types of PVD grinding wheels are used: flat with a double-sided groove and the same wheels with an external conical profile.

The A1-ZSHN-Z machine set includes five flat PVD circles with a double-sided groove and one flat circle with a double-sided groove and an external conical profile. The ZShN machine set includes one wheel with an external conical profile and six wheels with a straight profile. The A1-BShM-2.5 grinding machine uses eight abrasive wheels with a straight PP profile. Before installation in the machine, the circles are mounted on wooden bushings, outside diameter which is equal to the inner diameter of the hole of the circles. In this form, the circles are installed and secured on the shaft, forming a solid cylinder. Summary data of the abrasive wheels used in the A1-ZShN-Z, ZShN and A1-BShM-2.5 grinding machines are given in Table 1.

The main working body grinding machine RC-125 is a truncated conical drum, the side surface of which is covered with an artificial abrasive mass consisting of a mixture of emery, caustic magnesite and a solution of magnesium chloride. The grain size of the emery is selected taking into account the requirements for ensuring effective grinding of grain.

The worn-out rotor surface is usually restored in a cereal factory using the above technology for magnesium bonded abrasive products.

Sieve cylinders. In grinding machines, perforated cylinders of various designs are installed around abrasive wheels with a certain gap. Since the grain is processed between rotating abrasive wheels and a stationary perforated cylinder under the influence of friction forces, the cylinders are subject to intense wear.

The sieve cylinder of the A1-ZSHN-Z machine is made of perforated steel sheet with a thickness of 0.8... 1.0 mm with oblong holes measuring 1.2 x 20 mm. The cylinder is equipped with upper and lower rings. Two stops are attached to the upper ring to prevent circular movement of the cylinder while the machine is operating.

The design of the sieve cylinder for ZShN type machines is similar to that described above. Its internal diameter is 270 mm.

The sieve cylinder in the A1-BSHM-2.5 frame-type machine consists of two half-cylinders. The semi-cylinders are connected to each other at the top with bolts, and at the bottom with special clamps (hinged bolts). To make one half-cylinder, a sieve with oblong holes measuring 1.2 x 20 mm and a sheet thickness of 1 mm is used. Sheet dimensions 870 x 460 mm. The sieve is attached to the frame with easily removable races. This design of the sieve cylinder ensures a uniform working gap between it and the abrasive wheels, low labor intensity when replacing worn sieves and races, as well as installing cylinders in the machine. The service life of sieves with a thickness of 1 mm is about 200 hours.

Compressed air. The quantities characterizing air in a given state are called state parameters. Most often, the state of air is determined by the following parameters: specific volume, pressure and temperature. Using compressed air as a working agent for grain peeling, they use aerodynamic dependencies that explain and reveal the phenomena that occur during flow around solid(grains) with high-speed air flow. When an air flow flows over its surface, tangential frictional forces or viscous forces arise, creating tangential stresses.

A characteristic feature of air is elasticity and compressibility. A measure of the elasticity of air is the pressure that limits its expansion. Compressibility is the property of air to change its volume and density with changes in pressure and temperature.

The thermal equation of state of an ideal gas is widely used in the study of thermodynamic processes and in thermotechnical calculations.

In most problems considered in aerodynamics, the relative speed of gas movement is high, and the heat capacity and temperature gradients are small, so heat exchange between individual streams of moving gas is practically impossible. This allows us to accept the dependence of density on pressure in the form of an adiabatic law.

A characteristic of the energy state of a gas is the speed of sound in it. The speed of sound in gas dynamics is understood as the speed of propagation of weak disturbances in a gas.

The most important gas-dynamic parameter is the Mach number M = c/a - the ratio of the gas velocity c to the local speed of sound a in it.

Outflow of gases through nozzles. In practical tasks, various types of nozzles (nozzles) are used to accelerate air flow.

The exhaust velocity and air flow rate, i.e., the amount of air flowing out per unit time, are determined using dependencies known in aerodynamics. In these cases, first of all, the ratio P 2 /P 1 is found, where P 2 is the pressure of the medium at the outlet of the nozzle; P 1 - medium pressure at the nozzle inlet.

To obtain exhaust velocities above critical (supersonic speeds), an expanding nozzle or a Laval nozzle is used.

Energy indicators of compressed air. The process of grain peeling using a jet of air flow moving at critical and supercritical speeds is based on the basic laws of high-speed aerodynamics. It should be noted that the use of a high-speed air jet for peeling is an energy-intensive operation, since significant energy costs are required to produce compressed air.

So, for example, for two-stage compressors with a final pressure of 8-105 Pa, the specific power consumption (in kW min/m3) depending on the capacity (m 3 /min) is characterized by the following data:

The use of compressed air for peeling is effective in cases where the cost of the processed raw materials is several times higher than the cost of energy or when it is impossible to achieve the required processing of the product by other means.