MET -105

POWER TRANSMISSION

Definitions

A rotating machine is one in which the main working components rotate about a fixed center in a regular manner. Most such machines incorporate additional subsidiary mechanisms such as linkages, slides, gears and reciprocating components, and many of the operating principles that apply to the rotating assembly also apply to these other elements.

Although there are many different types of rotating machines, they can all be classified into three basic groups in terms of their function.

Driving machines (engines or prime movers)

This group includes all machines whose purpose is to drive other machines. Examples include :

Electric motors Steam turbines Diesel engines Petrol engines Air motorsThe common characteristic of these machines is that they convert an energy input of varying kinds into a mechanical output in the form of a rotating drive shaft.

Transmission machines

These are machines whose purpose is to transmit mechanical energy from a driving to a driven machine. Examples include : Gearboxes Differentials Variable speed drives Chain Belts

The mechanical energy transmitted often undergoes a speed transformation and these machines often incorporate some means of drive disengagement such as a clutch.Driven machines

These machines cannot operate independently and need to be coupled to a driving machine. Examples include:

Pumps Compressors Fans Generators Blenders Machine toolsThis group is by far the largest and includes a large number of different types of machines. The common characteristic is that the energy input is normally in the form of a rotating drive shaft while output may be in a variety of forms including kinetic or pressure energy of a fluid, electrical energy, kinetic or potential energy of solid materials, etc.

Power TransmissionThere are three major systems in use for transmission of rotary motion between adjacent shafts : belts, chains and gears.

BELT DRIVE

One of the most common elements in power transmission systems, belt drives give dependable and cost effective power transmission with a minimum of maintenance.There are four basic types of belts used in power transmission

1. Flat belts2. V-belts3. Toothed timing belts4. Ribbed

V-BELT TYPES

Most V-belt drives used in industrial applications fall into two categories: heavy duty (industrial) and light duty (fractional horsepower). There are primarily two types of industrial belts: the classic cross sections (A, B, C, and D) and the narrow cross sections (3V, 5V, and 8V) (Molded notch construction belts are usually designated with an X after the section letter. A 3V molded notch belt would be designated 3VX.)Fractional-horsepower belts are used most often on drives transmitting less than 1 horsepower. Fractional-horsepower belts are available in the following sections: 2L, 3L, 4L, and 5L. A V-belt is specified by cross section and length.

Principles of Operation

V-belts are normally used to transfer power between two shafts whose axes are parellel and some distance apart.

Figure 1. Typical V-belt arrangements

The belt is mounted on pulleys that are attached to the driving and driven shafts and the drive relies on friction between the belt and the pulleys for its operation. The belt sits in the groove of the pulley and makes contact with the sides of the groove as shown in Figure2.

Figure2

Classic cross section. Narrow cross section.

Molded notch belt. Joined belt cross section.

Heavy problem solving joined

Light duty belt dimensions.

Classic cross-sectional dimensions. Narrow cross-sectional dimensions.

V-Belt Length

V-belt length can be measured in three ways: outside circumference (OC), datum length (DL), and effective length (EL). The outside circumference is measured by wrapping a tape measure around the outside surface of the belt.

Datum Length. Datum length is a recent designation adopted by all belt manufacturers in order to retain standard belt and sheave designations while more accurately reflecting the changes that have occurred in belt pitch

length and pitch-line location within the belt (pitch length is the length of the neutral axis of the belt).

Effective Length. The effective length is defined as the measured center distance plus the outside circumference of one of the inspection sheaves.

BELT DRIVES

Total measuring force

Schematic of a V-belt measuring fixture.

Misalignment

There are three primary sources of misalignment in belt-drive systems:1. Driver and driven shafts are not parallel (both horizontal and vertical planes).2. Sheaves are not located in line axially with respect to one another on the shafts.3. Sheaves are tilted due to improper mounting (wobble while running).

Sheave groove inspection.

SYNCHRONOUS BELTSSynchronous belts are toothed belts in which power is transmitted through positive engagement between belt teeth and pulley or sprocket grooves rather than by the wedging friction of V-belts. advantages of synchronous belts over other modes of power transmission include a wider load/speed range, lower maintenance, increased wear resistance, and a smaller amount of required take up.

TYPES OF SYNCHRONOUS BELT

Synchronous belt profiles.

Modified Curvilinear Belts

The modified curvilinear belt tooth form is a refinement of the curvilinear system. The belt tooth and sprocket groove forms were optimized for smoother belt tooth entry/exit properties and improved belt tooth support in the sprocket grooves.

Curvilinear Belts

The curvilinear belt tooth form was developed to provide increased load capacity and performance over trapezoidal belts. The curvilinear belt consequently has a higher horsepower capacity than does a comparably sized trapezoidal belt

Timing BeltsTiming belts were the first family of synchronous belts introduced to the market and were designed with trapezoidal teeth. The belt horsepower ratings are relatively low compared to

curvilinear or modified curvilinear belts introduced later, but the synchronization qualities are excellent for accurate positioning or registration sensitive applications

Pitch

The word ‘pitch’ is commonly used in connection with many kinds of machinery and types of mechanical operations and calculations. Its definition, as applied to mechanical power transmission, is simple yet very important: the distance from a point to a corresponding point. In figure 1 are several examples of this measurement of distance from point to corresponding point.

Pitch Circle

Although the “pitch circle” is not visible, its dimension can be stated specifically as the pitch diameter of a gear, sheave, sprocket, etc. These dimensions are a necessary part of all rotary power-transmission calculations. These calculations are based on the concept of disc or cylinders in contact, as illustrated in Figure 2.

The rotation of one disc causes the disc with which it is in contact to rotate. This concept assumes that no slippage occurs between the surfaces of the discs. The surfaces then travel equal distances at equal surface speeds.

. One revolution of a 2-inch circle will result in a ½ revolution of a 4-inch circle

One revolution of a 4-inch circle will result in two revolutions of a 2-inch circle

Shaft speeds are inversely proportional to pitch diameters.

4” PITCH DIA.

2” PITCH DIA.

Using the term “driver” to indicate the driving gear, sprocket, or sheave, and the term “driven” to indicate the gear, sprocket, or sheave that is being driven, the ratio can be stated or expressed in the form of an equation, as the follows :

Driver Rotational Speed Driven Pitch DiameterDriven Rotational Speed Driver Pitch Diameter

The equation can be simplified by substituting letters and numbers for the words. Use the letter (N) to signify speed, the letter (D) to indicate pitch, the number (1) to indicate the “driver”, and the number (2) to indicate the “driven”.

Driver rotational speed = N1

Driven rotational speed = N2

Driver pitch diameter = D1

Driven pitch diameter = D2

The equation then becomes

N 2N1

=D1D2

These equations may be used to find unknown values by simple substitution of known values in the appropriate equation. Following is an example of the use of each of one of these equations.

Example 1

The pitch diameter of the driver unit that is turning at 100 rpm is 2”. What will be the speed of the driven unit of its pitch diameter is 4”?

=

2” PITCH DIA.

4” PITCH DIA.DRIVER

100 RPM

Figure 6.Known values :

N1 = 100D1 = 2D2 = 4

Unknown N2 =

D1 xN1D2

=N2=2x 1004

or2004or 50

Speed of driven unit is 50 rpm.Example 2

The driver unit is turning at a speed of 600 rpm. The driven unit is turning at 2000 rpm and its pitch diameter is 3”. What is the pitch diameter of the driver unit?

Figure 7.

N1 = 600 RPMN2 = 2000 RPMD2 = 3 “

Unknown D1 =

N 1 x D2D1

or D1=2000 x 3600

or6000600

or 10

The pitch diameter of the driver unit is 10”

Speed, Torque and Power

3” PITCH DIA.DRIVER

600 RPM

DRIVEN

2000 RPM

The power generated by driver equipment and transmitted to the driven is used at different rotational speeds.

If the power is assumed to be transmitted without any mechanical losses, so the total value of the power will be the same for both driver and driven.

As P = T.

P = T.

2πN60

Where

P = Mechanical Power = Angular speedT = Torque

Then, between driver and driven shafts and assuming no mechanical lossesP1 = P2 (Power is completely transmitted)

T 1⋅2πN 160

= T 2⋅2 πN260

T1.N1 = T2.N2

Note : The homogeneity of units in the application of these formulae is required.

In S.I. units :

P = watts (w) kW or 1 HP = 746 WattsT = N.m = radians / sec

efficiency () =

output powerinput power

In order to be able to transmit power, the belt must be under tension so that it is forced down into the groove. The depth of the groove is always greater than the thickness of the belt, however, and the belt should never bottom in the groove. The operation of the belt and its ability to transmit power depend on the size of the friction force and the arc of contact of the belt. The greater the arc of contact the more power the belt can transmit. (Figure 3).

Figure 3. Relationship between power and arc of contact

As well as performing its primary function of transmitting power, a V-belt can be used to change the speed of the driver output and hence the torque transmitted to the driven unit. There are three basic alternatives as shown on Figure 4.

Figure 4. Alternative arrangements for V-belt drives

ARC OF CONTACT

LARGER ARC OF CONTACT CAN TRANSMIT MORE POWER

SPEED RATIO

1:1

SPEED INCREASE

SPEED DECREASE

DRIVENDRIVER

The speed ratio between the two pulleys of a belt drive can be calculated from a simple formula.

Driven speed (RPM) =

driver pulley diameter (mm)driven pulley diameter (mm )

x driver speed (RPM )

It is generally accepted that V-belt drives are limited to belt speeds between 300 and 3000 meters per minute (1000-10,000 feet per minute). If required to operate at higher speeds then dynamic balancing of the pulleys becomes increasingly important

TYPES OF BELTS

There are many types of belts, some of them are commonly used and some other are rarely used. In the following the common types of belts:

1 round belts2. flat belts3. single V-belt4. banded V-belt5. linked V-belt6. timing belt7. V-ribbed belts

The common types of belts

BELT TENSION TECHNIQUES

1. Belt tension by using slotted holes of the bolts of the motor base2. Motor with slide rail for the whole base3. Pivoted motor base4. Belt tension by using idler pulley

CHECK OF BELT TENSION

1 By using human sense and experience ( by hand sensitivity )2 by depress the belt and measuring the deflection3 by using the mechanical belt tension tool4 by measuring the belt elongation after applying tension5 by measuring the belt vibration frequency ( advanced method )

Belt Tension Checking Gauge

CHAINS FOR POWERTRANSMISSIONChain drives consist of an endless series of chain links which mesh with toothed wheels, calledsprockets. The sprockets are keyed to the shafts of the driving and driven mechanisms.

A roller chain has two kinds of links—roller links and pin links—alternately assembled throughout the chain length

Dimensions for roller-chain identification

Chordal action is a serious limiting factor in roller-chain performance. It may be described as the vibratory motion caused by the rise and fall of the chain as it goes over a small sprocket

Chordal action.

Figure shows schematically a roller chain entering a sprocket (A); the line of approach is not tangent to the pitch circle. The chain makes contact below the tangency line, is then lifted to the tangent line (B),and then is dropped again (C) as sprocket rotation continues. Because of its fixed-pitch length, the pitch line of the link cuts across the chord between two pitch points on the sprocket and remains in this position relative to the sprocket until the chain disengages.

Principles of Operation

Chains and sprockets fulfil the same basic function as belts and pulleys in transferring power between two parallel shafts. Instead of relying on friction, a chain drive is a positive drive in which the links of the chain engage with specially formed teeth on the sprocket.

Standard roller chain is made up of alternate roller links and pin links.

The pitch of the chain is determined by the length of the side plates, and the bushings and pins are press-fitted into the side plates. The pins of a special joining link may be longer and grooved to take spring clips as shown in Figure 11.

Figure 9. Chain and Sprocket

. Standard roller chain

. Special joining link

Pitch, width and roller diameter are the critical dimensions of roller chain.

Types and Arrangements

Standard roller chain is available in single and multi-strands form, and the number of strands required will depend on then power to be transmitted. Double pitch chains are also available. They are cheaper, and are suitable for light loads and low speeds.

Chain drives are used most commonly as horizontal drives and any slack in the chain resulting from wear, should accumulate on the lower strands as shown in Figure 15.

Vertical drives should be arranged so that accumulated slack falls into the driven sprocket rather than away from it, to prevent misengagement.

Figure 14. Double pitch chain

Figure 15. In a horizontal drive, slack should accumulate on the lower strand

Where chain tensioners are used they should be used on the side of the chain where the slack is expected to accumulate (Figure 17).

RIGHT

WRONG

RIGHT WRONG

Figure shows Accumulated slack in a vertical drive should fall into, rather than away from the driven sprocket

Figure 17. Using a chain tensioner

DRIVE TYPEBelt Drives Chain Drives

1. Belt drives rely on friction between the belt and pulleys for their operation.

2. needs pulley alignment

3. needs belt tension adjustment

4. sensitive to temperature

5. Sensitive to any liquid, oil, dust and environmental conditions.

6. Considerable speeds

7. Slippage always exists so that it is inaccurate in speed transfer.

8. usually smooth running

9. cheap in the cost

1. Chain drive is a positive drive in which the links of the chain engage with specially formed teeth on the sprocket.

2. needs sprockets alignment (more sensitive for misalignment )

3. needs chain tension adjustment

4. needs lubrication

5. Sensitive to dust and environmental conditions

6. Limited in the transferred speed ( up to 1350 meters per minute )

7. Accurate speed transfer.

8. usually noisy running

9. expensive in the cost

GEARS AND GEAR BOXES

Gear Drives

Gear drives are used to transmit power from one machine to another where changes of speed, torque, direction of rotation or shaft orientation are required. They may consist of one or more sets of gears depending on the requirements. In most cases the gears are mounted on shafts supported by an enclosed casing which also contains a lubricant.

Principles of Operation

A gear is a form of wheel with teeth machined around the outer edge which allow it engage with similar wheel or rack. The most important features of a gear are the tooth profile or cross-sectional shape, and the number of teeth. In order to understand the geometry of gears. However, there is a limit to the torque that can be transmitted by friction and so teeth are cut into the outer edges of the discs to provide a means of positive engagement as shown in figure 1.

The imaginary circles on which the gears are cut are called the pitch circles, and the pitch circle diameter is the major dimension on which gear geometry is based. The other important dimension is the pressure angle. This is the angel between a tangent to the pitch circle and the line of contact of two mating teeth as shown in figure 2.

The Teeth provide a means of positive engagement

If two gears are to mesh properly they must have the same pressure angle. Standard pressure angles of 14.5 and 20 are used with 20 being the most common.

Fig.2 The pressure angle

In practice, gears are cut to provide running clearance between mating teeth. This known as backlash.

Figure 3. Terms used in circular gear geometry

PRESSUREANGLE

LINE OF ACTION

PRESSUREANGLE

In practice, gears are cut to provide running clearance between mating teeth. This is known as backlash (figure 4).

Figure 4. Gears are cut to provide backlashThe characteristics of mating gears are often described by the term, diametral pitch. This term refers to the ratio of the number of teeth to the pitch circle diameter of the gear and reflects the size and shape of the teeth. Hence two mating gears must also have the same diametral pitch as well as the same pressure angle.There are several ways in which diametral pitch can be calculated.

Diametral pitch =

πcircular pitch

Diametral pitch =

number of teethpitch circle diameter

Diametral pitch =

number of teeth + 2outside diameter

πD= PZ P : Pitch Circle

Z : Number of teeth

D : Diameter

The speed relationship between two mating gears depends on the number of teeth on each gear and can be determined as follows :

Speed of driven gear = spead of driver x no. of teeth on driver

(RPM) (RPM) no. of teeth on driven

Types and arrangements of Gear

A gear train consists of one or more gear sets intended to give a specific velocity ratio, or change direction of motion. Gear and gear train types can be grouped based on their application and tooth geometry.

Table 1. Gear Types Grouped According to Shaft Arrangement

arallel AxesIntersecting Axes

Non-Intersecting (Non-parallel) Axes

Rotary to Translation

Spur Gears Bevel gears: Hypoid gears Rack and Pinion

Helical Gears Straight bevel Crossed helical gears

Herringbone or double helical gears

Zerol bevel Worm gears

Spiral bevel

Spur gears (Fig. 5): Spur gears connect parallel shafts, have involute teeth that are parallel to the shafts, and can have either internal or external teeth. Notes:

1. Spur gears are inexpensive to manufacture. 2. They cause no axial thrust between gears. 3. They give lower performance, but may be satisfactory in low speed or simple

applications 4. Simple overall design and assembly.

Figure. 5 Spur Gears

Helical gears (Fig. 6): Helical gears also connect parallel shafts, but the involute teeth are cut at an angle (called the helix angle) to the axis of rotation. Note that two mating helical gears must have equal helix angle but opposite hand. These are found in automotive transmissions, and any application requiring high speed rotation and good performance. Notes:

1. Helical gears run smoother and more quietly than spurs (due to continuous tooth mating).

2. They have a higher load capacity (teeth have a greater cross section). 3. They are more expensive to manufacture. 4. Helical gears create axial thrust.

Figure 6. Helical gears

Herringbone gears (Fig. 7): To avoid axial thrust, two helical gears of opposite hand can be mounted side by side, to cancel resulting thrust forces. These are called double helical or herringbone gears

Figure. 7 Herringbone gears

Bevel gears (Fig. 8): Bevel gears connect intersecting axes, and come in several types (listed below). For bevel gears, the pitch surface is a cone, (it was a cylinder in spur and helical gears) and mating spiral gears can be modeled as two cones in rolling contact. Types of bevel gears:

1. Straight bevel: These are like spur gears, the teeth have no helix angle. Straight bevel gears can be a. Miter gears, equal size gears with a 90 degree shaft angle, b. Angular bevel gears, shaft angle other than 90 degrees, or c. Crown gears, one gear is flat, has a pitch angle of 90 degree.

2. Spiral bevel gears(Fig. 8a): Teeth have a spiral angle which gives performance improvements much like helical gears

3. Zerol bevel gears (Fig. 8b): Teeth are crowned, so that tooth contact takes place first at the tooth center. Zerol bevel gears offer performance that is equivalent to that of straight bevel gears and are spiral bevel gears with a spiral angle of 0°. They offer the advantage of low axial thrust over spiral bevel gears.

Bevel gears

Figure 8a. Spiral Bevel gears

Figure 8b. Zerol bevel gears

Hypoid gears (Fig. 9): Similar to spiral bevel gears, but connect non-parallel shafts that do not intersect. The pitch surface of a hypoid gear is a hyperboloid of revolution (rather than a cone, the pitch surface in bevel gears), hence the name. Hypoid pinions (the smaller driving gear) are stronger than spiral bevel pinions because the helix angle of the pinion is larger than that of the gear. Hypoid gears are found in auto differentials. I also know that a hypoid gear set is used in my NH baler, connecting the flywheel to the rear driveshaft.

Figure 9. Hypoid gears

Crossed helical gears (Fig. 10): Helical gears that connect skew shafts. The teeth have sliding motion and therefore lower efficiency. One application is connecting distributer to cam shaft in pre-electronic ignition vehicles.

Figure 10. Crossed helical gears

Worm Gears (Fig. 11): The driving gear is called a worm, and typically has 1, 2, or four teeth. The low number of teeth on the worm can result in a very large velocity ratio. These can also be designed to be non-backdriveable, and can carry high loads. Because of sliding action, efficiency is low.

Figure 11. Worm Gears

Rack and Pinion (Fig. 12): These transmit rotary motion (from the pinion) to translational motion (of the rack). The rack is a gear with infinite radius; its teeth, although flat sided, are involute. The rack and pinion is commonly used in steering units and jacks.

Figure 12. Rack and Pinion

Gear reduction arrangements

Whatever type of gear is employed, the arrangement may involve one or more pairs of gears depending on the degree of speed reduction required

Gears are generally made from steel or cast iron and are surface hardened in order to increase the wear resistance.

Figure 13. Gear Reduction

GEAR DRIVES AND SPEED REDUCERSCommon Gear Types

Common types of gears used in industrial gear drives include spur, helical, double-helical, bevel,spiral bevel, hypoid, zerol, worm, and internal gears

Double helical drives

Epicyclic Gear Drives

In an epicyclic gear drive, power is transmitted between prime mover and driven machinery through multiple paths. The term epicyclic designates a family of designs in which one or more gears move around the circumference of meshing, coaxial gears, which may be fixed or rotating about their own axis. Individual gears within an epicyclic drive may be spur, helical, or double-helical.Because of the multiple power paths, an epicyclic gear drive will normally provide the smallest drive for a given load-carrying capacity. Other advantages include high efficiency, low inertia for a given duty, high stiffness, and a high torque/power capability

The basic elements of an epicyclic drive are a central sunwheel, an internally toothed annulus ring,a planet or star carrier, and planet or star wheels. Depending on which of the first three

elements is fixed, three types of epicyclic drives are possible: a planetary gear drive, a star gear drive, or a solar gear drive. In a planetary gear drive the annulus ring is fixed.

(A) High-speed gearbox, enclosed. (B) High-speed gearbox, showing internal parts.

GEAR-TOOTH WEAR AND FAILUREExperience indicates that the vast majority of gear-tooth wear and failure types may be summed up under nine basic headings in two classifications:Classification A: Surface deterioration1. Wear2. Plastic flow3. Scoring4. Surface fatigue5. Miscellaneous tooth-surface deteriorationsClassification B: Tooth breakage6. Fatigue7. Heavy wear8. Overload9. Cracking

Ridging. Rolling.

Destructive pitting. Spalling.

Severe scoring. Initial pitting.

Rippling Slight scoring.

COUPLINGS

Couplings are the devices used to connect two shafts with a common axis of rotation. Couplings, no matter what type, all have one thing in common: they need proper alignment. Any coupling that isn’t aligned won’t perform properly. No matter how flexible its center member is, it’ll wear out. This is the primary point in maintaining couplings. There are two main types of couplings :

1. Rigid

2. Flexible

Rigid couplings

Rigid couplings usually require no further maintenance than correct alignment. Once they are aligned, corrected sized coupling bolts should be installed and then tightened to the correct torque for that bolt. Since there are no moving parts, no wear should occur. On inspection, the bolts should be checked to insure that they haven’t loosened.

Flexible couplings

Flexible couplings require more maintenance than do rigid couplings. They should be aligned to the same standards, as misalignment causes rapid wear. Flexible couplings can be divided into two classes:

Mechanical - Flexing Material - Flexing

Mechanical flexible couplings depend on some form of a mechanically flexible element. In this class falls the gear, chain, grid, spindle, and universal joint .These all require the use of lubrication to prevent wear. If they aren’t lubricated, they ‘ll wear excessively fast. The lubricant should be clean. If sufficient lubrication is not applied, metal-to-metal contact will occur between contacting metal parts under load and rapid wear will occur.

The following guidelines should be used :

1 Gear – half full of clean lubricant2 Chain – packed with clean oil3 Grid – packed with clean grease4 Spindle – same as gear5 Universal joint – dependant on application

Material flexible couplings use some form of flexible material between the two coupling halves to absorb some limited misalignment but this isn’t a cure-all. The inspector should give attention to the flexible member during his inspection. If the member has been hot or is cracked and showing wear, it should be replaced and the alignment checked. When the alignment is bad, it flexes the material, heats up, and wears more rapidly than it should.

FLEXIBLE COUPLINGSFOR POWER TRANSMISSIONA flexible coupling is a mechanical device used to connect two axially oriented shafts. Its purpose is to transmit torque or rotary motion without slip and at the same time compensate for angular, parallel,and axial misalignment.

CAUSES OF COUPLING FAILURE

Most failures due to internal faults are the result of improper or poor machining. Another major cause of failure due to internal faults is improper product design. On mechanical-flexing couplings, the major problem is to provide adequate lubrication between the sliding contact faces, since lack of a lubricating film between these high pressure surfaces will result in rapid wear. On material-flexing couplings, improper design of the flexing-element section and method of attachment to the hubs are the main causes of premature fatigue.Most common causes of failure due to external conditions have to do with improper selection, improper assembly, and excessive misalignment

Coupling SelectionProper selection as to the type of coupling is the first step of good maintenance. A well-chosen coupling will operate with low cross-loading of the connected shafts, have low power absorption, induce no harmful vibrations or resonances into the system, and have negligible maintenance costs. The primary considerations in selecting the correct type of flexible couplings, as well as its size and style, are

1. Type of driving and driven equipment2. Torsional characteristics3. Minimum and maximum torque4. Normal and maximum rotating speeds5. Shaft sizes6. Span or distance between shaft ends7. Changes in span due to thermal growth, racking of the bases, or axial movement of the connected shafts during operation8. Equipment position (horizontal, inclined, or vertical)9. Ambient conditions (dry, wet, corrossion, dust, or grit)10. Bearing locations11. Cost (initial coupling price, installation, maintenance, and replacement).