P E T R O F O D S K I Y

STATE UNIVERSITY

FOREST ENGINEERING FACULTY

Department of Traction Machines

FOREST MACHINES

(Lecture notes. Part 2)

This abstract of lectures does not claim to be complete, therefore, for a complete study of individual issues, it is necessary to use the recommended literature (each issue is considered in detail in the classroom).

The summary outlines the purpose and place of forest (mobile) machines in logging production, the general and traction dynamics of wheeled and tracked vehicles (traction balance of cars and tractors, traction and speed characteristics and power balance, throughput, stability and general dynamics of forest machines.). The types of transmissions, their design and principle of operation (advantages and disadvantages), the requirements for them are considered; the elements of schemes of mechanical and hydraulic transmissions (clutches, gearboxes, transfer boxes, cardan and final drives, differential and its kinematics and statics, mechanisms for turning tracked vehicles, the basics of the theory of turning tracked (skidding) machines, determining the main parameters of turning and braking systems, steering elements, installation of steered wheels, etc., hydraulic clutch and torque converter diagrams, their characteristics).

In conclusion, brief information is given on the running systems of wheeled vehicles, suspensions of wheeled and tracked vehicles.

The abstract can be used in the study of the following disciplines:

"Theory and design of wheeled and tracked vehicles",

"Transmissions of mobile machines",

"Transmissions and control mechanisms of forest machines",

"Forest vehicles"

"Forest machines"

and can be useful for students and graduate students involved in traction calculations of wheeled and tracked vehicles in course and diploma design, the study of traction and coupling qualities, the basics of the theory of rotation, and other forestry and general-purpose machines.

The abstract was developed by the professor of the department "Traction machines"

M. I. Kulikov

INTRODUCTION

The leading place in the mechanization of timber industry work is increasingly occupied by forest machines. Forest machines are machines used in the forestry industry for transporting timber, which includes hauling (skidding) and hauling timber (wheeled and caterpillar tractors, logging vehicles, etc.). The basis for most forest machines are general-purpose vehicles and tractors (ZIL, MAZ, Ural, KamAZ, KRAZ, T-130, MTZ-82, etc.). There are a number of requirements for forest machines, the main of which are:

1. Compliance with the design of the machine to the operating conditions and ensure high performance.

2. High traction and dynamic qualities, high cross-country ability, good grip of the mover with the ground, high maneuverability, good adaptability for operation in various climatic conditions, etc.

3. Prospectiveness of the design, which makes it possible to modernize the original basic model for a long time.

4.High reliability and wear resistance of parts, components and assemblies, their unification.

5.High efficiency - minimal costs for fuel, spare parts, maintenance, etc.

In addition, additional requirements are imposed on forest vehicles: an increase in the trip load, an increase in the speed of movement and an improvement in cross-country ability.

The fulfillment of these requirements is usually achieved by increasing the engine power per ton of the mass of the road train and increasing its total load capacity. From year to year, the power of automobile engines and the carrying capacity of road trains are increasing (ZIL-131-110 kW-12.0 t; MAZ-509-132 kW-17.0 t; KRAZ-255 - 176 kW-23.0 t; KRAZ-260-220 kW-29 .0 t).

Improving the transmission and running systems play a leading role in increasing the average speed of the vehicle and increasing its cross-country ability. Skidding of the forest is carried out by special tractors - skidders, transporting wood in a semi-submerged position. In recent years, intensive development of new designs of special machines has been carried out.

For the first time skidders were created in the USSR - 1946. Mostly in logging operations, caterpillar machines are used, which have better cross-country ability than wheeled ones (most logging is carried out in areas with low soil bearing capacity). However, the advantages of a wheeled mover - high speeds, smooth running, etc., forced the designers to take the path of developing new wheeled vehicles with increased cross-country ability (TLK-4, TLK-6, ShLK, etc.).

An increase in productivity and traction and coupling qualities of caterpillar tractors is achieved by increasing the load capacity and engine power.

ENGINE TORQUE TRANSMISSION TO DRIVE

FOREST MACHINE WHEELS. TRANSMISSION EFFICIENCY

On modern cars and tractors, both foreign and domestic, piston internal combustion engines are used, in the development of which a tendency has been established to increase their speed. This results in their compactness and low weight. However, on the other hand, this leads to the fact that the torque on the shaft of these engines is much less than the torque that must be supplied to the driving wheels of the machine, despite the relatively high power of these engines. Therefore, in order to obtain the necessary torque on the driving wheels, it is necessary to introduce an additional device into the system - "engine - driving wheels", which provides not only the transmission of engine torque, but also its increase. The role of this device on modern cars and tractors is performed by the transmission. The transmission includes a number of mechanisms: clutch, gearbox, cardan, main, final (onboard) gear, turning mechanisms, and additional gearboxes (transfer boxes) that set a constant gear ratio. The torque from the engine is transmitted to the gearbox through clutches. On modern machines, friction clutches are the main distribution. The ratio of the friction torque of the clutch M m to the rated torque of the engine Me is called the safety factor of the clutch β:

β=M m / M e (1)

The value of this coefficient varies in a wide range (1.5 - 3.8) for trucks and tractors and is selected from the conditions of the value of friction work during slipping during the acceleration of the tractor unit, as well as protection from damage to engine and transmission parts in case of possible overloads.

When choosing the coefficient β, one also takes into account a possible change in the coefficient of friction of the clutch discs, a decrease in the pressure force of the springs due to wear of the friction surfaces, etc. From the clutch, the torque is transmitted through the gearbox and other transmission elements to the drive wheels. In the absence of slipping between the driving and driven clutch discs (δ clutch \u003d 0), the gear ratio of the transmission is generally determined: i tr \u003d ω e / ω k \u003d n e / n k, (2)

where ω e and n e are, respectively, the angular velocity and rotation frequency crankshaft engine;

ω to and n to - respectively, the angular velocity and frequency of rotation of the drive wheels.

Equality (2) can be represented as:

i tr =i to ∙i rk ∙i ch ∙ii kp = i to ∙i rk ∙i o, (2΄)

where i k is the gear ratio of the gearbox;

i rk - gear ratio of the transfer case;

i ch - gear ratio of the main (central) gear;

i - gear ratio of the turning mechanism;

i kp - gear ratio of the final (onboard) gear;

i o - constant gear ratio implemented in the main, turning mechanism, and final gears, as well as in other transmission gearboxes.

The torque on the driving wheels of the machine is determined by:

M to =M e ∙i tr ∙η tr, (3)

η tr - transmission efficiency, which is determined from the ratio:

η tr =N to /N e =(N e - N tr)/N e =1-(N tr / N e) , (4)

where N to - the power supplied to the drive wheels;

N tr - power lost in the transmission.

The transmission efficiency η tr takes into account the mechanical losses that occur in bearings, gearbox gears, central and final drives and losses during oil agitation. Transmission efficiency is usually determined experimentally. It depends on the type of transmission design, the quality of manufacture and assembly, on the degree of loading, oil viscosity, etc. The efficiency of modern automobile and tractor transmissions at nominal operation is within 0.8..0.93 and depends on the number of pairs of gears connected in series η kp =0.97..0.98; η c.p. =0.975..0.990.

In accordance with this, the value of η tr can be approximately calculated:

η tr = η c.p. ∙η kp (4΄)

Excluding no-load losses:

η cold \u003d 1-M cold / M e, (5)

where M cold is the moment of resistance reduced to the input shaft of the transmission, which occurs when the transmission is idling.

m c, m To - the number of pairs of cylindrical and bevel gears, respectively.

Wheel rolling radii

A car (tractor) moves as a result of the action of various forces on it, which are divided into driving forces and forces of resistance to movement. The main driving force is the traction force applied to the drive wheels. Traction is generated by the operation of the engine and is caused by the interaction of the drive wheels with the road. The tractive force P to is defined as the ratio of the torque on the axle shafts to the radius of the driving wheels with a uniform movement of the car. Therefore, to determine the traction force, it is necessary to know the radius of the drive wheel. Since elastic pneumatic tires are installed on the wheels of the car, the radius of the wheel changes during movement. In this regard, the following wheel radii are distinguished:

1. Nominal - the radius of the wheel in the free state: r n \u003d d / 2 + H, (6)

where d is the rim diameter (tire diameter), m;

H is the total height of the tire profile, m.

2. Static r s is the distance from the road surface to the axis of the loaded stationary wheel.

r с =(d/2+H)∙λ , (7)

where λ is the tire radial deformation coefficient.

3. Dynamic r d is the distance from the road surface to the axis of the rolling loaded wheel. This radius increases with a decrease in the perceived load of the wheel G k and an increase in the internal air pressure in the tire p w.

With an increase in the speed of the car under the action of centrifugal forces, the tire is stretched in the radial direction, as a result of which the radius r d increases. When the wheel is rolling, the deformation of the rolling surface also changes in comparison with a stationary wheel. Therefore, the shoulder of the application of the resultant tangential reactions of the road r d differs from r s. However, as experiments have shown, for practical traction calculations, r s ~ r d can be taken.

The kinematic (rolling) radius of the wheel r k is the radius of such a conditional non-deformable ring, which has the same angular and linear speeds with a given elastic wheel.

At a wheel rolling under the action of a torque, the tread elements that come into contact with the road are compressed, and the wheel travels a shorter distance at equal speeds than during free rolling; for a wheel loaded with braking torque, the tread elements that come into contact with the road are stretched. Therefore, at equal speeds, the brake wheel travels a slightly longer distance than a freely rolling wheel. Thus, under the action of torque, the radius r to - decreases, and under the action of braking torque - increases. To determine the value of rk by the method of “chalk prints”, a transverse line is applied on the road with chalk or paint, on which the car wheel rolls, and then leaves prints on the road.

Measuring the distance l between the extreme prints, determine the rolling radius by the formula: r to = l / 2π∙n , (8)

where n is the wheel speed corresponding to the distance l .

In the event of complete wheel slip, the distance l = 0 and radius r to = 0. During the sliding of non-rotating wheels (“SW”), the rotational speed n=0 and r to
.

To select tires and determine the wheel rolling radii by their dimensions, it is necessary to know the load distribution over the axles.

In passenger cars, the distribution of the load from the total mass over the axles depends mainly on the layout. With the classic layout, the rear axle accounts for 52 ... 55% of the load of the total mass, for front-wheel drive vehicles 48%.

Wheel rolling radius r to is selected depending on the load on one wheel. The maximum load on the wheel is determined by the position of the center of gravity of the car, which is set according to a preliminary sketch or prototype of the car.

Therefore, the load on each wheel of the front and rear axles of the car, respectively, can be determined by the formulas:

P 1 = G 1 / 2, (6)

P 2 = G 2 / 2. (7)

where G 1 , G 2 - loads from the total mass on the front and rear axles of the vehicle, respectively.

The distance from the front axle to the center of mass is found by the formula:

a=G 2 *L/G a , (8)

where G a - the module of gravity of the car (N);

L is the base of the car.

Distance from center of mass to rear axle

We select tires based on the load on each wheel according to Table 1.

Table 1 - Car tires

For example: 165-13 / 6.45-13 with a maximum load of 4250 N, 165 and 6.45 - profile width mm and inches, respectively, rim diameter 13 inches. By these dimensions, you can determine the radius of the wheel, which is in a free state

r c = + b, (10)

where b is the width of the tire profile (mm);

d - tire rim diameter (mm), (1 inch = 25.4 mm)

The rolling radius of the wheel r to is determined taking into account the deformation, depending on the load

r k \u003d 0.5 * d + (1 - k) * b, (11)

where k is the coefficient of radial deformation. For standard and wide profile tires, k is 0.1 ... 0.16.

Calculation of the external characteristics of the engine

The calculation begins with determining the power N ev required to ensure movement at a given maximum speed V max .

With the steady movement of the car, the engine power, depending on road conditions, can be expressed by the following formula (kW):

N ev = V max * (G a * + K in * F * V ) / (1000 * * K p), (12)

where - the coefficient of total road resistance for cars is determined by the formula:

0.01 + 5 * 10 -6 * V. (13)

K in - streamlining coefficient, K in \u003d 0.3 N * s 2 * m -4;

F is the frontal area of ​​the car, m 2;

transmission efficiency;

K p – correction factor.

Coefficient of total road resistance for trucks and road trains

\u003d (0.015 + 0.02) + 6 * 10 -6 * V . (14)

The frontal area for cars is found from the formula:

F A \u003d 0.8 * B g * H g, (15)

where B g is the overall width;

H g - overall height.

Frontal area for trucks

F A \u003d B * H g, (16)

Engine speed

The engine speed n v corresponding to the maximum vehicle speed is determined from the equation (min -1):

nv = Vmax * , (17)

where is the engine speed factor.

For existing passenger cars, the engine speed ratio lies in the aisles of 30 ... 35, for trucks with a carburetor engine - 35 ... 45; for trucks with a diesel engine - 30 ... 35.

According to this Rule, additional indices of speed and their bearing capacity are introduced into the marking of automobile tires. Some indexes of speed and bearing capacity of car tires are presented in the table below.

Some indexes of speed and bearing capacity of car tires:

Examples of tire designation according to UNECE Regulation 30:

175 / 80R16Q88 - tires for the Niva;

175 / 80R16СN106 - tires for the Gazelle.

Free wheel radius

free radiusr 0 is the radius of the wheel in the free (not loaded) state. For example, for a low profile tire type 205/70-14 78 S(tire designation is given according to UNECE Regulation 30) this radius is found as:

r 0 = 0,5d+H= 0,5d+IN(N/A)10 -2; (100×N/V) – tire series; 1 inch equals 25.4 mm, that is:

r 0 = (0.5×14×25.4 + 205×0.7)×10 –3 = (177.8 + 143.5)×10 –3 = 0.321 m.

Static wheel radius

One of the determining factors when calculating the performance properties of a car is the value from the center of the wheel to the bearing surface of a stationary wheel loaded with a normal load (the weight of a stationary vehicle). Strictly speaking, given that the tire is elastic and deforms when a load is applied, this value is the distance from the center of the wheel to the chord, however, in the theory of the automobile, this value is usually called the static radius ( r st). In the technical data, the static radius is often not given, but the tire marking is indicated instead. Obviously, if we denote the diameter of the rim - d, tire profile width - B, the percentage of the height of the tire profile to its width (tire series) - P, the outer diameter of the tire - D, then the static radius is defined as:

For toroid tires:

;

For low profile tires:

;

For wide profile tires

.

Here: - coefficient of radial deformation of the tire. For passenger car tires with an internal pressure in the range of 0.15 - 0.25 MPa as a first approximation, one can take = 0.15, for truck tires with an internal pressure of 0.5 MPa = 0,1.

Pneumatic tire properties

Pneumatic tires are widely used due to their shock-absorbing properties. They greatly soften the bumps from the bumps in the road.

The physical and mechanical properties of the tire determine such performance indicators of the car as carrying capacity, economy, handling, cross-country ability, etc. Ultimately, all these indicators are determined by the value and type of tire deformation under the action of external forces.

There are four types of deformations of a pneumatic tire: radial (normal), circumferential (tangential), transverse (lateral) and angular.

Radial deformation of the tire measured by its normal deflection h n, equal to the difference of the free (r 0 ) and static ( r st) wheel radii:

h n =r 0 –r Art.

Under the action of a static vertical load (the weight of a stationary vehicle), as a result of deformation of the elastic structure of the tire, the distance from the wheel axis to the supporting surface decreases.

normal deflection- one of the most important characteristics of the tire, which determines its load capacity and smooth running. With an increase in deflection, the stresses in the structural elements of the tire increase, and the fatigue strength and service life decrease. The highest permissible value of the normal load, at which, despite the radial deformation, a given tire life is provided at a given air pressure in it, it is customary to call the tire load capacity. The value of the normal load is regulated by GOST or Rules 30 of the UNECE (for foreign-made vehicles).

The type and parameters of driving wheels for cars are selected (table 1) in accordance with the normal load on them. The standard provides for several allowable loads on the tire, depending on the air pressure in it. When choosing a tire for the calculated machine, you must be guided by the following rule. The normal load on the tire obtained by calculation should not exceed the maximum allowable according to the standard at the lowest air pressure in it from among the values ​​provided for by the standard.

When determining the load on the drive wheel, it is necessary to provide for the maximum possible load in the operation of the machine, taking into account its technological purpose.

With a uniform static distribution of the vehicle weight along the axles, the maximum load on one wheel should be determined based on its possible redistribution in operation. In this case, the load on the drive wheel from the gravity of the vehicle and the transported cargo, as well as from the vertical component of the traction force on the trailer hitch, is taken into account.

The parameters of the selected tire are compared with the type and parameters of the drive wheels of the prototype machine. When comparing the parameters of the selected wheel and the wheel of the prototype, it should be borne in mind that manufacturers of trucks sometimes use an increased tire size (if the requirements for the car allow it). “Oversized” tires are more durable, put less pressure on the ground and give the machine more traction. The use of such tires is most appropriate for trucks operating on dirt roads or roads with poor coverage.

Table 1.

Parameters of car tires (GOST 7463-89)

Normal tire deflection h n due to its deformation not only in the radial, but also in the circumferential and transverse directions. At the same time, 40% of the total tire compression load is spent on the deformation of its material and 60% on air compression.

Distinguish low, medium and high pressure tires. Tires of low pressure have the increased volume of air, smaller number of cord layers. They are softer to take shocks from bumps in the road and have better shock-absorbing properties, but with a lower carrying capacity. For low and medium pressure tires, the permissible normal deformation of the tire is 15 ... 20% of its height, and for high pressure tires - 10 ... 12%.

A car (tractor) moves as a result of the action of various forces on it, which are divided into driving forces and forces of resistance to movement. The main driving force is the traction force applied to the drive wheels. Traction is generated by the operation of the engine and is caused by the interaction of the drive wheels with the road. Traction force P to is defined as the ratio of the moment on the axle shafts to the radius of the driving wheels with a uniform movement of the car. Therefore, to determine the traction force, it is necessary to know the radius of the drive wheel. Since elastic pneumatic tires are installed on the wheels of the car, the radius of the wheel changes during movement. In this regard, the following wheel radii are distinguished:

1. Nominal - the radius of the wheel in the free state: r n \u003d d / 2 + H, (6)

where d is the rim diameter, m;

H is the total height of the tire profile, m.

2. Static r s is the distance from the road surface to the axis of the loaded stationary wheel.

r с =(d/2+H)∙λ , (7)

where λ is the tire radial deformation coefficient.

3. Dynamic r d is the distance from the road surface to the axis of the rolling loaded wheel. This radius increases with a decrease in the perceived load of the wheel G k and an increase in the internal air pressure in the tire p w.

With an increase in the speed of the car under the action of centrifugal forces, the tire is stretched in the radial direction, as a result of which the radius r d increases. When the wheel is rolling, the deformation of the rolling surface also changes in comparison with a stationary wheel. Therefore, the shoulder of the application of the resultant tangential reactions of the road r d differs from r s. However, as experiments have shown, for practical traction calculations, r s ~ r d can be taken.

4 Kinematic radius (rolling) of the wheel r k - the radius of such a conditional non-deformable ring, which has the same angular and linear speeds with a given elastic wheel.

At a wheel rolling under the action of a torque, the tread elements that come into contact with the road are compressed, and the wheel travels a shorter distance at equal speeds than during free rolling; for a wheel loaded with braking torque, the tread elements that come into contact with the road are stretched. Therefore, at equal speeds, the brake wheel travels a slightly longer distance than a freely rolling wheel. Thus, under the action of torque, the radius r to - decreases, and under the action of braking torque - increases. To determine the value of rk by the method of “chalk prints”, a transverse line is applied on the road with chalk or paint, on which the car wheel rolls, and then leaves prints on the road.

Measuring the distance l between the extreme prints, determine the rolling radius by the formula: r to = l / 2π∙n , (8)

where n is the wheel rotation frequency corresponding to the distance l .

In the event of complete wheel slip, the distance l = 0 and radius r to = 0. During the sliding of non-rotating wheels (“SW”) rotation frequency n=0 and r to .

Due to the large variety of types of deformation of a pneumatic tire, its radius does not have one specific value, as in a wheel with a rigid rim.

There are the following rolling radii of a wheel with a pneumatic tire: free g 0 , static r cv dynamic g a and kinematic g k.

free radius g 0- this is the largest radius of the treadmill of the wheel, free from external load. It is equal to the distance from the surface of the treadmill to the axis of the wheel.

The static radius r st is the distance from the axis of a stationary wheel loaded with a normal load to the plane of its support. The values ​​of the static radius at maximum load are regulated by the standard for each tire.

dynamic radius g i- this is the distance from the axis of the moving wheel to the point of application of the resulting elementary soil reactions acting on the wheel.

Static and dynamic radii decrease with increasing normal load and decreasing tire pressure. The dependence of the dynamic radius on the moment load, obtained experimentally by E.A. Chudakov, shown in Fig. 9, A, schedule 1. It can be seen from the figure that with increasing torque M wea, transmitted by the wheel, its dynamic radius is reduced. This is because the vertical distance between the wheel axle and its bearing surface is reduced due to the twisting deformation of the tire sidewall. In addition, under the action of torque, not only a tangential force arises, but also a normal component, which tends to press the wheel to the road surface.

Rice. 9. Dependencies obtained by E.A. Chudakov: a - change in dynamic (I and kinematic ( 2) wheel radii depending on the driving moment: b - change in the kinematic radius of the wheel under the action of driving and braking torques

The value of the dynamic radius also depends on the depth of the track when driving on deformable ground or soil. The deeper the track, the smaller the dynamic radius. The dynamic wheel radius is the application arm of the tangential reaction of the soil pushing the drive wheel. Therefore, the dynamic radius is also called the force radius.

The kinematic radius or rolling radius of the wheel is divided by 2k the actual distance traveled by the wheel in one revolution. The kinematic radius is also defined as the radius of such a fictitious wheel with a rigid rim, which, in the absence of slipping and slipping, has the same angular rotation speed and translational speed as the actual wheel:

where v K is the forward rolling speed of the wheel; wk - angular speed of rotation of the wheel; S K- the path of the wheel in one revolution, taking into account slipping or slipping.

From expression (5) it follows that with full wheel slip (v K = 0) the radius g to= 0, and with full slip (co k = 0) the kinematic radius is equal to ©o.

On fig. 9, A(schedule 2) presented received by E.A. Chudakov, the dependence of the change in the kinematic radius of the wheel on the action of the torque M ved on it. It follows from the figure that the magnitude of the change in the dynamic and kinematic radii is different depending on the action of the moment. The steeper dependence of the kinematic radius of the wheel compared to the dependence of the dynamic radius can be explained by the action of two factors on it. First, the kinematic radius is reduced by the same amount that the dynamic radius is reduced by the driving moment, as shown in Fig. 9, I, schedule /. Secondly, the driving or braking torque applied to the tire causes a deformation of compression or tension of the rolling part of the tire. The processes accompanying these deformations can be easily traced if the wheel is represented as a cylindrical elastic spiral with a uniform winding of coils. As shown in fig. 10, a, under the action of the driving moment, the running part of the tire (front) is compressed, as a result of which the total perimeter of the tire tread circumference decreases, the wheel path S K decreases with one turn. The greater the compression deformation of the tire in the running part, the greater the reduction in path S K , which, in accordance with (5), is proportional to the decrease in the kinematic radius g k.

When braking torque is applied, the opposite occurs. Tensile tire elements approach the bearing surface

(Fig. 10, b). Tire circumference and wheel path S K , passed in one revolution, increase as the braking torque increases. Therefore, the kinematic radius increases.

Rice. 10. Scheme of tire deformation from the action of moments M ved (a) and M t(b)

On fig. 9, b the dependence of the change in the radius of the wheel on the action of the torsional active L / ved and the brake M 1 moments with stable adhesion of the wheel to the supporting surface. E.A. Chudakov proposed the following formula for determining the radius of a wheel:

where r to 0 is the rolling radius of the wheel in the free rolling mode, when the driving moment and the rolling resistance moment are equal to each other; A, m - coefficient of tangential elasticity of the tire, depending on its type and design, which is found from the results of experiments.

In engineering calculations, the static radius of a given tire given in the standard at a given air pressure and maximum load on it is usually used as dynamic and kinematic radii. It is assumed that the wheel moves on an indestructible surface.

When driving on a track, the static radius is the distance from the wheel axle to the bottom of the track. However, when the wheel moves along the track, the point of application of the resultant elementary reactions of the soil, which forms the torque (leading or resistance), will be above the bottom of the track and below the soil surface (see Fig. 17). The dynamic radius in this case depends on the depth of the track: the deeper it is, the greater the difference between the static and dynamic radii of the wheels, the greater the calculation error from the assumption g l = g st