Screw application and meaning. Where's the screw? How to determine if a screw fits

The choice of propeller is a pressing issue for all people using motorized watercraft. Without it, the full and maximum implementation of the engine’s capabilities is impossible, since on boats it does not have the ability to change gears.

Using a properly selected device provides the following benefits:

1. Reduced fuel consumption.
2. Reduced background noise.
3. Improved engine performance.
4. Possibility of achieving higher speeds or increasing load capacity.
5. Reducing water resistance while the boat is moving.

Varieties

To select and purchase the most suitable propeller, you should first understand the existing classifications. There are many different criteria for dividing them, the most significant of them are discussed below:

1. An indicator of the distance that a screw can travel during one revolution. This criterion is called a step; sliding is not taken into account.
2. Diameter is the extreme points of the circle, which is created when the blades rotate.
3. The ratio of the total area of ​​all blades to the diameter area, this criterion is usually called the disk ratio.
4. The number of blades, which can be 2, 3, 4 or 5 pieces depending on the design features of the selected model. Today, three-blade options are the most common.
5. The material that was used for production. The most common models are made from various aluminum alloys, carbon steel, brass, stainless steel or plastic. Bronze devices are less popular because they are too expensive and have no visible advantages over their brass counterparts. Plastic models are made from modern and durable material, but metal options still remain more reliable and have a long service life.
6. Hub design features, on which the method of exhaust removal also depends.

Marking

Each propeller must have a special marking, which can be applied to its blades or directly to the hubs; All dimensions are indicated in inches.

Labeling of devices today is carried out in different ways, the most common examples are given below:

1. 1¼х15–G– in this example there are two numerical values, they indicate the diameter of the blades and the pitch of the device.
2. 3x10-3/8x11R- is a more detailed marking that shows that the device is equipped with three blades and has right-hand rotation.
3. 3213-101-14 – is a catalog marking; a decoding of the article numbers must be present in the attached instructions or on the packaging.

Making calculations

Today, there is a large number of different software that allows you to calculate the optimal propeller parameters taking into account the specified parameters.

It is believed that the most accurate calculations are generated by programs that use Pampel diagrams for these purposes. However, even in this case, errors are allowed, so the final selection of indicators is carried out only through test plants.

To obtain the most accurate calculations, the following significant factors must be taken into account:

1. Dimensions and weight of the boat.
2. Features of the shape of the bottom of a motor boat.
3. The volume of water that is displaced by a boat.
4. The presence of longitudinal or transverse steps that reduce resistance.
5. Engine performance.
6. Reduction indicators.

The main task is to acquire the skill to carry out the most accurate calculations with a minimum amount of information. To do this you will need to have the following information:

1. Gear ratio, which can be found in the documentation supplied with the engine.
2. Speed ​​of maximum engine power. This information is applied directly to the engine or in the engine compartment; if it is not available, this information can be checked with the manufacturer or viewed on its official website.
3. The maximum speed expected to be achieved. It is necessary to provide real indicators that can be obtained by comparing engine power and boat features.

The following formula is usually used to calculate the pitch:

(750x (desired maximum speed)) / number of shaft revolutions.

To accurately select a device, the resulting step indicator must be used as follows:

1. The diameter of the device and its pitch are interrelated indicators, but even with a known pitch there will be about 2-3 models with different diameters. Here it is necessary to take into account the maximum permitted engine power: the higher it is, the larger the propeller diameter should be.
2. If possible, it is recommended to ask the seller to conduct test tests or to take a deposit of a screw with a pitch that corresponds not only to the calculated indicator, but also to values ​​​​close to it. This will allow you to verify in practice the correctness of the calculations by measuring speed and select the most suitable option.

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3. Lures based pheromones.

Selection rules

Criterias of choice

In addition to the calculated indicators, there are a large number of features that you need to pay attention to when choosing a propeller:

1. The number of blades will primarily affect driving performance. It is recommended to choose three-bladed models; options with 2 or 5 blades are actually not used. Devices equipped with 4 blades are used when traction is required. Their use is advisable when there is a need for increased load capacity, if the features of the gearbox do not allow increasing the screw diameter.
2. The shape of the blades must also be selected correctly; here, first of all, it is taken into account that models with increased curvature accelerate cavitation. In addition, the leading edges should not be too sharp, as this will negatively affect the performance parameters.
3. Particular attention must be paid to the material of the propeller. Models made from new generation stainless steel are considered the most reliable, durable and durable. However, for boats equipped with low-power engines, especially when used in fresh water, devices made from aluminum-silicon or aluminum-magnesium metal alloys are also suitable.

How to determine if a screw fits

You can understand whether the existing propeller is suitable by taking measurements of the revolutions at maximum and minimum loads; the indicator must be within the limits defined by the manufacturer.

The following are specific examples of compliance and non-compliance of selected devices:

1. At minimal loads, the engine shows the number of revolutions declared by the manufacturer; at maximum loads there is no significant resistance to movement, it is possible to go on planing. This indicates the versatility of the propeller; it was chosen correctly.
2. Under no load the engine does not produce the stated number of revolutions, and problems arise when reaching planing. This situation clearly demonstrates that a propeller with too large a pitch was selected.
3. Torsions occur: the motor makes too many revolutions, exceeding the values ​​​​set by the manufacturer; At the same time, the speed of the boat is far from the maximum limit. This indicates that a propeller with a higher pitch is required.
4. A correctly selected cargo propeller will allow you to plan without any problems even when the craft is fully loaded; a slight loss of speed in this situation is normal.
5. Maximum engine speed and boat speed are achieved only when the boat is lightly loaded and the hydraulic lift is in the upper position; similar situations are observed when high-speed propellers are installed.

Propeller protection

Operating rules

Even the most durable and reliable propellers are highly vulnerable; they are the most fragile part of the boat. Below is a list of rules, compliance with which increases safety and has a positive effect on the service life of the device:

1. Reverse is allowed to be turned on only if you are absolutely sure that the propeller has enough depth. It’s better not to take unnecessary risks and push off from the shallow water several times with the help of oars.
2. It is necessary to monitor the condition of the blades, since any deformations, irregularities and potholes interfere with the full functioning of the propeller and can damage it.
3. When the vessel passes near the most problematic areas of the reservoir, which are shallow waters and various underwater obstacles, you must remember to use hydraulic lift.
4. You must constantly ensure that the propeller does not come into contact with the bottom surface, even briefly.– this is the main condition for ensuring long service.

To summarize, the following tips regarding propellers and their use can be given:

1. It is not recommended to additionally coat such devices with paint, since if it does not have water-repellent properties, the surface will soon begin to peel off greatly, which will impair the functioning of the screw. As a result, the speed will drop even as the number of revolutions made increases.
2. It is best to purchase propellers from large manufacturers that have proven themselves well and have a sufficient number of positive reviews. Such companies provide a long warranty on their equipment and often allow you to pre-test selected models.
3. To ensure the straightness of the vessel, two propellers with different directions of rotation can be installed. It must be remembered that installing several devices that have the same rotation will cause the boat to tilt to one side.

How does a propeller work? The propeller converts the rotation of the engine shaft into thrust - a force that pushes the ship forward. When the propeller rotates, a vacuum is created on the surfaces of its blades facing forward - in the direction of the movement of the vessel (suction), and increased water pressure on those facing backwards (pumping). As a result of the pressure difference on the blades, a force Y arises (it is called lifting). By decomposing the force into components - one directed towards the movement of the vessel, and the second perpendicular to it, we obtain the force P, which creates the thrust of the propeller, and the force T, which generates the torque, which is overcome by the engine.

The thrust largely depends on the angle of attack a of the blade profile. The optimal value for high-speed boat propellers is 4-8°. If a is greater than the optimal value, then the engine power is unproductively spent on overcoming a large torque, but if the angle of attack is small, the lift force and, consequently, the thrust P will be small, and the engine power will be underutilized.

In a diagram illustrating the nature of the interaction between the blade and water, a can be represented as the angle between the direction of the velocity vector of the flow W flowing onto the blade and the discharge surface. The flow velocity vector W is formed by the geometric addition of the vectors of the translational movement velocity Va of the propeller together with the ship and the rotational speed Vr, i.e., the speed of movement of the blade in a plane perpendicular to the propeller axis.

Helical surface of the blade. The figure shows the forces and velocities acting in one specific cross-section of the blade, located at a certain radius r of the propeller. The circumferential speed of rotation V depends on the radius at which the section is located (Vr = 2× p × r× n, where n is the speed of rotation of the propeller, rev/s), while the translational speed of the propeller Va remains constant for any section of the blade. Thus, the larger r, i.e., the closer the section under consideration is located to the end of the blade, the greater the peripheral speed Vr, and therefore the total speed W.

Since side Va in the triangle of speeds under consideration remains constant, then as the blade section moves away from the center, it is necessary to rotate the blades at a large angle to the propeller axis so that a maintains its optimal value, i.e., remains the same for all sections. Thus, a helical surface with a constant pitch N is obtained. Let us recall that the pitch of the propeller is the movement of any point of the blade along the axis in one full revolution of the propeller.

The drawing helps to visualize the complex helical surface of the blade. During operation of the propeller, the blade seems to slide along guide squares, which have a different base length at each radius, but the same height - pitch H, and rises in one revolution by the amount H. The product of the pitch and the rotation frequency (Hn) is the theoretical speed of movement of the propeller along the axis.

Vessel speed, propeller speed and slip. When moving, the ship's hull carries water along with it, creating a passing flow, so the actual speed of the propeller meeting the water Va is always slightly less than the actual speed of the ship V. For high-speed planing motorboats the difference is small - only 2 - 5%, since their hull slides along water and almost does not “pull” it along with it. For boats traveling at an average speed, this difference is 5-8%, and for low-speed, deep-draft displacement boats it reaches 15-20%. Let us now compare the theoretical speed of the screw Hn with the speed of its actual movement Va relative to the water flow.

The difference Hn - Va, called slip, determines the work on the mouth of the propeller at an angle of attack a to the water flow having a speed W. The ratio of slip to the theoretical speed of the propeller as a percentage is called relative slip:
s = (Hn-Va)/Hn.

The slip reaches its maximum value (100%) when the propeller is operating on a ship moored to the shore. The propellers of light racing motorboats have the least slip (8-15%) at full speed; for the propellers of planing pleasure motorboats and speedboats, the glide reaches 15-25%, for heavy displacement boats 20-40%, and for sailing yachts with an auxiliary engine, 50-70%.

Light or heavy propeller. The diameter and pitch of the propeller are the most important parameters on which the degree of use of engine power and, consequently, the possibility of achieving the highest speed of the vessel depends.

Each engine has its own so-called external characteristic - the dependence of the power removed from the shaft on the crankshaft speed when the carburetor throttle is fully open. Such a characteristic for the Whirlwind outboard motor, for example, is shown in the figure (curve 1). Maximum power of 21.5 l, s. the engine develops at 5000 rpm.

The power that is absorbed by the propeller on a given boat, depending on the engine speed, is shown in the same figure not by one, but by three curves - screw characteristics 2, 3 and 4, each of which corresponds to a specific propeller, i.e. a propeller of a certain pitch and diameter.

When both the pitch and the diameter of the propeller are increased above the optimal values, the blades capture and throw back too much water: the thrust increases, but at the same time the required torque on the propeller shaft also increases. Propeller characteristic 2 of such a propeller intersects with the external characteristic of engine 1 at point A. This means that the engine has already reached the limit - the maximum value of torque and is not able to turn the propeller at a high speed, i.e. it does not develop the rated speed and its corresponding rated power. In this case, the position of point A shows that the engine produces only 12 hp. With. power instead of 22 hp. With. This propeller is called hydrodynamically heavy.

On the contrary, if the pitch or diameter of the screw is small (curve 4), both the thrust and the required torque will be less, so the engine will not only easily develop, but also exceed the rated crankshaft speed. Its operating mode will be characterized by point C. And in this case, the engine power will not be fully used, and operation at too high speeds is associated with dangerously high wear of parts. It should be emphasized that since the propeller stop is small, the ship will not reach the maximum possible speed. This screw is called hydrodynamically light.

A propeller that allows a particular combination of ship and engine to fully utilize the power of the latter is called agreed upon. For the example under consideration, this agreed the propeller has characteristic 3, which intersects with the external characteristic of the engine at point B, corresponding to its maximum power.

The figure illustrates the importance of choosing the right propeller using the example of the Crimea motorboat with the Whirlwind outboard motor. When using a standard motor propeller with a pitch of 300 mm, a motorboat with 2 people. on board it reaches a speed of 37 km/h. With a full load of 4 people, the speed of the boat is reduced to 22 km/h. When replacing the propeller with another one with a pitch of 264 mm, the speed with full load increases to 32 km/h. The best results are achieved with a propeller having a pitch ratio H/D = 1.0 (pitch and diameter are 240 mm): the maximum speed increases to 40-42 km/h, the speed with full load is up to 38 km/h. It is easy to conclude about the significant fuel savings that can be obtained with a reduced pitch propeller. If with a standard propeller with a load of 400 kg, 400 g of fuel is consumed for each kilometer traveled, then when installing a propeller with a pitch of 240 mm, the fuel consumption will be 237 g/km.

It should be noted that agreed upon There are an endless variety of propellers for a given boat and engine combination. In fact, a propeller with a slightly larger diameter but a slightly smaller pitch will load the engine just as much as a propeller with a smaller diameter and a larger pitch. There is a rule: when replacing a propeller matched with the hull and engine with another, with similar values ​​of D and H (the discrepancy is permissible no more than 10%), it is required that the sum of these values ​​for the old and new propellers be equal.

However, from this set agreed upon screws, only one screw, with specific values ​​of D and H, will have the greatest efficiency. This screw is called optimal. The purpose of calculating a propeller is precisely to find optimal diameter and pitch values.

Efficiency. The efficiency of a propeller is assessed by its efficiency, that is, the ratio of useful power to expended engine power.

Without going into details, we note that the efficiency of a non-cavitating propeller mainly depends on the relative slip of the propeller, which in turn is determined by the ratio of power, speed, diameter and rotational speed.

The maximum efficiency of a propeller can reach 70 ~ 80%, but in practice it is quite difficult to choose the optimal values ​​of the main parameters on which the efficiency depends: diameter and rotation speed. Therefore, on small ships, the efficiency of real propellers may be much lower, amounting to only 45%.

The propeller reaches maximum efficiency at a relative slip of 10 - 30%. As slip increases, efficiency quickly drops: when the propeller operates in mooring mode, it becomes equal to zero. Similarly, the efficiency decreases to zero when, due to high speeds at a small pitch, the screw stop is zero.

However, the mutual influence of the housing and the screw should also be taken into account. During operation, the propeller captures and throws significant masses of water into the stern, as a result of which the speed of the flow flowing around the aft part of the hull increases and the pressure drops. This is accompanied by the phenomenon of suction, i.e. the appearance of an additional force of water resistance to the movement of the vessel compared to that which it experiences when towing. Consequently, the screw must develop a thrust that exceeds the body resistance by a certain amount Pe = R/(1-t) kg. Here t is the suction coefficient, the value of which depends on the speed of the vessel and the contours of the hull in the area where the propeller is located. On planing boats and motorboats, on which the propeller is located under a relatively flat bottom and does not have a sternpost in front of it, at speeds above 30 km/h t = 0.02-0.03. On low-speed (10-25 km/h) boats and motorboats, on which the propeller is installed behind the sternpost, t = 0.06-0.15.

In turn, the ship’s hull, forming a passing flow, reduces the speed of water flowing onto the propeller. This takes into account the associated flow coefficient w: Va = V (1-w) m/s. The values ​​of w are easy to determine from the data given above.

The overall propulsive efficiency of the ship-engine-propeller complex is calculated by the formula:
h = h p h ((1-t)/(1-w)) h h m = h p h h k h h m Here h p is the efficiency of the screw; h k - body influence coefficient; h m - efficiency of the shafting and reverse gear transmission.

The housing influence coefficient is often greater than unity (1.1 - 1.15), and losses in the shafting are estimated at 0.9-0.95.

Screw diameter and pitch. The elements of a propeller for a particular vessel can be calculated only by having a curve of water resistance to the movement of a given vessel, an external characteristic of the engine and design diagrams obtained from the results of model tests of propellers having certain parameters and blade shapes. To preliminarily determine the diameter and pitch of the screw, there are simplified formulas, which make no sense to present here, because suggested to use more accurate methods for calculating the optimal propeller. These methods are based on the approximation (approximate representation) of graphic diagrams by analytical dependencies, which makes it possible to perform fairly accurate calculations on a computer and even on microcalculators.

The diameter of the propellers, obtained either by an approximate formula or by accurate calculations, is usually increased by about 5% in order to obtain a deliberately heavy propeller and ensure its consistency with the engine during subsequent tests of the vessel. To “lighten” the screw, it is gradually cut in diameter until the nominal engine speed is obtained at the design speed.

However, for propellers of small vessels this need not be done. The reason is simple: the loading of pleasure craft varies widely, and a propeller that is a little "heavy" or "light" at one displacement will become consistent at another load.

Cavitation and features of the geometry of propellers of small ships. The high speeds of motorboats and motorboats and the speed of rotation of the propellers cause cavitation - the boiling of water and the formation of vapor bubbles in the vacuum region on the suction side of the blade. In the initial stage of cavitation, these bubbles are small and have virtually no effect on the operation of the propeller. However, when these bubbles burst, enormous local pressures are created, causing the surface of the blade to chip. During prolonged operation of a cavitating propeller, such erosion damage can be so significant that the efficiency of the propeller will decrease.

With a further increase in speed, the second stage of cavitation begins. A solid cavity—a cavern—encompasses the entire blade and can even close outside of it. The thrust developed by the propeller falls due to a sharp increase in drag and distortion of the shape of the blades.

Propeller cavitation can be detected by the fact that the speed of the boat stops increasing, despite a further increase in rotation speed. The propeller makes a specific noise, vibration is transmitted to the hull, and the boat moves irregularly.

The moment of onset of cavitation depends not only on the rotation speed but also on a number of other parameters. So, the smaller the area of ​​the blades, the greater the thickness of their profile and the closer the propeller is located to the waterline, the lower the rotation speed, i.e., the earlier cavitation occurs. The appearance of cavitation is also facilitated by a large angle of inclination of the propeller shaft, defects in the blades - bending, poor-quality surface.

The thrust developed by the propeller is practically independent of the area of ​​the blades. On the contrary, as this area increases, friction with water increases and engine power is additionally consumed to overcome this friction. On the other hand, it must be taken into account that with the same emphasis on wide blades, the vacuum on the suction side is less than on narrow ones. Therefore, a wide-bladed propeller is needed where cavitation is possible (i.e. on high-speed boats and at high propeller shaft speeds).

The working, or straightened, area of ​​the blades is taken as a characteristic of the propeller. When calculating it, the width of the blade is taken, measured on the discharge surface along the length of the circular arc at a given radius drawn from the center of the propeller. The characteristics of a propeller usually indicate not the straightened area of ​​the blades A itself, but its ratio to the area Ad of a solid disk of the same diameter as the propeller, i.e. A/Ad. On factory made screws, the disc ratio value is stamped on the hub.

For propellers operating in pre-cavitation mode, the disk ratio is taken within the range of 0.3 - 0.6. For heavily loaded propellers on high-speed boats with powerful high-speed engines, A/Ad increases to 0.6 - 1.1. A large disk ratio is also necessary when making screws from materials with low strength, for example, silumin or fiberglass. In this case, it is preferable to make the blades wider than to increase their thickness.

The axis of the propeller on a planing boat is located relatively close to the surface of the water, so there are frequent cases of air being sucked into the propeller blades (surface aeration) or the entire propeller being exposed when sailing on a wave. In these cases, the propeller thrust drops sharply, and the engine speed may exceed the maximum permissible. To reduce the influence of aeration, the pitch of the propeller is made variable along the radius - starting from the cross section of the blade at r = (0.63-0.7) R towards the hub, the pitch is reduced by 15~20%.

Boat propellers usually have a high rotation frequency, therefore, due to high centrifugal speeds, water flows along the blades in the radial direction, which negatively affects the efficiency of the propeller. To reduce this effect, the blades are given a significant tilt towards the stern - from 10 to 15°.

In most cases, the propeller blades are given a slight saber shape - the line of the middle sections of the blade is curvilinear with a convexity directed along the direction of rotation of the propeller. Due to the smoother entry of the blades into the water, such propellers are characterized by less vibration of the blades, are less susceptible to cavitation and have increased strength of the entering edges.

The most widespread among the propellers of small ships is the segmented plano-convex profile. The blades of the propellers of high-speed motorboats and speedboats designed for speeds over 40 km/h have to be made as thin as possible in order to prevent cavitation. To increase efficiency in these cases, a convex-concave profile (“hole”) is advisable. The profile concavity arrow is assumed to be equal to about 2% of the section chord, and the relative thickness of the segment profile (the ratio of the thickness t to the chord b at the design radius of the screw equal to 0.6R) is usually taken within the range t/b = 0.04-0.10.

A two-blade propeller has a higher efficiency than a three-blade propeller, but with a large disk ratio it is very difficult to ensure the necessary strength of the blade of such a propeller. Therefore, three-blade propellers are most widespread on small ships. Propellers with two blades are used on racing ships, where the propeller is lightly loaded, and on sailing and motor yachts, where the engine plays an auxiliary role. In the latter case, it is important to be able to install the propeller in a vertical position in the hydrodynamic wake of the sternpost to reduce its resistance when sailing.

0 Today, society has become catastrophically degraded, which is facilitated by the impoverishment of the population, democracy, freedom of speech and liberal values. People are already on the brink of survival, and the situation has worsened so much that many cannot afford to buy not only shoes or clothes, but the simplest and cheapest food products. Therefore, young people who could not find themselves in life have only one thing left, " jam"your consciousness with a variety of substances, ranging from legal drugs like tobacco or alcohol, and ending with heavy drugs. And since “drugs” also cost money, people choose the cheapest drugs, often homemade ones. Today, as you already understand, we will talk about such an eloquent word as Screw, which means you can find out a little below. Add our website to your bookmarks so that you can visit us periodically.
However, before I continue, I would like to show you a couple of other sensible publications on the subject of drug addict slang. For example, what is Xan; what does the word Weed mean? how to understand the term Shabit; who is called Stoned, etc.
So, let's continue, what does Screw mean in slang? This term comes from the word " pervitin", which in turn comes from the term " methamphetamine".

Screw is a slang name for a drug that is made at home. The Screw includes: ephedrine, iodine, methamphetamine. It can be classified as a stimulant drug. In its appearance, the Screw resembles a clear liquid that drug addicts inject into a vein using an injection.

I want to advise you to read a few more sensible articles on the topic of youth slang, for example, the translation of the word Reboot, the meaning of the exclamation Pff, what is Push, when Fart Bombs, etc.

Method of preparing Vinta

Red phosphorus and other substances that we will not mention are placed in a container and simmered over low heat. This is a rather lengthy process, so drug addict-the brewer carefully controls the process of creating a poisonous potion and checks his recipe.
Drug addicts who consider themselves the elite among ordinary junkies use separate disposable syringes. Such drug addicts have not yet destroyed their last brains with this poison. The injection is given in places hidden by clothing.
This one screw consumed orally, that is, orally, and in the same dosage. This dose is calculated so as to get pleasure and still live in this world.

Everyone has it " screw“I have an acquaintance or an acquaintance of an acquaintance who has his own laboratory for the production of this poison. Cookers Vinta people are quite knowledgeable in chemistry, they are respected among drug addicts. Therefore, if some junkie orders a cook to cook Vint for himself, he has the legal right to take half of this product for himself.
People taking Vint may suddenly discover new abilities, for example, a junkie suddenly feels an irresistible desire to get creative, he can sit down at an easel and draw a picture or even start writing poetry.

After the impact Vinta weakens, the person begins to feel emptiness, melancholy and irritation. As a rule, after Vint injections, the drug addict sleeps for quite a long time. They can walk all day long, but then they fall into bed and sleep 24 hours. Although this happens to new drug addicts.

After reading this short but useful article, you learned what is Vint in slang, and now you won’t find yourself in a difficult situation if you suddenly find this word again.

There was nowhere to dive. August is hot this year, the water in the ponds has decreased, and mud floats on the surface in islands. Small rivers are muddy because cows enter the water in herds to escape insects. And I really wanted to dive. Having gathered together, according to tradition, at my garage, we discussed for a long time where to go, and decided to scout out places in the wide floodplain of the Oka. There is no point in going into the Oka itself at this time of year - the Oka water is muddy. Hoping to admire the clear lake water, we drove along the approved route, four of us, in two cars.

Ivanovich sat with me as a navigator; Oleg and Sergei were in another car. To avoid getting lost, we turned on the navigator. Go! In Ivanovich’s hands, the navigator poured out a female voice:

The route has been completed! Turn right! Keep left!

Turn back!

Don't trust her! Let's go there! I know for sure!

As you say!

You have deviated from the route! After a hundred meters turn back.

How persistent! Go straight ahead!

Ivanovich! Do you remember? They took the car out of the river, the owners claimed, and just like that a female voice said: “The river is two hundred meters away!” And they immediately fell off a cliff!

I'm telling you! Do not believe!

I don't believe it, you see the food.

After two hundred meters turn left!

Here! I thought better of it, well done!

We turned and drove off, rattling and shaking along the dead-end rural road. The second car disappeared into clouds of dust. And only the female voice did not rattle, but firmly commanded:

The route has been rebuilt! Straight fifteen kilometers!

Believe? No?

No! What are you doing? Will lead to hell! It's the rise of the machines! Turn!

I didn’t deceive you, we ran out to the lake. Straight from the shore down! But there is no water in it - it is dry. Yes! Lucky! We rush to another. It's the same picture. But this is on the left bank, and what is on the right? Let's see! Where is our ferry crossing?

Wide river. Clear weather. There is no wind. A huge pontoon, a boat attached to a pontoon. Handrails, railings, gangways. Bustle of workers. The stinking breath of trucks. A regular ferry crossing on a hot summer day. First flight for today.

And amidst all this movement, a demanding voice:

Where's the screw? Where's the screw? Tear you in two!

Lost!

What are you going to row with now? With an oar?

We don't have a paddle. We need to look for the screw!

How? It's six meters! I'll choke!

I'll choke on you! Search!

Mom dear! Where will I find him?

And where you lost it, look for it! People are waiting! Search!

In the senseless bustle, tension was running high, the ferry was standing, people were waiting, cars were waiting, everyone was waiting. We, too, waited with our entire underwater team. Well, how can you not help people? We have all the equipment, three devices, flashlights. We're not going anywhere anyway. Well, the management accepted the offer. We started looking!

We ask the workers:

Show yourself where you lost the screw. Three hands in different directions at once!

Is there a landmark?

Yes! The seagull was swimming.

Car?

Nope... Bird!

Here the river is silver! Where to look?

There! And again in different directions!

And how to work here? Well, once you get started, you won’t quit! I go into the water, and Oleg directs me with signals. Indeed, six meters! At the bottom, stones of different sizes stick out with sharp edges from the sand. Visibility is disgusting, you can’t see an outstretched hand. I walk along the bottom in one direction, then in the other. I walked by, turned around, Oleg let me go about a meter, and I went back. Pieces of iron, something like a road sign, a rectangular plate, a chain-link mesh, pieces of wire, a rope, a cable. There is no screw.

I turned on the flashlight under the pontoon. The direct ray did not penetrate the muddy waters. Only by touch. I looked at the console! ABOUT! It's time to go out, the air in the tank is running out. There is no screw! Wide Oka, but nowhere to look, they didn’t remember the place!

Having worked in all areas near the pontoon and the pier, in all places where the propeller was likely to be found, having crawled the entire bottom in search of such a necessary piece of iron, having touched all the stones and snags with my fingers, I could not believe that I could not find such a sufficiently large search object.

Ivanovich! Empty!

Yes, however! But everything was covered.

Damn, Ivanovich! And where is he?

Here Oleg:
- Was there a “boy”?

Come on, look under his stern!

I climb under the boat, shine a flashlight on the shaft, rudder blade, and bottom. Such a gorgeous view through the darkness! Shaft! The propeller is missing, but the nut, key and cotter pin are still there. Paradox! Well, it happens! And such sad thoughts came into my head. You don’t need to look for the propeller at the bottom, but in a barn, warehouse, or on a nearby boat. Eh, people!

Photo by Olga Tsvetkova

Screw and its application

A screw (from German Gewinde - cutting, thread) is the simplest mechanism of a cylindrical shape, onto which a thread is applied in a spiral (a series of alternating grooves and protrusions). The screw is a fastening part.

The invention of the screw is attributed to the great ancient mathematician and physicist Archimedes (287-212 BC) from Syracuse.

And the Rhimedes screw is a mechanism for lifting water from bottom to top. The bottom of the cylinder is in water. Inside the cylinder there is a flat spiral (screw). When the handle was rotated, water from the bottom of the cylinder moved to the top. Such a device was intended for irrigation of land, as well as for pumping water from the holds of large cargo ships.

The process of making a screw in the Middle Ages was as follows: a strip of paper was wound onto a rod, and then sawed through with a file.

Later, larger threads were made by hot forging: the hot workpiece was struck with a forming tool. Fine carvings were made on primitive lathes, where the tool was held in the hands.

Then a screw-cutting machine appeared, which removed chips from the workpiece to obtain threads.

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At present, threads are produced by surface deformation. The screw blank is clamped between two dies and rolled between them. One die is movable, and the other is stationary.

And also by cold stamping method. The wire is fed into a machine, which cuts the rod to the required length, passes it through a series of forming dies, and a thread is rolled onto the resulting workpiece.

Screws are used to fasten parts. They are widely used in electrical devices, since when fastening parts they pass electric current through themselves.

For electrical products, screws are made of copper, bronze and brass. Screws used in mechanical engineering are made of steel.

The screw principle is used in tools such as jacks and vices. The screw is also used for its “direct purpose”: to move meat in a meat grinder, to rotate the gear.

The screw thread is an inclined plane, it always gives a gain in strength.

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Let us imagine that an inclined plane with height h and length l is rolled into a tube (Fig. 1).

By turning the nut placed on the bolt, a rotational and translational movement occurs, you lift it along an inclined plane (Fig. 2).

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the gain in force is equal to the ratio of the distance traveled by the point of application of force per revolution of the screw (circumference l = πD) to the distance traveled by the load along the axis of the screw.

During one revolution, the load moves the distance between two adjacent threads (a and b or b and c), which is called the thread pitch.

An example of converting rotational motion into translational motion using a screw is the adjustment of precision optical-mechanical instruments. Adjustment – ​​alignment of the device along the axial direction.