FIELD OF THE INVENTION

The present invention relates to frames that are used as canvas stretcher frames for painting, to which the canvas is fixed and stretched. More particularly, the invention relates to an improved structure of an aluminium-wood canvas stretcher frame which, if necessary, allows with simple techniques to further stretch the canvas after it has been fixed to the stretcher frame. Additionally, the stretcher frame according to the invention can be used for replacing an old stretcher frame during painting restoration. PRIOR ART OF THE INVENTION

For exhibiting a painted canvas or a printed canvas, it is inevitable to use a stretcher frame (kiilraam (in Estonian), stretcher frame (in English), Keilrahmen (in German), spielatten (in Dutch), blindrammer (in Danish), nodpaMHUK (in Russian)), on which the canvas is stretched (cases where the canvas has previously been glued on a stiff base material such as cardboard or plywood will not be considered here). The canvas is fixed to the frame with small nails or, in recent times, with staples. If necessary, it must be possible to later further stretch the canvas fixed this way, by hammering wedges into the slots in the corners of the frame. Stretching is necessary for the canvas (and the image on it) to remain taut, correctly exhibited. Namely, the painted canvas may sag over time due to temperature and air humidity. If the canvas is not stretched, it will slacken and move back and forth on the stretcher frame (depending whether the air in the room becomes depressurized or pressurized, e.g. when opening or closing the door).

In the article“About the choise of tension for canvas paintings”, CeROArt interactive magazine, 4/2009 : Les dilemmes de la restauration (https://ceroart.revues.org/1269) Antonio laccarino Idelson describes the problems concerning canvas stretching and the choice of stretching tension. The article gives an overview of the principles of stretching a canvas on a stretcher frame and of stretching tension calculations.

This known frame type was introduced in the 18th century. Further information: maalikunst.weebly.com/lotildeuend.html.

Wood is the classic material that stretcher frames have been made of over times. In our climatic zone, pine is a good wood material because it has sufficient strength, stability and is readily available. More broadly - in Europe -, spruce is also used. An important property of the stretcher frame is for it to remain straight, to resist the stress caused by the stretched canvas (i.e. remain straight so that the exhibited painting looks good), so knotless wood must be used. Bending occurs most easily where there is a sudden change of density in the material, usually at the location of a knot. Spruce contains significantly more knots than pine and is also softer than pine. Thus, pine is the preferred wood. The occurrence of knotless wood is evaluated to be around 3-5% from the entire volume of sawn pine timber.

In the construction of a stretcher frame, wood drying, reducing internal stresses, plays a significant role. However, some internal stresses remain. The slower the drying process, the more stable the resulting material is, e.g. optimally 24 months. Today, such wood is not available in industrial volumes. Technology has given us an alternative in wood that has been cleared of knots, finger jointed and glued together from boards (gluelam). A blank in the necessary size for producing the stretcher bar profile of the stretcher frame is either glued together directly or the necessary size is cut from a larger rectangular beam (by splitting the beam e.g. into 2 or 3 pieces). In this way, it is possible to get a more or less straight blank for producing the necessary profile. It is important to note the fluctuation of the properties of timber. When in metals a fluctuation of 1 % or more is considered a big flaw, then in glued laminated timber the fluctuation is at least 10% as the material is not homogeneous and its curvature is not smooth. For example, a part of a 6-metre beam can be with an almost imperceptible bend and then there occurs one sudden bend. From there on, the beam might again be with a smoother curvature.

Thus, we cannot state that glued laminated timber is always straight. So, we need to talk of a perceptible straightness. Only a theoretical line between two points is absolutely straight. Everything made of any substance is curved. It is for the user to decide whether the perceptible curvature is acceptable for them or not. So, the authors of this invention can, based on their experience of over 10 years, claim that the most labour-intensive part of producing wooden stretcher frames is cross-cutting the profiled bars into required (ordered) lengths. That is, deciding where it may/can be cut. As a result, we get blanks of stretcher bars in the right length with the curvature in the allowed fluctuation range.

Relating to the wide spread of the technology for printing pictures onto a (painting) canvas during the last 10-15 years, the demand for stretcher frames has increased noticeably. Many pictures are not meant to last very long - e.g. advertisements in stores, etc. Also, people get tired of the acquired picture and recycle it. Undoubtedly, the classical painting technique has not gone anywhere. The demand has rather increased here as well. Painting is a recreational activity for many people.

A substitute for a classic stretcher frame is the so-called fixed-corner frame. The profile is cut under a guillotine or sawn on a respective mitre saw to pieces of necessary length, the pieces are joined in one way or another and the frame is complete. However, after fixing the canvas on this frame, the canvas cannot be stretched any further.

When producing stretcher frames in bulk, the limiting factor is always the length of the bars. Sizes of pictures range from the smallest, e.g. 20 x 20 cm, to rather large ones. For example, frame size 200 x 300 cm is quite common. No (wholesale) trader can keep every possible size in some predetermined range in stock - e.g. all lengths ranging from 20 cm to 300 cm with a 5-mm step. Thus, making a choice in inevitable. Usually it is either 5 cm or 10 cm or 1 inch, etc. The longer the stretcher bars, the larger the interval between two lengths. The eternal question is - what length stretcher bars to order (make). On a long time-scale - such as a month or a year -, it is possible to say quite accurately how many bars and of which size are necessary. It is however almost impossible to predict the demand of the following day or the following week. This problem is solved either by keeping a stock that is large enough or by forcing the wishes of the client into the framework of available possibilities. Although, the latter is often not a solution. Artists usually have a very specific preference for the format. As is well known, stock means tied up money. Of course, there is always the possibility of ordering frames with custom dimensions. Distances are not too big, and it is possible to operate with the principle - directly from the producer to the consumer. On large markets - as in any larger European country - this is not feasible. Western European people have a deeply ingrained knowledge that any custom work, including custom sized painting canvas stretcher frames, are so expensive that ordering such a frame should not even be thought of. The factor of time for filling the orders is also important. A separate category is restorers who, in general, need stretcher frames for old pieces with a mm level accuracy. Here, forcing the piece into any framework is not an option. In the last couple of decades, price has become a dominating factor in assessing the suitability of the objects. In the hunt for cheap prices, other properties of the product are often forgotten or not paid enough attention to. This pressure has put the producers (and buyers) into a situation where every opportunity for cheaper production must be searched for without asking too much at the expense of what this is happening. One part of it is moving production into countries with low labour costs. Another significant part is decreasing the material-intensity of the product, offering products of lower quality under the sign of the previous good-quality product. In terms of stretcher frames, which is a material-intensive product - the largest part is earned by the producer of the raw material -, this has meant a continual reduction of the profile of the bars. This also means lower resistance to the tension of the canvas. Many pictures are surrounded with decorative frames. Here, the same trend occurs - less material. The groove of the moulding of a decorative frame has become so low that it does not fit the bar of the stretcher frame. For example, the thickness of the wood stretcher frame that can resist some considerable tension is at least 20 mm. However, the grooves of the mouldings of the decorative frame are in many cases 10-12-14 mm. Looking at a painting framed this way from the side, we can see how the stretcher frame has been attached to the decorative frame - all the screws and staples are visible. It can be argued whether this is important or not. It would probably be preferable not to have the work processes visible so that attention could focus on the final result - the exhibited painting.

In comparison - the so-called professional frames require a depth of 26-36 mm. The lip that distances the canvas from the frame should be min 4 mm, preferably 6-8 mm. Reducing the height of the lip has become especially popular. In so-called mass production, this is mostly 2 mm. Often less. For the canvas to stay firmly away from the frame, it must be tightly stretched. But it cannot be stretched too tight, as other dimensions are also heavily reduced compared to what the tension of the canvas requires.

It is possible to calculate the moment of inertia of the surface of every bar profile. It is also possible to determine the modulus of elasticity (or use data found from literature that might not apply to the specific batch of the product - NB: variability of wood properties). From here it is possible to calculate the deflection occurring on a specific profile when we apply some force (in the horizontal as well as vertical direction). This force is the force produced from the stretching of the canvas. Based on specialised literature and on the results of a conducted test, the authors of this invention can claim that this force is in the range of 150-200 N per running metre. A canvas stretched with this force acts as a drum when tapped with fingers.

Various types of solutions for canvas stretcher frames can be found from patent documentations. For example, document EP0655521 , Ebaris Co., Ltd. 31 .05.1995 describes a rectangular canvas stretcher frame, which uses special corners and fasteners for connecting the bars of the stretcher frame. Document W094/25296, Cousins Art Holdings Inc., 10.1 1 .1994 describes an external frame that can be attached to the canvas stretcher frame and that is compiled of aluminium profile elements with a profile that attaches the external frame onto the stretcher frame by pressing so that the flanges in the profile catch the stretcher frame.

In conclusion, there are a lot of problems associated with canvas stretcher frames today:

1 . There is very little suitable knotless solid wood;

2. Curvature of wood is chaotic;

3. Based on the previous, cutting the wood into pieces is labour-intensive (users want straight pieces!), due to non-smooth curvature is impossible automatization (for example using optimising saw);

4. The offered range of dimensions is always limited;

5. Making a custom stretcher frame is (very) expensive, time consuming (you need to wait in line);

6. Wishing to save material, the bars of the stretcher frame have been made increasingly thin; despite this, they cannot be“hidden” inside decorative frames.

BRIEF DESCRIPTION OF THE INVENTION

The inventors of this invention propose the use of an improved aluminium frame as a canvas stretcher frame. For a long time, the deficiencies of classic wooden stretcher frames have been tried to be compensated for by using aluminium or aluminium + wood stretcher frames. Maybe this has been mostly brought on by the wish to quickly produce a stretcher frame of any size from the profiles available on-site. The same frame that is made from wood but in some way stretchable. Aluminium profiles are not free of curvature, but it is smooth and, most of the time, noticeably smaller than in wood. Thus, the amount of work for cutting pieces decreases. However, there are many examples where aluminium has been used for making a stretcher frame with fixed corners.

It is not known that anyone has proposed an aluminium painting canvas stretcher frame, which has the following properties combined:

- the frame can be stretched from the corners as well as from the support bar

(many inventions have solved the stretching of the corners, but stretching of the support bar is either unsolved or poor or clumsy); - the frame is made entirely of aluminium, excluding a thin strip of wood for stapling

(there are many versions of aluminium + wood frames but the purpose of this bar is unclear, since for achieving a strength advantage compared to wood, the aluminium must have a contour that is significantly larger than that on e.g. the examples of Alers or NBframe; with the last two, a wooden bar could still be used instead of the proposed bar which would be significantly cheaper; this is based on strength calculations);

- the wooden bar is fixed to the aluminium rigidly, it does not move; at the same time no screws or other fasteners that could prevent the cutting of the profile have been used for fixing (e.g. the wooden bar on Alustretch frames has been fastened with a screw; if the screw happens to be in the cutting path of the circular saw blade, the blade will be ruined; on an Alers frame the wooden bar is attached by denting; in both cases the wooden part can move and if the part of the bar gets used that has no fastening, the wooden part will move of its own accord; immobility of the wood is important in the process of cutting the profile, in the frame it cannot move anymore);

- the support bars are either addable or removable without removing the canvas from the frame

(a very important property; often the need for using a support bar becomes apparent only after stretching the canvas, especially in the case of borderline cases; the support cross must be (temporarily) removed for restoration works, as this work is often also done from the back of the canvas; sometimes a need rises for adding extra supports to the support cross);

- all work operations for fixing and tightening the corner and support bar fastenings have been taken to the back side of the stretcher frame and as far away from the canvas as possible

(an important aspect for avoiding possible damage to the canvas either with a tool or with knuckles);

- there are no details protruding from the contour of the frame

(frames are good to lift without the fear of damaging your fingers; aesthetic);

- only one standard tool is needed for fastening all screws

(practical advantage; compared to the version of the NBframe, where you need a separate expensive tool for stretching that does not move along with the frame; in the case of Alustretch, two different standard keys are needed); - comer fixings are such that allow applying an equation for a fixed-corner frame on calculating deflection

(very important for calculating the deflection of the frame bar; this is 5 times smaller on a frame with fixed corners than on a similar frame with open corners; for example, the corners of an Alustretch frame are completely open, on an Alers frame the corner clamp deflects; in both cases, the corners will work as fixed corners only after some deflection occurs).

Furthermore, the object of this invention is to decrease the amount of different details that are included in the kit of the stretcher frame, i.e. the purpose is to unify the stretching details as much as possible. Another object is to optimize the measurements of the stretcher frame profile depth. To achieve this, strengthening grooves have been added to the profile that can simultaneously be used for guiding the stretching details. In addition to the grooves in the profile, the stamped grooves or flanges are also modified and used with corner and support clamps (support plates) to thus make the corner and support clamps (support plates) stiffer, which in turn makes the entire stretcher frame stiffer. This enables to use a thin-walled profile and reduce the weight of the stretcher frame.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in more detail below with reference to the drawings, where:

Figure 1 shows an assembled canvas stretcher frame according to the invention viewed from below;

Figure 2 shows the fastening of the canvas stretcher frame support bar in the stretcher bar, where the ends of the support clamps attached into the groove of the support bar have been placed into the side groove of the stretcher bar, this Figure also shows a stretching element according to the invention for fixing stretcher bars;

Figure 3 shows the cross section of the support bar and stretching element shown in Figure 2 to illustrate the arrangement of the grooves and flanges of the profile material of the support bar and the shape of the stretching element together with the fixing screws and stretching screw;

Figure 4 shows the connection of the stretcher bars of the stretcher frame to the corner clamps and stretching elements; Figure 5 shows a cross section along the line C-C on Figure 4 to illustrate the cross sections of the stretcher bar and the stretching element and the placement of the wooden bar that is used for fixing the canvas;

Figure 6 shows a cross section along the line D-D on Figure 4 to illustrate the cross sections of the stretcher bar and corner clamps and the stretching of the corner clamps in the grooves of the stretcher bar;

Figure 7 shows a cross section along the line E-E on Figure 1 to illustrate the cross section of the support bar and support clamps and the stretching of the support clamps in the grooves of the support bar;

Figure 8 shows a cross section along the line F-F on Figure 1 to illustrate the cross section of the stretcher bar and a cross section of an alternative embodiment of the corner clamps (the same cross section can be used to produce the support clamps used for fastening the support bars);

Figure 9 shows a cross section along the line F-F on Figure 1 to illustrate the cross section of the stretcher bar and a cross section of another alternative embodiment of the corner clamps (the same cross section can be used to produce the support clamps used for fastening the support bars);

Figure 10 shows the cross sections of the corner clamps with the alternative embodiment shown in Figure 8, i.e. the cross sections of the upper corner clamp and the lower corner clamp (the same cross section can be used to produce the support clamps meant for fastening the support bars);

Figure 1 1 shows the cross sections of the corner clamps with the alternative embodiment shown in Figure 9, i.e. the cross sections of the upper corner clamp and the lower corner clamp (the same cross section can be used to produce the support clamps meant for fastening the support bars).

Figure 12 shows the reinforced corner design of an assembled canvas stretcher frame having long stretcher bars with length more than 2m for example.

Figure 13 shows the cross section of the corner having reinforcement corner clamps arranged in cavity of stretcher bar profile.

Figure 14 shows the outside and inside reinforcement corner clamps.

Figure 15 shows top view of the fixing screw if the outside reinforcement corner clamp. Figure 16 shows arrangement and connection of the continuous stretcher bars with joint clamp and fixing screws in case if the stretcher bar is longer for example than 3m . Figure 17 shows the cross section of the continuous stretcher bars and arrangement of joint clamps and fixing screws.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a canvas stretcher frame 5 according to the invention that includes stretcher bars 1 , 2, 3, 4 that are joined together by metal corner clamps 6 and thus form a rectangular stretcher frame 5. The ends of stretcher bars have been cut at a 45-degree angle, which is a common angle for cutting bars, so that when assembling the stretcher frame 5, a rectangle would form with corners of exactly 90 degrees. A stretcher frame used with larger canvases/prints has additional support bars 7, 8 to support the stretcher bars of the stretcher frame 5 against deflection. Support bars may be horizontal 7 or vertical 8 in relation to, for example, the longest side of the stretcher frame.

Stretcher bar 1 , 2, 3, 4 has been cut from an aluminium profile that has a cross section on which different grooves (9 - groove for stretching element and corner clamps; 18 - groove for ends of support clamps, 31 - groove for wooden bar) and flanges (10 - groove for stretching element; 34 - flange of the groove meant for the end of the support clamp of the stretcher bar profile; 35, 36 - flange of the groove for wooden bar) can be seen that give the profile the necessary stiffness, furthermore, the grooves of the profile can be used for attaching a stretching element (stretching clamp, stretching device, etc.) 1 1 (grooves 9 and flanges 10), wherein the stretching elements work as a retainer as well as a stretching device, wooden bars 21 , 22, 23, 24 (grooves 31 and flanges 35, 36), to which the canvas to be attached to the stretcher frame 5 is fixed (e.g. with staples, fixing nails, etc.), and the support clamps for the support bars 61 (groove 18 and flange 34), wherein the support bars are necessary for stretching larger canvases when using larger stretcher frames.

On the back side of the stretcher bar profile there are grooves 9 into which the stretching clamp 1 1 and the corner clamps 6 used for joining the stretcher bars are placed when assembling the stretcher frame 5. The shape of the grooves 9 and the stretching clamp might be different, but their working principle is the same. Stretching clamp 1 1 consists of a cylindrical corpus and two flat side surfaces. Two holes 12 have been drilled and tapped perpendicularly in relation to the stretcher bar profile into the flat side surfaces (wings or ears) of the stretching clamp 1 1 and sharp point screws 13 have been driven into them. Stretching clamp 1 1 is placed into the desired position in the grooves 9 of the stretcher bar and the screws 13 are turned until they cut into the surface of the stretcher bar 1 , 2, 3, 4, in doing so, the upper surface of the stretching clamp is pressed against the flanges 10. Thus, a firmly stationary and fixed support is formed. Now the stretching screw 14 of the stretching clamp is turned, wherein the screw is parallel to the stretcher bar and has been driven into the hole drilled and tapped into the cylindrical corpus of the stretching clamp. The longitudinal axis of this hole is also the longitudinal axis of the cylindrical corpus of the stretching clamp. As the stretching clamp 1 1 is placed against the end of the pair of metal corner clamps 6 fixing two stretcher bars (e.g. 1 and 2, 2 and 3, 3 and 4, 4 and 1 ) together, the stretching screw 14 starts pushing the pair of corner clamps 6 away from itself. The stretching screw 14 has a blunt point, that way the stretching screw 14 is in contact with both corner clamps 6. Thus, the effect known from the state of art, wherein the wooden stretcher frames are stretched with wedges, is produced. When the desired stretching outcome is reached, the fixing screws 15 of the corner clamps 6 of the stretcher bar are fastened. Wherein a hole for the fixing screw 15 has been drilled and tapped into the upper corner clamp of the pair of corner clamps 6. The lower corner clamp of the pair of corner clamps 6 has no hole. In this solution, when tightening the fixing screw 15, the lower and upper corner clamp of the pair of corner clamps 6 are pushed away from each other, in doing so, the lower surface of the lower corner clamp is pushed against the stretcher bar profile and the upper surface of the upper corner clamp against the flanges 10 of the stretcher bar profile. More generally, the solution of corner clamps 6 is the used method of affixing that has been improved in this invention and that can be successfully used for assembling aluminium profile decorative frames. In the embodiment of the present invention, the corner clamps 6 are equipped with 2 + 2 fixing screws 15 to ensure sufficient stiffness to the corner of the stretcher frame 5 assembled from stretcher bars 1 , 2, 3, 4 after the opening of the corners resulting from the stretching. If necessary, additional reinforcement can be added into the cavity of the stretcher bar profile to achieve greater stability. In an alternative solution (see Figure 8 or 9, also Figures 10 and 1 1 ) the corner clamps as well as support clamps can be produced with a special profile. For example, Figure 8 shows the upper and lower corner clamp/support clamp with special profiles, where the cross section of the upper corner clamp/support clamp 602 is a U-shaped profile, which is formed by the base 605 of the corner clamp/support clamp and sides 604 (ears) protruding from its edges perpendicularly, and which has such a height that the upper corner clamp/support clamp 604 can move freely in the groove 9 meant for the corner clamps/support clamps. In the middle of the base 605 of the comer clamp/support clamp there is a strengthening guidance 606 parallel with the sides, which fits into the guide groove 609 of the lower corner clamp/support clamp 601 . The part of the guide groove pressed out of the plane of the lower corner clamp/support clamp in turn fits into the longitudinal indent 29 in the profile of the stretcher bar/support bar. This profile of the upper and lower corner clamp/support clamp firstly makes the corner clamp/support clamps stiffer and in turn makes the entire assembly of the stretcher bars into the stretcher frame stiffer.

In another alternative embodiment for the corner clamps/support clamps, the lower corner clamp/support clamp 609 has the same cross section but the profile of the upper corner clamp 603 has been altered. The cross section of the upper corner clamp/support clamp 603 is a U-shaped profile, which is formed by the base 605 of the corner clamp/support clamp and the sides 604 (ears) protruding from its edges perpendicularly, and which has such a height that the upper corner clamp/support clamp 604 can move freely in the groove 9 meant for the corner clamps/support clamps. In the middle of the base 605 of the corner clamp/support clamp there is a strengthening guidance 607 (a groove pressed upwards from the base in the direction of the sides) parallel to the sides and guide grooves 608 (pressed downwards from the base) formed on both sides of the strengthening guidance. The strengthening guidance fits into the guide groove 609 of the lower corner clamp/support clamp 601 . This use of the profile of the upper and lower corner clamp/support clamp firstly makes the corner clamp/support clamps stiffer and in turn makes the entire assembly of the stretcher bars into the stretcher frame stiffer. Furthermore, this enables producing lighter and thinner stretcher bars and support bars without the fear of them deflecting or breaking later, when the canvas fastened to the stretcher frame is stretched.

A novel solution on the canvas stretcher frame according to this invention is also the method for attaching the support bar 7, 8 to the stretcher bar 1 , 2, 3, 4 and the method for stretching the support bar. On the back side of the support bar 7, 8 there are grooves 19 identical with the grooves 9 on the stretcher bar, together with identical flanges 10. At least two metal support clamps 61 that are pushed apart from each other by blunt point fixing screws 17 are placed into the grooves 19 and fastened. As with the corner clamps of the stretcher bar, a hole has been drilled and tapped into the upper support clamp 61 for a fixing screw, there is no hole in the lower support clamp. In this solution, when tightening the fixing screw 17, the lower support clamp 61 and the upper support clamp are pushed away from each other, in doing so, the lower surface of the lower support clamp is pushed against the support bar profile and the upper surface of the upper support clamp against the flanges 10 of the support bar profile. The ends of the support clamps 61 extend beyond the end of the support bar 7, 8 and fit into the corresponding external groove 18 in the lower external edge of the stretcher bar (frame bar) 1 , 2, 3, 4. The width of the external groove 18 is smaller (a few tenths of a millimetre narrower) than the total width of the two (metal) support clamps 61 pushed apart from each other and the depth/width/height thus formed between them. Thus, strong friction occurs between the inner walls of the external groove 18 and the ends of the metal support clamps 61 . This frictional force is enough to keep the support bar 7, 8 in the desired position in the stretcher bar. The same method is used to join two support bars together at the intersection of the bars. The ends of the support clamps 61 are inserted into the external groove 18 of the other support frame profile.

Stretching of the support bar is conducted on the same principles as the stretching of the stretcher bar (side bar). First, the stretching clamp 1 1 is fastened into the grooves 9 of the support bar 13 with sharp point screws and then by turning the stretching screw 14, the support clamps (more specifically, the ends of the support clamps) 61 are pressed against the base of the external groove 18 of the stretcher bar. When the required tension is achieved on the stretcher bar and support bar and thereby on the canvas fixed to the stretcher frame, the support clamps 61 are stretched with fixing screws 17 in the grooves 9 of the support bar and the external groove 18 of the stretcher bar.

Pieces of stretching clamp 1 1 cut from the profile of the stretching clamp can also be used as the hanging loops of the stretcher frame. They must be pushed into the grooves 9 on the back side of the stretcher bar (side bar) before assembling the stretcher frame. For example, one fixing screw 13 is enough for fastening a piece of stretching clamp. A suitable wire or other hanging device 20 can be inserted through the hole for the stretching screw 14. As the profile of the stretcher bar has the flange 34 of the external groove 18, it is possible to hook a previously existing hook on the wall behind this. In this case, using the pieces of stretching clamp 1 1 and the hanging wire is not necessary.

This stretcher frame according to the invention and its fastenings stretch the stretcher frame together with the canvas and fasten the support bar in a minimalistic way, a solution that has not been proposed before. The construction according to the invention helps to save material used for producing stretcher bars and support bars, because the flanges, grooves and the longitudinal indent 29 in the profile as well as the profiles of the corner clamps and support clamps described above ensure an improved stiffness and stability to the entire stretcher frame. This is especially important in the restoration of older prints/paintings. Also owing to the lightness of the stretcher frame, it is significantly easier for the user to transport paintings/prints.

When designing the profile of the stretcher bar of the aluminium stretcher frame according to the invention, the main object was to find an easy system containing minimal details for stretching the corners and the support bar. Also, to achieve maximum resistance to the stress created by the tension of the canvas at a minimal weight per running metre. The main advantage of the aluminium frame as compared to wood is that it helps to bear greater tensions with the same contour of the profile. One desired outcome is lighter weight per running metre. Here, a rule applies, according to which the bigger the contour of the aluminium profile, the larger the advantage of weight in comparison with a wooden bar with an equivalent contour. This result is achieved at the expense of the cavities in the aluminium profile. A certain weight advantage is also achieved at the expense of a smaller amount of support bars. An aluminium frame does not need them in the same amount as a wooden frame. The profiles of the stretcher bar and support bar of the stretcher frame 5 are produced from an aluminium profile, which has cavities 32, 33 (on support bar 41 , 42, respectively) for reducing the weight of the stretcher bar/support bar, wherein the wall thickness and flanges of the profile have been developed according to the calculation of optimum deflections, and the deflections of the profiles of the stretcher bars and support frames of the present technical solution have been compared with the deflections of the widely used profiles that are, at present, commercially available. Comparison results are summarized in tables 1 and 2.

Table 1. Comparison of moments of inertia between commercially available profiles and the profile according to the invention

Table 2. Comparison of deflections between commercially available profiles and the profile according to the invention

Based on the test results, the canvas stretcher frame according to the invention that is composed of stretcher bars according to the invention has significantly better test results, therefore there is less material consumption and the stretcher frame is considerably stiffer after stretching, at the same time, if the canvas should stretch over time, stretching the canvas is easy.

On the lower side of the stretcher bar profile, the flanges 35 and 36 have been used to form grooves 31 into which wooden bars 21 , 22, 23, 24 are placed for fixing the canvas to the canvas stretcher frame. On the surface of the grooves of the profiles there are longitudinal flanges which are used to ensure tight fastening of the wooden bar in the groove so that the wooden bar would not move around in the groove even if the dimensions of the wooden bar should decrease due to low air humidity. Generally, wooden bars are made from wood, which has a minimal decrease in dimensions upon drying (e.g. oak, ash, birch, etc). In the middle of the profile there are grooves 9 formed by flanges 10 for holding the stretching clamp 1 1 and corner clamps 6 stationary and for fastening them. On the other side of the stretcher bar there is a groove 18 formed by the flange 34 into which the ends of support clamps are inserted that are used for connecting the stretcher bar and the support bar. In the middle of the base of the grooves 9 of the stretcher bar profile runs a longitudinal indent 29 which fits the cylindrical corpus of the stretching clamp. This way, when the stretching clamp has already been placed into the grooves, substantial movement in the direction of the longitudinal axis of the stretching clamp can be avoided. Furthermore, on the bases of the grooves on either side of the longitudinal indent 29, run longitudinal grooves into which the point of the sharp point screws is fixed for fastening the stretching clamp in the grooves. Using the longitudinal groove 29 also makes the profiles of the stretcher bar and the support bar stiffer and more stable.

The profile of the support bar 7, 8 is similar to the profile of the stretcher bar, excluding the fact that practically it is a symmetrical profile, where grooves 18 that are identical to the external groove 18 on the longitudinal bar are formed to both sides of the lower side of the profile by flanges 43 and where grooves 19 that are identical to the grooves 9 of the longitudinal bar are formed in the middle of the profile by flanges 10. Also, in the middle of the base of the grooves 19, runs a longitudinal indent 29 for supporting the cylindrical corpus of the stretching clamp 1 1 . On either side of the longitudinal indent 29, run longitudinal grooves for the sharp point screws 13 used for fastening the stretching clamp.

When the dimension of the canvas are over 2 meters in one side then the canvas stretcher frame may bend from the corners when canvas is stretched to said frame. In that case additional reinforcement corner clamps 620, 621 are arranged into cavity of stretcher bar profile. In this alternative embodiments the stretcher bars are not cut at a 45-degree angle, when assembling the stretcher frame (5) in case of small canvas, but the stretcher bars are cut at a 90-degree angle to enable to use the reinforcement corner clamps for more stable stretcher frame. The reinforcement corner clamps are L-shaped, one part of L-shaped clamp is inserted into cavity of one stretcher bar 1 and other part of L-shaped clamp is inserted into cavity of other stretcher bar 2. In figure 14 and 15 are shown the reinforcement corner clamps away from each other to illustrate the shape of clamps and arrangement of threaded hole for fixing screws. The outside reinforcement corner clamp has threaded hole for fixing screw 615, and inside reinforcement corner clamp is without holes. Inside clamp means the corner clamp arranged into cavity of stretcher bar towards the inner side the frame and outside clamp means corner clamp arranged into cavity of stretcher bar toward the outer side of frame (see figure 12). When both reinforcement corner clamps are arranged into cavity of stretcher bar 1 they are tightened into cavity of the stretcher bar by tightening the fixing screw 615, the outside reinforcement corner clamp 620 and the inside reinforcement corner clamp 621 are pushed away from each other, in doing so, the reinforcement corner clamps are pushed against the walls of the cavity 32 in the stretcher bar.

Sometimes the canvas or painting have frame with dimension more than 3 meters. Usually the stretcher frame parts (stretcher bars) are prepared in advance in workshop and transported to the place where stretcher frame in mounted and canvas stretched into said frame. Transportation of long stretcher bars is inconvenient and difficult. Therefor in alternative embodiment the stretcher bars are cut in parts (for example cut halfway or other suitable dimension) and joined later together with joint clamp 231 . Joint clamp 231 is arranged into groove 9 in the profile of the stretching bar. Joint clamp has upper joint clamp having threaded holes for fixing screws. The threaded holes arranged in staggered arrangement for better and uniform tightening. These fixing screw can be the same screws as fixing screw 15 for corner clamps. Tightening of the joint clamp is same as tightening of corner clamps, a hole has been drilled and tapped into the upper joint clamp 232 for a fixing screw, there is no hole in the lower joint clamp 233. When tightening the fixing screw 17, the lower joint clamp 233 and the upper joint clamp 231 are pushed away from each other, in doing so, the lower surface of the lower joint clamp is pushed against the support bar profile and the upper surface of the upper joint clamp against the flanges 10 of the support bar profile. The ends of the continuous stretcher bars 200, 201 and continuous wooden bar are cut for example at a 45-degree angle before joining the continuous stretcher bars together when assembling the stretcher frame (5).

From the point of view of the producer of the frame (also the wholesaler, warehouse keeper, etc.), the possibility to offer a stretcher frame of any dimensions at any time is a significant advantage. Production is fast and significantly less labour-intensive than the production of a wooden frame of specific dimensions. As a further advantage it is noticeably more affordable for the end user. There is also no need for keeping an immense stock with various bar lengths.

The only disadvantage is the higher price of aluminium compared to timber. Nevertheless, aluminium has many properties that timber will never have, no matter what the cost of the material.

List of details:

1 , 2, 3, 4 - stretcher bars

2 - stretcher bar

3 - stretcher bar

4 - stretcher bar

5 - stretcher frame

6 - corner clamps

61 - support clamp for support bar

7, 8 - support bars

8 - support bar

9, 19, 18, 31 - groove

9 - groove for stretching clamp and corner clamps

18 - groove for ends of support clamp

19 - groove for stretching clamp and support clamps

31 - groove for wooden bar

10, 34, 35, 36 - flange

10 - flange of the groove of the stretching clamp

34 - flange of the groove meant for the end of the support clamp of the stretcher bar profile

35, 36 - flange of the groove of the wooden bar

43 - flange of the groove meant for the end of the support clamp of the support bar profile

11 - stretching element (stretching clamp)

12 - drilled and tapped hole

13 - sharp point fixing screw of stretching clamp

14 - stretching screw of stretching clamp

15 - fixing screw of corner clamp

17 - blunt point fixing screw for corner clamps and support clamps for support bar 20 - hanging device

21 , 22, 23, 24 - wooden bars for fixing the canvas to the stretcher bars

22 - wooden bar

23 - wooden bar

24 - wooden bar

29 - longitudinal indent for cylindrical corpus of stretching clamp

32, 33 - cavity of stretcher bar profile

41 , 42 - cavity of support bar profile 601 - lower corner clamp/lower support clamp

602 - upper corner clamp/upper support clamp

603 - upper corner clamp/upper support clamp

604 - side profile (ears) of upper corner clamp/upper support clamp

605 - base of the upper corner clamp/upper support clamp profile

606 - guidance (extruded curvature) of upper corner clamp/upper support clamp

607 - guidance (extruded curvature) of upper corner clamp/upper support clamp

608 - guide groove of upper corner clamp/upper support clamp

609 - guide groove of lower corner clamp/lower support clamp 200, 201 - continuous stretcher bar

210, 220 - continuous wooden bar

231 - joint clamp

232 - upper joint clamp

233 - lower joint clamp

615 - fixing screw of outside reinforcement corner clamp

620 - outside reinforcement corner clamp

621 - inside reinforcement corner clamp