CLASS : JSS 3
SUBJECT : BASIC TECHNOLOGY
TOPIC : MOTION IN ENGINEERING SYSTEM (MACHINES).
DURATION :
PERIOD :
Motion is referred to as the movement of any object that changes its position with time. Motion in mechanisms is based on the motion that takes place in some engineering system such as an: Motor Vehicle, Bench vices, Sewing machine, bicycle, screw jack. A Mechanism is a working part of a machine.
TYPES OF MOTION IN MECHANISMS
There are two basic types of motions in mechanisms, which are :
1) THE LINEAR MOTION: The Linear motion is the movement in a mechanism that moves along a straight line from one point to another point. Linear motion can be applied in :
a. Lever Arrangement
b. Linkages
c. Slides Movement
d. Slots movement
e. Pulley
f. Needle of sewing machine.
a) Lever Arrangement: This is an arrangement used to do useful work easily in order to produce linear motion. A lever is a simple form of a machine which could be applied to a rigid body.
A lever has three essential parts namely:
I) Effort (E): This is the point where forces are applied to a machine to make the machine work i.e. the force is called the effort.
ii) Load (W): This is the point where work is overcome i.e. the object that is lifted is called load (W).
III) Fulcrum (F): This is the point or support on which a lever turns i.e. the point of pivoting is the fulcrum (F).
There are three classes of lever depending on the order of arrangement.
I) First order lever: This is the order in which the fulcrum (F) lies between the load (W) and the Effort (E).i.e. the load comes down lower than the effort. E.g. Crowbar, a pair of scissors, beams balance, pliers, claw hammer etc.
ii) Second order lever: this is the order in which the Load (W) lies between the Fulcrum (F) and the Effort (E). Here load which is usually bigger than the effort is moved through the shorter distance than the effort. E.g. bottle opener, Wheelbarrow.
III) Third order lever: this is the order in which the effort (E) lies between the Load (W) and the fulcrum (F). Here; the load is usually smaller than the effort. E.g. fishing rod, human arm, lifting stone, knife etc.
MECHANICAL ADVANTAGES (M.A)
The Mechanical advantage of a machine is the ratio of the output force to the ratio of the input force in which the load is more than the effort.
M.A = (load) (W) ÷ (effort) (E)
Where L > E
Example: Calculate the M.A of jack that requires an effort of 40N to lift a 250kg load.
Solution: Note: 1kg = 10N (Newton)
M.A = ?, Load = 250kg, Effort = 40N
W = 250 x 10N = 2500N
M.A = W ÷ E
= 2500N / 40N
M.A = 62.5
b) Linkage: This is an assembly of bars in which each bar is connected to at least two other bars by hinges or sliding joints. The linkages are used to transmit motion from one point (the driven arm as linear) to another driven arm.
c) Slides movement: this is a rigidly supported object over which another object can move on it on a straight line.
d) Slots movement: this is a method of using a circular hole with a cylindrical object to guide a rod along a straight line. The slot can used to obtain linear motion.
APPLICATION OF LINEAR MOTION.
a) A drawer in a cabinet follows a straight line because it is guided by a slot.
b) In a door locker, the locking device is constrained to move in a rectangular slotted hole as the handle is turned left or right.
c) The ink tube in a bore of a biro can move forward and backward with the hand, is a linear motion.
d) The handle of the drawers can move forward and backward in a wooden and backward in a wooden cabinet, is a linear motion.
ROTARY MOTION:
Rotary motion is the movement of a mechanism in a circular direction around an exist or a centre. Examples: rotation of electric fan blade, movement of grinding machine, turn tables of motors and movement of hands of wall clock or wrist watches.
TYPES OF ROTARY MOTION.
There are two types of rotary motion.
1) The one way (non-return) rotary motions: this is the mechanism, which rotates in one direction only. It always rotates in a clockwise direction only and it can only be brought to rest by the application of brakes. E.g. wall clock, electric fan, grinding machine etc.
2) The Reversible Rotary motion: This is the mechanism, which rotates in a clockwise direction and anti-clockwise direction. E.g. Cassette and tape recorder (rotates in one way direction when playing it while rotation in reversible direction is used for rewinding), wheel of motorcars, forklifts, Lorries, trains etc (perform reversible rotary motion by changing from forward gear to reverse gear).
- CONVERSION OF MOTIONS.
1) From Linear motion into Rotary Motion.
For instance, the conversion of linear motion into rotary motion takes place in the following mechanism:
a. The playing cassette
b. The ejecting of video tape
c. The movement of motor vehicle on the road wheel.
d. In an internal combustion engine cylinder.
2) From Rotary Motion into Linear Motion.
For instance, the conversion of rotary motion into linear motion takes place in the following mechanism.
a. The screw jack for lifting up a vehicle while exchanging tyres.
b. The vice used in holding device in a workshop.
c. The rope and bucket for lifting sand or water from the deep well.
d. There are other different arrangements that can be used to convert rotary motion to linear motion.
Examples:
a.Nut and Screw: if the screw is held in position by restricting its movement to revolve, then the nut will be forced to move, but depending on the direction of the revolving screw.
There are three devices that are necessary to control rotary motion in mechanisms, which are;
i. Brake: the brake is a device, which is widely used to control the motion of a moving vehicle and bring it to an eventual stop.
ii. Clutch: the clutch is a device, which is widely used in transmitting system to connect and disconnect, or engage and disengaged two revolving gears (two moving parts).
iIi. Ratchet: The ratchet is a mechanism, which is widely used in machinery to allow shat rotation in one direction only.