Movement and Force Ratio

The movement ratio is also called the velocity ratio (both these terms are used in the GES 1 test). It is the comparison between how far you have to move vs how far the load moves. I remember this as a slave pushing a lever to winch up an anchor, sorry for the example, but it stuck in my mind. The distance I have to walk around over the distance the anchor is winched up. This is effectively a simple level, its actually a circular lever

So as we are walking around in a circle, we will need the circumference, then divide this by the pitch of the screw jack. This is how much it will lift for a full turn. Just make sure you have everything in the same units, in this case, I used mm. One more thing is the large number always goes on top for these questions, so if you get mixed up, just make sure you put the largest number on top.

This gives a ratio of 680.667 to 1, but we just write it as 680.667. This means for every 680.667 mm I move, the load will be lifted 1mm.

You cannot have a machine that is 100% efficient, this is a law of physics, You always lose energy to a system through unwanted losses, such as heat, friction, and sound. You cannot have 100% and you certainly cannot have more than 100%! These are perpetual motion machines!

The force ratio is the force lifted (or the force we are wanting to apply, usually it is lifting, sometimes pushing) divided by the force we apply (the input force). I remember this as it is ALWAYS (for these tests) the larger number divided by the smaller number. The result, say 10, is the magnification effect the equipment has on your force, so I can lift a force ten times greater than the force I apply.

The velocity ratio (aka movement ratio) is the distance I have to move in order to move the load, divided by the load. Think of this as peddling a bike uphill in low gear, you have to peddle a whole bunch in order to move very slowly uphill! Again it's always the larger number divided by the smaller number, it's just in this case it is actually the other way around to the force ratio, again to avoid confusion, big over small.

Force ratio is a comparison of how much force can be exerted, over how much force is applied. to make this easier to remember, I think of this as the load lifted over the force applied. Again the big number always goes on top, it is a very similar layout to the previous question (but load and applied are flipped). These numbers come from actually measuring the equipment.

As we are lifting on earth, and against gravity, we find the force needed to lift the object, by converting into KG and then into Newtons, divide this but the effort, or force applied and we are done:

Efficency is a comparison between the actual advantage given by the equipment and the theoretical advantage given by the equipment.

This can be calculated by dividing the force ratio by the movement ratio.It can be given as a decimal or multiplied by 100 to convert it into a percentage. The number cannot be great than 1 (decimal) or greater than 100%, so this will help us to remember the order. The smaller number MUST go on top!

The efficiency of the machine is given by the Force ratio divided by the Velocity Ratio. In order to remember this, I use the acronym EFM (Efficeny =Force ratio / Movement ratio). These questions are always together,  so it's the first thing I write down.

One more thing to help me remember is that you cannot have efficiency greater than 100% (as a percentage) or greater than 1 (as a decimal), so the smaller number always needs to go on top. If you end up with a 150% efficient machine, you have got it the wrong way round!

Efficiency can be given as a percentage or a decimal, they are both valid.