TM 5-3895-373-20 1.20. TRACK DRIVE HYDRAULIC SYSTEM. The track drive system is a hydrostatic system consisting of two variable displacement propulsion pumps driven by the  engine  through  the  pump  drive  gearbox.    A  pump pilot  control  valve  with  integral  electronic  displacement control   is   mounted   on   each   propulsion   pump’s   pilot control  port.    The  pump  pilot  control  valve  receives  an electric  signal  from  the  control  handle  on  the  operator console gauge panel, which increases or  decreases  the output of the propulsion pump.  Each pump drives a two- speed,    variable    displacement    motor    connected    to    a gearbox with a 65:  1 reduction and built-in brake system. One gearbox is connected to each track drive.  Because both  tracks  are  identical,  the  following  applies  to  either track drive: The   control   handles   receive   12   VDC   from   the   circuit breaker  panel  (CB1).    Control  handle  position  in  either forward or reverse is tracked by an electronic potentiometer   on   a   circuit   board.      Within   the   control handle  are  two  cam-activated  micro  switches  which  are closed and opened when the control handles are moved out   of   neutral   detent.      When   the   control   handles   are moved out of neutral in either forward or reverse, a micro switch    closes,    passing    12    VDC    to    bias    the    output transistor   on   internal   circuit   boards.      The   signal   then passes   though   a   variable   resistor,   which   varies   the voltage  signal,  depending  on  control  handle  position  on the  potentiometer.  Output  voltages  from  3.6  to  1  volts control  reverse  propulsion,  and  voltages  from  4.0  to  7 volts  control  forward  propulsion.  This  voltage  is  sent  to the electrical displacement control valve on the propulsion pump, controlling pump output. The  propulsion  pump  is  a  variable  displacement  pump, supplying  a  constant  pressure  with  variable  flow  rates. The  output  from  the  propulsion  pump  is  determined  by the position of the control handle.  As the control handle is    moved,    an    increasing    voltage    is    applied    to    the displacement    control    on    the    pump.        As    the    signal increases,  the  displacement  control  changes  the  signal to  the  propulsion  pump.    The  variable  displacement  in the  pump  increases  or  decreases  the  tilt  (angle)  of  the swashplate   in   the   pump.      As   the   pump   turns,   the swashplate causes pistons to stroke in or out, forcing oil out of the pump. Hydraulic  oil  is  drawn  from  the  hydraulic  reservoir  into the  propulsion  pump  by  an  internal  charge  pump.    The charge  pump  is  a  fixed  displacement  pump  within  the hydraulic pump that supplies cool oil from the reservoir to the pump, and supplies  oil  to  operate  the  control  system.    The  charge pump  ensures  that  the  internal  oil  pressure  of  the  main pump stays constant at 350 psi (2400 kPa).  An internal relief  valve  controls  this  pressure.    The  charge  pump also    circulates    charge    pressure    oil    from    the    pump, through the charge filters, and back to the pump.  During operation  of  the  pump  and  motor,  a  small  amount  of hydraulic  oil  leaks  from  the  main  closed  circuit  and  is used   to   lubricate   the   rotating   parts   of   the   pump   and motor.    A  case  drain  to  the  hydraulic  reservoir  provides relief of this oil.  Hot oil flows from the low pressure side of  the  hydraulic  circuit  to  allow  for  cooling.    The  charge pump replaces any oil that is lost to the reservoir through the case drain. Because  the  propulsion  pump  and  motor  are  a  closed- loop   system,   no   return   other   than   the   case   drain   is required.  The main ports of the pump are connected by hydraulic  lines  to  the  input  port  of  the  motor.    Hydraulic oil  flows  in  either  direction  from  the  pump  to  the  motor and   then   back   to   the   pump.   Crossover   relief   valves protect  the  high  pressure  side  of  the  pump  by  limiting pressure to 5000 psi (34 500 kPa). The hydraulic oil from the propulsion pump is sent to the twospeed   propulsion   motor.      The   swashplate   in   the motor  is  limited  to  two  positions,  providing  two  speeds. As  the  oil  flows  through  the  motor,  the  volume  of  oil against the swashplates forces the pistons to compress, turning the motor. The displacement of the propulsion motor is determined by  the  two-position  speed  range  switch  on  the  operator gauge panel. With the speed range switch set to down, "tortoise" pave position,  the  charge  pressure  fluid  is  routed  though  the high speed shift solenoid valve to the variable displacement  motor.  This  sets  the  motor  swashplate  to 13°  for  maximum  displacement.    When  set  to  the  up, travel   "hare"   position,   charge   pressure   fluid   flow   is reversed in the high speed shift solenoid valve to set the swashplate at 7 1/2° for minimum displacement. The  propulsion  motor  is  coupled  to  a  speed  reduction gearbox at a 65:  1 ratio.  The output of the gearbox is attached  to  the  spring-actuated,  hydraulically  released brakes.  The brakes are sprocket-coupled to the paving machine  tracks.    When  hydraulic  pressure  is  removed from   the   propulsion   system   the   brakes   automatically apply, stopping the paving machine. 1-36