Hydraulic system working principle. The principles of hydraulics in the most simple language. Piping lines: connection of system parts

The hydraulic system is a device designed to convert a small effort into a significant one using some kind of fluid to transfer energy. There are many types of nodes that operate according to this principle. The popularity of systems of this type is primarily due to their high efficiency, reliability and relative simplicity of design.

Scope of use

Widespread use of this type of system found:

In industry. Very often, hydraulics is an element of the design of metal-cutting machines, equipment designed for transporting products, loading / unloading them, etc.
In the aerospace industry. Similar systems are used in different kind controls and chassis.
IN agriculture. It is through hydraulics that the attachments of tractors and bulldozers are usually controlled.
In the field of cargo transportation. Cars are often equipped with hydraulic
In the ship, in this case, it is used in steering, it is included in the design scheme of turbines.

Operating principle

Any hydraulic system works on the principle of a conventional liquid lever. The working medium supplied inside such a node (in most cases, oil) creates the same pressure at all its points. This means that by applying a small force on a small area, you can withstand a significant load on a large one.

Next, we consider the principle of operation of such a device using the example of such a node as a hydraulic structure. The design of the latter is quite simple. Its scheme includes several liquid-filled, and auxiliary). All these elements are connected to each other by tubes. When the driver presses the pedal, the piston in the master cylinder moves. As a result, the liquid begins to move through the tubes and enters the auxiliary cylinders located next to the wheels. After that, braking is activated.

Design of industrial systems

The hydraulic brake of a car - the design, as you can see, is quite simple. IN industrial machines and mechanisms, liquid devices are used more complicated. Their design may be different (depending on the scope of application). However, the concept hydraulic system industrial design is always the same. It usually includes the following elements:

Fluid reservoir with mouth and fan.
Coarse filter. This element is designed to remove various kinds of mechanical impurities from the liquid entering the system.
Pump.
Control system.
Working cylinder.
Two fine filters (on the supply and return lines).
Distribution valve. This structural element is designed to direct fluid to the cylinder or back to the tank.
Check and safety valves.

Hydraulic System Operation industrial equipment also based on the principle of fluid leverage. Under the influence of gravity, the oil in such a system enters the pump. Then it goes to the control valve, and then to the piston of the cylinder, creating pressure. The pump in such systems is designed not to suck the liquid, but only to move its volume. That is, the pressure is not created as a result of its work, but under the load from the piston. Below is a schematic diagram of the hydraulic system.

Advantages and disadvantages of hydraulic systems

The advantages of nodes operating on this principle include:

The ability to move loads of large dimensions and weight with maximum accuracy.
Virtually unlimited speed range.
Smoothness of work.
Reliability and long term services. All components of such equipment can be easily protected from overloads by installing simple pressure relief valves.
Efficiency in work and the small sizes.

In addition to the advantages, hydraulic industrial systems, of course, and certain disadvantages. These include:

Increased risk of fire during operation. Most fluids used in hydraulic systems are flammable.
Sensitivity of equipment to contamination.
The possibility of oil leaks, and therefore the need to eliminate them.

Hydraulic system calculation

When designing similar devices many different factors are taken into account. These include, for example, the kinematic fluid, its density, the length of pipelines, rod diameters, etc.

The main goals of performing calculations for such a device as a hydraulic system are most often to determine:

Pump characteristics.
The magnitude of the stroke of the rods.
working pressure.
Hydraulic characteristics of highways, other elements and the entire system as a whole.

The hydraulic system is calculated using various kinds of arithmetic formulas. For example, pressure losses in pipelines are defined as follows:

The estimated length of the lines is divided by their diameter.
The product of the density of the liquid used and the square average speed stream is divided into two.
Multiply the obtained values.
Multiply the result by the path loss factor.

The formula itself looks like this:

∆p i \u003d λ x l i (p) : d x pV 2: 2.

In general, in this case, the calculation of losses in the mains is carried out approximately according to the same principle as in such simple designs like hydraulic heating systems. Other formulas are used to determine pump characteristics, piston stroke, etc.

Types of hydraulic systems

All such devices are divided into two main groups: open and closed type. The schematic diagram of the hydraulic system considered by us above belongs to the first variety. An open design is usually used for devices of low and medium power. In more complex closed systems, a hydraulic motor is used instead of a cylinder. The liquid enters it from the pump, and then returns to the line again.

How is the repair done

Since the hydraulic system plays a significant role in machines and mechanisms, its maintenance is often entrusted to highly qualified specialists of companies engaged in this particular type of activity. Such firms usually provide a full range of services related to the repair of special equipment and hydraulics.

Of course, in the arsenal of these companies there is all the equipment necessary for the production of such work. Repairs to hydraulic systems are usually done on site. Before it is carried out, in most cases, various diagnostic measures must be taken. To do this, hydraulic service companies use special installations. The components necessary to fix problems are also usually brought by employees of such firms.

Pneumatic systems

In addition to hydraulic, pneumatic devices can be used to drive the nodes of various kinds of mechanisms. They work in much the same way. However, in this case, the energy of compressed air, not water, is converted into mechanical energy. Both hydraulic and pneumatic systems quite effectively cope with their task.

The advantage of devices of the second type is, first of all, the absence of the need to return the working fluid back to the compressor. The advantage of hydraulic systems in comparison with pneumatic ones is that the medium in them does not overheat and does not overcool, and therefore, no additional components and parts need to be included in the circuit.

A hydraulic drive is a system in which the transfer of energy from a source (usually a pump) to a hydraulic motor (hydraulic motor or hydraulic cylinder) is carried out by means of a dropping liquid.

Structurally, the hydraulic drive consists of a pump (s), control and distribution equipment, a hydraulic motor (s), a working fluid, a container (tank) for its maintenance and means (filters and coolers) that preserve its qualities, as well as connecting and sealing fittings.

On fig. 2.1. shows a diagram of the volumetric hydraulic drive under study, consisting of a pump 1, a safety valve 2, distributors 3 and 4, hydraulic motors - a hydraulic motor 5 and a hydraulic cylinder 6, a retarding device 7 for lowering the load 8, a tank and a filter 9 installed in the drain hydraulic line blocked by a valve 10.

Rice. 2.1 Scheme of the studied hydraulic drive.

The pump 1 is designed to convert the mechanical energy flow coming from the primary energy source 11 (electric or fuel engine) into a hydraulic energy flow, i.e. into the flow of working fluid under pressure, which, depending on the positions (positions) of the valves of the distributors 3, 4, can be directed directly (idle mode) or through one or both hydraulic motors 5, 6 together (working mode) into the tank. In this case, the pressure at the outlet of the pump depends on the set of resistances encountered by the flow of the working fluid on the way from the pump to the tank. In cases where distributors 3, 4 are in positions "A" (see Fig. 2.1), the flow of working fluid from pump 1 passes into the tank through the said distributors, hydraulic lines and filter 9 (idle mode). The pressure at the outlet of the pump is:

Where
are the pressures required to overcome the flow of the working fluid resistance, respectively, sections of the gyrolines, distributors and filter.

In those cases when, on command from the outside, one or both distributors 3, 4 are transferred to any position "B" or "C", one or both hydraulic motors, respectively, are switched on (s). The direction of movement of the hydraulic motors depends on the position "B" and "C" of their distributors. When only one hydraulic motor is turned on, for example, hydraulic motor 5, the operating pressure at the pump outlet will be:

Where
- pressure loss to overcome the resistance of the distributor 3, 4

– pressure loss on the hydraulic motor drive 5, depending on the load to be overcome on its shaft.

In the case when the hydraulic motor 5 and the hydraulic cylinder 6 are simultaneously included in the work, their joint operation is possible only at the same required pressures. If one of them has a lower required pressure than the other, then their joint operation is impossible, since the fluid flow will mainly go in the direction of lower resistance and disrupt the normal operation of the hydraulic drive as a whole.

If the required pressure in the hydraulic drive exceeds the allowable one, the safety valve 2 is activated and diverts the flow of working fluid from the pump 1 into the tank (overload mode), thereby limiting the pressure in the hydraulic drive and protecting its elements from destruction.

To ensure the smoothness of the lowered loads (working bodies) in hydraulic drives, retarding devices are used (see Fig. 2.1, item 7), usually consisting of a check valve and a throttle. When lifting the load (working body), the working fluid enters the cylinder through the check valve and throttle. When lowering the load, the liquid from the cavity of the cylinder goes into the tank only through the throttle, which provides resistance to it, the value of which depends on the magnitude of its flow, and this ensures the smoothness of its lowering. In this case, the opposite cavity of the hydraulic cylinder is filled with liquid supplied by the pump. In the event of an excess amount of liquid supplied by the pump, part of it will be discharged to the drain through the safety valve 2.

Pressure gauge 12 is designed for visual control of pressure in the hydraulic drive. To ensure the cleaning of the working fluid from solid contaminants (abrasives, wear products), filters of various designs are used in hydraulic drives.

hydraulic machines

Hydraulic machines (hydraulic machines) are called mechanical devices designed to convert the types of energy flows using dropping liquid as an energy carrier.

Hydraulic machines are divided into pumps and hydraulic motors.

Pumps are called hydraulic machines designed to convert a mechanical energy flow into a hydraulic energy flow.

Hydraulic motors are called hydraulic machines designed to convert hydraulic energy flow into mechanical energy flow.

Hydraulic motors, the output links of which perform linear reciprocating movements, are called hydraulic cylinders (hydraulic cylinders).

Hydraulic motors, the output links of which perform rotational movements, are called hydraulic motors (hydraulic motors).

Depending on the angle of rotation of the output link, hydraulic motors are divided into full
and semi-rotary
.

Hydraulic machines, in which the working process is based on the use of the kinetic energy of the fluid, are called dynamic, and those machines in which the working process is based on the use of the potential energy of the fluid are called volumetric.

The main feature of volumetric hydraulic machines is that they contain at least one working chamber, the volume of which varies
during the work cycle. In addition, each working chamber contains a movable element designed to change its volume. Usually the movable element of the working chamber is called the displacer. Pistons, plungers, gear teeth, balls, rollers, plates, membranes, etc. can be used as displacers.

During the operation of a volumetric hydraulic machine, each of its chambers communicates in turn with a low and high pressure line, i.e. the working chambers of the pump alternately communicate with the suction and discharge lines, and for engines - with the high pressure outlet line and the drain line.

The value of the pressure developed (implemented) by the pump depends on the resistance of the consumer (usually a hydraulic motor) and the connecting hydraulic fittings.

The value of the pressure of the working fluid consumed by the hydraulic motor depends on the value of the load it implements on the output link.

According to the type of displacers, hydraulic machines are divided into piston, plunger, ball, roller, gear (gear), lamellar, membrane, etc., and according to the number of working chambers into single and multi-chamber.

Hydraulic machines, in which the working chambers, together with the displacers, perform rotational movements, are called rotary.

The value of the changing volume of the working chambers of a hydraulic machine is called its working volume. The working volume of hydraulic machines is usually expressed in cubic centimeters.

The amount of working fluid supplied by the pump to the system per unit of time is called its supply.

If the working volume is known
pump and cycle rate , then its ideal supply can be determined by the formula

Due to the fact that there are leaks of the working fluid between the moving elements of the pump, the actual flow will always be less than the ideal, i.e.

Where
- the amount of leakage through the gaps;

– voluminous pump efficiency.

The ideal speed of the hydraulic motor is determined by the formula

and the actual

Where
- the value of the input flow of the working fluid;

- the working volume of the hydraulic motor;

is the volumetric efficiency of the hydraulic motor.

The volumetric efficiency of the hydraulic motor can be determined by the formula

Where
- the value of the flow of the working fluid, usefully used in the hydraulic motor;

- the amount of leakage through the gaps in the hydraulic motor.

The drive power of the pump can be determined by the formula

Where
- the power of the flow of the working fluid at the outlet of the pump;

– full efficiency of the pump;

- the value of pressure at the outlet of the pump;

– hydraulic efficiency of the pump;

- the pressure in the working (s) chamber (s) of the pump;

is the mechanical efficiency of the pump.

The energy quality of a hydraulic motor is characterized by its total efficiency, which can be defined as the ratio of the power on its output shaft
to the value of the power of the inlet fluid flow
, i.e.

Where
- torque;

is the angular velocity;

- pressure drop in the hydraulic motor.

Most positive displacement hydraulic machines are reversible, i.e. they are capable of operating both as pumps and as hydraulic motors.

In hydraulic drives of construction and road machines, gear (Fig. 2.2) and axial (Fig. 2.3) hydraulic machines are most widely used as pumps, and axial (Fig. 2.3) and radial (Fig. 2.4) as hydraulic motors.

Due to the fact that in rotary pumps there is a movement of working chambers with liquid from the suction cavity to the discharge cavity, they differ from simple piston (plunger) pumps in the absence of valve distribution of the liquid, which in turn increases their speed up to 85 s -1 and provides high uniformity of supply and pressure. All rotary hydraulic machines can only work with clean, non-aggressive fluids that have good lubricating properties and are designed for hydraulic drives.

Gear hydraulic machines

Gears are called rotary hydraulic machines with working chambers formed by the surfaces of the gears, housing and side covers.

Gear hydraulic machines are performed with gears of external (see Fig. 2.2, a) or internal (see Fig. 2.2, b) gearing. Such a hydraulic machine is a pair of (most often identical) gears 1 and 2, which are engaged and placed in a housing with small radial clearances (usually 10 ... 15 microns).

Rice. 2.2 Schemes of gear (gear) hydraulic machines.

The working process of the external gear pump is as follows. The drive gear 1 (see Fig. 2.2, a) drives the driven gear 2 in rotation. When the gears rotate in opposite directions in the chamber "A", their teeth disengage, which leads to an increase in the volume of the working chamber and to a decrease in the pressure of the working fluid to the vacuum value. Due to the resulting pressure difference between the reservoir (tank) and the suction chamber "A", the working fluid from the tank will flow into the chamber "A" and fill the cavities between the teeth of gears 1 and 2. With further movement of the gears, the working fluid in the cavities between the teeth is transferred from the zone suction (from chamber "A") to the injection zone (to chamber "B"). In the injection zone, the teeth of the gears engage and push the liquid out of the depressions into the injection hydraulic line under pressure, the value of which depends on the resistance of the consumer and the connecting hydraulic fittings.

In pumps with internal gear engagement (see Fig. 2.2, b), the drive gear is most often internal gear 1 with external teeth. Suction "A" and discharge "B" windows are made on the front side of the gear teeth in the side cover or pump casing. Female gear 2 with internal teeth rotates in a cylindrical bore of the body. Between the gears there is a separating sickle-shaped element 3, by means of which the suction cavity "A" is separated from the discharge cavity "B".

IN Lately in hydraulic power steering machines, hydraulic machines with internal gears with a special tooth profile (see Fig. 2.2, c), in which there is no separating element of cavities with different pressure levels, are widely used. Such hydraulic machines are called gerotoric or birotoric, i.e. with two rotors. The annular rotor (wheel) 1 has one tooth more than the internal one (gear) 2. Their axes are offset one relative to the other by an amount , forming the engagement of gears in the area of the upper separating jumper. The contact of the teeth when they pass the lower dividing bridge ensures the separation of the high and low pressure cavities. The input and output hydraulic lines with interdental cavities are connected by means of sickle-shaped windows "A" and "B".

Gerotor hydraulic machines are used as pumps operating at working fluid pressures up to 14 MPa and a shaft speed of 30 s -1 . They can be used as high-speed low-torque hydraulic motors. In some cases, gerotor hydraulic machines are capable of operating at pressures of 30 MPa at a rotation frequency of up to 60 s -1 .

The working process (suction and discharge) in internal gear pumps is similar to that in external gear pumps.

The overall dimensions and weight of pumps with internal gear are much smaller than pumps with external gear for equal working volumes.

The spur gear engagement of pump gears is characterized by a straight-line contact of the working surfaces (profiles) of the teeth over their entire width (length), with inaccurate manufacture of which, uneven movement of the driven gear and noise occur, and rapid wear of the working surfaces is also observed.

These shortcomings are eliminated in helical (spiral) and herringbone gears (see Fig. 2.2, d and e). The engagement and disengagement of the teeth in these gears occurs gradually, due to which errors in the tooth profile are reduced and smooth and relatively silent operation of the hydraulic machine is achieved.

In pumps with helical gears, the pulsation of the flow and torque, as well as the blocking of the liquid in the cavities, is much lower than in pumps with cylindrical gears. To reduce pressure fluctuations, it is necessary to ensure that the product
was equal to
etc., where - the angle of inclination of the teeth; - gear width; - tooth pitch. Corner are chosen so that the shift of the teeth along the circumference at the ends of the gears is half the step. In practice, this angle usually does not exceed 7…10.

During the operation of pumps with helical gears, axial forces arise that press the gears against the ends of the housing (covers). This disadvantage is eliminated in pumps with chevron gears (Fig. 8.2, e). Tooth angle chevron gears used in pumps is usually 20 ... 25.

Axial hydraulic machines

Axial hydraulic machines are characterized by the fact that the axes of their cylinders are parallel to the axis of rotation of the cylinder block or make an angle of no more than 45 with it.

The positive qualities of axial hydraulic machines include:

high working pressure (35…70 MPa);

speed (80 ... 550 s -1);

low metal consumption (0.5…0.6 kg/kW);

wide range of speed control of the hydraulic motor shaft 1:100 at variable and 1:1000 at constant loads;

the ability to operate hydraulic motors at low speeds (up to 0.01 s -1);

greater durability (up to 12000 hours);

high speed (feed change from zero to maximum and vice versa in 0.04…0.08 s);

low operating costs and fast payback.

Axial hydraulic machines come with an inclined block of cylinders (see Fig. 2.3, a, b) or with an inclined washer (see Fig. 2.3, c, d). They can be piston (see Fig. 2.3, a, b) or plunger (see Fig. 2.3, c, d) with a variable (adjustable) or constant (non-adjustable) working volume. In axial-piston hydraulic machines, there is: a small radial load on the piston, a large angle of inclination of the cylinder block (up to 45), as well as a higher efficiency (by 2 ... 3%) than that of a hydraulic machine with a swash plate.

On fig. 2.3, a diagram of an axial-piston adjustable hydraulic machine with an inclined block is presented. It consists of shaft 1, cylinder block 2, end distributor 3, central axle 4, pistons 5, connecting rods 6 and universal joint 8.

The described hydraulic machine in the function of the pump works as follows. The rotation of the drive shaft through the cardan 7 and the connecting rods 6 is transmitted to the cylinder block 2. With the coaxial arrangement of the shaft 1 and the cylinder block 2, the pistons 5 do not reciprocate and, therefore, the pump flow is 0. The deviation of the axis of the cylinder block from the axis of the drive shaft leads to reciprocating motion of the pistons.

For one revolution, each piston completes one working cycle. The strokes of the pistons depend on the angle of inclination of the cylinder block. When the angle of inclination of the cylinder block changes in the opposite direction from zero, the direction of pump delivery changes, i.e. the hydraulic machine provides reversal of the hydraulic drive.

Axial hydraulic machines with a swash plate are characterized by the following advantages compared to hydraulic machines with an inclined block of cylinders: the ability to work at higher pressures (up to 70 MPa); low noise level; small dimensions; low cost; simplicity of design and its manufacturability.

Rice. 2.3 Schemes of axial hydraulic machines.

On fig. 2.3, c shows a simplified diagram of an axial-plunger hydraulic machine with a swash plate. Plungers 2 are installed in the cylinders of its block 1, which are kinematically connected with the inclined washer 4 by means of springs 6 through shoes 3.

The described hydraulic machine in the function of the pump works as follows. Shaft 5 rotates cylinder block 1. In this case, plungers 2 reciprocate in the cylinder block. The stroke of the plungers, respectively, the pump flow, is determined by the angle of inclination of the washer 4. When the plungers, under the influence of springs 6, move out of the cylinder block, the process of suction of the working fluid occurs, and when they return, they are injected.

Swashplate axial-plunger hydraulic machines are often used in the functions of adjustable and fixed hydraulic motors, the principle of operation of which is similar to that of axial hydraulic machines with an inclined block of cylinders.

2015-11-15

Hydraulic drive(volumetric hydraulic drive) is a set of volumetric hydraulic machines, hydraulic equipment and other devices designed to transfer mechanical energy and convert movement through fluid. (T.M Bashta Hydraulics, hydraulic machines and hydraulic drives).

The hydraulic drive includes one or more hydraulic motors, fluid energy sources, control equipment, connecting lines.

The operation of the hydraulic drive is based on the principle

Let's consider the system.

In this system, the force created on the piston 2 can be determined by the dependence:

It turns out that force depends on the area ratio, the larger the area of the second piston, and the less area the first, the more significant will be the difference between the forces F1 and F2. Thanks to the hydraulic lever principle, you can get a lot of force with a small amount of force.

Winning in effort on the hydraulic lever, you will have to sacrifice movement, moving the small piston by l1, we get the displacement of piston 2 by l2:

Given that the piston area S2 is greater than the area S1, we get that the displacement l2 is less than l1.

The hydraulic drive would not be so useful if the loss in movement could not be compensated, and this was possible thanks to special hydraulic devices - .

check valve- this is a device for blocking the flow moving in one direction, and free passage of the reverse flow.

If in the considered example, on the outlet of the chamber with piston 1, install check valve so that the liquid can exit the chamber, but cannot flow back. The second valve must be installed between the chamber with piston 1 and an additional tank with liquid, so that the liquid can enter the chamber with , and from this chamber it cannot flow back into the tank.

The new system will look like this.

Applying a force F1 to the piston and moving it to a distance l1, we get the movement of the piston with a force F2 to a distance l2. Then we take piston 1 to the initial distance, the liquid cannot flow back from the chamber with piston 2 - the check valve will not allow - piston 2 will remain in place. Liquid from the tank will enter the chamber with the piston alone. Then, you need to apply force F1 again to piston 1 and move it to a distance l1, as a result, piston 2 will again move to a distance l2 with force F2. And in relation to the initial position, in two cycles, the piston 2 will move a distance of 2*l2. By increasing the number of cycles, it is possible to obtain a greater displacement of the piston 2.

It was the ability to increase the displacement by increasing the number of cycles that allowed the hydraulic lever to get ahead of the mechanical one in terms of the possible force being developed.

Drives where it is required to develop huge forces, as a rule, hydraulic.

A unit with a chamber and piston 1, as well as with check valves in hydraulics, is called pump. Piston 2 with chamber - hydraulic motor, in this case - .

Distributor in hydraulic drive

What to do if in the system under consideration it is necessary to return piston 2 to starting position? In the current configuration of the system, this is not possible. The liquid from under the piston 2 cannot flow back - the check valve will not allow, which means that a device is needed to send the liquid to the tank. You can use a simple tap.

But in hydraulics there is a special device for directing flows - distributor, allowing you to direct the flow of fluid in the desired direction.

Let's get acquainted with the work of the resulting hydraulic drive.

Devices in hydraulic drives

Modern hydraulic drives are complex systems, consisting of many elements. The design of which is not simple. In the presented example, there are no such devices, because they are designed, as a rule, to achieve the desired characteristics of the drive.

The most common hydraulic devices

Safety valves
Pressure reducing valves
Flow regulators
Chokes

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1.Hydraulic system

The most common control systems of the first group are hydraulic. In this case, the driver makes less effort to move the handles than with mechanical control, resulting in reduced driver fatigue. Structurally, the wiring of control systems with the help of hydraulic pipelines and hoses is more simply solved. An example is outrigger control. The combined system allows the use of lever-swivel gears before the control valve is put into operation. At the same time, the hydraulic distributors are placed in a separate block with the output of the handles in a place convenient for work.

The electro-hydraulic system has the following advantages: low effort on the control devices, the ability to remote control, high efficiency, low weight and low metal consumption due to a small number of wires. The disadvantage of this system is that when the mechanisms are suddenly switched on and stopped, significant dynamic loads occur. Electro-hydraulic control with proportional valves eliminates this disadvantage. For electrically driven machines electrical system management.

Drive control equipment is a system of devices from clutches, brakes, hydraulic valves, hydraulic distributors.

With a hydraulic control system for the working bodies of machines and their elements, all operations (lifting, lowering) are provided with the help of pumps, hydraulic distributors (control mechanisms), power actuating hydraulic cylinders, shut-off and safety valves and devices.

The hydraulic control system includes elements of the drive mechanism, consisting of one or more hydraulic pumps mounted either directly on the base machine engine and driven by it, or on a special power take-off gearbox, also driven by the base machine engine; elements of the control mechanism, consisting of a system of switchgears (one or more hydraulic distributors) installed, as a rule, in the driver's cab and designed to turn on and off certain actuators and a hydraulic servo system; elements of actuators and devices, consisting of hydraulic cylinders or hydraulic motors; elements of auxiliary devices, consisting of a tank for the working fluid, main filters, pipelines, locking devices (hydraulic valves, valves, plugs, etc.).

circuit diagram hydraulic system operation. From the tank, the working fluid flows through the suction pipeline to a gear or vane or other pump, which, as a result of the drive received directly from the engine of the base machine or a special gearbox, supplies it through the pipeline under pressure to switchgear(hydraulic distributor) and then also under pressure into one or another cavity of the executive hydraulic cylinder connected to one or another working body of the machine. When directing the working fluid into one or another cavity of the executive hydraulic cylinder, its rod, and with it the system of levers, actuates the working or other body of the machine, raising or lowering it or moving it to one side or the other.

In the hydraulic drive of machines, the rotational movement of the motor shaft turns into the rotational movement of the pump shaft, and the rotation of the latter turns into the translational movement of the piston of the power hydraulic cylinder and then through the hydraulic cylinder rod is transmitted to the executive working bodies.

From the hydraulic tank, through the suction pipeline, the working fluid enters the pump, which pumps it through the pressure line to the pump cavity of the hydraulic distributor. After that, the operation of the hydraulic drive depends on the position in which the handle and the hydraulic distributor spool associated with it will be placed.

The hydraulic distributor consists of a body placed in the axial hole of the spool body and a handle.

The axial opening of the hydraulic distributor housing is equipped with special branch cavities. The cavity connects the hydraulic distributor with the pump, the cavities and supply the working fluid to the hydraulic cylinder, and the drain cavities connect the hydraulic distributor with the hydraulic tank.

In position I, the spool bands block the access of the working fluid from the cavity to the cavity k, and also drain from them through the cavity i. In the case under consideration, the working fluid in the hydraulic cylinder is locked and the controlled element of the working equipment is stationary (is in the neutral position). Subsequently, the working fluid, flowing from the pump to the hydraulic distributor, increases the pressure in the pressure hydraulic line and, having overcome the resistance of the spring of the overflow valve 11 built into the hydraulic distributor through the channels, it drains back into the hydraulic tank.

In position II, when the spool is in the lower part of the axial bore of the hydraulic spool, the cavity is connected to the cavity of the hydraulic cylinder, and the cavity of the hydraulic cylinder is connected to the cavity. Then the piston of the hydraulic cylinder will move to the upper position.

In position III, when spool 6 is in the upper part of the axial bore of the hydraulic spool, the direction of the flow of the working fluid drain will change to the opposite, respectively, the hydraulic cylinder piston will move in the opposite direction.

When the spool is in the fully lowered position b (position IV), the cavity is isolated from both cavities and the hydraulic cylinder, which at this time are connected to the drain cavities. Thus, when exposed external load from the working equipment, the piston (respectively, the rod) of the hydraulic cylinder moves, freely pumping the working fluid contained in it from one cavity to another. This position is called "floating". It is used when moving working machines, when a machine, such as a bulldozer or a scraper, transports the collected soil without deepening the working body into the ground.

In hydraulic drives, mineral oils are used as a working fluid, which are selected depending on the operating conditions of the hydraulic system (summer or winter, climatic features, etc.).

2.Maintenance

In modern road-building machines, the hydraulic drive operates at high pressures, reaching up to 20-40 MPa. At the same time, during operation, the temperature of the working fluids of hydraulic systems ranges from -60 to + 100 ° C. Therefore, to ensure the necessary performance, working fluids must meet the basic requirements: the viscosity should change as little as possible with temperature fluctuations from -50 to + 50 ° C and there should be as little mechanical impurities as possible (as this leads to blockage of the oil-conducting paths) and aggressive substances; working fluids should not cause swelling of rubber products (seals, gaskets, etc.).

According to the principle of operation, hydraulic actuators are divided into two types - hydrostatic and hydrodynamic.

The hydrostatic drive consists of a pump as a driving link, receiving movement from the motor shaft or some intermediate shaft (power take-off shaft, etc.). The pump, taking the working fluid from the hydraulic tank, supplies it through the pipeline to the hydraulic distributor and then through the hydraulic distributor to the executive (working) body of the machine. The working fluid, having worked in closed system hydraulic drive, enters the hydraulic tank and then, under the action of the pump, goes to the hydraulic distributor, etc.

The hydrodynamic drive consists of a pump wheel as a leading link that receives movement from the engine shaft or some intermediate shaft (power take-off shaft, etc.), which, taking the working fluid from the hydraulic tank, supplies it to the turbine wheel, filling it and setting it in rotation. , and with it the executive (working) body of the machine or some other (other) element of the machine, for example, running wheels. The working fluid, having worked out in a closed system of a hydrodynamic drive, enters the hydraulic tank and then, under the action of the pump wheel, is directed to the turbine wheel, etc.

A hydrodynamic transmission with two impellers (pump and turbine) is called a fluid coupling, and with three or more (pump, reactor and turbine) - a torque converter.

In road-building machines for the drive of working bodies, the hydrostatic system has a predominant distribution. This system provides the ability to use and maintain a relatively large number of posts, rigid connection with the executive (working) bodies, easy and quick reversal of the executive (working) bodies, independent arrangement of controls from other elements and hydraulic drive devices, simple and easy control of the hydraulic distributor levers.

The positive properties of the hydrostatic system, in particular, ensuring the rigidity of the connection with the elements of the executive (working) bodies of machines (due to the incompressibility of liquids), make it possible to forcefully move and hold the working bodies of machines and equipment (for example, to deepen the cutting elements of the working bodies into the ground and keep them in the required position). At the same time, the system has a number of disadvantages: a small movement of the mechanisms and elements of the executive (working) bodies; low translational speeds of movement of the elements of the working bodies (no more than 0.2 m/s); the need to use special working fluids for operation, which, depending on climatic conditions (summer, winter), often have to be changed in the system; laboriousness and complexity of adjustment, adjustment, maintenance of the system.

The main equipment used for the operation of hydraulic systems and hydraulic drives includes pumps, hydraulic distributors, valves, pressure regulators. hydraulic drive gear pump

Pumps used in hydraulic drives of road construction machines are divided into axial piston, gear and vane pumps.

The most widely used are gear and bladed. However, axial piston pumps, which have the ability to create the most high pressures in hydraulic systems (considering modern tendencies development of hydraulic drives, aimed at increasing the pressure in the hydraulic systems of machines), are widely used.

The gear pump consists of two mated gears placed in a housing. When these gears rotate, the working fluid captured (absorbed) by them from the chamber through the spaces (between the gear teeth, as well as between the gear teeth and the pump housing) is directed into the discharge cavity and further under pressure into the pipelines. The drive gear shaft protruding from the pump housing has splined threads, through which the pump is connected to the power take-off shaft or to the gearbox shaft. Gear pumps are reversible, i.e. these pumps can work both as pumps and as hydraulic motors.

Vane (vane) pump consists of a stator placed in a housing with an inner surface in a shape close to an ellipse. On this surface, rotating, blades-vanes slide, moving in the cavities of the rotor. The pump rotor, mounted on a splined shaft, rotates between two liners together with the blades-blades. Each of the liners has four holes (windows), evenly spaced around the circumference, of which two diametrically opposite ones are connected to the suction channels in the pump housing, and the other two to the discharge channels. During the rotation of the pump rotor, the blades-blades under the action of centrifugal force and pressure of the working fluid, moving in the grooves, are pressed against inner surface stator. When the rotor rotates, the space (volume) between the adjacent pair of blades-blades, as well as the rotor and the stator, changes due to the elliptical shape of the inner surface of the stator, as a result of which, with an increase in the above space (volume), the working fluid is sucked in, and with a decrease in the space (volume) - injection. Therefore, for one revolution of the pump shaft, the process of suction and discharge occurs twice, which is why vane pumps are called double-acting pumps. The opposite arrangement of the suction chambers (inlet 6) and discharge chambers (drain hole) helps to balance the pressure of the working fluid on the rotor, freeing the pump trunnions from one-sided radial loads.

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Hydraulic jack

This system (Figure 1) consists of a reservoir with liquid, a system of valves and rods, and is a Pascal hydraulic lever. Moving the small rod (pump) down causes the large rod (lift cylinder) to be lifted up with a load. Since the pressure under the small and large rods is the same, but the areas of the rods (on which this pressure acts) are different, in accordance with Pascal's law, with a small force on the pump rod, a much greater force is achieved on the lifting cylinder.

Figure 1 shows the intake stroke at the top. The outlet check valve closes under load pressure and the suction check valve opens so that liquid from the reservoir fills the pumping chamber. IN bottom diagram Figure 1 pump plunger moves down. The inlet check valve closes under pressure and opens the outlet valve. A mass of fluid is pumped under the large piston to raise it. To lower the load, a third valve (needle valve) is provided in the system. When it is opened, the volume of liquid under the large piston communicates with the reservoir. The load pushes the large lift rod down and forces the liquid back into the reservoir.

up- intake stroke and load holding, at the bottom- tact of release and lifting of the load.

Figure 1 - Hydraulic jack

Reversible hydraulic motor

Figures 2 and 3 show a mechanically driven hydraulic pump and a hydraulic reversible rotary motor. A flow direction valve (reversing valve) directs fluid flow either to one side or the other of the motor and back to the tank. Thus, the possibility of working hydraulic motor With different direction rotation (reversibility) Safety valve protects the system from overpressure and can create a bypass to exit the fluid flow from the pump back to the tank if the pressure rises too high.

Figure 2 - Reversible hydraulic motor

Figure 3 - Reversible hydraulic motor (continued)

Open Center System

In this system, the control directional control valve must be opened in the center to allow oil flow to pass through the valve and return to the reservoir. Figure 4 shows this system in the neutral position. In order to handle multiple hydraulic functions at the same time, an open center system must have correct connections which are discussed below. The open center system is effective for single hydraulic functions and is limited to multiple functions.

Figure 4 - Hydraulic system with open center.

(1) Series connection. Figure 5 shows an open center system with hydraulic consumers/distributors connected in series. The oil flow from the pump is directed to three control valves in series. The center of each distributor is open in the neutral position to allow free flow of oil from the pump to the reservoir. The direction of oil flow is indicated by arrows. The flow from the outlet of the first valve is directed to the inlet of the second, and so on. When the control valve is operating, the incoming oil enters the cylinder, which is controlled by the corresponding control valve. The return fluid from the cylinder is directed through the return line and to the next valve.

Figure 5 - Open center hydraulic system with serial connection.

This system is only effective if one control valve is operating at the same time. When this happens, full oil flow and pump outlet pressure are available for this function. However, if more than one directional valve is in operation, the total amount of pressure and flow required for each function cannot exceed the system reset setting (reset valve setting).

2) Series-parallel connection. Figure 6 shows the change from a serial connection. Oil from the pump is directed through control valves in series as well as in parallel. Valves are sometimes "piled up" to provide additional flow passage. In the neutral position, fluid flows through the valves in sequence, as the arrows indicate. However, when either directional valve fires, the outlet on the operating valve is closed, but oil flow is made available to all other valves through a parallel connection.

Figure 6 - Open center hydraulic system with series-parallel connection.

When two or more valves operate at the same time, the cylinder that needs the least pressure will operate first, followed by the cylinder with the next lower pressure, and so on. This ability to operate two or more valves at the same time is an advantage over a series connection.

(3) Flow divider. Figure 7 shows an open center system with a flow divider. The flow divider receives the volume of oil from the pump and divides it between two functions. For example, the flow divider could be set to open the left side first in this case if both control valves were actuated at the same time. Or it can split the oil flow on both sides, equally or in different percentages. For such a split flow system, the pump must be powerful enough to control all functions at the same time. It must also supply liquid at maximum pressure to the most important of the hydraulic functions. And this means that a large number of horses are wasted when only one control valve is operating.

Figure 7 - Hydraulic system with open center and flow divider.

Closed center system

In this system, the pump can be idle (standby) when oil is not needed for the function to work. This means that the control valve (distributor) is closed in the center, stopping the flow of oil from the pump. Figure 8 shows schematically a closed center hydraulic system during the operation of the hydraulic function. In order for several functions to work simultaneously, the closed center hydraulic system has the following connections:

Figure 8 - Closed center hydraulic system.

(1) Pump with constant flowand battery. Figure 9 shows a closed center hydraulic system with an accumulator. This system has a small pump, but charges the battery at a constant volume. When the accumulator is charged to full pressure, the unloader valve diverts pump flow back to the reservoir. The check valve keeps oil under pressure in the circuit.

Figure 9 - Closed center hydraulic system with accumulator.

When the control valve is operated, the accumulator discharges its pressurized oil and drives the cylinder. As the pressure begins to drop, the unloader valve opens and directs the pump flow to the accumulator to recharge the flow. This system, using a small displacement pump, is effective when oil is needed only for a short period of time. However, when the hydraulic function needs a lot of oil for longer periods, an accumulator system may not be able to handle it unless the accumulator is very large.

(2) variable flow pump. Figure 10 shows a closed center hydraulic system with a variable displacement pump with the control valve in neutral. When the control valve is in the neutral position (center closed), oil is pumped in until the pressure rises to the set level. The pressure regulating valve allows the pump to turn itself off and maintain that pressure in the valve. The pump is in standby mode. The oil flow of the pump is close to zero (self-leakage in the pump is replenished), the pressure is equal to the settings of the pump standby pressure valve.

When the control valve is actuated (moves up), oil is diverted from the pump to the bottom of the cylinder cavity. The pressure drop caused by the communication between the pump pressure line and the lower cylinder cavity brings the pump from standby to operating mode to create oil flow and pressure on the bottom of the piston to lift the load.

Figure 10 - Closed center hydraulic system with variable flow pump.

At this time, the upper cavity of the cylinder is connected to the return line, which allows the oil to be pushed out of the piston to return to the reservoir or pump. When the control valve returns to the neutral position, the oil becomes locked on both sides of the cylinder, and the pressure supply from the pump to the hydraulic cylinder is tightly blocked. After this sequence, the pump goes back into standby mode. Moving the spool to the down position directs oil to the top of the piston cavity and causes the weight to move down. The oil from the bottom of the piston is sent to the return line to the reservoir.

Figure 11 shows the same closed center system, but with a booster pump (charging pump) that pumps oil from a reservoir to a variable flow pump. During operation of the feed pump, the necessary pressure is created for the main pump and the required amount of oil for it. All this makes the operation of the variable flow pump more efficient. The oil return from the operating hydraulic functions of the entire hydraulic system is directed directly to the inlet of the variable flow pump.

Figure 11 - Closed center hydraulic system with booster pump.

Because modern machines need more hydraulic power, a closed center hydraulic system is more advantageous. For example, on a tractor, oil may be required for power steering, brake booster, slave cylinders, three-point hitch, loader and other attachments. In most cases, each function requires a different amount of oil. In closed center systems, the amount of oil for each function can be set by line or valve size or by throttling with less internal heat generation compared to using flow dividers in a comparable open center system. Other advantages of the closed center system are:

Does not require unloading valves, as the pump simply turns off by itself when the standby pressure is reached. This prevents heat build-up in systems where relief pressure is often reached.
Has lines, valves and cylinders that can be adapted to the flow requirements of each function.
Oil flow reserve for full work and hydraulic system speed, available at low engine speeds per minute (RPM). More functions can be active at the same time.

Greater performance in some cases. For example, hydraulic functions such as brakes that require force but very little piston movement. By holding the valve open, in standby mode, pressure is constantly applied to the brake piston without loss of efficiency as the pump returns to standby mode.