because they can work at very high pressures. They are attractive for 
seawater applications, because leakage can be minimized through use of a 
long piston and piston grooves (based on the well-known labyrinth sealing 
principle) to produce a high pressure drop with limited leakage. An experi- 
mental high-pressure seawater pump (Figure 28) based on an axial piston 
design and using ceramic components has been developed by the Naval Ship 
Research and Development Center (Robbins, Schneider, and Mehnert, 1968). 
This pump may provide a basis for designing a suitable motor and pump for 
a seawater hydraulic system, although a more compact design for the motor 
would be desirable. 
The poor lubricating properties and corrosive nature of seawater 
limit the materials which are suitable for use in the construction of these 
pumps and motors to those which exhibit low friction, low wear, and low 
corrosion when wetted by seawater. One very promising material is the 99.5% 
pure alumina (Al,03) ceramic used in the development of the NSRDC seawater 
pump. This very hard material (resists 2,100 kg per mm?) exhibits good mechan- 
ical properties, low friction, and low wear when lubricated by seawater (Robbins, 
Schneider, and Mehnert, 1968). 
Although lubricated sliding systems are generally composed of a hard 
and a soft surface, these systems fail when adequate lubrication is not present. 
For unlubricated surfaces, two hard surfaces perform better than a hard and 
and a soft surface. Nonmetals, such as alumina, are better than metals because 
they do not spot weld and shear (Bowden and Tabor, 1950). Another approach 
is the use of self-lubricating materials. These materials may consist of a matrix 
for mechanical strength and a polymeric material for lubrication. One such 
material is the wood, lignum vitae (Lagally, 1967). Other possibilities include 
glass-filled Teflon sliding on stainless steel, Teflon fiber fabric and plastic alloys. 
However, self-lubricating plastics rubbing on hard surfaces usually show high 
rates of wear which may not be acceptable in critical clearance applications 
unless there Is a compensating mechanism. Another plastic exhibiting low 
friction in water is ‘‘Delrin,”” a polytetrafluoroethylene (PTFE) filled acetal 
resin (Anonymous, 1967b). This material has been used in the construction 
of small high-speed gears lubricated only by seawater (Glasgow and Bartilson, 
1968). 
One of the more common materials used in the construction of small 
hydraulic pumps and motors is aluminum. However, aluminum has been 
found to be unacceptable for use in a seawater hydraulic system (Anonymous, 
1967a; and Daly, 1969), at least as used to date. 
30 
