This is an airborne dry vacuum pump used on small aircraft. The round thing is called the "rotor". The vanes slide in and out of grooves in the rotor. The vanes push the air. Rotor and vanes are made from carbon. They are brittle.
Notice the extremely small clearance between the rotor and the housing. Any small particles that get between the rotor and housing will shatter the rotor.
Being carbon, there is a slight amount of carbon dust as the vanes wear. Any liquids will turn the dust into glue and your pump will slowly stop working. So how does liquid get inside your pump? Easy, just wash down the engine and spray water at the pump.
An oil leak at the back of the engine is another typical reason these pumps get contaminated with oil and fail. Water and oil enters through the drive shaft clearance.
Notice the remains of Teflon tape in the inlet port. Teflon tape caused this pump to fail because it got inside the pump and lodged between the rotor and housing. Nothing is used on the threads of dry vacuum pumps.
When you remove the failed pump, first thing is remove the fittings. You will need them for the replacement pump. Next, take a good look in the inlet and outlet port. Is it dry or wet?
If it is wet then your pump AND lines are contaminated. The replacement pump will also fail as the liquid is pulled into the pump. I remember one pump that had oil dripping out the port! The mechanic who removed the pump hadn't noticed.
Can I just buy a airborne vacuum pump repair kit with new rotor and vanes and rebuild my own vacuum pump?
There you go trying to beat the system and save money on aircraft maintenance. Now we get to sell you a kit, and then when that doesn't work, we get to sell you a pump - two sales in one!
Take a good look at the pump to the left. Notice the chatter marks in the cavity caused by wear. When the carbon van crosses these chatter marks it chips the van tip. Small bits of carbon then get stuck in the clearance between the rotor and housing and shatters the rotor.
Pumps that have lots of hours on them will have chatter marks in the cavity. So you need to evaluate the condition of the cavity and either purchase a new one along with the carbon or just replace the pump.
Picture of shear coupling. This prevents damage to the engine when the vacuum pump seizes.
When the carbon van wears too short it twists in the slot and breaks. How long does this take? It depends on the pump, on the aircraft, and on the envirnoment. Carbon uses moisture from the air as a lubricant. In humid climates the carbon vanes have lots of lubricant and have a low wear rate. In dry environments (Arizona) there is less humidity lubricant and the wear rate is greater.
The best method is to periodically measure the vane lengths. Newer vacuum pumps have a wear port so that you can measure vane length.
Shown to the left is a comparison of vane size between a dry vacuum pump and a Garwin G455 wet vacuum pump. The wet pump vane is the larger one. One reason why the wet pump has greater life expectancy.
The drive coupling on the wet vacuum pump may wear severely and spread hard steel particles throughout your engine if the pump is allowed to operate decade after decade. These shouldn't be run until they fail. Overhaul them at engine TBO.
This is what a a new wet pump drive coupling looks like. This coupling is designed to shear (break) if the pump freezes. This coupling is used on the Garwin G-455 and the Pesco 3P194F pump. Larger pumps for larger aircraft use a different coupling that shears at a higher load.
Blow out the inlet hose before installing a new vacuum pump.
When the pump fails small bits of carbon from the vanes and rotor are let loose inside the pump. With no vacuum pressure these bits can exit the pump and travel into the hose. If you install a new pump with bits of carbon in the inlet hose then as soon as the new pump starts these carbon bits are sucked into the pump. The pump rotor shatters when the pieces get lodged between the rotor and housing.
