# Joint (technique)

A technical joint is a connection between two rigid bodies that can move in a given manner . The respective mobility of a joint can be assessed quantitatively with the degree of freedom of the movement form rotating ( swivel joint ) and / or shifting ( sliding joint ) taking place in it .

In particular, these are those elements that connect the links of a mechanical transmission .

The joints that occur individually in machines , mechanical devices and structures such as bridges are i. d. Usually referred to as a warehouse ( machine warehouse , building warehouse ).

Combinations of two or three joints and the bodies in between have their own names as machine elements (e.g. the universal joint and the constant velocity joint) .

The bones - joints of vertebrates in the body of living beings are enveloped by the soft tissues ( muscles , blood and nerve tracts) so that the connected bones do not rotate around each other like technical bodies, but can only move against each other within a limited angular range . That is why the wheel does not appear in nature.

## Components and function

In a joint, the bodies to be connected are in constant contact. The two contact points, which are geometrically designed in a special way (e.g. as a bore and bolt ) are referred to as contact or joint elements . At best, they form a form fit . The type of contact is planar, linear or punctiform. The relative movement takes place in a sliding and / or rolling manner and thus largely free of force. In contrast, forces and moments in other directions are transferred.

### basic forms

The basic shapes of the joints are differentiated according to the shape of the joint elements, the type of relative movement between them and the degree of freedom f of the movement. Rotation and displacement can be combined in a joint.

 Fig. 1: Slide joint , f = 1 Fig. 2: Swivel joint , f = 1 Fig. 3: screw joint , slidably on the helical line, f = 1 Fig. 5: Plate joint, slide can be pushed in the plane and rotated perpendicular to it, f = 3 Fig. 6: Rotary sliding joint , axially slidable and rotatable about axis, f = 2 Fig. 7: Ball joint , f = 3 (ball guided between two sockets opposite each other in the outer part)
Crank handle. Model after Leonardo da Vinci in Codex Madrid I. in the Deutsches Museum Bonn

Joints are ubiquitous in objects of everyday use, in construction, in machines and vehicles. Their use ranges from simple swivel joints for hole punches, sliding joints for umbrellas to complex three-dimensional gears, e.g. B. in suspension. The inventors have always been concerned with the development of mechanisms for the most varied of applications.

In the simplest case, the body to be moved is connected directly by a joint with one degree of freedom. The sliding joint can only be pushed to z. B. with a telescopic arm. The swivel joint is found in every hinge, but also in the trailing arm axle . As a rule, however, joints are part of a kinematic chain , which creates mechanisms of the most varied of complexity. Joints with more than one degree of freedom, such as the swivel joint or the ball joint, are often used. The degree of freedom of the mechanism can be determined using the Grübler's equation .

### Curve joint

In the gear drives and cam drives u. a. the curved joint not included above is used. Rolling sliding (sliding and / or rolling) takes place between its joint elements, tooth flanks or generally curved flanks (e.g. those of a camshaft and a valve tappet in internal combustion engines ) . The degree of freedom is f = 2.

## camp

An articulated connection is called a bearing when one of the two bodies is firmly connected to the machine frame or the foundation and is therefore at rest. In contrast to a joint, a bearing is usually not part of a more complex mechanism ( gear ). In the foreground of his consideration is almost always only the only kind of flexible connection that occurs in him. Rotating shafts and linearly displaceable bodies ( linear bearings ) are supported in machines and devices, bridge girders or other point-like supported structures in construction. In contrast to those in machines and devices, only very small movements (shifts and rotations) take place in warehouses in the construction industry . They only serve to create statically determined bearings and to maintain them in the event of disturbances (subsoil subsidence, temperature-related changes in length, etc.).

Instead of plain bearings, roller bearings with rolling elements inserted between the joint elements are often used as bearings in machines and devices . These also correspond to the joint basic shapes rotating and sliding joint , while in the construction industry as a bearing so-called "rolling joints" used in its basic form curve joints are (when paired concrete to concrete only the only possible rolling).

## In construction

There are generally no visible rotations in joints in construction. They are used on the one hand to keep the eccentricities in the bearings and components low, on the other hand they reduce the static uncertainty . The latter reduces stresses due to deformations (e.g. constraints or settlements ). They are therefore often used in warehouses or tannery girders .

## Machine elements

### Universal joint

Cardanic suspension of a ship's compass. The outermost ring is firmly attached to the ship. The extensions of the outer circle, which look like pins in a bearing, are an indication of the inclination of the ship, otherwise they are vertical.
Cardan joint as a machine element for connection to two mutually pivoted shafts.
The intermediate piece, together with the pivot pin, has the shape of a cross.
The axes of all joints meet at one point. The four-part transmission obtained with the shaft bearing joints (not shown) and the machine frame is a spherical one .

The cross or cardan joint is a series of two rotary joints. The second contact surface of each of the two joints is located on the common cross-shaped intermediate piece (see figure on the right). The axes of the swivel joints intersect at 90 °. The point of intersection is the point of rotation at which the two joint "stems" arranged at right angles to the respective rotary joint axis can be pivoted relative to one another. The cardan joint has 2 freedom of movement. Compared to the ball joint, it lacks the freedom of rotation of the joint handles around its own axis. Its degree of freedom is f = 2.

The historically oldest application was cardanic suspension , in which pivoting around the two horizontal spatial axes is possible, but rotation around the vertical axis is not permitted. The intermediate piece is a ring.

The lack of freedom of rotation around the joint handles makes the universal joint also suitable for torque transmission between mutually pivotable shafts ( cardan shaft ).

### Constant velocity joints

Constant velocity joints , like the universal joint, are used for torque transmission between mutually pivotable shafts. You do not have the disadvantage of the not entirely synchronous, but periodically slightly fluctuating rotation transmission with the rotation. Like the universal joint, they are ball joints reduced to two swivel movements. Inside, as with the universal joint, there are intermediate parts whose joint elements are in contact with surfaces on varied spheres or sockets. At the Rzeppa joint there are z. B. balls in contact with grooved spherical surfaces.

## Spring joints in mechanical engineering

The spring joints belong neither to the basic joint forms nor to the bearings in which rigid bodies slide and / or roll over one another. They are available as approximate swivel and sliding joints with little mobility. Between the bodies that are supposed to move relative to one another, an elastically yielding body - z. B. a spring bar that is twisted or bent - permanently installed. Spring joints have the advantage of being free of friction, play and maintenance.

Torsion spring joints are often used in mechanical measuring instruments, whereby the torsion moment can be used as a restoring moment.

## Individual evidence

1. Johannes Volmer: Gear Technology: Guide . 1st edition. Vieweg, 1978, ISBN 978-3-528-04096-3 , pp. 24-27 . ( limited preview in Google Book search)
2. Kurt Luck, Karl-Heinz Modler: Transmission technology: analysis synthesis optimization . 1st edition. Springer, 1990, ISBN 978-3-7091-3890-8 , pp. 5-13 . ( limited preview in Google Book search)
3. Wolfgang Matschinsky: Wheel guides of road vehicles: statics, kinematics, elasto-kinematics and construction . 3. Edition. Springer, 2007, ISBN 978-3-540-71196-4 , pp. 12 . : ( limited preview in Google Book search)
4. Johannes Volmer, p. 30
5. Steffen Marx, Gregor Schacht: Joints in solid construction
6. ^ Siegfried Hildebrand : Feinmechanische Bauelemente , Hanser, p. 429