A Guide to Jack Screws
A jack screw, sometimes called a screw jack, is a type of jack utilized by turning a leadscrew. Jack screws are used to do many things, including lift moderately heavy weights such as vehicles, raise and lower the horizontal stabilizer of aircraft, and as adjustable supports for heavy loads such as the foundations of houses. In this blog, we will discuss jack screws in detail.
Application of Jack Screw
A jack screw comprises a heavy-duty vertical screw and a load table mounted on its top, which screws into a threaded hole in a stationary support frame with a wide base resting on the ground. A rotating collar on the head of the screw has holes into which a metal bar fits. When the handle is turned clockwise, the screw moves further out of the base, lifting the load resting on the load table. In order to support large load forces, the jack screw is formed with acme threads. Acme threads are screw threads with trapezoidal outlines that offer high strength and durability.
The main advantage of jack screws over other types of jacks is that they are self-locking. This means that once the rotational force on the screw is removed, the jack will remain stationary in the position it was left and not rotate backwards, no matter how much load it is supporting. This makes them safer than hydraulic jacks, which will move backwards under load if the force on the hydraulic actuator is inadvertently released. Nevertheless, jack screws have definite limitations as well. The main limitation of jack screws is their lifting capacity. Increasing the load increases friction within the screw heads, speeding up the process of wear. Furthermore, a fine pitch thread, which would address this problem, reduces the speed at which the jack can operate. Jack screws have now largely been replaced by hydraulic jacks.
The large area of sliding contact between the screw threads means jack screws have high friction and low efficiency when used as power transmission linkages - only about 30 to 50%. As such, they are not commonly used for continuous transmission of high power, but instead have many uses in intermittent positioning applications. Jack screws are used in technical applications such as actuators, despite their higher friction. This is because they are easy to manufacture, wear can be compensated for, and they are stronger than comparably sized square thread types. Jack screws are also used throughout the aviation industry, especially to raise and lower horizontal stabilizers. They are extremely important parts of an aircraft. The failure of a jack screw within the horizontal stabilizer can quickly cause an aircraft to become out of control.
Jack screws are also used in electrical connectors. They serve as the captive screws that bring the two parts of a D-subminiature electrical connectors together and hold them in place. When unscrewed, they allow the connector halves to be taken apart. These jack screws may have ordinary screw heads or extended heads, allowing the user to turn the jack screw by hand. Whatever use you have for jack screws, it is important you get them from a trusted source.
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Posted on April 19, 2021