pressure regulating valve is included in the oil system on the  pressure side of the pressure pump. A regulating valve system  controls the systems pressure to a limited pressure within the  system. It is more of a regulating valve than a relief valve  because it keeps the pressure in the system within certain  limits other than only opening when the absolute maximum  pressure of the system is exceeded. The regulating valve  Figure 6-37  has a valve held against a  seat by a spring. By adjusting the tension (increase) on the  spring, you change the pressure at which the valve opens, and  you also increase the system pressure. A screw pressing on the  spring adjusts the tension on the valve and the system pressure. Some large turbofan oil systems do not have a regulating valve.  The system pressure varies with engine rpm and pump speed.  There is a wide range of pressure in this system. A relief valve  is used to relieve pressure only if it exceeds the maximum limit  for the system.  [Figure 6-38]  This true relief valve system  is preset to relieve pressure and bypass the oil back to the  inlet side of the oil pump whenever the pressure exceeds the  maximum preset system limit. This relief valve is especially  important when oil coolers are incorporated in the system  since the coolers are easily ruptured because of their thin-wall  construction. Under normal operation, it should never open.  Oil jets (or nozzles) are located in the pressure lines adjacent  to, or within, the bearing compartments and rotor shaft  mesh screens that trap larger contaminants. Also, there are  fine-mesh screens called last chance filters for straining  the oil just before it passes from the spray nozzles onto the  bearing surfaces.  [Figure 6-36]  These filters are located at each bearing and help screen out contaminants that could  plug the oil spray nozzle. Most turbine engine oil systems are the pressure regulating  type system that keeps the pressure fairly constant. An oil  Figure 6-37.  Pressure regulating valve.  Figure 6-36.  Last-chance filter before spray nozzle.  Figure 6-35.  Turbine oil filter paper element.  6-25 Oil Pressure Regulating Valve Oil Jets Oil Pressure Relief Valve Filter element Pressure regulating valve