Unit - 2
Machine Element Design
Machine detail or hardware refers to an essential aspect of a gadget.
These factors encompass 3 simple types: structural additives including body members, bearings, axles, splines, fasteners, seals, and lubricants, mechanisms that manipulate motion in numerous approaches including tools trains, belt or chain drives, linkages, cam and follower systems, such as brakes and clutches, and manipulate additives including buttons, switches, indicators, sensors, actuators and pc controllers.
While commonly now no longer taken into consideration to be a gadget detail, the shape, texture and color of covers are an vital a part of a gadget that offer a styling and operational interface among the mechanical additives of a gadget and its users.
Machine factors are simple mechanical components and capabilities used because the constructing blocks of maximum machines.
Most are standardized to not unusual place sizes, however customs also are not unusual place for specialized applications.
Machine factors can be capabilities of a part (including screw threads or vital simple bearings) or they will be discrete components in and of themselves including wheels, axles, pulleys, rolling-detail bearings, or gears. All of the easy machines can be defined as gadget factors, and plenty of gadget factors include principles of 1 or extra easy machines.
For example, a lead screw includes a screw thread that is a willing aircraft wrapped round a cylinder.
Many mechanical design, invention, and engineering obligations contain an expertise of numerous gadget factors and a wise and innovative combining of those factors right into an aspect or meeting that fills a need (serves an application).
Objectives
- Develop a knowledge of the ideas and processes to Describe and quantify the metrics for key system factors together with bearing, springs, gears, and shafts.
- Apply middle engineering ideas with inside the layout of system factors.
- Understand and put in force machine stage evaluation processes to system detail layout.
- Apply fundamental standards of experimentation, measurement, and facts evaluation to system layout.
- Synthesize the above targets in a real-global system layout context.
Measurable Outcomes
- At the give up of the course, a scholar must have the ability to Identify, define, and formulate in quantitative terms, layout issues for system factors.
- Apply middle difficulty engineering knowledge (together with mechanics, materials, structures and dynamics and mathematics) in system detail layout issues.
- Define and follow the metrics for system factors and the metrics that symbolize the general system behavior.
- Implement laptop layout and simulation equipment with inside the layout of system factors.
- Understand the way to combine system factors right into a system with targeted function.
- Define, quantify, and degree the general thing of protection of a system.
General-cause gadget factors
These factors are the simple constructing blocks in lots of forms of machines. Parts which include fasteners (screws, nuts and bolts, rivets, etc.), shafts, chains, belts, bearings and keys are examples of popular-cause gadget factors.
They commonly carry out the identical characteristic in these kind of machines. In maximum cases, popular-cause gadget factors are available in shapes and sizes described via way of means of worldwide standards.
For example, hex bolts may be synthetic as in keeping with 18 exceptional standards, the maximum famous being DIN 931 and DIN 933. In maximum of those standards, they may be to be had in sizes from M3 to M48. This will increase their usability in loads of exceptional machines, as replacements are without problems to be had.
Special-cause gadget factors
These are mechanical factors that locate unique use in gadget layout. Examples of such components are the turbine in a jet engine, blades in a fan, pistons, crankshaft, etc.
The mechanical layout of those components is customised as in keeping with requirements. Let us take into account the instance of deliver engines.
They are available in exceptional designs, with the quantity of cylinders starting from 6 to 14. For every kind of engine, the dimensions of each thing is redesigned.
The exhaust valve, cylinder head, liner, piston, piston rings, connecting rod and crankshaft all are available in exceptional sizes for 2 exceptional forms of engines.
Types of Machine Elements
Both popular and unique cause gadget factors are simple mechanical additives that characteristic collectively to make a gadget paintings. Let’s see the diverse forms of not unusual place gadget factors and their uses.
Bearings
Bearings are one of the maximum not unusual place gadget factors in gadget layout. Their process is to do away with the friction among shifting components. The mechanical layout of rotating machines is incomplete without it.
The number one cause of bearings is to save you direct steel-to-steel touch of the 2 components and permit easy relative movement among them. They are available in diverse shapes and sizes.
The abundance of to be had bearing designs allows designers to pick out the maximum appropriate bearing for exceptional programs, making sure most reliability, efficiency, overall performance and durability.
Bearings locate use in a variety of various motions, which include linear (conveyors), rotational (crankshafts), hinge (doors, windows) and spherical (ball and socket joint).
They transmit radial hundreds, axial hundreds (thrust bearings), or a mixture of each from the rotating detail to the bearing housing. Some programs of bearings are:
Some programs of bearings are:
- Sliding doors/windows/drawers
- Engine crankshaft
- Conveyor pulleys and idlers
- Wind generators
- Motors Shafts
Shafts are commonly designed for the specific utility and CNC machined Shafts are long, cylindrical additives used for the switch of torque and mechanical energy among additives.
Designer’s use them while the gap among force educate additives is just too superb for an immediate connection, or in the event that they perform in exceptional environments.
Similarly, the steam generators powering the shipment oil pumps in oil tankers are remote from the pumps via way of means of a bulkhead to do away with the possibilities of ignition (exceptional environments utility).
Only the shaft passes via the bulkhead from the engine room to the pump room.
A shaft can be strong or whole, relying at the want. Solid ones are greater compact, however their whole opposite numbers have an extra load-wearing capability for the identical weight.
For shafts beneath heavy hundreds all through operation, designers select a whole shaft because it has better rigidity, stiffness and bending moments.
Some programs of shafts are: IC engine crankshafts/camshafts Vehicle axles Clocks and watches
Motors Pumps Keys gadget keys
In gadget layout, keys are small mechanical additives that join shafts to rotating factors. In a few cases, they'll be entirely accountable for the switch of torque among the 2 factors.
Keys are positioned among the shaft and the rotating detail and feature provisions reduce out in each of them to restoration the important thing in place.
The cutout with inside the hub is referred to as the keyway. The backside of the keyway wherein the important thing rests with inside the shaft is referred to as the key seat.
The entire meeting is referred to as a keyed joint. A keyed joint allows no relative rotational movement however might also additionally permit axial movement to a small quantity as keys are inserted with inside the axial direction.
Due to any such characteristic, keys should undergo excessive compressive and shearing stresses. Thus, crushing failure and shearing failure are vital concerns in a key’s mechanical layout.
The diverse forms of keys in gadget layout are available in many well-known shapes. The 5 essential forms of keys are round, saddle, spline, sunk and tangent.
It is available in diverse shapes and sizes which include rectangle, square, parallel sunk, woodruff, gab-head and feather. Some programs of keys are:
- Motors Marine propellers
- Gear drives
- Pulleys Sprockets
- Couplings shaft coupling
Couplings are mechanical additives that join rotating in-line shafts, with the number one cause of transmitting energy in mechanical layout.
The complete meeting rotates on the identical speed. A coupling can be inflexible or bendy, relying at the want.
A bendy coupling can take in any mounting mistakes in addition to any minor misalignments among the shafts that could increase over time.
Unlike clutches, couplings do now no longer have interaction and disengage.
These gadget factors additionally isolate the warmth switch among the 2 results in a few programs. For instance, a motor can warmth up appreciably all through operation.
A coupling prevents this switch of warmth from the motor to the paired gadget. Some couplings paintings like fuses.
If the torque exceeds a positive limit, they spoil and sever the relationship among the using and pushed additives to shield touchy equipment. Such a coupling is referred to as Overload Safety Mechanical Coupling and is generally used for the safety of automobiles and force structures in energy transmissions. Some programs of couplings are:
Generators Motion manipulate in robotics
- Automotive guidance linkages
- Paddle steamers
- Car differentials
- Fasteners
In mechanical engineering programs, exceptional forms of fasteners are used to maintain collectively or greater equipment additives. They create transient joints which may be disassembled while needed. Some machines paintings beneath intense conditions.
The number one cause of fasteners is to shield those machines in opposition to excessive pressures, immoderate forces and vibration. In gadget layout, it's far vital to be as unique as viable approximately the layout or choice of fasteners in programs.
This is to make sure that those gadget factors can manipulate the forces that the product can be subjected to in carrier and the machines can characteristic without failure.
Fasteners are commonly crafted from carbon steel, stainless steel, or alloy steel. Some examples of fasteners are screws, nuts/bolts, cut up pins, rivets and circles. And they may be used everywhere, unbiased of the industry.
Gears
CNC gadget bevel tools Gears are simple gadget factors with toothed wheels to switch energy and rotation among shafts.
They can boom or lower angular speed whilst concurrently lowering or growing torque, following the legal guidelines of strength conservation. In essence, they act as levers in a translating mechanical system.
The enamel on gears mesh with every different and switch energy from the using shaft to the pushed shaft.
Usually, the shafts are parallel, however unique gears are able to moving energy among intersecting in addition to non-parallel, non-intersecting shafts.
The functionality to paintings efficaciously in any orientation approach in addition they are available in loads of shapes.
Most gears are cylindrical in form with enamel alongside the circumference. Others are available in shapes that resemble a shaft (computer virus tools) or a rod (rack and pinion). Yet others have enamel at the face rather than the circumference (face gears).
Some programs of gears are:
- Clocks and watches
- Vehicle gearboxes
- Clocks and watches
- Mixers and blenders
- Washing machines and dryers
Key Takeaways:
- These are mechanical factors that locate unique use in gadget layout. Examples of such components are the turbine in a jet engine, blades in a fan, pistons, crankshaft, etc.
- The number one cause of bearings is to save you direct steel-to-steel touch of the 2 components and permit easy relative movement among them. They are available in diverse shapes and sizes.
- Only the shaft passes via the bulkhead from the engine room to the pump room.
Design of Joints:
Joints:
Joints are brief and everlasting in nature. Permanent joints are in addition categorized in welded joints and Riveted joints.
Advantages of riveted joints over welded joints:
- More dependable in packages subjected to vibrations and effect forces
- Riveted joints may be used for non-ferrous metals like aluminum alloy, copper, brass etc.
- Less harm to the linked components while the joint is dismantled
Properties of Rivets
- Given under are the crucial residences of rivets.
- The rivet have to be sound, loose from cracks, flaws, burrs, seams, pits and different defects
- The head of the rivet have to be concentric with the axis of the shank
- The cease of the rivet have to be rectangular with recognize to the axis
Terminology of Riveted Joints:
Pitch (p):
Distance among the center of 1 rivet to the center of the adjoining rivet with inside the identical row.
Margin (m):
Distance among the threshold of the plate to the centerline of the rivets with inside the nearest row.
Transverse pitch (returned pitch or row pitch):
The distance among consecutive rows of rivets with inside the identical plate is named as transverse pitch.
Diagonal pitch:
The center distance among one rivet and its adjoining rivet positioned with inside the adjoining row is known as diagonal pitch.
Strength of riveted joint:
The energy of the riveted joint is the pressure that the joint can bring without inflicting joint failure.
Rivets, welds and threaded fasteners based on different types of loading
In different words, whilst you operate a fastener to attach additives, you may do away with it, and the separate portions ought to come aside without struggling any harm. The identical wouldn’t be the case with a welded joint, for instance.
The one exception to this rule are rivets, which fall beneath the class of fasteners, however create everlasting joints.
Though maximum fasteners shape a non-everlasting joint, this doesn't imply you handiest use fasteners while you need with a purpose to take something aside, and it doesn’t imply the joint is vulnerable via way of means of any method. Fasteners can reliably keep collectively gadgets that may enjoy an outstanding diploma of stress.
That method they’re an outstanding preference for everlasting and non-everlasting joints alike, supplying versatility for the form of task you need to apply them for. Mechanical fasteners are available in many forms, consisting of screws, nails, nuts, bolts, washers, anchors and rivets.
Each of those exclusive forms of fasteners is a class unto itself, with many kinds to pick from. If you’ve ever been in a fastener aisle at a hardware store, you understand what number of exclusive forms of fasteners are out there. Let’s study every of those not unusual place examples of fasteners and spoil them down a piece further.
1. Screws
For many human beings, after they consider fasteners, screws are the primary issue to return back to mind. Screw fasteners are one of the maximum flexible forms of fasteners out there.
Their threaded shafts provide them long lasting maintaining energy, and not like a bolt, they don’t require something to keep them in area. Typically, you operate a drill to make a pilot hollow in a fabric after which use a screwdriver to power the screw in area. Screws are available in a huge type of kinds and sizes.
Some of the maximum not unusual place forms of screws encompass: Wood screws have coarse threads and a tapered head.
Deck screws are much like wooden screws, however encompass self-tapping designs and corrosion resistance for out of doors applications. Drywall screws function a self-tapping head that may countersink without inflicting harm to drywall. Sheet steel screws have sharper threads for connecting steel to different substances.
Machine screws have a uniform thickness and don’t taper off on the bottom. Masonry screws have a flat tip and commonly function a hex-head design. MDF screws can self-faucet without splitting the cloth.
2. Nails
Nails had been used given that historic times, and they're nevertheless a regular family item. You can without difficulty spot the distinction among a screw and a nail due to the fact a nail doesn’t have threading.
While a nail doesn’t offer the identical maintaining energy a screw does, it has more shear strength, making it the higher preference for a few applications. Many forms of nails take their names from their applications that could make it a snap to locate the proper type of nail for a job.
They commonly have particular traits designed for running with the substances or utility at hand. Some not unusual place styles of nails encompass the following:
Common nails have a thicker shank for more maintaining energy for lots exclusive applications, however the thick head might be seen on maximum projects. Box nails provide less difficult using with a metallic creation and diamond-factor tip. Brad nails can without difficulty mixture into wooden trim.
Finishing nails are for greater distinct paintings and feature a small, flat head designed to mixture in with task finishes, which includes the ones on crown molding.
Drywall nails are designed for clean concealment and decreased slippage. Flooring nails are designed to be used with numerous floors substances. Framing nails can encompass a number of styles, frequently designed for flush set up and clean concealment.
Roofing nails have a huge head and are frequently corrosion-resistant.
3. Bolts, Nuts and Washers
Nuts and bolts are any other not unusual place form of fastener. These portions paintings in tandem to keep additives collectively. You insert the bolt via the 2 components you need to join, then fasten it in area with a nut at the quit. Here are a number of the maximum not unusual place forms of bolts you’ll use:
Carriage bolts have domed or countersunk heads with a rectangular issue beneath the warmth to maintain the bolt from shifting even as tightening the nut. Flange bolts encompass a round flange underneath the top for dispensing hundreds evenly. Plow bolts are for heavy-obligation applications, which includes heavy equipment, with non-sticking out heads.
Hex-head bolts are six-sided with gadget threads. Square-head bolts have rectangular heads, supplying a less difficult grip for wrenches. Allen bolts have a hexagonal socket to be used with Allen wrenches.
4. Anchors
Anchors get their call due to the fact they serve a comparable characteristic to a boat’s anchor, which embeds itself with inside the seabed to maintain a deliver from shifting. Generally, human beings use those fasteners to attach something to a fabric like drywall or concrete. They embed themselves with inside the cloth and keep the item you’re affixing in area. Some not unusual place forms of anchors encompass: Internally threaded anchors
- Externally threaded anchors
- Masonry screw and pin anchors
- Screw anchors
- Hollow wall anchors
- Sleeve anchors
- Drive anchors
5. Rivets
A rivet creates an everlasting joint among gadgets. They include a cylindrical shaft with a head on one quit and a tail at the different. You use a completely unique device to make bigger the tail, so the rivet remains in area.
Rivets are remarkably long lasting, as compared to different forms of fasteners. Some not unusual place forms of rivets encompass:
- Blind rivets
- Semi-tubular rivets
- Solid rivets
- Split rivets
- Drive rivets
Key Takeaways:
- The center distance among one rivet and its adjoining rivet positioned with inside the adjoining row is known as diagonal pitch.
- For many human beings, after they consider fasteners, screws are the primary issue to return back to mind. Screw fasteners are one of the maximum flexible forms of fasteners out there.
- Brad nails can without difficulty mixture into wooden trim.
Boiler has a longitudinal joint in addition to circumferential joint. The longitudinal joint is used to sign up for the ends of the plate to get the desired diameter of a boiler.
For this, a butt joint with cowl plates is used.
Assumptions in Designing Boiler Joints
1. The load performing at the joint is similarly shared via way of means of all of the rivets. I.e. the shell and plate are inflexible and that everyone the deformation of the joint takes location with inside the rivets themselves.
2. The tensile strain is similarly disbursed over the phase of metallic among the rivets.
3. The shearing strain in all of the rivets is uniform.
4. The crushing strain is uniform.
5. There isn't any any bending strain with inside the rivets.
6. The holes into which the rivets are pushed do now no longer weaken the member.
7. The rivet fills the hollow after it's far pushed.
8. The friction among the surfaces of the plate is neglected.
Design of Longitudinal Butt Joint for a Boiler According to Indian Boiler Regulations (I.B.R), the subsequent process need to be followed for the layout of longitudinal butt joint for a boiler.
1. Thickness of boiler shell.
First of all, the thickness of the boiler shell is determined by using the thin cylindrical formula
i.e. t = P D / 2 σt × η l + 1 mm as corrosion allowance
Where,
t = Thickness of the boiler shell,
P= Steam pressure in boiler,
D = Internal diameter of boiler shell,
σt = Permissible tensile stress, and
ηl = Efficiency of the longitudinal joint.
The following points may be noted: (a) The thickness of the boiler shell should not be less than 7 mm.
(b) The efficiency of the joint may be taken from the DDHB table in Rivets chapter for the type of joint.
2. Diameter of rivets.
Diameter of the rivet hole (d)
Unwin's empirical formula, i.e. d =6 √t (when t > 8 mm)
But if the thickness of plate is much less than eight mm, then the diameter of the rivet hollow may be calculated with the aid of using equating the shearing resistance of the rivets to crushing resistance.
In no case, the diameter of rivet hollow have to now no longer be much less than the thickness of the plate, due to the fact there could be threat of punch crushing.
3. Pitch of rivets.
By equating the tearing resistance of the plate to the shearing resistance of the rivets we can find out the Pitch of rivets.
pmax = C × t + 41.28 mm
Where t = Thickness of the shell plate in mm, and C = Constant.
4. Distance between the rows of rivets.
As specified by Indian Boiler Regulations is as follows : (a) For equal number of rivets in more than one row for lap joint or butt joint, the distance between the rows of rivets (pb) should not be less than
0.33 p + 0.67 d, for zig-zig riveting, and 2 d, for chain riveting.
(b) For joints wherein the range of rivets in outer rows is 1/2 of the range of rivets in internal rows and if the internal rows are chain riveted, the gap among the outer rows and the following rows ought to now no longer be much less than
0.33 p + 0.67 or 2 d, whichever is greater.
The distance between the rows in which there are full number of rivets shall not be less than 2d
Cotter joints:
A cotter joint, additionally called a socket and spigot joint, is a technique of briefly becoming a member of coaxial rods.
One rod is outfitted with a spigot, which suits inner a socket on one give up of the alternative rod. Slots with inside the socket and the spigot align in order that a cotter may be inserted to fasten the 2 rods together.
The cotter used inside a cotter joint is commonly an elongated wedge, which may be pushed into the slots. Cotter joints are used to guide axial masses among the 2 rods, tensile or compressive.
Although a cotter joint will face up to rotation of 1 rod relative to the alternative, it need to now no longer be used to sign up for rotating shafts. This is due to the fact the cotter will now no longer be balanced and may go free below the aggregate of vibration and centrifugal pressure.
These additives have been traditionally used to sign up for connecting rods in steam engines and pumps used to empty mines.
Cotter joints have been preferred due to the fact they're especially smooth to layout and manufacture, they produce an inflexible connection and they're now no longer at risk of operating free in reciprocating machinery. Today they nonetheless locate programs in some of regions including anchor bolts. Knuckle joints are regularly used instead.
A cotter joint is fashioned with the aid of using first placing the spigot give up into the socket give up. The slots are then aligned earlier than the cotter is pushed thru the slots in each additives. The slots are positions in order that because the cotter is pushed into position, the spigot is drawn into the socket till a shoulder at the spigot rests securely towards the give up of the socket.
The attitude of the cotter is decided with the aid of using the coefficient of friction and the benefit of disassembly required.
Typically, the taper attitude is much less than the attitude of friction in order that no axial pressure might reason the cotter to push out of the slots. Additional wonderful locking gadgets can be used for cotter joints subjected to vibration.
Application of Cotter Joint
- A cotter joint is used to guide the axial load among coaxial rods that is the tensile and compressive load.
- Although a cotter joint will withstand the rotations of 1 rod relative to the other, it must now no longer be used to sign up for a rotating shaft.
- This joint use among the pistons rod and the tail of the pump rod. Cotter’s joints are used among the slide spindle and the fork of the valve mechanism.
- Cotter and Dowell set up to sign up for components of a flywheel.
- Foundation bolt: especially used for fastening basis and creation heavy machines
- In a vehicle engine, the cotter joint is used to attach the extension of the piston rod with the connecting rod with inside the crosshead.
- The Cottar joint has traditionally been used to attach connecting rods to steam engines and pumps of dumping mines.
- It is utilized in bicycles to attach the paddle to the sprocket wheel. Use a moist air pump to sign up for a tail rod with the piston rod.
- It is used to connect rods of identical diameter subjected to axial forces.
Knuckle joints:
A knuckle joint is a mechanical joint used to attach rods that are below a tensile load, while there's a demand of small quantity of flexibility, or angular second is necessary.
There is usually axial or linear line of motion of load.
The knuckle joint meeting includes the subsequent main components:
- Single eye.
- Double eye or fork
- Knuckle pin.
At one stop of the rod the unmarried eye is fashioned and a double eye is fashioned at the alternative stop of the rod.
Both, unmarried and double eye are linked with the aid of using a pin inserted via the eye. The pin has a head at one stop and at different stop there's a taper pin or cut up pin. For gripping purpose, the ends of the rod are of octagonal forms.
A knuckle joint is a form of pin joint used to sign up for additives which are loaded in anxiety. One factor has a fork with a hollow passing via each sides, forming a double eye, whilst the alternative factor has an unmarried eye.
The unmarried eye is inserted among the double eyes so that everyone 3 holes are concentric and a pin is used to stable the additives. Knuckle joints are regularly used to attach rods subjected to anxiety in systems consisting of roof ties, bridges, and cranes.
The hyperlinks of a series also are taken into consideration a sequence of knuckle joints. Simplicity approach that knuckle joints are effortlessly synthetic whilst being sturdy and inflexible below anxiety loads. They’re additionally effortlessly assembled and disassembled.
The cap potential to permit a few rotation may be high-quality in truss systems as this guarantees that the structural participants continue to be in a natural anxiety load and aren't subjected to bending moments that may boom stress. In this regard, it is able to be a downside that the joint handiest lets in angular movement in an unmarried plane. A knuckle joint continually has a fork quit, a watch quit, and a pin.
Typically, those are all mainly made elements despite the fact that well known rod ends are every now and then used for the attention quit. This has the benefit that a rod quit permits a few extra rotational motion perpendicular to the pin axis, presenting in addition safety towards bending stress. Various forms of pins also can be used.
The maximum not unusual place is a unique pin with a shoulder at one quit and a go hollow at the alternative to simply accept a collar and cut up pint. For big systems, consisting of bridges, bloodless rivets had been traditionally used. The pivot will also be bolted.
A knuckle joint paperwork a form of one diploma of freedom (1-DOF) kinematic pair called a revolute joint, or a pin joint. This approach that it permits handiest relative rotation approximately an unmarried axis.
All translations in addition to rotations approximately some other axis are prevented. Other names for this form of joint are a pivot point, whilst reading movement in dimensions, or a hinge.
Now, while the 2 eyes are pulled apart, the pin holds them together. The stable rod part of the joint in this example is a great deal more potent than the element via which the pin passes.
The modes of failure are:
- Shear failure of pin (unmarried shear).
- Crushing of pin in opposition to rod.
- Tensile failure of flat stop bar.
Application:
- Tie rod joint of roof truss.
- Tension hyperlink in bridge structure.
- Link of curler chain.
- Tie rod joint of jib crane.
- The knuckle joint is likewise utilized in tractor.
- Connecting rods among locomotive wheels.
Key Takeaways:
- In no case, the diameter of rivet hollow have to now no longer be much less than the thickness of the plate, due to the fact there could be threat of punch crushing.
- The distance between the rows in which there are full number of rivets shall not be less than 2d
- The cotter used inside a cotter joint is commonly an elongated wedge, which may be pushed into the slots. Cotter joints are used to guide axial masses among the 2 rods, tensile or compressive.
References:
1. Design of Machine Elements, V.B. Bhandari, Tata McGraw Hill
2. Mechanical Engineering Design, J.E. Shigley, C.R. Mischke, R.G. Budynas and K.J. Nisbett, TMH
3. Machine Design, Pandya and Shah, Charotar Book Stall
4. Fundamentals of Machine Component Design by R.C. Juvinall and K.M. Marshek, John Wiley & Sons.
5. Machine Drawing by N. Sidheswar, McGraw-Hill
