They constitute a necessary element for coupling large thin-walled tubes for air transport in buildings to one another or to installations.

This necessity arises from the inherently very high rigidity of these air ducts, combined with the building method where, when the ducts are run, it is not always possible to align all parts with one another or the installation. The installation or replacement of large installation components would also be very difficult in the case of an installation without flexible couplings.

Existing sleeve couplings are in general very satisfactory in functional terms.

However, new legislation demands substantial airtightness. It is thus important to clamp the fabric well all round and also to make the transition between the frame parts such that there is a good air seal.

A disadvantage of existing sleeve couplings is that the production method is frequently complex and demands specific tools. As a result of the very diverse sizes, this process is difficult to automate and in practice this frequently leads to unergonomic operations on the part of the production personnel.

In order to fix the fabric in the frame use is frequently made of components that clamp the fabric with very high force. These components themselves also demand a great deal of force for fitting. This is one of the reasons for the unergonomic production operations and the need for production in a factory environment.

Another disadvantage of existing sleeve couplings in larger sizes is that they have to be produced in a factory using aids. Transport of the highly voluminous sleeve couplings, which are also vulnerable in these sizes, is difficult and regularly causes damage to the frames.

Finally, a relatively large proportion of the materials used in the frame is often needed for fixing the fabric, without making a substantial contribution to the strength or (torsional) stiffness of the frame. This increases the costs.

A coupling for ducts consisting of the construction described above, where the fabric is clamped by the opposing walls, which are made resilient, is disclosed in DE 101 08 085 and US 4 940 264. It has been found that where these walls have a relatively low clamping force insertion is simple, but the risk of slipping out is high. If the clamping force is substantial it is then impossible to insert the fabric. Therefore, with these systems there is additional clamping that, for example, can consist of pop rivets or the like, which makes use of the system labour intensive.

The aim of the invention is to offer a solution to the above drawbacks. According to the invention clamping means are fitted with an air duct coupling as described above and the gap between said walls is such that said fabric and said clamping means can be accommodated in this.

Furthermore, the invention consists of components that can easily be pushed into one another by hand without aids or great force, as a result of which assembly is simple and ergonomic and is even possible on site and no longer has to be done in the factory.

Finally, in a preferred embodiment the frame parts in the form of sections can consist of aluminium sections with a tubular part therein, which substantially improves the torsional rigidity of the product.

According to a further advantageous embodiment of the invention, the clamping means are compressible in the direction transverse to the walls. Apart from the resilient characteristics of the opposing upright walls into which the clamping means are pressed, the clamping means themselves are also made resilient.

For this purpose these can have any shapes conceivable in the state of the art and consist of any conceivable resilient material.

According to an advantageous embodiment of the invention, the frames are made up of corner pieces in combination with parts in the form of sections. These parts in the form of sections can be produced easily, for example by extrusion, and can be shortened and combined with the corner components to give the frames on site or beforehand. Frames of various sizes can thus be provided in a simple manner.

Clamping between the corner components and the section components can be achieved in any way known in the art. According to an advantageous embodiment of the invention this clamping is achieved by fitting the fabric. That is to say, as a result of the insertion of the clamping means and the fabric a locking effect is produced between corner components and section components.

In order to clamp the fabric firmly in a secure manner in the gap between the two upright wall sections, according to an advantageous embodiment of the invention there is a acutely angled edge in one of the upright wall sections. Moreover, the fabric can be provided with a corresponding deformation.

According to a further particularly advantageous embodiment there are locking means that act between the clamping means and the upright wall sections, so that snap- fitting of the clamping means in the space between the upright wall sections is possible without too many problems, but removal of the clamping means in the opposing direction is not possible. Such removal is possible only by sliding the section components optionally used in the longitudinal direction.

The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing: Fig. 1 shows, diagrammatically, the flexible duct coupling according to the invention; Fig. 2 shows a detail of a frame used for this; Fig. 3 shows, diagrammatically, the way in which such a frame is combined with a fabric; Fig. 4 shows, in cross-section, a detail of the combination of the clamps for the fabric in the frame.

A flexible air duct coupling is shown in Fig. 1. This consists of two frames located some distance apart with a flexible fabric arranged between them. These frames 10 and 11 can be fixed to two ducts in a conventional manner. As a result a flexible transition is produced between two such ducts.

An example of such a frame is shown in Fig. 2.

With this arrangement the frames 10 and 11 are made up of section components 1 and corner components 2, both of which are provided with upright walls 6,7. In this illustrative embodiment these section components and corner components are pushed into one another by hand. This produces an accommodating groove 8 that runs all round. A fabric 5 that has been produced accurately to size is inserted into this groove stretched to some extent. The frame is held in place as a result. The fabric 5 is then confined in the groove 15 and clamped firmly with the aid of resilient parts 3,4, in a preferred embodiment consisting of U-shaped components 4 at the location of the corner pieces 2 and consisting of tubular elastic section components 3 in the section components 1.

One or a number of acutely angled edges 8 are made in the seat in the frame over a

large portion of the periphery. The resilient components 3,4 press the fabric around these edges. In a preferred embodiment the fabric consists of a polyester woven fabric provided with a PVC coating. Under pressure, this fabric will assume the shape of the acutely angled edges in the frame in the course of time, as a result of which a strong join is produced.

Partly as a result of this, the force required to pull out the fabric is very high whilst nevertheless a very low force is required to push in the resilient components.

As can be seen from Fig. 4, the clamping means 3 are provided with a protruding ridge 12. Upright wall 6 is provided with a corresponding recess 13. When the clamping means or resilient component 3 or 4, respectively, is pushed into the groove delimited between the upright wall sections 7 and 8, ridge 12 will snap into recess 13 so that removal in the reverse direction is no longer possible.

The solution described above leads to low production costs, simple and ergonomic production operations for personnel and a product that can also be supplied to the customer as components in very compact form and without the risk of damage and can be assembled on site by unskilled personnel.

In a preferred embodiment the part of the frame in the form of a section consists of an aluminium section 1 in which at least one tube 9 is incorporated, as a result of which very high torsional rigidity is produced.

In preferred embodiments the resilient press-in component 3 in the form of a section is made of lightweight and inexpensive resilient material, such as a plastic extruded section or a rubber string. As a result a very high proportion of the high grade aluminium used benefits the strength and rigidity of the frame in an optimum manner.