Lesson 2: Power Train Basic Componen Components ts
Fig. 1.2.1 Basic Power Tr Train ain 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.
n o i t a r e p O f o y r o e h T n i a r T r e w o P : 2 n o s s e L
Unit 1 Lesson 2
1-2-2
Power Train I
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. surfaces. This resistance is is better known as friction (Figure 1.2.3). While friction is useful for transmitting 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, machinery, unchecked friction can quickly lead to damaged parts and equipment breakdowns.
Unit 1 Lesson 2
1-2-3
Power Train I
Fig. 1.2.4 Bearings on Shafts
Bearings on Shafts
Usually, the bearing supports a moving part. The bearing must allow Usually, 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 • Dec Decrea rease se Fric Frictio tion, n, Heat Heat and Wear Wear • Support Support Static Static Weight Weight of Shafts Shafts and Machinery Machinery • Suppo Support rt Radi Radial al and and Thr Thrust ust Loads Loads • Allow Allow Tigh Tighter ter Fit Tolera Tolerance nces s • Easier Easier to Replace Replace and and Less Less Expens Expensive ive than than Shaft Shafts s
Fig. 1.2.5 Bearing Functions
Bearing Functions
Bearings were invented early early in history. 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 surfaces of wood or leather lubricated with grease such as animal fat. Modern bearings are often designated into into friction and anti-friction anti-friction types. Neither type of bearing is completely frictionless but both are efficient in reducing friction.
Unit 1 Lesson 2
1-2-4
Power Train I
In machinery, the most common methods used to reduce friction, heat and wear are lubrication lubrication and bearings. Oil provides lubrication 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 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 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 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 thrust load. Bearings absorb radial loads and thrust loads to prevent shafts shafts from moving.
Unit 1 Lesson 2
1-2-5
Power Train I
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 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 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 less than direct proportion to the 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.
Unit 1 Lesson 2
1-2-6
Power Train I
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 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. 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. other. The sleeves of most bearings therefore are lined with brass, bronze, 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.
Unit 1 Lesson 2
1-2-7
Power Train I
Fig. 1.2.10 Split-half Bearing
Split-half Bearing
A second type of solid bearing is the split-half bearing (Figure (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 that are bolted to the piston rods. These bearings can be replaced if they wear excessively. 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. metal. The bearing face material is often often an alloy such as aluminum, which is softer than steel and a good conductor of heat. The relative softness 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
Unit 1 Lesson 2
1-2-8
Power Train I
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 rac Inner racee or co cone: ne: 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 ra Outer race: ce: 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 stationary..
Unit 1 Lesson 2
1-2-9
Power Train I
Balls or Roll Balls Rollers: ers: Between the races are the actual friction reducing components. These may be hardened steel steel balls, straight straight or tapered rollers, or thin rollers rollers called needles. The balls or rollers turn 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 more surface contact for for greater support against radial loads.
Fig. 1.2.15 Tapered Tapered Roller Bearings
Tapered Roller 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 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.
Unit 1 Lesson 2
1-2-10
Power Train I
Fig. 1.2.16 Needle Bearings
Needle Bearings
Needle bearings (Figure 1.2.16) work the same way as straight rollers, providing line line contact. Because of the small 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).
Unit 1 Lesson 2
1-2-11
Power Train I
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 earliest times engineers have devised different means to keep lubricant around moving parts and keep out water, water, dust and dirt. 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.
Unit 1 Lesson 2
1-2-12
Power Train I
Fig. 1.2.20 Seal Types
Seal Types
A seal is defined as a piece of material material or a method that prevents or decreases the flow of fluid or air between two surfaces. surfaces. The sealed surfaces may be stationary or have movement between them. Some of the many duties of a seal are to: - Pr Prev event ent lubr lubric icant ant leak leakag agee - Keep out dirt dirt and othe otherr forei foreign gn bodie bodiess - Keep diff different erent fluid fluidss such such as oil and water water apart apart - Remain flexi flexible ble enough enough to to allow allow some some movement movement between parts without leaking - Se Seal al rou rough gh sur surfa face cess - Wear faster faster than than the more more expensive expensive parts parts with which they they are used Seals (Figure 1.2.20) can be classified into into two basic types: types: static seals and dynamic seals. 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
1-2-13
Power Train I
Fig. 1.2.21 Gaskets
Gaskets
Gaskets are one of the most common seals used to seal small clearances between static static machinery parts. 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 the two surfaces. surfaces. The compressed ring ring provides the seal. The ring may be used as a static seal in a manner similar to a gasket.
Unit 1 Lesson 2
1-2-14
Power Train I
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. 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 for dirt, cuts and scratches. Do not twist or stretch the O-ring 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 resistant to movement between the two parts they are sealing. Lip seals are relatively easy to remove for service replacement.
Unit 1 Lesson 2
1-2-15
Power Train I
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 specific applications. Internal lip seals have the seal lip on the inside diameter diameter of the seal. 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 1.2.26 Garter Spring
Garter Gar ter Spri Spring ng
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
1-2-16
Power Train I
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. grooves. Do not use lip seals with a broken lip. Do not use lip seals if the the lip is "turned under". Lip seals must be removed 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 minimum maintenance. They must be resistant resistant to shaft 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
1-2-17
Power Train I
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 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. dry. Before assembly, 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 install the seal with a correct and even application of force. force. Duo-cone seal rings must always be kept in pairs.
Unit 1 Lesson 2
1-2-18
Power Train I
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. tasks. For proper operation, meshing 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 1.2.29 Straig Straight ht Cut or Spur Gear Gearss
Straight Stra ight Cut or Spur Gea Gears rs
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
1-2-19
Power Train I
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 in the form of a helix. Helical gears are suitable for heavy loads because the gear teeth come together at an acute angle angle rather than than at 90° as in spur gearing gearing.. Engagem Engagement ent 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 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
1-2-20
Power Train I
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 side is counterbalanced by the thrust on the other side. side. Usually a small channel is machined between the 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. generators.
Unit 1 Lesson 2
1-2-21
Power Train I
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 driving gear is called 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 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 the gears. The teeth overlap considerably,, so they can carry greater loads. Spiral bevel gears considerably reduce speed and increase force.
Unit 1 Lesson 2
1-2-22
Power Train I
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° 90° an angl gle. e.
Unit 1 Lesson 2
1-2-23
Power Train I
Fig. 1.2.36 Worm Worm Gear Application
Worm Wo rm Gear Applicati Application on
Figure 1.2.36 is an example of a worm gear application.
Fig. 1.2.37 Rack and Pinion Pinion Gear Set
Rack and and Pinion Pinion Gear 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
1-2-24
Power Train I
Fig. 1.2.38 Rack and Pinion Pinion Gear Set
Fig. 1.2.39 Rack and Pinion 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 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. gear. Operation of the planetary gear set is explained in Lesson 3.
