MECHANICAL GEARS

Example of a balanced lever

​​​​​​Mr BILLINGTON

Mechanical Devices - Types of Motion

Issac Newton defined 3 laws of motions that relate to how an object behaves on earth;
1. Inertia - An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force
2. Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object)
3. Reaction - for every action there is an equal and opposite reaction

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There are 4 primary types of mechanical motion;

• LINEAR - Motion that moves an object in a straight line from its starting point. Newtons 1st law suggests an object will continue to travel in a straight line unless a unbalanced force is applied. Forces such as drag, gravity and friction will prevent this on earth
• RECIPROCATING - Motion that pivots back and forth in a linear direction. This could be likened to a saw blade or a pneumatic drill
• ROTARY - Motion that turns the object in a complete 360° circle around and axis, often repeatedly. This motion is simple to remember as wheel or fan. The strength of this motion is defined as torque and the repetitions are counted in RPM (Rev. per min)
• OSCILLATING - Motion is a combination of rotary and reciprocating. The object is offset from the axis and rotates back and forth in a arc. This is best likened to the swing of a pendulum

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Most mechanical systems use a combination of these 4 motions to output the required force and are known as 'mechanisms'. The transfer of kinetic energy between each motion type can be achieved in many ways;

The use of a cam-and-follower can convert rotary motion into reciprocating motion. A cam is a rounded profile whereby it's central axis is offset. A follower is a straight piece that's placed on top of the cam and is forced up by the uneven profile and down by gravity

The distance traveled by the follower is know as the 'throw' and each rotation is known as a 'cycle'. A range of cam profiles can be used to achieve different reciprocating motions. They are found in a variety of mechanisms and vital to the automotive industry for components such as the crank shaft to transfer power from the engine

The shape of the follower can be altered to suit different applications;

• Knife-edge follower comes to a point on top of the cam is the simplest but is prone to wear and rarely used
• A Roller is a rotating cylinder pinned at end to rotate with the cam profile. Commonly used but can jam whilst in use
• Flat-faced followers are ideal for quick changes in direction but any misalignment can cause higher stress, fatigue & friction
• Spherical-faced followers compensate for this misalignment maintaining precise contact with the cam but with similar friction

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Another method for transferring motion is the use of Levers. A lever is one of the simplest and oldest mechanisms that takes advantage of an unbalanced force pivoting on a Fulcrum. On one side of the fulcrum is the effort arm, where the dynamic force is applied. On the other is the resistance arm where a static force (load) is applied

> Class 1 Levers- These are defined by the placement of the effort and resistance forces being on opposite sides of the fulcrum as in a see-saw. They are typically unbalanced and through calculated placement of the fulcrum can be used to lift far greater forces than are applied

> Class 2 Levers- These are defined by locating the load and the effort forces on the same side of the fulcrum. In this type of lever, the movement of the load is in the same direction as that of the effort. The effort arm must be longer than the load arm to be advantageous

> Class 3 Levers- These are defined by locating the forces on the same side of the fulcrum with the effort arm closer than the load arm. The load  and effort arm go all the way to the fulcrum with the load arm always longer than the effort. Both forces move in the same direction

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Class 1 & 2 increase the input force applied through 'effort' to output far greater force upon the load. This is defined as a 'moment'. In order to calculate the moment you must know size of the forces and their distance from the pivot. The equation used to calculate each force is;

Force (Newton) x Distance (metre) = Moment (Nm)

Sir Isaac Newton

1642 - 1726