# Connection technology

Connection between the rail and threshold :

positive locking transversely of the rail and frictional connection in the direction of the rail (slips in thermal expansions), traction under the screw heads and the mother (by spring washers improved)

The connection technology describes the constructive methods of assembling technical structures ( machines , equipment , apparatus , equipment and modern buildings ) from their individual parts . The process of connecting is called joining and is a matter of manufacturing technology .

As a rule, these are fixed connections. Connections that only restrict the mobility between two parts are joints (e.g. swivel or sliding joints).

The connections can be detachable if the connection can be detached again without damaging the components ( e.g. screw connection or Velcro fastener ), not detachable if the components have to be destroyed ( e.g. welded connection or adhesive bond ), or conditionally detachable if only the auxiliary joining parts must be destroyed, but not the components (knocking off the rivets for riveted connections ).

The classification according to physical operating principles is: form-fit , force-fit and material-fit.

## Form fit

Principle of form and force fit. The upper cube can only be moved a little because it is fitted into a recess. The lower cube is pressed onto the base by a load and can therefore only be moved if the force applied for this exceeds the static friction of the material on the base.

Positive connections are created by the interlocking of at least two connection partners. This means that the connection partners cannot disengage even with or without power transmission. In other words, with a positive connection, one connection partner is in the way of the other . Under normal operating conditions, compressive forces act normally, i.e. at right angles to the surfaces of the connection partners. Such “blocks” occur in at least one direction. If a second homogeneous pair of surfaces is arranged opposite, the opposite direction is also blocked (see figure, principle illustration and bung ). If the pair consists of two mutually coaxial cylinder surfaces, there is a form fit in all directions of the plane perpendicular to the cylinder axis. An example is the pin inserted into a hole that can be removed again. The hole is advantageously a blind hole so that the pin cannot fall through. There is also a one-sided form fit in the axial direction. Pin-like connecting elements are also rivets and screws , screw connections usually being both positive and non-positive.

Primarily only two components have to be connected to one another in a form-fitting manner, but this is often achieved with the help of a third part - the special connecting element. An example is the connection of two overlapping sheet metal edges using rivets or screws. In addition to the plane of the sheet metal, the form fit must also be established perpendicular to it. The metal sheets should be held on top of one another and the connecting elements should not fall out. For this purpose, the rivets have heads on both sides. The screw has its head , and opposite is the nut (if the screw is not screwed into the sheet metal).

Riveted connections of an old railway bridge

With thin sheets or large forces between the sheets in the direction of their planes, there is a risk of plastic deformation or destruction at the hole edges of the sheets (hole embedment ) and the shear in the pins. Sheet metal connections are usually additionally provided with a frictional connection or even exclusively designed so that the frictional connection alone withstands the stress. The rivets and screws are elastically stretched axially, which is done by shrinking after hot riveting or by tightening the screws firmly.

Further examples:
solvable:

not solvable:

## Frictional connection

Non-positive connections require a normal force on the surfaces to be connected. Their mutual displacement is prevented as long as the counter-force caused by the static friction is not exceeded. The frictional connection is lost and the surfaces slip on each other if the tangentially acting load force is greater than the static friction force, for example between wheel and rail or road surface in vehicles with their own drive. In the friction clutch of a car , the frictional connection is interrupted when it is kicked. If it is only partially kicked against the built-in spring causing the normal force, it grinds.

${\ displaystyle F _ {\ text {Last}} \ leq F _ {\ text {Haft}} = \ mu \ cdot F _ {\ text {N}}}$

${\ displaystyle F _ {\ text {Last}}}$= tangential load force (blue)     = tangential static friction force (red) = static friction coefficient     = normal force (green) ${\ displaystyle F _ {\ text {Haft}}}$
${\ displaystyle \ mu}$${\ displaystyle F _ {\ text {N}}}$

Frictional connection is the cause of the self-locking of loaded wedges or screws . The static friction between the active surfaces prevents the wedge from slipping out or the screw starting to turn. Screws are therefore firmly tightened, even if their preload is not required to create a force fit between the parts they connect (for example in a sheet metal connection, see above). This means that tightly tightened screw connections are also force-fit connections.

The terminal is also a positive connection: something between thumb and forefinger or with a spring clip hold.

Two ropes that are knotted together only transmit a tensile force via a frictional connection, if one neglects that the ropes have a residual stiffness against bending, i.e. that a form fit is also involved to a small extent.

## Material connection

Electrically welded seam (left) and after processing with a slag hammer and wire brush (right)

Cohesive connections are all connections in which the connection partners are held together by atomic or molecular forces. They are also non-detachable connections that can only be separated by destroying the connecting means:

## literature

• Gottfried W. Ehrenstein: Handbook of plastic connection technology. 1st edition, Hanser Verlag, Munich 2004, ISBN 978-3-446-22668-5 .
• Manfred Neitzel, Peter Mitschang, Ulf Breuer (eds.): Manual composite materials. Materials, processing, application, 2nd updated and expanded edition, Hanser Verlag, Munich 2014, ISBN 978-3-446-43696-1 .

## Web links

Commons : fasteners  - collection of images, videos and audio files