Hydrodynamic Lubrication

Lubrication

The main object of lubrication is to separate moving metallic parts by inserting a layer of fluid between them in the form of a thin film which reduces friction. Before going on to study the actual mechanism of lubrication system in a marine diesel engine, let us first see which all parts of the engine need lubrication. It requires just an elementary knowledge of the engine construction and a little forethought to imagine which all parts are moving relative to each other and hence require lubrication.

There are mainly four types of motions present in a marine engine and hence the lubrication can be provided based on the type of motion. These types are as follows.

1. There is purely rotational motion between certain components and these include the bearings of various parts such as the camshaft, crankshaft and so forth.

2. There is purely sliding motion between components such as the piston ring/cylinders, crosshead guide

3. There is oscillatory motion between parts such as the rocker arms

4. There is very small point or line type friction in certain parts such as meshing gear teeth, chain and sprocket wheels and so forth.

Types of Fluid Lubrication

Whenever a layer of lubrication fluid exists between two surfaces it has to be under one of the following conditions.

· When there is a continuous thick film of lubricant between the moving surfaces without any break, it is referred to as perfect or thick film lubrication.

· When the layer is just a few molecules thick the lubrication is said to be boundary lubrication

· When the layer is imperfect and the two objects in relative motion are in partial contact, the lubrication is known as imperfect lubrication or partial film lubrication.

We have cleared the basic concept of lubrication in this article and will further proceed to study the exact mechanisms of lubrication used in various types of moving parts in marine engines in the subsequent articles.

Hydrodynamic Lubrication

The lubrication concept related to situations such as those found in a journal bearing is known as hydrodynamic lubrication wherein a wedge of the lubricating oil is formed and helps to separate the two surfaces. The journal bearing or other types of bearings experience this phenomenon and this can be understood more clearly with the diagram shown below.

The diagram shows a green coloured shaft rotating within a bearing a section of which is shown in red colour. When the shaft is at rest there is hardly any oil between the area of contact between the shaft and the bearing especially if the shaft under consideration is quite heavy as that would simply squeeze out due to the weight of the shaft. When the shaft starts to move slowly there exists imperfect lubrication between the surfaces and there are chances of we and tear. It is the property of oiliness or stickiness which would be helpful for lubrication at this stage. Finally when the shaft is at rotation at a high speed the normal hydrodynamic lubrication takes place and separates the two surfaces

In cases where the speed of the shaft is not large enough for the thick film lubrication to take place forced lubrication is used by pumping oil using some kind of a pump from the bottom end of the bearing as has been shown using a thick black coloured line which depicts the lubricating oil line¹. Sometimes a splash system of some kind can also be used which simply splashes oil over the requisite surfaces just as you would manually pour oil intermittently over two moving surfaces to keep them lubricated.

Apart from the speed of relative motion between the shaft and the bearing other factors which affect the formation of the lubrication film between the two surfaces are the viscosity of the lubricant and the shape of the surfaces in contact as well.

Crosshead Lubrication

The type of motion of the crosshead of a marine diesel engine is oscillatory in nature wherein it oscillates to a few degrees on either side of the center of oscillation. Apart from that the speed of oscillation is not very significant which in turn means that it will not generate a pumping action for lubrication unlike the case of the fast rotating shaft in a bearing which we saw in the previous article.

Due to the nature of the two stroke cycle both in the normal 2 stroke marine diesel engines as well as supercharged 2 stroke marine diesel engines, the force acting on the crosshead bearing is always acting in the downward direction throughout the cycle without any such time period for which this load might be relieved. This certainly presents a challenging situation for effective lubrication to take place.

To overcome these challenges it is important not only to find a suitable lubrication technique but also to improvise on the design of the bearings so that lesser lubrication effect is required. Some of the steps which can be taken in this regard are as follows.

· The connecting rod design can be modified in a manner which provides better support for the crosshead pin.

· Apart from that the pin is also made thicker that what is required from calculations of load so that the large surface area of the thicker pin reduces load per unit area.

· The pin is also finished to a high surface finish and made more wear resistant so that the oiliness of the oil keeps it sticking to the surface

· Bearing shells are lined with a special kind of antifriction white metal that helps it to attain higher strength.

The hand drawn sketch below shows a typical crosshead just in case you haven’t got the idea about the arrangement of the crosshead bearing.

Apart from taking the above mentioned measures the lubricating oil is pumped at high pressure to the loaded region using high pressure pumps. This is necessary since the slow oscillatory motion of the pin is not sufficient to produce oil pressure that would keep the surfaces separate from each other.

Antifriction bearing

A machine element that permits free motion between moving and fixed parts. Antifrictional bearings are essential to mechanized equipment; they hold or guide moving machine parts and minimize friction and wear.

In its simplest form, a bearing consists of a cylindrical shaft, called a journal, and a mating hole, serving as the bearing proper. Ancient bearings were made of such materials as wood, stone, leather, or bone, and later of metal. It soon became apparent for this type of bearing that a lubricant would reduce both friction and wear and prolong the useful life of the bearing. Petroleum oils and greases are generally used for lubricants, sometimes containing soap and solid lubricants such as graphite or molybdenum disulfide, talc, and similar substances.

Materials

The greatest single advance in the development of improved bearing materials took place in 1839, when I. Babbitt obtained a United States patent for a bearing metal with a special alloy. This alloy, largely tin, contained small amounts of antimony, copper, and lead. This and similar materials have made excellent bearings. They have a silvery appearance and are generally described as white metals or as Babbitt metals.

Wooden bearings are still used for limited applications in light-duty machinery and are frequently made of hard maple which has been impregnated with a neutral oil. Wooden bearings made of lignum vitae, the hardest and densest of all woods, are still used.

Some of the most successful heavy-duty bearing metals are now made of several distinct compositions combined in one bearing. This approach is based on the widely accepted theory of friction, which is that the best possible bearing material would be one which is fairly hard and resistant but which has an overlay of a soft metal that is easily deformed. Figure 1 shows bearings in which graphite, carbon, plastic, and rubber have been incorporated into a number of designs illustrating some of the material combinations that are presently available.

Bearings with (a) graphite; (b) wood, plastic, and nylon

Rubber has proved to be a surprisingly good bearing material, especially under circumstances in which abrasives may be present in the lubricant. The rubber used is a tough resilient compound similar in texture to that in an automobile tire. Cast iron is one of the oldest bearing materials. It is still used where the duty is relatively light.

Porous metal bearings are frequently used when plain metal bearings are impractical because of lack of space or inaccessibility for lubrication. These bearings have voids of 16–36% of the volume of the bearing. These voids are filled with a lubricant by a vacuum technique. During operation they supply a limited amount of lubricant to the sliding surface between the journal and the bearing. In general, these bearings are satisfactory for light loads and moderate speeds.

Lubricants

The method of supplying the lubricant and the quantity of lubricant which is fed to the bearing by the supplying device will often be the greatest factor in establishing performance characteristics of the bearing. For example, if no lubricant is present, the journal and bearing will rub against each other in the dry state. Both friction and wear will be relatively high. The coefficient of friction of a steel shaft rubbing in a bronze bearing, for example, may be about 0.3 for the dry state. If lubricant is present even in small quantities, the surfaces hydrodynamic pressure in film become contaminated by this material whether it be an oil or a fat, and depending upon its chemical composition the coefficient of friction may be reduced to about 0.1. Now if an abundance of lubricant is fed to the bearing so that there is an excess flowing out of the bearing, it is possible to develop a self-generating pressure film in the clearance space as indicated in Fig. 2. These pressures can be sufficient to sustain a considerable load and to keep the rubbing surfaces of the bearing separated.

Hydrodynamic fluid-film pressures in a journal bearing

The types of oiling devices that usually result in insufficient feed to generate a complete fluid film are, for example, oil cans, drop-feed oilers, waste-packed bearings, and wick and felt feeders. Oiling schemes that provide an abundance of lubrication are oil rings, bath lubrication, and forced-feed circulating supply systems. The coefficient of friction for a bearing with a complete fluid film may be as low as 0.001.

Fluid-film hydrodynamic bearings

If the bearing surfaces can be kept separated, the lubricant no longer needs an oiliness agent. As a consequence, many extreme applications are presently found in which fluid-film bearings operate with lubricants consisting of water, highly corrosive acids, molten metals, gasoline, steam, liquid refrigerants, mercury, gases, and so on. The self-generation of pressure in such a bearing takes place no matter what lubricant is used, but the maximum pressure that is generated depends upon the viscosity of the lubricant. Thus, for example, the maximum load-carrying capacity of a gas-lubricated bearing is much lower than that of a liquid-lubricated bearing. The ratio of capacities is in direct proportion to the viscosity. Gas is the only presently known lubricant that can be used for operation at extreme temperatures. Because the viscosity of gas is so low, the friction generated in the bearing is correspondingly of a very low order. Thus gaslubricated machines can be operated at extremely high speeds because there is no serious problem in keeping the bearings cool.

The self-generating pressure principle is applied equally as well to thrust bearings as it is to journal bearings. The tiltingpad type of thrust bearing (Fig. 3a) excels in low friction and in reliability. A typical commercial tthrust bearing (Fig. 3b) is made up of many tilting pads located in a circular position. One of the largest is on a hydraulic turbine at the Grand Coulee Dam. There, a bearing 96 in. (2.4 m) in diameter carries a load of 2,150,000 lb (9,560,000 newtons) with a coefficient of friction of about 0.0009.

Tilting-shoe-type bearing

Fluid-film hydrostatic bearings

Sleeve bearings of the self-generating pressure type, after being brought up to speed, operate with a high degree of efficiency and reliability. However, when the rotational speed of the journal is too low to maintain a complete fluid film, or when starting, stopping, or reversing, the oil film is ruptured, friction increases, and wear of the bearing accelerates. This condition can be eliminated by introducing high-pressure oil to the area between the bottom of the journal and the bearing itself, as shown schematically in Fig. 4. If the pressure and quantity of flow are in the correct proportions, the shaft will be raised and supported by an oil-film whether it is rotating or not. Friction drag may drop to one-tenth of its original value or even less, and in certain kinds of heavy rotational equipment in which available torque is low, this may mean the difference between starting and not starting. This type of lubrication is called hydrostatic lubrication and, as applied to a journal bearing in the manner indicated, it is called an oil lift. Hydrostatic lubrication in the form of a step bearing has also been used on various machines to carry thrust.

Fluid-film hydrostatic bearing

Large structures have been floated successfully on hydrostatic-type bearings. For example, the Hale 200-in. (5-m) telescope on Palomar Mountain (California Institute of Technology/Palomar Observatory) weighs about 1,000,000 lb (450,000 kg); yet the coefficient of friction for the entire supporting system, because of the hydrostatic-type bearing, is less than 0.000004. The power required is extremely small and a ¹/₁₂-hp (62-W) clock motor rotates the telescope while observations are being made.

Rolling-element bearings

Everyday experiences demonstrate that rolling resistance is much less than sliding resistance. This principle is used in the rolling-element bearing which has found wide use. In the development of the automobile, ball and roller bearings were found to be ideal for many applications, and today they are widely used in almost every kind of machinery.

These bearings are characterized by balls or cylinders confined between outer and inner rings. The balls or rollers are usually spaced uniformly by a cage or separator. The rolling elements are the most important because they transmit the loads from the moving parts of the machine to the stationary supports. Balls are uniformly spherical, but the rollers may be straight cylinders, or they may be barrel- or cone-shaped or of other forms, depending upon the purpose of the design. The rings, called the races, supply smooth, hard, accurate surfaces for the balls or rollers to roll on. Some types of ball and roller bearings are made without separators. In other types there is only the inner or the outer ring, and the rollers operate directly upon a suitably hardened and ground shaft or housing. Figure 5 shows a typical deep-grooved ball bearing, with the parts that are generally used.

Deep-groove ball bearing

These bearings may be classified by function into three groups: radial, thrust, and angular-contact bearings. Radial bearings are designed principally to carry a load in a direction perpendicular to the axis of rotation. However, some radial bearings, such as the deep-grooved bearings shown in Fig. 5, are also capable of carrying a thrust load, that is, a load parallel to the axis of rotation and tending to push the shaft in the axial direction. Some bearings, however, are designed to carry only thrust loads. Angular-contact bearings are especially designed and manufactured to carry heavy thrust loads and also radial loads.

A unique feature of rolling-element bearings is that their useful life is not determined by wear but by fatigue of the operating surfaces under the repeated stresses of normal use. Fatigue failure, which occurs as a progressive flaking or sifting of the surfaces of the races and rolling elements, is accepted as the basic reason for the termination of the useful life of such a bearing.

Machine rotational speeds reached a point where lubricated metal journal bearings became hydrodynamically isolated so that there was no metal-to-metal contact. The initial discovery was certainly by accident, but soon an empirical methodology was developed followed by the science of tribology related to journal bearings.

Fluid Film Bearings

Historically speaking, fluid film bearings were the first type of rotary bearing to appear simply because they are more simple than rolling-contact bearings. Early sliding bearings were almost certainly first made from wood after the invention of the wheel in prehistoric times, and these were lubricated with grease derived from animal fat. Metal wheel journal bearings followed. When the industrial revolution began in the eighteenth century, machine rotational speeds reached a point where lubricated metal journal bearings became hydrodynamically isolated so that there was no metal-to-metal contact. The initial discovery was certainly by accident, but soon an empirical methodology was developed followed by the science of tribology related to journal bearings.

A Journal Bearing Has Hydrodynamic Lubricating Layer.

When a journal bearing begins rotating, there is very little lubricant between the hole and shaft at the contact point, H₀, and rubbing occurs. Therefore much friction needs to be overcome when starting a hydrodynamic journal bearing. When the bearing has reached sufficient speed, the lubricant begins to wedge into the contact area and hydrodynamic lift is attained.

Various ways have been devised for keeping a journal bearing lubricated. The lubricant often cools and cleans the bearing in addition to its lubrication function. For instance, in a siphon wick lubrication arrangement, as illustrated below, gravity provides the pressure head needed to keep the lubricant flowing to the bearing surface.

Siphon Wick Journal Bearing Lubrication

Another method of delivering lubricant can be done by a capillary wick, as illustrated below. The necessary pressure head is developed through capillary action within the fibers of a textile wick, as with the phenomenon of a candle wick.

Capillary Wick Lubrication Method

For applications that require significantly decrease the amount of starting friction, a hydraulic lift can be introduced.

Hydraulic Lift Lubricated Journal Bearing