ISO 4409 is the most important international standard for measuring the efficiency of hydraulic pumps and motors, the latest edition being 4409:2007. The standard describes methods for determining the steady-state performance in terms of overall efficiency. It also defines equations for calculating the volumetric efficiency of pumps and motors. The hydro-mechanical efficiency is only defined for motors, not for pumps. This paper analyses the efficiency and losses of pumps and motors in an alternative way. The preference is on loss analysis instead of efficiencies. Especially the effects of the bulk modulus are considered in a different and more inclusive manner. The new methodology results in a higher total loss for motor and a lower total loss for pumps than the current ISO 4409 standard. Furthermore, it results in significant changes of the hydro-mechanical and volumetric losses. The differences between the new methodology and ISO 4409 become larger for high load pressures. The new methodology demands knowledge about the minimum volume of the displacement chamber. The ratio between this volume and the full displacement of a single displacement chamber strongly influences the hydro-mechanical and volumetric losses of the pump or motor. The new methodology is valid for all positive displacement hydrostatic pumps and motors. The volumetric efficiency, as defined in ISO 4409, can still be used as a flow rate factor, but should not be regarded as an energy conversion efficiency. The importance of adopting the proposed methodology is further demonstrated by analyzing and comparing the measurement data about a fixed displacement pump and motor, showing the differences in the loss analysis by means of ISO 4409 and the new equations. The methodology, observations and validation results presented in this paper are significant and can pave the road for improving the current ISO 4409:2007 standard, which would ultimately benefit the industry.

The increasing focus on energy efficient systems and energy recuperation functionalities calls for multi actuator hydraulic systems to be tightly integrated with regards to energy distribution. Such systems need to allow power to pass back and forth between loads and the supply, while also enabling the ability to store energy. Here two approaches are obvious; namely the usage of hydraulic transformers interconnected via common pressure rails containing accumulators, and variable-speed pumps interconnected via a common electric dc-bus containing capacitors and/or batteries, both having the potential for energy storage and power sharing. A main question is when to apply which of these technologies, when considering specific requirements to actuator dynamics and control features. This paper presents an investigation into the analogy between hydraulic transformers and variable-speed pumps. The investigation takes offset in model based analyses of these technologies regarding their actuation dynamics related to torque generation, input-to-output pressure dynamics with and without a cylinder load. Finally a numerical study is presented, verifying and comparing their transient characteristics and energy losses. It is found that, disregarding the conventional speed control loop in variable-speed pumps, these are in fact equivalent with hydraulic transformers regarding secondary control abilities, dynamics and energy recovery, wheres hydraulic transformers generally must be expected to be more efficient than variable-speed pumps.

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 [1]. 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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In search for sustainable and clean solutions, the hydraulic industry is forced to develop more efficient alternatives to traditional systems. For mobile applications, battery driven machines are becoming an essential solution. But, electric driven hydraulic systems set completely different demands than classical systems. Since batteries are expensive and bulky, it is no longer acceptable that the majority of the battery stored energy is lost in the hydraulic system. One of the promising solutions for efficiency increase is the application of electrohydraulic actuators (EHAs). Aside from all the inherent control advantages, EHAs deliver energy to each load on demand. This makes them much more efficient than current valve controlled systems, at least in principle. In practice, EHAs require both low and high-speed operation of pumps. Almost all hydrostatic pumps have high friction losses, strong wear and often also high volumetric losses at speeds below 500 rpm. Additionally, it is obvious that the pumps must have the highest efficiency possible. Given these constraints and demands it is understandable that information is needed about the performance of pumps and motors. In the past years, Innas has measured and tested several positive displacement machines and published a comprehensive report about these measurements. This paper will analyse the outcome of the test results, with a special focus on the application in EHAs.