Mechanical System

Muhajir Ab. RahimSchool of Mechatronic Engineering

Introduction

• Mechanisms are devices which can be considered to be motion converters, in that they transform motion from one form to some other required form

• e.g; Linear motion to rotational motion

Other functions

1. Force amplification, (levers)

2. Change of speed (gears)

3. Transfer rotation about one axis to rotation about another (timing belt)

4. Particular types of motion

Types of motion

1. Translational motion

movement which can be resolved into components along one or more of the three axes.

2. Rotational motion

rotation which has components rotating about one or more axes

3. Complex motion = translational + rotational motions

(combinational motions)

Translational motion Rotational motion

Freedom and Constraints

• Involve the orientation and arrangement of the elements and parts

• A free body in space, has 6 DOF• A constraints is used to limit or prevent DOF

Constrained to move along a line

(1 DOF)

Constrained to move along a plane

(2 DOF)

Kinematic Chains

• A sequence of joints and links = kinematic chain• Link = part of mechanism which has motion

relative to some other part• Nodes = two or more point of attachment to

other links• Joint = connection between two or more links at

their nodes and which allows some motion between the connected links.

• For a kinematic chain to transmit motion, one link must be fixed.

Links with (a) 2 nodes, (b) 3 nodes, (c) 3 nodes

(a)(b)

(c)

Cams and Follower

• A cam is a body which rotates/oscillates and in doing so imparts a reciprocating/oscillatory motion to a second body, called the follower.

• As the cam rotates so the follower is made to rise, dwell and fall.• The shape of the cam determine the length of times spent at

each position.• Rise section = drives the follower upward• Fall section = drives the follower downward• Dwell section = allows the follower remains at the same level

(does not fall or rise)• Motion of the follower will depend on the shape of the cams and

the type of follower used.

dwell

rise fall

Cam Profiles

• The shape of the cam is called the PROFILE

• Example of cam profilesa) Pear shaped cams are used on the shafts of cars. The follower remains motionless for about half of the cycle of the cam and during the second half it rises and falls.

b) Circular cams or eccentric cams produce a smooth motion. These cams are used in steam engines

c) Heart shaped cams allow the follower to rise and fall with ‘uniform’ velocity.

d) Snail/drop cams produce sudden drop or fall of the follower.

Pear Eccentric Heart Snail

Types of Follower

Flat Point/Knife Roller Offset

Crank and Slider

• This mechanism is composed of three important parts:• The crank which is the rotating disc, the slider which

slides inside the tube and the connecting rod which joins the parts together.

• As the slider moves to the right the connecting rod pushes the wheel round for the first 180 degrees of wheel rotation. When the slider begins to move back into the tube, the connecting rod pulls the wheel  round to complete the rotation

Gears

• Gears are machine elements that transmit motion by means of successively engaging teeth

• Gear ‘A’ is called the ‘driver’ because this is turned by a motor. As gear ‘A’ turns it meshes with gear ‘B’ and it begins to turn as well. Gear ‘B’ is called the ‘driven’ gear.

Spur Gears

• Spur gears are the most common type of gears. They have straight teeth, and are mounted on parallel shafts. Sometimes, many spur gears are used at once to create very large gear reductions

Helical Gears

• Helical gears are similar to spur gears except that the gears teeth are at and angle with the axis of the gears.  

• A helical gear is termed right handed or left hand as determined by the direction the teeth slopes away from the viewer looking at the top gear surface along the axis of the gear

Gear Details• The circle marked in red shows

the outer limit of the teeth whilst the green circles are known as the pitch circles.

• The pitch circle of a gear is very important as it is used by engineers to determine the shape of the teeth and the ratio between gears (ratios will be explained later).

• The pitch of a gear is the distance between any point on one tooth and the same point on the next tooth.

• The root is the bottom part of a gear wheel.

• The pitch point is the point where gear teeth actually make contact with each other as they rotate

Bevel Gears

• Bevel gears can be used to change the direction of drive in a gear system by 90 degrees.

• A good example is seen as the main mechanism for a hand drill. As the handle of the drill is turned in a vertical direction, the bevel gears change the rotation of the chuck to a horizontal rotation.

Compound Gears

• Compound gears are used in engines, workshop machines and in many other mechanical devices.

• In the diagram, gear ‘A’ is actually two gears attached to each other and they rotate around the same centre. Sometimes compound gears are used so that the final gear in a gear train rotates at the correct speed.

Gear Ratio

• The reason bicycles are easier to cycle up a hill when the gears are changed is due to what is called Gear Ratio (velocity ratio).

• Basically, the ratio is determined by the number of teeth on each gear wheel, the chain is ignored and does no enter the equation.

Gear ‘A’ has 30 teeth and gear ‘B’ has 20 teeth. If

gear ‘A’ turns one revolution, how many times will

gear ‘B’ turn ? Which gear revolves the fastest?• When gear 'A' completes one revolution gear 'B'

turns 1.5 revolutions (1½ times)

A basic rule of gears is - if a large gear (gear ‘A’) turns a small gear (gear ‘B’) the speed increases.On the other hand, if a small gear turns a large gear the opposite happens and the speed decreases

Worms and Worm Wheel

• The worm, which in this example is brown in colour, only has one tooth but it is like a screw thread.

• The wormwheel, coloured yellow, is like a normal gear wheel or spur gear.

• The worm always drives the worm wheel round, it is never the opposite way round as the system tends to lock and jam.

Rack and Pinion

• The ‘pinion’ is the normal round gear and the ‘rack’ is straight or flat. The ‘rack’ has teeth cut in it and they mesh with the teeth of the pinion gear.

• rotary motion’ changes to ‘linear motion’.

Ratchet and Pawl

• A ratchet mechanism is based on a wheel that has teeth cut out of it and a pawl that follows as the wheel turns.

• As the ratchet wheel turns and the pawl falls into the 'dip' between the teeth.

• The ratchet wheel can only turn in one direction - in this case anticlockwise

Belts

• A belt drive is a method of transferring rotary motion between two shafts.   

• A belt drive includes one pulley on each shaft and one or more continuous belts over the two pulleys.  

• The motion of the driving pulley is, generally, transferred to the driven pulley via the friction between the belt and the pulley.  

• Synchronous/timing belts have teeth and therefore do not depend on friction.

Four main types of belts1. Flat

2. Round

3. V

4. Timing

Chains

• A chain is a reliable machine component, which transmits power by means of tensile forces, and is used primarily for power transmission and conveyance systems.

• The function and uses of chain are similar to a belt.

• Basic types of chain1. Roller 2. Ladder 3. Timing

Bearings

• The function of bearing is to guide with minimum friction and maximum accuracy the movement of one part relative to another

Ball Bearings

• Ball bearings, as shown below, are probably the most common type of bearing. They are found in everything from inline skates to hard drives. These bearings can handle both radial and thrust loads, and are usually found in applications where the load is relatively small.

• In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner race.

• Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly.

• But it also means that there is not very much contact area holding that load, so if the bearing is overloaded, the balls can deform or squish, ruining the bearing.

Roller Bearings• Roller bearings like the one illustrated below are used in

applications like conveyer belt rollers, where they must hold heavy radial loads.

• In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the bearing to handle much greater loads than a ball bearing. However, this type of bearing is not designed to handle much thrust loading.

• A variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. This allows the bearing to fit into tight places.

Ball Thrust Bearings

• Ball thrust bearings like the one shown below are mostly used for low-speed applications and cannot handle much radial load.

Roller Thrust Bearing

• Roller thrust bearings like the one illustrated below can support large thrust loads. They are often found in gearsets like car transmissions between gears, and between the housing and the rotating shafts.

• The helical gears used in most transmissions have angled teeth -- this causes a thrust load that must be supported by a bearing.

Tapered Roller Bearings

• Tapered roller bearings can support large radial and large thrust loads.

• Tapered roller bearings are used in car hubs, where they are usually mounted in pairs facing opposite directions so that they can handle thrust in both directions.