Understanding the Friction Forces in Power Screws

06 May.,2024

 

Understanding the Friction Forces in Power Screws

Hello

So at the moment, I am reading through Shigley's Mechanical Engineering Design book. I've come, at, last, to the first application chapter. Screws, Fastners.. etc. Since I lack experience, what I am trying right now is to visualize every single diagram in this chapter and move through equations slowly. Attached to this post is an image. It is written in the book as follows 'First, imagine that a single thread of the screw is unrolled for exactly a single turn. One edge of the thread will form the hypotenuse o a right triangle whose base is the circumference of the mean-thread diameter and whose height is the lead' To be honest, I've tried hard visualizing this and searched all over the web, but I have no clue. I can't imagine it. Can someone give me a hand over here?? I think if we unwrap one turn the hypotenuse would be the circumference??... Moreover, in the attached picture, what is F (the load we want to push up or down)?? and why is the nut stationary and is it stuck to the bolt.. Why is there F/2 on the each side if we only have ONE nut?? I will be very interactive in this post because I had very hard time to understand it and look through the internet.

Thanks A LOT to whoever contributes with an answer no matter how small. Thanks in advance and sorry for the long post.

MERRY CHRISTMAS!

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9.4 Screw Threads

Contrast the different types of screw motion, with and against applied loads, and match the motion cases to their corresponding equation.

A screw thread is uniform shape which spirals around the inside or outside surface of a cylinder or cone. Like wedges, screws are simple machines. They are essentially a ramp or inclined plane wrapped into a helix, and the input to screws is torque rather than linear force. The mechanical advantage of a screw depends on its lead, which is the linear distance the screw travels in one revolution.

Screws used to fix objects in place are called fasteners, and screws used to move objects are called power screws or lead screws. In this chapter we will focus on power screws.

A power screw assembly includes a nut with matching internal threads which fits around the screw. There are two ways that a power screw can operate based on the movement of the screw and nut. In a scissors jack, the operator rotates the threaded rod with a crank fitted to the nut on the right, which is not threaded but acts as a thrust bearing. The nut on the left moves along the stationary screw to raise the load. In the C-clamp, the nut is stationary and the screw translates through as it rotates. In either case, a small moment on the screw can cause enormous forces on the nut, with the added benefit of the force being applied at a precise location as controlled by the screw.

(a)

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A scissors jack uses fixed screw and a moving a nut.

(b)

A C-clamp screw rotates and translates through a fixed nut.

Figure

9.4.1

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