Rapco Vac Pump

ksandrew

ksandrew

Pre-takeoff checklist
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Oct 22, 2014
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1GA6 Dawson Georgia USA
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ksandrew
Hi All.

Has anyone on here replaced the shear shaft/coupler on the rapco vacuum pump. The way they are designed and made I think that time is what deteriorates them rather than hours of use.
My main pump failed and I replaced it with a pump that I had removed with only 380 hours on it but had been stored for 9 years. the coupler sheared right away on startup. my carbon impellers are in fine shape and moves freely.
The part number is Rapco RA03-002. $15.99 from Spruce.

Life is a barrel of fun.

Ken Andrew
 
Thanks for the info, I am adding that to my annual checklist, On my plane it is less than a one hour job to R&R and the cost is OK.

Life is a barrel of fun.

Ken Andrew
 
the coupler is deigned to shear at a certain force of drag. it is there to save engine damage when the pump goes. if the shaft shears, there is a problem in the pump, not the coupler. i have never seen a copuler fail that did not have a worn pump hooked to it.
 
Sounds like the pump was not centered correctly when it was installed.
 
unsafervguy said:
i have never seen a copuler fail that did not have a worn pump hooked to it.
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FYI: one of the couplers that failed which I was involved in was due to solvent deterioration and not the pump itself per the OEM after I sent the pump in. That is when I found out about the 6 year recommendation which I believe every pump OEM recommends. Cessna even came out with an SB on this back in the 90s.
 
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Bell206 said:
FYI: one of the couplers that failed which I was involved in was due to solvent deterioration and not the pump itself per the OEM after I sent the pump in. That is when I found out about the 6 year recommendation which I believe every pump OEM recommends. Cessna even came out with an SB on this back in the 90s.
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Exactly right. From http://www.rapcoinc.com/pdf/Service letters/Rapco Service Letter RASL-008.pdf

.....we read:

Compliance with this service letter is recommended to be accomplished every (6) years. The life of the urethane shear coupler can be shortened by number of cycles, heat, oil, and solvents. The Shear Coupler or Shear Coupler Kit must be installed by an authorized mechanic.

It was one item we didn't bother tracking in the flight school, since the pumps wore out long before they were six years old. Continental engines spin their pumps faster than Lycomings do, so they'll wear faster. Vane wear is usually at limits by around 1200 hours. We bought Rapco or Tempest since they had provision for checking vane wear. I don't know why anyone would buy a new pump without that. We checked the vane wear as per Instructions, like so:

The inspection procedure listed below should be accomplished after 500 hours time in service on all 200 series pumps and after 200 hours time in service on all 400 series pumps. After the initial inspection, the pump should be re-inspected after 100 hours time in service or annually (which ever occurs first).
 
Not sure if it’s the same model, but I have a pump from a O320E2D that I’m going to be listing soon. We removed the whole vacuum system and no longer need it.
 
The hardest part is that "@#%&! lower nut on the flange to r&r the pump. Those of you who have done this job know of which I speak.
 
Yep, they have special little wrenches for that. I made one that works pretty well.
 
Jdm said:
Yep, they have special little wrenches for that. I made one that works pretty well.
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What tools do you all use to make tools you use to make tools with? ...

sigh......
 
Last Rapco vacuum pump I replaced came with a magic wrench.
 
Ive taken apart a couple of pumps. Both had walls that looked like a wash boards where the vanes contacted them. I figured that the vane sitting in a valley in the housng would make it likley to shear the shaft. Or the shaft was just wore out and gave up the ghost. Im glad new technology is available now. The old vacuum pump served us as best it could
 
Scott MacMoyle said:
Ive taken apart a couple of pumps. Both had walls that looked like a wash boards where the vanes contacted them. I figured that the vane sitting in a valley in the housng would make it likley to shear the shaft. Or the shaft was just wore out and gave up the ghost. Im glad new technology is available now. The old vacuum pump served us as best it could
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When the vanes get short they start to stick in the rotor slots and will start to chatter across the bore, making those washboard marks. Any vanes that do that are worn way beyond limits as prescribed by Tempest or Rapco, which is the reason they have a means of measuring vane length. Sooner or later a worn vane cocks so much that it just jams and shatters the rotor. And it almost always happens in flight, of course. Nice. If it breaks on the ground because you turned the prop backwards, be grateful it busted then and not in IMC or on a black night. It was about to fail you.
 
If I had a worthless, dangerous dry vacuum pump I would never fly IMC until I replaced it with a wet pump which go thousands of hours trouble free. You can rebuilt wet pumps for about $700 no core..
 
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