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RAPCO Vacuum Pump Failure Mode - Info Only

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Bob Axsom

Well Known Member
I pulled my failed RAPCO Vacuum Pump yesterdayand found that the plastic coupling shaft between the drive gear and the pump body had broken. Both ends of the system (the engine input and the pump rotor) appear to be functioning properly. Info only.

Bob Axsom

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My Rapco pump broke the same way about 3 months ago. It looks like my coupling might have become brittle (yellow and almost crystallized) from engine heat. Sure didn't look like the new opaque white one that was on my replacement pump. I considered just replacing the coupling but there seemed to be some internal friction in the pump. Better to be safe and just replace the whole thing.

Karl
 
This coupling is intentionally the weak point in the system. The coupling is designed to shear to avoid broken gears locking up everything in the accessory case.
 
250 Hours on Mine

I had 250 hours on my pump. It was part of the complete RAPCO package offered by Aircraft Spruce. It was shipped to me on December 14, 2000. The first flight did not occurr until March 21, 2004. The operational characteristics were never great. I could never get a good vacuum reading at idle but during runup (after I adjusted the regulator) I got a good "5" (differential) on the gauge. No observed leaks but no good test of the system beyond the in panel gauge either. Not very comforting for IFR operation until proper gyro function was verified in visual flight conditions. It always worked but I always had a concern. On the day of the failure it was a local flight under 6,000ft overcast so no sweat but the AI rolled like a dead fish on climbout and the gauge read "0". The new pump is installed and I am adding the cooling shroud offered by Aircraft Spruce today. The shroud is STC'd for virtually all of the Beech, Cessna and Piper aircraft so I expect some real life enhancement. We shall see.

Bob Axsom
 
rapco

Well.... It's been my experience that Rapco pumps dont last very long. I dont think they replace the bushing. I rebuilt mine myself for about 150 in parts to include everything new except the ends of the pump.
The pumps will last a lot longer if you get some 600 grit paper and place it on a piece of glass. then remove the vains from the carbon wheel inside and sand the outer ends slightly. Then if you have a fresh bushing... the pump should last 12-1300 hrs. You are only allowed a certain amount of clearance on the inside wheel of carbon as well. This is something I would recommend for your IA to do.... not the owner.
Brian Wallis
Owner Wallis Aviation
AP/IA
 
OK Thanks for the good information

brianwallis said:
Well.... It's been my experience that Rapco pumps dont last very long. I dont think they replace the bushing. I rebuilt mine myself for about 150 in parts to include everything new except the ends of the pump.
The pumps will last a lot longer if you get some 600 grit paper and place it on a piece of glass. then remove the vains from the carbon wheel inside and sand the outer ends slightly. Then if you have a fresh bushing... the pump should last 12-1300 hrs. You are only allowed a certain amount of clearance on the inside wheel of carbon as well. This is something I would recommend for your IA to do.... not the owner.
Brian Wallis
Owner Wallis Aviation
AP/IA
Click to expand...

That is very good information Brian. I don't have an AI. I retired a couple of years ago at 68 after 50 years in aerospace and I built the airplane because I was tired of poor maintenance more than the cost actually and I really want to do it myself. For now I will just keep plugging in new vacuum pumps but this technology's days are numbered even in my airplane. Are you staying with vacuum pumps? Some of the new (well new to me) Tru Trak units with internal back up batteries are looking awefully good.

Bob Axsom
 
Bob Axsom said:
I had 250 hours on my pump. It was part of the complete RAPCO package offered by Aircraft Spruce. It was shipped to me on December 14, 2000. The first flight did not occurr until March 21, 2004. The operational characteristics were never great. I could never get a good vacuum reading at idle but during runup (after I adjusted the regulator) I got a good "5" (differential) on the gauge. No observed leaks but no good test of the system beyond the in panel gauge either. Not very comforting for IFR operation until proper gyro function was verified in visual flight conditions. It always worked but I always had a concern. On the day of the failure it was a local flight under 6,000ft overcast so no sweat but the AI rolled like a dead fish on climbout and the gauge read "0". The new pump is installed and I am adding the cooling shroud offered by Aircraft Spruce today. The shroud is STC'd for virtually all of the Beech, Cessna and Piper aircraft so I expect some real life enhancement. We shall see.

Bob Axsom
Click to expand...

Very similar to my situation. Purchased about the same time. First flight was July 2003, 175 hrs at failure. I also had (and still have) a very low reading at idle. I just assumed that was normal. In my case, the first indication I had of failure was my DG spinning like a top. It's funny that my first reaction was to look outside to make sure I wasn't really spinning like that.

This coupling is intentionally the weak point in the system. The coupling is designed to shear to avoid broken gears locking up everything in the accessory case.
Click to expand...

I'm very aware of the coupling being the weak point. The point I was trying to make is: It looks like the coupling may have been weakened by heat. It was yellow and crystallized. If it was weaked enough from the heat, it may have failed under normal conditions (may be nothing wrong with the pump). I chose the conservative route and replaced the whole pump.
 
New Pump seems better

The gauge regesters around a 3 at fast idle and went up over 5 in the ground check. I did not study the new pump in detail but there are some apparent differences like the socket screw in the side wall so hopefully the reliability has been improved.

Bob Axsom
 
Just for info... both of my pumps are Rapco 215CC. My understanding is they don't make the 211CC anymore. They also don't make a rebuild kit for the 215CC. And you can't buy overhauled 215CC pumps. Just new.

I also bought the cooling shroud but haven't installed it yet. If I get another early failure, I will probably go with a different brand next time... even if it costs more.

Karl
 
The shroud looks like a good idea

I have installed the shroud and it appears to be a good idea and a good high quality implementation of the concept. The shroud was a very good fit but I did apply the optional edge sealing to the shroud (Dow 736 - the red RTV) with a cut down icecream stick. I riveted the air pickup flange to the rear baffle over cylinder #3 and routed the hose to the shroud inlet on the right side of the pump. There was no way I could locate it on the opposite side of the pump from the ports as suggested. I also made up a backup plate that looks like a big washer, to sandwich the plastic air pickup flange between it and the baffle in the riveted installation. I put Dow 736 in the interface between the flange and the baffle. The rivets I used were AN470AD4-10s cut off using 4 spacer tabs on the Aircraft Spruce rivet cutter. The new 1.125" diameter hole in the rear baffle causes a little air speed regret but ... safety first.

Bob Axsom
 
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