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Figure  2-22.An  automobile  brake  system.
Figure 3-1.Saybolt viscometer.

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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CHAPTER 3 HYDRAULIC FLUIDS During the design of equipment that requires fluid  power,  many  factors  are  considered  in selecting the type of system to be used—hydraulic, pneumatic,  or  a  combination  of  the  two.  Some of the factors are required speed and accuracy of operation,  surrounding  atmospheric  conditions, economic  conditions,  availability  of  replacement fluid, required pressure level, operating tempera- ture  range,  contamination  possibilities,  cost  of transmission lines, limitations of the equipment, lubricity,  safety  to  the  operators,  and  expected service  life  of  the  equipment. After  the  type  of  system  has  been  selected, many  of  these  same  factors  must  be  considered in selecting the fluid for the system. This chapter is devoted to hydraulic fluids. Included in it are sections   on   the   properties   and   characteristics desired  of  hydraulic  fluids;  types  of  hydraulic fluids;  hazards  and  safety  precautions  for  working with,  handling,    and   disposing   of   hydraulic liquids;  types  and  control  of  contamination;  and sampling. PROPERTIES If fluidity (the physical property of a substance that  enables  it  to  flow)  and  incompressibility  were the  only  properties  required,  any  liquid  not  too thick  might  be  used  in  a  hydraulic  system. However,  a  satisfactory  liquid  for  a  particular system must possess a number of other properties. The most important properties and some charac- teristics are discussed in the following paragraphs. VISCOSITY Viscosity  is  one  of  the  most  important properties of hydraulic fluids. It is a measure of a  fluid’s  resistance  to  flow.  A  liquid,  such  as gasoline, which flows easily has a low viscosity; and a liquid, such as tar, which flows slowly has a   high   viscosity.   The   viscosity   of   a   liquid   is affected  by  changes  in  temperature  and  pressure. As  the  temperature  of  a  liquid  increases,  its viscosity  decreases.  That  is,  a  liquid  flows  more easily  when  it  is  hot  than  when  it  is  cold.  The viscosity of a liquid increases as the pressure on the liquid increases. A  satisfactory  liquid  for  a  hydraulic  system must  be  thick  enough  to  give  a  good  seal  at pumps,  motors,  valves,  and  so  on.  These  com- ponents  depend  on  close  fits  for  creating  and maintaining   pressure.   Any   internal   leakage through these clearances results in loss of pressure, instantaneous   control,   and   pump   efficiency. Leakage  losses  are  greater  with  thinner  liquids (low viscosity). A liquid that is too thin will also allow rapid wearing of moving parts, or of parts that  operate  under  heavy  loads.  On  the  other hand, if the liquid is too thick (viscosity too high), the  internal  friction  of  the  liquid  will  cause  an increase  in  the  liquid’s  flow  resistance  through clearances  of  closely  fitted  parts,  lines,  and internal passages. This results in pressure drops throughout   the   system,   sluggish   operation of   the   equipment,   and   an   increase   in   power consumption. Measurement of Viscosity Viscosity  is  normally  determined  by  measuring the  time  required  for  a  fixed  volume  of  a  fluid (at  a  given  temperature)  to  flow  through  a calibrated  orifice  or  capillary  tube.  The  instru- ments  used  to  measure  the  viscosity  of  a  liquid are  known  as  viscometers  or  viscosimeters. Several types of viscosimeters are in use today. The  Saybolt  viscometer,  shown  in  figure  3-1, measures  the  time  required,  in  seconds,  for  60 milliliters  of  the  tested  fluid  at  100°F  to  pass through a standard orifice. The time measured is 3-1







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