R975 Radial engines

Malcolm Towrie · #1 25-09-18, 06:47

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 · #2 25-09-18, 15:18

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 · #3 25-09-18, 17:07

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.

David Herbert · #4 25-09-18, 22:36

Thanks Perry,
It also says it in the operating section of the manuals for the various radial engined Sherman variants and of course private owners are really good at reading manuals and doing what they are told.....

David

rob love · #5 25-09-18, 23:22

So if a guy needs to overhaul one of these, is it doable, or does it need to be sent somewhere that specializes in the radials? The boss keeps talking about wanting to get the Sexton going. I's hard at this point to even know what the problems with the engine are....something initially caused it to be parked, and about 6 years back a volunteer tore out the magneto and all the ignition wiring. We just got back the magneto (untouched) after about 5 years of it sitting at the local air museum.

Any recommendations for rebuilders? We had company in Winnipeg (standard aero) who used to do all the Dakota engines a few decades back, until the air force retired them in 1989. Or are the C1 radials too oddball for a regular commercial rebuilder?

I think I saw in the US a company that said they do them for around the $30K mark. They warned that they were often fixing other rebuilder's mistakes.

Matthew Noonan · #6 26-09-18, 01:09

Speaking of R975 mods

https://imgur.com/a/cnnMRnk

Are these towers? with the repositioned exhaust?

David Herbert · #7 26-09-18, 01:39

There are other photos of Rams with this same modification. The engine is completely standard but the exhaust is routed out through the engine decks instead of the standard arrangement as built with two mufflers side by side just bellow the engine deck discharging through fishtails that are hidden behind the rear upper hull and discharge downwards.

The other photos that I have seen were gun tower conversions for towing 17pdrs and I had assumed that the relocation was to make hooking up easier but the photos Matthew has found appear to be ordinary Kangaroos so it may be related to the fact that all these Rams are equipped for the fitting of wading trunking. However the standard wading trunking did not require the relocation of the exhaust and was very similar to the radial engine Sherman design which never had the exhaust relocated.

David

Bob Phillips · #8 26-09-18, 01:39

Hi Rob,
I think your question about overhauling a radial might best be directed to Jesse as I recall he tackled the overhaul himself. Would welcome your comments Jesse. On a related note I think that with care and patience the overhaul might be much easier to do using the TM manuals as they are very detailed compared to aircraft OH books. There are a number of engine companies that specialize in radial overhauls. The price quoted (30K US) is not out of line. Last year I talked to a large company overhauling a pair of C4s, they needed a lot of work (including salvaging badly smashed up cylinders) and the offhand comments about cost put it much higher than the 30k number.

Malcolm Towrie · #9 26-09-18, 03:55

Quote:

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

Malcolm Towrie · #10 26-09-18, 04:12

We acquired a Sexton a few years ago and I pulled the R975 out of it and dismantled it. It was a total write-off, 5 of the 9 rods snapped. So the engine is quite an impressive static display in our museum now.

I would agree with Jesse. There's is nothing particularly high tech or mysterious about the design. But you would have to be SO careful with reassembly. I regret I didn't have the opportunity.

Malcolm