Basic Components Of Power Train

Fig. 1.2.1 Basic Power Train Components

Introduction This lesson covers power train basic components, which includes bearings, seals and gears. Objectives After completing this lesson the student will be able to demonstrate an understanding of basic components, including bearings, seals and gears, by selecting the proper responses on the quiz.

Lesson 2: Power Train Theory of Operation

Lesson 2: Power Train Basic Components

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Fig. 1.2.2 Bearings

Bearings A bearing (Figure 1.2.2) is a mechanical device for decreasing friction in a machine in which a moving part exerts force on another part.

Fig. 1.2.3 Friction

Friction When objects move against one another, a degree of resistance is produced by the contacting surfaces. This resistance is better known as friction (Figure 1.2.3). While friction is useful for transmitting motion from one object to another, it is also a force that works against movement. Continuous friction causes heat to build up and results in wear of the contacting surfaces. In machinery, unchecked friction can quickly lead to damaged parts and equipment breakdowns.

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Fig. 1.2.4 Bearings on Shafts

Bearings on Shafts Usually, the bearing supports a moving part. The bearing must allow the moving part one type of motion, such as rotation, while preventing it from moving in any other way, for example, sidewise. Bearings are generally found at the rigid supports of rotating shafts (Figure 1.2.4) where friction is the greatest.

BEARING FUNCTIONS • Decrease Friction, Heat and Wear • Support Static Weight of Shafts and Machinery • Support Radial and Thrust Loads • Allow Tighter Fit Tolerances • Easier to Replace and Less Expensive than Shafts

Fig. 1.2.5 Bearing Functions

Bearing Functions Bearings were invented early in history. When the wheel was invented, it was mounted on an axle, and where wheel and axle touched was a bearing. Early bearings had surfaces of wood or leather lubricated with grease such as animal fat. Modern bearings are often designated into friction and anti-friction types. Neither type of bearing is completely frictionless but both are efficient in reducing friction.

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In machinery, the most common methods used to reduce friction, heat and wear are lubrication and bearings. Oil provides lubrication and cooling but does not provide support. Bearings are particularly useful because they also support both the static weight and dynamic loads of the rotating driveshafts, gears, connecting rods, etc. For example, wheel bearings support the weight of the entire heavy machine. Crankshaft journal bearings support the shaft against the forces produced by the piston rods. The primary functions of bearings in a machine are as follows: - Decrease friction, heat and wear - Support the static weight of shafts and machinery - Support radial and thrust loads produced by rotating shafts - Allow tighter fit tolerances to prevent "slop’ in rotating shafts - Easier to replace and less expensive than shafts

Fig. 1.2.6 Radial and Thrust Bearing Loads

Radial and Thrust Bearing Loads As gear shafts operate in a machine they produce a number of different loads that bearings must support. First, there is the static load of the weight of the shaft and gears that are mounted on it (Figure 1.2.6, top diagram). The direction of the load is toward the center line (or axis) of the shaft. This is called the radial load. As the shaft rotates it also tries to move to the left or right along the center line of shaft (Figure 1.2.6, bottom diagram). This is called a thrust load. Bearings absorb radial loads and thrust loads to prevent shafts from moving.

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While many specific varieties of bearings are used in modern machinery, bearings are classified into two main types: solid (plain) bearings and anti-friction bearings.

Fig. 1.2.7 Solid Bearings

Solid Bearings Solid bearings (Figure 1.2.7) are classified as sleeves or bushings and split-half. Solid bearings are also referred to as friction bearings.

Fig. 1.2.8 Shaft Supported by Oil (solid theory)

In a solid bearing, the shaft turns on the bearing surface. The shaft and the bearing are separated by a thin layer of lubricating oil. When rotating at operational speeds, the shaft is often supported by the thin layer of oil and not by the bearing itself. As the rotational speed increases, the oil film becomes thicker, so that the friction increases in less than direct proportion to the speed. At lower speeds, the oil film is thinner if other factors are unchanged. At extremely low speeds, the film may break and the two pieces come into contact. Therefore, friction is high when a machine is started in motion, and the bearing may fail if high stresses are put on it during starting.

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Fig. 1.2.9 Sleeve Bearing

Sleeve Bearing The simplest types of solid bearings are one-piece sleeve bearings also called bushings. Sleeve bearings have been used in wheels and other rotating shafts since the earliest times. Sleeve or journal type bearings are simpler than anti-friction bearings in construction but more complex in theory and operation. Figure 1.2.9 shows a type of sleeve bearing and a camshaft. The camshaft is supported at the journals by sleeve bearings in the engine block. The shaft supported by the bearing is called the journal, and the outer portion, the sleeve. If journal and sleeve are both made of steel, the bearing surfaces, even if well lubricated, may grab or pick up small pieces of metal from each other. The sleeves of most bearings therefore are lined with brass, bronze, or Babbitt metal. Bronze sleeve bearings are widely used in oil pumps and electric motors. Solid bearings are lined metals that are softer than the shafts that turn on them so that the bearing will wear before the shaft does. It is typically less difficult and much less costly to replace a worn bearing than it would be to replace the shaft or assembly that rests on the bearing. Sleeve bearings are generally pressure-lubricated through a hole in the journal or from the housing that contains the bearing. The sleeve is often grooved to distribute the oil evenly over the bearing surface.

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Fig. 1.2.10 Split-half Bearing

Split-half Bearing A second type of solid bearing is the split-half bearing (Figure 1.2.10). Split half bearings are probably most recognizable because of their use in automotive engines. Crankshaft rod bearing caps are split bearings that are bolted to the piston rods. These bearings can be replaced if they wear excessively. Split half bearings, in addition to oil holes, often incorporate grooves that allow oil to flow freely around the face of the bearing. Split half bearings may also have locking tabs that fit into notches in the bearing cap. These tabs prevent the bearing from sliding horizontally on the shaft. Although they are described as solid, split-half bearings are most often made of two types of metal. The bearing face material is often an alloy such as aluminum, which is softer than steel and a good conductor of heat. The relative softness of aluminum allows foreign particle that enter the oil to become embedded in the face of the bearing avoiding scratches on the more costly crankshaft.

Benefits of Solid Bearings • Less Expensive • Handle heavy radial loads

Fig. 1.2.11 Benefits of Solid Bearings

Benefits of Solid Bearings - Less Expensive - Handle Heavy radial loads

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Fig. 1.2.12 Anti-friction Bearings

Anti Friction Bearings Anti-friction bearings use rolling action to reduce friction and have lower starting friction than plain bearings. Anti-friction bearing (Figure 1.2.12) designs include ball bearings, roller bearings and needle bearings.

Fig. 1.2.13 Anti-Friction Bearing Components

Anti-friction bearing assemblies (Figure 1.2.13) consists of most or all of the following components: Inner race or cone: The inner race is a hardened steel ring with a machined channel or groove that the balls or rollers travel in. The inner race is often attached to the rotating shaft that the bearing supports. Outer race: Similar to the inner race, the outer race is a hardened steel ring with a channel or grove for the balls or rollers to travel in. The outer race is normally a separate component often mounted so it remains stationary.

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Balls or Rollers: Between the races are the actual friction reducing components. These may be hardened steel balls, straight or tapered rollers, or thin rollers called needles. The balls or rollers turn freely between the inner and outer races. Cage: The cage is positioned between the inner and outer races and is used to maintain the correct spacing between the balls or rollers.

Fig. 1.2.14 Bearing Contact Area

Anti-friction bearings reduce friction by providing both rolling action and a narrow contact area (Figure 1.2.14). Balls have point contact with the races that support them allowing high speed operation. A thin layer of oil separates the components. Straight rollers have a line contact. The line provides more surface contact for greater support against radial loads.

Fig. 1.2.15 Tapered Roller Bearings

Tapered Roller Bearings Tapered rollers work the same way as straight rollers. The rollers and the surface of the races are tapered at an angle to the centerline of the shaft they support. The angle provides resistance to thrust loads. Tapered bearings (Figure 1.2.15) are often used on both ends of a shaft and work together to counteract thrust loads from both directions.

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Fig. 1.2.16 Needle Bearings

Needle Bearings Needle bearings (Figure 1.2.16) work the same way as straight rollers, providing line contact. Because of the small diameters of the needles, they can be used for minimum clearance applications.

Fig. 1.2.17 Caged Needle Bearings

Caged Needle Bearings Needles have the highest load capacity for the same radial space of all bearings but application is limited to bore diameters of less than 10 inches (254 mm).

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BENEFITS OF ANTI-FRICTION BEARINGS • No Wear on the Shaft • Less Power Loss • Allows Higher Speeds

Fig. 1.2.18 Benefits of Anti-friction Bearings

Anti-friction Bearings The benefits of anti-friction bearings are listed below: - No wear on the shaft - Less power loss - Allow higher speeds

Fig. 1.2.19 Seal Failure

Seals and Gaskets For smooth operation with minimal wear, most gears and bearings require constant lubrication. Since the earliest times engineers have devised different means to keep lubricant around moving parts and keep out water, dust and dirt. Given the conditions under which construction machines typically operate, effective seals are particularly important. Seal failure (Figure 1.2.19) results in machinery breakdowns and the resulting lost time and money.

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Fig. 1.2.20 Seal Types

Seal Types A seal is defined as a piece of material or a method that prevents or decreases the flow of fluid or air between two surfaces. The sealed surfaces may be stationary or have movement between them. Some of the many duties of a seal are to: - Prevent lubricant leakage - Keep out dirt and other foreign bodies - Keep different fluids such as oil and water apart - Remain flexible enough to allow some movement between parts without leaking - Seal rough surfaces - Wear faster than the more expensive parts with which they are used Seals (Figure 1.2.20) can be classified into two basic types: static seals and dynamic seals. Static seals are used when there is no movement between the two sealed surfaces. Dynamic Seals are used when there is movement of the sealed surfaces in relation to each other. Static Seals include O-ring seals, gaskets and liquid gasket material. Dynamic Seals include O-ring seals, lip seals, Duo Cone seals and packing rings.

Unit 1 Lesson 2

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Fig. 1.2.21 Gaskets

Gaskets Gaskets are one of the most common seals used to seal small clearances between static machinery parts. They are made of materials that prevent the passage of air, gas or liquid between stationary surfaces. Some of the places that gaskets are used are between the cylinder head and the block and between the block and the oil pan. Surfaces where gaskets are used must be flat, clean, dry and free of scratches. The pressure of the fasteners used to join the surfaces produces an important part of the sealing action of gaskets. It is essential to tighten fasteners to the specified torque to prevent leaking.

Fig. 1.2.22 O-ring Seal

O-rings An O-ring (Figure 1.2.22) is a smooth circular ring made from natural or synthetic rubber or plastic. In operation the ring is usually compressed between the two surfaces. The compressed ring provides the seal. The ring may be used as a static seal in a manner similar to a gasket.

Unit 1 Lesson 2

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Fig. 1.2.23 Backup Ring

In extreme high pressure sealing applications above 5500 kPa (800 psi), backup rings (Figure 1.2.23) are sometimes used in conjunction with the O-rings to prevent extrusion of the O-ring into the clearance space between the sealed parts. The pressure backup rings are usually made of a plastic material and extend the life of the O-ring. While the most commonly used O-rings have a circular cross section there are other types that are used for specific applications. Make sure that all surfaces where O-rings are installed are free from dirt and dust. Inspect the O-ring for dirt, cuts and scratches. Do not twist or stretch the O-ring during installation. When removing an O-ring use tools that will not damage the surface of the part.

Fig. 1.2.24 Internal Lip Seals

Lip Seals Lip seals are some of the most important dynamic seals used in construction equipment. Lip seals endure operation in all types of severe conditions and resist breakdown due to heat build-up or contact with lubrication or hydraulic fluids. They are also resistant to movement between the two parts they are sealing. Lip seals are relatively easy to remove for service replacement.

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The two most common types of lip seals are radial lip seals and dirt excluding lip seals. Dirt excluding lip seals are used as "scrapers" or "wipers" on hydraulic cylinders. Radial lip seals are used to prevent leaks on rotating shafts and are manufactured in many different shapes and sizes to suit specific applications. Internal lip seals have the seal lip on the inside diameter of the seal. Some of the most common internal lip seals are shown in Figure 1.2.24.

Fig. 1.2.25 External Lip Seals

External radial lip seals External radial lip seals (Figure 1.2.25) have the seal lip on the outside diameter of the seal.

Fig. 1.2.26 Garter Spring

Garter Spring Radial lip seals are held against the surface of the shaft they seal by fluid pressure and a garter spring (Figure 1.2.26). The garter spring provides additional force when fluid pressure is less. The seal actually operates on a thin film of oil between the seal lip and the shaft. This permits lubrication of the seal lip without allowing leakage.

Unit 1 Lesson 2

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Sometimes thin metal cylinders called shaft wear sleeves are used in conjunction with lip seals to provide a replacement smooth surface for the seal and avoid replacement of expensive, highly machined shafts. The sleeves are most often found on U-joints and large crankshafts. Make sure that surfaces where lip seals are used are clean and free of scratches and grooves. Do not use lip seals with a broken lip. Do not use lip seals if the lip is "turned under". Lip seals must be removed with a special tool.

Fig. 1.2.27 Duo Cone Seal Components

Duo-Cone Seal Duo-cone seals are designed to keep large amounts of dirt out and lubricant in. Because of the harsh conditions where they are used, duo cone seals must be resistant to corrosion so they last for a long time with minimum maintenance. They must be resistant to shaft bends, end play and shock loads. The Duo-cone seal consists of two rings, usually made of rubber mounted on two grooved metal retaining rings.

Unit 1 Lesson 2

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Fig. 1.2.28 Duo-Cone Seal

In operation, the rubber or toric rings hold the metal rings together to form a seal. They also provide a cushion for the metal rings and keep the sealed faces in alignment when the shaft moves during machine operation. The smooth surfaces of the metal rings combine with the viscosity of the oil to a seal the shaft. Duo cones must be "exercised" to maintain the metal-to-metal seal. If a machine is idle for a long time, the seals may begin to leak. This does not mean the seals should be replaced. Use published operation guidelines to determine whether Duo-Cone seals have failed. When servicing Duo-Cone seals, thoroughly remove all traces of protection layers or oil from new duo cone rings. Use a solvent and make sure all surfaces are dry. Before assembly, wipe clean the seal faces and using a tissue moistened with light machine oil carefully apply a layer of oil on the metal seal face. Do not put oil on the rubber ring. Use an installation tool to install the seal with a correct and even application of force. Duo-cone seal rings must always be kept in pairs.

Unit 1 Lesson 2

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Gears Since the work of a gear is done by the teeth, gears are usually named according to the way the teeth are cut. As machinery has developed over the years many different gear patterns have been devised to perform specific tasks. For proper operation, meshing gears must have teeth of the same size and design. Also, at least one pair of teeth must be engaged at all times although gear tooth patterns allow for more than one pair of teeth to be engaged. The following are the most common gears found in modern industrial machines.

Fig. 1.2.29 Straight Cut or Spur Gears

Straight Cut or Spur Gears The teeth of straight cut or spur gears are cut straight parallel with the axis of the gear rotation. Straight cut gears are prone to produce vibration. These gears also tend to be noisy in operation and are generally used in slower speed applications. Straight spur gears are often used in transmissions because the straight teeth allow gears to be more easily slid in and out of mesh allowing easier shifting.

Unit 1 Lesson 2

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Fig. 1.2.30 Helical Gears

Helical Gears Helical gears have teeth that are not parallel to the axis of the shaft but are spiraled around the shaft in the form of a helix. Helical gears are suitable for heavy loads because the gear teeth come together at an acute angle rather than at 90° as in spur gearing. Engagement of the gears begins and rolls down to the trailing edge allowing a smoother transfer of power than on a straight cut. This also permits quieter operation and the ability to handle more thrust. So helical gears are more durable than straight gears. A disadvantage of simple helical gears is that they produce a sideways thrust that tends to push the gears along shafts. This produces additional load on the shaft bearings.

Unit 1 Lesson 2

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Fig. 1.2.31 Herringbone Gears

Herringbone Gears The thrust produced by helical gears can be balanced by using double helical, or herringbone, gears. Herringbone gears have V-shaped teeth composed of half a right-handed helical tooth and half a lefthanded helical tooth. The thrust produced by one side is counterbalanced by the thrust on the other side. Usually a small channel is machined between the two rows of teeth. This is to allow for easier alignment and to prevent oil being trapped in the apex of the ‘V’. Herringbone gears have the same advantages as helical gears, but are expensive. They are used in large turbines and generators.

Unit 1 Lesson 2

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Fig. 1.2.32 Plain Bevel Gears

Plain Bevel Gears Bevel gears permit the power flow in a gear train to turn a corner. The gear teeth are cut straight on a line with the shaft but are beveled at an angle to the horizontal axis of the shaft. Bevel gear teeth are tapered in thickness and in height. The smaller driving gear is called the pinion while the larger driven gear is known as the ring gear. Plain bevel gears are used in applications where speed is slower and there is no high impact present. For example, hand wheel type controls often use plain bevel gears.

Fig. 1.2.33 Spiral Bevel Gears

Spiral Bevel Gears Spiral bevel gears are designed for applications where more strength is needed than a plain bevel gear can provide. Spiral gear teeth are cut obliquely on the angular faces of the gears. The teeth overlap considerably, so they can carry greater loads. Spiral bevel gears reduce speed and increase force.

Unit 1 Lesson 2

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Fig. 1.2.34 Hypoid Gears

Hypoid gears Hypoid gears are variations of helical bevel gears that are used when the axes of the two shafts are perpendicular but do not intersect. The smaller pinion is located below the center of the larger ring gear it drives. One of the most common uses of hypoid gearing is to connect the drive shaft and the rear axle in automobiles. Helical gearing used to transmit rotation between shafts that are not parallel is often incorrectly called spiral gearing.

Fig. 1.2.35 Worm Gears

Worm Gear Another variation of helical gearing is provided by the worm gear, also called the screw gear. A worm gear is a long, thin cylinder that has one or more continuous helical teeth that mesh with a helical gear. Worm gears differ from helical gears in that the teeth of the worm slide across the teeth of the driven gear instead of exerting a direct rolling pressure. Worm gears are used chiefly to transmit rotation, with a large reduction in speed, from one shaft to another at a 90° angle.

Unit 1 Lesson 2

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Fig. 1.2.36 Worm Gear Application

Worm Gear Application Figure 1.2.36 is an example of a worm gear application.

Fig. 1.2.37 Rack and Pinion Gear Set

Rack and Pinion Gear Set Rack and pinion gears can be used to convert straight-line motion into rotary motion or rotary motion into straight-line motion depending whether the rack or the pinion is driven. The teeth on the rack are straight cut while those on the pinion are curved. Common uses of a rack and pinion gear set is in automotive steering systems or in an arbor press.

Unit 1 Lesson 2

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Fig. 1.2.38 Rack and Pinion Gear Set

Fig. 1.2.39 Rack and Pinion Gear Set

Figures 1.2.38 and 1.2.39 are examples of different rack and pinion gear set applications.

Fig. 1.2.40 Ring and Planet Gear

Ring (internal tooth) Gear Ring gears are used in planetary gear sets. The planetary gear set includes a ring gear with internal teeth which mates with teeth on smaller planetary gears. The planetary gears mate with a sun gear. Operation of the planetary gear set is explained in Lesson 3.