TM 5-4940-228-14 CHAPTER 9 REPAIR OF COMPRESSOR 9-1. General. The   air   compressor   is   a   four   cylinder   recip- rocating  type  with  a  capacity  of  80  psi  to  15  cfm. Reed type valves with unloaders are used to allow for continuous operation. Unloaders are controlled by the unloader pilot valve which maintains a pres- sure  range  of  65  psi  to  80  psi  in  the  pulsation chamber. 9-2. Operation. Consider a piston at the top of stroke and begin- ning to go down into the cylinder. As the piston is pulled downward by crankshaft rotation, a vacuum is  formed  in  the  cylinder  allowing  atmospheric  pres- sure  to  force  the  intake  valves  open.  Air  is  drawn through the filter, past the intake valves and into the cylinder. Discharge valves are held closed by air pressure in the pulsation chamber. When the piston starts  its  upward  stroke,  pressure  in  the  cylinder  ex- ceeds  intake  air  pressure,  forcing  intake  valves closed. As the piston continues upward air is com- Key to Figure 9-1: 1.   Pipe 2.  Drain  cock 3.  Bolt  (4) 4.  Pulsation  chamber 5.  Screw  (6) 6.  Filter  retainer  (2) 7.  Outer  filter  (2) 8.  Inner  filter  (2) 9.  Screen  (2) 10.  Bolt,  long  (4) 11.  Lock  washer  (2) 12.   Unloader   (2) 13.  Bolt,  short  (8) 14.  O-Ring  (2) 15.   Head   (2) 16.   Gasket   (2) 17.  Valve  assembly  (2) 18.   Gasket   (2) 19.  Nut  (12) 20.  Lock  washer  (12) 21.   Cylinder   (2) 22.  Gasket  (2) 23.  Lock  wire  (8) 24.  Bolt  (8) 25.  Piston  (4) 26.  Nut  (2) pressed  and  increases  in  pressure  until  it  exceeds pulsation chamber pressure. The discharge valve is forced open, allowing cylinder air to enter the pul- sation   chamber   until   the   piston   reaches   top   of stroke. As the piston starts down, cylinder pressure becomes  less  than  pulsation  chamber  pressure which closes the discharge valves and the process is repeated. Unloading occurs when pressure from un- loader  pilot  valves  forces  unloader  pistons  down against  the  intake  valves.  Since  intake  valves  are now  open  continuously,  cylinder  pressure  cannot build up enough to open valves. No air is pumped into the pulsation chamber. Internal lubrication is automatic by splash from dippers on the connecting rods.  Intake  air  filters  and  silencers  are  pad  type  at- tached directly to the cylinder head. Air cooling is by  fins  cast  into  cylinders  and  heads,  also,  pulley has cast blades which force air over fins 9-3. Disassembly. Use Figures 9-1, 9-2, 9-3, and 9-4 as guides and proceed as follows: 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. Bolt (2) Pulley Key Breather cap (2) Spring (2) Valve (2) Seat (2) Pipe plug Coupling Nipple Street elbow Dipstick O-Ring Bolt (5) Rear, carrier Gasket Oil seat Bolt (5) Front,  carrier Gasket Bearing cup (2) Bearing cone (2) Crankshaft Stud (12) Crankcase 9-1