Bentley BR2

[John Roberts]

Attached are some photos of 1/4 scale my master rod assembly. One can see the difference in construction. I chose to use shielded bearings which I hope will be OK.

Regarding the involute springs you used , do you have the specifications on these. That would be very helpful.
I have obtained some main spring which I may try to make , but don't hold much hope getting a consistent hardness.

 

[joerom]

John,
I have not made my springs yet..Mine is the 1/3 scale one, and the springs would be very different..Have you tried to find something ready made that would be about what you think you need...
My master rod was already made..I am just modifying it to do what I want it to do..As far as the bearing being shielded or not, I have seen engines using shielded and engines using not shielded, so I am not sure which would be better..I guess it is all an experiment..Also, I am going to try a different oil passage on the rod journals..Nothing is etched in stone on building an engine..You just do what you think may work best, and give it a try, because that is what this is all about anyway...…
I just want everyone to be sure of understanding what I am doing..I purchased two part built engines that were representative of a BR2..I am in the process of trying to make one more closer to scale than was intended by the original builder, and in doing so, I am making major changes in a lot of things, and it may seem like I am building an engine......But, I am only doing this on one engine...………...

 

[petertha]

The bearing shield is a good question. On RC engines its quite common to remove both shields for fully internal bearings or just leave the outer shield on an external facing bearing. But those engines typically have oil premix in the fuel so the coating is pretty much everywhere. I think another consideration is that the fuel can act as a solvent & rapidly dissolve the bearing grease anyways (probably creating more issues if it gets up into the glow plug). At least I think that's the case of methanol fueled engines. I haven't taken apart many gasoline model engines to know if they adopt this or to what degree.

 

[joerom]

petertha,
That is a great reply for what I have been saying, and that is just do what you may think will work, and see what will happen..I would bet that you would get three different replies from 3 different people..I have always listened, and then taken what I think I could use, and then did my own thing...It is all good...……………….

 

[John Roberts]

Yes, grease getting flushed out by fuel mixture was also my thought. I may remove the shields in time.
More photos of my build to date.

 

[58tux]

John

Looks like you are making good progress. Your work is bring back a lot of memories of when I built mine.

I used open bearings on my BR2.

Rich

 

[joerom]

I am still here and working on the BR2..The things that I am doing now are just things to put everything back to normal because of all the changes I made..Things like relocating oil passages and finishing things that were either shortened or redone, re-tapping holes which would not lend to more pictures..I hope to soon get started on finishing the cams, and that would get back to doing pictures again..
These engines are beautiful pieces of machinery, but when you have to make 9 of everything, it seems to take forever...……..

 

[mayhugh1]

petertha,
That is a great reply for what I have been saying, and that is just do what you may think will work, and see what will happen..I would bet that you would get three different replies from 3 different people..I have always listened, and then taken what I think I could use, and then did my own thing...It is all good...

I'll add the third reply. Another consideration is metal contamination in open bearings. As the engine 'wears in' and that includes the rings, there will be metal circulating (or, in the Bentley, suspended) in the oil, and the shields can help keep it out of the bearings. The engines we build with oil pumps seldom have oil filters, and so it's probably a goid idea to at least include a magnetic drain plug to capture the ferrous debris. - Terry

 

[joerom]

My next project which will be the Gipsy Major, has an oil pump and filter included in the build..

 

[HWF]

In the case of the BR2, the oil is not recirculated. Hopefully the metal contaminants will be exhausted along with the spent fuel and oil.

I suspect that is was the intent of the original design that all of the parts be bathed in an oil fuel mixture which is why an oil that is not soluble in gasoline was used.

Therefore I would probably opt for unsealed bearings.

 

[58tux]

As far as using shielded or unshielded bearings it these models I do not think it makes much difference. Most of the model we build do not get run for that many hours.
I used all unshielded bearings in my BR2 except the front support bearing.
TBO for a BentleyBR2 was only 40 to 50 hours.

Terry, how much time do you have running your Merlin or your 18 cylinder radial?

Rich

 

[petertha]

I think micro metal shrapnel is real. Some other RC specific factors I failed to elaborate on was

- shields, at least the more common urethane coated ones that always seem to correspond to our bearings, were considered a bit of extra drag on HP engines (real or imagined). Or at least if they were distorted & were not reinstalled properly. The front shield of front bearing is always left on. That's where runway crap always gets in. Or a 'deep loam massage' on an unlucky day LOL

- I suspect the bigger reason relates to proactive bearing maintenance. High grade methanol & probably nitro too are very hyrgoscopic which means rust. Many of the common synthetic oils in these fuels have a nasty habit of creating acids, so even more corrosion. You can see stained metal where a small puddle line of unspent fuel was in the crankcase within a week. It is very common to liberally add after run oil through the plug(s) & carb & turn over the prop. Then always drain before starting the engine so do chance of hydraulic lock given the low squish clearance. People have their own recipes, but for winter storage especially in humid climate, the engine is liberally coated with something like pneumatic oil & stored in a ziplock. Looks kind of like an oily marinating steak but it does work. The other thing is (at least in HP circles) usually you don't wait for a bearing to growl, because any scoring that occurs on the (ringless) piston liner usually means bye-bye power or worse. Bearing replacement is cheap by comparison so its common to just pull them after X runs.

 

[joerom]

Back to machining..I did start on the cams for the BR2..In the picture below , that is what I received with the engine minus a few holes..The cams were also not sized for thickness..

I next trimmed the inside of the ring gears below..They are still oversize, and they will be finished when the cam gears are done..

The next thing I did was to machine the one side of the cam gear for the tappets, shown below as it was being done...

And this is how it looks after finishing that operation...

Next I removed the excess metal from the cam gear below, and down to the right thickness.This is as far as I got and I need to finish the gears yet, and that is for another day...….

 

[Cogsy]

If bearing damage from small contaminants is potentially an issue, you can buy 'hybrid' bearings that have one of the rolling elements replaced with a ceramic rolling element. These ceramics are far harder than the raceways and most contaminants as well. During operation, the ceramic roller/ball will somewhat self-repair the micro damage caused by contamination by flattening the raceway and crushing/ejecting the contaminant as well. So the bearing will have a far greater life in a dirty environment than a standard type. When I was in the bearing industry these hybrids were available for many of the common sizes (although primarily metric as the vast majority of the bearings produced and used around the globe are metric).

 

[John Roberts]

Thanks to all that have commented on the bearing debate. Very interesting comments.
My engine is progressing slowly, currently making the Maniton shaft and crankshaft. I was pleased how my slot attachment worked. My intention is to screw the crankshaft into the crank and Loctite. Hopefully this will be strong enough and save me some tedious machining.

 

[petertha]

Neat!
- what is the brass stem coming out of the slotting housing?
- is the slotting tool something like drill rod & you hardened the cutting edge?
- about how much do you progress the cutter every new stroke?

 

[John Roberts]

The brass stem engages in a slot in the steel ram. The cutter was made from 8mm silver steel. Machined on the mill and hardened and tempered. The amount of stroke was about 0.001" each time. The slot was 2mm wide by 1mm deep in medium carbon steel.

 

[John Roberts]

I mean depth of cut for each stroke!

 

[joerom]

I am working on the cam gears again..The blanks that I got with the engine were not quite right for me, so I am making new ones that suit me better..They look like the ones pictured above, just a slight change in diameter, so I did not bother to photograph..I want to finish up the cam gears and rings, and also the crankshaft, and there is nothing actually new to show, even though I am still working on it...………
If I decide to do the other engine I have, I now know all the changes needed to make a more scale BR2 than what was originally intended, and it would be a lot easier to do so now...I think that it looks very much like a proper BR2 now, and at least for me, it is worth the extra work..The other way would have been to do it as it was, and I would have a rotary that looks loosely like a BR2, but with a lot less work..The machine work for the most part that had been done by the original builder was actually very good, I just don't think I would have been happy with that...….

 

[58tux]

Joe A word of caution. When you install the cam rings in the cam box, use a steel key. I used a brass key the first time and then got to make a second set of cams and cam rings when the brass key failed.