Engineering mechanics and hydraulics

Students Learn About:

orders of levers
mechanical advantage, velocity ratio and efficiency

Students Learn To:

apply mathematical and/or graphical methods to solve problems of biomedical engineering practice

orders of levers

class 1 lever

For the Class 1 lever the pivot lies between the effort and load. A see saw in a playground is an example of a Class 1 lever where the effort balances the load.

class 2 lever

For the Class 2 lever the load is between the pivot and the effort (like a wheelbarrow). The effort force needed is less than the load force, so there is a mechanical advantage.

Standing on tip toes is a Class 2 lever. The pivot is at your toe joints and your foot acts as a lever arm. Your calf muscles and achilles tendon provide the effort when the calf muscle contracts. The load is your body weight and is lifted by the effort (muscle contraction).

class 3 lever

For a Class 3 lever the load is further away from the pivot than the effort. There is no mechanical advantage because the effort is greater than the load. However this disadvantage is compensated with a larger movement. This type of lever system also gives us the advantage of a much greater speed of movement.

A bent arm is a Class 3 lever. The pivot is at the elbow and the forearm acts as the lever arm. The biceps muscle provides the effort (force) and bends the forearm against the weight of the forearm and any weight that the hand might be holding.

The three orders of levers each have characteristic MA and VR relationships.

First-order lever: MA and VR may be greater than, or less than, one depending on the distances of the effort and load from the pivot.

Second-order lever: MA and VR are always greater than one. Thus the effort is lower than the load but a greater distance is moved by the effort compared to the load.

Third-order lever: MA and VR are always less than one. Thus a high effort is required but a small distance is moved by the effort, as compared to the load.

mechanical advantage, velocity ratio and efficiency

mechanical advantage

Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever. Machine components designed to manage forces and movement in this way are called mechanisms.[1] An ideal mechanism transmits power without adding to or subtracting from it. This means the ideal mechanism does not include a power source, is frictionless, and is constructed from rigid bodies that do not deflect or wear. The performance of a real system relative to this ideal is expressed in terms of efficiency factors that take into account departures from the ideal.

velocity ratio

velocity ratio is The ratio of the distance moved by the point at which the effort is applied in a simple machine to the distance moved by the point at which the load is applied, in the same time. In the case of an ideal (frictionless and weightless) machine, velocity ratio = mechanical advantage. Velocity ratio is sometimes called distance ratio.

efficiency

An ideal machine is one that is 100% efficient. That means all energy put into the machine is used. In reality, however, this never occurs. There is always some type of energy loss (usually as a result of friction) that results in the efficiency being below the ideal 100%. In the case of levers there may be friction in the pivots or the lever may bend slightly. VR is always the same irrespective of efficiency, since there no change in the distances the effort and load move. But MA is effected by a less than ideal efficiency; thus the MA will always be less than the VR for machines with efficiencies below 100%. The percentage efficiency is found by the following formula,

apply mathematical and/or graphical methods to solve problems of biomedical engineering practice

Identify an example in body for each class of lever
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pulleys

The pulley allows the user to apply a force at a different location or in a different direction. For example, a tractor can pull out a stump going at right angles to the required direction using a pulley.

Another use for the pulley is to be placed together with other pulleys and used as a block and tackle to make lifting heavier objects easier. The pulley can also be used to transfer rotational motion from one shaft to another. An example of this is the drive belt on a tractor connecting the fan, alternator and crankshaft together.

Note that the size of a pulley transmitting rotational motion determines how fast one shaft spins compared to the other.

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