Steam engine piston rings

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General

Pistons are describe in detail at http://en.wikipedia.org/wiki/Piston. The seal against the cylinder wall is formed by one or more piston rings.

Piston rings need to be cut so that they can slide over the piston into a machined groove. Cast iron rings are brittle, so care must be taken when fitting them. Cuts come in several types:

• Step Cut - common for noncritical sealing applications
• Butt Cut - common where high leakage rates are permitted
• Hook Cut - used in transmissions (etc) where oil passage may be needed.
• Angle Cut - used in rotating seals. Restricts bypass in the joint.
• Angle Step - injection moulding.
• Step Seal - a step cut with in inner seal.
• Two Ring Set - butt cut inner and a step but outer ring. Good for high temps and high pressure seals.
• Three Ring Set - 1 butt cut inner and two butt cut outers (staggered).
• Locking Ring Set - two rings with a locking element at the joints.

Other features of piston rings include:

• Chamfer
• Radius
• Scraper
• Vent
• Groove
• Bronze Insert
• Taper

Cast iron rings are used because they are low-friction and resist wear. The mechanical structure of cast iron produces consistent pressure against the bore (cylinder walls).

Manufacturing Description

At Federal-Mogul grey iron piston rings are cast as individual rings in a noncircular shape; there are other ring manufacturers who cut the individual rings from pots or cuffs. The rings are generally machined to the required shape by means of double cam turning, a process in which the ring blank, already axially ground, is copy turned simultaneously on the inside and outside diameters. After a segment equivalent to the free gap is cut from the ring it assumes the free shape that will give it the required radial pressure distribution when fitted into the cylinder. Once inside the cylinder the ring is completely light tight on its outside diameter and exerts the predefined radial pressure against the cylinder wall.

Besides using double cam turning, ring blanks can also be shaped by machining the inside and outside diameters separately. This involves cam turning the outside diameter of the noncircular blank and machining the inside diameter with the ring in the compressed state. The gap is cut out in a step between O.D. and I.D. machining. Heat forming as a means of shaping piston rings should be mentioned to complete the range of options, but this process is seldom used.

Steel piston rings are made from a profiled wire. The rings are first coiled into a circular shape and then the gap is cut out. The necessary shape is obtained using a heat treatment process in which the rings are mounted onto an arbor appropriately designed to impart the required radial pressure distribution.

Profiling of the running faces of taper faced, Napier and slotted oil rings is carried out, depending on the ring design, on automatic O.D. lathes or profile grinding machines using special profile cutting tools before or after coating.

ISO Standards for Piston Rings

From Federal Mogul

ISO Designation	Edition	Title
ISO 6621-1	11 / 2007	Vocabulary
ISO 6621-2	11 / 2003	Inspection measuring principles
ISO 6621-3	09 / 2000	Material specifications
ISO 6621-4	10 / 2003	General specifications
ISO 6621-5	12 / 2005	Quality requirements
ISO 6622-1	12 / 2003	Rectangular rings made of cast iron
ISO 6622-1	10 / 2003	Rectangular rings made of steel
ISO 6623	02 / 2004	Scraper rings made of cast iron
ISO 6624-1	08 / 2001	Keystone rings made of cast iron
ISO 6624-2	10 / 2003	Half keystone rings made of cast iron
ISO 6624-3	08 / 2001	Keystone rings made of steel
ISO 6624-4	10 / 2003	Half keystone rings made of steel
ISO 6625	12 / 1986	Oil control rings
ISO 6626	07 / 1989	Coil spring loaded oil control rings
ISO 6626-2	10 / 2004	Coil spring loaded oil control rings of narrow width made of cast iron
ISO 6626-3	neu	Coil spring loaded oil control rings made of steel
ISO 6627	09 / 2000	Expander / segment oil control rings