404012 - KINEMATICS AND DESIGN OF MACHINERY - CHAP 8.pdf

NGUYÊN LÝ MÁY

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CHAPTER 8: Screw Mechanisms
8.1 Introduction
8.2 Screw Kinematics
8.3 Screw Forces and Torques
404012 -Kinematics and design of machinery
This slide is based on the chapter 12 of this course’s main textbook:
“Myszka D. H., [2012], Machines and mechanisms: Applied kinematic analysis,
4e., Pearson”

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8.1. Introduction
404012 -Kinematics and design of machinery
•Ingeneral,screwmechanismsaredesigned
toconvertrotarymotiontolinearmotion.
•Becausethemotionofanutonathreadis
strictlylinear,graphicalsolutionsdonot
provideanyinsight.

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8.1. Introduction
404012 -Kinematics and design of machinery
Thetwomostcommonfeaturesofathreadarethepitchandpitch
diameter.
Thepitch,p,isthedistancemeasuredparalleltothescrewaxis
fromapointononethreadtothecorrespondingpointonthe
adjacentthread.
Thepitchdiameter,d,isthediametermeasuredfromapoint
halfwaybetweenthetipandrootofthethreadprofilethroughthe
axisandtothecorrespondingpointontheoppositeside.
Theminordiameter,theleadangle,andtheincludedangle.
IntheU.S.CustomaryUnitSystem,thenumberofthreadsperinch,
n,alongthelengthofthescrewismorecommonlyusedthanthe
pitch.Thethreads-per-inchvalueisrelatedtothepitchthroughthe
followingequation:

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8.1. Introduction
404012 -Kinematics and design of machinery

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404012 -Kinematics and design of machinery
8.1. Introduction

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404012 -Kinematics and design of machinery
Unified threads is described as a sharp, triangular tooth.
8.1. Introduction

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404012 -Kinematics and design of machinery
Unified threads are designated as either coarse pitch (UNC) or fine pitch (UNF).
A standard unified thread is specified by the size, threads per inch, and
coarse or fine pitch.
9-32 UNF
½-14 UNC
8.1. Introduction

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404012 -Kinematics and design of machinery
Metric thread forms are also described as sharp, triangular shapes, but
with a flat root.
8.1. Introduction

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404012 -Kinematics and design of machinery
8.1. Introduction

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404012 -Kinematics and design of machinery
8.1. Introduction

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404012 -Kinematics and design of machinery
Typical screws have lead angles that range from
approximately 2°to 6°.
8.1. Introduction

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404012 -Kinematics and design of machinery
The condition that must be met for self-locking is:
8.1. Introduction

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8.2. Screw Kinematics
404012 -Kinematics and design of machinery
A is the part that is allowed to rotate.
B is the other part joined by the screw joint.

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8.2. Screw Kinematics
404012 -Kinematics and design of machinery
I.Translation of the nut as the screw rotates
II.Translation of the screw as the nut rotates
III.Translation of the screw as it rotates
IV.Translation of the nut as it rotates
4 cases:

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8.2. Screw Kinematics
404012 -Kinematics and design of machinery
Example: A single thread, 3⁄4 -6 ACME screw shaft moves the slide
The screw is rotated at 80 rpm, moving the slide to the right. Determine the speed
of the slide. Also determine the number of revolutions to move the slide 3.5 in.

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8.2. Screw Kinematics
404012 -Kinematics and design of machinery
Example: Thescrewhasasingle1⁄2*10ACMEthread,bothinaright-
handandleft-handorientation,asshown.Thehandlerotates
counterclockwiseat45rpmtodrivethepressureplate
downward.Inthepositionshown,withβ=25°,determinethe
velocityofthepressureplate.

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery
Force and torque on a screw.

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery
The motion of the nut occurs in the opposite direction from
the applied force acting on a nut.The required torque is:

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery
The motion of the nut is in the same direction as the
force acting on the nut.

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery
Efficiency e:

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery
For ball screws, the operational torque equations can be estimated as

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery
Example:
Single-thread,1–5ACMEscrew,toraisethejack.Screwcannotrotate.
Thenutrotatesat300rpm.Determinetheliftingspeedofthejack,the
torquerequired,andtheefficiencyofthejack.

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8.3. Screw Forces and Torques
404012 -Kinematics and design of machinery

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ASSIGNMENT
Review: [1]: 8.1 to 8.3.
Do homework.
404012 -Kinematics and design of machinery