Name: why shafts develop local stress peaks
Uploaded: 2024-05-23T12:59:41.000Z
Duration: 1 min 18 s
Channel: JoVE

Ricordiamo la formula della torsione elastica, applicabile ad un albero circolare a sezione uniforme. Qui, si presume che gli alberi siano caricati alle loro estremità da piastre rigide solidamente attaccate ad essi.
Tuttavia, le coppie vengono spesso applicate all'albero tramite giunti a flangia o ingranaggi collegati da chiavette inserite in sedi chiavette.
Ciò altera la distribuzione delle sollecitazioni vicino al punto di applicazione della coppia, divergendo dalle distribuzioni previste dalla formula di torsione.
Bruschi cambiamenti di diametro potrebbero anche portare a una distribuzione non uniforme delle concentrazioni di stress, soprattutto vicino alle articolazioni.
Queste sollecitazioni possono essere ridotte implementando un raccordo. Il valore massimo della sollecitazione di taglio sul raccordo può essere espresso in termini di fattore di concentrazione della sollecitazione.
Il fattore di concentrazione delle sollecitazioni, che dipende dai rapporti del diametro dell'albero e dalle dimensioni del raccordo, può essere precalcolato e memorizzato per l'uso pratico.
Questo metodo di analisi rimane valido finché il valore massimo di sollecitazione non supera il limite elastico del materiale. Se si verificano deformazioni plastiche, si tradurranno in valori di sollecitazione massima più bassi.

concentrazioni di sollecitazioni negli alberi circolari

Consideriamo la formula della torsione elastica, che si applica ad un albero circolare di sezione costante. Questa formula presuppone che le estremità dell'albero siano caricate con piastre rigide saldamente fissate. Tuttavia, in molti casi, le coppie vengono applicate all'albero attraverso meccanismi come giunti a flangia o ingranaggi, collegati tramite chiavette inserite nelle scanalature. Questo metodo di applicazione, modifica la distribuzione delle sollecitazioni in prossimità del punto di applicazione della coppia, facendola deviare dalle distribuzioni previste dalla formula di torsione. Inoltre, spostamenti improvvisi del diametro, possono anche causare una distribuzione irregolare delle concentrazioni di stress, in particolare attorno alle aree articolari.

Queste sollecitazioni possono essere mitigate incorporando un raccordo. Il fattore di concentrazione dello stress può rappresentare il valore più alto dello stress di taglio sul raccordo. Questo fattore, che si basa sui rapporti tra il diametro dell'albero e la dimensione del raccordo, può essere calcolato in anticipo e salvato per riferimento futuro e applicazione pratica. Questo metodo di analisi rimane efficace se il valore massimo della tensione rimane entro il limite elastico del materiale. Se si verificano deformazioni plastiche, porteranno a valori di picco di stress più bassi, sottolineando che la comprensione di questi fattori e il loro impatto sulla distribuzione dello stress è cruciale per applicazioni accurate e pratiche della formula della torsione elastica.

Concentrazioni di Sollecitazioni negli Alberi Circolari

Ricordiamo la formula della torsione elastica, applicabile ad un albero circolare a sezione uniforme. In questo caso, si presume che gli alberi siano caricati ...

Education

Didattica

JoVE Core

Mechanical Engineering

Ingegneria Meccanica

Unit 1

Unità 1

Torsion

Torsione

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

Stress Concentrations in Circular Shafts

why shafts develop local stress peaks

Circular shafts can develop local stress peaks when torque is applied through real machine parts. The elastic torsion formula works best for a circular shaft with a uniform cross-section and rigid plates fixed to its ends. In practice, torque is often passed through flange couplings or gears, and these parts use keys in keyways to connect to the shaft.

That loading method changes the stress pattern near the point where torque enters the shaft. The stress distribution no longer matches the simple pattern predicted by the torsion formula. Sudden changes in shaft diameter can also create stress concentrations, especially near joints.

A fillet, which is a rounded transition, can reduce these concentrated stresses. The highest shearing stress at the fillet is described by a stress concentration factor. Engineers can calculate this factor in advance from the ratios of the shaft diameter and the fillet size, then save it for later use.

This approach is valid as long as the maximum stress stays within the material’s elastic limit. If plastic deformation occurs, the peak stress becomes lower than the elastic estimate. Understanding these effects helps make the elastic torsion formula more useful in practical designs.

Why Shafts Develop Local Stress Peaks

stresses in a shaft

Torque and Shear in a Twisted Shaft

A shaft under torque develops internal shear forces as it twists. In this setup, equal and opposite torques act on the shaft PQ. To study the effect, a section is cut perpendicular to the shaft axis at an arbitrary point R. The free-body diagram of the QR segment shows the shearing forces applied by the PR portion. These forces act as the shaft resists twisting. Using equilibrium on the QR segment links the internal shear forces in the cut section to the internal torque. Here, r is the...

deformation in a circular shaft

Shear Strain in a Circular Shaft

Shear strain in a circular shaft describes how the shaft’s surface changes shape under torsion. A circular shaft keeps each cross-section flat and undistorted as it twists. Each section behaves like a rigid slab that rotates in place. To study the strain inside the shaft, consider a cylindrical section with length L and radius R. One end of this section is fixed. The radius at a point inside the section is labeled r. Before the load is applied, a small square element lies on the surface of...

circular shaft - stresses in linear range

Torsion Formula for Circular Shafts

Torsion in a circular shaft is analyzed when the applied torque stays within Hooke's law. In this linear range, the shaft does not undergo permanent deformation. The discussion uses shearing strain, shearing stress, and the modulus of rigidity to build the stress equation. The derivation also requires the moments of the small forces on a cross-section to equal the applied torque. When the shearing stress expression is rewritten, an integral term appears. This term represents the polar moment...

angle of twist - elastic range

Elastic Twist in a Circular Shaft

Elastic twist in a circular shaft describes how a cylindrical shaft rotates when torque is applied. The shaft has a length L and a uniform radius r. A torque at the free end creates shearing strain, which is greatest at points farther from the shaft axis. When the shaft stays within the elastic range, the shearing strain can be written in terms of the applied torque, radial distance, polar moment of inertia, and modulus of rigidity. The polar moment of inertia is a geometric measure of how the...

angle of twist: problem solving

Shaft Torsion Between Pulleys B and C

Shaft torsion between pulleys B and C is found by analyzing how torque twists an aluminum shaft. An electric motor applies 700 N·m of torque and produces stable rotation. The shaft carries pulleys B and C, which experience torques of 300 N·m and 400 N·m. The calculation starts with a section cut between pulleys B and C. A free-body diagram is used to study the cut cross-section. Because pulley B has an anticlockwise torque, the torque at the cut must balance it in the opposite direction. The...

design of transmission shafts

Sizing a Transmission Shaft by Torque

A transmission shaft is sized from the power it must carry and the speed it rotates at. These two design limits guide the choice of shaft material and cross-sectional size. The goal is to keep the maximum shearing stress within the elastic limit while still transmitting the needed power. Power is directly linked to torque, which is the twisting force applied to the shaft. By rearranging the power relation, the torque can be found from the required power and the shaft speed. That torque value...

stress concentrations in circular shafts

Circular shafts can develop local stress peaks when torque is applied through real machine parts. The elastic torsion formula works best for a circular shaft with a uniform cross-section and rigid plates fixed to its ends. In practice, torque is often passed through flange couplings or gears, and these parts use keys in keyways to connect to the shaft. That loading method changes the stress pattern near the point where torque enters the shaft. The stress distribution no longer matches the...

plastic deformation in circular shafts

Torsional Failure and Plastic Strain in Shafts

Plastic deformation in circular shafts happens when torsional force pushes the material past its yield strength. At that point, the shaft does not return to its original shape. Instead, it keeps a permanent strain. To measure this change, the stress pattern inside the shaft must be understood. The maximum shearing stress is calculated first. Then a shearing-stress-strain diagram can be drawn to find the maximum shearing strain. Shearing strain changes linearly with the distance from the shaft...

circular shafts - elastoplastic materials

Torsion Limits in Circular Shafts

Circular shafts under torsion show a clear change from elastic behavior to plastic deformation as torque increases. In the elastic limit, stress rises in direct proportion to the distance from the shaft’s center. The outer material carries the highest stress first, and the shaft can still return to its original shape. At a critical stress level, the shaft begins to yield. This marks the maximum torque the shaft can carry without permanent deformation. Beyond this point, a plastic region forms...

residual stresses in circular shafts

Residual Stress After Shaft Torsion

Residual stress can remain in a circular shaft after torque causes plastic deformation. This happens in elastoplastic materials, which show both elastic and plastic behavior. The plastic change can come from high shearing stress or large strain. Residual stress is the internal stress that stays in the material after the external force is removed. In a shaft under torque, the angle of twist does not return fully to its original value. That lasting change shows that residual stress is still...

torsion of noncircular members

Torsion in Square and Rectangular Bars

Torsion in square and rectangular bars shows how shape changes the way a material twists. Circular shafts keep their cross-sections nearly unchanged because they are axis-symmetric. That symmetry helps the stress spread evenly, so the shaft can resist torsion without distorting. Square bars behave differently because they do not have the same axial symmetry. When twisted, their cross-sections distort, although the diagonals and the lines connecting the midpoints stay as exceptions. This means...

thin-walled hollow shafts

Shear Flow in Thin-Walled Shafts

Shear flow in a thin-walled hollow shaft describes how torsional force is carried through the wall. When a small segment of width dx is isolated, it is still in equilibrium, but it experiences torsional shearing forces at its ends. These forces come from the longitudinal shearing stress on the segment's minor surface multiplied by that surface area. The result is a uniform shear flow through the structure. To study the wall more closely, a small section with length ds is considered. The force...

key terms and concepts

Learn how torque entry points, keyways, diameter changes, and fillets affect stress in circular shafts, and how stress concentration factors help predict peak shear stress.

Why Shafts Develop Local Stress Peaks | JoVE K12

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Why Shafts Develop Local Stress Peaks

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