The floating cup principle is a new concept for hydrostatic pumps, motors and transformers. It features a large number of pistons, which enables a strong reduction of the pressure pulsations and fluid borne noise. The pistons are arranged in a double ring, back-to-back configuration, and are locked onto the rotor. Each piston has a separate, cuplike cylinder, which is floating on a rotating barrel plate. This article will discuss the consequences of the floating cup design on the power density of pumps and motors. For current axial piston units the trend is towards larger tilt angles of the barrel. In slipper type machines, the tilt angle can be as large as 21°, whereas in bent axis machines the tilt angle can even be increased to 45°. For a 24 piston floating cup machine, the tilt angle of each barrel is however limited to about 12°. The object of the article is to prove that the reduced tilt angle does not need to have a detrimental effect on the power density of the hydrostatic machine. After giving a brief description of the floating cup principle, the article will focus on the design aspects that limit the barrel tilt angle. After this, the main parameters that govern the power density are discussed. A comparison with a slipper type pump is made.
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