Improved part for an undercarriage assembly of a tracked vehicle, undercarriage and continuous track comprising such improved part and process for producing an improved part for an undercarriage assembly

Abstract

The invention concerns an improved part (53) of an undercarriage assembly (1) of a tracked vehicle. The undercarriage assembly (1) comprises an undercarriage frame assembly (3) to which a crawler track (5) can be assembled so as to be able to run relative to the undercarriage frame assembly (3). The undercarriage assembly (1) further comprises a first friction surface (7, 540) and a second friction surface (9, 560) configured for sliding one against another. The first (7) and the second friction surface (9) are formed by the undercarriage frame assembly (3) and / or by the crawler track (5). The improved part (53) is comprised in the undercarriage frame assembly (3) or in the crawler track (5) and forms at least the first friction surface (7). The first friction surface (7) is made of a hard coating obtained through an EHLA coating process.

Description

IMPROVED PART FOR AN UNDERCARRIAGE ASSEMBLY OF A TRACKED VEHICLE , UNDERCARRIAGE AND CONTINUOUS TRACK COMPRIS ING SUCH IMPROVED PART AND PROCESS FOR PRODUCING AN IMPROVED PART FOR AN UNDERCARRIAGE ASSEMBLYField of the invention

[0001] The present invention relates to an improved wearresistant part of a tracked vehicle such as bulldozers , excavators , loaders , diggers , dumpers , road finishers , mobile drills , farm tractors , military tanks and to a process for manufacturing said improved wear-resistant part .Background art

[0002] A metal continuous track of a tracked earth moving machine usually comprises a plurality of bushings each of which is axially crossed by a pin which transversally connect the two main track link bodies arranged substantially aside one to another and hinge two subsequent main track links one with another, so as to form an articulated chain of a plurality of track links .

[0003] During operation the outer flanks of the bushings slide against the teeth of the sprocket wheels of the undercarriage and are stressed by a particularly severe wear .

[0004] The inner side of the bushings is also stressed by the wear caused by the friction against their respective pins .

[0005] The intermittent contact and sliding between the bushings and the teeth worsens the wear damages of the bushings and causes also fatigue stresses , hence these bushings need to have a suf ficient toughness .

[0006] These wear and fatigue stresses are among the mostsevere and the bushings have one of the shortest operative li fe of all the components in an undercarriage .

[0007] For withstanding these stresses the material of the bushing needs to be chosen with particular care and the bushings currently undergo particular thermal and / or chemical treatments . Such treatments are time-consuming and cost-intensive .

[0008] For example in a known manufacturing process the bushing is extruded, carburi zed and finally quenched and tempered .

[0009] A scope of the present invention is overcoming the above described shortcomings and in particular providing a bushing or another undercarriage part able to withstand the wear and other mechanical stresses even i f obtained through a simpler and / or cheaper manufacturing process than the known ones .Summary of the invention

[0010] This scope is achieved, according to a first aspect of the present invention, with an improved part for an undercarriage assembly of a tracked vehicle having the features according to claim 1 .

[0011] The Extreme High Speed Laser application (EHLA) treatment allows to improve hardness only the zones of the bushings of a continuous track -or other improved undercarriage parts- which are subj ect to wear avoiding to harden other zones which are not or negligibly af fected by wear .

[0012] These coating processes can be carried out at -or close to- the end of the manufacturing process of the bushings or other improved part since they do not substantially alter the dimensions , shape and hardness of the coated substrate .

[0013] More particularly these coating processes can be advantageously carried out after thermal treatments and machining of the bushing, more generally and possibly at or near the end of the manufacturing process of the bushing 53 or another improved part.

[0014] This way, if some manufacturing steps are carried out by external providers, the semimanufactured parts need be handled less times, for example packaged and unpackaged when delivered to or by the external suppliers; furthermore the production line of the main manufacturer needs to be retooled less times if not at all.

[0015] In an improved part according to a particular embodiment of the invention, said hard coating has one or more of the following contents in weight of elements: -a content of carbon comprised between 0, 5-5,0 % wt;-a content of chromium comprised between 5,0-30 % wt; -a content of nickel comprised between 2,0-15 % wt; -a content of silicon comprised between 0,5-10 % wt;-a content of boron comprised between 1,0-20 % wt;-a content of iron balancing to 100% the contents of one or more of the previous elements.

[0016] In an improved part according to a particular embodiment of the invention, said hard coating has all of the contents in weight of the following elements:-a content of carbon comprised between 0, 5-5,0 % wt; -a content of chromium comprised between 5,0-30 % wt; -a content of nickel comprised between 2,0-15 % wt; -a content of silicon comprised between 0,5-10 % wt;-a content of boron comprised between 1,0-20 % wt;-a content of iron balancing to 100% the contents of the previous elements.

[0017] In an improved part according to a particular embodiment of the invention, said hard coating has one or more of the following contents in weight of elements: -a content of carbon comprised between 1,0-3, 5 % wt;-a content of chromium comprised between 10-18 % wt; -a content of nickel comprised between 3,5-10 % wt; -a content of silicon comprised between 1, 0-5,0 % wt; -a content of boron comprised between 2, 5-8,0 % wt .

[0018] In an improved part according to a particular embodiment of the invention, said hard coating has one or more of the following contents in weight of elements: -a content of carbon comprised between 2, 0-2, 5 % wt;-a content of chromium comprised between 13-14 % wt; -a content of nickel comprised between 5, 5-6, 5 % wt; -a content of silicon comprised between 2, 0-2, 5 % wt; -a content of boron comprised between 3, 5-4,0 % wt .

[0019] In an improved part according to a particular embodiment of the invention, said hard coating has all of the contents in weight of the following elements:-a content of carbon comprised between 2, 0-2, 5 % wt; -a content of chromium comprised between 13-14 % wt; -a content of nickel comprised between 5, 5-6, 5 % wt; -a content of silicon comprised between 2, 0-2, 5 % wt; -a content of boron comprised between 3, 5-4,0 % wt;-a content of iron balancing to 100% the contents of the previous elements.

[0020] In an improved part according to a particular embodiment of the invention, said hard coating comprises one or more of the following materials: tungsten carbide, molybdenum boride, vanadium carbide, martensitic steel, nickel or nickel alloys, cobalt or cobalt alloys, chromium carbide.

[0021] In an improved part according to a particular embodiment of the invention, said hard coating has one or more of the following substance contents :-a content of tungsten carbide comprised between 0-50- wt ;-a content of cobalt , cobalt alloys , nickel or nickel alloys comprised between 0-30% wt ;-a content of molybdenum boride or chromium carbide comprised between 0-40% wt ;-a content of martensitic steel comprised between 40-99% wt ;-a content of molybdenum boride or chromium carbide comprised between 0-40% wt ;-a content of vanadium carbide comprised between 0-40% wt .

[0022] In an improved part according to a particular embodiment of the invention, said hard coating has one or more of the following substance contents :-a content of tungsten carbide comprised between 5-45% wt ;-a content of cobalt , cobalt alloys , nickel or nickel alloys or martensitic steel comprised between 10-14 % wt ; -a content of molybdenum boride or chromium carbide comprised between 5-15% wt ;-a content of martensitic steel comprised between 50-90% wt ;-a content of vanadium carbide comprised between 5-30% wt .

[0023] In an improved part according to a particular embodiment of the invention, said hard coating has a content of titanium comprised between 1 , 0-8 , 0 % wt , more preferably comprised between 2 , 0-7 , 0 % wt or between4 , 0-5 , 0 % wt .

[0024] Thanks to the previous composition the hard coating has a great hardness and wear resistance and can for example reduce wear of the base material by about 70% , wherein this percentage has been obtained testing the improved part for an undercarriage assembly in design conditions , in particular for the number of hours or cycles corresponding to the operating li fe of the part considered in design conditions .

[0025] In an improved part according to a particular embodiment of the invention, said hard coating has a hardness comprised between 50-100 HRC .

[0026] The previous hardness values allow for example a metal bushing connecting di f ferent track links of a continuous track to validly withstand the wear due to the engagement with a sprocket wheel of an undercarriage assembly of a tracked vehicle or more generally of a tracked vehicle , reducing at the same time the manufacturing costs of the bushing itsel f .

[0027] The li fetime of such a bushing is signi ficantly improved .

[0028] In an improved part according to a particular embodiment of the invention, said hard coating has a hardness comprised between 60-80 HRC .

[0029] These hardness values are an even more optimal balance between wear resistance and reduction of manufacturing costs .

[0030] In an improved part according to a particular embodiment of the invention, said hard coating has an average thickness comprised between 0 , 1-1 millimetres .

[0031] This thickness choice is particularly ef fective for both enhancing wear resistance and keeping manufacturingcosts low .

[0032] In an improved part according to a particular embodiment of the invention, said hard coating obtained through said Extreme High Speed Laser application, also referred to as EHLA, has one or more following features : -a surface roughness Rz equal to or lower than 300 micrometres ;-a surface roughness Rz equal to or lower than 23 micrometres ;-a surface roughness Ra equal to or lower than 50 micrometres ;-a surface roughness Ra equal to or lower than 6 micrometres .

[0033] These relatively low values of surface roughness , which can be obtained through a relatively quick and cheap EHLA process , further enhance the wear resistance of said hard coating and of said first friction surface , further reducing wear of the base material .

[0034] An improved part according to a particular embodiment of the invention comprises a base part coated by the hard coating, the base part comprises a base material chosen from the following group :-a steel which has not been previously carburi zed or case hardened;-a steel having a content of carbon equal to or lover than 0 , 5% wt , or equal or lower than 0 , 45 % wt or equal or lower than 0 , 30%wt ;-a steel having a content of carbon comprised between 0 , 1-0 , 5% wt ;-a steel having a content of carbon comprised between 0 , 4-0 , 45% wt .

[0035] Avoiding carburi zation or case-hardening stepsreduces the time and costs for manuf cturing a bushing of a continuous track or another improved undercarriage part .

[0036] An improved part according to a particular embodiment of the invention is a bushing (53) .

[0037] In an improved part according to a particular embodiment of the invention, said base part has a substantially tubular shape.

[0038] In an improved part according to a particular embodiment of the invention, the base part has been formed at least by extrusion.

[0039] In a second aspect of the invention, the abovementioned scope is achieved through an undercarriage assembly having the features according to claim 14.

[0040] In a third aspect of the invention, the abovementioned scope is achieved through a continuous track having the features according to claim 15.

[0041] In a continuous track according to a particular embodiment of the invention:-the continuous track (5) comprises a plurality of track links, a plurality of hinging pins 52 and a plurality of pin bushings (53) ;-each track link comprises a respective hinging pin (52) and a respective pin bushing (53) , is fixed and articulated to a subsequent track link by means of the respective hinging pin (52) ;-each hinging pin (52) axially crosses a respective pin bushing (53) ;-each pin bushing (53) form flanks;-the undercarriage frame assembly (3) comprises at least one sprocket wheels (32, 34) configured for engaging thehinging pins ( 52 ) resting and / or sliding against the flanks of the pin bushings ( 53 ) ;-at least some of the pin bushings ( 53 ) , and possibly all of them, have the features according to claim 11 .

[0042] In a fourth aspect of the invention, the abovementioned scope is achieved through a process for producing an improved part for an undercarriage assembly having the features according to claim 17 .

[0043] Avoiding carburi zing or case hardening the whole pin bushing of a continuous track or other improved undercarriage part simplifies and renders the manufacturing process less expensive ; this advantage i s particularly remarkable since carburi zing and case hardening are thermochemical processes , that is special manufacturing processes requiring special and expensive systems , equipment and plants .

[0044] Concluding, avoiding carburi zing or case hardening greatly reduces the manufacturing costs improving -or at least without worsening- the performances of the improved undercarriage parts .

[0045] The dependent claims are directed to further features of the invention .

[0046] The advantages which can be achieved with the present invention will appear more evident for the skilled reader, from the following detailed description of a particular and non limiting embodiment , described with reference to the following schematic figures .List of FiguresFigure 1 shows a perspective view of an undercarriage assembly according to a particular embodiment of the present invention;Figure 2 shows a perspective view of a section of thecontinuous track of the undercarriage assembly of Figure1 ;Figure 3 shows a sectioned view, according to the section hplane crossing the axes of the pins , of a section of the continuous track of Figure 2 ;Figure 4 shows a perspective view of a pin bushing of the continuous track of Figure 2 ;Figure 5 shows a sectioned view, according to the section plane crossing the axis o f the pin, of a section of the pin bushing of Figure 4 ;Figure 5A shows an enlarged detail of Figure 5 ;Figure 6 shows a perspective view of a portion of a sprocket wheel of the undercarriage assembly of Figure 1 .Detailed description

[0047] Figures 1-6 relate to an undercarriage assembly according to a particular embodiment of the invention, referred to as with the overall reference 1 .

[0048] The undercarriage assembly 1 can be part of a tracked vehicle such as bulldozers , excavators , loaders , diggers , dumpers , road finishers , mobile drills , farm tractors , military tanks .

[0049] The undercarriage assembly 1 comprises an undercarriage frame assembly 3 to which a crawler track 5 can be assembled so as to be able to run relative to the undercarriage frame assembly 3.

[0050] The crawler track 5 can comprise a plurality of track links , each of which comprising for example a track link main bodies 50 and a shoe 51 fixed to the latter, and the track links can be articulated together so as to form a chain, preferably a continuous chain closed on itsel f ( Figure 1 , 2 ) ; in this case the crawler track ispreferably and mostly made up of steel or another metallic material .

[0051] Preferably the crawler track 5 comprises a plurality of pin assemblies connecting a first track link to a second and subsequent track link; for example each pin assembly can connect and hinge a first and a second track link main body 50 which are part of the first and second and subsequent track link respectively ( Figure 2 ) .

[0052] Each pin assembly can comprise a hinging pin 52 and a pin bushing 53.

[0053] The track links can be assembled and hinged together for example by press- fitting the two opposite ends of each hinging pin 52 in two pin bores made in the ends of the link main bodies 50 of an adj acent track link, so as to prevent the hinging pin 52 from rotating relative to the link main bodies 50 of the adj acent track link; thi s way each hinging pin 52 connects transversally the link main bodies 50 of a track link .

[0054] Each end of the pin bushing 53 is preferably press- fitted in a bushing bore of two ends of two respective track links .

[0055] Since the hinging pin 52 axially extends through and beyond its respective pin bushing 53 and is able to rotate relative to the bushing 53, each pin assembly connects and articulates two adj acent track links .

[0056] The wall of the pin bushing 53 can have a maximum radial thickness WBS for example comprised between 5-50 millimetres and a minimum radial thickness comprised for example between 5-20 millimetres ( Figure 5 ) .

[0057] Advantageously the pin bushing 53 can have an enlarged middle section 530 and two coupling bushing ends 532 having a smaller average- or maximum diameterthan the middle section.

[0058] The enlarged middle section 530 is conceived for engaging the drive sprocket 32 driving the endless track 5 and causing the tracked vehicle to proceed and for withstanding the wear caused by the drive sprocket during operation .

[0059] Each coupling bushing end 532 is preferably separated from the middle section by a shoulder 534.

[0060] The ratio LTN / LB between the length LTN of the enlarged middle section 530 and the overall length LB of the bushing 53 is preferably comprised between 0,2-0, 8, more preferably between 0,3-0, 6 or 0,4-0,45.

[0061] The ratio DB1 / LB between the outer diameter DB1 of the enlarged middle section 530 and the overall length LB of the bushing 53 is preferably comprised between 0,2-0, 8, more preferably between 0,3-0, 6, 0,4-0, 5 or 0,45-0,5.

[0062] The undercarriage frame assembly 3 can comprise for example one or more of the following elements:-a frame 30;-wheels such as one or more sprocket wheels 32, and one or more idler wheels 34; at least one sprocket wheel is preferably a driving wheel 32 or idler wheels 34, like in Figure 1, the idler wheel 34;-pulleys, roller assemblies 38, 40 resting against the crawler track 5 and / or over -like for example the roller assembly 40- or under which -like for example the roller assemblies 38-said crawler track 5 rolls and / or runs;-motors or other actuators, gearboxes or joints driving the crawler track 5, power supply lines such as electric or hydraulic power lines powering said motors or other actuators .

[0063] The frame 30 can comprise for example welded metal plates, shells and / or beams each of which having a more or less complex shape.

[0064] Preferably the undercarriage assembly 1 comprises one or more supporting roller assemblies 38, 40 configured for resting against the crawler track 5 and / or rolling over or under the crawler track 5.

[0065] In the embodiment of Figure 1 the supporting roller assemblies 38 roll over the crawler track 5 and the supporting roller assemblies 40 roll under the crawler track 5.

[0066] The roller assemblies 38, 40 are arranged preferably between a sprocket wheel 32 and one or more idler wheels 34 -the latter being also referred to as idlers 34.

[0067] Preferably the roller bodies 39, 41 of the roller assemblies 38, 40 respectively have a smaller outer diameter than the sprocket wheel 32 and idler or idlers 34.

[0068] Preferably the sprocket wheels 32, 34 change the direction of the crawler track 5 substantially more than what each roller assembly 38, 40 does.

[0069] The undercarriage assembly 1 further comprises a first friction surface 7 and a second friction surface 9 configured for sliding one against another (Figure 4, 6) .

[0070] The first 7 and the second friction surfaces 9 are formed by the undercarriage frame assembly 3 and / or by the crawler track 5.

[0071] Fo r example the first 7 and the second friction surfaces 9 can be formed both by the undercarriage frameassembly 3, both by the crawler track 5 or one of them -the first 7 or the second friction surface 9- by the undercarriage frame assembly 3 and the other one of them -the second 9 or the first friction surface 7 respectively- by the crawler track 5 .

[0072] According to an aspect of the present invention, the undercarriage comprises an improved part 52 which can be comprised in the undercarriage frame assembly 3 or in the crawler track 5 and forms at least the first friction surface 7 .

[0073] The first friction surface 7 is made of a hard coating 5301 obtained through an extreme high speed laser application (EHLA) process .

[0074] In the present description the expression " extreme high speed laser application (EHLA) " indicates the coating process defined in claim 1 of the German patent application DE102011100456A1 , that is a laser deposition welding process in which a molten bath is produced on a surface with at least a filler material -such as a metallic material- molten through a laser beam directed onto the molten bath, wherein :-a powder of the filler material is molten through the laser beam;-the filler material is molten through the laser beam at a distance from the molten bath and is brought in fully molten form into the molten bath .

[0075] An EHLA process according to the present description can be carried out according to the teachings and the di f ferent embodiments described in the abovementioned patent application DE102011100456A1 .

[0076] For example according to a particular embodiment ofthe present invention the laser beam can be focused in a focus , which is spaced apart from the molten bath and in which the filler material is molten .

[0077] According to a particular embodiment of the present invention the molten bath and the focus are displaced parallel one to another and relative to the surface to be coated, at a speed preferably equal to or greater than 20 m / min, more preferably equal to or greater than 50 m / min, more preferably equal to or greater than 100 m / min and even more preferably equal to or greater than 130 m / min .

[0078] According to a particular embodiment of the present invention said distance from the molten bath, at which the filler material is molten, can be equal to or greater than 0 , 2 millimetres , preferably equal to or greater than 1 millimetre , more preferably equal to or greater than 4 millimetres , even more preferably equal to or greater than 5 millimetres and / or equal to or smaller than 7 millimetres or 6 millimetres .

[0079] According to a particular embodiment of the present invention the density of said powder is arranged in such a way that the laser power of the laser beam reaching the molten bath is preferably lower than 80% of the laser power before the contact of the laser beam with the powder, and more preferably is lower than 60% or than 50% or 30% of said laser power before the contact of the laser beam with the powder .

[0080] According to a particular embodiment of the present invention the EHLA can be carried out with the features according to claim 1 of the German patent DE102011100456B4 .

[0081] In known laser cladding processes the powder of thefiller material is fed into the melt pool generated by the laser on the surface of the part to be coated, that is on the surface of the so-called substrate ; this implies in particular that some machining, grinding or thermal treatments need to be carried out after traditional laser cladding for eliminating excessive deformations of the coated part .

[0082] On the contrary in the EHLA process the powder of the filler material is fed into the laser beam above the melt pool and the substrate , saving energy and powder, reducing the heated portion of the substrate -that is the surface of the part to be coated- and a much higher processing speed .

[0083] Since the EHLA coating process heats a much smaller portion of the part to be coated, EHLA does not substantially alter the dimensions , shape and hardness of the coated substrate and it can be carried out at - or close to- the end of the manufacturing process of the bushings or other improved part .

[0084] More particularly the EHLA can be advantageously carried out after thermal treatments and machining of the bushing, more generally and possibly at or near the end of the manufacturing process of the bushing 53 or another improved part .

[0085] This way, i f some manufacturing steps are carried out by external providers , the semimanufactured parts need be handled / less often, for example packaged and unpackaged when delivered to or by the external suppliers ; furthermore the production line of the main manufacturer needs to be retooled less often, i f not at all .

[0086] The sprocket wheels 32 , 34 preferably form arespective crown of teeth (Figure 6) ; during operation said teeth preferably engage the flanks of the pin bushing 53, for example a substantially middle section 530 of the flanks.

[0087] Said first friction surface 7 can be for example said middle section 530 of the flanks of the pin bushing 53, or more generally the whole or a part of the flanks of the pin bushing 53.

[0088] Said second friction surface 9 can comprise for example one or more of the following entities: the flanks of the teeth of the sprocket wheels 32, 34, the whole or a part of the surface of the recesses present between two adjacent teeth of the sprocket wheels 32, 34 and periodically receiving the flanks 530 of the bushings 53 during operation (Figures 5, 6) .

[0089] Advantageously the hard coating has one or more of the following contents in weight of elements:-a content of carbon comprised between 0, 5-5,0 % wt;-a content of chromium comprised between 5,0-30 % wt;-a content of nickel comprised between 2,0-15 % wt;-a content of silicon comprised between 0,5-10 % wt;-a content of boron comprised between 1,0-20 % wt;-a content of iron balancing to 100% the contents of one or more of the previous elements.

[0090] More advantageously the hard coating has all of the contents in weight of the following elements:-a content of carbon comprised between 0, 5-5,0 % wt;-a content of chromium comprised between 5,0-30 % wt;-a content of nickel comprised between 2,0-15 % wt;-a content of silicon comprised between 0,5-10 % wt;-a content of boron comprised between 1,0-20 % wt;-a content of iron balancing to 100% the contents of theprevious elements.

[0091] More advantageously the content of carbon is comprised between 1,0-3, 5 % wt and more preferably between 2, 0-2, 5 % wt .

[0092] More advantageously the content of chromium is comprised between 10-18 % wt and more preferably between 13-14 % wt.

[0093] More advantageously the content of nickel is comprised between 3,5-10 % wt and more preferably between 5, 5-6, 5 % wt .

[0094] More advantageously the content of silicon is comprised between 1, 0-5,0 % wt and more preferably between 2, 0-2, 5 % wt .

[0095] More advantageously the content of boron is comprised between 2, 5-8,0 % wt and more preferably between 3, 5-4,0 % wt .

[0096] Preferably said hard coating comprises one or more of the following materials: tungsten carbide, molybdenum boride, vanadium carbide, martensitic steel, nickel or nickel alloys, cobalt or cobalt alloys, chromium carbide .

[0097] Preferably the hard coating has one or more of the following substances contents:-a content of tungsten carbide comprised between 0-50-% wt, or between 10-20 % wt, 20-30% wt, 30-40% wt or 40- 50% wt;-a content of cobalt, cobalt alloys, nickel or nickel alloys comprised between 0-30% wt, or between 5-20% wt, 7-15% wt, 10-30% wt or 10-14% wt and for example equal to 12% wt;-a content of martensitic steel comprised between 40-99% wt or between 50-95% wt or 50-90% wt;-a content of molybdenum boride or chromium carbide comprised between 0-40% wt, or between 10-40% wt, 5-20% wt, 5-15% wt;-a content of vanadium carbide comprised between 0-40% wt, or between 5-30% wt;-a content of titanium comprised between 1, 0-8,0 % wt, more preferably comprised between 2, 0-7,0 % wt or between 4 , 0-5, 0 % wt .

[0098] Preferably said hard coating comprises a matrix and an one or more enriching substances.

[0099] Preferably the enriching substances form between 0- 30% wt, or between 5-20% wt or 5-15% wt of the hard coating .

[0100] Preferably the matrix form the remaining content by weight of the hard coating 5301.

[0101] The matrix preferably comprises cobalt or cobalt alloys and / or nickel and / or nickel alloys.

[0102] Preferably the enriching substances comprise one or more of the following substances: martensitic steel, cobalt, for example substantially 99% pure cobalt, 90% pure cobalt or other cobalt alloys having a major amount of cobalt, for example at least 50% cobalt, at least 60% cobalt, at least 70% cobalt or at least 80% cobalt, chromium carbide, molybdenum carbide, vanadium carbide.

[0103] Preferably at least one of the enriching substances comprise one or more of the following substances: tungsten carbide, molybdenum boride, vanadium carbide.

[0104] Preferably said hard coating has a hardness comprised between 50-100 HRC, 55-90 HRC or 60-80 HRC .

[0105] Preferably said hard coating has an average thickness comprised between 0,2-4 millimetres, between 0,5-0, 4 millimetres, or 0,5-3, 5 millimetres, 0, 5-2, 5millimetres, 0,5-2 millimetres or 0,2-1 millimetres.

[0106] Thanks to the previous compositions and / or dimensions the hard coating has a great hardness and wear resistance and can for example reduce wear of the base material by about 70%, wherein this percentage has been obtained testing the improved part for an undercarriage assembly in design conditions, in particular for the number of hours or cycles corresponding to the operating life of the part considered in design conditions.

[0107] Advantageously at least the first friction surface 7 made of said hard coating made through EHLA has a surface roughness Rz equal to or lower than 300 pm (micrometres) and / or a surface roughness Ra equal to or lower than 50 pm.

[0108] More preferably said surface roughness Rz is equal to or lower than 285 pm or than 75 pm or 25 pm or 23 pm or 20 pm or 10 pm.

[0109] More preferably said surface roughness Ra is equal to or lower than 48 or 49 pm, or than 15 pm, or than 12 pm, or than 10 pm or than 6 pm or than 5 pm.

[0110] Said surface roughness Rz can be for example comprised between 8-300 pm, between 10-300 pm, between 10-20 pm, between 20-25 pm, between 25-75 pm, between 75-285 pm or between 285-300 pm.

[0111] Said surface roughness Ra can be for example comprised between 5-50 pm, between 5-6 pm, between 6-10 pm, between 10-12 pm, between 12-15 pm, between 15-48 pm or between 48-50 pm.

[0112] The previous values of surface roughness, which can be obtained through a relatively quick and cheap EHLA process, further enhance the wear resistance of thefirst friction surface 7 or other EHLA coated surface and further reduce wear of the base material .

[0113] The previous measures allow a pin bushing 53 be obtained for example through the following process :-a semimanufactured bushing is extruded from a metal bar ;-the whole semimanufactured bushing is induction hardened, cooling the bushing for example from the outer diameter and causing a martensitic trans formation for example over its whole thickness , preferably at least over its outer and inner surfaces ;-the bushing is induction hardened and cooled from the inner diameter, tempering the previously quenched portion;-the bushing then undergoes stress relieving ( e . g . by stress relieving annealing) , can be then machined and its outer flanks can be coated through the abovementioned EHLA process .

[0114] Preferably said improved part comprises a base part 5300 coated by the hard coating 5301 and comprising -possibly consisting of or made mostly of- a base material chosen from the following group ( Figure 5A) : -a steel or other metal which has not been previously carburi zed or case hardened, for example because it has never been carburi zed after the blank or another semimanufactured bushing has been produced;-a steel having a carbon content equal to or lower than 0 , 5% wt , or equal or lower than 0 , 45 % wt or equal or lower than 0 , 30%wt .

[0115] The base part 5300 comprises , or is made of a steel having a carbon content preferably equal to or higher than 0 , 2 % wt , more preferably equal to or higher than0,25 % wt or 0,3 % wt .

[0116] Preferably the steel forming or however contained in the base part 5300 has a content of carbon comprised between 0,1-0, 5% wt and more preferably between 0,4- 0,45% wt .

[0117] The base part 5300 can for example be or comprise said blank or semimanufactured bushing or a part of them.

[0118] The base part 5300 can form for example a major part of the finished bushing 53, and form for example at least the half of the weight of the finished bushing, or at least 0,7 times, at least 0,8 times, at least 0,9 times or at least 0,75 times the weight of the finished bushing .

[0119] When the improved part is a pin bushing 53 its base part is preferably a substantially tubular blank or another substantially tubular body.

[0120] Avoiding carburization or case-hardening steps reduces the time and costs for manufacturing the bushings 53 or another improved undercarriage part.

[0121] More generally, a further aspect of the present invention relates to a process for producing the bushing 53 or another improved part of an undercarriage assembly of a tracked vehicle, comprising the following steps: -providing a semimanufactured base part, for example by extrusion, and made of or however comprising a steel which has not previously been carburized or case hardened;-coating at least a part of the semimanufactured base part through an EHLA process so as to make the hard coating forming the first friction surface 7; wherein said process for producing the improved part does not comprise the step of carburizing or casehardening the semimanufactured base part or the finished base part .

[0122] Avoiding carburi zing or case hardening of the whole pin bushing 53 or other improved part 53 simpli fies and renders the manufacturing process less expensive ; this advantage is particularly remarkable since carburi zing and case hardening are thermochemical processes , that is special manufacturing processes requiring special and expensive systems and plants .

[0123] EHLA treatments allow to harden only the zones o f the bushing 53 -or other improved parts- which are subj ect to wear avoiding to harden other zones which are not or negligibly af fected by wear .

[0124] Concluding, avoiding carburi zing or case hardening greatly reduces the manufacturing costs improving -or at least without worsening- the performances of the improved parts .

[0125] More generally an EHLA coating process allows making a thick hard coating 5301 greatly reducing the wear of the friction surface 7 and / or 9.Furthermore an EHLA coating process is more ef ficient in comparison with other coating proces ses , since about 90% of the coating powder inj ected in the laser beam reaches the molten bath created by the beam on the surface to be coated instead of being dissipated in the surrounding environment , and because the EHLA can be carried out at very high speeds and allow to coat for example about 1000 cmA2 / min ( square centimetres per minute ) .

[0126] The embodiments previously described can be modi fied in several ways without departing from the scope of the present invention .

[0127] For example an improved part according to the present invention can be not only a pin bushing 53 but for example also a part of the roller assemblies 38, 40, the shoe grousers 510 or the track link rail.

[0128] Any reference in this specification to "One embodiment", "an embodiment", "example embodiment," etc, means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention .

[0129] The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment.

[0130] Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments.

[0131] All constructional details can be replaced with technically equivalent elements.

[0132] A wording such as "A comprises B, C, D" or "A is made up of B, C, D" is to be understood as encompassing and disclosing the particular case of "A consists of B, C, D".

[0133] Wordings such as "A comprises an element B" when not differently specified is to be understood as "A comprises one or more elements B" .

[0134] Mentions of a "first, second, third... n-th entity" merely intend to distinguishing the entities one from another but the mention of a n-th entity does notnecessarily imply the existence of a first, second, third ... (n-l)-th entity.

[0135] Examples and lists of possible alternatives are to be understood as non-exhaustive .

Claims

CLAIMS1) Improved part (53) of an undercarriage assembly (1) of a tracked vehicle, wherein:-the undercarriage assembly (1) comprises an undercarriage frame assembly (3) to which a crawler track (5) can be assembled so as to be able to run relative to the undercarriage frame assembly (3) ;-the undercarriage assembly (1) further comprises a first friction surface (7, 540) and a second friction surface (9, 560) configured for sliding one against another;-the first (7) and the second friction surface (9) are formed by the undercarriage frame assembly (3) and / or by the crawler track (5) ;-the improved part (53) is comprised in the undercarriage frame assembly (3) or in the crawler track (5) and forms at least the first friction surface (7) ;-the first friction surface (7) is made of a hard coating obtained through an extreme high speed laser application (EHLA) deposition process.2) Improved part according to claim 1, wherein said hard coating has one or more of the following contents in weight of elements:-a content of carbon comprised between 0,5-5 % wt;-a content of chromium comprised between 5,0-30 % wt;-a content of nickel comprised between 2,0-15 % wt;-a content of silicon comprised between 0,5-10 % wt;-a content of boron comprised between 1,0-20 % wt;-a content of titanium comprised between 1, 0-8,0 % wt;-a content of iron balancing to 100% the contents of one or more of the previous elements.3) Improved part according to claim 2, wherein said hard coating has one or more of the following contents in weight of elements:-a content of carbon comprised between 2, 0-2, 5 % wt; -a content of chromium comprised between 13-14 % wt; -a content of nickel comprised between 5, 5-6, 5 % wt; -a content of silicon comprised between 2, 0-2, 5 % wt; -a content of boron comprised between 3, 5-4,0 % wt; -titanium in a content comprised between 2, 0-7,0 % wt -a content of iron balancing to 100% the contents of one or more of the previous elements.4) Improved part according to one or more preceding claims, wherein said hard coating comprises one or more of the following materials: tungsten carbide, molybdenum boride, vanadium carbide, martensitic steel, nickel or nickel alloys, cobalt or cobalt alloys, chromium carbide .5) Improved part according to one or more preceding claims, wherein said hard coating has one or more of the following substance contents:-a content of tungsten carbide comprised between 0-50-% wt ;-a content of cobalt, cobalt alloys, nickel or nickel alloys comprised between 0-30% wt;-a content of molybdenum boride or chromium carbide comprised between 0-40% wt;-a content of martensitic steel comprised between 40-99%wt ;-a content of vanadium carbide comprised between 0-40% wt ;-a content of titanium comprised between 4 , 0-5 , 0 % wt .6) Improved part according to one or more preceding claims , wherein said hard coating has one or more of the following substance contents :-a content of tungsten carbide comprised between 5-45% wt ;-a content of cobalt , cobalt alloys , nickel or nickel alloys or martensitic steel comprised between 10-14 % wt ; -a content of molybdenum boride or chromium carbide comprised between 5-15% wt ;-a content of martensitic steel comprised between 50-90% wt ;-a content of vanadium carbide comprised between 5-30% wt .7 ) Improved part according to one or more preceding claims , wherein said hard coating has a hardness comprised between 50-100 HRC .8) Improved part according to claim 7 , wherein said hard coating has a hardness comprised between 60-80 HRC .9) Improved part according to one or more preceding claims , wherein said hard coating has an average thickness comprised between 0 , 1-1 millimetres .10) Improved part according to one or more preceding claims , wherein at least the first friction surface ( 7 )made of said hard coating obtained through the Extreme High Speed Laser application, also referred to as EHLA, has one or more following features:-a surface roughness Rz equal to or lower than 300 micrometres ;-a surface roughness Rz equal to or lower than 23 micrometres ;-a surface roughness Ra equal to or lower than 50 micrometres ;-a surface roughness Ra equal to or lower than 6 micrometres .11) Improved part (53) according to one or more preceding claims, comprising a base part coated by the hard coating, the base part comprises a base material chosen from the following group:-a steel which has not been previously carburized or case hardened;-a steel having a content of carbon equal to or lower than 0,5% wt, or equal or lower than 0,45 % wt or equal or lower than 0,30%wt;-a steel having a content of carbon comprised between 0, 1-0, 5% wt;-a steel having a content of carbon comprised between 0,4-0, 45% wt .12) Improved part according to one or more preceding claims, said improved part being a bushing (53) .13) Improved part according to claim 11 or 12, wherein said base part has a substantially tubular shape.14) Undercarriage assembly (1) comprising an improved part (53) according to one or more of the preceding claims .15) Continuous track (5) comprising an improved part(53) according to one or more of the preceding claims.16) Continuous track (5) according to claim 15, wherein: -the continuous track (5) comprises a plurality of track links, a plurality of hinging pins 52 and a plurality of pin bushings (53) ;-each track link comprises a respective hinging pin (52) and a respective pin bushing (53) , is fixed and articulated to a subsequent track link by means of the respective hinging pin (52) ;-each hinging pin (52) axially crosses a respective pin bushing (53) ;-each pin bushing (53) form flanks;-the undercarriage frame assembly (3) comprises at least one sprocket wheel (32, 34) configured for engaging the hinging pins (52) resting and / or sliding against the flanks of the pin bushings (53) ;-at least some of the pin bushings (53) , and possibly all of them, have the features according to claim 10.17) Process for producing an improved part (53) of an undercarriage assembly (1) of a tracked vehicle according to one or more claims 1-13, comprising the following steps:-providing a semimanufactured base part made of or however comprising a steel which has not been previously carburized or case hardened;-coating at least a part of the semimanufactured base part through an EHLA deposition process so as to make a hard coating forming a first friction surface ( 7 ) ;-wherein said process for producing the improved part ( 53 ) does not comprise the step of carburi zing or case hardening the semimanufactured base part or the finished base part .

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