Hydrostatic machines often have multiple hydrodynamic bearing interfaces, which also serve as a sealing interface. In axial piston machines, the bearing and sealing interface between the barrel and the port plate is a well known example. At reasonably high operating speeds, hydrodynamic effects create an oil film between the barrel and the port plate. This oil film will then, to a certain extend, lift the barrel from the port plate, thereby avoiding metal-to-metal contact. The disadvantage of hydrodynamic bearings is, that they need a relatively high velocity of the sliding components, in order to reduce the friction. Below a certain speed, mixed lubrication and finally solid friction will occur. This results in strongly increased friction losses and wear. Low speed operation has always been of interest for hydrostatic motors, which are often operated at close to zero speed or at low rotational speeds. But low and near zero speed operation has also become of importance for pumps when being operated in electro-hydraulic actuators (EHAs). Many of the existing pump principles are not allowed to be operated below a certain minimum speed, due to excessive wear which results from coulomb friction conditions. Furthermore, the stick-slip-behaviour creates additional nonlinear behaviour of the EHA-operation, and makes it difficult to control EHAs. In order to overcome the disadvantages of hydrodynamic bearings, a new hydrostatic bearing has been developed. In the new bearing, the sealing land of the barrel is divided into three concentric rings. In the middle ring, so called pockets are created. Each pocket has a direct connection with the corresponding port by means of a small groove. The new bearing not only lifts the barrel to a certain height, but also helps to counteract the tilting torque of the barrel. The size of the pocket grooves determines the height of the oil film, and therefore also the leakage and viscous friction of the bearing and sealing interface. In a recent research project, INNAS has performed a number of experiments to measure the influence of the groove size on the overall efficiency, as well as on the leakage and torque loss. Measurements have been performed on a 24 cc floating cup pump in a speed range between 500 and 4000 rpm and a pressure range between 100 and 400 bar. At the end of the project, the range has been extended to a speed range between 0.23 and 4400 rpm, and a pressure range between 50 and 450 bar. This paper describes some of the results of these experiments. The measured width of the pocket grooves is taken as a characteristic parameter for the size of the flow area and resistance of the pocket grooves.
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