GB1600569A

GB1600569A - Piston rings and method and apparatus of their manufacture

Info

Publication number: GB1600569A
Application number: GB2533778A
Authority: GB
Original assignee: Associated Engineering Italy SpA
Current assignee: Associated Engineering Italy SpA
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-10-21

Description

(54) PISTON RINGS, AND METHOD AND APPARATUS OF THEIR MANUFACTURE (71) We, ASSOCIATED ENGINEERING ITALY S.p.A., an Italian Company, of Strada Valdellatore, 10091 Alpignano, Turin, Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement; This invention relates to piston rings for reciprocating intemal combustion engines and to methods of their manufacture.

It will be appreciated that when a piston ring, particularly (but not exclusively) a compression ring having a single gap of either a plain rectangular cross-section or any other known or convenient cross-section, is installed in an engine, the outer periphery of the ring should be circular and should have an outward resilient load so that it maintains sealing contact with the cylinder bore around the entire periphery. When the ring and bore are new the gap should be closed or substantially closed.

It is also desirable that as the cylinder bore and/or piston ring wear during operation, the resilient sealing contact between the ring and the bore should be maintained around the entire periphery (although it will be obvious that as the diameter of the piston ring increases by say 0.5%of the diameter, the gap will increase correspondingly).

Hitherto, in order to solve this problem, piston rings have been made by one of two methods, which may be outlined as follows: In the first method the piston ring is machined to have both its inner and outer diameters circular before cutting the gap; the gap is then cut and the piston ring is heat formed by placing on a mandrel and subjecting it to a high temperature so that the ring is expanded within its elastic limit to have a "permanent set". The extent of this is such that on assembly of the piston ring in an engine with the gap closed the outer diameter is circular and the ring has an inherent resilient load which maintains it in contact with the cylinder bore. The term "closed" in this context refers to the condition in which the ring is contracted to fit in the cylinder bore, and does not necessarily imply that the free ends are in contact.In most cases, in fact, there is still a small gap in the "closed" position.

The second known method is to turn the piston ring before cutting the gap so that its inner and outer diameters have a predetermined non-circular form, usually determined by the shape of a cam on a machine of which the cutting tool follows the movement of the cam follower. A predetermined gap is then cut, so arranged that on assembly, when the gap is closed, the external periphery of the ring is circular and has an outward resilient load.

The latter method has the disadvantage that the calculations of the non-circular form assume that the material is homogeneous, and in practice this is not always so.

In accordance with the present invention therefore in one aspect, a method of manufacture of a piston ring includes the steps of making a ring blank of which the dimension over the outer periphery is greater and the dimension within the inner periphery is less than that required in the finished ring, cutting a gap in the ring blank to afford two free ends, applying a force only to the two free ends to close them towards one another, to a closed position in which the gap is reduced or the free ends are lightly in abutment with one another, clamping one or more ring blanks to hold them in said closed position, and then machining the inner and outer peripheries to be circular.

Preferably the method includes the step of centralising the ring in the closed position by means of either its inner or outer periphery and machining the centralised ring on the other of its inner and outer peripheries, and then centralising the ring on said other periphery and machining the hitherto unmachined periphery.

The method may also include the step of drilling or otherwise forming two holes (either wholly or partially through the width of the ring), one near each free and of the ring blank (either before or after cutting the gap), and locating the ring in the closed position by engagement of said holes with pins set at a predetermined spacing, before clamping the ring blank or blanks.

The clamping means may bear on the side faces of the ring or rings, and where there are two or more rings being machined simultaneously, their side faces will be in abutment and they will be substantially concentric.

In accordance with the present invention in another aspect, apparatus for the manufacture of a piston ring includes means to apply a force only to the two free ends of the ring blank, after the gap has been cut therein, to close them towards one another to a closed position in which the gap is reduced or the free ends are lightly in abutment with one another, means to clamp one or more of said ring blanks to hold them in said closed position, and means then to machine the inner and outer peripheries of the clamped ring or rings to be circular.

In accordance with a feature of this aspect of the invention the apparatus includes a pair of substantially parallel pins set at a predetermined spacing to engage in a pair of holes formed in the ring blank near its free ends, whereby to locate the ring blank in said closed position before being clamped.

In rings of suitable dimensions, the holes may be drilled completely throughh the width of the rings near the free ends, and in this case the gaps and the holes in each of the rings of a stack may be aligned, and a pair of suitable rods passed through the aligned holes, so that by moving the free ends of the rods together, the free ends of each of the rings of the stack will be brought into the closed position, and the stack of rings then clamped by their side faces.

One embodiment of the invention will now be specifically described by way of example and with reference to the accompanying drawings, of which Figure 1 is an elevation of a piston ring, Figure 2 is a cross-section on the plane of symmetry of the ring, showing apparatus for its manufacture, and Figure 3 is aview similar to Figure 2, showing further apparatus.

In piston ring practice it is conventional to refer to the radial dimension of the ring as the thickness and the axial dimension of the ring as the width.

The ratio of diameter to thickness of a piston ring typically varies from 20 1 to 30 1 depending on the material.

The typical ring illustrated has, when finish machined, an outside diameter of 60 mm, a thickness of 2.5 mm and a width of 2 mm. It is shown of plain rectangular cross-sectionn, though it may be of any of a large number of known or convenient cross-sections. The invention has particular application to compression a and scraper rings, though it may also be applied in suitable cases to oil control rings, for example to the plain rings of multi-piece oil control rings.

The ring may be cast, or produced by any known or convenient method and preferably has a small degree of ovality, though in certain cases - e.g. where the required gap is small - may be truly circular; the rings thus produced will have a machining tolerance, the reason for which will become apparent; thus the outer diameter is larger and the inner diameter smaller than the finished dimensions, each by, say, 1 mm. The side faces are ground parallel and to the required width.

The compression ring shown is formed with a hole 11 of 1 mm diameter through the entire width of the ring near its free ends, for example by drilling. Usually the two holes will be drilled before the gap is milled, so that the correct spacing on the ring blank can readily be achieved. These will be on each side of the position where the gap is to be, indicated by a notch or similar feature on the blank. However, if desired, the gap may be cut before drilling the holes. The gap is then milled in the ring; a plain gap is shown though, as is well known, this may have different forms.

A manual tool resembling a pair of pliers with two projecting pins to fit in the 1 mm holes is then employed, the pins being inserted in the holes and the jaws of the tool closed to bring the free ends of the ring into a predetermined closed or partly closed position. The distance apart of the two holes in this position will be determined by means of a stop on the tool.

The ring blank 10 will then be placed so that the holes 11 in the ring blank engage with two pins 12 on a lathe or equivalent machine, spaced at the same predetermined distance apart. The ring blank is then clamped in this predetermined position, and centralised for example by means of a male conical or part-conical member 13 engaging against the corner formed by one side face and the inner periphery, clamped against a member 14 engaging the other side face of the ring blank. The outer periphery may then be turned to circular form by tool 15.

Following this, the ring blank 10 is centralised by a female conical or part-conical member 16 engaging its outer diameter and clamping it against a member 17 in contact with the opposite side face, and the inner periphery turned to be truly circular by tool 18.

These two operations may be repeated. Moreover, any "features" such as grooves, steps, bevels, etc. will now be machined.

It will be evident that when the ring is freed from the machine, its free ends will move apart, increasing the gap, as a result of the resilience of the material.

Moreover, it will also be evident that, when the ring is installed in the engine, with the gap closed to the predetermined position, the outer diameter will be truly circular and of the correct dimension to fit in the cylinder bore.

Instead of using a manual tool, the apparatus for the manufacture of the rings may include a pair of slides, each formed with a projecting pin.

mounted to move in converging slideways.

In one position of the slides the distance apart of the two pins may be such that the ring blank, in its free state, after formation of the holes near its two free ends and of the gap, readily engages with the slides, by the two holes engaging over the projecting pins. The two slides are then moved on their converging slideways by equal amounts to their second position, so that the free ends of the ring blank are still op posite one another and at equal radius from the centre, but have moved towards one another, and to the closed position in which the gap is reduced (or the free ends are lightly in abutment with one another).

The ring blank is then centralised, clamped, and machined as described above.

It has been found that with rings manufactured as described the resilient action of the ring against the cylinder bore is maintained around the entire periphery for a longer time, in terms of the wear of the cylinder bore, than with a conventional ring. For example, with a 60 mm diameter ring, as described, a conventional ring may maintain good sealing contact for an increase in cylinder bore size of 0.15 mm on diameter. It has been found that rings made in accordance with the present invention retain good sealing characteristics until about double this amount of wear has taken place.

In addition the fact that the rings may be machined by normal turning to circular form, rather than on a special machine designed to give a particular non-circular form, tends to reduce the cost of machining.

Instead of the holes being drilled wholly through the axial width of the ring, where the ring is of sufficient width, the holes may be drilled part way only through the width. The criterion is that the pins, whether mounted on a hand tool or on a face of a machine tool, when inserted in the holes, should have sufficient bearing area to enable the gap to be maintained at the predetermined "closed" dimension. Moreover where the holes are drilled through the entire width of the ring, pins may be inserted in the holes with an interference fit, the projecting portion of the pins enabling the gap to be closed by a suitable tool, and the projecting portion then being ground off.

Other variations within the scope of the invention will be apparent to those skilled in the art.

WHAT WE CLAIM IS: 1. A method of manufacture of a piston ring, including the steps of making a ring blank of which the dimension over the outer periphery is greater and the dimension within the inner periphery is less than that required in the finished ring, cutting a gap in the ring blank to afford two free ends, applying a force only to the two free ends to close them towards one another to a closed position in which the gap is reduced or the free ends are lightly in abutment with one another, clamping one or more ring blanks to hold them in said closed position, and then machining the inner and outer peripheries to be circular.

2. A method as claimed in Claim 1, including the step of forming two holes. one near each free end of the ring blank (either before or after cutting the gap), and locating the ring in the closed position by engagement of said holes with substantially parallel pins set at a predetermined spacing, before clamping the ring blank or blanks.

3. A method as claimed in Claim 1 or Claim 2 including the step of clamping said one or more ring blanks between two members one at least of which is conical or has conical surfaces and engages with an inner or outer corner of the ring blank cross-section, whereby to centralise the ring blank while the outer or inner peripheral surface respectively is machined.

4. A method as claimed in any of Claims 1 to 3, including the step of grinding the side faces of the ring blank flat, before closing the free ends together.

5. A method as claimed in Claim 3, or Claim 4 when dependent on Claim 3, including first centralising a ring blank by engagement of an inner corner of the ring blank cross-section with a member having a male conical surface or surfaces, clamping it in this position, machining the outer periphery to be circular, then centralising the partly-machined blank by engagement of an outer corner of its cross-section with a member having a female conical surface or surfaces, clamping it in this position, and then machining the inner periphery to be circular.

6. Apparatus for the manufacture of a piston ring, including means to apply a force only to the two free ends of the ring blank, after the gap has been cut therein, to close them towards one another to a closed position in which the gap is reduced or the free ends are lightly in abutment with one another, means to clamp one or more of said ring blanks to hold them in said closed position, and means then to machine the inner and outer periphery of the clamped ring or rings to be circular.

7. Apparatus as claimed in Claim 6, including a pair of substantially parallel pins set at a predetermined spacing to engage in a pair of holes formed in the ring blank near its free ends, whereby to locate the ring blank in said closed position before being clamped.

8. Apparatus as claimed in Claim 6 or Claim 7, wherein said means to clamp said ring blanks in the closed position includes means having a conical or part-conical surface to engage with an inner or outer corner of the cross-section of the ring blank, whereby to centralise the latter before machining.

9. Apparatus as claimed in Claim 6 and including a pair of slides having projecting pins and arranged to move in converging slideways from a first position, where the spacing of the pins is such as to allow the pins to engage readily in holes in the ring blank in its free state, to a second position in which the free ends of the ring blank are in said closed position.

10. A method for the manufacture of piston rings, substantially as herein described with reference to and as illustrated in the ac

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. readily engages with the slides, by the two holes engaging over the projecting pins. The two slides are then moved on their converging slideways by equal amounts to their second position, so that the free ends of the ring blank are still op posite one another and at equal radius from the centre, but have moved towards one another, and to the closed position in which the gap is reduced (or the free ends are lightly in abutment with one another). The ring blank is then centralised, clamped, and machined as described above. It has been found that with rings manufactured as described the resilient action of the ring against the cylinder bore is maintained around the entire periphery for a longer time, in terms of the wear of the cylinder bore, than with a conventional ring. For example, with a 60 mm diameter ring, as described, a conventional ring may maintain good sealing contact for an increase in cylinder bore size of 0.15 mm on diameter. It has been found that rings made in accordance with the present invention retain good sealing characteristics until about double this amount of wear has taken place. In addition the fact that the rings may be machined by normal turning to circular form, rather than on a special machine designed to give a particular non-circular form, tends to reduce the cost of machining. Instead of the holes being drilled wholly through the axial width of the ring, where the ring is of sufficient width, the holes may be drilled part way only through the width. The criterion is that the pins, whether mounted on a hand tool or on a face of a machine tool, when inserted in the holes, should have sufficient bearing area to enable the gap to be maintained at the predetermined "closed" dimension. Moreover where the holes are drilled through the entire width of the ring, pins may be inserted in the holes with an interference fit, the projecting portion of the pins enabling the gap to be closed by a suitable tool, and the projecting portion then being ground off. Other variations within the scope of the invention will be apparent to those skilled in the art. WHAT WE CLAIM IS:

1. A method of manufacture of a piston ring, including the steps of making a ring blank of which the dimension over the outer periphery is greater and the dimension within the inner periphery is less than that required in the finished ring, cutting a gap in the ring blank to afford two free ends, applying a force only to the two free ends to close them towards one another to a closed position in which the gap is reduced or the free ends are lightly in abutment with one another, clamping one or more ring blanks to hold them in said closed position, and then machining the inner and outer peripheries to be circular.

companying drawings.

11. Apparatus for the manufacture of piston rings, substantially as herein described with reference to the accompanying drawings.

12. A piston ring made by the method of any of Claims 1--5 and 10.