Hydraulic System and Levers
The principle of levers is widely used in modern tractors for lifting purposes such as tipping trailers, fore-end loaders and mounted implements. The force required for lifting is transmitted through a liquid and we usually refer to the liquid as the hydraulic fluid. The force to operate the lever is increased as a result of a small piston, fitting closely in a cylinder full of oil, forcing the oil through a pipe to larger cylinder also containing a close-fitting piston.
If the area of the small piston is 10mm2 and the force on it is 30N, the pressure transmitted to the large piston is equal to force/ area, i. e. 30N/10mm2. If the area of the large piston is 40mm2, the total force exerted on the piston is force per unit area X area of the large piston = 3N/mm2 x 40mm2 = 120N. Therefore, the mechanical advantage is equal to resulting force/initial force= 120/30 or 4.
It is sometimes necessary to compare the distance moved by the effort with the distance moved by the load. The ratio of these two distances is called the velocity ratio which has the formula:
velocity ratio=distance moved by effort/distance moved by load
In the above example the small piston would need to move 4mm to move the large piston 1mm and thus the velocity ratio is 4. As with other machines, friction—in this case between the piston and cylinder wall—-prevents the efficiency from reaching 100 %. Therefore, it can be seen that the principles are the same as with mechanical levers on many machines, such as the fore-end loader, both hydraulic and mechanical levers are used.
Jack and Tractor Lifting System
An important fundamental principle of hydraulics, which permits an understanding of the action of such machines as the hydraulic jack and the power lift on a tractor, may be stated as follows:
If a fluid at rest has any pressure applied to any part of its surface, that pressure is transmitted equally to all parts of the fluid.
For example, if the vessel is filled with oil and a given pressure is exerted on the small cylinder, the same pressure will be exerted all over the large cylinder. Thus, if the small piston is 10mm in diameter and the large is 80mm, then the area of the large piston is sixty-four times that of the small, and a force of IN exerted on the small piston will support a force of 64N on the large piston or ram.
The pressure in the system {neglecting friction) is obtained by piding the total force on either piston by the area. Thus, considering the larger piston.
The hydraulic jack is a simple practical example of an application of this principle. The pump is hand-operated, and consists of a tiny plunger which is caused to reciprocate in a cylinder. Fluid is sucked from a reservoir into this cylinder through a non-retum valve and is forced via another non-retum valve into the ram cylinder, which is comparatively large. The action of the non-retum valves is entirely automatic, as with the usual type of piston pump, and the only control that the operator has to manipulate is a valve which releases the oil from the ram cylinder when it is desired to lower the jack. This valve returns the oil to the reservoir from which the pump draws its supply.
The action of a tractor's hydraulic lift system is similar in essentials, though the pump is in this case power-driven and the control more complex.
Some parts that usually use in machines:
1.Shafts and couplings
Virtually all machines contain shafts. The most common shape for shafts is circular and the cross section can be either solid or hollow (hollow shafts can result in weight savings).  Rectangular shafts are sometimes used, as in screwdriver blades, socket wrenches and control knob stems.
A shaft must have adequate torsional strength to transmit torque and not be overstressed. It must also be torsionally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. Generally speaking, the angle of twist should not exceed one degree in a shaft length equal to 20 diameters. 机械结构英文文献和中文翻译(5):http://www.751com.cn/fanyi/lunwen_6770.html