Examples of such a shaft can be found in U.S. Pat. No. 6,203,439 B1, U.S. Pat. No. 6,264,567 B1, or DE 199 59 836 A1.
Such a shaft is usually used as cardan shaft to transmit an orbiting and rotating movement of a first element of a hydraulic machine to a purely rotating movement of a second element of a hydraulic machine. An example for such a hydraulic machine is a hydraulic steering unit or a hydraulic gerotor motor.
Such a cardan shaft is often named “dog bone” because it has some similarities with a dog bone, i.e. a shaft section having a smaller diameter and two tooth geometries at both ends having a larger diameter.
As mentioned above, the cardan shaft is used to transmit an orbiting and rotating movement of a first element to a purely rotating movement of a second element. This requires that the cardan shaft must have the possibility to pivot with respect to both elements during one rotation. This pivoting movement is possible due to the form of the outer tooth curve and due to the form of the bottom curve having a rising slope from the outer end, i.e. from the end opposite said shaft section, in a direction towards the shaft section. In other words, the radius of the bottom curve increases starting from the outer end of the cardan shaft. The second end has a negative slope, i.e. the radius of the bottom curve is decreasing towards the shaft section.
When the cardan shaft is used in a hydraulic machine to transmit large torques the tooth geometry tends to wear.