IT201900002849A1 - Set screw for piston rings - Google Patents

Description

"Set grain for piston rings"

"Retaining pin for piston rings"

Technical field of the invention

The invention refers to a locking dowel, or stop, for piston rings, in particular a specific piston for high performance 2-stroke engines, but it can also be applied on 4-stroke pistons as a locking dowel of the rotation of the segments, for example in the application to boxer engines. More particularly, the invention relates to the grain surface treatment which is a nitriding treatment process.

The stopper grain of the invention is therefore a nitrided grain, preferably with plasma nitriding techniques, and can be mounted both in a central position and as an internal grain in accordance with the ISO 6621 Standard.

Known art

Modern two- and four-stroke internal combustion engines, both diesel and petrol, have high operating pressures and temperatures, which subject the piston, in particular the piston head, to increasingly extreme thermal and mechanical stresses. The 2 and 4-stroke pistons are generally made of die-cast aluminum alloys and have housings and seats on the head for piston rings and set screws.

Generally, the piston rings, also called piston rings, or simply rings, are metal rings inserted in suitable seats obtained in the upper part of the piston. It is very important, when assembling the piston rings, to respect the correct position with respect to the piston. For this reason, the piston generally returns a dowel in the seat obtained to house the piston and this dowel serves as a reference for a correct assembly of the band. In fact, once mounted, the band (or segment) must never surmount the grain which instead must be found in the only gap found in the elastic band.

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

The evolution of modern 2-stroke engines has led to ever higher rotational speeds (over 14000 rpm and even more) and to the consequent increase in the openings of the transfer ports inside the cylinder, in order to improve the thermal efficiency. fluid dynamics and therefore the performance of the engine. These modifications have led to increasingly higher stresses on the elastic sealing segments; with very high tangential thrusts (a sort of hammering action) and rotational thrusts on the part of the ends (points) of the segment, against the stop grain itself.

To ensure greater reliability of the piston and therefore of the set grain, in recent years grains in new materials and types of steel have appeared on the market more and more frequently (such as steels belonging to the family of stainless steels, which have an expansion coefficient comparable with that of aluminum, so as to maintain the interference of grain pressing (grain forcing) inside the piston, even at high temperatures (even over 450 ° C). It should be borne in mind that aluminum it has an expansion coefficient higher than that of steel and therefore a grain made of aluminum would no longer be forced inside its seat in the piston, with possible possible slipping and relative exit of the same from the piston, with easily imaginable catastrophic consequences.

The locking dowel is inserted by force into the piston with special machines specially built for this purpose, according to very precise dimensional constructive tolerances and driving force; too high a driving force can lead to yielding and plasticization of the grain seat, while a too low driving force may not be sufficient to prevent the grain from slipping out during operation.

For this purpose, grains in materials that are too hard, such as tempered steel in UNI 100Cr6 and therefore with high hardness and mechanical resistance to wear, have a coefficient of thermal expansion, incompatible with high performance applications, while stainless steel which has a coefficient of thermal expansion compatible with that of aluminum, they do not have sufficient hardness and therefore wear resistance capable of withstanding the hammering and abrasion conditions created by the ends of the tips of the sealing segment. Normally these steel grains, which we can define as “soft”, are easily sheared by 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 a nitriding on the surface of the stainless steel grain, so as to obtain an increase in surface hardness, an increased wear resistance, comparable to that of UNI 100 Cr6 steel. quenched and tempered goes beyond 500 HV of Vikers hardness, with a thickness of the nitriding layer of 0.05 mm, for example, while maintaining the thermal expansion characteristics of stainless steels.

It is therefore an object of the present invention to provide a surface nitrided stop grain for the piston to resist wear, provided with a nitriding layer with a thickness of 25-110 µm, preferably 30-90 µm, more preferably 50-80 µm and characterized from surface hardness ≥500 HV. The grain is advantageously made of metal and is characterized by an external nitrided surface layer inside which there is an internal core of non-nitrided metal material. Therefore the nitrided outer surface layer has a higher hardness than the inner core. Preferably the hardness of the nitrided outer surface layer is at least 50 ± 10HV greater than that of the inner core.

Another purpose of the invention is a grain that is housed in the hole of a piston in which the hole has a diameter that satisfies the following relationship, the values being expressed in mm:

(diameter of the grain to be planted - 0.1) 2 x (Thickness of the nitriding layer)

Another purpose of the invention is a piston for an internal combustion engine

which is provided with at least one hole for housing a locking dowel

as specified above, where the hole has a diameter that satisfies the above relationship.

Another purpose of the invention is a piston equipped with at least one

surface nitrided stop grain according to the invention.

Still another object is an internal combustion engine, for example an engine

two or four stroke or boxer, equipped with pistons bearing at least one

nitrided grain according to the invention.

Still another object of the invention is a mechanical apparatus,

such as an internal combustion engine comprising at least one grain

stop and at least one piston as described above.

Further aspects, purposes and advantages of the grain and its process

of nitriding will be evident from the detailed description of the invention that follows.

Brief description of the Figures

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

Figure 1 is a partial and schematic perspective view

a detail of the piston housing (hole) in which a dowel will be inserted;

Figure 2a is a cross sectional schematic view of a piston

with Alfin ring;

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

Figure 3a is a cross sectional schematic view of a piston

without Alfin ring;

Figure 3b is the detail in circle B of Figure 3a;

Figure 4 is an electron micrograph (100x magnification) of a nitrided grain with the nitrided layer highlighted;

Figure 5 is an electron micrograph (200x magnification) of a nitrided grain with the imprints of the durometer highlighted outside and within the nitrided layer;

Figure 6 is an electron micrograph (200x magnification) showing the imprint of the durometer within the nitrided layer;

Figure 7 is an electron micrograph (200x magnification) showing the imprint of the durometer at the heart of the grain of Fig.6;

Figures 8a and 8b illustrate a manual Vickers micro-hardness tester from ZWICK used for micro hardness measurements.

Detailed description of the invention

In the context of the invention, the term segment or segments means the piston rings, and the terms band and segment are to be considered equivalent.

The piston is an element of an internal combustion engine, which may for example be an automobile or motorcycle engine, two or four stroke, or a boxer engine, and will typically be a high-performance engine, such as a sports car engine. and / or racing.

With particular reference to the embodiment of the present invention illustrated in Figures 1-3b, the following legend is provided:

1 Housing or Grain hole

2 Segment or Piston ring

3 Piston

4 Wheat

5 Alfin ring

In Figures 1, 2a, 2b, 3a and 3b, the partial view of the housing 1 or hole (also called pre-hole below) of a stop grain 4 in a piston 3 of a generic internal combustion engine is schematically shown. In Fig.2a and 2b the segment 2 and the Alfin 5 ring are also represented with the grain 4 inserted in its housing, while in the Fig.3a and 3b the piston 3 does not carry any Alfin ring.

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

The locking dowel is inserted by force inside the piston with special machines specially built for this purpose, according to precise dimensional constructive tolerances and driving force known to the expert in the field; too high a driving force can lead to yielding and plasticization of the grain seat, while too low a driving force may be insufficient to prevent the grain from slipping out during operation. The driving forces typically employed are in the range of 150-400 kg.

The grains according to the invention are advantageously subjected to nitriding on their surface. Nitriding is in fact a process that produces a surface hardening layer, dense, without cracks, resistant to corrosion.

Advantageously, the nitriding is carried out with conventional techniques known per se, for example that described in E. Angelini et al. Metallurgical Science Technology Vol. 6 [2] (1988) 33-39. Plasma nitriding (also called ionic nitriding) is preferred as it guarantees a homogeneous treatment of the entire surface of the grain in contact with the segment; moreover, it does not allow the formation of hard and at the same time fragile surfaces or the formation of a white blanket or the stratification of compounds into discrete particles (peculiarity of gas nitriding or carbonitriding) which, detaching from the grain, could compromise the proper functioning of the engine, as these hard particles can detach and settle in sensitive areas, such as the piston pin flow rates, segment slots, piston skirt, etc.,

In addition, the smooth and homogeneous surface that is achieved on the grain with plasma nitriding allows tolerances on the driving forces that are optimal for the specific application as it facilitates the centering of the window of said driving forces.

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

A mathematical relationship has now been found experimentally

which binds the size of the hole (also called pre-hole) for driving the

nitrided grain with the nitriding thickness; this relationship facilitates the centering of the window of the driving forces to make the

grain during operation, in order to compensate for the subsequent thermal expansion between the cylinder and the nitrided grain.

On the basis of our experience it has been determined that, by applying a driving force between 150-400 kg or even 250-400 kg, it is necessary to make a calibrated hole, suitable for receiving the nitrided grain.

(in the traditional diametral tolerance according to ISO standard) equal to the following rule:

(diameter of the nitrided grain to be planted in mm - 0.1) 2x (Nitriding thickness in mm) = hole diameter

The nitriding thickness is the effective nitriding thickness,

which is conventionally measured as effective thickness, i.e. identifying the point where in the radial seam of micro-hardness, the

hardness reaches at least 50 HV ± 10 or more than the non-nitrided core.

The radial seam is experimentally determined through measurements

conducted with a Vickers micro-durometer through the construction of a

micro-hardness measurement curve, carried out in a radial direction along the thickness of the nitriding.

For example: if the effective thickness of plasma nitriding is

0.08mm and the grain is has a diameter of 2.5mm, you need to prepare

a grain planting hole equal to:

(2,5-0,1mm) + (effective nitriding thickness 80 micron / - 5

microns) = diameter of the hole

therefore the pre-hole must have a diameter of 2.40 0.16 / - 0.05 = 2.56

+/- 0.05 => range 2.51-2.61 mm

This relationship is particularly useful when applied for pistons completely in aluminum, for pistons in aluminum alloys, for example eutectic and hypereutectic alloys in aluminum and alloys with similar characteristics. A non-limiting example is made up of 22% Aluminum / Silicon alloys or UNI AlSi12Cu1Mg1Ni1P, UNI AlSi12Cu3MgNi3PVZrTi, UNI AlSi18Cu1Mg1Ni1P alloys and alloys with similar characteristics. Advantageously, the above relationship can also be used for pistons that have a cast iron ring (also called ring carrier or Alfin ring, normally used in diesel and 4-stroke engines to increase the wear resistance of the piston rings. obtained in the pistons to improve the resistance of the segment / piston group to the bursting pressures to which the group is subjected).

The grains on which the surface nitriding coating according to the invention can be carried out are metallic grains or made of metal alloys having thermal expansion comparable to that of aluminum, in particular they can be austenitic and martensitic stainless steels with thermal expansion comparable to that of aluminum. 'aluminum

For example, steels that can be used to make the set grains according to the invention are listed in table 1.

Table 1

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

Nitriding on stainless steel grains allows to obtain an increase in surface hardness, for wear resistance, similar to that of UNI 100 Cr6 steel or other tempered steels.

The invention will now be described with reference to embodiment examples that are not to be considered limitative of its scope. Examples

The main technical characteristics of the plasma nitriding process used for the piston set screws were:

Process control and regulation By control unit and PC SW

The following table 1 provides the surface hardnesses that have been achieved after ionic nitriding of the most common steels:

Table 1

The following Table 2 illustrates the hardness and thickness data of the nitrided layer performed on the specimens of three grains (measurements performed three times).

The steel was a 46S20 + C steel and the specimens were grains Ø 2.5 x 20 mm prepared for metallographic analysis.

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

Table 2

Claims (11)

CLAIMS 1. Set screw for pistons of an internal combustion engine made of metallic material comprising a nitrided outer surface layer which surrounds an inner core of the non-nitrided metal material and is characterized by the fact that the nitrided outer surface layer has one thickness of 25-110 µm. 2. Set screw according to claim 1 wherein the thickness of the nitrided layer is 30-90 µm, preferably 50-80 µm. 3. Stop grain according to any of claims 1-2 in which the metal material is selected from metals and metal alloys having thermal expansion comparable to that of aluminum 4. A stop nut according to any one of claims 1-3 wherein it Nitrided outer surface layer has a higher hardness than the core internal. 5. Stop grain according to claim 4 wherein the hardness of the nitrided outer surface layer is at least 50 ± 10 HV greater than that of the inner core. 6. A set screw according to any one of claims 1-5 which is housed in the bore of a piston where the bore has a diameter which satisfies the following relationship: (diameter of the grain to be planted - 0.1) 2 x (Thickness of the nitriding layer x 10) 7. Piston of an internal combustion engine bearing at least one hole for housing a locking dowel according to any one of claims 1-6. 8. A piston according to claim 7 wherein the bore has a diameter which satisfies the following relationship: (diameter of the grain to be planted - 0.1) 2 x (Thickness of the nitriding layer x 10) 9. Piston according to any one of claims 7-8 which is made in a material chosen from: aluminum, aluminum alloys, eutectic and hypereutectic aluminum alloys, 22% Aluminum / Silicon alloys, UNI AlSi12Cu1Mg1Ni1P, UNI AlSi12Cu3MgNi3PVZrTi, UNI AlSi18Cu1Mg1Ni1P alloys. 10. Mechanical apparatus comprising at least one set dowel according to any one of claims 1-6 and at least one piston according to any one of claims 7-9. 11. Mechanical apparatus according to claim 10 which is a motor a outbreak.