HYDRAULIC CONTROLS
Designing and calculating a control is preceded by the preparation of a hydraulic circuit diagram, based on the rule that one inlet and one outlet valve each or one double-acting slide valve each is required for one reciprocal motion of the plunger. Consequently, a four valve control is to be provided for the advance and return motion or the up-and-down motion of the piston of a horizontal or vertical press respectively.
A two valve control also answers the purpose when, for example, the pullback cylinders of an auxiliary device of the press are connected directly to the pressure conduit. In such case the main piston, while advancing, has to overcome the constantly acting pullback power and must therefore have correspondingly larger piston and valve cross-sections. Such simplified control with two valves is recommended for various auxiliary motions under full pressure and low piston forces. It would be less suitable, however, for the shifting device, because the controlled cylinder and the piston would have to be doubled in size. A pullback device working under constant pressure, may by no means be employed for motions comprising one idle and one power stroke. The reason for this being that the main cylinder is filled with low-pressure water or oil during the idle stroke, the pressure of which is not sufficient to overcome the constant pullback power.
All modern extrusion presses are equipped with controls. on the one hand pressure-water or oil is consumed only for the power strokes proper and, on the other hand, relatively high speed rates are attained during the idle strokes. The socalled filling valve being nothing but a large water or oil dischange valve which is balanced by a small pilot valve and which is pushed open by a drive piston the cylinder of which is connected to the pullback piping.
- Hand-lever Controls
Fig.178 illustrates a consisting of a forged block with slide-valves. Slide-valves are employed for oil-drive only。The control housing is fixed on a trestle in which is set the drive shaft for the opening and closing of the slide-valves.
The valves may be used for both pressure-water and pressure-oil. There is quite a variety of types on the market which have also been described in detail already. Pressure inlet valves which are opened by means of a simple stem, are, as illustrated in Fig.178, provided with a balance pin which serves to equilibrate the control shaft. If this was not provided, the force of the stem after opening an inlet valve, would cause a torsional moment on the control shaft which would impede the operation of the control. A balance pin is, however, not required in valves equipped with a through stem which is sealed in the upper valve cap and is thus perfectly balanced.
The free valve cross-section f depends on the velocity v and the cross-sectional area F of the main plunger, as well as on the rate of flow V of the fluid in the valve seat.
Therefore Fv=fV
And f=Fv/V
以上是毕业论文文献综述，课题毕业论文、任务书、外文翻译、程序设计、图纸设计等资料可联系客服协助查找。
