Vac pump - gradual loss of vac at low revs

[peter-h]

I have noticed, over the two pumps I have had, that there is a gradual loss of vacuum when the engine is at idle (700rpm; IO540-C4).

It starts to be noticed after say 200hrs. If the revs are raised even slightly, the vac is back in the green band and all is fine.

I wonder if others see this too.

The carbon vanes wear gradually and this must show up somewhere.

My first pump was changed at 700hrs at SB569-time, as a precaution, and the 2nd one is on 300hrs.

 

[gismo]

I have noticed, over the two pumps I have had, that there is a gradual loss of vacuum when the engine is at idle (700rpm; IO540-C4).

It starts to be noticed after say 200hrs. If the revs are raised even slightly, the vac is back in the green band and all is fine.

I wonder if others see this too.

The carbon vanes wear gradually and thisust show up somewhere.

My first pump was changed at 700hrs at SB569-time, as a precaution, and the 2nd one is on 300hrs.

Given their construction, I see no basis for a gradual loss of suction as the carbon vanes wear in a dry pump. The shortened vanes simply slide a bit further out of the rotor leaving a bigger gap inside the rotor where there is no airflow. It is possible that the rotor itself wears slightly providing the opportunity for air to leak around the vanes inside the rotor slots but I'd expect such leakage to miniscule compared to the "leaks" represented by the air powered gyros. That said I have heard that it's common for dry pumps to exhibit a noticeable drop in suction shortly before they fail although I've never experienced this myself.

I can see a related issue on a pressure system where the particle filter between the pump and the gyros begins to get plugged up with debris from the wearing vanes. It's even marginally possible that the filter on a suction based system might coincidentally become plugged as the pump vanes wear since they are normally changed at the same time.

Finally, for pumps that only see the relatively light duty of powering a gyro or two, heat and contamination are the chief killers. You can mitigate the heat issue by installing a cheap ($60) cooling shroud and for the contamination problem it's not a bad idea to plug any vacuum system openings and cover the pump when hosing an engine down with solvent.

 

[taters]

check the pressure regulator...if the filter is coal black you have likley found the problem(s). What ever you do dont have the pressure simply turned up..there is a system problem..usually the pump though...

 

[peter-h]

If the revs are raised even slightly, the vac is back in the green band and all is fine.

 

[kgruber]

What brand of vacuum pump?

For a "Sigma Tek" that kind of behavior is completely normal.

 

[Dan Thomas]

check the pressure regulator...if the filter is coal black you have likley found the problem(s). What ever you do dont have the pressure simply turned up..there is a system problem..usually the pump though...

In a vacuum system the filter doesn't get the carbon; it's blown overboard. The filter is upstream of the gyros and filters cabin air only.

Lycoming's vacuum pump drives run more slowly than Continental's. Vacuum fall-off at idle isn't unusual. When replacing a pump, get the Rapco or Tempest pumps that have the vane wear inspection ports in them. You get the max life out of the pump that way and aren't replacing it needlessly, nor are you risking failure due to extreme wear. When worn, the vanes get short and start cocking in the rotor, and the rotor's slots get wedge-shaped and lose their seal, too. If the vanes cock enough they'll stick and the rotor shatters. Busted rotors are bad; the residual vacuum volume inside the instrument cases can suck carbon bits and dust back into the gyros and wreck them.

Dan

 

[SCCutler]

My dry pump showed reduced pressure at low revs as it neared end-of-life; still had no problem delivering spec pressure until the day it just flat died, sheared is elastomeric coupling.

 

[Tom-D]

check the pressure regulator...if the filter is coal black you have likley found the problem(s). What ever you do dont have the pressure simply turned up..there is a system problem..usually the pump though...

Clogged filters in a VAC systems equal high vac readings, Leaks equal low readings.

Vac pumps are constant displacement type pumps, that means they displace the same volume for each revolution, the faster they spin the more volume per minute. you change the volume by changing the speed.

In this case I'd suspect a leak in the system that the pump can keep up with at a higher RPM.

If this aircraft has the old AN style gyros, they may need overhauling because the flow thru them is too high, remember pressure is the resistance to flow, no resistance, no pressure.