of lubrication. Since these passages are usually rather small,  they are easily clogged. pump inlet when the outlet pressure exceeds a predetermined  limit.  [Figure 6-32]  The regulating valve can be adjusted, if  needed, to bring the oil pressure within limits. Also shown  is the shaft shear section that causes the shaft to shear if the  pump gears should seize up and not turn. The gerotor pump, like the gear pump, usually contains  a single element for oil pressure and several elements for  scavenging oil. Each of the elements, pressure and scavenge,  is almost identical in shape; however, the capacity of  the elements can be controlled by varying the size of the  gerotor elements. For example, the pressure element may  have a pumping capacity of 3.1 gallon per minute (gpm) as  compared to 4.25 gpm capacity for the scavenge elements.  Consequently, the pressure element is smaller since the  elements are all driven by a common shaft. The pressure is  determined by engine rpm with a minimum pressure at idling  speed and maximum pressure at intermediate and maximum  engine speeds. There are several types and locations of filters used for  filtering the turbine lubricating oil. The filtering elements  come in a variety of configurations and mesh sizes. Mesh  sizes are measured in microns, which is a linear measurement  equal to one millionth of a meter (a very small opening).  A main oil strainer filter element is shown in  Figure 6-34 .  The filtering element interior is made of varying materials  including paper and metal mesh.  [Figure 6-35]  Oil normally  flows through the filter element from the outside into the  filter body. One type of oil filter uses a replaceable laminated  paper element, while others use a very fine stainless steel  metal mesh of about 25–35 microns. Most filters are located close to the pressure pump and consist  of a filter body or housing, filter element, a bypass valve, and  a check valve. The filter bypass valve prevents the oil flow  from being stopped if the filter element becomes clogged. The  bypass valve opens whenever a certain pressure is reached.  If this occurs, the filtering action is lost, allowing unfiltered  oil to be pumped to the bearings. However, this prevents the  bearings from receiving no oil at all. In the bypass mode,  many engines have a mechanical indicator that pops out to  indicate the filter is in the bypass mode. This indication is  visual and can only be seen by inspecting the engine directly.  An antidrain check valve is incorporated into the assembly to  prevent the oil in the tank from draining down into the engine  sumps when the engine is not operating. This check valve is  normally spring loaded closed with 4 to 6 psi needed to open it. A typical set of gerotor pumping elements is shown in  Figure 6-33 . Each set of gerotors is separated by a steel plate,  making each set an individual pumping unit consisting of  an inner and an outer element. The small star-shaped inner  element has external lobes that fit within and are matched  with the outer element that has internal lobes. The small  element fits on and is keyed to the pump shaft and acts as a  drive for the outer free-turning element. The outer element fits  within a steel plate having an eccentric bore. In one engine  model, the oil pump has four elements: one for oil feed and  three for scavenge. In some other models, pumps have six  elements: one for feed and five for scavenge. In each case,  the oil flows as long as the engine shaft is turning. The filters generally discussed are used as main oil filters;  that is, they strain the oil as it leaves the pump before being  piped to the various points of lubrication. In addition to  the main oil filters, there are also secondary filters located  throughout the system for various purposes. For instance,  there may be a finger screen filter that is sometimes used  for straining scavenged oil. These screens tend to be large  Filters are an important part of the lubrication system  because they remove foreign particles that may be in the  oil. This is particularly important in gas turbines as very  high engine speeds are attained; the antifriction types of ball  and roller bearings would become damaged quite rapidly  if lubricated with contaminated oil. Also, there are usually  numerous drilled or core passages leading to various points  Figure 6-34.  Turbine oil filter element.  Figure 6-33.  Typical gerotor pumping elements.  Turbine Oil Filters 6-24