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The flat thread, though rarely encountered today, is a fascinating relic from the early days of mechanical engineering. It was one of the first motion threads used in the 19th century for lifting machines and threaded spindles, designed to convert rotational movement into linear motion. While it may seem unusual by modern standards, the flat thread had some unique characteristics that made it highly efficient—especially in terms of reducing friction and radial pressure.
What makes the flat thread special is its profile: it consists of square-shaped threads with no crest, and the flanks are at a 0° angle. This means the thread has no taper or slope, just straight sides. The pitch is measured as the distance between two adjacent corners on the squares, and each square’s width and height equal half the pitch. Although this design was effective, it also posed significant challenges in manufacturing.
Over time, the flat thread fell out of favor due to its difficulty in production and susceptibility to wear. Its sharp edges were prone to damage, which limited its lifespan and increased maintenance costs. As a result, more practical alternatives emerged, such as the trapezoidal thread, which retained many of the flat thread’s advantages while being easier to produce and more durable.
Today, the metric trapezoidal thread (DIN 103) is the most common successor in many countries, featuring a 30° flank angle. In the U.S., the ACME thread is widely used, with a slightly smaller flank angle of 29°, and a variation known as the STUB ACME thread, which has a reduced depth. These threads are now the standard for lead screws and screw presses.
While the flat thread is no longer in widespread use, there are still niche applications where it might be found, often as custom-made components. For example, antique machinery or historical equipment may still feature flat threads, making them of interest to collectors or restorers.
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