Винтовая резьба: устройство, характеристики и применение.

Helical structure used to convert between rotational and linear movement or force

A screw thread is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener. The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied, as long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight elastic deformation.

Every matched pair of threads, external and internal, can be described as male and female. Generally speaking, the threads on an external surface are considered male, while the ones on an internal surface are considered female. For example, a screw has male threads, while its matching hole (whether in nut or substrate) has female threads. This property is called gender. Assembling a male threaded fastener to a female threaded one is called mating.

The included angle characteristic of the cross sectional shape is often called the thread angle. For most V threads, this is standardized as 60 degrees, but any angle can be used. The cross section to measure this angle lies on a plane which includes the axis of the cylinder or cone on which the thread is produced.

Lead ('/l//iː//d/) and pitch are closely related concepts. They can be confused because they are the same for most screws. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw thread (360°). Pitch is the distance from the crest of one thread to the next one at the same point. Because the vast majority of screw threadforms are single start threadforms, their lead and pitch are the same. Single start means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of one ridge. "Double start" means that there are two "ridges" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of two ridges. Another way to express this is that lead and pitch are parametrically related, and the parameter that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to pitch times the number of starts. Whereas metric threads are usually defined by their pitch, that is, how much distance per thread, inch based standards usually use the reverse logic, that is, how many threads occur per a given distance. Thus, inch based threads are defined in terms of threads per inch (TPI). Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a 1/4 20 thread has 20 TPI, which means that its pitch is 1/20 inch
As the distance from the crest of one thread to the next, pitch can be compared to the wavelength of a wave. Another wave analogy is that pitch and TPI are inverses of each other in a similar way that period and frequency are inverses of each other.

Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a saw or file, or between coarse grit and fine grit on sandpaper. The common V thread standards (ISO 261 and Unified Thread Standard) include a coarse pitch and a fine pitch for each major diameter. For example, 1/2 13 belongs to the UNC series (Unified National Coarse) and 1/2 20 belongs to the UNF series (Unified National Fine). Similarly, M10 (10 mm nominal outer diameter) as per ISO 261 has a coarse thread version at 1.5 mm pitch and a fine thread version at 1.25 mm pitch. The term coarse here does not mean lower quality, nor does the term fine imply higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter. Coarse threads are more resistant to stripping and cross threading because they have greater flank engagement. Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque.

There are three characteristic diameters (⌀) of threads: major diameter, minor diameter, and pitch diameter: Industry standards specify minimum (min.) and maximum (max.) limits for each of these, for all recognized thread sizes. The minimum limits for external (or bolt, in ISO terminology), and the maximum limits for internal (nut), thread sizes are there to ensure that threads do not strip at the tensile strength limits for the parent material. The minimum limits for internal, and maximum limits for external, threads are there to ensure that the threads fit together.

The major diameter of threads is the larger of two extreme diameters delimiting the height of the thread profile, as a cross sectional view is taken in a plane containing the axis of the threads. For a screw, this is its outside diameter (OD). The major diameter of a nut cannot be directly measured (as it is obstructed by the threads themselves) but it may be tested with go/no go gauges. The major diameter of external threads is normally smaller than the major diameter of the internal threads, if the threads are designed to fit together. But this requirement alone does not guarantee that a bolt and a nut of the same pitch would fit together: the same requirement must separately be made for the minor and pitch diameters of the threads. Besides providing for a clearance between the crest of the bolt threads and the root of the nut threads, one must also ensure that the clearances are not so excessive as to cause the fasteners to fail.

The minor diameter is the lower extreme diameter of the thread. Major diameter minus minor diameter, divided by two, equals the height of the thread. The minor diameter of a nut is its inside diameter. The minor diameter of a bolt can be measured with go/no go gauges or, directly, with an optical comparator. As shown in the figure at right, threads of equal pitch and angle that have matching minor diameters, with differing major and pitch diameters, may appear to fit snugly, but only do so radially; threads that have only major diameters matching (not shown) could also be visualized as not allowing radial movement. The reduced material condition, due to the unused spaces between the threads, must be minimized so as not to overly weaken the fasteners. In order to fit a male thread into the corresponding female thread, the female major and minor diameters must be slightly larger than the male major and minor diameters. However this excess does not usually appear in tables of sizes. Calipers measure the female minor diameter (inside diameter, ID), which is less than caliper measurement of the male major diameter (outside diameter, OD). For example, tables of caliper measurements show 0.69 female ID and 0.75 male OD for the standards of "3/4 SAE J512" threads and "3/4 14 UNF JIS SAE J514 ISO 8434 2". Note the female threads are identified by the corresponding male major diameter (3/4 inch), not by the actual measurement of the female threads.

The way in which male and female fit together, including play and friction, is classified (categorized) in thread standards. Achieving a certain class of fit requires the ability to work within tolerance ranges for dimension (size) and surface finish. Defining and achieving classes of fit are important for interchangeability. Classes include 1, 2, 3 (loose to tight); A (external) and B (internal); and various systems such as H and D limits.

Thread limit or pitch diameter limit is a standard used for classifying the tolerance of the thread pitch diameter for taps. For imperial, H or L limits are used which designate how many units of 0.0005 inch over or undersized the pitch diameter is from its basic value, respectively. Thus a tap designated with an H limit of 3, denoted H3, would have a pitch diameter 0.0005 × 3 = 0.0015 inch larger than base pitch diameter and would thus result in cutting an internal thread with a looser fit than say an H2 tap. Metric uses D or DU limits which is the same system as imperial, but uses D or DU designators for over and undersized respectively, and goes by units of Generally taps come in the range of H1 to H5 and rarely L1. The pitch diameter of a thread is measured where the radial cross section of a single thread equals half the pitch, for example: 16 pitch thread = 1/16in = 0.0625in the pitch actual pitch diameter of the thread is measured at the radial cross section measures 0.03125in.

To achieve a predictably successful mating of male and female threads and assured interchangeability between males and between females, standards for form, size, and finish must exist and be followed. Standardization of threads is discussed below.

Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, yielding a final thread depth that can be expressed as a fraction of the pitch value. The UTS and ISO standards codify the amount of truncation, including tolerance ranges. A perfectly sharp 60° V thread will have a depth of thread ("height" from root to crest) equal to 0.866 of the pitch. This fact is intrinsic to the geometry of an equilateral triangle — a direct result of the basic trigonometric functions. It is independent of measurement units (inch vs mm). However, UTS and ISO threads are not sharp threads. The major and minor diameters delimit truncations on either side of the sharp V.

The nominal diameter of Metric (e. g. M8) and Unified (e. g. 5/16 in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by 0.866/4 of the pitch from the dimension over the tips of the "fundamental" (sharp cornered) triangles. The resulting flats on the crests of the male thread are theoretically one eighth of the pitch wide (expressed with the notation 1/8p or 0.125p), although the actual geometry definition has more variables than that. A full (100%) UTS or ISO thread has a height of around 0.65p. Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 75% of the 0.65p value. For example, a 75% thread sacrifices only a small amount of strength in exchange for a significant reduction in the force required to cut the thread. The result is that tap and die wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed. This additional truncation is achieved by using a slightly larger tap drill in the case of female threads, or by slightly reducing the diameter of the threaded area of workpiece in the case of male threads, the latter effectively reducing the thread's major diameter. In the case of female threads, tap drill charts typically specify sizes that will produce an approximate 75% thread. A 60% thread may be appropriate in cases where high tensile loading will not be expected. In both cases, the pitch diameter is not affected. The balancing of truncation versus thread strength is similar to many engineering decisions involving the strength, weight and cost of material, as well as the cost to machine it.

Tapered threads are used on fasteners and pipe. A common example of a fastener with a tapered thread is a wood screw. The threaded pipes used in some plumbing installations for the delivery of fluids under pressure have a threaded section that is slightly conical. Examples are the NPT and BSP series. The seal provided by a threaded pipe joint is created when a tapered externally threaded end is tightened into an end with internal threads. For most pipe joints, a good seal requires the application of a separate sealant into the joint, such as thread seal tape, or a liquid or paste pipe sealant such as pipe dope.

The screw thread concept seems to have occurred first to Archimedes, who briefly wrote on spirals as well as designed several simple devices applying the screw principle. Leonardo da Vinci understood the screw principle, and left drawings showing how threads could be cut by machine. In the 1500s, screws appeared in German watches, and were used to fasten suits of armor. In 1569, Besson invented the screw cutting lathe, but the method did not gain traction and screws continued to be made largely by hand for another 150 years. In the 1800s, screw manufacturing began in England during the Industrial Revolution. In these times, there was no such thing as standardization. The bolts made by one manufacturer would not fit the nuts of another.

Standardization of screw threads has evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users. The standardization process is still ongoing; in particular there are still (otherwise identical) competing metric and inch sized thread standards widely used. Standard threads are commonly identified by short letter codes (M, UNC, etc.) which also form the prefix of the standardized designations of individual threads. Additional product standards identify preferred thread sizes for screws and nuts, as well as corresponding bolt head and nut sizes, to facilitate compatibility between spanners (wrenches) and other tools.

The most common threads in use are the ISO metric screw threads (M) for most purposes, and BSP threads (R, G) for pipes. These were standardized by the International Organization for Standardization (ISO) in 1947. Although metric threads were mostly unified in 1898 by the International Congress for the standardization of screw threads, separate metric thread standards were used in France, Germany, and Japan, and the Swiss had a set of threads for watches.

The first historically important intra company standardization of screw threads began with Henry Maudslay around 1800, when the modern screw cutting lathe made interchangeable V thread machine screws a practical commodity. During the next 40 years, standardization continued to occur on the intra and inter company levels. No doubt many mechanics of the era participated in this zeitgeist; Joseph Clement was one of those whom history has noted. In 1841, Joseph Whitworth created a design that, through its adoption by many British railway companies, became a standard for the United Kingdom and British Empire called British Standard Whitworth. During the 1840s through 1860s, this standard was often used in the United States as well, in addition to myriad intra and inter company standards. In April 1864, William Sellers presented a paper to the Franklin Institute in Philadelphia, proposing a new standard to replace the US' poorly standardized screw thread practice. Sellers simplified the Whitworth design by adopting a thread profile of 60° and a flattened tip (in contrast to Whitworth's 55° angle and rounded tip). The 60° angle was already in common use in America, but Sellers's system promised to make it and all other details of threadform consistent. The Sellers thread, easier to produce, became an important standard in the U. S. during the late 1860s and early 1870s, when it was chosen as a standard for work done under U. S. government contracts, and it was also adopted as a standard by highly influential railroad industry corporations such as the Baldwin Locomotive Works and the Pennsylvania Railroad. Other firms adopted it, and it soon became a national standard for the U. S.,

There are many ways to generate a screw thread, including the traditional subtractive types (for example, various kinds of cutting [single pointing, taps and dies, die heads, milling]; molding; casting [die casting, sand casting]; forming and rolling; grinding; and occasionally lapping to follow the other processes); newer additive techniques; and combinations thereof.

Another common inspection point is the straightness of a bolt or screw. This topic comes up often when there are assembly issues with predrilled holes as the first troubleshooting point is to determine if the fastener or the hole is at fault. ASME B18.2.9 "Straightness Gage and Gaging for Bolts and Screws" was developed to address this issue. Per the scope of the standard, it describes the gage and procedure for checking bolt and screw straightness at maximum material condition (MMC) and provides default limits when not stated in the applicable product standard.