Differences between Structure and Machine

 Differences between Structure and Machine

A structure is an assemblage of a number of resistant bodies/members having no relative motion between them and meant for carrying loads having straining action.

Examples of a structure are: A railway bridge, Machine frame etc.

Machine is a mechanism or combination of mechanisms which, apart from imparting definite motions to the parts, also modifies and transmits available mechanical energy into some desired work.

Examples of a machine are: Internal combustion engine, Shaping machine etc.

The parts of a machine move relative to one another, whereas the members of a structure do not move relative to one another.

A mechanism transforms the available energy into some useful work, whereas a structure does not transform the available energy into useful work.

The links of a mechanism may transmit both power and motion, whereas the members of a structure transmit only forces.

What are rigid and resistant bodies?

 What are rigid and resistant bodies?

A Rigid body is the one, which does not suffer any distortion under the action of forces.

Examples: Crank shaft, connecting rod of an IC engine. 

A resistant body is the one, which is rigid for the purposes they have to serve. In other words, which is normally flexible, but under certain loading conditions act as a rigid body for the limited purpose.

Some examples of resistant bodies are Belt in belt drive, Fluid in hydraulic press.

Difference between Linkage and mechanism

 Difference between Linkage and mechanism

A linkage is obtained if one of the links of a kinematic chain is fixed to the ground.

A mechanism is also obtained if one of the links of a kinematic chain is fixed to the ground, but the basic difference between a linkage and a mechanism is, in a mechanism motion of any movable links results in definite motions of the others.

However, this distinction between a mechanism and a linkage is hardly followed and each can be referred in place of the other.

Difference between Kinematics and Dynamics

 Difference between Kinematics and Dynamics

Kinematics deals with the relative motion of different parts of a mechanism without taking into consideration the forces producing the motions.

Hence, Kinematics is the study from a geometric point of view to find the displacement, velocity and acceleration of the mechanism.

Dynamics deals with the forces, which produce relative motion of different parts of a mechanism. 

Dynamics further divided into Statics and Kinetics.

Statics deals with the forces that act on the parts which are assumed to be massless and motionless.

Kinetics on the other hand deals with the inertia forces arising out of the combined effect of mass and the motion of the parts.

Difference between Mechanism and Machine

 Difference between Mechanism and Machine

Mechanism is a combination of rigid / resistant bodies so formed and connected that they move with respect to each other with definite relative motion.

Hence the function of a mechanism is to transmit and modify motion.

Crank, connecting rod and piston of an Internal combustion engine is an example of a mechanism (Slider crank mechanism). A slider crank mechanism converts reciprocating motion of the piston into rotary motion of the crank or vice-versa.

Machine is a mechanism or combination of mechanisms which, apart from imparting definite motions to the parts, also modifies and transmits available mechanical energy into some desired work.

Internal combustion engine is an example of a machine. Which converts force on the piston into the torque of the crankshaft.

In spirit, all the machines are mechanisms. If a mechanism transmits a substantial amount of forces along with definite relative motion, it can be called a machine.

Difference between kinematic Analysis and Synthesis of mechanisms:

 Difference between kinematic Analysis and Synthesis of mechanisms

Kinematic analysis aims at determining the performance of a given mechanism such as Displacement, velocity, acceleration etc.

Whereas kinematic synthesis aims at determining the dimensional parameters of the mechanism / designing a mechanism which can satisfy prescribed motion characteristics, such as displacement, velocity, acceleration etc.

Obtaining cam profile for generating specified type of follower motion falls in the category of synthesis of mechanisms, whereas obtaining displacement, velocity, acceleration etc. of the cam falls under the category of analysis of mechanisms.

Kinematic Inversion, a comprehensive note

 Kinematic Inversion, a comprehensive note

A mechanism is a kinematic chain in which one of the links is fixed. By fixing the links of a kinematic chain one at a time, we get as many different mechanism as the number of links in the chain. This method of obtaining different mechanism by fixing different links of the same kinematic chain, is called as Inversion of the mechanism.

Number of inversions possible for a kinematic chain equals the number of links in the parent kinematic chain.

Relative motion, displacement, velocity and acceleration between any two links does not change with inversion. This is because relative motion between different links is a property of parent kinematic chain.

Absolute motion of points on various links measured with respect to the fixed link may, however, change drastically from one inversion to the other.

The concept of inversion enables us to categorize a group of mechanisms arising out of inversions of a parent kinematic chain as a family of mechanisms. Members of this family have a common characteristic in respect of relative motion.

In case of direct inversions, as relative velocity and relative acceleration between two links remain the same, it follows that complex problems of velocity and acceleration analysis may often be simplified by considering a kinematically simpler direct inversion of the original mechanism.

In many cases of inversions, by changing proportions of lengths of links, desirable features of the inversion may be accentuated and many useful mechanisms may be developed.

Four bar (Crank rocker) mechanism - Input torque versus Output torque

 Four bar (Crank rocker) mechanism - Input torque versus Output torque 

In the below picture shown is a four bar mechanism (crank rocker) with links L1 fixed, L2 crank, L3 coupler and L4 follower. Angle between crank and fixed link is theta2, angle between horizontal/fixed link and coupler is theta 3 and angle between horizontal and follower link is theta 4.

In the below picture equations written to find angles theta 3 and theta 4.

Torque T1 (CW) is applied on the crank, on the coupler torque T2 will be experienced due to T1 on crank. in the below pictures described how to derive equation to establish relationship between T1 and T2.

In the below table, a problem is solved using Microsoft excel. Fixed link length L1 is 220 mm, Crank length L2 is 100 mm, coupler length L3 is 200 mm and Follower link length L4 is 150 mm are considered. and a clock wise direction torque of T1 is applied on the crank. Using the above equations, Theta 3, theta 4 and torque T2 are calculated. Clock wise direction torque is considered as negative and counter clockwise direction torque is considered as positive.

In the below graph, crank angle theta 2 zero to 360 degrees versus torque on the follower link T2 are plotted. Theta two along horizontal axis and torque T2 along vertical axis.

Drag link (Double crank) four bar mechanism - Transmission angle

 Drag link (Double crank) four bar mechanism - Transmission angle

In the below picture shown is a Four bar (drag link/double crank mechanism. In double crank / Drag link mechanism both crank and follower links make complete rotation. Link L1 is a fixed link, L2 is crank link, L3 is coupler and L4 is the follower link. Angle between Fixed link and crank is theta 2 and angle between coupler and follower link is called "Transmission angle" gamma.

In the below picture described how to calculate transmission angle gamma for a given crank angle theta 2.

In the below picture shown maximum and minimum transmission angles. Maximum transmission angle occurs when crank angle theta 2 is 180 degrees. Minimum transmission angle occurs when crank angle theta 2 is zero degrees. Equations to find maximum and minimum transmission angles are shown below.

After having derived all the necessary equations to find Transmission angle for a given crank angle, minimum transmission angle and maximum transmission angle, a problem is solved using Microsoft excel. In the below problem fixed link length L1 is 100 mm, crank link length L2 is 150 mm, coupler link length L3 is 220 mm and follower link length L4 is 200 mm are considered. In the below table transmission angles for crank angles from zero to 360 degrees calculated and the same results plotted in the below graph. From the below table we can find that at crank angle theta 2 equals to zero, transmission angle is minimum (15.54 degrees) and when theta 2 is 180 degrees transmission angle is maximum (72.88 degrees).