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Transcript of POWER SCREW
TYPES OF SCREW THREAD
These threads have high
efficiency but they are difficult to manufacture and are expensive
-Acme threads have a 29° thread angle, which is easier to machine than square threads.
-They are not as efficient as square threads, due to the increased friction induced by the thread angle.
-The thread has an optimum efficiency of about 70% for helix angles between 25 degree and 65 degree. Outside this range the efficiency falls away.
Buttress threads are of a triangular shape.
These are used where the load force on the screw is only applied in one direction.
They are as efficient as square threads in these applications, but are easier to manufacture.
A power screw is a drive used in machinery to convert a rotary motion into a linear motion for power transmission. It produces uniform motion and the design of the power screw may be such that
(a) Either the screw or the nut is held at rest and the other member rotates asit moves axially. A typical example of this is a screw clamp.
(b) Either the screw or the nut rotates but does not move axially. A typical example for this is a press.
The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread.
They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.
The "M" designation for metric screws indicates the outer diameter of the screw, in millimeters (e.g. an M6 screw has an outer diameter of 6 millimeters).
The function of a power screw is to provide a means for obtaining a large mechanical advantage and at the same time transmitting power by converting angular, into linear motion.
Common applications include lifting jacks, presses, vices, and lead screws for lathe machines.
ADVANTAGES AND DISADVANTAGES
Large load carrying capability
Simple to design
Easy to manufacture; no specialized machinery is required
Large mechanical advantage
Precise and accurate linear motion
Smooth, quiet, and low maintenance
Minimal number of parts
Most are self-lockin
The disadvantages are that most are not very efficient.
Due to the low efficiency they cannot be used in continuous power transmission applications.
They also have a high degree for friction on the threads, which can wear the threads out quickly.
For square threads, the nut must be replaced; for trapezoidal threads, a split nut may be used to compensate for the wear.