## 2 Function, Types, Shapes and Hydraulic-Seal Materials  
The general function of a seal built in between two surfaces in a component is to seal an area of higher pressure against an area of lower pressure, of either liquid or gas. This generalised definition applies both to the sealing of two stationary surfaces or a combination of a stationary and a moving surface. Seal effectiveness is measured in terms of leakage. A leak-free seal through the contact surfaces must be ensured over the entire range of operating parameters, which is achieved both with the appropriate shape and material of the seal. The form of seals has developed gradually from the earliest forms of seals, just some space between two surfaces filled with sealing material, to today's more efficient forms of seals. The first advance in sealing technology was the development of V-shaped and U-shaped sealing rings (called U-Cups). These forms of seals are relatively effective, but they already require changes in the design of the sealing space. Additionally, a sealing effect is only in one direction of movement, and is only activated when a pressure is present. Thus, two seals are needed to seal in both directions. Apart from that, the operating pressure was also limited--in the case of U-Cups, to around 100 bar, and in the case of V-rings, to approx. 140 bar, which is a fairly low operating pressure for today's hydraulic systems. it was necessary to install a package, e.g., three or more V-seals, which requires a larger installation space for the seals. Further development in the field of seals led to ring-shaped seals, in the case of the most commonly used circular cross-section, called O-rings. Apart from the circular cross-section, as in the case of O-rings, ring-shaped seals also have other cross-sectional shapes: square rings, D-rings, X-rings, T-seals and cup seals--Figure 1. Due to their simple shape, standardisation of dimensions, and their simplicity and effectiveness in sealing, ring-shaped seals are indispensable today too, used for sealing stationary surfaces or at the low-movement speed of one surface--s.c. static seals. Although the O-ring has been the most popular design for static seals, square rings are more effective static seals because they provide a wider seal-contact area and thus a more effective seal effect; plus, they do not twist and roll when moving. However, due to the larger contact surface, greater friction occurs during movement, which is reduced by the other mentioned forms of ring seals. In the case where one surface moves relative to another, it is necessary to use a more suitable shape that ensures both good sealing and low friction. In this case, it is a question of dynamic seals, which, depending on the place of installation and, thus, the size, can be further divided into seals suitable for sealing pistons and piston-rod seals, as is the case with hydraulic cylinders. In the area of fluid power technology, different profiled seals are implemented in regard to application. These are mainly different forms of U-cup seals, in the version without or with a support ring, generally known as lip seals. Some examples of the usually very wide range and variety of different piston-seal designs as produced by specialist seal manufacturers are shown in Figure 2.  
In order to achieve an effective seal, not only is the shape of the seal important, but so is the material of the seal. Here, the compatibility of various seal materials with hydraulic fluids is at the forefront of this paper. The earliest seals were nothing more than stuffing boxes packed with waxed string or cord, leather strips, pieces of rubber, cotton, wool, or even paper. Later, rubber was used in the case of the O-ring. In these early examples of materials, there was no emphasis on material compatibility, as the fluid used was first water, and, later, mineral-based hydraulic oil.