Materials for axles and shafts. Shafts and axles general information and design basics
ShaftsAndaxes.Are commonintelligence
Shaft is a machine part intended for torque transmission along its center line. In most cases, shafts support parts that rotate with them (gears, pulleys, sprockets, etc.). Some shafts (for example, flexible, cardan, torsion) do not support rotating parts. Machine shafts, which, in addition to gear parts, carry the working parts of the machine, are called main shafts. The main shaft of machine tools with the rotational movement of a tool or product is called a spindle. The shaft that distributes mechanical energy among individual working machines is called a transmission shaft. In some cases, shafts are made as one piece with a cylindrical or bevel gear (shaft-gear) or with a worm (shaft-worm).
According to the shape of the geometric axis, shafts can be straight, cranked And flexible(with variable axis shape). The simplest straight shafts have the shape of bodies of revolution. The picture shows smooth(s) And stepped (b) straight shafts. Stepped shafts are the most common. To reduce weight or to be placed inside other parts, shafts are sometimes made with a channel along the axis; in contrast to solid ones, such shafts are called hollow.
An axis is a part of machines and mechanisms that serves to support rotating parts, but not transmitting useful torque. There are axles rotating(A) And motionless(b). The rotating axis is mounted in bearings. An example of rotating axles is the axles of railway rolling stock, and an example of non-rotating axles is the axles of the front wheels of a car.
From the definitions it is clear that during operation the shafts always rotate and experience torsional or bending and torsion deformations, and the axles only experience bending deformations (tensile and compression deformations that arise in some cases are most often neglected).
Structural elements of shafts and axles
The supporting part of the shaft or axle is called a journal. The end pin is called a tenon, and the intermediate pin is called a neck. The end journal designed to carry the predominant axial load is called the fifth. The shaft pins and journals rest on bearings; the supporting part for the heel is the thrust bearing. The shape of the axles can be cylindrical, conical, spherical and flat (heels).
The annular thickening of the shaft, which forms one whole with it, is called a shoulder. The transitional surface from one section to another, which serves to support parts mounted on the shaft, is called a shoulder.
To reduce stress concentration and increase strength, transitions in places where the diameter of the shaft or axis changes are made smooth. The curved surface of a smooth transition from a smaller section to a larger one is called a fillet. Fillets come in constant and variable curvature. A shaft fillet recessed beyond the flat part of the shoulder is called an undercut.
The shape of the shaft along its length is determined by the distribution of loads, i.e., diagrams of bending and torque moments, assembly conditions, and manufacturing technology. Transition sections of shafts between adjacent steps of different diameters are often made with a semicircular groove for the exit of the grinding wheel.
The landing ends of the shafts, intended for installing parts that transmit torque in machines, mechanisms and devices, are standardized. GOST 12080-66* establishes the nominal dimensions of cylindrical shaft ends of two designs (long and short) with diameters from 0.8 to 630 mm, as well as the recommended dimensions of threaded shaft ends. GOST 12081-72* establishes the main dimensions of the conical ends of shafts with a taper of 1:10, also in two designs (long and short) and two types (with external and internal threads) with diameters from 3 to 630 mm.
Materials of shafts and axles. The performance requirements of shafts and axles are most fully met by carbon and alloy steels, and in some cases by high-strength cast iron. The choice of material, thermal and chemical-thermal treatment is determined by the design of the shaft and supports, technical specifications for the product and its operating conditions.
For most shafts, heat-treated steels 45 and 40Х are used, and for critical structures - steel 40ХН, ЗОХГТ, etc. Shafts made of these steels are subjected to improvement or surface hardening with high-frequency heat.
High-speed shafts rotating in plain bearings require high hardness of the journals, so they are made from case-hardened steels 20Kh, 12Kh2N4A, 18KhGT or nitrided steels such as 38Kh2MYuA, etc. Chrome-plated shafts have the greatest wear resistance.
Typically, shafts are subjected to turning, followed by grinding of the seating surfaces and journals. Sometimes the seating surfaces and fillets are polished or hardened by surface peening (ball or roller treatment).
CalculationshaftsAndaxes
During operation, shafts and rotating axes, even under constant external load, experience alternating bending stresses of a symmetrical cycle, therefore, fatigue failure of the shafts and rotating axes is possible. Excessive deformation of shafts can interfere with the normal operation of gears and bearings, therefore The main criteria for the performance of shafts and axles are material fatigue resistance and rigidity. Practice shows that the destruction of shafts of high-speed machines usually occurs as a result of material fatigue.
For the final calculation of the shaft, it is necessary to know its design, type and location of supports, and places where external loads are applied. However, the selection of bearings can only be carried out when the shaft diameter is known. That's why calculation of shafts is carried out in two stages: preliminary(design) and final(testing) (we will not consider the second stage).
Preliminary calculation of shafts. The design calculation is carried out only for torsion, Moreover, to compensate for bending stresses and other unaccounted factors, significantly reduced values of permissible torsional stresses are taken, for example for the output sections of gearbox shafts = (0.025...0.03), where is the temporary resistance of the shaft material. Then the diameter of the shaft will be determined from the strength condition
,
The resulting diameter value is rounded to the nearest standard size in accordance with GOST 6636-69* “Normal linear dimensions”, which establishes four rows of main and a number of additional dimensions; the latter may be used only in justified cases.
When designing gearboxes, the diameter of the output end of the drive shaft can be taken equal to the diameter of the electric motor shaft to which the gearbox shaft will be connected by a coupling.
After establishing the diameter of the output end of the shaft, the diameter of the shaft journals is assigned (slightly larger than the diameter of the output end) and bearings are selected. For ease of assembly, the diameter of the mounting surfaces of the shafts under the hubs of the mounted parts is larger than the diameters of the adjacent sections. As a result, the stepped shaft is close in shape to a beam of equal resistance.
Lecture 6. Shafts and axles.
Study questions:
1. Purpose, design and materials of shafts and axles.
2. Performance criteria and calculation of shafts and axles.
3. Calculation of shafts.
4. Keyed and splined connections.
5. Calculation of the strength of connections with parallel keys.
6. Pin connections.
1. Purpose, design and materials of shafts and axles.
Shaft call a part (usually smooth or stepped cylindrical shape) designed to support pulleys, gears, sprockets, rollers, etc. installed on it, and to transmit torque.
During operation, the shaft experiences bending and torsion, and in some cases, in addition to bending and torsion, the shafts may experience tensile (compression) deformation.
Some shafts do not support rotating parts and only work in torsion.
Shaft 1 (Fig. 8.1, p. 204 Markhel) has supports 2 called bearings. The part of the shaft covered by the support is called pin . I call the end pins spikes 3 , and intermediate ones - necks 4 .
Classification of shafts and axles.
By purpose shafts are divided into:
Gear shafts (gear parts are installed on them);
Main shafts (the working parts of the machine are also installed on them).
By geometric shape shafts are divided into:
Straight (see Fig. 8.1);
Crank (Fig. 8.3, A);
Crankshafts (Fig. 8.3, b);
Flexible (Fig. 8.3, V);
Telescopic (Fig. 8.3, G);
Cardan shafts (Fig. 8.3, d).
Crank and crankshafts are used to convert reciprocating motion into rotational motion (piston engines) or vice versa (compressors); flexible - for transmitting torque between machine components that change their position during operation (construction mechanisms, dental machines, etc.); telescopic - if it is necessary to axially move one shaft relative to another.
By design features: smooth shafts and axles (Fig. 8.2); stepped shafts and axles (see Fig. 8.1); gear shafts (see Fig. 3.36; 3.46, V); worm shafts (see Fig. 5.1, pos. 1 ).
By section type shafts and axles are:
Solid (see Fig. 8.2, a);
Hollow (see Fig. 8.2, b);
Combined (Fig. 8.3, d).
Sites 1 axes and shafts (Fig. 8.4), with which they rest on bearings when perceiving axial loads, they are called heels . Footrests serve as supports for the heels 2 . The seating surfaces of shafts and axles for the hubs of mounted parts are made cylindrical, conical or spherical. Cylindrical axles are widely used in mechanical engineering; conical and ball pins; conical and ball journals are rarely used.
Question : What are the names of the trunnions shown in Fig. 8.5?
-in Fig. 8.5, a – cylindrical axle;
- in Fig. 8.5, b – conical;
- in Fig. 8.5, c – ball.
Transition sections (fillets) between the stages of shafts and axles are made to reduce stress concentrations and increase durability. The ends of the shafts and axles are made with chamfered, i.e. lightly grind them at the end. The seating surfaces of shafts and axles are processed on lathes and grinding machines.
- Question : What is called a fillet?
-Fillet is the surface of a smooth transition from a smaller section (axis) to a larger one.
Materials for shafts and axles .
The materials most often used for axles and shafts are carbon and alloy steels (rolled steel, forgings and, less commonly, steel castings), as well as high-strength modified cast iron and non-ferrous metal alloys (in instrument making). For non-critical light-loaded shaft and axle structures, carbon steels without heat treatment are used. Critical, heavily loaded shafts are made from alloy steel 40ХНМА, 25ХГТ, etc. Without heat treatment, steels 35 and 40, St5, St6, 40Х, 40ХН, 30ХН3А are used, with heat treatment - steels 45, 50, etc.
In the automotive and tractor industries, engine crankshafts are made of ductile or ductile iron.
Question : Indicate the most common grades of steel used for the manufacture of shafts and axles.
- In the manufacture of shafts and axles, steel grades St3, St4, St5, 35, 40, 45, 45, 50, 40Х, 40ХН are used.
APPLIED MECHANICS AND
DESIGN BASICS
Lecture 8
SHAFT AND AXLES
A.M. SINOTIN
Department of Technology and Production Automation
Shafts and axles General information
Gears, pulleys, sprockets and other rotating machine parts are mounted on shafts or axles.
Shaft designed to support parts sitting on it and to transmit torque. During operation, the shaft experiences bending and torsion, and in some cases additional tension and compression.
Axis- a part intended only to support the parts sitting on it. Unlike a shaft, an axle does not transmit torque and therefore does not experience torsion. The axes can be stationary or rotate together with the parts mounted on them.
Variety of shafts and axles
According to their geometric shape, shafts are divided into straight (Figure 1), cranked and flexible.
1 – spike; 2 – neck; 3 – bearing
Figure 1 – Straight stepped shaft
Crankshafts and flexible shafts are special parts and are not covered in this course. Axles are usually made straight. In design, straight shafts and axles differ little from each other.
The length of straight shafts and axles can be smooth or stepped. The formation of steps is associated with different tensions of individual sections, as well as manufacturing conditions and ease of assembly.
According to the type of section, shafts and axles can be solid or hollow. The hollow section is used to reduce weight or to be placed inside another part.
Structural elements of shafts and axles
1 Trunnions. The sections of the shaft or axis lying in the supports are called axles. They are divided into spines, necks and heels.
Thorn called a journal, located at the end of a shaft or axis and transmitting predominantly radial load (Fig. 1).
Figure 2 – Heels
Neck called a journal located in the middle part of the shaft or axis. Bearings serve as supports for the necks.
Spikes and necks can be cylindrical, conical or spherical in shape. In most cases, cylindrical pins are used (Fig. 1).
Fifth called a journal that transmits axial load (Figure 2). Thrust bearings serve as supports for the heels. The shape of the heels can be solid (Figure 2, a), ring (Figure 2, b) and comb (Figure 2, c). Comb heels are rarely used.
2 Landing surfaces. The seating surfaces of shafts and axles for the hubs of mounted parts are cylindrical (Figure 1) and less often conical. When pressing fits, the diameter of these surfaces is taken to be approximately 5% larger than the diameter of adjacent areas for ease of pressing (Figure 1). The diameters of the seating surfaces are selected in accordance with GOST 6336-69, and the diameters for rolling bearings are selected in accordance with GOST standards for bearings.
3 Transitional areas. The transition sections between two stages of shafts or axles perform:
With a rounded groove for the exit of the grinding wheel in accordance with GOST 8820-69 (Figure 3, a). These grooves increase stress concentration and are therefore recommended at end sections where bending moments are small;
Figure 3 – Transition sections of the shaft
with a fillet * of constant radius according to GOST 10948-64 (Figure 3, b);
With a fillet of variable radius (Figure 3, c), which helps reduce stress concentration and is therefore used on heavily loaded areas of shafts and axles.
Effective means for reducing stress concentration in transition areas are turning relief grooves (Figure 4, a), increasing the fillet radii, and drilling in large diameter steps (Figure 4, b).
Figure 4 – Methods for increasing the fatigue strength of shafts
SHAFT AND AXLES
Basic information
The parts on which rotating machine parts are mounted (for example, pulleys, gears) are called shafts and axles. Distinguish shafts and axles according to loading conditions:
· shafts transmit torque along its axis of rotation and experience bending, compression, tension and torsion stresses;
· axles do not transmit torque and are loaded only with bending stresses.
Shafts and axles have similar shapes and one general function– support the parts mounted on them (the classification of shafts is presented in Table 1.1).
Table 1.1
Shaft classifications
It should be noted that smooth shafts are more technologically advanced than stepped shafts, and that sometimes shafts and axles are made hollow both to reduce weight and to install other rotating parts inside the shaft. Hollow shaft with a ratio of the diameter of the internal hole to the outer diameter of the shaft equal to 0.75, Almost 2 times lighter than a solid equal-strength shaft.
In mass production, hollow welded shafts made of steel tape wound along a helical line are sometimes used. This saves up to 60% of metal.
According to their design, axles are divided into 2 main groups:
1) moving axes , rotating in supports together with parts mounted on them (Fig. 1.1, a);
2) fixed axes , serving as supports for parts rotating on them (Fig. 1.1, b).
Rice. 1.1. Examples of axle designs:
A - movable axis; b – fixed axis
Axles and shafts are usually constructed in the form of bars consisting of a number of cylindrical sections of various diameters. Parts mounted on axles and shafts are secured using keys or splines. In the axial direction, the parts are fixed relative to the shafts and axes using spacer rings (or bushings), as well as due to the presence of shoulders and shoulders on the shafts.
Stepped shaft shape or axis is also determined by the desire to bring their outlines closer to the shape of a beam of equal resistance to bending. A beam of equal bending resistance is a beam in which the highest bending stresses are the same in all cross sections. Such a beam of circular cross-section has the shape of a cubic paraboloid along its axis.
However, it is very difficult to produce a beam having the shape of a cubic paraboloid, and this shape is inconvenient for fitting the parts associated with it onto the shaft. Therefore, the shaft (axis) is made consisting of cylindrical and conical sections of different diameters (Fig. 1.2). This is done to ensure that the shaft material is loaded as evenly as possible throughout its entire volume.
Rice. 1.2. Stepped Shaft Design Example
The axles and shafts rest on fixed supporting parts - bearings and thrust bearings. The areas of axles and shafts that are in direct contact with the supports are called trunnions . The end journals are called spikes , and the intermediate journals – necks . The ends that rest against a fixed support and prevent axial displacement of the shaft (axis) are called heels. They can be flat, spherical or conical.
The difference between two adjacent sections of the shaft is called step , For example: one of the shaft stages– shank diameter d and an adjacent section with a diameter D (see Fig. 1.2). The minimum step size is 2...3 mm per side, i.e. difference in radius. At the same time, the diameters D And d must be consistent with normal linear dimensions in accordance with GOST 6636-69.
The end surfaces of the shaft (axis) steps are called shoulders . The difference between the diameters of adjacent cylindrical sections of the shaft (axis) should provide sufficient dimensions of the shoulders for axial fixation of rotation parts mounted on the shaft (axis).
The pairing of two adjacent sections of a shaft step (axis), called fillet , it is advisable to perform through a smooth arc transition as wide a radius as possible. The fillet radius is usually taken in the range from 0.05. d before 0.10. d (see Fig. 1.2).
The fillet reduces stress concentration at the transition point from one shaft diameter to another . This is especially important with variable loads on the shaft.
Rice. 1.3. Types of fillets on the shaft steps:
A - constant radius; b – two radii;
V - constant radius and with a groove that relieves stress concentration; G - with undercut into the shaft shoulder
The transition from one shaft diameter to another, made according to Fig. 1.4, A, is irrational, since the groove is a strong stress concentrator. The influence of the groove can be somewhat mitigated by performing it according to Fig. 1.4, b.
Rice. 1.4. Grooves on the shaft: A - without fillets ; b – with roundings
The design of shafts and axles is determined by their operating conditions. A number of agricultural machines use long (up to 20 m) composite shafts used to transmit torque. Such shafts are called transmission. Used in piston engines and compressors crankshafts, having a broken axis of rotation.
To transmit torque between units with spatially displaced axes of the input and output shafts, flexible shafts are used that have a curved geometric axis during operation. These shafts have high torsional rigidity and low bending rigidity. An example is the flexible shaft of a dental drill.
Rotating machine parts are mounted on shafts or axes that ensure a constant position of the axis of rotation of these parts.
Shafts are parts designed to transmit torque along their axis and to support rotating machine parts.
Shafts according to their intended purpose can be divided into gear shafts, load-bearing parts of gears - gears, pulleys, sprockets, couplings (Fig. , A and b), and on main shafts machines and other special shafts that, in addition to transmission parts, carry the working parts of machines, engines or implements - turbine wheels or disks, cranks, clamping chucks, etc. (Fig. V And d)
According to the shape of the geometric axis, shafts are divided into straight and cranked.
Axles– parts designed to support rotating parts and not transmitting useful torque.
Rice. 12.1 Main types of shafts and axles:
a – smooth transmission shaft; b – stepped shaft;
c – machine spindle; g - steam turbine shaft; d – crankshaft;
e – axis of the rotating carriage; g – non-rotating axis of the trolley.
The supporting parts of shafts and axles are called trunnions. Intermediate axles are called necks, terminal – spikes.
Straight shafts according to form divided into shafts of constant diameter (transmission and multi-span ship shafts, Fig. , A, as well as shafts that transmit only torque); stepped shafts (most shafts, Fig. god); shafts with flanges for connection along the length, as well as shafts with cut gears or worms. According to the cross-sectional shape, the shafts are divided into smooth, splined, having a gear (spline) connection profile along a certain length, and profile.
Shaft length length determined by the distribution of loads along the length.
The diagrams of moments along the length of the shafts, as a rule, are significantly uneven. Torque is usually not transmitted over the entire length of the shaft. Bending moment diagrams usually go to zero at the end supports or at the ends of the shafts. Therefore, according to the conditions of strength, it is permissible and advisable to design shafts of variable cross-section approaching bodies of equal resistance. In practice, I make stepped shafts. This form is convenient to manufacture and assemble; Shaft shoulders can absorb large axial forces.
The difference in the diameters of the steps is determined by: the standard diameters of the seating surfaces for hubs and bearings, a sufficient supporting surface to absorb axial forces at given radii of rounding of edges and chamfer sizes, and, finally, the conditions of the assemblies.
Trunnions Shaft (necks) operating in plain bearings are: a) cylindrical; b) conical; c) spherical (Fig.). The main application is for cylindrical pins. To facilitate assembly and fixation of the shaft in the axial direction, end journals are usually made of a slightly smaller diameter than the adjacent section of the shaft (Fig.).
Shaft journals for rolling bearings (Fig.) are characterized by a shorter length than journals for plain bearings.
Trunnions for rolling bearings are often made with threads or other means for securing the rings.
Rice. 12.4 Design means of increasing endurance
shafts in landing areas: a – thickening of the hub part of the shaft;
b – rounding of the hub edges; c – thinning of the hub; g – unloading
grooves; d – bushings or fillings in the hub made of material with a low modulus
elasticity.
Shaft endurance is determined by relatively small volumes of metal in areas of significant stress concentration. Therefore, special design and technological measures to increase the endurance of shafts are especially effective.
Design means of increasing the endurance of shafts at landing sites by reducing edge pressures are shown in Fig. .
By strengthening the hub parts with surface peening (roller or ball rolling), the endurance limit of shafts can be increased by 80–100%, and this effect extends to shafts with a diameter of up to 500–600 mm.
The strength of shafts in places of keyed, toothed (splined) and other detachable connections with the hub can be increased: by using involute spline connections; spline connections with an internal diameter equal to the diameter of the shaft in adjacent areas, or with a smooth exit of the splines to the surface, ensuring a minimum stress concentration; keyways made with a disk cutter and having a smooth exit to the surface; keyless connections.
Axial loads and onto the shafts from the parts mounted on them are transferred in the following ways. (rice.)
1) heavy loads - by focusing parts on the ledges on the shaft, by fitting parts or mounting rings with interference (Fig. , A And b)
2) medium loads - with nuts, pins directly or through mounting rings, terminal connections (Fig. ,c – d);
3) light loads and protection from movement by random forces - locking screws directly or through mounting rings, terminal connections, spring rings (Fig. , d – g).
