POWER OR TRANSLATIONAL SCREWS
INTRODUCTION Power or Translational screws are used to t o convert rotary motion into translatory motion. m echanical device used for converting Power or Translational screw is a mechanical rotary motion into linear motion and transmitting power Applications of Power Screw To obtain accurate motion in machining operations -For example, in the case of the lead screw of lathe, the rotary motion is available but the tool has to be advanced in the direction of the cut against the cutting resistance of the material. To raise load - In case of screw jack, a small force applied in the horizontal plane is used to raise or lower a large load. To clamp a work piece - Power screws are also used in vices, testing machines, presses To load a specimen- Universal testing machine
In
most of the power screws, the nut has axial motion against the resisting axial force while the screw rotates in its bearings.i.e lead screw In some screws, the screw rotates and moves axially against the resisting force while the nut is stationary i.e. screw jack
INTRODUCTION Power or Translational screws are used to t o convert rotary motion into translatory motion. m echanical device used for converting Power or Translational screw is a mechanical rotary motion into linear motion and transmitting power Applications of Power Screw To obtain accurate motion in machining operations -For example, in the case of the lead screw of lathe, the rotary motion is available but the tool has to be advanced in the direction of the cut against the cutting resistance of the material. To raise load - In case of screw jack, a small force applied in the horizontal plane is used to raise or lower a large load. To clamp a work piece - Power screws are also used in vices, testing machines, presses To load a specimen- Universal testing machine
In
most of the power screws, the nut has axial motion against the resisting axial force while the screw rotates in its bearings.i.e lead screw In some screws, the screw rotates and moves axially against the resisting force while the nut is stationary i.e. screw jack
INTRODUCTION Advantages of Power screws
Power screw has large load carrying capacity
Compact in construction
Manufactu Manufacturing ring of power power screw is easy ² no SPM are required
Power screw has large Mechanical Advantage
Power screw provides precise control and highly accurate linear motion
Provides smooth and noiseless service without any maintenance
Power screws can designed with self locking property
INTRODUCTION Disadvantages of Power screws
Power screw has poor efficiency about 40%
High friction between threads causes wear of screw sc rew or nut
TYPES OF SCREW THREADS USED FOR TRANSLATIONAL SCREWS Three types of screw threads are mostly used for translational screws: 1. Square thread. A square thread, as shown, is adapted for the transmission of power in either direction
This thread results in maximum efficiency and minimum radial or bursting pressure on the nut. It is difficult to cut with taps and dies. It is usually cut on a lathe with a single point tool and it can not be easily compensated for wear. The square threads are employed in screw jacks, presses and clamping devices.
TYPES OF SCREW THREADS 2. Acme and trapezoidal thread. An acme or trapezoidal thread, as shown, is a modification of square thread. 30
The slight slope given to its sides lowers the efficiency slightly than square thread and it also introduces some bursting pressure on the nut, but increases its area in shear.
It
Wear may be taken up by means of an adjustable split nut.
is used where a split nut is required and where provision is made to take up wear as in the lead screw of a lathe. An acme thread may be cut by means of dies and hence it is more easily manufactured than square thread.
TYPES OF SCREW THREADS 3. Buttress thread. A buttress thread, as shown is used when large forces act along the screw axis in one direction only.
This thread combines the higher efficiency of square thread and the ease of cutting and the adaptability to a split nut of acme thread. It is stronger than other threads because of greater thickness at the base of the thread. The buttress thread has limited use for power transmission. It is employed as the thread for light jack screws and vices
MULTIPLE THREADED SCREWS
The power screws with multiple threads such as double, triple etc. are employed when it is desired to secure a higher travelling speed. It also referred as multiple start screws
Such type of threads are usually found in high speed actuators.
MULTIPLE THREADED SCREWS Advantages of multiple threaded screws o
o
It
provides large axial motion per revolution of screw. Efficiency of multiple threaded screws is more than single threaded screws due to increase in helix angle
Disadvantages of multiple threaded screws o
o
The Mechanical advantage obtained with multiple threaded screws is lower than that of single threaded screw Self locking property is lost in multiple threaded screws
TERMINOLOGY OF POWER SCREW (i) Pitch: The pitch is defined as the distance, measured parallel to the axis of the screw, from a point on one thread to the corresponding point on the adjacent thread. It is denoted by the letter 'p¶ . (ii) Lead: The lead is defined as the distance, measured parallel to the axis of the screw, that the nut will advance in one revolution of the screw . It is denoted by the letter 'l', For a single-threaded screw, the lead is same as the pitch, for a double-threaded screw, the lead is twice of the pitch, and so on.
TERMINOLOGY OF POWER SCREW (iii) Nominal diameter: Nominal diameter is the largest diameter of the screw, It is also called major diameter. It is denoted by the letter 'd'. (iv) Core diameter: Core diameter is smallest diameter of the screw thread. It also called minor diameter. It is denoted by letters 'dc'.
TERMINOLOGY OF POWER SCREW (v)Helix angle: The helix angle is defined as the angle made by the helix of the thread with a plane perpendicular to the axis of the screw . Helix angle is related to the lead and the mean diameter of the screw. It is also called lead angle. Helix angle is denoted by .
