Types of kinematic pairs

Types of kinematic pair :

Kinematic pair can be classified according to :

• The nature of the contact.
• The nature of mechanical constraint.
• The nature of relative motion.

Kinematics pairs according to nature of contact :

Lower pair: A pair of links having surface area contact between the member is known as a lower pair.

In the lower pair, the contact surfaces of the two links are similar.

For example : 

• Nut turning on a screw shaft.
• Shaft rotating in a bearing.
• All pairs of slider-crank mechanism.
• Universal joint.

Higher pair: A pair has a point or line contact between the links is known as a higher pair.

In the higher pair, the contact surfaces of the two links are dissimilar.

For example : 

• Rotating on a surface 
• Cam and follower pair 
• Tooth gear 
• Ball-bearing  
• Roller bearings 

Kinematic pairs according to the nature of mechanical constraint :

Closed pair: When the elements of a pair are held together mechanically it is known as a closed pair.
The two elements are geometrically identical one is solid and full and other is hollow or open. The letter not only envelops the former but also and closed it. 
All the lower pairs and some of the higher pairs are closed pair.
A screw pair belong to the closed pair category also. 
  

Unclosed pair: When two links of a pair are in contact but due to some spring action or force of gravity, they constitute an unclosed pair.
In this, the links are not held together mechanically. 

Kinematics pair according to nature of relative motion :

Sliding pair: If two links have a sliding motion related to each other, they form the sliding pair. 

For example - A rectangular rod in a rectangular hole in a prism

Turning pair: When one link has a turning on revolving motion relative to others, they constituted a turning pair.

They also called revolving pair.

For example - In a slider-crank mechanism all pairs except the slider and guide pair are turning pair.

A circular shaft revolving inside a bearing.

Rolling pair: When the link of a pair has a rolling motion relative to each other, they form a rolling pair.

For example - Rolling wheel on a flat surface

Ball and roller bearing in a ball bearing 

The ball and shaft constitutive and rolling pair where is the ball and the bearing is a second rolling pair.

Screw pair : If two mating links have a turning as well as sliding motion between them, they form of screw pair. This can be achieved by cutting matching threads on the two links.

For example - The lead screw and the nut of a late is a screw pair.

Spherical pair : When one link in the form of sphere turn inside a fixed link, it is a spherical pair.

For example - The ball and socket joint

Types of constrained motion

Types of constrained motion :

• Completely constrained motion 

When the motion between two elements of a pair is indefinite direction irrespective of the direction of the force applied it is known as completely constrained motion.

The constrained motion may be linear or rotary.

Example of completely constrained motion :

• The sliding pair 
• The turning pair 

In sliding pair, the inner prism can only slide inside the hollow prism.

In case of the turning pair, the inner shaft can have only rotary motion due to the collar at the ends. 
In each case of force has to be applied in a particular direction force required motion.

• Incompletely constrained motion 

When the motion between two elements of a pair is possible in more than one direction and depend upon the direction of the force applied that motion is known as incompletely constrained motion. 

Example of incompletely constrained motion :

• The inner shaft may have sliding or rotary motion depending upon the direction of the force applied if the turning pair does not have a collar.

Each motion is independent of the other.

• Successfully constrained motion 

When the motion between two elements of a pair is possible in more than one direction but with the using of some external means motion is made to have only in one direction so that motion is known as successfully constrained motion.

Example of a successfully constrained motion :

• A shaft in a footstep bearing may have vertical motion apart from rotary motion but due to the load applied on the shaft, it is a constraint to move in that direction and thus is successfully constrained motion.
•  A Piston in a cylinder of an internal combustion engine is made to have only reciprocating motion and no rotary motion due to constraining of the piston pin. 
• The value of an IC engine is kept on the seat by the force of a spring.

Types of joints in kinematics

Types of joints :

• Binary joint 
• Ternary joint 
• Quaternary joint 

Binary joint :

If two links are joint at the same connection it is known as a binary joint.

Ternary joint :

If three links are joint at a connection it is known as a ternary joint. 
It is considered equivalent to the two binary joints since fixing of anyone link constitute two binary joints with each of the other two links.

Quaternary joint :

If four links are joint at a connection it is known as a quaternary joint.
It is considered equivalent to three binary joints since fixing of anyone link constitutes three binary joints.

If n number of links are connected at a joint, it is equivalent to ( n -1 ) binary joints. 

What is kinematic link

What is the kinematic link?

A mechanism is made of a number of resistant bodies out some may have motions relative to each other. A resistant body or a group of resistant bodies with rigid connections preventing their relative movement is known as a link.
This link is also known as kinematic link or element.

The link can be classified into binary, ternary and quaternary depending upon the end on which revolute or turning pairs can be placed.

A link may also be defined as a member or a combination of members of a mechanism, connecting other member and having motion relative to them. Thus, a link may consist of one or more resistant bodies. 

For example, a slider-crank mechanism consists of four links: frame and guides, crank, connecting rod and slider. however, the frame may consist of bearings of for the crankshaft. The crank link may have a crankshaft and flywheel also, forming one link having no relative motion of this.

Mechanism and machine

In this article, we will discuss the mechanism and machine.

What is the mechanism :

When one of the links of a kinematic link is fixed, then the chain is known as a mechanism.

A mishmash of a number of bodies assembles in such a way that it causes the constrained and predictable motion to the other is known as a mechanism.
Thus, the function of the mechanism is to transmit and modify a motion.

What is a machine :

Machine is a mechanism which receives energy and transforms it into some useful work.

A collection of mechanisms which, transmits and modifies available mechanical energy into some kind of desired work and also imparting definite motions to the parts is called machine.

It is neither a source of energy nor a producer of work that helps in proper utilization of the same. The motive power has to be derived from external sources.

Example :

A slider-crank mechanism converts the reciprocating motion of a slider into rotary motion of the crank or vice-versa. however, when it is used as an automobile engine by adding valve mechanism it is it becomes a machine which converts the available energy into the desired energy.  

Some other examples of mechanisms are typewriter, clocks, watches, spring toys etc. 
In each of these force or energy provided is not more than what is required to overcome the friction of the path and which is utilized just to get the desired motion of the mechanism not to obtain any useful work.

Some useful notes :

A machine is tangible, but the mechanism is not. Only the effect of the mechanism is observable.

In a simple way, a mechanism is a system that is followed by a machine to achieve the particular function while a machine is a combination of tools and parts that performs specific functions at the expense of energy.

For Example :

We think a car is a machine. It made of various mechanisms such as the wipers, piston and crankshaft, differential.
All this parts are simple a mechanism following below : 

• Wipers is a 4 link crank lever mechanism
• Piston and crankshaft is a slider crank mechanism
• Differential is a gear mechanism. 

Thus a machine is made of number of mechanism to carryout a particular task.

Mechanism and structure

Mechanism :

Linkage is obtained if one of the links of kinematic chain is fixed to the ground. If the motion of any of these movable ink results in definite motion of the other, the linkage is known as the mechanism.

However, this distance between a mechanism and linkage is hardly followed and it can be referred in place of others.

In a mechanism, links are connected with temporary fasteners. The links can move relative to each other.

This is facilitated by the presence of joints between the links. So a degree of freedom is >=1

Structure :

If one of the links of the redundant chain is fix it is known as a structure.
It is also known as a locked system.
To obtain constrain or definite motion of some of the links of linkage, it is necessary to know how many inputs are needed. In some mechanism, only one input is necessary that determine the motion of other links and it is said to have one degree of freedom. While other mechanisms, two inputs necessary to determine to constrain motion of the other links so we said they have two degrees of freedom. 

In a structure again a combination of links connected to each other but no relative motion exists between them. So a degree of freedom is zero.

A structure who have a negative degree of freedom is known as a superstructure.

A mechanism is a force manipulating device, while the structure is a load bearing device.

Degree of freedom in kinematics

Degree of freedom :

Degree of freedom is related to motion possibilities of rigid bodies.

An unconstrained rigid body moving in space can describe the following independent motions :

• Translational motion along any three manual perpendicular axis x y and Z.
• Rotational motion about this axis.

Thus, rigid body processes 6 degrees of freedom.

The concept of degree of freedom in the kinematics of machines is used in three ways :

• A body is relative to a reference frame.
• Kinematic joints
• A mechanism.

The connection of a link with another imposes certain constraints on their relative motion.
The number of restraint can never be zero or six.

Degree of freedom of a pair is defined as the number of independent relative motions, both translational and rotational a pair can have. 

Degree of freedom = 6 -  Number of restraints 

The general equation to find out degrees of freedom of a planar mechanism is given below. This equation is also known as Kuthbach equation.

D.O.F = 3 ( N-1 ) - 2L_p - H_p

Here N = Total number of links in the mechanism. 

L_P and H_P = Number of lower pairs and higher pairs respectively.

If the mechanism is 3 dimensional in nature, you could easily derive an equation for mobility using the same concept. So the equation for the degree of freedom would be following below :

D.O.F = 6 ( N-1 ) - 5P₅ - 4P₄ - 3P₃ - 2P₂ - 1P₁

Where P_n = number of pairs which block 'n' degrees of freedom. 

The main thing here will be the determination of nature of the kinematic pair. 

TCI Vs CDI

TCI and CDI both are improved technology of ignition system used by a different automobile company.  First of all, you should know the full form of these two terms. 

Full form of TCI - Transistorised Coil Ignition

Full form of CDI - Capacitor Discharge Ignition

The main difference between them is capacitor discharge ignition is when a capacitor stored energy is used for ignition while transistor controlled ignition is when the ignition coil current is driven by a transistor. 

Let us have a deep insight into the comparison between transistorised coil and capacitor discharge ignition. 

Difference between TCI and CDI :

• CDI ignition system is independent on time while in TCI ignition system is dependent on time.
• CDI Ignition makes the spark by discharging a capacitor loaded with high voltage about 200 to 450 volt from the ignition coil by using an SCR known Thyristor while TCI Ignition charges the coil with the current before the spark is done. Spark is done when the current is cut suddenly.
• CDI coils have low impedance and inductance is about XL< 1 ohm and can reach high RPM makes high power and short sparks while TCI coils have high impedance is about XL >1 ohm and can reach lower RPM thus spark duration may be longer.

Full form of TCI

What is the full form of TCI?

Answer :

• Transistorized Coil Ignition system

What does TCI mean?

TCI is one type of ignition system that reduces the use of mechanical devices, and its purpose is to improve the efficiency of the ignition system. 

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Submerged arc welding

Introduction :

Submerged are welding is a common arc welding process in which the formation of an arc between a continuously fed electrode and the workpiece. It is mainly used for doing faster welding joints. The arc is produced while the consumable electrode wire is continuously fed into the weld zone as in gas metal arc welding. The welding zone is completely covered by means of a large amount of flux. The arc occurring between the electrode and the workpiece is completely submerged under the flux and is not visible from outside. A part of the flux melts and forms the slag, which covers the weld metal. The unused flux is collected and reused.

How it works?

This welding process may be automatic or semi-automatic. The main principle of this process is the flux starts depositing on the joint to be welded. The power source used with submerged arc welding can either be AC or DC. Both constant-voltage and constant current type machines can e effectively used. The arc may be stuck either by touching the electrode with the workpiece or by placing steel wool between electrode and job before switching on the welding current. The arc is maintained between the end of the bare wire electrode and the weld. The electrode is constantly fed into the arc as it is melted. In all cases, the arc is stuck under the cover of flux. Flux otherwise is an insulator but once it melts due to the heat of arc, it becomes highly conductive and hence the current flow is maintained between the electrode and workpiece through the molten flux. The upper portion of flux is in contact with the atmosphere which is visible and remains unused. The lower portion of flux becomes slag, which is waste material and it is removed after the welding process is done.

The electrode at a constant predetermined speed continuously fed to the joint to be welded. The arc length is also kept constant by using the principle of self-adjusting arc because if the arc length decreases the voltage will increases. Therefore burn off rate will increases. 

In this process also some backing plate of the material like steel or copper may be used for control penetration and to support a large amount of molten metal associated with this process. 

SAW process mainly depending upon the following factor :

• Arc voltage or Arc length 
• Electrode travel speed 
• Electrode stick out or Contact tip to work 
• Current type AC or DC 
• Wire feed speed 

Advantages of submerged arc welding :

• High deposition rate.
• High operating factors in some applications.
• Deep weld penetration so welded joints are strong.
• It prevents hot materials from splattering and splashing onto workers because arc is always covered under the blanket of flux.
• High speed welding for thin plates.
• SAW is suitable for indoor and also outdoor works.
• Single pass welds can be made in thick plates. 
• Most of the flux are reused in this process. 
• Very neat appearance and smooth weld shapes can be got.
• Good ductility and corrosion resistance and good impact strength are formed in joints.
• In this process we did not required to add pressure because it already generated by electrode.

Limitations of submerged arc welding :

• This process is limited to steel, stainless steels and nickel.
• Limited to the 1F, 1G and 2F positions. 
• Limited to high thickness metal plates.
• Flux is subjected to contamination that may caused porosity in welded joints.
• The flux needs replacing on the joint which is not always possible.
• Requires backing strips for proper penetration.
• Flux and slag residue can present a health and safety concern.

Electroslag welding

Introduction :

The electro slag welding process is developed mainly to weld very large metal plates without any edge preparation. This process is most of the cases single-pass process using a consumable electrode for filling the gap between the two heavy plates. The heat required for melting the plates and the electrode is obtained initially by means of an arc so that flux will form the molten slag. Once the molten slag is formed, the arc is extinguished and the heat of welding is obtained by the resistance heating of the slag itself. 

How it works?

First of all, in this process, the gap between the two workpieces is filled with a welding flux. It required to maintain a satisfactory amount of slag is fairly small in the order of 0.2 to 0.3 kg per meter of weld length, irrespective of plate thickness. The welding is initiated by an arc between the electrode and workpiece thus, heat utilized for melting the slag is much less. Most of the heat supplied in electroslag welding it melts the fluxing powder and forms the molten slag. The slag, having low electrical conductivity and is maintained in a liquid state due to the heat produced by the electric current.

By this process, a plate of 200 mm thickness can easily be welded in a single pass. The slag reaches a temperature of about the 1930 ⁰C. This temperature is sufficient for melting the consumable electrode and workpiece edges. Metal droplets fall to the weld pool and join the workpiece.

For effective welding, it is necessary to maintain a continuous slag pool and therefore the best way to maintain it. The slag pool is contained in the groove with the help of two water-cooled copper dam plates which move along with the weld.

Electroslag welding is mainly used for welding the steels.

The quality of weld in this process mainly depends upon :

• Slag depth.
• The ratio of a width of the weld pool and its maximum depth is known as the form factor.
• Weld current and voltage.
• A number of electrodes used.

Advantages of electro slag welding :

• Most of the cases welding are done easily by a single pass.
• If any gas is present easily bubbles out through the slag and therefore, a better weld can be done.
• The heating and cooling of the edge are more gradual.
• Whatever be the thickness of the plate,  no edge preparation is required.
• It is also useful for very thick plates. 
• High deposition rate - up to 45 lbs/h (20 kg/h).
• Low slag consumption and low distortion.

Disadvantages of electro slag welding :

• The coarse grain structure of the weld.
• An only vertical position of the weld is possible.
• Low toughness of the weld. 

Application of electro slag welding :

• Fabrication of high-pressure vessels.
• Frames of heavy mechanical and hydraulic presses. 
• In Rolling mill frames. 
• Ship hulls and locomotive frames.

Resistance welding

Introduction :

All welding process are fusion welding process. The resistance welding process is also a fusion welding process where both heat and pressure are applied on the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name is resistance welding.

How it works?

In this process, a low voltage and very high current are passed through the joint for a very short time. This high heats the joint, due to the contact resistance at the joint and melts it. The pressure on the joint continuously maintained and the metal fuses together under this pressure. The heat generated in resistance welding can be expressed as 

H = k I² R t

Where 

H = The total heat generated in the work ( J )

I = electric current ( A )

t = time for which the electric current is passing through the joint ( s )

R = Resistance of the joint ( ohms ) 

k = A constant to account for the heat losses from the welded joint 

In this process, the amount of heat released is directly proportional to the resistance. It is likely to be released at all of the mentioned points below :

• The resistance of the electrodes
• Contact resistance between the electrode and workpiece
• Contact resistance between the two workpiece plates
• The resistance of the workpiece plates

All of above a large amount of heat is to be generated to have an effective fusion is at the interface between the two workpiece plates. 

Because of the squaring in the above equation, the current I needs to be precisely controlled for any proper joint.

At first, apply force and current through the electrodes contacted with metal parts to be welded and resistance heat is generated at the interface of metal parts and hence the metal parts melt and form the joint. Through a large current flows, there is no danger of an electric shock because the only low voltage is impressed.

Electrodes for resistance welding :

The electrodes in resistance welding carry the very high current required for fusion, as also transmit the mechanical force to keep the plates under pressure and in alignment during fusion. They also help to remove the heat from the weld zone thus preventing overheating and surface fusion of the work. For both of the purpose in this process, the electrode should have higher electrical conductivity as well as high hardness. 
Copper is alloyed form is generally used for making electrodes. Though pure copper has high electrical and thermal conductivity, it is poor in mechanical properties.

Copper-cadmium alloys have the highest electrical conductivity with moderate strengths and therefore are used for welding non-ferrous materials such as aluminium and magnesium alloys.

Copper-chromium alloys have slightly lower electrical conductivities than above but better mechanical strength. These are used for resistance welding of low-strength steels such as mild steels and low alloy steels. 
When cobalt and beryllium are added to copper, its conductivity is decreased to a great extent but the strength is increased. Hence, these are used for welding higher heat-resisting alloys such as stainless steels and steels with tungsten and other such alloying elements.  

Advantages of resistance welding :

• High-speed welding.
• Economical welding process.
• Easily automated.
• Easy operation so that suitable for high rate of production.
• No flux require such as solder is necessary.
• Electric facility is required in some cases due to use of large current.
• Possible to weld dissimilar metals as well as metal plates of different thickness.
• Very little skill is required to operate resistance welding machine.

Disadvantages of resistance welding :

• Visual inspection is difficult because welded portion can't be checked from the outside.
• Initial equipment cost is high.
• Lower tensile and fatigue strength of welded joint that formed by using this process.
• This welding process are limited only to lap joints.

The various resistance welding processes of interest are mentioned below :

• Resistance spot welding 
• Resistance seam welding 
• Projection welding 
• Upset welding 
• Flash welding

Thermit welding

Introduction :

Thermit welding is a process, which was traditionally used for the welding of very thick plates. Though this was used for welding large sections such as locomotive rails, ship hulls and broken large castings, its use has decreased nowadays because of the availability of other simpler methods such as submerged arc welding. 

A thermit mixture that uses for welding steels is aluminium and iron oxide. One question arises in your mind that what is thermit mixture?
The heat source utilized for fusion in this welding process is the exothermic reaction of the thermit mixtures. The thermit reaction starts when the mixed thermit powder is brought to its ignition temperature of 1200 ⁰C. 

How it works?

The thermit welding in which the molten metal obtained by the thermit reaction is poured into the refractory cavity made around the joint. It is a similar process as casting. The two pieces to be joined are properly cleaned and the edge is prepared. Then wax is poured into the joint so that a pattern is formed where the weld is to be obtained. Around the joint moulding, a flask is kept and sand is rammed carefully around the wax patterns as shown in below figure providing necessary pouring basin, risers and sprue. For the run off the molten wax, the bottom opening is also provided. The wax is melted through this opening which is also used to preheat the joint and make it ready for welding.

The thermit mixture which is mixed with fluxes is filled into a ladle through the bottom opening. The opening is initially closed. The igniting mixture which is normally barium peroxide or magnesium is placed at the top of the thermit mixture. The igniting mixture is lighted by means of a heated metal rod, whereby the complete reaction takes place and molten metal is produced. The bottom plug of the ladle is opened and the metal is allowed to flow into the mould prepared. The weld joint is allowed to cool slowly. Thus weld is formed. If we making a fast weld, thermit welding also provides a reasonably strong weld. The strength if thermit welded joint reaches that of a forged metal without any defects.

Applications :

The main application of this welding technique is in the repair works of rails in railways.

Advantages of thermit welding :

• Very large and heavy parts are also joined.
• No external power source is required it just use the heat of the chemical reaction.
• It is also used for building up large wobblers.

Disadvantages of thermit welding :

• Weld may contain gas mainly hydrogen so that the slag inclusion problem occurs.
• The metal which has low melting point can't weld by this process.
• Low deposition rate with operating factor.
• Only ferrous parts may be weld by this process.
• It is the high-temperature process so that cause distortion and changes in Grain structure in the weld region.