Patent Application
20180133859
The present invention relates to a blade with wear indicating means.
Blades, be it for propellers on aircrafts, boats, in pumps or for all sorts of scraping and cutting tools, all owe their efficiency to a particular geometry. For instance, the blades used to propel fluids must feature a sharp edge for cutting through the fluid without effort and a smooth surface to minimize frictions. Besides, functional cutting and scraping blades obviously require a sharp edge. However, the lifetime of a blade subjected to intensive use may not exceed a few hours until it becomes unsuitably worn out. The progressive erosion of the sharp edge of the blade is due to the repetitive friction against the object to cut, to scrape or the fluid to propel. A blunt blade represents a waste of time and energy, since more force and more time are required to achieve the same result. In addition, the blade does not wear out uniformly, which may result in a dented and rough cutting edge. For example, cavitation is a significant cause of erosion on the sides of propelling blades for liquids. A dented blade has disastrous consequences: instead of a clean cut, the material in the way of a cutting or scraping blade may be torn out. Similarly, a dented propelling blade may generate chaotic turbulences rather than a strong laminar flow. Eventually, the blade cannot be used any more and it has to be discarded.
For the sake of simplicity, the following introduction and description shall focus on blades for cutting, scraping and the like. It will be appreciated, though, that other types of blades, e.g. propelling blades for fluids, face similar problems. Thus, it is obvious to any person skilled in the art to apply the solution described herein to these other types of blades.
In the context of a hand-held cutting tool, dull blades not only slow down or compromise the quality of the cutting process. The extra force required for cutting also intensifies the strain in the user's hand and arm and may cause musculoskeletal disorders, more commonly known as Repetitive Strain Injuries (RSI). RSI are classified as occupational diseases in most developed countries. Besides, the additional force exerted on the material during the cut increases the risk of slipping and the severity of the probable resulting injury. Therefore, it is a matter of prime concern to monitor the sharpness of the blade's cutting edge in order to identify the stage at which it should be deemed unsuitable for cutting. Alas, the sharpness of the blade declines gradually and it is hard for the user to determine when the blade or knife should be replaced. Besides, a visually unaltered cutting edge doesn't necessarily mean that the blade isn't unsuitably sharp.
US 2014/0182144 discloses a cutting blade whose visual appearance changes when it is used. The metallic blade is coated with a colored indicator which wears away during the use of the blade by abrasion against the object to cut. So, the disappearance of the indicator provides a clear signal that the blade has worn out. The indicator layer is meant to have a poor wear resistance, in order to wear away upon the first use of the blade. Therefore, this allows the user to distinguish a brand-new blade from a used one. However, blades are not necessarily dull after the first use. This method therefore doesn't allow the user to detect the wear level, which ideally indicates the precise moment when the blade should be deemed unsuitably dull for the intended use.
EP 0499215 discloses a surface coated with an indicator layer which isn't completely worn away upon the first use of the object, but only much later, when the core material has worn out as well. This is achieved with an indicator layer whose wear resistance is at least as high as the one of the core material. However, the object only changes color when the surface has already worn out to a certain extent and lost its properties (smoothness, regularity). Thus, this invention also fails to signal the precise moment when the object should be deemed unsuitably dull for the intended use, e.g. when the cutting edge of a blade gets dull. Indeed, cutting edges are so sharp that it is impossible to identify their visual appearance or color with the naked eye. Therefore, the change of appearance of the blade will only be visible on the sides of the blade. However, the cutting force is transmitted from the blade to the material to cut via the cutting edge, which is therefore subjected to much more intense abrasion than the sides of the blade. So, if the wear layer located on the sides of the blade has the same wear resistance as the core material forming the cutting edge, it will wear away much slower and it may fail to signal the precise moment when the blade should be discarded.
Of course, even if the wear layer has the same wear resistance as the core material of the blade, one could resolve the above-mentioned issue by making the wear layer thinner. Alas, this raises difficulties specific to the deposition of very thin layers. Let's take the following example:
Then, in order to have the wear layer disappearing totally from the sides of the blade when 50 μm of material is worn away from its cutting edge, the wear layer should be 5 μm thick. Depositing such thin layers with a uniform and accurate thickness is difficult and costly. What's more, even if the coating technique allows the deposition of layers with an outstanding accuracy of ±1 μm, this would still cause variations of thickness of ±20% on a 5-μm-thick layer. Thus, the amount of time the wear layer takes to disappear completely is only determined with a 20% precision. So, taking the example of a blade which gets dull after approximately 5 days of intensive use, the visual appearance of the blade may change one entire day too early or too late, either leaving the user with a blunt blade, or advising to discard a blade which is still suitably sharp.
The main object of the present invention is to provide a blade whose visual appearance changes under the effect of wear. The change of visual appearance occurs as soon as the initial geometry is changed by abrasion, providing a clear visual signal that it needs to be replaced.
In the drawings:
Preferred embodiments of the blade are described according to the drawings. These preferred embodiments are to be understood as exemplary embodiments and any detailed description shall not be interpreted as limiting. In particular, the present blade may be used for any purpose, be it for cutting or scraping or propelling fluids, for example in pumps, on boats or on aircrafts. Alternate embodiments obvious to one skilled in the art will not be described in detail or will be omitted to prevent the relevant details of the invention to be overlooked.
The blade is made of a metallic or ceramic core material 20. A wear layer 21 is located on top of the blade, thus concealing the underlying core material from the view of the user (
This principle can be used to monitor the gradual erosion of the blade either on its whole surface or in areas of particular interest. So, the wear layer may be applied on the whole surface of the blade (
An important feature of the present invention is the careful calibration of the wear resistance and thickness of the wear layer. If the wear layer wears away too easily, the change of visual appearance of the blade may happen while the cutting edge is still suitably sharp (
Therefore, the wear resistance of the wear layer should be smaller than the wear resistance of the core material of the blade. This way, it is possible to calibrate the thickness of the wear layer so that its disappearance occurs at the optimal moment for replacing the blade (
There are at least two possibilities of achieving a layered structure where the wear layer has a different visual appearance than the core layer, and a lower wear resistance. On the one hand, both core and wear layers may be made of the same material, but with structural differences that account for the different visual appearance and the lower wear resistance of the wear layer. For example, the wear layer may be formed by etching a metallic or ceramic core layer in order to make it porous down to a certain depth. The porous structure of the wear layer will then have a different visual appearance and a lesser strength than the untouched core layer. On the other hand, core and wear layers may be made of two different materials. For example, here is a non-exhaustive list of possible combinations of core material and wear layer:
Ceramics are well known for having the best wear resistance, so they are often used as core material for high-performance blades. Metals are less resistant to wear and therefore represent suitable materials for the wear layer. In addition, ceramics and metals have very different visual appearances: polished metals have a shiny grayish color, whereas ceramics are matte, mostly white and can easily be pigmented in many different colors. A further possibility is to use two different metals, a stronger one for the core material and a weaker one for the wear layer. In this case, the layered structure may be achieved by sputtering, plating or Physical/Chemical Vapor deposition (PVD and CVD) for example. If despite their different nature, both metals still have the same visual appearance, they can be given different colors via implantation of ions. Seeding colored nanoparticles onto the surface of a metallic or ceramic core material is another possibility for making a colored wear layer. Besides, ceramic or metallic cores may be coated with various polymers like resins or various organic materials like paints. The advantage of polymers and organic materials in general is that they are cheap, easy to process and can easily be given any desired color. For example, paint is applicable at room temperature and the blade may be coated with a polymer precursor and then UV- or heat-cured.
In order to create a strong direct bond between two materials, especially if they are very different, an intermediary bonding layer 20a may be required to ensure the adhesion of the wear layer 21 to the core layer 20 (
In further embodiments, the blade may not only feature one wear layer, but several stacked layers (
In a further embodiment of the invention, rather than locally coating the core material 20 of the blade with the wear layer 21 and therefore creating a difference in height between the coated and non-coated regions, a first step may involve creating one or several small holes or grooves 13 on the surface of the core material 20 of the blade. This can be done by conventional mechanical machining means, e.g. with a drill, a reamer, a punch, a blade or an indenter. Alternatively, a laser may be used in order to machine high-precision holes or grooves 13 in the core material. These holes or grooves 13 can have any desired shape. In a second step, these holes or grooves 13 are filled with a material building the wear layer 21. As the blade wears out, the depth of the hole or groove 13 and the thickness of the wear layer 21 continuously decreases until it eventually disappears completely, causing a visual change of appearance of the blade. The holes or grooves 13 may also be filled with several successive wear layers with different visual appearances. In a specific embodiment of the invention, the blade may be provided with a series of holes 13 of different depths, each filled with the wear layer material 21 (
In another embodiment of the invention, the core material of the blade 20 may be provided with one or several grooves 13 of increasing depth, filled with a material forming the wear layer 21 (
For some uses an alternative embodiment includes a sensor, which can be included with the knife or blade holder. The sensor can be a separate device or incorporated on the blade. It can be an optical sensor which detects the color of the partially worn blade and sends a signal to the user to indicate that the blade should be replaced. The sensor could also measure the resistance of the blade, which depends on the thickness of the wear layer, to enable the precise detection if the blade should be replaced. Such an embodiment including a sensor is particularly useful in environments, where visibility is limited due to dust or other particles in the air, or when there is only limited lighting.
This description and the accompanying drawings show exemplary embodiments of the invention. The invention, however, should not be interpreted as being limited to these particular embodiments. Variations of the embodiments can be made by those skilled in the art without departing from the scope of this invention as defined by the claims.
