question: Why are washers (flat steel rings) used before fixing nuts and bolts?
comment: They are normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring (Belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer).
passages: Many knuckle threads follow neither the DIN nor API standards, and are custom designed and fabricated for the particular application.
  
  Section::::Applications.
  
  This thread form's good debris tolerance is the source of its use in oilfields, where it provides a leak-free connection in field use. Many European automobiles use a knuckle thread for their "tow eye," an external threaded recess where a steel towing lug can be fitted. For example, some Porsche cars use DIN 405 Rd 20 x 1/8", and some BMW use a thread similar to DIN 405 Rd 16 x 1/8".
   Eye bolts are often installed into masonry and so versions that form their own anchor bolt are commonly available. Most of these screw into some form of shield anchor. Some lightweight forms are not screwed, but rely on just the pull on the ring itself to expand the anchor.
  
  Section::::Eye bolts.:Ring bolts.
  
  A ring bolt is an eye bolt with a captive ring passing through it. The loose ring is articulated, such that it can rotate in at least one axis, and can thus reduce bending forces on the bolt when loaded and will present the minimum obstruction when "flat".
   No special tools are required for installation or removal. Duck billed pliers can be used in installing and removing external spiral rings.
  
  Section::::Circular push-on.
  
  A Circular push-on ring resembles a toothed washer, commonly fabricated in metal. These are installed by pressing onto the end of a grooved shaft, until the nut's inner teeth snap into the groove. The use of push nuts avoids the cost of threading a nut onto the end of the shaft during the manufacturing process.
  
  Section::::Protective finishes.
  
  The following are various surface finished used on retaining rings:
  
  BULLET::::- Passivation
  
  BULLET::::- Phosphate
   Often Conrad-style ball bearings will exhibit contact ellipse truncation under axial load. That means that either the ID of the outer ring is large enough, or the OD of the inner ring is small enough, so as to reduce the area of contact between the balls and raceway. When this is the case, it can significantly increase the stresses in the bearing, often invalidating general rules of thumb regarding relationships between radial and axial load capacity. With construction types other than Conrad, one can further decrease the outer ring ID and increase the inner ring OD to guard against this.
   Thread galling and seizing is significantly reduced because mating bolts or studs are loaded in pure tension without twisting.
   Section::::History.
  
  The original forms of workholding on lathes were between-centers holding and "ad hoc" fastenings to the headstock spindle. The spike-style centers still used on wood lathes represent an ancient method. "Ad hoc" fastening methods in centuries past included anything from pinning with clenching or wedging; nailing; lashing with cords of leather or fiber; dogging down (again involving pinning/wedging/clenching); or other types. Faceplates have probably been around at least since the era of medieval clock-makers.
   While friction can be reduced, it can never be eliminated, and sometimes hundreds of tons of force are required to push the machine and liner into the ground. A large “jacking frame” containing hydraulic rams is required to produce these forces. In most cases the entrance must be modified to support this frame and the forces it generates. 
   The line of General Purpose (GP) Acme threads (ASME/ANSI B1.5-1997) are not designed to sustain external radial loads and both the nut and bolt are, ideally, independently supported, the nut by a linear guide and the screw by shaft bearings. This is due to the need to avoid "wedging" of the thread flanks when subjected to radial loads, which would contribute substantially to friction forces and thread wear. However, there is a centralizing Acme-thread standard (also specified in ASME/ANSI B1.5-1997) which caters to applications where the threads are not radially supported, where the roots and crests of opposing threads are designed to come into contact before the flanks do, under radial loads. This adds the requirement that the sum of the allowances and tolerances on the major diameters of nut and bolt be less than the sum of the allowances on the pitch diameters (PD). The drawback is that for a given amount of end play (axial clearance due solely to PD deviations), closer tolerances and a cleaner work environment are necessitated in the application of a centralizing Acme thread.
   Section::::Alternatives.
  
  There are also other systems of fastener retention that do not rely on safety wire at all, such as lock washers, locknuts, jam nuts, thread-locking fluid, castellated nuts and cotter pins, all of which accomplish the same objective as safety wire, which is to prevent vibration or other forces from causing the undesired loosening of a mechanical fastener.
  
  Section::::Further reading.
  
  BULLET::::- Aviation Publication (AvP) 970 dated 1959:Design Requirements for Service Aircraft
  
  BULLET::::- Air Publication (AP) 970 2nd Edition dated 1924:Handbook of Strength Calculations
  
  BULLET::::- Handbook (HB) 806 1st Edition dated 1918:Handbook of Strength Calculations
   Examples of products produced using this method include axles, tapered levers and leaf springs.
  
  Section::::Processes.:Net-shape and near-net-shape forging.
   Because there is effectively a head on each end of an installed rivet, it can support tension loads. However, it is much more capable of supporting shear loads (loads perpendicular to the axis of the shaft).
  
  Fastenings used in traditional wooden boat building, such as copper nails and clinch bolts, work on the same principle as the rivet but were in use long before the term "rivet" was introduced and, where they are remembered, are usually classified among nails and bolts respectively.
  
  Section::::Types.
  
  There are a number of types of rivets, designed to meet different cost, accessibility, and strength requirements:
   When initially developed, the relative importance of "protection" was higher than it is today, owing to the fact that steels were more prone to corrosion than their modern-day replacements. For this reason, the formulation tends to become somewhat gummy over time. While this property is helpful in the prevention of corrosion, it can cause issues in applications where higher viscosity is an issue, examples of which include bearings, shafts, or delicate machinery such as sewing machines.
   Section::::Construction types.:Relieved race.
   Internally and externally locked structural blind rivets can be used in aircraft applications because, unlike other types of blind rivets, the locked mandrels cannot fall out and are watertight. Since the mandrel is locked into place, they have the same or greater load-carrying capacity as solid rivets and may be used to replace solid rivets on all but the most critical stressed aircraft structures.
   BULLET::::- Thrust-collar type MJTs are used in applications where it is difficult to use threaded tensioners, such as on rolling mills where the joint would typically need to be tightened by crane wrenching. Jackbolts push a threadless nut against a retainer ring, which is fitted into a groove and transfers the jacking force to the main bolt or stud. Because there are no threads, they can also be used in press column applications, eliminating the problems and hassles associated with large nut threads.
  
  Section::::Mechanical advantage.
   Buttress threads have often been used in the construction of artillery, particularly with the screw-type breechblock. They are also often used in vises, because great force is only required in one direction.
   Subsequent frames were made from cattle horn or from wood, but were consistently designed with a connecting rivet for tension.
  
  Section::::Varieties.:C-bridge.
   Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank (called the ) as this makes for a better, stronger dowel. The presence of the unthreaded shank has often been given as characteristic of bolts vs. screws, but this is incidental to its use, rather than defining. 
   BULLET::::- The flush rivet displaced the domed rivet in the 1930s and pneumatic rivet guns work in combination with a heavy reaction bucking bar; not depending on plastic deformation, specialist rivets were developed to improve fatigue life as shear fasteners like the Hi-Lok, threaded pins tightened until a collar breaks off with enough torque.
   In machinery, the buttress thread form is designed to handle extremely high axial thrust in one direction. The load-bearing thread face is perpendicular to the screw axis. or at a slight slant (usually no greater than 7°) The other face is slanted, often at 45°. The resulting thread form has the same low friction properties as a square thread form but at about twice the shear strength due to the long thread base. This thread form also is easy to machine on a thread milling machine, unlike the difficult to machine square thread form. It can also compensate for nut wear using a split nut, much like the Acme thread form.
   These resemble an expanding bolt except the shaft snaps below the surface when the tension is sufficient. The blind end may be either countersunk ('flush') or dome shaped.
  
  One early form of blind rivet that was the first to be widely used for aircraft construction and repair was the Cherry friction-lock rivet. Originally, Cherry friction-locks were available in two styles, hollow shank pull-through and self-plugging types. The pull-through type is no longer common; however, the self-plugging Cherry friction-lock rivet is still used for repairing light aircraft.
   Section::::Construction types.:Fractured race.
  
  Another way of fitting more balls into a radial ball bearing is by radially 'fracturing' (slicing) one of the rings all the way through, loading the balls in, re-assembling the fractured portion, and then using a pair of steel bands to hold the fractured ring sections together in alignment. Again, this allows more balls, including full ball complement, however unlike with either slot fill or relieved race constructions, it can support significant axial loading in either direction.
  
  Section::::Construction types.:Rows.
   Most rolling-element bearings feature cages. The cages reduce friction, wear, and bind by preventing the elements from rubbing against each other. Caged roller bearings were invented by John Harrison in the mid-18th century as part of his work on chronometers.
  
  Typical rolling-element bearings range in size from 10 mm diameter to a few metres diameter, and have load-carrying capacity from a few tens of grams to many thousands of tonnes.
  
  Section::::Design.:Ball bearing.
   Security locknuts are found in a variety of industrial applications. They are most commonly used in high vibration environments where non-locking nuts could come loose. In the rail industry, security locknuts can be found most commonly on railroad fishplates, frogs, switches, and crosses. Mining equipment. features security locknuts on conveyor systems, crushers, shakers, mining chains, among other applications. In addition, mills use security locknuts to hold mill liners in place. Heavy vibration equipment manufacturers. use security lockouts on woodchippers, grinders, pulverizers, shakers, feeders, and more. 
   The first bolts had square heads, formed by forging. These are still found, although much more common today is the hexagonal head. These are held and turned by a spanner or wrench, of which there are many forms. Most are held from the side, some from in-line with the bolt. Other bolts have T-heads and slotted heads. 
  