Hooping

The hooping is the assembly of two parts thanks to a tight adjustment. The external part is called “hoop”, the interior part is known as “hooped”.

The assembly is carried out with tolerances of Usinage which prohibit its assembly with the hand or even with the press. The simplest solution, when it is possible without deterioration of material, is to heat the hoop to dilate it before threading it on the element which should be hooped. One can contrary cooling the interior element with liquid nitrogen or the carbonic ice to contract it and engage it in the hoop, but these solutions are more expensive. In certain cases, for example for tools of sintering or forging, one is obliged to practice at the same time the dilation of the hoop and the contraction of the hooped element.

Not to confuse the fr' e' ttage with the Fr ''' I ''' ttage, which is a method of acquisition parts per powder material agglomeration.

Hooping is generally practiced on parts of revolution for which it is easier to control the allowances machining and the constraints generated in materials.

Use

Most of the time one practices hooping to ensure the cohesion of a whole of elements or to avoid the bursting of an element under pressure.

  • the circles which girdle the barrel X to ensure the sealing of the Douelle S, the Bandage S of old the Roue S of tanks out of wooden, are hoops.
  • the rings which one sees around the low parts of the pressure pipes, or those which girdle the tools of sintering, spinning by shocks, etc, are also hoops.

One can also have recourse to hooping for

  • to create thanks to the assembly of two parts a new too complex part to be machined only, such as for example a driveshaft comprising of the brought back pinions. Here a few years, the Vilebrequin of the Moteur S of the 2CV Citroen consisted of several assembled elements by contraction with liquid nitrogen, returning the connecting rod assembly indémontable.
  • to create a part made up of two parts of different matters.
  • to create a part made up of two parts of characteristics or different uses. For example, the wheels of railroad generally comprise a hooped binding heated by induction with the assembly. This binding is carried out in a steel different from that of the wheel, with much better strength properties to wear. When binding is too worn, it is dismounted and it is replaced.
  • the rotors low pressure of the steam turbines of the power stations of the type REFERENCE MARK 900 MW use hooping for their assembly. Discs, corresponding to the various pressure stages, are hooped on the tree of each body. On these discs the wings are then fixed. From an exploitation point of view, this technique imposes important constraints during starting. Indeed, the discs being heated or cooling (into relative) more quickly than the tree, there is a risk of surfrettage (appearance of splits) or of hoop removing (displacement of the disc). These rotors are replaced little by little by rotors of the welded type.

It rather often arrives, in particular in the tools of manufacture when the operating conditions are very severe, that hooped assemblies are them-even locked up in one or more other hoops. One counts sometimes up to 4 or 5 successive hoops, increasingly large.

Certain buildings in masonry are girdled by rings or wire ropes tended acting to the manner of hoops, to consolidate them and prevent their bursting. It is thus, for example, of the bell-tower of the basilica of Torcello, celebrates brick construction built on one of the islands of the Lagune of Venice. These elements can be envisaged as of the construction of the building, and it is then extremely imprudent to remove them, or added in the event of noted weakness of the structures.

The wire rolling up coated with resin and strongly tended to form a composite envelope of Material around certain containers (bottles of gas strongly compressed for the diving, tanks of Propellent S for the rockets,…) puts the walls of these containers in compression and constitutes a form of hooping.

Mechanics

A hooped whole with mechanical properties different from those of a of the same unit composition but which would not be hooped. Hooping carries out a powering of the part external and a compression of the interior part. However, in general the materials do not have the same compressive strength and to the tension. Thus

  • hooping is favorable if the interior part must undergo a setting in tension at the time which it is made in a material which resists it badly: compression initale will cancel whole or part of the tension, in order to leave material in a favorable field of application. Of course, that refers on the hoop: it was already in tension, therefore it will undergo a still increased tension, with the risk to make it break earlier; but it is enough to choose it in a material which resists the tension well so that is not a disadvantage.

  • and vice versa for a hoop which must undergo a compression whereas it is made in a material which resists it badly.

There all the interest of hooping is seen: it will make it possible to ensure at least one of the two components of the conditions more favorable to the material of which it is made, once placed under the conditions of use envisaged.

The traditional barrel, of ringed iron wood, is an good example in this respect: wood resists the tension only in the direction of fiber, but in the other direction it is split. With hooping, wood is compressed with manufacture, thus, even with pressure of the liquid which it will contain, there will remain compressed, therefore under good conditions for its mechanical resistance. Of dimensioned sound, the iron hoop undergoes without disadvantages the additional tension which it will undergo during the use. Lastly, thanks to the good resistance of wood to the tension in the direction of fiber, it is not necessary to hoop on all the surface of the barrel, but some circles are enough.

See too

The technique of the concrete prestressed rests exactly on the same principle.

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