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If a high degree of stiffness and/or freedom from backlash is required, then the ball screw and nut system must be pre-loaded. To this end the nut must be in two parts (double nut). In the upper part a double nut with an external thread is shown. The pre-load is applied by turning one half of the nut: subsequent keying then prevents it turning in the opposite direction. In the low¬er half of the diagram calibrated spacing discs are placed between the nut halves to produce the pre-load.
Whether the ball screw is to be pre-loaded with a tensile or compressive load is governed by the direction of the largest externally applied load. To maintain a good stiffness, care must be taken to ensure that a minimum pre-load remains even after the application of an externally applied load.
The Basic Principle of Hydraulic System
Hydraulic Pumps
The heart of a hydraulic system is its pump. Like the human heart the hydraulic pump is responsible for sending fluid to all the components of the system. The pump does this by converting mechanical energy, delivered through its input shaft, into hydraulic energy. The hydraulic energy is then transmitted through the system fluid to the various actuators, so that it can be turned again to mechanical energy to do useful work.
The pump delivers only flow, not pressure. Pressure is caused by resistance to the pump flow. As a result, pressure is, in a sense, a by¬product of flow, or an indication of how much work the flow is doing.
To deliver flow, the pump creates a region of low pressure, or partial vacuum, on its inlet side. Fluid in the reservoir, or inlet line, is forced into this low-pressure region by atmospheric pressure acting on the reservoir fluid. The fluid is then passed through the pump and forced into the outlet line. Consequently, the fluid is not "sucked" into the pump; it is forced in by atmospheric pressure.
All pumps in tractor hydraulic systems are of the positive displacement type i.e. ,the pump "seizes" small, definite volumes of fluid, fluid conveys them through the pump, and ejects them on the outlet side A positive-displacement pump differs sharply from a non-positive-displacement pump, such as an automotive water pump, which merely impels the fluid in a certain direction.
Now let's take a look at the main types of pump used in agricultural hydraulics. Positive-displacement pumps are usually one of three types—gear, vane or piston. Also, vane and piston pumps may be of either fixed or variable displacement.
Gear Pumps
In a gear pump, fluid is trapped between teeth of two or more rotating gears and conveyed from the inlet to the outlet of the pump. Where the teeth mesh, a positive seal against fluid leakage is provided. Nearly any gears that mesh can be made to provide a pumping action, but the two most popular types are: gear on gear, consisting of two external spur gears; and gear within gear, in which a large internal gear is driven by a smaller external gear that rides within it.
Vane Pumps
In a vane pump, a rotor with retractable vanes performs much the same function as the teeth in a gear pump. The vanes move the fluid from the inlet to the outlet, and seal against leakage .Normally, in a fixed-displacement vane pump, the rotor is located inside a cam ring that has an elliptical shape, with a set of inlet and outlet ports at each tip of the ellipses. This configuration provides two pressure zones at opposite sides of the pump, thus balancing the pump and minimizing loads on the bearing, for greater life.
Piston Pumps
Several categories of piston pumps are used, with the inpidual types determined by the way the pistons work and which direction they lay. In radial-piston pumps, fairly unusual for agricultural applications the pistons extend outward from the shaft, like those in old radial aircraft engines. The pistons are driven by the shaft, either directly or indirectly to produce a pumping motion.
Axial-piston pumps, on the other hand, have pistons lying alongside the shaft. As the shaft rotates, the pistons are driven back and forth to provide a pumping action.