Pulleys are a better way to lift large masses onto tall heights and are examples of simple machines.
The types of pulleys that had been researched included:
- Fixed pulleys:
- Movable Pulleys
- Compound Pulleys
- Block and tackle
For the project we decided to use the the fixed pulley system. This is a fixed or class 1 pulley with a fixed axle. That is, the axle is "fixed" or anchored in place. A fixed pulley is used to change the direction of the force on a rope (called a belt). A fixed pulley has a mechanical advantage of 1. A mechanical advantage of one means that the force is equal on both sides of the pulley and there is no multiplication of force
NOTE that The number of pulleys used in a system may increase or decrease the mechanical advantage of the system. Generally, the higher the mechanical advantage is, the easier it is to lift the object. This means no matter how easy it is to use the pulley system, the system itself is not very efficient due to the force of friction. For example, one has to pull two meters of rope of cable through the pulleys in order to lift an object one meter.
Corrosion Theory Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons (Iron Age, Bronze Age). With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the application of a considerable amount of energy. Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. Some excerpts of that document are used here. Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations. The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown in the simple model on the left, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges. The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion.
We have come to the conclusion as a group that our crane will either have to be made up of Aluminium or Steel whilst taking into account their respective weight, cost, and durability in the long run.
Similarities
To start with the similarities of aluminium and steel:
- Structural applications of aluminium and steel are mostly similar;
- Design problems/processes are similar so an identical approach is used;
- The design rules for aluminium and steel (EC9 and EC3) are purposely very similar, see the first two reasons.
Differences
However, there are important differences in physical as well as mechanical properties which have to be accounted for in the design process. The table on this page gives a comparison between the most important physical properties of aluminium and steel. The differences in properties, the consequences for structural behaviour and how to deal with that in structural design will be elucidated below.
First of all, the low density of aluminium is the main driver for using it in many structural applications. The high strength to weight ratio is the number one reason for the development of the aircraft industry. Although its low weight is a favourable property, it can in some cases be a disadvantage; for example with cyclic loading the ratio live load/dead load is disadvantageous as compared to steel and so fatigue must be considered early in the design stage.
The low density makes an aluminium structure prone to vibrations and in these cases the dynamic behaviour of the structure has to be considered. The Young modulus, E is important for the structural behaviour. Its value is about 1/3 that of steel, but contrary to density, this is a disadvantage compared to steel.
The low value of the Young modulus, E has a big influence on the deformations of an aluminium structure. A well-known example is the bending of beams, where the stiffness EI is the governing factor and IAl = 3 ISteel to arrive at the same stiffness as a steel beam which is illustrated in the table below.
The above indicates that in designing aluminium structures, it is often not the strength, but in many cases the deformation, that is the governing factor. So in building and civil engineering it is frequently the alloy which does not have the highest strength that has to be considered.
The low Young modulus is also responsible for the higher sensitivity to stability problems in aluminium structures (buckling). The critical stress for buckling is linearly related to the Young modulus. Moreover, aluminium designs often have very slender, thin walled sections which make it even more important to consider their stability in designing structures.
Finally, there is cyclic loading, where the Young modulus is responsible for the lower fatigue strength of aluminium – circa half that of steel. This, in combination with the low density, means that fatigue design should be considered more carefully than with steel structures. Similar to the Young modulus is the shear modulus G which is also about 1/3 of that for steel. This means that the resistance against shear forces, shear deformations and shear stability (for example lateral torsional buckling of beams) can be an important aspect in the design.
