External forces in strength of materials are divided into active And reactive(connection reactions). Loads are active external forces.
Loads by application method
According to the method of application, loads can be volumetric (own weight, inertial forces), acting on each infinitesimal element of volume, and surface. Surface Loads are divided into concentrated loads And distributed loads.
Distributed Loads characterized by pressure - the ratio of the force acting on a surface element normal to it to the area of this element and are expressed in International system units (SI) in pascals, megapascals (1 PA = 1 N/m2; 1 MPa = 106 Pa), etc., and in technical system– in kilograms of force per square millimeter, etc. (kgf/mm2, kgf/cm2).
In compromising materials, they are often considered surface loads, distributed along the length of the structural element. Such loads are characterized by intensity, usually denoted q and expressed in newtons per meter (N/m, kN/m) or in kilograms of force per meter (kgf/m, kgf/cm), etc.
Loads according to the nature of changes over time
Based on the nature of changes over time, they are distinguished static loads- increasing slowly from zero to its final value and then not changing; And dynamic loads causing big
Impacts experienced by the stand from the hand bent (see Fig. 42), the board from the load (see Fig. 44), the cylindrical rod of the bolt when screwing the nut wrench(see Fig. 45), etc., represent external forces or loads. The forces arising in the places where the rack is secured and the board is supported are called reactions.
Rice. 42
Rice. 44
Rice. 45
According to the method of application, loads are divided into concentrated and distributed (Fig. 49).
Types and classification of loads:
Concentrated loads transmit their effect through very small areas. Examples of such loads are the pressure of the wheels of a railway car on the rails, the pressure of a hoist trolley on a monorail, etc.
Distributed Loads operate over a relatively large area. For example, the weight of the machine is transmitted through the frame to the entire area of contact with the foundation.
Based on the duration of action, it is customary to distinguish between constant and variable loads. An example of a constant load is the pressure of a plain bearing - the support of shafts and axles - and its own weight on the bracket.
Variable load It is mainly the parts of periodic action mechanisms that are affected. One such mechanism is a gear transmission, in which the teeth are in the contact zone of adjacent pairs gear wheels experience variable load.
By the nature of the action loads may be static And dynamic. Static loads remain almost unchanged during the entire operation of the structure (for example, the pressure of trusses on supports).
Dynamic loads and last for a short time. Their occurrence is associated in most cases with the presence of significant accelerations and inertial forces.
Dynamic loads are experienced by parts of impact machines, such as presses, hammers, etc. Parts of crank mechanisms also experience significant dynamic loads during operation from changes in the magnitude and direction of speeds, that is, the presence of accelerations.
Strength of materials. Main tasks of the section. Classification of loads.
The science of the strength and deformability of a material.
Tasks.
A) Calculation of strength: strength is the ability of a material to resist loads and destruction;
B) Calculation of rigidity: rigidity is the ability of a material to resist deformation;
C) Calculation of stability: stability is the ability to maintain a stable balance.
Classification of loads.
During operation, structures and structures perceive and transmit loads (forces).
The forces can be:
A) Volumetric (gravity, inertia, etc.);
B) Surface (surface water, water pressure);
Surface loads are:
Focused
Distributed Loads
Depending on the nature of the load:
A) static – constant in value or slowly increasing;
B) dynamic - rapidly changing loads or shock;
C) re-variable load - loads that change over time.
Calculation schemes. Hypotheses and assumptions.
They simplify calculations.
Calculation schemes.
Design diagrams are a part that is subject to calculations for strength, rigidity, and stability.
All the variety of parts designs comes down to 3 design diagrams:
A) Beam - a body in which one of the dimensions is larger than the other 2 (beam, log, rail);
B) Shell - a body in which one of the dimensions is smaller than the other two (rocket body, ship hull);
C) An array is a body in which all 3 sides are approximately equal (machine, house).
Assumptions.
A) All materials have a continuous structure;
B) The material of the part is homogeneous, i.e. has the same properties at all points material;
C) All materials are considered isotropic, i.e. they have in all directions identical properties;
D) The material has ideal elasticity, i.e. after removing the load, the body completely restores its shape and size.
Hypotheses.
A) Hypothesis of small movements.
The displacements that occur in the structure under the influence of external forces are very small, so they are neglected in the calculations.
B) Linear deformability assumptions.
Movement in structures is directly proportional to the acting loads.
Section method. Types of loads (deformations)
Section method.
Consider a loaded load external forces P1, P2, P3, P4. Let's apply the section method to the beam: cut it with a plane L into 2 equal parts, left and right. Let's discard the left one, leave the right one.
The right side - left - will be in equilibrium, because In the cross section, internal force factors (IFF) will arise, which balance the remaining part and replace the actions of the discarded part.
A) N – longitudinal force
B)Qx – shear force
B) Qy – shear force
D) Mz – torque
D) Mx – bending moment
E) My – bending moment.
Types of deformations (loads)
A) Tension, compression: such deformation in which only the longitudinal force N (spring, button accordion, self-phone) acts in the cross section;
B) Torsion - such a deformation in which only the torque Mz (shaft, gear, nut, spinning top) acts in the section;
B) Bending – deformation during which a bending moment Mx or My acts in the section (bending of a beam, bending of a balcony);
D) Shear is a deformation in which a transverse force Qx or Qy acts in the section (shear and crushing of the rivet).
The deformations considered are considered simple.
Complex view deformation.
Deformation in which 2 or more internal force factors act simultaneously in a section (combined actions of bending and torsion: a shaft with a gear).
Conclusion: the section method makes it possible to determine the VSF and the type of deformation. To assess the strength of a structure, the intensity of internal stress forces is determined.
Mechanical stress.
Mechanical stress- called the value of the internal force factor per cross-sectional area.
Tensile and compressive deformation. VSF, voltage.
Tension, compression deformation.
This is a deformation at which a longitudinal force N appears in the section. Example (spring, button accordion, cable).
Conclusion: Stretching– deformation in which the force is directed from the section, compression – towards section.
Voltage at R-S:
Conclusion: with R-S, normal stresses arise, i.e. they, like the longitudinal force N, are perpendicular to the section.
Calculations of tensile and compressive strength.
There are 3 strength calculations:
A) Strength test
B) Selection of section
B) Determination of permissible load
Conclusion: strength calculations are needed to predict destruction.
Hooke's law in tension and compression.
E – Young’s modulus (or elastic modulus).
E.I. like tension.
The Young's modulus for each material is different and is selected from reference material.
Normal voltage directly proportional to longitudinal deformation - Hooke's law .
Young's modulus characterizes the rigidity of a material under tension and compression.
Crumpling. Calculations for crushing.
If the thickness of the parts being connected is small, and the load acting on the connection is large, then a large mutual pressure arises between the surface of the parts being connected and the walls of the hole.
It is designated - Sigma see
As a result of this pressure, a rivet, bolt, screw... is wrinkled, the shape of the hole is distorted, and the tightness is broken.
Strength calculations.
Slice Shear calculations.
If 2 sheets of thickness S are connected to each other with rivets or a bolt, then shearing will occur along planes perpendicular to the axial lines of these parts.
Shear calculations.
Torsion. Pure shift. Hooke's law in torsion.
Torsion – deformation at which a torque Mz occurs in the cross section of the part (shaft, gear, worm).
Torsion can be achieved by pure shear of a thin-walled pipe.
On the faces of the selected element a,b,c,d, a shear stress τ(tau) arises – this is what characterizes pure shear .
In pure shear, a direct relationship has been established between the tangential stresses τ and the shear angle γ(gamma) – Hooke's law in torsion :τ=G*γ
G - shear modulus, characterizes the shear rigidity of the material.
Measured – MPa.
2) G=E*E(Young's modulus)
For the same material, there is a relationship between the shear modulus G and Young’s modulus (3).
The shear modulus is determined from the formula by calculation, taking values from the reference material.
Torsional stresses. Distribution of tangential stresses in a section.
Ws is the polar moment of resistance to the section.
The tangential stress is distributed in the section according to a linear law, tmax is located on the contour of the section, t=0 in the center of the section, all other t are between them.
Ws – for the simplest sections.
Torsional strength calculations.
Conclusion: Torsional strength calculations are necessary to predict failures.
Calculations for torsional stiffness.
Precise shafts are calculated for rigidity in order to lose spring accuracy.
Relative twist angle.
Both quantities can be measured in degrees or radians.
Bend. Types of bends. Examples of bends.
Bend – deformation at which the bending moment acts (Mx, My).
Examples : bend in a construction beam, desk, balcony.
Kinds :
Straight bend
Oblique bend
Classification of mechanical gears
- based on the principle of motion transmission: friction transmission and gear transmission; within each group there are transmissions by direct contact and transmissions by flexible communication;
- according to the relative position of the shafts: gears with parallel shafts (cylindrical, gears with intersecting shaft axes (bevel), gears with crossed shafts (worm, cylindrical with a screw tooth, hypoid);
- by the nature of the gear ratio: with a constant gear ratio and with a continuously variable gear ratio (variators).
Depending on the ratio of the parameters of the input and output shafts, transmissions are divided into:
-gearboxes(downshifts) - from the input shaft to the output shaft they reduce the rotation speed and increase the torque;
-animators(overdrive gears) - from the input shaft to the output shaft, the rotation speed is increased and the torque is reduced.
Friction gears
Friction transmission - a mechanical transmission used to transmit rotational motion (or to convert rotational motion into translational motion) between shafts using friction forces arising between rollers, cylinders or cones mounted on shafts and pressed against one another.
Friction transmissions are classified according to the following criteria:
1. By purpose:
With unregulated gear ratio (Fig.9.1-9.3);
With stepless (smooth) gear ratio control (variators).
2. According to the relative position of the shaft axes:
Cylindrical or conical with parallel axes (Fig. 9.1, 9.2);
Conical with intersecting axes (Fig. 9.3).
3. Depending on working conditions:
Open (run dry);
Closed (work in an oil bath).
4. Based on the operating principle:
Irreversible (Fig.9.1-9.3);
Reversible.
Advantages of friction gears:
Simplicity of design and maintenance;
Smooth transmission of motion and speed control and quiet operation;
Great kinematic capabilities (conversion of rotational motion into translational motion, stepless speed change, the ability to reverse on the move, switching gears on and off on the move without stopping);
Uniform rotation, which is convenient for devices;
Possibility of stepless regulation of the gear ratio, and on the move, without stopping the transmission.
Disadvantages of friction gears:
Inconstancy of the gear ratio due to slippage;
Low transmitted power (open transmissions - up to 10-20 kW; closed transmissions - up to 200-300 kW);
For open gears, the efficiency is relatively low;
Large and uneven wear of the rollers when slipping;
The need to use specially designed shaft supports with clamping devices (this makes the transmission cumbersome);
For power open gears, low peripheral speed (7 - 10 m/s);
Large loads on shafts and bearings due to downforce, which increases their size and makes the transmission cumbersome. This disadvantage limits the amount of power transmitted;
Large friction losses.
Application.
They are used in mechanical engineering relatively rarely, for example, in friction presses, hammers, winches, drilling equipment, etc. These gears are used primarily in devices where smooth and quiet operation is required (tape recorders, players, speedometers, etc.).
Transmission Screw-nut
The screw-nut transmission consists of : a screw and a nut in contact with screw surfaces. The screw-nut transmission is designed to convert rotational motion into translational motion.
There are two types of screw-nut gears:
Sliding friction transmissions or screw pairs sliding friction;
Rolling friction transmissions or ball screws. The driving element in the transmission is usually a screw, the driven element is a nut. In rolling screw-nut transmissions, helical grooves (threads) of a semicircular profile are made on the screw and in the nut, serving as raceways for the balls.
Depending on the purpose of the transmission, the screws are:
- cargo, used to create large axial forces.
- running gear, used for movements in feed mechanisms. To reduce friction losses, trapezoidal multi-start threads are mainly used.
- installation, used for precise movements and adjustments. Have metric thread. To ensure backlash-free transmission, the nuts are doubled.
Main advantages:
1.opportunity to get a big win in power;
2. high precision of movement and the ability to obtain slow movement;
3. smooth and quiet operation;
4. big load bearing capacity at small overall dimensions;
5. simplicity of design.
Disadvantages of screw-sliding nut gears:
1.high friction losses and low efficiency;
2. difficulty of use at high rotation speeds.
Application of screw-nut transmission
The most typical applications for screw-nut transmissions are:
Lifting loads (jacks);
Loading in testing machines;
Implementation of the working process in machine tools (screw processes);
Aircraft tail control (flaps, directional and altitude arms, landing gear release mechanisms and changes in wing sweep);
Movement of the robot's working parts;
Precise division movements (in measuring mechanisms and machine tools).
Gears
A mechanism in which two moving links are gears forming a rotational or translational pair with a fixed link is called gear transmission . The smaller of the transmission wheels is usually called a gear, and the larger one is a wheel, a gear link that makes rectilinear movement, is called a rack.
Classification:
- according to the relative position of the wheel axes: with parallel axes, with intersecting axes with crossed axes) with motion transformation
- by the location of the teeth relative to the forming wheels: straight teeth; helical; chevron; with a circular tooth;
- in the direction of oblique teeth there are: right and left.
- by design: open and closed;
- by number of steps: single-multi-stage;
Worm gears
Worm Gear (or Helical Gear)- a mechanism for transmitting rotation between shafts by means of a screw and an associated worm wheel. The worm and the worm wheel together form a higher gear-screw kinematic pair, and with the third, fixed link, lower rotational kinematic pairs.
Advantages:
· Smooth operation;
· Low noise;
· Self-braking - at certain gear ratios;
· Increased kinematic accuracy.
Flaws:
· Increased requirements for assembly accuracy, the need for precise adjustment;
· At some gear ratios, transmission of rotation is possible only in one direction - from the screw to the wheel. (for some mechanisms this may be considered an advantage).
· Relatively low efficiency (it is advisable to use at powers less than 100 kW)
· Large friction losses with heat generation, the need for special measures to intensify heat removal;
· Increased wear and tendency to seize.
Wormsare distinguished by the following characteristics:
According to the shape of the generating surface:
· cylindrical
· globoid
In the direction of the coil line:
By number of thread starts
· single-pass
· multi-pass
· according to the shape of the screw thread surface
· with Archimedean profile
· with convolution profile
· with involute profile
trapezoidal
Gearbox
Gearbox (mechanical)- a mechanism that transmits and converts torque, with one or more mechanical gears.
Main characteristics of the gearbox -Efficiency, gear ratio, transmitted power, maximum angular speeds of shafts, number of drive and driven shafts, type and number of gears and steps.
First of all, gearboxes are classified according to the types of mechanical transmissions : cylindrical, conical, worm, planetary, wave, spiroid and combined.
Gear housings : V serial production Standardized cast gearbox housings are widely used. Most often, in heavy industry and mechanical engineering, housings are made of cast iron, less often of cast steel.
Classification of gearboxes
- Worm gearboxes
- Helical gearboxes
- Classification of gearboxes depending on the type of gears and the number of stages
Belt drives
Device and purpose
Belting refers to transmissions friction with flexible connection and can be used to transmit motion between shafts located at a considerable distance from one another. It consists of two pulleys (driver, driven) and an endless belt covering them, put on with tension. The drive pulley forces the friction forces that arise on the surface of contact between the pulley and the belt due to its tension, causing the belt to move. The belt, in turn, causes the driven pulley to rotate.
Application area
Belt drives are used to drive units from electric motors of low and medium power; for drive from low-power motors internal combustion.
Chain transmissions
Chain transmissions - these are transfers engagement And flexible connection, consisting of a driving and driven sprocket and a chain enclosing them. The transmission also often includes tensioning and lubrication devices and guards.
Advantages:
1. possibility of application in a significant range of interaxle distances;
2. smaller dimensions than belt drives;
3. no slippage;
4. high efficiency;
5. relatively small forces acting on the shafts;
6. the ability to transfer movement to several sprockets;
7. Possibility of easy chain replacement.
Flaws:
1. the inevitability of wear of chain joints due to the lack of conditions for fluid friction;
2. variability of chain speed, especially with a small number of sprocket teeth;
3. the need for more precise installation of shafts than for V-belt transmission;
4. the need for lubrication and adjustment.
Chains by appointment divided into three groups:
1. cargo – used to secure cargo;
2. traction – used to move goods in continuous transport machines (conveyors, elevators, escalators, etc.);
3. drive – used to transmit movement.
Application: Gears are used in agricultural, material handling, textile and printing machines, motorcycles, bicycles, cars, and oil drilling equipment.
Mechanisms
Mechanism- the internal structure of a machine, device, apparatus that puts them into action. Mechanisms serve to transmit motion and convert energy (gearbox, pump, electric motor).
The mechanism consists of 3 groups of links:
1. Fixed links - racks
2. Drive links - transmits movement
3. Driven links - perceive movements
Classification of mechanisms:
1. Lever mechanisms: crank mechanism - crank (rotational movements), connecting rod (calibrating), slider (translational).
Application: Piston pumps, steam engines.
Shafts and axles
In modern machines, rotational movement of parts is most widely used. Less common is translational motion and its combination with rotational motion (helical motion). The movement of progressively moving machine parts is ensured special devices, called guides. To carry out rotational movement, special parts are used - shafts and axles, which with their specially adapted sections - pins (spikes) or heels – rest on supporting devices called bearings or thrust bearings.
They call it a shaft a part (usually a smooth or stepped cylindrical shape) designed to support pulleys, gears, sprockets, rollers, etc. mounted on it, and to transmit torque.
During operation, the shaft experiences bending and torsion, and in some cases, in addition to bending and torsion, shafts may experience tensile (compression) deformation. Some shafts do not support rotating parts and work only in torsion (drive shafts of cars, rolls of rolling machines, etc.).
The axis is called a part intended only to support the parts installed on it.
Unlike the shaft, the axis does not transmit torque and only works on bending. In machines, the axles can be stationary or they can rotate together with the parts sitting on them (moving axles).
Lassification of shafts and axles
By purpose shafts are divided into:
Gear- carrying only various parts of mechanical transmissions (gears, belt pulleys, chain sprockets, couplings, etc.),
Indigenous- bearing the main working parts of machines (rotors of electric motors and turbines, connecting rod-piston complex of internal combustion engines and piston pumps), and, if necessary, additionally parts of mechanical transmissions (machine spindles, drive shafts of conveyors, etc.). The main shaft of machines with rotational movement of a tool or product is called spindle .
By geometric shape shafts are divided into: straight; crank; crank; flexible; telescopic; cardan shafts .
According to the manufacturing method, they are distinguished: solid and composite shafts.
By appearance cross sections Shaft sections distinguish between solid and hollow shafts with round and non-circular cross-sections.
Bearings
Bearing - An assembly unit that is part of a support or stop and supports a shaft, axle or other movable structure with a given rigidity. Fixes position in space, provides rotation, rolling or linear movement (for linear bearings) with the least resistance, absorbs and transmits the load from the moving unit to other parts of the structure.
Based on the principle of operation, all bearings can be divided into several types:
· rolling bearings;
· sliding bearings;
Rolling bearings
Represents a ready-made unit, the main elements of which are rolling bodies - balls or rollers, installed between the rings and held at a certain distance from each other.
Advantages:
1. Low cost due to mass production.
2. Low friction losses and low heating during operation.
3. Small axial dimensions.
4. Simplicity of design
Flaws:
1. Large radial dimensions.
2. There are no detachable connections.
Classification:
1. According to the shape of the rolling elements: ball, roller.
2. According to the direction of action: radial-thrust, thrust, thrust-radial.
3. According to the number of rolling elements: homogeneous, two-row, four-row.
4. According to the main design features: self-aligning, non-self-aligning.
Application: In mechanical engineering.
Plain bearings
Sliding bearing - consists of a housing, liners and lubricating devices. In their simplest form, they are a bushing (insert) built into the frame of the machine.
Lubrication is one of the basic conditions reliable operation bearing and provides low friction, separation of moving parts, heat dissipation, and protection from harmful environmental influences.
Lubrication can be:
- liquid(mineral and synthetic oils, water for non-metallic bearings),
- plastic(based on lithium soap and calcium sulfonate, etc.),
- hard(graphite, molybdenum disulfide, etc.) and
- gaseous(various inert gases, nitrogen, etc.).
Classification:
Sliding bearings are divided into:
depending on the shape of the bearing hole:
- single or multi-surface,
- with displacement of surfaces (in the direction of rotation) or without (to maintain the possibility of reverse rotation),
- with or without center offset (for final installation of shafts after installation);
in the direction of load perception:
- radial
- axial (thrust, thrust bearings),
- radial thrust;
by design:
- one-piece (sleeve; mainly for I-1),
- detachable (consisting of a body and a cover; basically for all except I-1),
- built-in (frame, integral with the crankcase, frame or frame of the machine);
by number of oil valves:
- with one valve,
- with several valves;
where possible regulation:
- unregulated,
- adjustable.
Advantages
- Reliability in high speed drives
- Capable of withstanding significant shock and vibration loads
- Relatively small radial dimensions
- Allows the installation of split bearings on crankshaft journals and does not require dismantling of other parts during repairs
- Simple design in slow-moving cars
- Allows you to work in water
- Allows adjustment of the gap and ensures precise installation of the geometric axis of the shaft
- Economical for large shaft diameters
Flaws
- Requires constant supervision of lubrication during operation
- Relatively large axial dimensions
- Large friction losses during startup and poor lubrication
- High consumption lubricant
- High requirements for temperature and lubricant cleanliness
- Reduced efficiency
- Uneven wear of bearing and journal
- Use of more expensive materials
Application: For oxen of large diameters; low-speed vehicles; Appliances.
coupling- a device (machine part) designed to connect the ends of shafts and parts freely sitting on them to each other to transmit torque. They are used to connect two shafts located on the same axis or at an angle to each other.
Classifications of couplings.
By type of management
· Controlled - coupling, automatic
· Uncontrollable - constantly operating.
Permanent connections.
Welded connections
Welded joint- permanent connection made by welding.
A welded joint includes three characteristic zones formed during welding: the weld zone, the fusion zone and the heat-affected zone, as well as the part of the metal adjacent to the heat-affected zone.
Zones of the welded joint: the lightest is the base metal zone, the darker is the heat-affected zone, the darkest area in the center is the weld zone. Between the heat-affected zone and the weld zone there is a melting zone.
Weld seam- a section of a welded joint formed as a result of crystallization of molten metal or as a result of plastic deformation during pressure welding or a combination of crystallization and deformation.
Weld metal- an alloy formed by molten base and deposited metals or only remelted base metal.
Base metal- metal of the parts being welded.
Fusion zone- zone of partially fused grains at the border of the base metal and weld metal.
Heat Affected Zone- a section of the base metal that has not undergone melting, the structure and properties of which have changed as a result of heating during welding or surfacing.
Adhesive connections.
Adhesive joints are increasingly used in connection with the development of high-quality synthetic adhesives. The most widely used adhesive lap joints are shear joints. If it is necessary to obtain particularly strong connections, I use combined connections: glued screws, adhesive rivets, glue-welded ones.
Areas of application of adhesives.
The largest consumers of adhesive materials are the woodworking industry, construction, light industry, mechanical engineering, aviation industry, shipbuilding, etc.
Adhesives are used in communication, signaling and power supply devices.
Combined connections: glue-welded, glue-threaded, adhesive-riveted - significantly improve specifications parts and mechanisms, provide high strength and, in some cases, tightness of structures.
Adhesives have found application in medicine for gluing bones, living tissues and other purposes.
Detachable connections.
Keyed connections
Keyed connections are used to secure rotating parts (gears, pulleys, couplings, etc.) to a shaft (or axle), as well as to transmit torque from the shaft to the hub of the part or, conversely, from the hub to the shaft. Structurally, on a groove is made in the shaft, into which a key is placed, and then a wheel, which also has a keyway, is put on this structure.
Depending on the purpose of the key connection, there are keys different shapes:
A) Parallel key with a flat end;
b) Parallel key with a flat end and holes for mounting screws;
c) Key with a rounded end;
d) Key with a rounded end and holes for mounting screws;
e) Segment key;
e) Wedge key;
g) Wedge key with stop.
Spline connections
Spline joints are used to connect shafts and wheels due to protrusions on the shaft and in depressions in the wheel hole.
According to the principle of operation, spline connections resemble keyed connections, but have a number of advantages:
· better centering of parts on the shaft;
· transmit more torque;
· high reliability and wear resistance.
Depending on the tooth profile, there are three main types of connections:
a) Straight-sided teeth (number of teeth Z = 6, 8, 10, 12), GOST 1139-80;
b) Involute teeth (number of teeth Z = 12, 16 or more), GOST 6033-80;
c) Triangular teeth (number of teeth Z = 24, 36 or more).
Spline connections are widely used in mechanisms where it is necessary to move the wheel along the axis of the shaft, for example, in car speed switches.
Spline connections are reliable, but not technologically advanced, so their use is limited due to the high cost of manufacturing.
Threaded connections
A threaded connection is a detachable connection of the component parts of a product using a part with a thread.
A thread consists of alternating projections and depressions on the surface of a rotating body, located along a helical line. The body of revolution can be a cylinder or round hole- cylindrical threads. Sometimes used tapered thread. The thread profile corresponds to a certain standard.
Types of threaded connections
| Name | Image | Note |
| Bolted connection |  | Used for fastening parts of small thickness. If the thread breaks, it is easily replaced. |
| Screw connection |  | The screw can have any head. The thread is cut directly into the body of the part. Disadvantage - the threads in the housing may be damaged, which leads to the replacement of the entire housing. |
| Pin connection |  | Tightening is done with a nut. The pin is screwed into the body. If a thread in the body breaks, a new thread of a larger diameter is cut or, if this is not possible, the entire body is replaced. |
| Pin connection |  | Tightening is done with two nuts. If the thread breaks, it is easily replaced. |
Basic structural forms of bolt and screw heads 
a) Hex head for tightening with a wrench; b) Round head with a slot for tightening with a screwdriver; c) Countersunk head with a slot for tightening with a screwdriver.
Fastening and sealing threads. They are used in threaded products intended both for fastening parts and for creating a seal. These include threads: cylindrical pipe, conical pipe, conical inch, round inch.
Set screws and connections.
Set screws are used to fix the position of parts and prevent them from moving.
a) With a flat end, used for fixing small thickness parts. b) Tapered shank. c) Stepped shank.
Stepped and tapered shanks are used for fastening pre-drilled parts.
Example of using a set screw with a tapered shank.
Bolts and connections for special purposes.
| Foundation bolts. Special fasteners made in the form of a threaded rod. They serve mainly for fastening various equipment and building structures. They are used in places where strong and reliable fastening of structures in concrete, brick, stone or other foundations is necessary. The bolt is placed in the base and filled with concrete. |  |
| Eye bolt (loaded bolt) - designed for gripping and moving machines and parts during installation, development, loading, etc. |  |
| Hook with a loaded bolt - designed for hooking and moving various loads. |  |
Nuts.
In detachable threaded connections, bolts and studs are equipped with nuts. The nuts in the holes have the same thread as the bolts (type, diameter, pitch). Threaded hole
1.4. Depending on the duration of the load, one should distinguish between permanent and temporary (long-term, short-term, special) loads.
1.5. Loads arising during the manufacture, storage and transportation of structures, as well as during the construction of structures, should be taken into account in calculations as short-term loads.
Loads arising during the operation stage of structures should be taken into account in accordance with paragraphs 1.6-1.9.
a) the weight of parts of structures, including the weight of load-bearing and enclosing building structures;
b) weight and pressure of soils (embankments, backfills), rock pressure.
The forces from prestressing remaining in the structure or foundation should be taken into account in calculations as forces from permanent loads.
a) the weight of temporary partitions, grouting and footings for equipment;
b) the weight of stationary equipment: machines, apparatus, motors, containers, pipelines with fittings, supporting parts and insulation, belt conveyors, permanent lifting machines with their ropes and guides, as well as the weight of liquids and solids filling the equipment;
c) the pressure of gases, liquids and granular bodies in containers and pipelines, excess pressure and rarefaction of air that occurs during ventilation of mines;
d) loads on floors from stored materials and racking equipment in warehouses, refrigerators, granaries, book depositories, archives and similar premises;
e) temperature technological influences from stationary equipment;
e) the weight of the water layer on water-filled flat coverings;
g) the weight of industrial dust deposits, if its accumulation is not excluded by appropriate measures;
h) loads from people, animals, equipment on the floors of residential, public and agricultural buildings with reduced standard values given in table. 3;
i) vertical loads from overhead and overhead cranes with a reduced standard value, determined by multiplying the full standard value of the vertical load from one crane (see clause 4.2) in each span of the building by the coefficient: 0.5 - for groups of operating modes of cranes 4K-6K ; 0.6 - for the 7K crane operating mode group; 0.7 - for the 8K crane operating mode group. Groups of crane operating modes are accepted according to GOST 25546 - 82;
j) snow loads with a reduced standard value, determined by multiplying the full standard value in accordance with the instructions in clause 5.1 by the coefficient: 0.3 - for snow region III: 0.5 - for region IV; 0.6 - for regions V and VI;
k) temperature climatic influences with reduced standard values, determined in accordance with the instructions of paragraphs. 8.2 - 8.6 provided = 
=
=
=
=0,
=
= 0;
l) impacts caused by deformations of the base, not accompanied by a fundamental change in the structure of the soil, as well as thawing of permafrost soils;
m) impacts caused by changes in humidity, shrinkage and creep of materials.
a) loads from equipment arising in start-up, transition and test modes, as well as during its rearrangement or replacement;
b) the weight of people, repair materials in equipment maintenance and repair areas;
c) loads from people, animals, equipment on the floors of residential, public and agricultural buildings with full standard values, except for the loads specified in clause 1.7, a, b, d, e;
d) loads from mobile lifting and transport equipment (forklifts, electric vehicles, stacker cranes, hoists, as well as from overhead and overhead cranes with full standard values);
e) snow loads with full standard value;
f) temperature climatic effects with full standard value;
g) wind loads;
h) ice loads.
A) seismic impacts;
b) explosive effects;
c) loads caused by sudden disturbances technological process, temporary malfunction or breakdown of equipment;
d) impacts caused by deformations of the base, accompanied by a radical change in the structure of the soil (when soaking subsidence soils) or its subsidence in mining areas and karst areas.
As practice shows, the topic of load collection raises the greatest number of questions among young engineers starting their professional activity. In this article I want to consider what permanent and temporary loads are, how long-term loads differ from short-term ones and why such a separation is necessary, etc.
Classification of loads by duration of action.
Depending on the duration of action, loads and impacts are divided into permanent And temporary . Temporary loads are in turn divided into long-term, short-term And special.
As the name itself suggests, permanent loads valid throughout the entire period of operation. Live loads appear during certain periods of construction or operation.
include: own weight of load-bearing and enclosing structures, weight and soil pressure. If prefabricated structures (crossbars, slabs, blocks, etc.) are used in the project, the standard value of their weight is determined on the basis of standards, working drawings or passport data of the manufacturing plants. In other cases, the weight of structures and soils is determined from design data based on their geometric dimensions as the product of their density ρ and volume V taking into account their humidity under the conditions of construction and operation of structures.The approximate densities of some basic materials are given in table. 1. Approximate weights of some rolled and finishing materials are given in table. 2.
Table 1
Density of basic building materials
Material |
Density, ρ, kg/m3 |
Concrete:- heavy- cellular |
2400400-600 |
Gravel |
1800 |
Tree |
500 |
Reinforced concrete |
2500 |
Expanded clay concrete |
1000-1400 |
Brickwork with heavy mortar:- made of solid ceramic bricks- made of hollow ceramic bricks |
18001300-1400 |
Marble |
2600 |
Construction waste |
1200 |
River sand |
1500-1800 |
Cement-sand mortar |
1800-2000 |
Mineral wool thermal insulation boards:- not subject to load— for thermal insulation of reinforced concrete coverings— in ventilated facade systems— for thermal insulation of external walls followed by plastering |
35-45160-19090145-180 |
Plaster |
1200 |
table 2
Weight of rolled and finishing materials
Material |
Weight, kg/m2 |
Bituminous shingles |
8-10 |
Plasterboard sheet 12.5 mm thick |
10 |
Ceramic tiles |
40-51 |
Laminate 10 mm thick |
8 |
Metal tiles |
5 |
Oak parquet:— 15 mm thick— thickness 18 mm— thickness 22 mm |
111315,5 |
Roll roofing (1 layer) |
4-5 |
Sandwich roofing panel:— thickness 50 mm— thickness 100 mm— thickness 150 mm— thickness 200 mm— thickness 250 mm |
1623293338 |
Plywood:— thickness 10 mm— 15 mm thick— 20 mm thick |
710,514 |
Live loads are divided into long-term, short-term and special.
relate:— load from people, furniture, animals, equipment on the floors of residential, public and agricultural buildings with reduced standard values;
— loads from vehicles with reduced standard values;
— weight of temporary partitions, grouts and footings for equipment;
— snow loads with reduced standard values;
— weight of stationary equipment (machines, motors, containers, pipelines, liquids and solids, filling equipment);
— pressure of gases, liquids and granular bodies in containers and pipelines, excess pressure and air rarefaction that occurs during ventilation of mines;
— loads on floors from stored materials and shelving equipment in warehouses, refrigerators, granaries, book depositories, archives of similar premises;
— temperature technological influences from stationary equipment;
— weight of the water layer on water-filled flat surfaces;
— vertical loads from overhead and overhead cranes with a reduced standard value, determined by multiplying the full standard value of the vertical load from one crane in each span of the building by the coefficient:
0.5 - for groups of operating modes of cranes 4K-6K;
0.6 - for the 7K crane operating mode group;
0.7 - for the 8K crane operating mode group.
Groups of crane modes are accepted according to GOST 25546.
relate:— the weight of people, repair materials in areas for maintenance and repair of equipment with full standard values;
— loads from vehicles with full standard values;
— snow loads with full standard values;
— wind and ice loads;
— loads from equipment arising in start-up, transition and test modes, as well as during its rearrangement or replacement;
— temperature climatic influences with full standard value;
- loads from movable lifting and transport equipment (forklifts, electric vehicles, stacker cranes, hoists, as well as overhead and overhead cranes with full standard values).
relate:— seismic impacts;
— explosive effects;
— loads caused by sudden disruptions in the technological process, temporary malfunction or breakdown of equipment;
- impacts caused by deformations of the base, accompanied by a radical change in the structure of the soil (when soaking subsidence soils) or its subsidence in areas of mining and karst.
