Piston Rings

Abstract: A method is described for the manufacture of piston rings, the method comprises the steps of producing a tube (10) and machining the desired ring free shapes on the inner (24) and outer (22) diameters, providing cross sectional profiles by grinding, for example, and then cutting the individual rings (34) from the tube. The ring gaps are preferably formed after cutting the ring from the tube. Coatings on the ring outer surface may be applied and shaped whilst in tube form.

Description: The present invention relates to a method for the manufacture of piston rings. It is known to produce piston rings from cast materials, usually cast iron, by two distinct methods. The first method comprises casting a "tree" having individual piston ring blanks attached to a central casting gate. The second method, as disclosed in GB-A-416,622, comprises casting a tube, usually by a centrifugal casting technique, forming the tube to the shape of a selected curve for the rings, for example, by turning, and slicing the tube into individual pieces from which piston rings are produced. Both these methods have the disadvantage that for many of the production operations the pieces are being handled individually whilst for other operations they must be brought together in a stack involving a large number of handling operations. The handling operations virtually all require dedicated tooling for virtually each part number produced, whilst the handling operations themselves lead to damage and increased scrap. Other disadvantages include large numbers of machines to perform the operations, large numbers of people to set and operate the machines, high levels of work in progress and large factory areas to house the machines and work in progress. These disadvantages consequently lead to less flexibility of manufacture and slower response time to changes in customer requirements. It is an object of the present invention to overcome or reduce some of the above disadvantages. According to the present invention a method for the production of piston rings, the method including producing a tube of the material from which substantially the rings are to be made, and machining the outer surface and the bore of the tube to desired ring profiles, is characterised by the following sequence of operations, in first machining steps of the tube there are provided two datum registers comprising either an end face of the tube and a bore, or a bore having an internal shoulder, and a machined datum feature is provided on said tube end face for location and reference, all subsequent machining operations on the tube are calculated from these datum registers and feature, machining the outer surface and the bore of the tube to desired profiles, which profiles substantially correspond to the required unconstrained free-shape of the rings, then providing ring cross sectional profile features by employing both machining and non-machining steps, and partially completed rings subsequently are separated from the tube. The machined datum feature on the tube end face for location and reference, may comprise a notch in the end face or a drilled hole and a mating dowel etc. Ring cross sectional profile features are formed by, for example, grinding prior to cutting the ring from the tube. In one embodiment of the method the tube may be made by centrifugal casting. The ring gap may be provided in each ring either before separation from the machined tube or after separation. Preferably, however, the ring gap may be produced after the individual ring has been removed from the tube. The method may be used to produce plain rings having no coating on the cylinder wall rubbing face, electro-plated rings having a chromium plated coat, for example, or rings having a coating produced by a physical vapour deposition (pvd) process, e.g. molybdenum by plasma spraying.Where the rings are to be coated by electro-plating or a pvd process the outer diameter of the tube may be machined to produce a series of grooves or other features corresponding to that required on each ring. The outer diameter of the tube may be plated or otherwise coated and machined by, for example, grinding, to a profile corresponding to the final desired ring surface profile. Some advantages of this method are that problems encountered with the plating step during the conventional ring manufacturing process, and which occur when. gap sealing is not satisfactory, are eliminated. For example, gas emanating from gaps causes an inconsistent chromium plate deposit on adjacent rings. Furthermore, loading and tooling techniques with the present invention are easier and faster. Current transfer is improved as there are fewer interfaces involved and, therefore, less likelihood of high resistances being set up. A particular advantage of the process of the present invention is that the part-off operation may be carried out near the end of the operation sequence. This gives considerable improvement in productivity due to the radial wall thickness of the tube being at a minimum at this stage. A piston ring having a nominal 125 mm outer diameter when closed and a 5 mm radial width would require a tube having a wall thickness of approximately 11 mm when made by a conventional route. Thus, a wall thickness of less that 50% of that normally encountered exists at the part-off stage. This allows greater acuracy of part-off width due to the reduced thickness to be cut and minimises subsequent grinding operations. Material is also saved by allowing a narrower part-off tool/wheel width which may be translated into additional ring capacity for each machined tube length. In order that the present invention may be more fully understood an example will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows an axial and radial section through a centrifugally cast cast iron tube; Figure 2 shows the tube of Figure 1 having a register bore and radial location notch machined therein; Figure 3 shows the tube of Figure 2 having the basic O.D. and bore machined; Figure 4 shows the tube of Figure 3 having a required profile machined on to the O.D. prior to chromium plating; Figure 5 shows two tubes joined together and having been chromium plated; Figure 6 shows a chromium plated tube being profile ground; Figure 7 shows a detail of Figure 6 showing two adjacent ring surface profiles; Figure 8 shows the ground tube of Figures 6 and 7 being split into individual rings; Figure 9 shows the ring gaps being produced in a stack; and Figure 10 which shows an end view of the rings of Figure 9 with the gap being produced. Referring now to the drawings and where the same features are shown by common reference numerals. A centrifugally cast cast iron tube is shown at 10 (Fig.l). The tube is then proof turned on the outer surface 12, provided with a datum register bore 14 by turning, and proof bored 16. A milled notch 18 is provided for location and reference. The end face 20 is machined as a datum for axial reference (Fig.2). The outer diameter and bore are then machined by a CNC technique to desired profiles 22 and 24 respectively and which substantially correspond to the required unconstrained free-shape of the ring (Fig.3). The outer diameter is then machined with a series of circumferential grooves 26 which correspond to the surface of each ring which is to be coated, in this case by chromium electro-plating (Fig.4). Multiples of two machined tubes 10 are placed together in a jig (not shown) with their notches 18 in alignment for treatment and chromium plating by a known technique in known apparatus. The entire outer surface of the tube including the grooves 26 are coated with a desired thickness of chromium 28 (Fig.5). The chromium coated surface 28 of the tube is now profile ground with a profiled grinding wheel 30 having recesses 32 corresponding to the desired ring profile (Fig.6). Between each eventual ring blank 34 is a thickness of material 36 to allow for parting-off of the blanks 34 from the tube 10 (Fig.7). Parting-off may be accomplished with a tool or elastic wheel 38 comprising, for example, cubic boron nitride (Fig.8). The individual ring blanks are given a side face grinding operation to bring them within the desired tolerance after which a stack of individual ring blanks 34 are brought together for provision of the ring gap which is made by removing the piece of metal 40 by sliting wheels 42 (Figs. 9 and 10).

Claims: A method for the production of piston rings, the method including producing a tube (10) of the material from which substantially the rings are to be made, and machining the outer surface (12) and the bore (16) of the tube to desired ring profiles is characterised by the following sequence of operations, in first machining steps of the tube there are provided two datum registers comprising either an end face (20) of the tube and a bore (14), or a bore having an internal shoulder (-), and a machined datum feature (18) is provided on said tube end face for location and reference, all subsequent machining operations on the tube are calculated from these datum registers and feature, machining the outer surface (12) and the bore (16) of the tube to desired profiles (22 and 24 respectively), which profiles substantially correspond to the required unconstrained free-shape of the rings, then providing ring cross sectional profile features (26,28,-), and partially completed rings (34) subsequently are separated from the tube. A method according to claim 1 characterised in that said machined datum feature (18) on the tube end face (20) is spaced from the longitudinal extending axis of the tube (10). A method according to claim 1 or claim 2, characterised in that a plurality of circumferential grooves (26) to comprise a cross sectional profile feature for each ring, each groove to receive a coating (28) for the corresponding ring, are provided by machining the outer surface (12) of the tube (10) before separating the plurality of partially completed rings (34) from the tube (10). A method according to claim 1, or claim 2, or claim 3, characterised in that, before separating the plurality of partially completed rings (34) from the tube (10), each partially completed ring is provided, by machining, with a required non-linear radially outer periphery (-) to comprise a cross sectional profile feature for the ring. A method according to any one preceding claim, characterised in that, before separating the plurality of partially completed rings (34) from the tube (10), a ring coating (28) is provided on the radially outer periphery (-) of each partially completed ring to comprise a cross sectional profile feature for the ring. A method according to any one preceding claim, characterised in that a gap (-) is provided for each ring before the partially completed rings (34) are separated from the tube (10). A method according to any one of claims 1 to 5, characterised in that a gap (-) is provided for each ring after the partially completed rings (34) are separated from the tube (10). A method according to any one of the preceding claims characterised in that a tube end is provided with a machined feature (18) for location and reference. A method according to any one of the preceding claims characterised in that the radial datum register (14) is a tube end face, and is provided with a machined feature (18) spaced from said longitudinally extending axis. A method according to Claim 8 characterised in that the coating is molybdenum deposited by a spraying process. A method according to any one of Claims 6 to 10 characterised by further including the step of providing a circumferential groove (26) corresponding to each piston ring for receiving the coating. A method according to any one preceeding claim characterised in that the desired ring shapes are machined by a CNC technique. A method according to any one preceeding claim characterised in that the ring cross-sectional surface profile is substantially formed on the tube prior to cutting of the ring therefrom.

Inventors: WOOD GRAHAM REGINALD PEEL HOUS, WOOD, GRAHAM REGINALD, PEEL HOUSE
Applicants: AE PISTON PRODUCTS, AE PISTON PRODUCTS LIMITED
Application year: 1990
Main classification: B23P15
IPC codes: B21D53, C23C30, B22D13, B23P15, F16J9