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Mechanisms are devices that allow machines to move or to transmit the motion

from one part of the machine to another one.

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They transmit the movement from a part to another but always linear. Ex: levers, pulleys.

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A lever is a bar which pivots around a fixed point called the fulcrum or pivot (fulcro o punto de apoyo).

The elements in a lever are:

● Load “L” (carga o resistencia): is the force we have to lift or

overcome.

● Effort “E” (esfuerzo o potencia): is the force exerted by the

user.

● Load arm “La” (brazo de resistencia): distance between the

load application point and the fulcrum.

● Effort arm “Ea” (brazo de potencia): distance between the

effort application point and the fulcrum.

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They do the same of the ones above but in this case the movement is circular. Ex.: pulleys and belts,

friction pulleys, gears ...

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In this mechanisms, a belt passes around two pulleys. When the driver pulley moves, it moves the belt and the

belt moves the driven pulley. Both pulleys rotate in the same direction. This mechanism is reversible, which

means that we can use both wheels as driven or as driving wheels.

The transmission speed ratio is the same as in the friction wheels. So the equation is exactly the same:

N1 x D1 = N2 x D2

D1 = diameter of the driver wheel.

D2 = diameter of the driven wheel.

N1 = speed of the driver wheel, usually in rpm (revolutions per minute).

N2 = speed of the driven wheel, usually in rpm.

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A pulley is a wheel with a groove (canal) that turns around a fixed axle.

The drive element of a pulley mechanism is usually a rope, cable, belt or chain that runs over the pulley inside the

groove.

A simple pulley (also called fixed pulley) doesn’t increase the speed of the rope nor

reduce the effort needed to move a load. E = L

The only advantage is to reduce friction and to change the direction of the rope to

help us with our own weight.

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Gears are toothed wheels that mesh each other to transmit the circular motion or to change the speed between two

axles. The gears rotate in opposite direction. The speed of the driven gear depends on the number of teeth of both

gears:

N1 x Z1 = N2 x Z2

Where

Z1 = number of teeth of the driver wheel.

Z2 = number of teeth of the driven wheel.

N1 = speed of the driver wheel, usually in rpm (revolutions per minute)

N2 = speed of the driven wheel, usually in rpm.

Sometimes the driver gear is called pinion.

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Driver wheel (rueda motriz o conductora ) is the wheel moved directly by a

motor or a crank.

Driven wheel (rueda conducida) is the one to which the motion is transmitted.

The motion to the driven wheel is transmitted by means of friction existing

in the contact of both wheels. Consequently, both wheels rotate in opposite

direction.

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It is a system similar to the belts and pulleys but built in toothed wheels and a chain. The driver toothed

wheel meshes with the chain and pulls it as it rotates. As the chain moves, the driven wheel turns.

Bicycles and motorbikes use sprockets and chains

because of their greater strength and the fact that

they do not slip.

The speed ratio is the same as with the spur gears.

As well as the spur gears it is a reversible mechanism.

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Circular movement transformation to linear.

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Circular movement transformation to linear.

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One part of the machine produces rotary movement and it is transformed into circular in another part

of it. Ex.: Rack and pinion, nut and screw, winch.

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Circular movement transformation to alternative linear

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Circular movement transformation to alternative linear

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. Ex. Crank-connecting rod system,crankshafts, cams, eccentrics

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A rack and pinion consists of a pair of gears which convert rotational motion into linear motion.

When the pinion (circular gear) turns, it meshes with the teeth of a linear “gear” bar, so it moves linearly.

This mechanism is reversible, which means that the rack can also make the pinion turn.

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If we turn the screw while locking the nut, then the screw will move back or forth depending on the turning direction

of it.

And if we turn the nut while locking the screw, then the nut will move back or forth depending on the turning

direction of it.

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A winch is made up of a spool with a rope or cable and a crank. When we make the crank turn, the rope will roll up or

unroll depending on the turning direction to the crank. This way, the load attached to the rope will move up or down.

It is the system used to lift buckets in water wells or in fishing rods.

We can use the lever’s law in a whinch, considering the

length of the crank as the Effort arm and the radium of

the spool as the Load arm

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It transforms a rotary movement in the crank (manivela) or an eccentric axle (eje excéntrico) into alternative linear

movement in the rod (biela). It is a reversible mechanism, so if we make the rod move back and forth, we will get a

rotary motion in the crank or the eccentric.

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A cam (leva) is a specially shaped piece of material, which is fixed to a rotating shaft. The follower is an element

designed to move up and down as it follows the edge of the cam. The same function of the cam can do it an

eccentrics (excéntrica)