AN OIL-SCRAPER RING"

Cross-Reference to related applications

This patent application claims priority from Italian patent application no. 102018000007888 filed on 06/08/2018, the entire disclosure of which is incorporated herein by reference .

Technical field of the Invention

The present invention relates to a sealing ring, such as one of the elastic bands used on pistons of endothermic engines, and in particular to an oil-scraper ring for high performance vehicles.

Prior Art

In modern four-stroke diesel engines it is very important that the consumption of lubrication oil is as low as possible, both for economic reasons and for limiting emissions into the environment.

The function of oil consumption control is entrusted to the entire sealing package of the elastic bands, but the oil-scraper ring is the component that most influences the result .

Its function is to control the film of oil that wets the cylinder liner, which is to be the minimum sufficient to ensure the correct tribological behaviour between the sealing package of the elastic bands and the cylinder liner, reducing as far as possible the amount of oil leaking towards the combustion chamber, which then, burning, contributes to the emission of harmful particles into the environment. This is done by allowing the film of oil to form during the piston stroke towards the TDC (Top Dead Centre) and vice versa by scraping off as much oil as possible during the expansion stroke towards the BDC (Bottom Dead Centre) .

In the past, the oil-scraper ring, or hereinafter simply "the oil-scraper", was made of cast iron and was made with two asymmetrical sliders with differentiated sides, where the term "slider" refers to the portion of the oil-scraper ring, usually radially protruding from the body of the oil- scraper ring, intended to cooperate in a sliding manner with the cylinder liner.

These sliders were made with machine tools such as the lathe; the slider facing the BDC was made with a negative draft, so as to increase the scraping effect, while the slider facing the TDC was made with a positive draft, with a lesser scraping effect.

Here and hereafter, the term "draft" means the angle formed between the perpendicular to the direction of rectilinear movement of the piston (which coincides with the direction of movement, in use, of the oil-scraper, which is borne integral by the engine piston) and the surface of the slider side facing in the direction of displacement of the piston, starting from the perpendicular. As a result, the slider facing the TDC had a positive draft, since the angle was formed "in front" of the perpendicular with respect to the direction of movement, while the slider facing the BDC had a negative draft, since the angle was formed "behind" the perpendicular, again with respect to the direction of movement .

The higher loads occurring in the meantime (higher pressures, higher temperatures and higher piston translation speeds) have made it essential, in order to ensure the necessary reliability and durability of the elastic bands, to coat the sliders with a material with greater wear resistance, such as a galvanic hard chrome insert.

However, the elevated hardness of the insert no longer allows the shaping of the side of the sliders on the lathe, which are shaped by grinding, which, however, is a technology that does not allow the creation of sides with a negative draft, but only with a positive draft.

This solution therefore led to the abandonment of the asymmetrical scraper slider because the negative draft slider could not be made by grinding, at least industrially. Therefore, oil-scraper sliders are now made with both sides (the one facing the TDC and the one facing the BDC) with positive draft resulting in less effective oil removal during the expansion stroke, in which the engine piston moves towards the BDC .

Summary of the invention

The object of the present invention is to overcome the drawbacks of the prior art by providing a sealing ring or "elastic band", specifically an oil-scraper ring, for an endothermic engine piston, designed to combine the advantages of the two solutions of the prior art, avoiding the drawbacks .

According to the invention, a sealing ring for an endothermic engine piston and a method for making it having the characteristics set forth in the appended claims, are therefore provided.

The proposed solution consists of making a composite slider differentiated in the sliding surface that cooperates in a sliding manner with the cylinder liner, creating a first stretch or section, having a predominant surface extension, in hard chromium or other material with elevated hardness, having a first predetermined axial extension, suitable to withstand greater loads, and a second stretch or section, having a minor surface extension, in cast iron and having a second axial extension significantly smaller than the first axial extension and which does not intervene substantially in the load-bearing function.

Here and hereinafter, the term "axial extension" means the extension or development of the first and second stretches of the sliding surface in the alternate sliding direction of the engine piston and its sealing ring between the TDC and the BDC .

The surface of the side of the slider immediately adjacent to the first stretch of the sliding surface is made by positive draft grinding. With the same operation, the entire sliding surface is also ground. The surface of the side of the slider immediately adjacent to the second stretch of the sliding surface is instead turned. This way, this side of the slider can be made with a negative draft, thus creating the best scraping effect.

Preferably, the sealing ring of the invention is an oil-scraper having two axially separated and spaced sliders, obtained in positions adjacent to respective opposite ends of the sealing ring facing, in use, the TDC and the BDC, respectively .

These two sliders are separated by an annular groove obtained by turning a radially outer lateral surface of the sealing ring. Making the groove by turning is possible by making the turning tool come out of the ground area at a safe distance (a few hundredths of centimetre) from the chrome insert, thus avoiding damage to the same.

Brief description of the drawings

The invention will now be described with reference to the appended drawings, which illustrate a non-limiting embodiment thereof, wherein:

figure 1 schematically illustrates a cross-section side view according to a radial cross-section plane of a sealing ring according to the invention; figure 2 illustrates, on an enlarged scale, a micrograph of a radial cross-section of a sliding portion or slider of a sealing ring made according to figure 1; and

figures 3 and 4 illustrate, on a more enlarged scale, micrographs of the same radial cross-section in figure 2 of two different sliding portions or sliders of a sealing ring made according to figure 1.

Detailed description

With reference to figures 1 to 4, reference numeral 1 denotes as a whole a sealing ring for a piston 2 (known and illustrated only partially and schematically by a dotted line for simplicity) of an endothermic engine 3, also known and of which for simplicity only the piston 2 and a respective cylinder liner 4 are illustrated, inside which the piston 2 with the sealing ring 1 integrally coupled to it, moves in reciprocating linear motion between a Top Dead Centre (TDC) and a Bottom Dead Centre (BDC) of the endothermic engine 3 parallel to an axis of symmetry A of the sealing ring 1, as well as of the piston 2 and the cylinder liner 4. The direction and orientation of the reciprocating movement of the sealing ring 1 together with the piston 2 is indicated by the arrows in figure 1.

Preferably, the sealing ring 1 according to the invention is an oil-scraper ring, as is clearly illustrated in figure 1. In any case, what will be said can be applied, if necessary, to any sealing ring or "elastic band" of a sealing package with which the piston 2 is normally equipped.

The sealing ring 1 comprises an annular body 5 having the axis A as its axis of symmetry and configured in a known manner to couple, in use, with the piston 2 such that the axis of symmetry A is coaxial to the piston 2 and aligned with a rectilinear reciprocating movement axis of the sealing ring 1 together with the piston 2 between the TDC and the BDC of the engine 3.

The sealing ring 1 further comprises at least a first and a second slider or annular sliding portion, respectively indicated by numbers 6 and 7.

The annular sliders 6 and 7 are shaped to protrude radially outwardly from the annular body 5 and are configured to slidingly cooperate, in use, with the cylinder liner 4 of the endothermic engine 3.

The annular sliders 6 and 7 are both radially delimited on the outside by a respective essentially cylindrical sliding surface 8. In particular, the annular body 5 and the annular sliders 6 and 7 are made in one piece in the same metal alloy consisting of cast iron.

According to one aspect of the invention, the sliding surface 8 of each slider 6 and 7 comprises a first stretch 9 made by an insert 10 in a metal or ceramic material of greater hardness than the cast iron with which the body 5 and the annular sliders 6 and 7 are made.

According to a preferred aspect of the invention, the insert 10 is a hard chrome insert.

According to the invention, the sliding surface 8 further comprises a second stretch 11, arranged immediately adjacent to the first stretch 9, made of the same cast iron with which the annular body 5 and the annular sliders 6 and 7 are made .

In particular, the sliding surface 8 of both annular sliders 6 and 7 is formed exclusively by the two stretches 9 and 11, immediately adjacent to each other, in hard metal (chrome) or ceramic and in cast iron.

According to an important aspect of the invention, moreover, the first stretch 9 of the sliding surface 8 has a first predetermined axial extension Tl and such that a surface extension of the first stretch 9 is predominant with respect to that of the second stretch 11.

The second stretch 11 has in fact a second axial extension T2 significantly smaller than the first axial extension Tl, so as to have a surface extension smaller with respect to that of the first stretch 9.

"Significantly smaller" means here and hereinafter that the axial extension T2 is half or less than half of the axial extension Tl .

In particular, the first predetermined axial extension Tl of the first stretch 9 defined by the hard chromium insert 10 is at least double the size of the second predetermined axial extension T2 of the second stretch 11 made of cast iron .

According to another important aspect of the invention, the first stretch 9 of the sliding surface 8 of each slider 6 and 7 is facing towards (namely is arranged on the side of) a first axial end 12 of the annular body 5 configured to be facing, in use, the TDC, and the second stretch 11 of the sliding surface 8 of each slider 6 and 7 is facing towards (namely is arranged on the side of) a second axial end 13 of the annular body 5, opposite to the first end 12 and configured to be facing the BDC, in use.

Both the sliders 6 and 7 are axially delimited, towards the end 12, by a lateral surface 14 of a corresponding first side 15 thereof, immediately adjacent to the respective first stretch 9 of the sliding surface 8 and, toward the end 13, by a lateral surface 16 of a corresponding second side 18 thereof, immediately adjacent to the respective second stretch 11 of the sliding surface 8 and opposite the first side 15.

According to one aspect of the invention, the surface 14 has a positive draft, while the surface 16 has a negative draft, for example in the range of -15° to -25°.

The first side 15 of both sliders 6, 7 is ground, while the second side 18 of both the first and second slider 6, 7 is turned.

In particular, the first and second sliders 6, 7 are obtained in positions immediately adjacent to the opposite first and second ends 12, 13 of the annular body 5 and are axially separated and spaced from one another by an annular groove 19 which has been obtained by turning a radially outer lateral surface 20 of the annular body 5 without compromising the respective ground portions 21 of the first side 15 of both sliders 6, 7 and, in particular, the rear slider 7.

The ground portions 21 are arranged throughout a lateral portion of the inserts 10 positioned towards the sides 14.

In particular, the side 15 of each of the sliders 6, 7 comprises a top stretch 22, arranged immediately adjacent to the sliding surface 8 of each slider 6, 7, provided with an annular recess 23 that extends up to, and throughout a radial thickness of, the hard material insert 10 (harder than cast iron) . From the above description it is evident that the invention also relates to a method for making a sealing ring 1, in particular an oil-scraper ring, for a piston 2 of an endothermic engine 3, the sealing ring 1 being movable, in use, with a reciprocating movement between a TDC and a BDC of the endothermic engine 3 and comprising an annular body 5 having an axis of symmetry A and at least a first and a second annular slider 6, 7 externally radially protruding from the annular body 5 and externally radially delimited by respective essentially cylindrical sliding surfaces 8.

The method according to the invention comprises the following steps: i)- obtaining the annular body 5 and the first and second annular sliders 6, 7 integral in one piece with one another in a cast iron, preferably by casting and by possible subsequent mechanical machining;

ii)- obtaining a first stretch 9 of the sliding surface 8 of each first and second slider 6, 7 facing towards the side of a first axial end 12 of the annular body 5 configured to be facing, in use, the TDC, by means of an insert 10 made of a metal material or a ceramic material having a hardness greater than the cast iron with which the body 5 and the annular sliders 6, 7 are made, preferably hard chromium; iii)- obtaining in the cast iron, with which the body

5 and the annular sliders 6,7 are made, a second stretch 11 of the sliding surface 8 of each first and second slider 6,7 in a position immediately adjacent to the first stretch 9 and facing towards the side of a second axial end 13 of the annular body 5 configured to be facing the BDC in use;

iv) where, in said steps ii) and iii), the first stretch 9 of the sliding surface 8 is made having a first predetermined axial extension Tl significantly greater than a second axial extension T2 with which the second stretch 11 is made, so that a surface extension of the first stretch 9 is predominant with respect to the smaller extension of the second stretch 11.

In addition, a surface 14 of a first side 15 of the first and second sliders 6, 7 facing towards the first end

12 and immediately adjacent to the first stretch 9 of the sliding surface 8, is obtained by grinding with a positive draft, also carrying out the grinding of the entire sliding surface 8; while a surface 16 of a second side 18 of the first and second sliders 6, 7 facing towards the second end

13 and immediately adjacent to the second stretch 11 of the sliding surface 8 is obtained by chip removal with a negative draft .

Lastly, it is evident from the above description that the invention also relates to an endothermic engine 3 comprising at least one cylinder liner 4 inside which a piston 2 moves in reciprocating motion between a TDC and a BDC of the engine 3, the piston being equipped with at least one sealing ring 1, preferably an oil-scraper ring, made as indicated above.

The oil-scraper 1 described, as shown in figures 2 to 4, is able to combine the advantages of both prior solutions:

• covering the load-bearing part (i.e. which supports the radial stresses) of the slider 6, 7 with a hard chromium insert 10 or similar, able to withstand the most severe conditions existing in the cylinder 4;

• making the side 15 of the sliders 6, 7 facing the TDC with a positive angle draft able to ensure the formation of a sufficient film of oil between the cylinder liner 4 and the sealing package formed of a plurality of sealing rings of the type described and suitable for the correct tribological behaviour of the system;

• achieving the best scraping condition, and therefore the lowest oil consumption for the part managed by the oil-scraper 1, by making the side 18 of the slider 6, 7 facing the BDC with a negative draft.

Greater scraping efficiency requires at the same time a lower specific pressure on the sliding surface 8 that compensates for the smaller chrome sliding surface defined by the insert 10 alone.

In this way all the objects of the invention are achieved .