MECHANISMS

IES PROFESOR MÁXIMO TRUEBA...................................... 1º ESO - Dpto. Tecnología. Sección bilingüe Seccion_Bil_Worktext_Mechanisms_1_Transmission And Conversion Of Motion ............................. Page 1

MECHANISMS I: TRANSMISSION AND

CONVERSION OF MOTION

1) PRELIMINARIES All machines have in common the following things:

• they involve a kind of motion

• they involve a kind of force

• they make a job easier to do

• they need some kind of input to make them work

• they produce some kind of output

The four basic types of motion are:

• Rotary: going round and round. This is the most common kind of movement

• Oscillating, swinging backwards and forwards

• Linear, in a straight line

• Reciprocating, backwards and forwards in a straight line

A machine is a device that helps make work easier to perform. (Remember, Work = Force

X Distance). A machine makes work easier to perform by accomplishing one or more of

the following functions:

� transferring a force from one place to another,

� changing the direction of a force,

� increasing the magnitude of a force, or

� increasing the distance or speed of a force.

2) TRANSMISSION OF MOTION a) Gear

A gear is a component within a transmission device that transmits rotational force to

another gear or device.

A gear is a round wheel which has linkages ("teeth" or "cogs") that

mesh with other gear teeth, allowing force to be fully transferred

without slippage.

The most common situation is for a gear to mesh with another gear, but

a gear can mesh with any device having compatible teeth, such as

other rotational gears, or linear moving racks.

A gear's most important feature is that gears of unequal sizes

(diameters) can be combined to produce a change of the rotational

speed of the second gear.


So that the equation to refer this mechanical advantage is:

S1·D1 = S2·D2

Where S= rotational speed and D= diameter.

Or

S1·T1 = S2·T2

Where T refers to the number of teeth on gears.

Comparison with other drive mechanisms

The fact that there is no slippage on gears gives gears an advantage over other drives

(such as traction drives and V-belts) in precision machines such as watches that depend

upon an exact velocity ratio.

In cases where driver and follower are in close proximity gears also have an advantage

over other drives in the reduced number of parts required

The downside is that gears are more expensive to manufacture and their lubrication

requirements may impose a higher operating cost.

b) Types of Gears

Spur gears:

Spur gears are the simplest, and probably most

common, type of gear. Their general form is a

cylinder or disk (a disk is just a short cylinder).

Helical gears:

Helical gears offer a refinement over spur gears. The

leading edges of the teeth are not parallel to the

axis of rotation, but are set at an angle. The angled

teeth engage more gradually than do spur gear

teeth.

This causes helical gears:

• to run more smoothly and quietly than spur

gears.

• also offer the possibility of using non-parallel

shafts.


Bevel gears:

Bevel gears are essentially conically shaped. Bevel

gears transmit motion between angled shafts.

Crown gears:

A crown gear or contrate gear is a particular form of

bevel gear whose teeth project at right angles to the

plane of the wheel; in their orientation the teeth

resemble the points on a crown. They mesh or

connect aligned axles.

Worm gears:

A worm is a gear that resembles a screw. Its body is

usually fairly long in the axial having actually just one

tooth wrapped along the gear. It is these attributes

which give it its qualities.

• The prime feature of a worm-and-gear set is

that it allows the attainment of a high gear

ratio with few parts, in a small space.

• This system achieves a huge speed reduction,

and so that, since the worm has a tooth, if the

gear has 50 teeth, a 1:50 speed reduction is

achieved.

• The motion transmission is always from the

worm to the gear, ie., moving the gear we

cannot make the worm rotate

Rack and pinion:

A rack is a toothed bar or rod that can be thought

of as a sector gear with an infinitely large radius of

curvature. Rotation can be converted to linear

motion by meshing a rack with a pinion: the pinion

turns; the rack moves in a straight line, so that there is

a conversion of motion from rotary to linear and vice

versa.

Such a mechanism is used in automobiles to convert

the rotation of the steering wheel into the left-to-right

motion of the tie rod(s).


c) Belt and pulley systems A belt and pulley system is characterized by two or more

pulleys in common to a belt. This allows for mechanical

power, and speed to be transmitted across axes and,

if the pulleys are of differing diameters, a change of

speed will occur.

A belt drive is analogous to that of a chain drive,

however a belt may be smooth.

Belt and pulley systems are systems that can be very

efficient, with stated efficiencies up to 98%, the problem

is when the belt slips on the pulley.

d) Chain and sprocket systems

Chain drive is a way of transmitting motion from one place to

another. It is often used to convey power to the wheels of a

vehicle, particularly bicycles and motorcycles. It is also used

in a wide variety of machines besides vehicles.

The gear is turned, and this pulls the chain putting

mechanical force into the system.

e) Chains versus belts Drive chains are similar to drive belts in many ways, and which device is used is subject to

several design tradeoffs.

• Drive chains are most often made of metal, while belts are often rubber, plastic, or

other substances. This makes drive chains heavier, so more of the work put into the

system goes into moving a chain versus moving a belt.

• On the other hand, well-made chains are often stronger than belts.

• Also, drive belts can often slip (unless they have teeth) which means that the output

side may not rotate at a precise speed, and some work gets lost to the friction of

the belt against its rollers.

• Chains often last longer.


3) CONVERSION OF MOTION Many technology problems involve movement in a straight line (or linear motion). This

motion can be produced by the conversion of rotary motion using different systems

a) Crank and slider mechanism A crank is an arm at right angles to an axle or spindle, used to change circular into

reciprocating motion, or reciprocating into circular motion. The arm may be a bent

portion of the shaft, or a separate arm keyed to it.

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which

translates reciprocating linear piston motion into rotation. The crankshaft was invented in

the 13th century by al-Jazari, an Arabian

b) Cam and follower Yet another mechanism which can convert rotary into

liner motion. A cam is specially shaped piece of metal

which is fixed to an axle or shaft. The follower is the

device designed to move up and down as it follows the

shape.

c) Rack and pinion Finally another system to convert motion is the rack and pinion system which was

explained in the topic before when explaining gears

Documents

MECHANISMS