Qualitative Of Machine Component Design

8/13/2019 Qualitative Of Machine Component Design

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Complex machines classified as :

MOBILE moves themselves in doing useful work such as transportation.STATIONARY fixed while doing useful work such as production machineryPORTABLE can be carried by the user such as power toolsalso be classified as:Prime Movers that converts chemical energy of the fuel into mechanical energy which isused to propel the entire vehicle.Secondary Movers because it receives energy directly or indirectly from a generator.Power Driven or power absorption unit that utilizes energy to do useful work.

Bucheles Laws of Machines express relationships between design and use ofmachine.Newtons Laws of Motion express relationship between the matter and motion.

1st Laws of Machines states that : Any operation performed by human hands can

be performances by a machine or series of machine

2nd Laws of Machines states that : Any operation performed by a machine or theseries of machine can be done faster and cheaper and with an improvement inquality of product by another machine or series of machines

3rd Laws of Machines states that : Any uncontrolled mechanized, profitableproduced product or service will be in over production.


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Electric Motor as the Power Source

Great flexibility and cleanlinessPower easily be increased or decreased

to meet varying loadreliable, high efficiency, operate quietlyEasy to install, use and maintainHas high starting torque and requires

just a coupling without a clutch and thusgives quick start

Greater power to weight ratio thanengine

I.Combustion Engine as the Power Source

Lower power to weight ratio than electricmotor

Good variable speed-torquecharacteristics

Power flow not smooth and continuousHas low starting torque and requires a

clutch so that it can gain speed and powerbefore engagement

Gives high torsional vibrations that isharmful if transmitted without suitabledamping to the driven machine

Air or Hydraulic Motor as the Power Source

Used where electric motors are impractical for lack of speed variation orbecause of wet or explosive environments.

Has high starting torque and operate with little vibration even at high speed


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Basic Aspects of Design1. Design Concept--Function, safety, reliability and maintainability2. Design of Elements--Size, shape, material, life3. Manufacturability--Machining process to achieve optimal design

4. Cost--Materials, construction, maintenance

4 basic modes of failure1. Lack of strength (rupture and destruction)2. Lack of rigidity (excess elastic deformation)3. Lack of stability (buckling or overturning)

4. Wear (removal of vital surface material)

STRENGTH - Ability to resist loads (forces, bending moments, torques) and isexpressed in terms of ultimate strength, yield strength and fatigue strength.

Maximum induced stress


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STABILITY - Ability to resist displacement or restore its initial condition. Laws ofequilibrium (i.e. M,F,T =0) applies to make sure that the machine system is stable !Maximum load


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Types of Wear1. Adhesive Wear - occurs when two solids are in sliding contact.2. Abrasive Wear - caused by abrasive particles carried by the lubricating oil

between two rubbing surfaces.

3. Corrosive Wear - chemical action that causes destruction of metal surfaces.4. Fatigue Wear occurs when a surface is repeatedly stressed and unstressed (e.g.

pitting of gear tooth).

Methods of Reducing Wear

1. Replacement design for easy and inexpensive replacement of vital machineelements, notably bearings.

2. Dissimilar Materials use dissimilar materials for parts in sliding contact (e.g. steelwith brass).

3. Compensation adjustment and take-up provision for automatic compensation

(e.g. spring loaded tensioner). 4. Metallizing coating of the surface of base metal.5. Hard-facing welding the facing edge subjected to excessive wear.6. Lubricating keeping sliding metals apart through the use of a lubricant.7. Seals and Gaskets retaining lubricants from dust, dirt, and contaminations.


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An axle is use to support the vehicles weight on the ground. Dead axles may rotate with the wheels but do not transmit power. Life axles transmit torque to provide mobility for self-propelled vehicle. A shaft is a device that supports pulleys, sprockets, gears, cranks, levers, wheels,

and other attachments and often transmits power between them. The shaft itself is mounted on bearings so that it can turn freely in the housing. It

may be subjected to reverse flexural loadings or torque loadings or more likelyboth.

A countershaft is an intermediate shaft placed between a driving and a driven shaftto make possible variations in speed ratio.

A flexible shaft permits direct transmission of power between 2 axles at an angle toeach other.

A shaft must be design for both its rigidity and strength. Designers must therefore be able to analyse the bending and torsional deflections

on the shaft. Designers must also be able to calculate the stresses, often caused by a

combination of loads, at various points on the shaft and be able to evaluate thesestresses by means of a suitable theory of failure. Excessive deflection may cause wearing on the available bearings, gear tooth

wear on the available gears on the shaft and shaft vibrations. Keys, pins, snap rings, and clamp collars are used on shafts to secure rotating

elements. Shoulders are made available on shafts to axially positioned gears,pulleys, sprockets and bearings.

AXLES AND SHAFTS


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Shaft materials are generally cold-drawn or machine from hot-rolled, plain-carbon steel to minimized deflection.

Cold drawn raises the value of ultimate and yield strength of steel. Alloy steels are used where toughness, shock resistance and greater strength

are needed. Carburizing grade steels are used if surface wear is more important than

strength. Carburizing only hardening the surface with the inside portion of thematerial is left to its original condition.

8 predominant factors in choosing a shaft and axle material

1. Rigidity or stiffness (ability to resist change of form)2. Strength (ability to resist load)3. Wear resistance4. Corrosion resistance5. Weight & Cost (expense of production)6. Size and availability7. Machinability

Positive drive transmission : Any drive that dependent on the positive contact ofinter-meshing teeth or other parts.

Negative drive transmission : Any drive that dependent on the frictional contact

that exist between the driving and driven surfaces.


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o Gears predominates because of their versatility (permits power transmission over awide speed range), compactness, and high transmission efficiency. But they aregenerally used for short center distances.

o Chains are suitable for high speed and high torque transmission since they have

many teeth engaged on the sprocket that is in contrast to gear where only one or twopairs of teeth are engaged at one time,o Belts are suitable for high speed and low torque transmission. They do not need

lubrication as compared to gears and chains but required more frequent tightening toprevent slippage at high torque.

Hydraulic drives are used in preference to mechanical systems when :1. Power to be transmitted is far apart for chains and belts2. High torque at low speed is required3. Very Compact unit is needed4. Smooth transmission and free vibration is required5. Easy control and direction is necessary6. Output speed must be varied steplessly


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Disadvantages: Need for a separate power unit (pump, sump, drive motor, andcooler), higher initial cost and lower transmission efficiency.

Belt drive transfer power from one shaft to another by using a belt and connecting

pulleys on the shafts.Belt drives are smooth running, quiet, resistant to momentary surges or overloads,clean (requiring no lubrication), and inexpensive to maintain.Their disadvantages, compared with chains and gears, are lower strength anddurability.Belts are used when large distances between shaft makes gears impractical or whenthe designated speed is too high for chain drives 4 main belt types : Flat, V, round and timing.Flat-belt drives produces very little noise and absorb more torsional vibration fromthe system than other belt drives. Satisfactory at high speed and relative low power.Transmission efficiency of around 98%, which is closed to that of a gear drive.V-belt drive can transmit more power than a flat drive however its transmissionefficiency varies between 70 to 96%.Timing belt drive is constructed with ribs and teeth underside with transmissionefficiency ranges from 97 to 99%.


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A chain is a power transmission element made as a series of pin-connected linksthat engages mating toothed wheels known as sprockets.

Power is transmitted solely by the tension side of the chain (i.e F1) while theslack side of the chain (i.e F2) is zero.

Adequate tension is important for chain drive. Chain life will be shorten if chainsare run too tight or too loose.

A chain that is too tight may carry an unnecessary additional load. A chain that is too loose causes surge in the slack strand which will result to wear

and fatigue failure.

Advantages of chains compared with belt drives: 1.Positive drive (no slip) and therefore greater power capacity. 2.Greater transmission efficiency because of no slip. 3.Synchronized motions.

4.Less sensitive to dust and humidity and not adversely affected by sun, oil orgrease.

5.Smaller transverse shaft load since power is transmitted solely by tension side. 6.Requires smaller and less costly bearings and shafts because of smaller

traverse shaft load.

7.Endless, can be set at any required length.


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Disadvantages of chains compared with belt drives :1.Require frequent or in some cases continual lubrication.2.Accept very little misalignment and so should be used only on parallel and horizontalshafts.3.Provide no overload protection because they will not slip (for this reason, chainsprockets are sometimes equipped with shear pins as overload protection).4.Cost more for the same application.

6 main types of chain : Roller chain, Rollerless chain, Silent chain, Detachable linkchain, Printle chain, and Block Chain.The roller chain is the most widely used type for transmitting power.Roller chains have high transmission efficiency because of the ability of the rollers torotate when contacting sprocket teeth.Even number of links could be connected together the available pink link.Odd number of links could be connected together with the use of an offset link. Offsetlinks wear faster than straight pin and should be avoided


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CHORDAL ACTIONChordal action is the rise and fall of each link as it engages a sprocket that causesrepeated chain & speed variations.Chordal action and speed variation decreases as the number of teeth in the inputsprocket is increased and in most application become negligible with 21 teeth or moreteeth in the sprocket.The line of approach is not tangent to the pitch circle of the sprocket. The line makescontact below the tangent line and is then lifted up to the top of the sprocket at adistance of (R - r).

LUBRICATION OF CHAINProper lubrication is needed to1.Reduce wear2.Protect against rust, corrosion, and heat3.Prevent seizing of pins and bushings

4.Cushions shock loadMethods of lubrication1.Drip feed lubrication2.Shallow bath lubrication3.Disc or slinger lubrication

4.Oil steam lubrication


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Gear are toothed, cylindrical wheels used for transmitting motion and power from onerotating shaft to another.The teeth of a driving gear mesh accurately in the spaces between teeth on a drivengear.The driving teeth push on the driven teeth, exerting a force perpendicular to the radiusof the gear.Thus torque is transmitted and because the gear is rotating, power is also transmitted.Gears are the most rugged and durable means of power transmission. They havetransmission efficiency as high as 98% and are generally the most costly means ofpower transmission.

4 main types of gears : spur, helical, bevel, and worm gears.Spur gears have teeth that are straight and arranged parallel to the axis the shaft thatcarries the gear. The faces of the gear teeth have involute curve geometry that makes itpossible for two gears to operate together smooth with positive power transmission. Theshafts carrying the gears are parallel.Helical gears have teeth that are arranged at an angle known as helix angle withrespect to the axis of the shaft. parallel to the axis of rotation. Typical helix angles are10 to 30. The helical gear operate more smoothly with lower stresses than equivalentspur gear and can be made smaller for a given power transmission capacity. Onedisadvantage is that an axial force known as a thrust force is generated in addition to the

driving force that acts tangent to the teeth,


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This thrust force need to be considered when selecting bearings that will hold tosupport the shaft. The shafts carrying the gears are parallel. However crosshelical gears with 45 helix angles have their shafts operates at 90 to each other.

Bevel gears have teeth arranged on conical surface being wider at the bottomand narrower at the top of the cone. The shafts carrying the gears are

perpendicular to each another. The teeth can straight gear (similar to spur gear )or spiral gear (similar to helical gear). The spiral bevel gear operate moresmoothly than straight bevel gear and can be made smaller for a given powertransmission efficiency.

Worm gears have teeth similar to screw thread and can either be straight likespur gear teeth or spiral like helical gear. They are used to accomplish a ratherlarge speed reduction ratio but have somewhat lower transmission efficiencybecause of extensive rubbing contact between the gears. The shafts carrying thegear are perpendicular to each other.

INVOLUTE CURVE

The teeth of spur gears are generated on the basis of involute curve so that theteeth comes in good tooth with the two meshed gears. When two such gear teethare in mesh and rotating, there is a constant angular velocity ratio between them.The resultant action of the two gears is very smooth. If not, there would be somespeeding up and slowing down during engagement, with the resultingaccelerations causing vibration, noise, and dangerous torsional oscillations.


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Parallel helical gear set is used for transmitting power between two parallel shafts. Crossed helical gear set is used for transmitting power between two nonparallel and

nonintersecting shafts. Herringbone gear refers to a helical gear having half its face cut with teeth of one

hand and the other half with the teeth of opposite hand. Crossed helical gears have a contact point rather the line of contact as in most

regular gear sets. Parallel helical gear is normally preferred because it has a much higher power

transmitting capacity than the crossed helical gear of similar size.

Advantages of helical gears over spur gears : Having more teeth in contact with gradual and uniform load being transferred (i.e. not

sudden) as successive teeth come in engagement. Thus make the gear to run moresmoothly and more quietly. The lower average load per tooth allows a greater powertransmission capacity for a given size of gear, or a smaller gear can be designed tocarry the same power.

Disadvantages of helical gears over spur gears : Greater cost and has an axial thrust load as the result of the inclined arrangement of

the teeth. The bearings that hold the shaft carrying the helical gear must be capableof reacting against thrust load.


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Straight bevel gears are the most economical among the various available types ofbevel gears.They are used primarily for relatively low-speed applications with pitch line velocities upto 1000 ft./min, where smoothness and quietness are not important.Bevel gears are non-interchangeable and are therefore made and replaced as matchedpinion-gear sets.The size and shape of the teeth are defined at the large end, on the back cones.The pitch angles (also called pitch cone angles) are pitch cones joining at the apex.Spring is a mechanical element that exhibit elastic deformation when loaded, andrecover its initial configuration when the load is removed.

It is a resilient device specially configured to exert desired forces or torques, to provideflexibility, or to store potential energy of strain for release at a later time.

Functions :1. Control of forces on shock loading or vibrations as in vehicle suspension

2. Restraint of motion as in valves of internal combustion engine3. Exertion of forces as in brakes and clutches4. Storage of mechanical energy as in timing devices5. Measurement of forces as in scales6. Compensation for heat expansion of machine parts


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BUCKLING AND SURGING


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BUCKLING AND SURGINGBuckling or elastic instability may occur for long and skinny helical compression spring that issubjected to axial external compressive loads.Surging or erratic operation may occur if the axial operating frequency on the helicalcompression spring approach the axial natural frequency of the helical compression spring.

The following graph is to be used to check if the designed helical compression spring is safe fromthe failure due to buckling.The coordinate for slenderness and critical deflection ratio should falls on the safe (stable) areaof the respective graphs.

Screws are used both in hold things together as fasteners and to move loads as so called power

screws. A bolt is a threaded fastener designed to be inserted through hole in the mating members and tobe secured by tightening a nut from the end opposite end the head of the bold. A screw is a threaded fastener designed to be inserted through a hole in one member to be

joined and into a threaded hole in the mating member. A stud refers to a headless fastener, threaded on both ends and screwed into the holes in the

one of the members being connected.Flat or plain washer is used to increase the area of contact between the bolt head or nut andclamp partSplit lock washer act as a spring under the nut and helps to prevent spontaneous loosening ofnut due to vibration.Power screws are designed to convert rotary motion to linear motion and to exert the necessary

force to move a machine element along a desired path.

POWER SCREW


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POWER SCREW A power screw or sometimes called linear actuator or translation screw, is a threadedshaft with an attached thrust collar at one end and the other end that engaged to amating nut. With suitable constraint, either the nut may be rotated to cause axialtranslation of the threaded shaft (screw) or the screw may be rotated to cause axial

translation of the nut.Common examples include jack screws, C-clamps, vises, lead screws for machine tools,and positioners for control rod drives.Various thread forms are available to maximize axial load-carrying capacity and tominimize frictional drag. The most common thread forms are the square thread, themodified square thread, the ACME thread, and the buttress thread.

Square thread provides the best strength and efficiency and also eliminates any radialcomponent forces between the screw and nut. However, it is more difficult to cutbecause of its perpendicular face.Modified square thread (include angle, 2 = 10 and thread angle =5) improves themanufacturing ability of it equivalent square thread.

ACME thread (include angle, 2 = 29 and thread angle = 14.5) is easy tomanufacture and allows the use of a split nut that can be squeezed radially to adjust forwear.BUTTRESS thread provides greater strength for unidirectional loads.

A b ll i d i i i d h d f i i d d h lidi f i i b


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A ball screw is used to minimized thread-friction drag and the sliding friction betweenscrew and nut threads.

A ball screw can support greater load than that of ordinary power screws of identicaldiameter. The smaller size and lighter weight are the advantages.

Cleanliness and thin film of lubricant are important to ball screw operations.Mounting parts are used on shafts to attach the hub of gears, pulleys, sprockets, andflywheels.

Some of the common mounting parts include setscrews, keys, pins, and splines.Each has its own advantages and disadvantages.

Couplings are used to connect two shafts. They can be grouped as rigid or flexible

couplings. A rigid coupling locks the two shafts together giving no provision formisalignments between shafts nor does reducing shock or vibration between the twoshafts.

In the design of any mounting parts or couplings, rated shaft torque must betransmitted without slip and premature failure must no be induced in any part of theoperating machine.

A - Setscrews Setscrews is used on shaft-to-hub connections that are for light service only.

Generally, a hole is drilled radially through the hub and sometimes a short distanceinto the shaft.

The hole in the hub section is threaded, and a setscrew is inserted and turned until it

is firmly seated on the shaft.


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Setscrews are available in various head and point configurations. Headless type ismore popular because it can be flush mounted with the hub surface and gives no workhazard when the shaft is rotating.

B- KeysMost common used key types are square and rectangular sections. The grooves madein the shaft-hub connections in which the keys fit are known as the keyways or keyseats. The advantages of keys are simplicity, low cost, and ease of assembly. Thedisadvantage lies in the weakening of the shaft due to the presence of keyway.Induced T by the shaft lead to a shaft force F acting on one half side of key and a hub

reaction F on the other half side of the key.

This induced force on the key and keyways leads to 4 possible modes of failure :1.the key may fail in direct shear2.the side of the key may be crushed

3.the side of the keyway in the shaft may be crushed4.the side of the keyway in the hub may be crushed

Modes (3) and (4) cannot be tolerated and are avoided by using higher strengthmaterials in both shaft and hubs.

Therefore, designing of key is based on Modes (1) and (2).

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Qualitative Of Machine Component Design