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

נזכיר את נוסחת הפיתול האלסטי, החלה על פיר עגול בעל חתך רוחב אחיד. כאן, ההנחה היא כי פירים נטענים בקצותיהם על ידי לוחות קשיחים מחוברים אליהם היטב.
עם זאת, מומנטים מוחלים לעתים קרובות על הפיר באמצעות צימודי אוגן או הילוכים המחוברים על ידי מפתחות המותקנים במפתחות.
פעולה זו משנה את התפלגות המאמץ הקרובה לנקודת יישום המומנט, תוך סטייה מההתפלגות הצפויה על ידי נוסחת הפיתול.
שינויים פתאומיים בקוטר יכולים גם להוביל לפיזור לא אחיד של ריכוזי הלחץ, במיוחד ליד המפרקים.
ניתן להפחית לחצים אלה על ידי יישום פילה. הערך המרבי של לחץ הגזירה בפילה יכול לבוא לידי ביטוי במונחים של גורם ריכוז המתח.
גורם ריכוז הלחץ, התלוי ביחסי קוטר הפיר ובגודל הפילה, ניתן לחישוב מראש ואחסון לשימוש מעשי.
שיטת ניתוח זו נשארת תקפה כל עוד ערך הלחץ המרבי אינו חורג מהגבול האלסטי של החומר. אם מתרחשים עיוותים פלסטיים, הם יגרמו לערכי מתח מרביים נמוכים יותר.

ריכוזי מתח במוטות מעגליים

<div style="direction: rtl;">
 חשבו על נוסחת הפיתול האלסטי, החלה על מוט עגול עם חתך עקבי. נוסחה זו מניחה שקצות המוט עמוסים בלוחות קשיחים המחוברים היטב. עם זאת, במקרים רבים, מומנטים מופעלים על המוט באמצעות מנגנונים כמו צימודי אוגן או גלגלי שיניים, המחוברים באמצעות מפתחות המוכנסים לפתחי מפתח. שיטת יישום זו משנה את התפלגות המתח בסמוך לנקודת הפעלת המומנט, וגורמת לה לסטות מההתפלגות החזויה על ידי נוסחת הפיתול. בנוסף, שינויים פתאומיים בקוטר עלולים לגרום לפיזור לא סדיר של ריכוזי מתח, במיוחד סביב אזורי המפרק.

ניתן להפחית מתחים אלו על ידי שילוב עיגול פינות. גורם ריכוז המתח יכול לייצג את הערך הגבוה ביותר של מתח הגזירה בפינה המעוגלת. את הגורם הזה, המסתמך על יחסי קוטר המוט וגודל הפינה המעוגלת, ניתן לחשב מראש ולשמור לעיון עתידי וליישום מעשי. שיטת ניתוח זו נשארת יעילה אם ערך המתח המרבי נשאר בגבול האלסטי של החומר. אם קורים עיוותים פלסטיים, הם יובילו לערכי מתח שיא נמוכים יותר, תוך שימת דגש שהבנת גורמים אלה והשפעתם על התפלגות המתח היא חיונית ליישומים מדויקים ומעשיים של נוסחת הפיתול האלסטי.

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נזכיר את נוסחת הפיתול האלסטי, החלה על פיר עגול בעל חתך רוחב אחיד. כאן, ההנחה היא כי פירים נטענים על שלהם ...

Education

JoVE Core

Mechanical Engineering

הנדסת מכונות

Unit 1

יחידה 1

Torsion

פיתול

KEY TERMS AND CONCEPTS

מושגים ומונחים מרכזיים

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

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

Why Shafts Develop Local Stress Peaks

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