What is a power screw?
A power screw is a mechanical component which is used to convert rotary motion into the linear motion.
Sometimes a power screw is also known as translation screw.
It uses helical motion of screw to transmit the power rather than holding the parts together.
Attribution: By Unknown – Downloaded original drawing from Erik Oberg, A. L. Valentine, Joseph M. Stabel (1910) Screw thread cutting, 3rd Ed., The Industrial Press, New York, p.4, fig.2 on Google Books, Public Domain, https://commons.wikimedia.org/w/index.php?curid=14780818
Applications of power screws
- It is used to raise the load e.g. screw jack
- It is used to obtain a precise motion e.g. lead screw of lathe
- It is used to load a specimen e.g. universal testing machine
- It is used to clamp a workpiece e.g. vice
Parts of power screws
A power screw has following three parts.
- It consists a Screw
- It consists a Nut
- It consists a part which holds either nut or bolt in place
Depending on the type of holding arrangement power screws can be divided into two parts
- Screw moves in axial direction and nut kept stationary. E.g. screw jack and vice
- Nut moves in axial direction and screw kept stationary. E.g. lead screw of a lathe.
Advantages offered by power screws
- It has large load carrying capacity
- It can be designed with self-locking property. Screw jack is the perfect example of this.
- Because of few parts it is cheap and reliable
- It gives smooth and noiseless service
- It requires almost no maintenance
- It provides very precise motion which is required in machine tool applications
- It gives very high mechanical advantage hence used in screw jacks, clamps, valves and vices.
- Its manufacturing is easy and does not required any specialized machinery
- It is simple to design
- Overall dimensions of power screw are small which results in compact construction
Disadvantages of power screws
- It has poor efficiency
- Due to high friction, wear is a serious problem in power screws
There are two types of applications of power screws
- Where high efficiency is desired. E.g. lead screws and presses
- Where low efficiency is desired. E.g. screws jacks, clamps and vices
To increase the efficiency of power screws the sliding friction is reduced by rolling friction. This principal is used in recirculating ball screws.
Types of threads used in power screws
- Square threads
- Trapezoidal threads
- Acme threads
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