上海充通大学 Shanghai Jiao Tong University Objective of This Lecture Learn to analyze power screws and to specify suitable sizes for a given application Describe the operation of a power screw and the general form of square threads,Acme threads, and buttress threads as they are applied to power screws. Compute the torque that must be applied to a power screw to raise or lower a load Compute the efficiency of power screws √ Compute the power required to drive a power screw Analyze the failure modes Screw mounting(How are they fastened to other machine parts?)
11 Objective of This Lecture Learn to analyze power screws and to specify suitable sizes for a given application Describe the operation of a power screw and the general form of square threads, Acme threads, and buttress threads as they are applied to power screws. Compute the torque that must be applied to a power screw to raise or lower a load Compute the efficiency of power screws Compute the power required to drive a power screw Analyze the failure modes Screw mounting( How are they fastened to other machine parts?)
-Major diameter 上游充通大率 Mean diameter Shanghai Jiao Tong University -Minor diameter Pitch p Basic Terminology3 chan ·Pitch,p(螺距) Root- Crest -Thread angle 2a >Distance between adjacent thread forms measured parallel to the thread axis. ·Ma jor Diameter,d(大径) >Largest diameter of a screw thread. ·Minor Diameter,d,(小径) >Smallest diameter of a screw thread ·Pitch Diameter,d。(中径) >The effective diameter of the s screw >Sometimes called the mean diameter,dm 12
12 Basic Terminology 3 • Pitch, p（螺距） Distance between adjacent thread forms measured parallel to the thread axis. • Major Diameter, d（大径） Largest diameter of a screw thread. • Minor Diameter, d r （小径） Smallest diameter of a screw thread • Pitch Diameter, d p （中径） The effective diameter of the screw Sometimes called the mean diameter, d m
上海充通大学 Shanghai Jiao Tong University Basic Terminology3 Major diameter Mean diameter -Minor diameter Pitch p 45° chamfer Nut- Root Crest Thread angle 2o FR F 13
13 Basic Terminology 3
-Major diameter 上游充通大率 Mean diameter Shanghai Jiao Tong University -Minor diameter Pitch p Basic Terminology ·Lead,I(导程) Root- Crest Thread angle 2a >Distance the i nut moves s parallel to the screw axis when the nut is given one turn ·Lead Angle,.入（导程角、升角） >Angle between the perpendicular of the s screw axis and the rise of the thread Helix Angle,Ψ (螺旋角) >Angle between the s screw axis and the rise of the thread Thread Angle, （牙型角、牙侧角) >Angle between the perpendicular to the s screw axis and the crest of the thread. 14
14 Basic Terminology 3 • Lead, l（导程） Distance the nut moves parallel to the screw axis when the nut is given one turn • Lead Angle, （导程角、升角） Angle between the perpendicular of the screw axis and the rise of the thread • Helix Angle, （螺旋角） Angle between the screw axis and the rise of the thread • Thread Angle, （牙型角、牙侧角） Angle between the perpendicular to the screw axis and the crest of the thread
上游充通大率 Shanghai Jiao Tong University Basic Terminology3 External Threads(外螺纹) >the threads on a screw ·Internal Threads(内螺纹) >the threads on a nut ·Engagement Length,L。(旋合长度) >length over which the nut and the screw a are engaged. 15
15 Basic Terminology 3 • External Threads（外螺纹） the threads on a screw • Internal Threads（内螺纹） the threads on a nut • Engagement Length, L e （旋合长度） length over which the nut and the screw are engaged