FIELD OF THE INVENTION

The present invention relates to a method for providing a steel- or iron-based friction face on a machine part, i.e. the friction face has properties to establish a high resistance to mutual shear movement along said face when forcedly being held to stationarily contact with a smooth mating counter face of plastically deformable metallic material, such mating face normally being located on an other machine part. Also, the invention relates to machine parts that are provided with a friction face. Further, the invention relates to a method for providing high static friction between two machine parts that are clamped together or otherwise mutually pressed against each other by means of a substantial force normal to the frictiously contacting mating faces to avoid relative shear movement.

BACKGROUND OF THE INVENTION

In many constructions it is important to obtain high static friction between mating faces of machine parts that are clamped together to avoid shear movement, i.e. the clamped parts must resist forces in the direction of the mating surfaces well. Examples of such constructions are two machine parts with planar or other non-interlocking shapes of curved mating friction surfaces that are forced towards one another by a clamping force and must resist shear movement, e.g. mutual rotation of an axially pre-stressed tapered plug in a mating tapered hole. Augmented friction between mating faces has shown beneficial to avoid mutual shear movement of clamped connections.

Static friction coefficient between smooth steel surfaces is in the range of 0,07 to 0,2. Various techniques for increasing the static friction are known in the art. WO 2007/131744 discloses a technique including thermal spraying hard-particle deposition on one mating face of a set .

However, vibrations occurring during use of the machine or construction break down some of the said type hard- particles and thereby generate loose fine hard-particle dust that can contaminate the machine or construction. If this fine hard particle material finds its way in lubrication oils the results can be highly detrimental.

A said hard particle coating can also be damaged during installation or maintenance and hard-particles may find their way into other parts of the construction where they are highly undesirable, e.g. as wear promoting abrasive in hydraulic fluid or lubrication oil.

Prior art friction faces that have been created with thermal spraying hard-particle deposition cannot be re- clamped due to setting of the material, and therefore they are usually applied in the form of a friction disk or shim that can be replaced.

US2008/0247817 discloses a method for providing a fraction face on a steel machine part for creating high resistance to shear movement when the friction face is in contact with a metallic material mating face. DISCLOSURE OF THE INVENTION

On this background, it is an object of the present invention to provide a method for creating a steel- or iron-based friction face of a machine part, which overcomes or at least reduces the drawbacks described above.

This object is achieved by providing a method for providing a steel or iron friction member with an adapted thickness for creating high resistance to shear movement when said friction member is squeezed between two mating metallic surfaces of at least two machine parts by at least two clamping devices per friction element for providing the squeeze force, said friction member is provided with at least one friction face to mate with a mating face of one of said machine parts, characterized by roughening said friction face followed by non-depositing surface hardening the roughened friction face.

The roughened and hardened friction face provides sufficient hard peaks that can penetrate the softer material of an opposing mating face. Vibrations of the construction in which the friction face is used will not cause any hard-particle dust and therefore there is no risk of such abrasive material entering into parts of the construction where they are potentially damaging. Further, tests have shown that friction surfaces that are produced according to the method above can be reused and still provide satisfactory static friction coefficients. Providing friction surfaces in accordance with the method above has also proven to be less expensive and more reliable and repeatable than the known techniques of thermal spraying hard-particle deposition. It was also found that the dimensions of the machine part in the inventive process above are relatively easy to control when compared to the known techniques in which hard-particle deposition on one mating face is provided by thermal spraying. Thereby, the dimension in the thickness direction in a correctly squeezed assembly can be controlled down to very low tolerances.

In an exemplary embodiment the roughening of the friction face is performed to obtain a surface roughness equal or above approximately Ra 3 μm.

In another exemplary embodiment the roughening of the friction face is performed to obtain a surface roughness between approximately Ra 3 μm and approximately Ra 20 μm. Relatively high roughness values have resulted in good static and dynamic friction coefficients.

According to another exemplary embodiment the roughening of the friction face is performed to obtain a surface roughness between approximately Ra 6 μm and approximately Ra 16 μm.

In an exemplary embodiment the roughening of the friction face is performed by grit blasting a smooth, i.e. otherwise untreated machined face. Grit blasting is a relatively economic and simple and well proven procedure that provides repeatable results.

According to an exemplary embodiment the grit blasting is performed with size 0.25 to 3.0 mm grit. The indicated size range for the grit provides the desired roughness for the friction face, of course also being related to the applied process parameters. Preferably, the grit blasting is performed with aluminum oxide grit. Aluminum oxide is a suitable and economical grit material.

The roughening of a said friction face can also be obtained by direct mechanical deformation resulting from one or more overrolling (s) or stamping (s) by a rigid member having a contact surface profile reflecting the desired roughness.

In an exemplary embodiment the roughened face is hardened to a value equal to or above 400 HV. Good results have been obtained by hardening the roughened face to a value between approximately 400 HV and approximately 3000 HV.

According to an exemplary embodiment the roughened face hardening process includes one or more of the following: Plasma Nitriding, Chemical Vapor Deposition, Plasma Enhanced Chemical Vapor Deposition, Physical Vapor Deposition, Toyota diffusion process, Gas Nitriding, Ion implantation, Chromating, Laser impregnation, Laser hardening, Flame hardening, or Induction hardening or quenching.

The object above is also achieved by providing a steel or iron friction member with a predetermined adapted thickness for creating high resistance to shear movement when said friction member is squeezed between two mating metallic surfaces of at least two machine parts by at least two clamping devices per friction element for providing the squeeze force said friction member is provided with at least one friction face to mate with a mating face of one of said machine parts, wherein the friction face has been created by a roughening step followed by non-depositing surface hardening the roughened friction face. Preferably, the friction element is a flat body with the distance between two opposing planar faces defining the thickness of the friction member.

The friction member may accordingly have a combination of circular solid round or ring-shaped forms.

The friction member may have one or more holes for passage of said at least two tension members.

The friction member may have a friction face on one side plate like body thus having a smooth face to the other side of said plate like body.

The friction member may have a friction face on both sides of said plate like body, e.g. from adequate general surface machining/conditioning .

The friction member may have a specified thickness for defining the distance even within narrow tolerances between the opposed mating faces of the respective machine parts when said opposed mating faces are correctly squeezed against a respective side of the friction member.

The friction may comprise an electronic tag that carries information about the friction member.

The object above is also achieved by assembly comprising at least a face from a first machine part and a face from a second machine part, wherein said faces mutually being secured against shear movement along said faces by being squeezed against a respective side of an intermediate friction element according to the present invention. Preferably, at least part of the external contour and/or of an internal contour of the mating surfaces of both machine parts and the friction faces of the friction element are identical.

Hereby some geometric functionality over an assembled set of element's and parts' edges is allowed, e.g. a combined cylinder inner running face for a sliding piston, or a combined rod being swept externally by some packing; the common contour may be realized by exact mutual positioning during an assembling operation of suitably prepared respective contours; or the common contour may be realized after the assembly operation by some machining. Hereby, the possibility is opened to further "produce" on a combined "semi-integral" part at an earlier stage being inventively assembled, e.g. application of a common hard chrome layer traversing an underlying smooth inventive assembling area of e.g. some piston rod combination.

Preferably, at least one pair of mutually squeezed adjacent faces of the assembly have at least one area not covered or deformably squeezed from the mating face.

Hereby over- and/or under-lapping of element-/parts ' - areas is forseen. Further, the clamping stress over the element's surface extension can hereby be suitably located.

Preferably, at least one first machine part has at least two face areas to respectively be part of a said assembly (combination) . The assembly may have at least one second machine part that is provided with at least two face areas, respectively to be part of a said (combined) assembly.

Thus, a possibility to "chain" assemblies from one part (combination) to a next part (combination) is established too.

Preferably, at least two face areas of a first machine part respectively are assembled to a corresponding number of face areas of a second machine part.

Hereby, it becomes possible to make an assembly between at least two pairs of mating faces from two machine parts, whilst still having other assembling possibilities from either part. Also, e.g. assembling (i.e. stiff fixing) of two opposed tripods with ball and shell mutual mating faces automatically establishing mutual alignment can be achieved.

Two assemblies between said a machine part and said second machine part can be co-acting to define a specific dimension of an opening located between said parts and between at least two of said assemblies.

Thus, it is possible to create a spanned-around hole with dimension adjusted via the thickness of the co-acting friction elements.

The first machine part of the assembly may be the bedplate for supporting the main bearings for a crankshaft of an internal combustion engine of the crosshead type, and the second machine part may be a main bearing cap, the opening being the space for bearing shell means located around a main journal, and the at least two co-acting friction elements by their respectively specified thicknesses are defining the bearing clearance when correctly assembled around the bearing combination.

The object above is also achieved by providing a steel or iron machine part comprising a first friction face for mating with a second suitable metallic, steel or iron face of a machine part, whereby the first friction face is a roughened and non-depositing hardened steel or iron face.

In an exemplary embodiment the mating second face has a surface hardness that is less than the hardness of the first face.

Preferably, said mating second face is substantially- smooth, i.e. only being adequately prepared for the shape mating and the plastic deformation during the intended squeezing against a first mating face".

According to an embodiment the machine parts are parts of a combustion engine.

In an exemplary embodiment the surface roughness of the first friction face is equal or above approximately Ra 3 μm.

In another exemplary embodiment the surface roughness of the first face is between approximately Ra 3 and approximately Ra 20 μm.

Alternatively, the surface roughness of the first face can be between approximately Ra 6 and approximately Ra 16 μm. Preferably, the hardness of the first face is equal to or above 400 HV.

Good results have been achieved with a hardness of the first face between approximately 400 HV and approximately 3000 HV.

The object above is also achieved by providing a method for providing high static and dynamic friction between mating friction faces of two metallic machine parts, said method comprising providing said machine parts with mating friction faces, one of said faces being smooth and the other of said faces being of steel or iron and being roughened and thereafter surface hardened by a non- depositing surface hardening treatment.

BRIEF DESCRIPTION OF THE FIGURES

In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawing's figures, in which:

Figure 1 is a view of a main crankshaft bearing of a large two-stroke diesel engine,

Figure 2 is a is a view of a friction increasing shim that is placed between the bearing support and the bearing cap of the bearing shown in figure, and

Figure 3 is a photo of a cross-section of a friction surface used on the shim illustrated in Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One area in which the present invention can be used is the assembly of a main bearing of a large two-stroke uniflow scavenged diesel engine, such as those of the MAN B&W® brand. However, the invention can also be used in other areas, in particular in the field of combustion engines such as 4-stroke diesel engines, such as those of the MAN® brand.

Figure 1 is a side view of a main bearing of a large two- stroke diesel engine. The main bearing is supported by the bedplate .

The bedplate consists of high, welded, longitudinal girders and welded cross girders 10 with bearing supports 20, i.e. the bedplate is an integral welded construction made from rolled metal plate material. The main bearing support 20 can be a cast steel element or it can be a rolled steel slab or plate. The cross girders 10 consist of two webplates with a main bearing support integrated therebetween .

The main bearing support 20 carries in its bore a thin bottom bearing shell 23 lined with bearing metal.

A cast iron bearing cap 40 is mounted on the bearing support 20 with shims 50 there between. The bearing cap 40 is provided with an arc shaped support surface for the upper bearing shell 43, which is also of the thin shell type, formed by a steel plate with bearing metal deposited thereon. The bearing cap 40 is held in place by hydraulically tensioned bolts 42 that are anchored in threaded bores that are provided in the body of the main bearing support 20.

The main bearing support 20 is provided with an abutment face 30 for friction and abutment surface 32 for friction. The bearing cap 40 is provided with an abutment face 44 for friction and an abutment surface 46 for friction. The oppositely disposed abutment surfaces 30/44 & 32/46 serve to support and fixate the bearing cap 40.

Here, the friction and abutment faces 30,32,44,46 are perfectly horizontal, and are substantially perpendicular to the respective bolts 42. The abutment faces are substantially smooth and may be a ground smooth surface with a roughness of Ra 0,4 μm or less.

Accordingly, the tension forces through the bolts 42 pass transversely though the faces 30,32,44,46. No lateral forces are generated by the bolt tension. Lateral forces that are generated during engine operation are transmitted between the bearing support 20 and the bearing cap 40 by friction.

A shim 50 provided with inventive static friction increasing faces 47 (Fig. 2) is placed between abutment faces 30 and 44 and similarly a shim 50 provided with static friction increasing faces is placed between abutment faces 32 and 46. The shims 50 act as a spacer and a friction member, here double sided. Bearing cap 40 which is of relevant physical size to allow easy processing can on one/more of faces 44,46 be provided with an a static friction increasing face; the mating shim 50 face thus being smooth, while the opposite shim 50 face to contact abutment faces 30, 32, respectively, are provided with an static friction increasing face, as the bedplate's abutment faces 30, 32 in this example were found too unhandy to submit an inventive process. Preferably, all processing to provide a friction increasing face is concentrated onto the smallest, i.e. the most handy part, here a shim 50, which consequently can have static friction increasing face applied to both opposed contacting faces. Another benefit herefrom is a larger freedom in choice of metallic material for mating smooth faces of normally much larger and heavier engine parts.

Figure 2 shows a shim 50 in greater detail. The shim 50 is provided with bores 54 for receiving the bolts 42 and is made from steel plate material.

Apart from the friction increasing function the shims 50 also serve to adjust the dimensions of the main bearing, thus having upper and lower faces 47 parallel and preferably planar. This adjustment is achieved by using the appropriate thickness t for the shim 50. The shim 50 is supplied either as a stiff slab or as a relatively flexible item from a band or roll.

Now, the upper face 47 and the lower face 47 of the shim 50 have a surface structure and characteristics that provide for high static friction when these friction faces 47 are clamped into contact with a smooth mating face of plastically deformable metal. In the . present exemplary embodiment the mating surfaces are the abutment faces 30,32,44,46.

The appropriate properties for the friction face 47 are obtained by roughening a smooth face 47 of the shim 50 followed by a non-depositing surface hardening treatment of the face 47.

In this context the term "Non-depositing" means that the process does not deposit a layer of hard material onto face 47. However, the term "non-depositing" does include all processes in which the iron or steel surface absorbs elements in the hardening process, e.g. absorption of chrome or nitrogen. Good results for the roughening process have been obtained with grit blasting. However, other surface roughening processes, such as etching or cold forming can also be used.

Tests have shows that a surface roughness equal or above approximately Ra 3 μm is required to obtain acceptable properties for the friction face 47. Good results have been obtained when the roughening of the friction face is performed to obtain a surface roughness between approximately Ra 3 and approximately Ra 20 μm, with the best results when the roughening of the friction face is performed to obtain a surface roughness between approximately Ra 6 and approximately Ra 16 μm.

Good results have been obtained when the grit blasting is performed with size 0.25 to 3.0 mm grit. A suitable material for the grit is aluminum oxide, but other materials can be used. Preferably, the grit should be virgin.

Tests have shown that the roughened face needs to be hardened to a value equal to or above 400 HV (Vickers Pyramid Number) . Good results have been obtained when the roughened face is hardened to a value between approximately 400 HV and approximately 3000 HV.

Instead of using dynamic effects resulted from impact of high-speed stiff massy grit particles, to establish the surface roughness, direct mechanical methods for application of the deformation force may be utilized. Thus, the roughening of the surface to become a friction face can be achieved by overrolling with a force-loaded wheel/roller having a suitable hardness and surface texture to "roll into" the receiving material surface. Also stamping with an adapted tool face onto the receiving face is a useful method of mechanical surface deformation to obtain the required surface roughness. Al such directly mechanically acting methods, can be performed as one of more treatments in a succession, what can be utilized as e.g. "part-of-a- time" treatment of a large surface being un-treatable by one overrolling or full-face stamping. Besides such part surface treatment also combinations of successive treatments on same surface area can be useful. Overrollings of same surface area from various directions with a same "linear pattern" roller e.g. can be used to obtain a desired cross-pattern roughening of the future friction surface. Also successive treatments of a same surface area with different mechanically acting surface deformation tools can be used to obtain the desired surface roughness.

The following surface hardening treatments can be used for hardening the roughened face. It should be noted that this is not an exhaustive list and there may be other surface hardening treatments that can be used which are not described herein: Plasma Nitriding, Chemical Vapor Deposition, Plasma Enhanced Chemical Vapor Deposition, Physical Vapor Deposition, Toyota® diffusion process, Gas Nitriding, Ion implantation, Chromating, Laser impregnation, Laser hardening, Flame hardening, and Induction hardening or quenching, the common feature of such hardening processes being their ability establish adequate material hardness without deposition of a further material layer.

The overall thickness of the shim 50 needs to be precisely controlled since it will influence the size of the main bearing ( ¹ S clearance/squeeze). In practice the desired size of the bearing clearance is achieved by using shims 50 with the appropriate adapted thickness t. The processes that are used for obtaining the friction surfaces 47 are highly repeatable and provide good control over the dimensional changes that occur during these processes to be taken into account.

The grit blasting roughening process causes a small increase in the overall thickness of the shim 50 (essentially no shim material is removed during grit blasting, but the peaks in the rough face create the increase in thickness of the shim 50) .

Depending on the type of surface hardening process, there is also an increase in the thickness t of the shim 50 during the surface hardening process. In processes where molecules are absorbed into the near-surface material of the shim, such as absorption of chrome or nitride formation, there is a small increase in the overall thickness t of the shim 50. Such increase is also well controllable and repeatable and therefore shims 50 with precise thickness tolerances can be relatively easily produced with the described processes. The resulted shim 50 has a well specified thickness t for dimensional adjustment of the main bearing. Thus a proper distance shim 50 for clamping of a main bearing is obtained. The shim 50 is a typical spare part that will be replaced during the engine's life span.

According to an embodiment a machine part is provided with two opposed external smooth faces each of shape and surface area extension to mate a respective roughened and non- depositing hardened friction surface according to the invention, located on one/more other machine part.

According to an embodiment the shim (or other machine part with the friction surface 47) is furnished with an electronic tag storing specific information about the machine part. The information may include a part's measured material analysis spectrum profile and/or data related to said machine part's installation and/or use history.

The face for mating with roughened and subsequently non- depositing hardened steel or iron friction face is according to an embodiment a smooth mating face of plastically deformable metallic material with a surface hardness value lower than that of the roughened and subsequently non-depositing hardened steel or iron friction face .

Figure 3, is a photo of a cross section through an inventive friction face. The material is nitriding steel. The material has been grit-blasted and plasma nitrided. After preparation of the cross-section, the surface has been etched. The picture is taken in an area, not submitted to clamping. The peaks and dents caused by the surface roughening process are clearly visible. The light colored layer nearest to the surface has been hardened by nitriding. The hardness of the top material and of that deeper in the diffusion zone are indicated in the figure and show that only the top surface layer has been substantially hardened. "B" indicates the base material. "D" indicates the diffusion zone with precipitated nitide needles and "U" indicates the unaffected base material. An indication of the scale and the hardness of the zones is indicated in the photo.

The optimum surface structure and characteristics of the friction faces will depend on the specific use and be influenced by e.g. the load on the components, the clamping pressure, the hardness of the machine parts, other properties of the opposing mating surface and vibration levels during use. When clamped with a common force preferably all sets of friction faces comprising an inventive static friction increasing face, of such assembly are mutually adjusted to perform satisfactorily submitted the same common clamping force. Such situation is relevant e.g. when more shims like 50 are piled as unitary thickness modules to achieve a given resulted overall thickness when clamped. Preferably, in such situations at least one shim of a pile has one smooth surface to contact a mating static friction increasing surface (of other shim or part) , while it's opposed face has an inventive static friction increasing surface. By such arrangement shims can be inserted/removed anywhere in a pile without affecting the friction achieved between mutual contacting faces - an adequate practical design of course provided. Such design preferably can be a "fingered" planar shape within the circumscribed contour to facilitate insertion/removal even without removal, but only loosening of the squeezing means 42. From e.g. the above described use for adjustment of bearing clearance one can easily imagine the risk of selection of a shim of a faulty thickness among many almost similar shims, as the resulted thickness accuracy to obtain is within e.g. 0.01 mm. Therefore secure identification of an inventive shim or other part provided with an inventive static friction increasing surface is mandatory. Such identification can on larger parts easily be furnished onto the relevant part by means of e.g. a stamped or engraved number or the like for secure referencing. Preferably such data may be located on an extending ear or exposed tab, possibly also having means for exact (mutual) positioning during the assembly operation. Because of many different parameters contributing to the overall functionality of an inventive friction member, it has however shown advantageous to embed all relevant information about such member into an electronic "tag" which remotely can be read by an appropriate device to result in fast display of tagged information. Such reading can even be performed in narrow environments, where access for the human eye is impossible. Preferably, such tag is mounted to the machine part so that it can be "read" /decoded during use in the mounted/installed situation to - by being able to "tell its own history" - facilitate decision about e.g. ordering in advance of a shim of other thickness for e.g. compensation for bearing wear at next service inspection. Of course, e.g. actual installation time and conditions also can be read into a part's tag for better supervision. Also, process history and authentications means like e.g. information about the part ' s material analysis spectrum profile can be incorporated in the tag content to hinder or demonstrate incorrect use.

The embodiments described above have been illustrated with reference to a main bearing of a large two-stroke diesel engine. However, the invention can be used in connection with any appropriate set of mating faces that are clamped or pressed together, i.e. the mating friction faces do not need to be planar, they can also be curved. The friction faces do not need to be part of the main bearing, they may- just as well be any other machine component that is secured by pressing two mating surfaces together. Also, the friction faces can be applied to machine parts that are not so easily replaced, such as on the bearing support (faces 30,32) or the bearing cap (faces 44,46) since a friction face according to the invention substantially preserves it's frictional characteristics even after multiple re- clamping operations. In case both the bearing support and the bearing cap are provided with the high friction faces, the shims can be simple sheet/plate material with smooth mating faces.

The term "comprising" as used in the claims does not exclude other elements or steps. The term "a" or "an" as used in the claims does not exclude a plurality.

The reference signs used in the claims shall not be construed as limiting the scope.

Although the present invention has been described in detail for purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the scope of the invention.