Segmanlar

Abstract

Segmanların Motorun çalışması sırasında, zamanların oluşumunda önemli görevleri vardır. Bir hava pompası gibi çalışan motorun çalışması sırasında, meydana gelen yüksek sıcaklık nedeniyle, pistonun genleşerek sıkışıp kalmasını önlemek için, özellikle piston başında fazla boşluk bulunur. Bu nedenle, piston basma takılan segmanlar, dört zamanın oluşumunda çok önemli görevler yaparlar. Piston başında bulunan segmanlar, silindir cidarlarına belli bir basınç yaparak, pistonla silindir arasında sızdırmazlık temin ederler. Segmanlar silindir cidarındaki fazla yağı sıyırarak, pistonla silindir arasında ince bir yağ filminin oluşumunu temin ederek, hem silindirlerin yağlanmasını sağlarlar ve hemde motorun yağ yakmasını önlerler. Ayrıca segmanlar piston dışındaki yüksek ısıyı, silindir cidarına ve oradan da soğutma suyuna iletilerek, pistonların soğumasına yardım ederler. Motorun kalbini oluştururlar. Genellikle dökme demirden, bazı özel hallerde de çelikten imal edilirler. Gerilmemiş halde oval durumdadırlar. Yuvalarına oturabilmeleri için belli bir açıklığı oluşturacak biçimde belli bir yerlerinden kesilmişlerdir, yani bütün bir halkadan küçük bir parça uygun şekilde kesilerek çıkarılmıştır. Segmanlar üzerindeki yuvalarda bu suretle belli bir basınç altında otururlar. Konuyla ilgili detaylı açıklamalar tezin içinde mevcuttur.

Piston rings in engines and compressors have three main functions, to seal the working chamber from the crankcase, to asisst in the flow of heat from piston to cylinder wall and to control oil consumption[l]. In order to achieve efficient sealing the piston ring should make a good fit with both the cylinder wall and either the top or bottom of the piston ring groove. The radial fit is achieved by the inherent spring force of the ring together with the pressure of the working medium acting from behind the ring. In the case of an engine this working medium is of course the combustion gas. The axial position of the ring within its groove is determined mainly by gas pressure and inertia forces and alternates between the top and bottom of the groove. Piston rings are also used in increasing numbers as metallic seals for rotating shafts. The operating behaviour of piston rings is to a great extent dependent on the design of the engine, the thermal stresses, the combustion pressure, the type of cylinder and its surface finish, the piston, the lubricating oil used, the fuel, and lastly an the quality of the rings themselves. When selecting the material for a ring the following factors should be considered: -The ring should have good running characteristics under conditions of both adequate and inadequate lubrication, -The ring should have good elastic properties rings - Particularly at higher temperatures - The ring should be a good conductor of heat. There is an additional requirment of strength in the case of an engine operating with a high gas pressure and at high speeds. Heat forming is used for the manufacture of the piston rings for shock obsorbers and generally for rings ove 450 mm diameter. However, the loss of tension of vn these rings, under subsequent exposure to thermal stresses is larger than for cam- turned rings. In addition it is very difficult to introduce deviations from a constant pressure distribution. Surface treatments serve to improve the running-in of piston rings and within limits do inhibit corrosion. This latter feature is only relevant in the case of prolonged storage. In order to lengthen the life of piston rings and cylinders the running surfaces of piston rings are protected with a wear resistant coating. The oil consumption of an engine is dependent not only an the oil scraping rings but also on a large number of other factors such as correct choice of compression rings, clearances of the ring in its groove as well as piston and cylinder design. According to their function, a distincition is made between when the speed of the engine is raised, there is a sudden increase in the amount of blowby. This phenomenon is called fluttering. Occurrence of this fluttering is said to be due to the vibration of lifting of the piston rings and the preventing measure is the use of high modulus of elasticity material. Also, radial thickness is taken large and width small to obtain small mass inertia. As for the wall pressure distribution, consideration is also given to suppress vibration by making the redial pressure distribution at the joint high. As a counter measure against sticking of piston ring due to temperature rise of the ring from heavy load or from deterioration of oil by postponement of oil changing intervals, the use of wedge (keystone) type ring is very effective although measures to taken by the engine side are also important. At present, there is a tendency for wide use of this type of ring in high speed diesel engines and small two-cycle gasoline engines. In regard to wear of piston ring, consideration must be given to both the circumferential wear and the side wear Abnormal wear is due to poor initial wear-in property and to heavy load. For improving this initial wear-in property, the use of barrel face shaped ring having no edge looding is very effective. From the standpoint of surface treatment, wet plating (Refer to Surface Treatment) is appeared to improve wear-in property. For wear from heavy load, there are measures such as providing grooves on the working face to obtain good lubrication property or applying porous Mo-Ceramic mi-ture patented on to the weaking face[2]. Piston ring shope is one of factors, having in fluence upon piston ring function. Piston rings are classified according to principal joints and cross section as follows 1- Cross section: a- Compression ring Mainly used for compression b- oil control ring Manily used for oil scraper and control VUl c- oil control ring (spring loaded and spring backed) 2- joint: a- Butt joint b- Step joint Piston ring shape shall be decided in view of strick-resistance, breakage resistance, as well as gas tightness, running-in, oil scraping and oil controlling. Simple shape is recommended as piston ring shape, but only if needed, complex shape is adapted. Simple shape of piston ring is more desirable if complex shape is not essential. 1- Cross Section : a-Compression Rings, there are some samples of compression rings are stated as below ; - Plain compression ring : The compression ring with a rectangular cross section and with its geometrically simple form provides an adequate seal under normal operating conditions. - Taper faced ring : This compression ring has a tapered running surface which is intended to shorten its running-in period. At the start, the ring and cylinder are in line contact only but with high unit pressure. The gas pressure which also acts at the running surface of the ring provides however some shight relief. Due to their oil scraping action, these rings are often used as an aid in controlling the oil consumption of an engine. - Keystone ring : The compression ring with both sides tapered and is used in these cases when ring sticking can be expected. Due to its wedge shape, any radial movement of the ring will alter its axial clearance and thuse minimise the build up of combustion residues. - Internally bevelled or stepped ring : Compression rings can be provided with one side bevelled or stepped. This asymmetrical change in cross section causes the ring to twist slightly inwords when installed and brings its bottom edge into contact with cylinder wall. Under operating conditional dynamic stress is produced in the ring since gas pressure forces its bottom side flat against the piston groove. - L-shaped ring : Compression ring with L-shaped cross section are commonly installed in small 2-stroke engines in order to improve control of the gas flow through the cylinder ports. Due to its resistance to flutter, such a ring is also used in high speed racing engines. IX b-Oil control rings : There are some examples of oil control rings are stated as below; - Napper ring : This ring can be considered as a compression ring with an oil scraping property. The recess in the ring causes it to twist in a manner similar to the compression ring. - Slotted oil control ring : The slotted oil control ring with parellel sides and two contact lands. Due to the narrow lands of this type of ring, a high unit pressure is achieved. - Bevel - egded oil control ring : In order to achieve a father increase in unit pressure and thereby a better oil scraping effect the outside egdes of both lands are chamfered. - Double bevelled oil control ring : Similar to the type bevel-egded ring but by chamfering the egdes of both lands in the same direction, the oil scraping effect is even firmer improved. c- Oil control ring ( spring loaded and spring backed ) : There are some examples of oil control ring are stated as below ; The current trend towards reduced engine weight often results in a higher degree of cylinder deformation. Such deformation lead to increased oil consumption and control rings with good conformability are recommended in order to achieve an economical level. Such rings are generally flexible and having a small cross section (low section moduls) they achieve their tension by means of an additional rear mounted spring. The spring loaded oil control ring differs from the spring backed version in that its spring does not bear upon the bottom of the piston groove but is self supporting i.e. independent of the piston. ? Expender spring backed oil control ring : In this case the inherent pressure of the piston ring against the wall of the cylinder is reinforced by a serpentine expander spring placed behind it. This auxiliary spring contacts alternately yhe inside of the piston ring and the bottom of the piston ring groove. ? Coil spring loaded oil control ring : An oil control ring the radial pressure of which is increased by means of a cylindrical coil spring. This spring acts equally in all directions against the inside of the ring. The annuler groove in the back surface of the ring can be semicircular or V-shaped. The V-shaped groove prevents the egdes of the oil slots from damaging the spring. ? Three piece oil control ring : Three piece ring comprising two split steel rails principally serve the same purpose as the spring loaded cast iron rings in providing from an effective oil scraping action resulting from their good comformability and high contact pressure. 2- Types of ring joints and typical joint stops : The most commen type of ring joint is the butt joint. Whereas the angle and overlapped versions are still to be found in compressors and hydraulic applications they are being used less and less for engines. In 2-stroke engines, piston ring have to travel over port openings and it is therefore necessery to privede some means of preventing the ring from rotating in its groove and allowing the ends of the ring to spring into the ports. This can lead to subsequent fracture of the piston ring.