Endless yarn tensioning strip and method for producing the same

Abstract

An endless yarn braking strip for a yarn tensioning device consists of precipitation hardened stainless steel (S). The endless yarn braking strip is produced by first manufacturing an endless blank from a sheet of precipitation hardening steel, shaping the blank into a truncated cone in its endless form, and then hardening the strip by precipitation hardening.

Description

[0001] The invention relates to an endless yarn braking strip according to the preamble part of claim 1 and to a method for manufacturing an endless yarn braking strip according to the preamble part of claim 2.

[0002] In a yarn brake comprising a frustoconical, endless braking strip the inner surface of the braking strip is pressed coaxially against a yarn withdrawal surface of e.g. a storage body, the yarn withdrawal surface in most cases being rounded. In this way a contact area is created between the inner surface of the braking band and the yarn withdrawal surface which contact area is used as a braking zone. The yarn is stored on the storage body in the form of windings and is withdrawn from the windings overhead of the storage body and below the braking strip. The yarn passes the contact area between the braking band and the withdrawal rim in order to be braked, or to create a substantially uniform yarn tension, respectively. By the withdrawal of the windings an orbiting rotation of the withdrawn yarn is generated similar to the motion of the hand of a clock. The braking strip undergoes a deformation following the orbiting motion. The yarn rubs at the braking strip. The storage body and the braking strip are maintained substantially stationarily. For this reason the braking strip needs to have flexibility, good spring property and high wear resistance against the friction load of the respective yarn material and against the friction load at the withdrawal surface defining the braking zone together with the braking strip, which withdrawal surface in most cases is metallic.

[0003] A frustoconical endless braking strip is known from WO 98/23520. The braking strip consists of a metal or a metal alloy. In this case beryllium-copper is mentioned as an example. Beryllium-copper is a material which needs extreme care during machining. In case of certain operation conditions or for certain yarn qualities the braking strip, however, may become worn out relatively rapidly and markedly.

[0004] U.S. Pat. No. 5,546,994 discloses a braking strip consisting of a metal sheet with a thickness of 0.1 mm or less and which is produced by stamping and deep drawing. The braking strip is described as being flexible.

[0005] U.S. Pat. No. 5,678,779 discloses an endless braking strip of this kind consisting of a metal alloy.

[0006] U.S. Pat. No. 5,409,043 discloses such a braking strip which is provided in the form of a very thin metallic layer at the inner side of an outer Kevlar carrier cone. The braking strip, alternatively, may consist of a steel sheet of a thickness of 0.05 to 0.1 mm. The active surface of the strip may be chromium plated or nickel plated, respectively.

[0007] The braking strip has to be endless, springy, smooth despite the small thickness and wearproof at its active surface for a proper performance. The spring property and the wearproofness could be achieved by hardened conventional steel types. However, hardened conventional steel normally cannot be formed anymore. On the other hand, hardening of prior deformed conventional steel hardly could be carried out until now. Hardening of conventional steel types after the forming process could result in form deficiencies of the frustocone in case of such thin wall thicknesses (maximum a few tenths of a millimetre) which form deficiencies destroy the necessary evenness of the braking strip at the active surface and which do not allow to achieve a homogeneous deformation performance of the braking strip in operation. For those reasons such yarn braking strips until now were produced from other metallic materials.

[0008] A flexible yarn guiding sleeve known from U.S. Pat. No. 2,218,976 A consists of steel. For improving the wear resistance only a free end section of the yarn guiding sleeve is hardened by a heat treatment with subsequent quenching.

[0009] The article W. T. LANKFORD, Jr. & AL.: “The Making, Shaping and Treating of Steel” 1985, HERBICK & HELD, PITTSBURGH, Pa. XPOO2193211, pages 1335, 1339, 1344, 1345 discusses the compositions of steels having high amounts of Cr-Ni of the class “Precipitation Hardening Stainless Steel” as well as method steps carried out during precipitation hardening of those steel classes.

[0010] The section Beitz, K: H: Grote, Dubbel: “Taschenbuch für den Maschinenbau”, 19 Edition, Springer Verlag, Berlin, 1997, ISBN 3-540-62467-8, E37-E39, discloses steel hardening processes for different steel classes, the consequences of the hardening processes and several application examples for the hardened steels.

[0011] It is an object of the invention to provide a yarn braking strip of the kind as mentioned at the beginning as well as a method for manufacturing of such a yarn braking strip which allow to fulfil the requirements of wearproofness, uniform surface smoothness, uniform spring property and industrial large series production in a cost effective way.

[0012] The object can be achieved by the features of claim 1 and in terms of the method by the features of claim 2.

[0013] Precipitation hardened stainless steel containing a high Cr-Ni amount of the class “Precipitation Hardening Stainless Steel” as is normally used for constructing aircrafts unexpectedly precisely fulfils the requirements which are valid for a braking strip of a yarn brake. That is, the steel can be formed comfortably prior to the hardening process in order to form the frustoconical shape of the braking strip from a flat blank without form deficiencies and then can be hardened such that it has the required springiness, the smooth surface and mainly the wear resistancy at the active surface. Precipitation hardening stainless steel is available in the desired thicknesses for fair costs, and can be simply machined mechanically and hardened comfortably.

[0014] In terms of the method the deformability of the precipitation hardenable stainless steel is used in order to form first the frustoconical shape of the braking strip from the flat blank. Then the good hardening property is used in view to high springiness and good wearproofness at the active surface. In this case it astonishingly has been found out that the hardening process can be carried out despite the thin wall thickness such that a smooth surface results and such that no form deficiencies of the frustocone of the yarn braking strip occur.

[0015] The flat blank, expediently, is formed by stamping. This allows to achieve a high production rate with sufficient preciseness. The cold deformation into the form of the frustocone coat is expediently carried out by deep drawing in a tool. The hardening process is carried out in three steps, namely in an austenite conditioning step, a subsequent austenite-martensite-transforming step and finally in an ultimate precipitation hardening step.

[0016] During the austenite conditioning step the cold deformed blank is heated to about 955° C. and is then maintained about ten minutes at this temperature. Thereafter the blank is cooled down in ambient air to ambient temperature. Before an hour has expired the cooled down, cold deformed blank is cooled down to about −73° C. and is maintained at this temperature for eight hours. Then the blank is allowed to warm in ambient air again to ambient temperature (transformation step). Ultimately the blank is heated to about 510° C., is maintained at this temperature for about 90 minutes, and finally is cooled down in ambient air to ambient temperature. By this the precipitation hardening is terminated. The braking strip then can undergo further conventional machining steps or may be directly integrated into the yarn brake, respectively.

[0017] In view to precise dimensions and homogeneous properties of the braking strip, it may be expedient, to stamp the blank as a flat annulus over-dimensioned in radial direction and to cut the blank only after the cold deformation and prior to the hardening process to the target dimensions.

[0018] The material provided in radial direction in the over-dimensions is expedient to compensate for material displacements occurring during the stretching process accompanying the cold deformation. As a result of the subsequent cutting then uniform properties will be achieved in the yarn braking strip up to the final cutting edges.

[0019] Alternatively, the final cutting even may be carried out after the hardening process.

[0020] During the cold deformation, expediently, a uniform wall thickness is set between 0.01 mm to 0.5 mm. A thickness range of about 0.05 mm to about 0.3 mm is of particular advantage for such yarn braking strips made from this precipitation hardened steel.

[0021] In order to avoid corrosion during storing and transportation or the like which corrosion might disturb during the machining of the steel, and to achieve a better machinability, the blank ought to be stamped from an annealed sheet metal. The term “annealed” is to be understood as a corrosion protection measure already employed by the sheet metal producer. This may, e.g., comprise a state which is called “Mill Annealed”, i.e., a state achieved by a treatment called “Solution Heat Treated And Rapid Cooled”.

[0022] The invention will be explained with the help of the drawing. In the drawing is:

[0023] FIG. 1 a perspective view of an endless braking strip for a yarn brake,

[0024] FIG. 2 the step of manufacturing a flat blank, in a vertical section,

[0025] FIG. 3 schematically the cold deformation of the flat blank of FIG. 2 into the shape of a frustocone, including a subsequent cutting of the blank into the final dimensions, and

[0026] FIG. 4 a vertical section of the braking strip, including indications to the carried out annealing or hardening steps.

[0027] An endless braking strip B has in FIG. 1 the shape of a frustocone coat with a smaller diameter di and a larger diameter da, a height h in the direction of the cone axis X, a strip width b in the direction of the generatrice, and a wall thickness y. The wall thickness y lies between about 0.01 mm and 0.5 mm and is uniform within the entire braking strip B. The braking strip B consists of a precipitation hardened stainless steel S (precipitation

[0028] An endless braking strip B has in FIG. 1 the shape of a frustocone coat with a smaller diameter di and a larger diameter da, a height h in the direction of the cone axis X, a strip width b in the direction of the generatrice, and a wall thickness y. The wall thickness y lies between about 0.01 mm and 0.5 mm and is uniform within the entire braking strip B. The braking strip B consists of a precipitation hardened stainless steel S of the class “Precipitation Hardening Stainless Steel”. A good operation performance can be achieved with the wall thickness of 0.08 mm, e.g. in case of a yarn braking strip having an outer diameter of about 110 mm, and inner diameter of about 85 mm, and a cone apex angle between about 90° and 120°.

[0029] The inner surface of the braking strip B is the active braking surface which has to be smooth and wear resistant. Furthermore, the strip has to be unstretchable but must be deformable in radial direction or must be springy in radial direction.

[0030] For manufacturing the braking strip B of FIG. 1 the following steps are carried out:

[0031] According to FIG. 2 first an annular flat blank Z is formed from a flat sheet metal M, e.g. by stamping. The inner diameter of the annulus is smaller than the target inner diameter di of the braking strip. The outer diameter of the blank is larger than the target outer diameter da of the braking strip B.

[0032] According to FIG. 3 the flat blank Z is brought to the shape of the frustocone or of a frustoconical intermediate product Z1 in a tool W, e.g. by deep drawing. Since unavoidably material displacements occur during a deep drawing process the blank Z is oversized in radial direction. The over dimensions then will be used during the deformation step of FIG. 3 in order to allow a cold flow of the material. After the deformation steps the blank is cut such that the target diameters di and da result.

[0033] The blank Z1 now having already the final dimensions then is treated as follows:

[0034] During an austenite conditioning step I the blank Z1 first is heated to e.g. 955° C. and is maintained for about ten minutes at this temperature. Then the blank is allowed to cool down in ambient air again to ambient temperature RT.

[0035] In a second austenite-martensite-transforming step II which starts within one hour after the first step I the blank Z1 is cooled down to about −73° C. and is then maintained at this temperature for about eight hours before the blank is allowed to again warm in ambient air to ambient temperature RT.

[0036] In a subsequent precipitation hardening step III blank Z1 again is heated, optionally to about 510° C., is maintained for ninety minutes at this temperature, and then is allowed to cool down in ambient air to ambient temperature RT. Then the braking strip B consisting of the precipitation hardened stainless steel S is manufactured.

[0037] A subsequent machining is not necessary, however, may occasionally be carried out.

[0038] Among other ingredients precipitation hardenable stainless steel of the class “Precipitation Hardening Stainless Steel” contains chromium and nickel as main alloy ingredients. This steel type per se is intended for manufacturing springs, clips, frame structures in aircrafts and pressure tanks. For such applications the wear resistance of this steel is of secondary importance. According to the invention to the contrary, mainly the wear resistance of the precipitation hardenable steel is used as an extremely favourable side effect for braking yarns.

Claims

1. Endless braking strip (B) for a yarn brake, the braking strip having the shape of a frustocone coat produced by cold deformation of a flat blank (Z) made from thin metal sheet (M), characterised in that the braking strip (B) consists of precipitation hardened stainless steel (S) of the class “Precipitation Hardening Stainless Steel” containing chromium and nickel as main alloy ingredients.

2. Method for manufacturing an endless braking strip for a yarn brake according to claim 1, wherein the method comprises the following sequential method steps, cutting the endless flat blank (Z) from the sheet metal (M), cold forming of the blank (Z) into the shape of a frustocone, precipitation hardening of the frustoconical blank (Z1).

3. Method as in claim 2, characterised by stamping the blank (Z), deep drawing the stamped blank (Z) into the frustoconical shape, and precipitation hardening of the deep drawn blank (Z1) an in an austenite conditioning step (I), a subsequent austenite-martensite-transforming step (II) and a final precipitation hardening step (III) .

4. Method as in claim 2, characterised by heating the deep drawn blank (Z1) to slightly below 1000° C., preferably to 955° C., and maintaining this temperature for about ten minutes, and cooling down in ambient air to ambient temperature (RT) during the austenite conditioning step (I), cooling the blank (Z1) subsequently within one hour to about −73° C., maintaining this cooling temperature over about eight hours during the austenite-martensite-transforming step (II), and warming in ambient air to ambient temperature, and heating the blank to somewhat above 500° C., preferably 510° C., maintaining this temperature over about ninety minutes, and cooling down in ambient air to ambient temperature (RT) during the precipitation hardening step (III).

5. Method as in claim 2, characterised by stamping the blank (Z) as a flat annulus with over dimensions in radial direction and cutting the blank in radial direction to the target dimensions (di, da) first after the deep drawing and prior to the precipitation hardening process.

6. Method as in claim 2, characterised by stamping the blank (Z) as a flat annulus with over dimensions in radial direction and cutting the blank in radial direction to the target dimensions (di, da) first after the precipitation hardening step.

7. Method as in claim 2, characterised by deep drawing the blank into a uniform wall thickness within a range (y) between 0.01 mm and 0.5 mm, preferably between 0.05 mm and 0.3 mm.

8. Method as in at least one of claims 2 to 7, characterised by annealing the sheet metal (M) prior to stamping the blank (Z).