Hydraulic hydrostatic drive

Hydrostatic drives allow for selection of any travel speed up to the maximum without a concurrent variance in engine speed. The engine can be operated at the governed speed to provide proper operating speeds for auxiliary elements, eg, the threshing section of a combine. A frill range of travel speeds is available to adjust to terrain or crop conditions. Industrial applications for hydraulic systems and hydrostatic transmissions include the following (16) ... 

Depend.s directly on the hydraulic (hydrostatic) ptessuren the renal vasculature (created hy the puropirtg beam, uhirii tends to drive water and solutes out of the glonieniltir cupiUa. -ies into Bowman s space... 

A hydraulic drive generally consists of a constant speed AC motor driving a hydraulic pump, which, in turn, drives a hydraulic motor and, of course, the associated controls. The entire package is often referred to as a hydrostatic drive. Some of the advantages of a hydrostatic drive are stepless adjustment of speed, torque, and power smooth and controllable acceleration ability to be stalled without damage and easy controllability. 

Despite the attractive features of hydrostatic drives, they are rarely used on regular (non-reciprocating) extruders. The reason for this situation is not obvious since the hydrostatic drive is in many respects competitive with, for instance, the SCR DC drive and in some respects better e.g., there is no need for a gearbox. A possible reason is that the hydrostatic drive is still regarded with suspicion by many people. This is because early hydraulic drives were not very reliable and accurate. However, this situation has changed dramatically over the years, but the hydrostatic drive still seems to suffer from its early unfavorable reputation. Very few U.S. companies supply hydraulic drives for extruders Feed Screws Division of New Castle Industries and Wiimington Plastics Machinery. It is claimed [5] that hydraulic drives are less expensive than DC drives on smaller extruders, up to about 90 mm. 

The line-to-shaft efficiency of DC motors is around 0.85 and about 0.80 for variable frequency AC motors. Considering that a typical two-stage gearbox has an efficiency of about 0.95, the overall efficiency for a DC drive is about 0.80 and about 0.75 for a variable frequency AC drive. The overall efficiency of a well-designed hydrostatic drive is around 0.70. Thus, the hydrostatic drive compares reasonably well with the AC and DC drives with regards to overall efficiency [31]. The drive efficiency of the DC motor drive and the adjustable frequency drive increases with rated speed, whereas the drive efficiency of the hydraulic drive is relatively independent of rated speed. The efficiency of the DC motor is better than other motors in the range of 20 to 100% rated speed below 20% rated speed the hydraulic drive is more efficient. 

Fluid film thrust bearings have been applied to extruders on a few occasions. Their load carrying capability at low speed is generally poor and a loss of fluid film would have disastrous results. If a hydraulic drive is used to turn the screw, application of hydraulic thrust bearings may deserve some consideration. Reference 28 describes an extruder with hydraulic drive that incorporates a patented thrust bearing assembly with hydraulic axial screw adjustment. By measuring the pressure of the hydrostatic chamber of the thrust bearing, the pressure in the polymer melt at the end of the screw can be determined. 

Fig. 8 (a) Schematic illustration of a hydrostatically coupled actuator a DBA membrane (A) is coupled to a chamber containing a fluid (B) which provides electrical insulation and hydraulic transmission for driving a tactile surface (Q (b) example of a prototype display ((b) Reproduced with permission from Lee et al. 2014)... 

As with drag flow rtieometers, there are two basic design types one features controlled drive pressure and measurement of flow rate, and the other uses controlled flow rate and measures pressure drop. Pressure is controlled by a hydrostatic head, external gas or hydraulic pressure, or even a weight. Flow rate can be controlled by motion of a driving piston. Whoriow (1992) has an extensive review of pressure driven iheometer design. 

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