PPT: Power Screws | Design of Machine Elements - Mechanical Engineering PDF Download

 Page 2


Power Screw 
Definition: 
 A device which is converting rotary motion into linear motion, to transmit the 
power is called as power screw. 
Example: 
1) Lead Screw in Lathe Machine 
2) Screw Jack  
3) Power Screw in Machine Vice, Testing Machine, Presses etc. 
Advantages: 
1) Large load carrying capacity 
2) Compact construction 
3) Simple in design and manufacturing 
4) Large mechanical advantage 
5) Precisely controlled motions 
6) Low cost and more reliable 
7) Smooth and noiseless working 
Disadvantages: 
1) Poor efficiency less than 40% 
2) Rapid wear 
3) High friction 
Page 3


Power Screw 
Definition: 
 A device which is converting rotary motion into linear motion, to transmit the 
power is called as power screw. 
Example: 
1) Lead Screw in Lathe Machine 
2) Screw Jack  
3) Power Screw in Machine Vice, Testing Machine, Presses etc. 
Advantages: 
1) Large load carrying capacity 
2) Compact construction 
3) Simple in design and manufacturing 
4) Large mechanical advantage 
5) Precisely controlled motions 
6) Low cost and more reliable 
7) Smooth and noiseless working 
Disadvantages: 
1) Poor efficiency less than 40% 
2) Rapid wear 
3) High friction 
Power Screw 
Terminology of Power Screw: 
1) Nominal Diameter (do): Largest diameter of screw. 
2) Core Diameter (dc): Smallest diameter of screw. 
3) Mean Diameter (d): Average of nominal diameter 
and core diameter of screw. 
4) Pitch (p): A successive distance between two adjacent 
threads is called as pitch. 
5) Lead (l): The distance through which a screw advances 
axially in one revolution of nut is called as lead. 
6) Helix Angle ( a): The angle made by helix of thread 
with a plane of perpendicular to the axis of screw. 
Page 4


Power Screw 
Definition: 
 A device which is converting rotary motion into linear motion, to transmit the 
power is called as power screw. 
Example: 
1) Lead Screw in Lathe Machine 
2) Screw Jack  
3) Power Screw in Machine Vice, Testing Machine, Presses etc. 
Advantages: 
1) Large load carrying capacity 
2) Compact construction 
3) Simple in design and manufacturing 
4) Large mechanical advantage 
5) Precisely controlled motions 
6) Low cost and more reliable 
7) Smooth and noiseless working 
Disadvantages: 
1) Poor efficiency less than 40% 
2) Rapid wear 
3) High friction 
Power Screw 
Terminology of Power Screw: 
1) Nominal Diameter (do): Largest diameter of screw. 
2) Core Diameter (dc): Smallest diameter of screw. 
3) Mean Diameter (d): Average of nominal diameter 
and core diameter of screw. 
4) Pitch (p): A successive distance between two adjacent 
threads is called as pitch. 
5) Lead (l): The distance through which a screw advances 
axially in one revolution of nut is called as lead. 
6) Helix Angle ( a): The angle made by helix of thread 
with a plane of perpendicular to the axis of screw. 
Power Screw 
Terminology of Power Screw: 
7) Right Hand Thread: When the axis of screw is 
vertical and if the thread slopes upward from left to 
right is known as right hand thread. 
8) Left Hand Thread: When the axis of screw is 
vertical and if the thread slopes downward from left to 
right is known as left hand thread. 
9) Multiple Thread Screw: When the desired axial 
movement is maximum at the same dimensional screw, 
then the number of threads of identical pitch will be 
manufactured side by side on the rod. 
Multistart threads are used when large lead with fine 
threads or high efficiency is required.  
Multiple start threads are double, triple, quadruple start 
screws.  
Page 5


Power Screw 
Definition: 
 A device which is converting rotary motion into linear motion, to transmit the 
power is called as power screw. 
Example: 
1) Lead Screw in Lathe Machine 
2) Screw Jack  
3) Power Screw in Machine Vice, Testing Machine, Presses etc. 
Advantages: 
1) Large load carrying capacity 
2) Compact construction 
3) Simple in design and manufacturing 
4) Large mechanical advantage 
5) Precisely controlled motions 
6) Low cost and more reliable 
7) Smooth and noiseless working 
Disadvantages: 
1) Poor efficiency less than 40% 
2) Rapid wear 
3) High friction 
Power Screw 
Terminology of Power Screw: 
1) Nominal Diameter (do): Largest diameter of screw. 
2) Core Diameter (dc): Smallest diameter of screw. 
3) Mean Diameter (d): Average of nominal diameter 
and core diameter of screw. 
4) Pitch (p): A successive distance between two adjacent 
threads is called as pitch. 
5) Lead (l): The distance through which a screw advances 
axially in one revolution of nut is called as lead. 
6) Helix Angle ( a): The angle made by helix of thread 
with a plane of perpendicular to the axis of screw. 
Power Screw 
Terminology of Power Screw: 
7) Right Hand Thread: When the axis of screw is 
vertical and if the thread slopes upward from left to 
right is known as right hand thread. 
8) Left Hand Thread: When the axis of screw is 
vertical and if the thread slopes downward from left to 
right is known as left hand thread. 
9) Multiple Thread Screw: When the desired axial 
movement is maximum at the same dimensional screw, 
then the number of threads of identical pitch will be 
manufactured side by side on the rod. 
Multistart threads are used when large lead with fine 
threads or high efficiency is required.  
Multiple start threads are double, triple, quadruple start 
screws.  
Power Screw 
Thread Profile Used for Power Screws: 
1) Square Thread: Flanks are 90° with the axis of screw. 
Force applied on both sides. Transmits power without any 
side thrust. Used in screw jack and clamping devices. 
2) T rapezoidal Thread: Flanks are 30° with each other. 
Force applied on both sides. Used in machine tool lead 
screw and power transmission devices. Manufactured on 
thread milling machine with multipoint cutting tool. 
3) Acme Thread: Flanks are 29° with each other. Force 
applied on both sides. Used in machine tool lead screw and 
power transmission devices. Manufactured by dies, hence 
easy to manufacture. Efficiency less than square threads. 
4) Buttress Thread: Flanks are inclined at 45°. It can take 
a thrust in one direction only. Used in vices and clamping 
devices.  Stronger than other type of threads.