The fundamental operation in manufacture is the creation of shape - this includes assembly, where a number of components are fastened or joined together either permanently by welding for example or detachably by screws, nuts and bolts and so on. Since there is such a variety of shapes in engineering to be assembled, it is hardly surprising that there is more variety in demountable fasteners than in any other machine element. Fasteners based upon screw threads are the most common, so it is important that their performance is understood, and the limitations of the fastened assemblies appreciated.

There are two distinct uses for screw threads and they usually demand different behaviour from the threads :

• a   power screw such as a lathe leadscrew or the screw in a car lifting jack which transforms rotary motion into substantial linear motion (or vice versa in certain applications), and
• a   threaded fastener similar to a nut and bolt which joins a number of components together again by transforming rotary motion into linear motion, though in this case the translation is small.

A typical hexagon headed bolt and nut are shown at ( i). The diameter of the bolt shank is usually the same as the outside diameter - the   major diameter or briefly   size - of the thread. The radiused fillet at the junction of shank and head reduces stress concentration.
The shank diameter of a 'waisted' bolt ( ii) is less than the thread diameter thus allowing a radiused thread runout which reduces stress concentration - beneficial in fatigue applications. The assembly illustrated incorporates a washer under the nut which promotes uniformity of contact - minimising damage to the underlying parts and again lessening stress concentration. The bolt head may be equipped with an optional washer face. A bolt's 'grip' is the combined thickness of the fastened parts.

A screw ( iii) is similar to a bolt - the names are often loosely interchanged - though strictly a bolt is equipped with a nut which is rotated to tighten the assembly, whereas a screw is itself rotated and engages with a threaded ( or 'tapped' ) hole in a stationary component such as an engine block casting. The screw illustrated has no shank, being threaded right up to the head. There is a great variety of screw head forms available.

The ubiquitous socket headed setscrew shown here is tightened by a hexagon wrench rather than by a spanner.

A   stud ( iv) has no head and is threaded at both ends. The ends are not necessarily the same. One end is screwed into one of the components usually before the second component is assembled. The sketch illustrates also :

• a   clearance hole through a component, typically 15-20% larger than the bolt/stud size to facilitate assembly and to clear any shank/ head fillet;
• a   tapped hole which is drilled smaller than the root or   minor diameter of the thread - see the enlargement ( v) below;
• the illustrated tapped hole is blind, extends deeper than the stud and ends in a conical point of 120o approximately;
• a stud's depth of engagement is typically 11/4 to 11/2 times its size;
• a threaded length sufficient for the nut to be tightened whilst leaving a couple of threads 'exposed' ( ie. not engaged ) to cater for variations in thickness of the assembled components - though too much exposed thread should be avoided;
• a thin 'locknut' may be jammed against the ordinary nut to assist assembly or to prevent loosening under severe vibration, though resistance to inadvertent loosening is usually effected by a thread locking fluid, a lock washer or a lock nut such as shown here :
  Screws may be supplied complete with captive lock washer to ease assembly - they are then known as 'sems' and they come in many forms, including self-tapping screws for joining sheet metal.

Salient geometric features of the thread are illustrated in ( v). The distance between similar points on adjacent threads is the thread's   pitch.
The load on the bolt   F_b passes from the nut gradually through the engaged threads into the bolt, however the whole load must pass through transverse cross-sections X-X at the exposed threads outside the nut. Neglecting stress concentration, the tensile stress in way of the exposed threads is therefore :

( 1)         σ   =   F_b / A_s where   A_s is the   stress area - a function of thread size and geometry.

Since the stress area is less than the cross- sectional area of a normal (non-reduced) shank, the exposed threads are usually the most critically loaded part of the assembly - this is why failure of threaded joints occurs most commonly close to the nut face.

A thread can be likened to a piece of string wound in a tight helix around a cylinder - or around a conical frustum in the case of pipe thread designed to eliminate leakage.
When a nut on a screw is rotated by one turn, it travels along the screw a distance known as the   lead L. Developing one turn of the thread at the mean diameter   d_m ( the average of major and minor diameters ) gives the   lead angle ( or helix angle )   λ   as   tanλ = L /π d_m .

Power screws may employ multiple threads, or   starts, so   L = p ∗ number of starts as illustrated. Fasteners on the other hand are almost invariably single start ( L = p ). They are also right handed to avoid confusion in tightening, though LH screws appear in turnbuckles and in certain bicycle parts where the prevailing torque would tend to loosen RH fasteners.

Thread geometry

A thread 'system' is a set of basic thread proportions which is scaled to different screw sizes to define the thread geometry. Whitworth, Sellers, British Standard Pipe (BSP) are just three of the many systems which proliferated before the adoption of the ISO Metric thread system. Since this last is now universal, it alone is examined here.

The basic profile of ISO Metric threads is built up from contiguous equiangular triangles of height   h disposed symmetrically about a pitch line which becomes the   pitch cylinder of diameter   d₂ when the profile is rotated about the axis to form the thread. The distance between adjacent triangles - the pitch - is   p = 2 h /√3. The tips of the triangles are truncated by   h/8 to form the major diameter ( size )   d of the thread, and the bases are truncated by   h/4 to form the minor diameter   d₁ . It follows that   d₁ =   d - 5 h/4   =   d - 1.08 p. This leads to the rule of thumb for suitable tapping size drills in normal materials :   d_tapping = d - p.

The basic profile becomes a   maximum material profile for external threads ( on screws ) and internal threads ( in nuts ) through the use of suitable radii and tolerances, so that there is adequate clearance when internal and external threads engage. The relatively large radius at the minor diameter of external threads tends to equalise the strengths of external and internal threads. AS 1721 sets out comprehensive geometric data including fits and tolerances, however knowledge of these many details is not required here.

At its most basic, a thread definition comprises a combination of size and corresponding pitch. Thus   M14x1.25 refers to a Metric thread whose major diameter   d is 14 mm and whose pitch   p is 1.25 mm. The stress area of an external thread corresponds to a diameter   d_s = d - ¹³/₁₂ h,   that is   A_s = ^π/₄ ( d - 0.9382 p )² . Other salient features follow from the underlying geometry.
Most threaded fasteners in general engineering are manufactured to the ISO Metric Coarse Pitch ( First Preference ) Series outlined in Table 1. Fine pitch and constant pitch series are used for special purposes such as IC engine spark plugs and externally threaded thin-walled pipes.

The 60^o thread form is not suitable for power screws which transform motion and which therefore must have high efficiency. The 'square' thread offers the best efficiency but is generally impractical. The 'Acme' thread form offers the best compromise between efficiency, ease of manufacture, assembly and wear take-up using split nuts. The stress area of Acme threads is based upon the average of the minor and mean diameters :   d_s = d - ³/₄ p .