basic components of power train

Lesson 2: Power Train Basic Components

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Introduction

This lesson covers power train basic components, which includesbearings, seals and gears.

Objectives

After completing this lesson the student will be able to demonstratean understanding of basic components, including bearings, seals andgears, by selecting the proper responses on the quiz.

Fig. 1.2.1 Basic Power Train Components

Fig. 1.2.2 Bearings

Fig. 1.2.3 Friction

Friction

When objects move against one another, a degree of resistance isproduced by the contacting surfaces. This resistance is better knownas friction (Figure 1.2.3). While friction is useful for transmittingmotion from one object to another, it is also a force that worksagainst movement. Continuous friction causes heat to build up andresults in wear of the contacting surfaces. In machinery, uncheckedfriction can quickly lead to damaged parts and equipmentbreakdowns.

Bearings

A bearing (Figure 1.2.2) is a mechanical device for decreasingfriction in a machine in which a moving part exerts force on anotherpart.

Unit 1 1-2-2 Power Train ILesson 2

Fig. 1.2.4 Bearings on Shafts

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 wasinvented, it was mounted on an axle, and where wheel and axletouched was a bearing. Early bearings had surfaces of wood orleather lubricated with grease such as animal fat. Modern bearingsare often designated into friction and anti-friction types. Neither typeof bearing is completely frictionless but both are efficient in reducingfriction.

Bearings on Shafts

Usually, the bearing supports a moving part. The bearing must allowthe moving part one type of motion, such as rotation, whilepreventing 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.


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 ofdifferent loads that bearings must support. First, there is the staticload 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 thecenter line (or axis) of the shaft. This is called the radial load. As theshaft rotates it also tries to move to the left or right along the centerline of shaft (Figure 1.2.6, bottom diagram). This is called a thrustload. Bearings absorb radial loads and thrust loads to prevent shaftsfrom moving.

In machinery, the most common methods used to reduce friction, heatand wear are lubrication and bearings. Oil provides lubrication andcooling but does not provide support. Bearings are particularly usefulbecause they also support both the static weight and dynamic loads ofthe 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 forcesproduced 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. 1.2.7 Solid Bearings

Fig. 1.2.8 Shaft Supported by Oil (solid theory)

In a solid bearing, the shaft turns on the bearing surface. The shaftand the bearing are separated by a thin layer of lubricating oil. Whenrotating at operational speeds, the shaft is often supported by the thinlayer of oil and not by the bearing itself.

As the rotational speed increases, the oil film becomes thicker, so thatthe friction increases in less than direct proportion to the speed. Atlower speeds, the oil film is thinner if other factors are unchanged.At extremely low speeds, the film may break and the two piecescome into contact. Therefore, friction is high when a machine isstarted in motion, and the bearing may fail if high stresses are put onit during starting.

Solid Bearings

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


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

Fig. 1.2.9 Sleeve Bearing

Sleeve Bearing

The simplest types of solid bearings are one-piece sleeve bearingsalso called bushings. Sleeve bearings have been used in wheels andother rotating shafts since the earliest times. Sleeve or journal typebearings are simpler than anti-friction bearings in construction butmore complex in theory and operation. Figure 1.2.9 shows a type ofsleeve bearing and a camshaft. The camshaft is supported at thejournals by sleeve bearings in the engine block.

The shaft supported by the bearing is called the journal, and the outerportion, the sleeve. If journal and sleeve are both made of steel, thebearing surfaces, even if well lubricated, may grab or pick up smallpieces of metal from each other. The sleeves of most bearingstherefore are lined with brass, bronze, or Babbitt metal. Bronzesleeve bearings are widely used in oil pumps and electric motors.Solid bearings are lined metals that are softer than the shafts that turnon them so that the bearing will wear before the shaft does. It istypically less difficult and much less costly to replace a worn bearingthan it would be to replace the shaft or assembly that rests on thebearing.

Sleeve bearings are generally pressure-lubricated through a hole inthe journal or from the housing that contains the bearing. The sleeveis often grooved to distribute the oil evenly over the bearing surface.


Fig. 1.2.10 Split-half Bearing

Split-half Bearing

A second type of solid bearing is the split-half bearing (Figure1.2.10). Split half bearings are probably most recognizable becauseof their use in automotive engines. Crankshaft rod bearing caps aresplit bearings that are bolted to the piston rods. These bearings canbe replaced if they wear excessively. Split half bearings, in additionto oil holes, often incorporate grooves that allow oil to flow freelyaround the face of the bearing. Split half bearings may also havelocking tabs that fit into notches in the bearing cap. These tabsprevent the bearing from sliding horizontally on the shaft.

Although they are described as solid, split-half bearings are mostoften made of two types of metal. The bearing face material is oftenan alloy such as aluminum, which is softer than steel and a goodconductor of heat. The relative softness of aluminum allows foreignparticle that enter the oil to become embedded in the face of thebearing 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

Fig. 1.2.12 Anti-friction Bearings

Fig. 1.2.13 Anti-Friction Bearing Components

Anti-friction bearing assemblies (Figure 1.2.13) consists of most orall of the following components:

Inner race or cone: The inner race is a hardened steel ring with amachined channel or groove that the balls or rollers travel in. Theinner race is often attached to the rotating shaft that the bearingsupports.

Outer race: Similar to the inner race, the outer race is a hardenedsteel 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 itremains stationary.

Anti Friction Bearings

Anti-friction bearings use rolling action to reduce friction and havelower starting friction than plain bearings. Anti-friction bearing(Figure 1.2.12) designs include ball bearings, roller bearings andneedle bearings.


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

Fig. 1.2.14 Bearing Contact Area

Fig. 1.2.15 Tapered Roller Bearings

Tapered Roller Bearings

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

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


Fig. 1.2.16 Needle Bearings

Fig. 1.2.17 Caged Needle Bearings

Caged Needle Bearings

Needles have the highest load capacity for the same radial space ofall bearings but application is limited to bore diameters of less than10 inches (254 mm).

Needle Bearings

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


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

Fig. 1.2.19 Seal Failure

Seals and Gaskets

For smooth operation with minimal wear, most gears and bearingsrequire constant lubrication. Since the earliest times engineers havedevised different means to keep lubricant around moving parts andkeep out water, dust and dirt. Given the conditions under whichconstruction machines typically operate, effective seals areparticularly important. Seal failure (Figure 1.2.19) results inmachinery breakdowns and the resulting lost time and money.

Anti-friction Bearings

The benefits of anti-friction bearings are listed below:

- No wear on the shaft

- Less power loss

- Allow higher speeds


Fig. 1.2.20 Seal Types

Seal Types

A seal is defined as a piece of material or a method that prevents ordecreases the flow of fluid or air between two surfaces. The sealedsurfaces may be stationary or have movement between them. Someof 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 betweenparts without leaking

- Seal rough surfaces

- Wear faster than the more expensive parts with which theyare used

Seals (Figure 1.2.20) can be classified into two basic types: staticseals and dynamic seals. Static seals are used when there is nomovement between the two sealed surfaces. Dynamic Seals are usedwhen there is movement of the sealed surfaces in relation to eachother.

Static Seals include O-ring seals, gaskets and liquid gasket material.Dynamic Seals include O-ring seals, lip seals, Duo Cone seals andpacking rings.


Fig. 1.2.22 O-ring Seal

Fig. 1.2.21 Gaskets

Gaskets

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


O-rings

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

Fig. 1.2.23 Backup Ring

Fig. 1.2.24 Internal Lip Seals

Lip Seals

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

In extreme high pressure sealing applications above 5500 kPa (800psi), backup rings (Figure 1.2.23) are sometimes used in conjunctionwith the O-rings to prevent extrusion of the O-ring into the clearancespace between the sealed parts. The pressure backup rings areusually made of a plastic material and extend the life of the O-ring.

While the most commonly used O-rings have a circular cross sectionthere are other types that are used for specific applications.

Make sure that all surfaces where O-rings are installed are free fromdirt and dust. Inspect the O-ring for dirt, cuts and scratches. Do nottwist or stretch the O-ring during installation. When removing an O-ring use tools that will not damage the surface of the part.


Fig. 1.2.25 External Lip Seals

External radial lip seals

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

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


Fig. 1.2.26 Garter Spring

Garter Spring

Radial lip seals are held against the surface of the shaft they seal byfluid pressure and a garter spring (Figure 1.2.26). The garter springprovides additional force when fluid pressure is less. The sealactually operates on a thin film of oil between the seal lip and theshaft. This permits lubrication of the seal lip without allowingleakage.

Fig. 1.2.27 Duo Cone Seal Components

Duo-Cone Seal

Duo-cone seals are designed to keep large amounts of dirt out andlubricant in. Because of the harsh conditions where they are used,duo cone seals must be resistant to corrosion so they last for a longtime with minimum maintenance. They must be resistant to shaftbends, end play and shock loads.

The Duo-cone seal consists of two rings, usually made of rubbermounted on two grooved metal retaining rings.

Sometimes thin metal cylinders called shaft wear sleeves are used inconjunction with lip seals to provide a replacement smooth surfacefor the seal and avoid replacement of expensive, highly machinedshafts. The sleeves are most often found on U-joints and largecrankshafts.

Make sure that surfaces where lip seals are used are clean and free ofscratches and grooves. Do not use lip seals with a broken lip. Do notuse lip seals if the lip is "turned under". Lip seals must be removedwith a special tool.


Fig. 1.2.28 Duo-Cone Seal

In operation, the rubber or toric rings hold the metal rings together toform a seal. They also provide a cushion for the metal rings and keepthe sealed faces in alignment when the shaft moves during machineoperation. The smooth surfaces of the metal rings combine with theviscosity 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. Thisdoes not mean the seals should be replaced. Use published operationguidelines to determine whether Duo-Cone seals have failed.

When servicing Duo-Cone seals, thoroughly remove all traces ofprotection layers or oil from new duo cone rings. Use a solvent andmake sure all surfaces are dry. Before assembly, wipe clean the sealfaces and using a tissue moistened with light machine oil carefullyapply a layer of oil on the metal seal face. Do not put oil on therubber ring. Use an installation tool to install the seal with a correctand even application of force. Duo-cone seal rings must always bekept in pairs.


Straight Cut or Spur Gears

The teeth of straight cut or spur gears are cut straight parallel with theaxis of the gear rotation. Straight cut gears are prone to producevibration. These gears also tend to be noisy in operation and aregenerally used in slower speed applications.

Straight spur gears are often used in transmissions because thestraight teeth allow gears to be more easily slid in and out of meshallowing easier shifting.

Fig. 1.2.29 Straight Cut or Spur Gears

Gears

Since the work of a gear is done by the teeth, gears are usually namedaccording to the way the teeth are cut. As machinery has developedover the years many different gear patterns have been devised toperform specific tasks. For proper operation, meshing gears musthave teeth of the same size and design. Also, at least one pair ofteeth must be engaged at all times although gear tooth patterns allowfor more than one pair of teeth to be engaged. The following are themost common gears found in modern industrial machines.


Fig. 1.2.30 Helical Gears

Helical Gears

Helical gears have teeth that are not parallel to the axis of the shaftbut are spiraled around the shaft in the form of a helix. Helical gearsare suitable for heavy loads because the gear teeth come together atan acute angle rather than at 90° as in spur gearing. Engagement ofthe gears begins and rolls down to the trailing edge allowing asmoother transfer of power than on a straight cut. This also permitsquieter operation and the ability to handle more thrust. So helicalgears are more durable than straight gears.

A disadvantage of simple helical gears is that they produce asideways thrust that tends to push the gears along shafts. Thisproduces additional load on the shaft bearings.


Fig. 1.2.31 Herringbone Gears

Herringbone Gears

The thrust produced by helical gears can be balanced by using doublehelical, or herringbone, gears. Herringbone gears have V-shapedteeth composed of half a right-handed helical tooth and half a left-handed helical tooth. The thrust produced by one side iscounterbalanced by the thrust on the other side. Usually a smallchannel is machined between the two rows of teeth. This is to allowfor easier alignment and to prevent oil being trapped in the apex ofthe ‘V’.

Herringbone gears have the same advantages as helical gears, but areexpensive. They are used in large turbines and generators.


Fig. 1.2.32 Plain Bevel Gears

Fig. 1.2.33 Spiral Bevel Gears

Spiral Bevel Gears

Spiral bevel gears are designed for applications where more strengthis needed than a plain bevel gear can provide. Spiral gear teeth arecut obliquely on the angular faces of the gears. The teeth overlapconsiderably, so they can carry greater loads. Spiral bevel gearsreduce speed and increase force.

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 beveledat an angle to the horizontal axis of the shaft. Bevel gear teeth aretapered in thickness and in height. The smaller driving gear is calledthe pinion while the larger driven gear is known as the ring gear.

Plain bevel gears are used in applications where speed is slower andthere is no high impact present. For example, hand wheel typecontrols often use plain bevel gears.


Fig. 1.2.34 Hypoid Gears

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 thathas one or more continuous helical teeth that mesh with a helicalgear. Worm gears differ from helical gears in that the teeth of theworm slide across the teeth of the driven gear instead of exerting adirect rolling pressure. Worm gears are used chiefly to transmitrotation, with a large reduction in speed, from one shaft to another ata 90° angle.

Hypoid gears

Hypoid gears are variations of helical bevel gears that are used whenthe axes of the two shafts are perpendicular but do not intersect. Thesmaller pinion is located below the center of the larger ring gear itdrives. One of the most common uses of hypoid gearing is toconnect the drive shaft and the rear axle in automobiles. Helicalgearing used to transmit rotation between shafts that are not parallelis often incorrectly called spiral gearing.


Fig. 1.2.36 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 motioninto rotary motion or rotary motion into straight-line motiondepending whether the rack or the pinion is driven. The teeth on therack are straight cut while those on the pinion are curved. Commonuses of a rack and pinion gear set is in automotive steering systems orin an arbor press.

Worm Gear Application

Figure 1.2.36 is an example of a worm gear application.




Figures 1.2.38 and 1.2.39 are examples of different rack and piniongear 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 setincludes a ring gear with internal teeth which mates with teeth onsmaller planetary gears. The planetary gears mate with a sun gear.Operation of the planetary gear set is explained in Lesson 3.

basic components of power train

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