Mechanical Motion

AHL Topic 10

Mechanical Design

from

http://www.bisnet.or.id/vle/mod/resource/index.php?id=155

Mechanisms are devices that have been designed to make jobs easier. They all have certain things in common:

They involve some kind of motion. They involve some kind of force. They must have some kind of input to make them work. They produce some kind of output. If we connect mechanisms together we can build mechanical systems called machines.

Machines help to make our lives easier and more comfortable.

Linear motion• Linear motion is motion in a straight line. Steady linear motion

is known as velocity (uniform motion in a straight line).

• An example of linear motion is the cutting arm of a paper guillotine (photo below) as it travels from one side of the machine to the other.

Rotary Motion

• Motion in a circle is called rotary motion. The number of complete revolutions made per minute (rpm), is called rotary velocity.

Intermittent MotionIntermittent motion is motion which starts and stops regularly. For example, in a cinema projector the film needs to be moved on one frame at a time then held stationary while the light projects it onto the screen. This is usually done with a Geneva stop as shown here.

Intermittent motion is usually the end result of a mechanism rather than the starting point for conversion.

Oscillating Motion

• Oscillating motion is motion backwards and forwards in a circular arc. E.g. playground swings (photo) and clock pendulums.

Reciprocating Motion• Reciprocating motion is linear

motion backwards and forwards in a straight in a straight line. Sewing machines make use of this type of motion.

Jigsaws and scroll saws which are often used in school workshops have blades that cut by reciprocating motion.

Irregular Motion

Irregular motion is motion which has no obvious pattern to its movement. It is often needed in automata to recreate the movements of living things.

Irregular motion is usually created using a cam or series of cams

LinkagesLinkages are used in mechanical control systems to allow forces and motion to be transmitted to where they are needed. They consist of a series of levers which can change the direction of a movement, alter the size of a force or make things move in a particular way.

Reverse Motion Linkages are used to change the direction of motion. A single lever with a pivot at its centre reverses an input motion without affecting the input force. The input force and length of transmitted movement can be increased or decreased by altering the position of the pivot.

Mechanical Motion in Context

• Hydraulic digger

• Bicycle

• Car Jack

• Hand drill

Discuss the following products, what motions do they use?

Torque• Definition:

Rotational force commonly measured in units of Newton metres.

A simple example is to look at what happens when a nut is tightened by a spanner.

A force F (newtons) is applied by hand at a distance r (metres) from the centre of the nut.

F x r = Torque (Nm)

100 x 0.2 = 20Nm (newton metres)

If the same force is applied to a longer spanner at distance r from the centre of rotation, the torque is greater.

Ratchet and Pawl

In mechanical engineering, a ratchet is a device that allows linear or rotary motion in only one direction, while preventing motion in the opposite direction. Ratchets are used in many other mechanisms, including clocks, capstans, turnstiles, spanners, winders, cable ties, jacks, screwdrivers, and hoists. The term is also used for a socket wrench.

Ratchets consist of a gearwheel or linear rack with teeth, and a pivoting spring-loaded finger called a pawl that engages the teeth.

CamsA Cam is a machine component that either rotates or moves back and forth (reciprocates) to create a prescribed motion in a contacting element known as a follower. The shape of the contacting surface of the cam is determined by the prescribed motion and the profile of the follower.

Cam followersAs the cam turns, driven by the circular motion, the cam follower traces the surface of the cam transmitting its motion to the required mechanism.

Cam follower design is important in the way the profile of the cam is followed. A fine pointed follower will more accurately trace the outline of the cam. This more accurate movement is at the expense of the strength of the cam follower.

Conversion of motion

Mechanisms can be used to convert one form of motion into another

REMEMBER: A rack and pinion is a pair of gears which convert rotational motion into linear motion.

A bell crank is a type of crank that changes motion around a 90 degree angle.

Toggle clamp

Toggle clampsToggle clamps are used to lock things into position. They are quick to use and it is possible to obtain a large clamping forces.   Industry makes considerable use of toggle clamps to secure work being machined, pressed or formed.

A toggle clamp usually consists of two links on a common pivot. The free end of one link is attached to a fixed pivot. The free end of the second lever is constrained to move in a straight line. When a force is applied to the common pivot, the lower end of the toggle clamp is pushed downwards against the item being clamped. The clamp locks in position once the common pivot is pushed slightly 'over centre', usually against a stop. Only by applying a force from the side can the clamp be unlocked. The toggle principle is used on push chairs to hold them open in use.

Parallel MotionParallel linkages are used to make two or more parts of a mechanism move together and stay parallel to each other as the linkage moves.

The photo shows a petrol engine cross cut wood saw. Rotary motion  from the engine output shaft  is transformed into reciprocating motion through a crank and slider. This reciprocating motion is then transmitted to the saw blade via a parallel motion linkage.

Treadle linkage

A treadle linkage changes either rotary to oscillating motion or oscillating to rotary motion. It can be used to power a rotary machine by working the treadle by foot.

Bell CrankBell Cranks are useful for changing the direction of motion or transmitting it round a corner. E.g. mountain bike brakes.

The photo on the right shows a bell crank linkage connecting the foot brake pedal in a car to the front and back drum brakes.

To see a range of simple animations and further explanations of mechanical motion visit the following website:

www.flying-pig.co.uk