CA2273048C - Connecting element for the frictional connection of components - Google Patents

Abstract

A connecting element for the friction- increasing connection of workpieces which are to be joined, which is a thin, flexible element which bears particles of defined size at its surface, the particles are made from a material with a compressive and shear strength which exceeds that of the workpieces to be joined.

Description

Conaeoting Elem~at ~'or Ths 8riational Conaeation Of Compoaenta The invention relates to an element for the frictional connection of components.
8_aokq~ouad of tho Iav~ation Frictional connections are used in all sectors of mechanical engineering, often to, transmit transverse forces or torque. The magnitude of the force which can be transmitted in each case depends, in addition to the design features, primarily on the coefficient of static friction (friction coefficient) of the component surfaces which are joined to, one another. Steel/steel pairings typically have friction coefficients of 0.15, which is frequently insufficient to provide reliable frictional connection given the increasingly rising demands placed on machine components. Measures for increasing the friction coefficient, e.g. in shrink joints, have been known from the very early times of mechanical engineering: for example, as early as 1860 it was recommended to introduce sand in the joint gap, in order to improve the seating of gearwheels on shafts. The grains of sand are pressed into the surfaces of the components to be joined together under the effect of the shrink forces and bring about a certain form fit, with sand grain penetration depths of a few tenths of a millimeter. In practice, however, it is difficult to incorporate loose particles~or particles which have been mixed in a spreadable carrier media evenly~into the joint gap.
Although this method is effective in principle, the effect of relatively coarse particles in the joint gap entails an increased risk of long-term fracture. 3f the prevailing operating conditions indicate that such a risk exists, the impressions which the particles, used for force transmission make in the component surfaces must not be significantly deeper than the peak-to-valley heights caused by prior machining.

Various methods are known for ;incorporating hard particles uniformly and reproducibly in a joint gap. DOS
23 64 275 of 07.10.1975 (corresponds to GB 1,483,124) describes the application of a layer containing hard-y material bodies onto one of two interacting surfaces by vapor deposition, spraying on, sintering on or diffusion of a foreign material into the component surface.
In "ant-Antriebstechnik [Drive Engineering]" 20, No.
1-2, Jan.-Feb. 1981, Peeken et al. propose surface layers for the frictional transmission of moments which are produced using an electrodeposition method by jointly depositing fine grains of hard material and a metallic binding phase. By means of such layers, the static friction of a shaft-hub shrink joint is more than doubled. These layers even allow the friction coefficients under rotating 'flexural loading to be even better than under purely static loading.
The measures which are described in the literature for increasing the static friction coefficient are all based on directly plating one of the two components to be connected with friction-increasing layers. However, in practice the desired coating often cannot be applied to either of the two components for process engineering reasons. If the surfaces which are to be joined together are planar surfaces, intervening plates which are covered on both sides with a friction-increasing layer can be used to alleviate this problem. However, such plates are expensive to produce, since they must be precisely plane-parallel and,_ in addition, must only have a low roughness. In practice, only precision-ground plates are suitable. However, rigid plates with a significant inherent weight are often impractical during assembly, since they have to be fixed in the desired position at additional cost until the two parts to be joined together are connected to one another.
The present invention is' based on the object of providing a friction-increasing connection between workpieCes to be joined together, which connection avoids the drawbacks of the prior art.
Brief D_~acriptioa of the Invsatio:n The object is achieved according to the invention by means of a connecting element which comprises a thin, flexible layer which bears particles of defined size at its surface, these particles consisting of a material with a compressive and shear strength which exceeds that of the workpieces to be joined together.
The particles of a material with a compressive and shear strength which exceeds that of the workpieces to be joined together are referred to herein as hard particles.
Thin layers are preferably to be understood as meaning layers with a thickness of 5 0.2 mm.
~,5 The connecting element of the invention has the following advantages over known frictional connections:
a) the difficulties associated with partial coating of relatively large or bulky components do not occur:
b) it is possible to connect components which are not suitable for direct coating;
c) the friction coefficient of frictional connections is increased by at least 50~r;
d) it is economical to produce;
e) it is easy to adapt even to joint surfaces of complex shape or to nonplanar joint surfaces;
f) it does not require any significant additional expenditure during assembly:
Brief Description of the Dra~~iaqa Fig.~.1 is a~view in cross-section of a thin resin film carrying hard particles.
' Fig. 2 is a view in cross-section of a thin metal film carrying hard particles on two sides.
Detailed Description of tha Invention The hard particles preferably consist of a material which, under the particular conditions of use, does not react chemically either with the materials of the components to be joined together or with environmental media. It is.
preferably. an inorganic material.
Preferably, the hard particles are selected~from the group consisting of hard materials. Examples of hard materials are carbides, such as SiC, WC and BBC, nitrides, such as Si3N, and cubic BN, borides, SiOz. A1z03, and diamond..
The size of the hard particles is selected in such a way that the damage to the joint surfaces caused by the particles being pressed into the surfaces does not reach an impermissible level. Preferably this is ensured if the particle diameter is not greater than about three times the peak-to-valley height of the joint surfaces, which peak to valley height results~from machining of the joint surfaces.
A particle size with a maximum diameter of about 0.1 mm generally fulfils this requirement. Preferably, hard particles with a maximum diameter of about 15 dun are used.
Ideally, the hard particles are of identical size.
However, this is technically impossible to achieve within .
the preferred grain size range. Several of the above-mentioned preferred hard materials are commercially available in very narrow grain size ranges, in which the.
scatter about a given nominal diameter amounts to no more than about t 50%. This is th4 case in particular with diamond and cubic BN, and to a limited extent also with A1203, SiC, B4C. Such grains within the size ranges are preferred as hard particles in the component according to the invention. from the diameter range of up to about 15 ~m which is suitable. it is preferred to select commercially available grain size ranges of 6 ucn or 12 Eun average 'diameter.
The number of hard particles per unit surface area of the contact surfaces of the components to be joined together is preferably selected in such a way that the normal force which is available for joining the components together is sufficient to ensure that the particles are pressed into the opposite surface. This will generally be the case if no more than about 30% of the surface of the friction film/foil is covered with hard particles.
An insufficient covering leads to the hard particles being pressed completely into the joint surfaces and direct contact between the metal of the joint surfaces occurs, with the risk of so-called fretting rust being formed, which can reduce the force which can be transmitted. This is the case if less than 3% of the friction film/foil is covered with to particles.
Preferably, friction film/foils are designed~in such a Way that the particles embedded therein cover about 5 to about 15%~of the friction film/foil.
Depending on whether relatively coarse or relatively fine particles are selected for the application envisaged, the friction film/foil can be of a single-layer or multilayer structure, as demonstrated by way of the examples.
Embodit~at 1: Single-laytar friction film/fail:
A thin, flexible film of the friction film/foil according to the invention may, for example, be a film, preferably made of organic material, the thickness of which is less than the diameter of the incorporated hard particles. Such a single-Layer friction film is illustrated in Fig. 1. In this embodiment, the force is transmitted directly by the hard particles, which are in contact with each of the two surfaces to be joined. In Fig. 1, 1 is the film of the invention, with hard particle 3 embedded in an organic material 2.
~mbodina~nt 2: ~dultilmyer frictioa f3.ha/foil:
The thin, flexible layer may be formed as a strip with sufficient inherent strength, e.g. made of metallic material, on which the hard particles are fixed by means of a binding phase. In this case, the binding phase is preferably applied to the thin, flexible layer by means of electroless deposition methods. Such a multilayer friction foil is illustrated in Fig. 2. Where 1 is the friction foil comprising a thin metal foil 4, having hard particles 3 by a binding phase 3 which can be an electroless metal layer. In this embodiment, the hard particles are only in contact in each case with one of the surfaces to be joined and the force is transmitted by means of an interlayer of sufficient inherent strength.
l0 Organic carrier films are primarily suitable for the production. of a carrier film which is thinner than the particles of hard material in accordance with Embodiment 1, since a metallic binding phase is extremely sensitive to fracture in self-supporting form. In addition, organic films have the advantage of being able to adapt to three-dimensional, multiply curved surfaces largely without cracks.
Suitable materials for organic carrier films of this nature are, for example, paint resins, such as for example polyvinyl butyral (trade name PIOLOFORMs"") or vinyl acetate/vinyl chloride copolymers (VINNOLT"" paint resins), but also dispersions, such as for example ACRYLATE LL 979T"' (manufacturer: blacker-Chemie GmbH, Munich). As is known, the composition of such films can be varied within wide limits, in order to adjust various properties, such as elongation at break, elasticity, tear strength, or self-adhesive performance.
The production of such a film/foil containing particles of hard material can be carried out analogously to methods which are known and are used, for example, ~in the production of abrasive agents on a substrate (abrasive paper) or laminate layers. For example, during the production of abrasive paper, a tightly packed layer of grains of abrasive is applied to a carrier strip, which has been coated with glue, by gravity spattering or by electrostatic scattering. The particles, which initially only adhere lightly, are fixed.on the substrate by means of a second layer of glue.
A substrate which is provided with a nonstick coating or release coating is advantageously used to produce a self-supporting film which is studded with particles. Such nonstick coatings are known, for example, from the adhesive label and transferant. Either an organic layer which is studded with a single layer of particles, can be applied to~
the release coated substrate using the above-mentioned processes which are conventional when producing abrasive agents, or else a prepared mixture of binder and particles of hard material can be applied in a uniform, single layer using the.processes which are known for in the production of laminates.
The quantity and composition of the organic binding agent are set in such a way that the thickness of the film which remains after drying or curing is less than the average diameter of the embedded particles of hard material.
Preferably, the thickness of the film which remains after drying or curing amounts to at most about 50~ of the average diameter of the embedded particles of hard material.
It is unimportant here whether the particles protrude from the film on both sides or only on one side, since the normal forces which occur during subsequent use exceed the shear strength of the particle/film composite. The quantity of .
particles introduced into the film is such that the advantageous surface-coating proportions described above are achieved.
The finished self-supporting film can be pulled off .
the substrate which is provided with the nonstick or release coating in a simple manner. It represents an embodiment of the component according to the invention which can easily be applied to joint surfaces of components which are to be connected.

In a further embodiment, the friction film/foil can be made self-adhesive by adjustment of the organic binder, in order to facilitate application to joint surfaces.
If because of sensitive components only very small particles, e.g. < 10 dun, are required for the transmission of forces, as a rule it is not reliably possible to handle a single-layer friction film/foil. It is therefore necessary to select a carrier material on which the particles can be anchored and which itself has sufficient strength to 'transmit the forces occurring. Such a friction film/foil is illustrated in Fig. 2. In the case of highly stressed frictional connections, these are generally metallic components, preferable made of fexrous materials, so that the demand for 'sufficient strength" of the carrier material is essentially fulfilled by steel. The further demands relating to coatability, planarity, plane parallelism, flexibility and elasticity are fulfilled to a satisfactory extent by a strip-metal preferably strip steel, in particular spring steel strip. Therefore, for the preferred embodiments of friction film/foil which is coated on.both sides, commercially available, unalloyed spring steel strip having a thickness of about 0.1 mm is preferred.
In principle, the force-transmitting particles can be applied to the carrier material using the methods which have been described for the production of a self-supporting, particle-studded film, with the nonstick coating of the carrier material being omitted, so that a firmly adhering, particle-studded film is applied to both sides of the carrier material.
3o However, in practice it is difficult to apply particles with a diameter of only a few micrometers uniformly to a carrier using an organic film whose thickness is preferably only half the diameter of the particles. The particles are therefore preferably fixed on both sides 'of the carrier material by means of a deposited layer of metal.
In this case, the hard materiai/metal layer is preferably s produced by means of electroless deposition processes, e.g.
an external current-free process (e. g. chemical nickel plating). Such processes are known and are described, for example, in the literature references which have already been mentioned. Chemically plated nickel layers can be hardened by means of heat treatment at up to about 400°C, with the result that adhesion to the base metal is improved and the inherent hardness of the layer is increased.
In principle, the components according to the to invention are useful as friction films/foils in any type of frictional connection throughout the field of mechanical engineering, and in particular if the 'forces which can be transmitted by the component surfaces which are imposed by the design are insufficient. This may be the case in particular with clamp or press joints in the presence of lubricants, but may also be useful in dry pairings.
Steel/steel combinations typically have friction coefficients of 0.15 in the unlubricated state. The trend toward smaller, more lightweight structures is increasingly leading to the normal forces which can be achieved being too low to transmit the transverse forces required given friction coefficients of 0.15. A higher friction coefficient is required in order to keep the structure capable of functioning.
~xa~lr 1: Broduction Of A Connecting El~w~st Bcaording To Embodi.~nwnt 2 The connecting element is an annular plate of steel foil which is 0.1 mm thick and is coated on both sides. The friction-increasing coating consists of diamond particles with an average diameter of 6 ~m in a binding phase of electroless deposition nickel in a layer thickness of 4 ~tm.
The coating density of the diamond particles on the foil surface is 7 area %.
To produce this connecting element, firstly an annular plate of the required dimensions is punched from an uncoated spring steel sheet with a thickness of 0.1 rnm.
Although in principle it is possible to produce such plates' from sheet metal which has already been provided with a friction-increasing coating on both sides, this causes. a very large amount of expensive waste owing to scrap.
Usually, a relatively large number of plates are treated simultaneously.
The prepared plates are placed on suitable mounts and are pre-treated in accordance with the general rules of to electroless deposition by degreasing, pickling and activating.
Then, the carrier bearing the plates is immersed in a chemical nickel bath in which diamond powder with an average particle diameter of 6 Eun is dispersed. The quantity of dispersed diamond powder is selected in such a way that under the parameters prevailing in the coating bath (agitation, deposition rate), the desired proportion of diamond' in the deposited layer of metal reaches the desired thickness of slightly more than half the diameter of the diamond particles. Under customary process conditions, the immersion time amounts to approximately 15 minutes.
The carrier comprising the plates, which are now coated, is then removed from the coating bath and cleaned in an ultrasonic bath, to remove diamond particles which are only loosely attached.
The cleaned plates are taken off the carrier and are subjected to a heat treatment for 2 h at 350°C. The heat treatment increases the adhesion of the chemically deposited nickel layer to the steel foil and the seating of the particles in the layer itself.
E~xm~ple 2: Production Of A Connaoting El~nant According To Embodiment 1 The connecting element consists of a self-supporting organic film which is 15 dun thick and in which silicon carbide particles with an average diameter of 30 Eun are to evenly distributed in a quantity which is such that they cover a total of approximately 10% of the surface area.
The connecting element is produced in the following way: using the principles of paint technology, a solvent s containing paint comprising a VINNOLT"' paint resin is made "soft", in order to obtain a film which is ductile after drying. The solvent proportion amounts to fi0%. Silicon carbide powder with an average particle size of 30 dun is mixed into the paint in a quantity of 5% by volume. The material prepared in this way is applied with the aid of a doctor blade uniformly, in ~a thickness of approx. 40 ~,un, to a carrier film/foil which has been release coated. Following evaporation of the solvent, a film with a thickness of approx. 15 dun remains on the carrier film/foil, from which film the embedded silicon carbide grains project in accordance with their diameter. When viewed from above, the silicon carbide grains take up approx. 12% of the coated surface area.
The fiim/foil produced in this way is ready for use as and is cut to an appropriate size for the application. The release carrier film/foil can easily be separated from the particle-studded film, which far its part, as a result of the ."soft" setting mentioned at the outset, can be handled in a freely supporting manner and is easy to apply to the assembly location.
F.~cample 3: Uae of s coanatstiag slsmant in accordaaca ~rith Example 1~
A gyrating mass made of gray iron is to be attached at the end side to a rotating shaft made of heat-treated steel. It is not possible to achieve a form-fitting connection by means of press fitting, since precise positioning takes place only during assembly. Attachment is carried out using a clamping screw Which provides a normal force of 16,000 N. The presence of lubricants at the contact surfaces is not to be .ruled out. The holding moment required amounts to 500 Nm, but only 350 Nm (dry) or 290 Nm (lubricated) are achieved.
By inserting a connecting element in accordance with Example 1, a holding moment of 540 Nm is achieved at the available normal force of 16,000 N. In order, with the given structure, to allow the foil to be inserted, the latter has to be bent elastically.
~zamplo 4: Use of a aoanoating al~mant in :aaardanae ~rith Ezampla 2 An auxiliary unit is to be attached in a nondisplaceable manner to a motor casing made of cast light metal, by means of 4 M 6 screws. The tightening torque of the screws is limited, to prevent the threads in the light-metal casing from being destroyed. In test operation, the operation-induced vibrations lead to a change in the position of the auxiliary unit after 60 hours. It is required that the assembly position should be reliably maintained after 1000 hours.
This requirement is fulfilled by means of a connecting element in accordance with Example 2. This film is cut to an appropriate size and is applied to the contact surface before the components are screwed together. The position of the auxiliary unit attached in this way did not changed after a long-term test lasting 1000 hours.

Claims (8)

What is claimed is:

1. A connecting element for friction-increasing connection of workpieces which are to be connected, which element comprises a thin, flexible element of a metallic material which bears particles of a defined size at its surface, the particles comprising at least one material selected from the group consisting of carbides, borides and nitrides, the particles being fixed by means of a binding phase which is an electroless metal layer of the metallic material.

2. The connecting element as claimed in Claim 1, wherein the particles have a diameter of no more than three times the peak-to-valley height of joint surfaces, which height stems from a machining of the joint surfaces.

3. The connecting element as claimed in Claim 1, wherein a scatter of a size of the particles about a nominal diameter amounts to no more than about ~ 50%.

4. The connecting element as claimed in Claim 1, wherein the number of particles per unit surface area of the connecting element is selected so that the normal force, which is available for joining the workpieces together, is sufficient to ensure that the particles are pressed into an opposite surface of the workpiece.

5. The connecting element as claimed in Claim 1, wherein not more than 30% and no less than 3% of the surface of the thin flexible element is covered with hard particles.

6. The connecting element as claimed in Claim 4, wherein the particles cover 5 to 15% of the surface of the thin flexible element.

7. The connecting element as claimed in Claim 1, wherein the thin, flexible element has a thickness which is less than the diameter of the incorporated hard particles.

8. The connecting element of Claim 1, wherein the hard particle consists essentially of at least one material selected from the group of SiC, WC, B4C, Si3N4, cubic BN, SiO2, Al2O3 and diamond.