R975 Radial engines

[Bob Phillips]

Piasecki helicopters;
hup25.jpg

you tube video, hopefully will work

www.youtube.com/watch?v=5gTyZ7grjeA

[Perry Kitson]

Here are a couple photo's of a new C4 supercharger impeller. Outside diameter is 8.875", 16 vanes, .750" 6 tooth spline.

In my opinion, the bane of the radial is the seperation of the two angular contact bearings on the supercharger impeller shaft. Angular contact bearings are meant to be used in pairs, as they are on the impeller shaft, BUT they are designed to be installed back to back to give the bearings the proper preload. Seperating the bearings on the impeller shaft necessitates very precise shimming of the housings so the bearings run neither too loose or too tight, as they run up to very high speeds.

[Bob Phillips]

Perry thanks very much for the impeller info! Looks a whole lot like the -46 impeller, does anyone know the significance of the holes through to the back?
As I mentioned earlier in this post tghere were weak points in the 975 engines, the impeller shaft/ bearings were certainly one. A second was the master rod/ crank bearing which often failed due to abuse or possible oil starvation. Any other known issues?
I also need to thank Perry for the opportunity to photograph his C4 cylinders next to a -46 cylinder. Please note -46 cylinder does not have an exhaust elbow attached.
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[David Herbert]

I would suggest that the holes in the impeller are to more balance the pressure on each side of it as without them it would draw itself towards the side with the vanes on it (as it sucks air from that side) causing high axial load on its bearings. A certain amount of recirculation will happen but it may be that that was judged to be the lesser evil.

In my experience the primary cause of failure of these engines (in preservation) is that people let them idle at too low a speed and the master rod to crank bearing fails due to oil starvation as most of the lubrication pressure of this bearing is from centrifugal force within the crank itself and that is negligable below 800rpm. They will idle nicely at 500rpm so people think that that is better - wrong !

David

[Malcolm Towrie]

I looked after about 1500 pumps in a nuke plant and I agree with Perry. Angular contact bearing pairs must be shimmed or matched so that the axially unloaded bearing (not the loaded) is still preloaded enough to ensure the balls roll and don't skid. Skidding causes nasty damage to happen. Modern Vortec blowers which spin up to thousands of rpm are supported by tiny angular contact bearing pairs and your best bet for a good rebuild is to send them back to Vortec, IMO. It's that tricky to set them up.

And I agree with David. The holes in the impeller back shroud are to reduce axial load on the shaft bearings. This is common practice in pump design. The holes bleed the discharge pressure seen on the back side of the impeller back to the suction side. This reduces the thrust load towards the suction side.

Malcolm

[Jesse Browning]

I learned from a pump manufacturer that the holes are to relieve the vacuume pressure, not to protect the bearings, but so that sewage (or oil in engine application) doesn’t get sucked past the seal.

[Perry Kitson]

David,

Good point on the low idle oil pressure issue. I was once told by a pilot with decades of experience flying radial engined aircraft, that after a proper warm up, RPM was kept no lower than 1000 RPM for that reason.

[Malcolm Towrie]

Originally Posted by Jesse Browning

I learned from a pump manufacturer that the holes are to relieve the vacuume pressure, not to protect the bearings, but so that sewage (or oil in engine application) doesn’t get sucked past the seal.

Gotta disagree, Jesse. As far as I know, those holes are to reduce axial load on the pump, or in this case supercharger, bearings.
What can cause oil getting sucked in past the shaft seal on an overhung impeller design like this is having the impeller back shroud too close to the supercharger rear housing. The tight gap between the shroud and casing causes a pumping effect which can reduce the pressure down at the shaft below atmospheric.

Malcolm