Power screws (translation screws) are used to transmit power by converting rotary motion into linear motion.
The power screw is also known as the translation screw.
It is a simple machine that can be used to create very large forces. Power screw is different from simple screw as simple screw are common fasteners while power screws are mechanical devices.
Some examples of Power Screw are:-
1) In the lead screw of a lathe, the rotary motion is available, but the tool has to be advanced (traveled) axially in the direction of the cut against the cutting resistance of the material.
2) In a screw jack, a small force (effort) applied in the horizontal plane, is used to raise or lower a large load.
3) In machine vices, testing machines, mechanical presses, etc.
Components Of Power Screw:-
The main components of a power screw are:
(i) Screw and
*Torque is applied to any one (generally a screw), causing it to rotate and move either itself or the other element in the axial direction.
* One holding arrangement is necessary to hold either a screw or nut in its place.
Depending upon the holding arrangement, power screws operate in two different ways:
(a) In the case of the lead screw of a lathe, the screw rotates in its bearing, while the nut has axial motion.
(b) In the case of a screw jack or machine vice, the nut is kept stationary and the screw moves in axial direction.
Materials used for screws and nuts:-
- The most suitable material for making screws is steel.
- Suitable materials for nuts are phosphor bronze and Brass.
Advantages Of Power Screws:-
1) t has a large carrying capacity.
2) Compact constructions due to small overall dimensions.
3) It has a very simple design and is easy to manufacture.
4) It has a very high Mechanical advantage (M.A).
Note:- Mechanical Advantage is the ratio of output force to input force in a system.
Both Fo and Fi are in Newton.
5) It provides precisely controlled and highly accurate linear motion required in machine tool application.
6) It has a self-locking capability which means they resist back-driving forces. Once the load is lifted and stopped, it maintains its position without the need of an external braking mechanism or continuous power input.
7) It has a very less cost and is more reliable due to less number of parts.
8) It has smooth and noiseless service without any requirement for maintenance.
9) It is very simple to design.
Disadvantages Of Power Screw:-
1) It has very less efficiency which is nearly 40%. So it is not preferred in continuous power transmission elements like machine tools except the lead screw of the lathe. It has high mechanical advantage but low efficiency.
2) The screws and nuts wear a lot due to the high friction of threads.
3) Power screws are much slower as compared to other motion conversion mechanisms like belts or gears.
4) It is very complex to have multi-axis motion using Power Screw.
Note:- The efficiency of power screw is increased
Applications Of Power Screw:-
1) Power Screws are mainly used for intermittent transmission i.e occasionally required for lifting the load such as car, bus or for actuating any mechanism.
2) Application of different types of power screws:-
Screw Jack:- To raise the load.
Lead Screw Of The Lathe:- To have accurate motion in machining operations.
Vice:- To clamp a workpiece.
Universal Testing Machine:- To load the specimen
Terminology Of Power Screw:-
1) Nominal Diameter:-
It is defined as the largest diameter of the screw thread. It is also called major diameter. It is denoted by do.
2) Core Diamater or Rout Diameter:-
It is defined as the smallest diameter of screw thread. It is also called a minor diameter. It is denoted by dc.
It is defined as the distance from one point on one thread to the corresponding point on the adjacent thread. It is denoted by P.
For square threads,
dc = do – P
d0 = dc + P
Types Of Threads Used in Power Screw:
1) Square Threads:-
# Square Thread is the most used thread for power screws.
# Square Thread has a zero profile thread angle. The flanks in square thread are perpendicular to each other.
# Square threads are used when force is applied in both directions.
# As they transmit power without any side thrust, so they are used in screw jacks and clamping devices.
# Square threads are manufactured in thread milling machine using a single-point cutting tool.
2) Trapezoidal Threads:-
# In trapezoidal threads, the thread angle is 30 degrees.
# They are manufactured on a thread milling machine with the help of multi-point cutting tool. Manufacturing using multi-point putting tool is economical as compared to single point cutting tool which is used to machine square threads.
# They are used in lead screw and other power transmission devices used in machine tools.
3) Acme Threads:-
# Acme thread is a special type of trapezoidal thread.
# Acme thread and trapezoidal thread are identical except the thread angle which 29o in the acme thread and 30o in the trapezoidal thread.
# Acme threads are used for lead screws and other power transmission devices.
# Acme threads are easy to manufacture as they can be cut by dies.
# Acme thread has less efficiency than square thread because of the slight slope of the flanks.
4) Buttress Threads:-
# In butter threads, the flanks are inclined at an angle of 45o.
# As they take unidirectional thrust, they are used in vice and clamping devices where force is to be applied in one direction only.
# Buttress threads are strongest among all threads.