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DESCRIPTION

Technical field of the invention

The invention relates to a retaining or stopping pin for piston rings, in particular to a specific piston for high-performance 2-stroke engines, but it can also be applied to pistons of 4-stroke engines as a retaining pin of the rotation of the segments, for example either in the application to boxer engines or where the piston is in an almost vertical position and there is the need to keep the segment in a fixed position being not free to rotate. More particularly, the invention relates to the treatment of the pin surface which is a nitriding treatment process.

The retaining pin of the invention is therefore a ferrous-based pin, such as for example an austenitic or martensitic stainless steel, preferably nitrided with plasma nitriding techniques, and can be mounted both in a central position and as an internal pin in accordance with the ISO 6621 standard.

Known art

Modern internal combustion engines, both diesel and petrol, with two and four strokes, have high operating pressures and temperatures, which subject the piston, in particular the piston head, to increasingly extreme thermal and mechanical stresses. The pistons of 2 and 4 stroke engines are generally made of die-cast aluminum alloys and have overhead housings and seats for piston rings and retaining pins . Generally piston rings, also called elastic segments, or simply segments, are metal rings inserted in suitable seats obtained in the upper portion of the piston. It is very important, when assembling the piston rings, to respect the correct position of the piston. For this reason, generally the piston brings a pin into the seat obtained to house the piston and this pin serves as a reference for correct assembly of the ring. In fact, once mounted, the ring (or segment) must never overcome the pin which instead must be present in the only gap present in the piston ring.

One of the most frequent problems in modern high- performance 2-stroke engines is the loosening of the pin in its seat and the removal/damage due to wear of the retaining pin of the piston and the possible introduction of the ends of the elastic segment inside the ports of the cylinder with consequent cutting of the segment tips, relative seizure and therefore inevitable destruction of the thermal unit.

The evolution of modern 2-stroke engines has led to higher and higher rotational speeds (over 14,000 rpm and even more) with the consequent increase in the opening of the transfer ports inside the cylinder, in order to improve the thermal-fluid dynamic efficiency and therefore the engine performances. These modifications have led to higher and higher stresses on the elastic sealing segments, with very high tangential thrusts (a sort of hammering action) and rotational thrusts by the ends (tips) of the segment, against the retaining pin itself.

In order to ensure a greater reliability of the piston and therefore of the retaining pin, in recent years pins made of new materials and types of steel have appeared on the market even more frequently (such as steels belonging to the family of stainless steels, which have an thermal expansion coefficient comparable with that of aluminum, so as to maintain the interference of pin hammering (pin forcing) inside the piston, even at high temperatures (even over 450°C) . The interference mounting is a type of coupling widely used in mechanics for locking a cylindrical body inside a hole, with the cylindrical body having a diameter greater than that of the hole; locking of the two components is obtained by forced insertion by means of a pressure introduction so as to exploit the elasticity of the materials and to obtain the locking of the two parts (pressure assembly is normally obtained by heating the perforated component and cooling the cylindrical body, so that at the return of the bodies at an ambient temperature or to that of normal operation, the locking of the parts is obtained) .

It must be kept in mind that aluminum has a thermal expansion coefficient greater than that of steel and therefore a pin made of aluminum would no longer be forced inside its seat in the piston with increasing operating temperature (as the hole inside the piston expands much more than the diameter of the retaining pin, thus losing the elastic ability to hold the retaining pin) , eventually with a possible removal and relative exit of the same from the piston, with catastrophic consequences easily imaginable.

The retaining pin is forcedly inserted into the piston with special machines specially built for this purpose, according to precise dimensional construction and planting force tolerances; a too high planting force can lead to the yielding and plasticizing of the pin seat, whereas a too low planting force may not be sufficient to prevent the pin from slipping out during operation .

For this purpose, pins made of too hard materials, such as UNI 100Cr6 tempered steel and therefore with high hardness and high mechanical wear resistance, have a low thermal expansion coefficient compared to aluminum, as in fact the thermal-linear expansion coefficient of the tempered carbon steel is about 1.2 10 ⁵ , the one of piston aluminum alloys is about 2-2.3 10 ⁵ , the one of stainless steel is 2.3-2.4 10 ⁵ , therefore it can be noted that carbon steel is incompatible with applications with high operating temperatures, whereas stainless steel which has a thermal expansion coefficient compatible with that of aluminum, does not have sufficient hardness and therefore its wear resistance is not able to withstand the conditions of hammering and abrasion made by the ends of the tips of the sealing segment. Normally these steel pin, which can be defined as "soft", are easily cut out from the sealing segment.

Consequently, the present invention is aimed at overcoming the drawbacks, limitations and disadvantages of the state of the art .

Summary of the invention

The drawbacks of the state of the art are overcome by carrying out nitriding on the surface of the stainless steel pin, so as to obtain an increased surface hardness, an increased wear resistance, comparable to that of UNI 100 Cr6 tempered steel, which exceeds 500 HV of Vickers hardness, with a thickness of the nitriding layer for example of 0.05 mm, while maintaining the thermal expansion characteristics of stainless steels.

It is therefore an object of the present invention to provide a retaining pin of a piston nitrided on its surface in order to resist wear, provided with a nitriding layer of 25-110 pm thickness, preferably of 30-90 pm, more preferably 50-80 pm and characterized by a surface hardness ³ 500 HV. The pin can advantageously be made of metal material and is characterized by an external nitrided surface layer inside which there is an internal core made of the non-nitrided metal material. Therefore the nitrided outer surface layer has a greater hardness than the inner core. Preferably the hardness of the nitrided outer surface layer is at least 50±10 HV greater than that of the inner core.

Another object of the invention is a pin which is housed in the blind hole of a piston in which the blind hole has a diameter which satisfies the following relationship, the values being expressed in mm:

(diameter of the retaining pin - 0.1) + 2x (thickness of the nitrided layer)

Another object of the invention is a piston for an internal combustion engine provided with at least one blind hole for housing a retaining pin as specified above, wherein the blind hole has a diameter which satisfies the relationship indicated above.

Another object of the invention is a piston provided with at least one surface nitrided retaining pin according to the invention. Still another object is an internal combustion engine, for example a two or four stroke or boxer engine, provided with pistons bringing at least one nitrided pin according to the invention.

Still another object of the invention is a mechanical apparatus, such as for example an internal combustion engine comprising at least one retaining pin and at least one piston as described above.

Still another object of the invention is a method of assembling a piston of an internal combustion engine, comprising a pressure insertion of a retaining pin into a blind hole, previously formed in the piston, so that the retaining pin opposes to a rotation of a piston segment into the respective seat.

Further aspects, objects and advantages of the pin and of the relative nitriding process will become clear from the detailed description of the invention which follows .

Brief description of the Figures

Features and advantages of the invention will be explained in the following description also with the aid of the drawings in which:

Figure 1 is a partial schematic perspective view of a detail of the housing (hole) of the piston in which a pin will be inserted;

Figure 2a is a schematic cross view of the section of a piston with an Alfin ring;

Figure 2b is the detail in the circle A of Figure 2a;

Figure 3a is a cross-sectional schematic view of the section of a piston without an Alfin ring; Figure 3b is the detail in the circle B of Figure 3a;

Figure 4 is an electronic micrograph (lOOx magnification) of a nitrided pin with the nitrided layer highlighted;

Figure 5 is an electronic micrograph (200x magnification) of a nitrided pin with the impressions of the durometer highlighted outside and within the nitrided layer;

Figure 6 is an electronic micrograph (200x magnification) with the impressions of the durometer highlighted within the nitrided layer;

Figure 7 is an electronic micrograph (200x magnification) with the impressions of the durometer highlighted at the core of the pin of Fig. 6;

Figures 8a and 8b show a ZWICK Manual Vickers Micro- durometer used for micro-hardness measurements.

Detailed description of the invention

Within the invention, the term segment or segments refers to piston rings, and the terms ring and segment are to be considered as equivalent.

The piston is an element of an internal combustion engine, which can for example be an engine for cars or motorcycles, with two or four strokes, or a boxer engine, and is generally a high-performance engine, such as a sports car and/or racing engine.

With particular reference to the embodiment of the present invention shown in Figures l-3b, the following reference list is provided:

1 Housing or pin hole

2 Segment or piston ring 3 Piston

4 Pin

5 Alfin ring

In Figures 1, 2a, 2b, 3a and 3b, a partial view of the housing 1 or blind hole (also called hereafter pre hole) of a retaining pin 4 in a piston 3 of a generic internal combustion engine is schematically shown. In Figures 2a and 2b also the segment 2 and the Alfin ring 5 are shown with the pin 4 inserted in the relative housing, whereas in Figures 3a and 3b the piston 3 does not bear any Alfin ring.

The piston can be made of aluminum alloy or steel or in other materials (composites, sintered etc.) known per se.

The retaining pin is forcedly inserted into the piston with special machines suitably built for this purpose, according to the dimensional construction tolerances and planting force, which are very precise and known to the expert in the field; a too high planting force can lead to the yielding and plasticization of the pin seat, whereas a too low planting force may be insufficient to prevent the pin from slipping out during operation. The planting forces (press-in force) typically used are in the range of 150-400 kg.

The pins according to the invention are advantageously subjected to nitriding on their surface. In fact, nitriding is a process that produces a dense, crack-free, corrosion-resistant hardening surface layer. It is evident that the nitriding process in any case involves the surface of the pin, but according to the present invention, the nitriding process is carried out so that the thickness of the nitrided material is suitably contained.

This allows to guarantee that the overall hardness of the pin is not too high to avoid the yielding of the pin seat during its insertion. First of all this occurs since the tribology of the nitrided pin in relation to aluminum is better than the non-nitrided pin, and also as the increased surface hardness allows the surface of the pin not to be damaged during the pressing step inside the hole. In fact, during the pressing of a non- nitrided pin, the surface of the same, not particularly hard, could be damaged by scratching, etc., so hindering the insertion of the same or even going to ruin the coupling due to the presence of any rising or burrs .

Advantageously, nitriding is carried out with conventional techniques known per se, for example the one described in E. Angelini et al. Metallurgical Science Technology Vol . 6 [2] (1988) 33-39. A nitriding plasma (also called ion nitriding) is preferred as it guarantees a homogeneous treatment of the whole surface of the pin in contact with the segment; moreover, it does not allow either the formation of hard and at the same time fragile surfaces, or the formation of a white layer or the stratification of compounds in discrete particles (peculiarity of gas nitriding or carbo- nitriding) which, detaching themselves from the pin, could compromise the proper operation of the engine, as such hard particles can detach and settle in sensitive areas, such as the piston pin seats, hollow segments, piston skirt etc.

In addition, the smooth and homogeneous surface obtained on the pin with plasma nitriding allows tolerances on the optimal planting forces for the specific application, as it facilitates the centering of the interval of said planting forces.

The main feature of the following invention is the application of the plasma nitriding process on steel pins (also called dowel pins or retaining pins) in order to give the pin itself anti-wear properties without compromising the planting characteristics inside the piston.

A mathematical relationship was now experimentally found between the blind hole size (also called a pre hole) for planting the nitrided pin with the nitriding thickness; such relationship facilitates the centering of the planting force interval in order to make the pin reliable during operation, so as to compensate for the subsequent thermal expansion between the cylinder and the nitrided pin.

On the basis of our experience it has been determined that, by applying a planting force between 150-400 kg or even of 250-400 kg, it is necessary to realize a calibrated hole, adapted to receive the nitrided pin (in its traditional diametrical tolerance according to ISO standard) according to the following rule :

(diameter of the nitrided pin to be planted in mm - 0.1) + 2x (nitriding thickness in mm) = hole diameter The nitriding thickness is the effective nitriding thickness, which is measured by identifying the micro hardness radial seam, that is the area where the hardness increases by at least 50 HV ± 10 or more compared to the non-nitrided core.

The radial seam is determined experimentally through measurements carried out with a Vickers micro- durometer through the construction of a curve of micro hardness measurements, performed in a radial direction along the nitriding thickness.

Since the retaining pin has a substantially longitudinal shape, the aforementioned radial direction is identified perpendicularly starting from a development axis of the pin itself.

The relationship between pin nitriding thickness and yield strength of the material defining the blind hole, that is the pin seat, is difficult to explain. It has been experimentally found through the measurement of the extraction force of the pin from the piston and it has been observed that by increasing the insertion force, the extraction force is very much reduced, indicating that the elastic load is practically exhausted. The empirical formula given above provides a good correlation between the diameter of the nitrided pin to be planted and the nitriding thickness, in order to size the hole diameter.

For example: if the effective thickness of plasma nitriding is 0.08 mm and the pin has a diameter of 2.5 mm, it is necessary to prepare a pin planting hole equal to: (2.5-0. lmm) + (effective nitriding thickness 80 microns +/- 5 microns) = hole diameter

in which the pre-hole must have a diameter of 2.40 +0,16 +/- 0.05 = 2.56 +/- 0.05 = > range 2.51-2.61 mm.

It has been experimentally verified that the relationship indicated above allows to prevent the yielding effect of the pin seat.

Such relationship is particularly useful when applied to fully aluminum pistons, pistons in aluminum alloys, for example, eutectic alloys and hypereutectic aluminum and alloys of similar characteristics. A non limiting example is constituted by 22% aluminum-silicon alloys or UNI AlSil2CulMglNilP, UNI AlSil2Cu3MgNi3PVZrTi, UNI AlSil 8CulMglNilP alloys, and alloys with similar characteristics. Advantageously, the relationship referred to above can also be used for pistons having cast iron co-molten rings (also called ring carrier or Alfin ring, normally used in diesel and 4-stroke engines in order to increase the wear resistance of the segments cavities obtained in the pistons, for improving the resistance of the segment/piston group to the bursting pressures to which the group is subjected) .

The pins on which the nitriding surface coating according to the invention can be carried out are metal pins or pins made of ferrous-based metal alloys having thermal expansions comparable to those of aluminum, in particular austenitic and martensitic stainless steels with thermal expansions comparable to those of aluminum and its alloys, of about 2.3-2.4 10 ^_5 °C ^_1 . For example, steels that can be used to produce the retaining pins according to the invention are listed in table 1. Table 1

* indicative values only, since the coefficient of thermal expansion also varies according to the temperature .

The nitriding process is advantageous when used with stainless steels which have a thermal expansion coefficient comparable with that of aluminum, but in themselves do not have sufficient hardness and therefore wear resistance capable of withstanding the conditions of hammering and abrasion realized from the ends of the tips of the sealing segment. Normally these so-called "soft" steel pins are easily cut by the sealing segment, however the treatment of the invention has improved its characteristics.

The nitriding on the stainless steel pins allows to obtain an increase in surface hardness, due to the wear resistance, similar to that of UNI 100 Cr6 steel or other hardened and tempered steels.

The invention will now be described with reference to embodiments which are not to be considered limitative of its relative scope.

Examples

The main technical characteristics of the plasma nitriding process used for the piston retaining pins were :

The following table 2 provides the surface hardness values which were achieved after ion nitriding of most of common steels:

Table 2

The following Table 3 shows the hardness and thickness data of the nitrided layer performed on three pin specimens (measurements performed three times) . The steel was a 46S20+C steel and the specimens were pins of 0 2,5 x 20mm prepared for metallographic analysis.

The dimensions of the nitrided layer are shown in Figure 4. The micro-hardness tests were performed with a Zwick manual micro-durometer (Figures 8a, 8b) with the possibility of movement along the XY axes with centesimal accuracy. Measurements were performed on pin sections, as shown in Figures 6 and 7.

5 Table 3

Constructive variations to the non-limiting examples described are possible, without however departing from the scope of protection of the present 0 invention, including all the equivalent embodiments for a person skilled in the art, to the content of the claims .

From the above description the person skilled in the art is able to realize the object of the invention 5 without introducing further construction details.