Quarter Scale Merlin V-12

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Terry
I am not a pro but the one piece bearing I'm woried about. The two pistons move a little bit and a solid bearing would hold it tight. Or am I looking at it wrong?
Nelson
 
Nelson,
I'm sorry, but I don't quite understand your question. Could you please elaborate a bit further? The two rods are free to rotate with respect to each other if that is what you are asking. - Terry
 
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Terry
I am not a pro but the one piece bearing I'm woried about. The two pistons move a little bit and a solid bearing would hold it tight. Or am I looking at it wrong?
Nelson

I think what he is looking at, is there are two con rods on a single bearing set, how will the rods move independent of each other?

Cheers
 
The bearing is gripped by the fork rod, and so the bearing and fork rod as a pair are free to rotate on the rod journal. The blade rod, on the other hand, is free to rotate on the bearing. Therefore, the blade and fork rod are free to rotate with respect to each other. Because of the angle of the opposing cylinder banks, though, the fork and blade rods really only have to move over a very limited range with respect to each other. Hope that makes it clearer. -Terry
 
Mayhugh1, thank you for the reply, and clearing this up, just that in post #459, 7th photo, it showed the bearing would be a tight fit on the single rod, then in the 8th photo it showed the bearing shells fitted to the forked bearing.
Cheers
 
Hi Terry:

Regarding the circularity of the bearings:
As with the rest of your superior workmanship you have thought this issue out further than most anyone else would have. As you point out the result is what counts and you have proven it out.
Amazing work.

Thanks

Sage
 
Terry
I am more lost now than what I was earlier. If the center rod is going to rotate any amount. When it comes to rotation with the bearing on the crank is the normal thing, but to rotate the rod end on the outside of the bearing, that movement no matter how small will have negative results and the rpm that motor spins at how long will it take to gald and lock up. I think it will hurt to see that thing self destruct.
Nelson
 
Hello, Henry Ford used similar type bearings in his V8 engines from 1932 to 1942. How many thousands of those motors were sold and performed. Norm
 
I think Nelson is worried that there is no lubrication being supplied to the outer surface of the rod bearing for the blade rod other than some splash type lubrication. I had a similar thought when I first saw the design.

The bearing stays in place on the fork rod, and is lubricated against the crank journal which is like a conventional engine and oil flows through the crank providing oil to that surface, but it appears there is no such system for the blade rod.
 
I suppose I could drill a hole through the two bearing halves. This would provide a path for pressurized oil from the rod journal to reach the interface between the bearing and blade rod. In the current design, the pressurized oil from the rod journal will flow out around the ends of the bearing, and some will inevitably be wicked in between the bearing and blade rod. In addition, there will be splash from windage.
Although I had my own misgivings about the blade rod's lubrication, I reasoned it was no different from the problem of lubricating the rods' small end bearings. In a model engine these usually rely only on splash lubrication, and many designs don't even include a dedicated bronze bearing. I've seen some model engine rod designs using an end-drilled hole at their small ends to help oil entry, but many do not. I'm pretty sure I read someone's convincing analysis that concluded its small benefit wasn't worth the stress riser it could introduce on a typical model engine rod end which typically has a marginal amount of material around the wrist pin bore. -Terry
 
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Mayhugh1, Your work is brilliant and I bow down to your ability, With the blade rods, maybe a 1/32" or 1/16" hole in the bearing shell now will avert any possible problems with the rods further down the road, will keep watching as you progress with the build on this masterpiece.

Cheers
Andrew
 
steel crank moving on bronze bearings = OK

Aluminum rod moving on bronze bearing........not so much

I suggest every rod has its own bearings so they stay in a fixed position in the rods but move freely on the crank, yes that means 1 bearing set becomes 3 sets, but will work better in the long run.

Or try to build as the real rods were, more difficult but will prevent the possible failures you may have down the road with aluminum moving over bronze.

http://www.enginehistory.org/phpbb/viewtopic.php?p=562&sid=36c9010b4523bceaaa2899b3971040ac has information on the real rod design, notice the inner bearing / outer bearing system they had.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&ved=0ahUKEwjajvyauvbPAhVB6CYKHUiMC5cQFghVMAk&url=http%3A%2F%2Fwww.enginehistory.org%2Fmembers%2Farticles%2FCrankpinBearings.pdf&usg=AFQjCNH-gJWgo8p6k7t_SwjfTgOR7tkPig&cad=rja is a PDF on how the bearings were developed.

These guy's http://www.51-factory.com/merlin_overhaul.html would be able to help more as they are the experts on the real deal.
 
IceFyre13th,
Thank you for the references and comments. I had briefly thought about adding a second bearing for the blade rod because from an engineering perspective it just felt better. Because of limited clearances around the rods, though, it would have been necessary to remove material from the blade rods in order to make room for the bearings. And, the blade rods already look a bit anemic to me.

I totally agree with you that putting (6061) aluminum against a bronze bearing doesn't feel right. The Brinnel hardness of 6061 is 65 while the 936 bronze I'm using has a hardness of 60. There's essentially no difference between the two, and so the two parts would essentially wear each other out instead of one 'wearing in' to the other. However, I used 7075 aluminum for the rods, and it has a hardness of 150. My justification for staying with the stock design's single bearing was that this difference would probably be adequate for the bearings a model engine. I admit I have little to back this up especially since there are so few running examples of this engine, and even for those I don't know what modifications the builders might have made. - Terry
 
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IceFyre13th,
Thank you for the references and comments. I had briefly thought about adding a second bearing for the blade rod because from an engineering perspective it just felt better. Because of limited clearances around the rods, though, it would have been necessary to remove material from the blade rods in order to make room for the bearings. And, the blade rods already look a bit anemic to me.

I totally agree with you that putting (6061) aluminum against a bronze bearing doesn't feel right. The Brinnel hardness of 6061 is 65 while the 936 bronze I'm using has a hardness of 60. There's essentially no difference between the two, and so the two parts would essentially wear each other out instead of one 'wearing in' to the other. However, I used 7075 aluminum for the rods, and it has a hardness of 150. My justification for staying with the stock design's single bearing was that this difference would probably be adequate for the bearings a model engine. I admit I have little to back this up especially since there are so few running examples of this engine, and even for those I don't know what modifications the builders might have made. - Terry

To me, and being "small scale" I would just split the single bearing into 3 bearings.....leave < 0.005" clearance or so between the "rod" and "blade rod" then you will be steel on bronze for all. Also you wont be running the split of the rod cap past the split of the bearing, something that may accelerate galling.

And like you, nothing to back up this idea.......it just sounds more "right" to me.

I still cant wait to see this marvel running!!!!!
 
I would also be concerned about galvanic corrosion between the aluminium and the bronze. (Brass and aluminuim are particularly bad - ask anyone whose used those pretty aluminium tyre valve caps on the brass tyre valve stems - after a couple of months they are never coming off).

In an oiled environment its not that big a problem as the oil inhibits current flow but if the engine stands for long period with "acidified" used oil it will tend to generate stains at point of contact.
This is mild corrosion and I'm not sure of the long term implications.

I would be tempted to replace the oil any time it becomes significantly discoloured.

At very least I would keep a careful eye on it.

Thanks for your impressive documentation that allows all us awe struck mortals to follow your awesome project.

Can't wait to hear it burble into life.

Regards,

Ken
 
The wrist pins were machined from a length of 5/16" diameter O1 drill rod. I used the same drill, reamer, and holder to machine the holes for the wrist pins in both the pistons and the rods so their diameters would all be identical. From a couple different lots of drill rod that I had on hand, I selected a length that was a close slip fit in the bores. The wrist pins float and can move side-to-side in their cylinders, and so the liners need to be protected from them. The centers of the pins were drilled out, and a soft aluminum rivet was pressed into each end in order to buffer them from the liners. I also drilled tiny air escape holes through the centers of the rivets before they were installed.

The pins were hardened before the rivets were pressed in place. I've created extra work for myself in the past by hardening cylindrical parts after they had already been carefully fitted to their mating bores. So, I ran an experiment on a trial pin before launching the dozen or so parts I'll eventually need. After all its machining was completed, the diameter of the annealed wrist pin measured .3126" at room temperature. The pin was then placed in a triple-folded stainless steel envelope filled with argon and heated for 45 minutes at 1475F. After quenching, and a return to room temperature, the pin's scale-free diameter had increased by a whopping six tenths to .3132" and would no longer fit any of the bores. After a one hour temper at 450F and an overnight return to room temperature, the pin diameter remained at .3132". I had to polish a half thousandth off the pin's diameter to re-fit it. I searched through my stock of drill rod once more and found another length that measured .3122" that, in its annealed state, had a loose fit in the bores. I used it to make the rest of the wrist pins. After hardening, I still had to polish off a couple tenths, but polishing left the pins fitting perfectly and with nice bearing surfaces.

The amount that heat treatment affected these particular pins was dependent on a number of factors including their size, form, material and, most likely, a number of factors involved with the quench. Looking back on my notes for the 18 cylinder radial I built, I remembered that its O1 tappets had grown only three tenths from an identical heat treatment.

Assembling the heads to the cylinder blocks and then installing the combinations onto the crankcase will be a complicated step that I hope to perform only once. The original Quarter Scale design would have allowed the rods with their attached pistons to be installed down through the tops of the cylinder blocks before the heads are installed, which might have made the process a bit easier. Because of my liner modifications, though, the rods' big ends will no longer fit through the liners. Instead, each cylinder block will now have to be inserted down over 14 studs and all six rods and pistons simultaneously after they've been pre-assembled to the crankshaft. An online ancient Rolls factory video shows the full-scale engines also being assembled this way.

My plan is to prepare everything I possibly can ahead of time to make this assembly go smoothly, including locating and clearing any binds or interferences beforehand. I've had a major concern about potential errors between the axes of the rods and cylinders that could cause piston scuffing or, even worse, binding. The Merlin's design which separated the cylinder blocks from the crankcase is a result of reliability issues discovered during testing of its original uni-block design. There was no time in its wartime production schedule to fix the problems associated with the original design, and so the Rolls engineers decided to break up the block into three separate castings. This decision added considerable complexity to the engine's manufacturing, but it evidently solved the reliability issues.

In the Quarter Scale, this three casting combination resulted in some dozen machining operations in as many different setups that affected the alignments of the rods in their cylinder bores. This issue first became a real concern of mine over a year ago while working on one of the machining setups for the crankcase. When I realized how errors from that setup, as well as from so many other set-ups to come, could stack up to affect this alignment, my hiatal hernia woke up.

With the rods, pistons, and wrist pins completed, I could finally begin checking the rod alignments. An important first test was to install a pair of rods and their liners, one opposing pair at a time, so I could individually check the smoothness of motion of the reciprocating piston pair as the crank was rotated. For this test I torqued the main bearing caps to their final values, and I also installed the main cap cross studs which were made much earlier during the crankcase machining. In order to secure the liners in their registered positions on the crankcase, a couple simple Delrin fixtures were constructed. The same pair of fixtures were usable in all six cylinder locations. To my great relief, I couldn't detect any rod issues in any of the cylinders.

The installed rods were checked by carefully watching the un-ring'd pistons for any twist or side-to-side rocking motion while feeling for any bind in the crank was as it was slowly rotated. Then, with the pistons at various depths in their cylinders, the rods were moved back and forth axially on their journals to the limits of their clearances while the small ends were watched for the same motion on their wrist pins. These tests weren't quantitative and they didn't include the cylinder blocks, but I came away feeling a lot better about the crankcase and crankshaft machining. A similar test will be repeated later with all the rods, pistons, and cylinder blocks installed before the heads are added.

The two head/block assemblies will eventually be secured to the crankcase with twenty-eight 8-32 studs. I was lucky to find a package of 50 four-inch long studs on a Cabin Fever vendor's table earlier this year. The packaging claimed these commercially made studs were fabricated from 60 kpsi steel, and their threads rolled. In addition to being shortened a bit, the center three inches of each stud had to be reduced to 1/8" diameter in between its threaded ends. The clearance this operation provides will allow the head/block assemblies to more easily slide down over them during assembly and reduce the difficulties involved with trying to simultaneously register all six liners and fourteen oil seals to the crankcase. It was possible to turn down only about a half inch at a time on each stud before repositioning it in the lathe's chuck. This tedious operation on the whole lot of studs required several days, including lots of procrastination time, and was one of the least fun parts of the build.

All the rods were finally assembled to their journals using full length steel rod bolts and the custom washers made earlier. Before installing them, I drilled holes through each rod bearing pair so that pressurized oil from the rod journals could better reach the blade rods. I drilled a single hole in the fork cap bearing shell near the journal hole and a pair of escape holes through the opposite bearing half. After digesting everyone's comments and doing some more research, I think these holes and the use of 7075 for the blade rod is a good compromise and should probably have been part of the original Quarter Scale design. During my research I found an online drawing showing similar oil holes in the full-size Merlin's bearings. Finally, the cylinder block studs were threaded into place in order to verify their fits in the cylinder blocks. - Terry

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After quenching, and a return to room temperature, the pin's scale-free diameter had increased by a whopping six tenths to .3132" and would no longer fit any of the bores.

Beautiful work as usual, Terry.

I'm glad you mentioned this issue because these fit-up subtleties are a source of head scratching (at least for me). I experienced the exact same post-hardening wristpin growth & suspect even more variation among my samples which I chocked up to my primitive torch & dunkaroo methods. Anyway, my nice sliding (annealed state) wristpin fit was no longer.

My first inclination was to lap the wristpins back down to size because they could be counted on to always grow. The plan was drill all pistons/rods identically. That lapping test turned out to be fussy work. They are short with not much grip area for the collet in order to lap the entire length eventually by flipping them around.

On a scrap (7075 Al) connecting rod I went a different route & lapped the hole with a piece of very mildly tapered drill rod to match the wristpin as a set. That actually had a nice fit & was pretty easy to do, but of course a micro bore angle. But it was more of a trail towards getting a needle lap when the time comes.

Then I got thinking about my RC engines, Many times I have had to mildly heat the piston to expand & the wristpin would then push through. I used to think it was gunk factor or maybe even distorted in duty. But I've also had brand new assemblies where this is the case. They slide nice on a brand new rod, but are much tighter in the piston. Maybe a function of those high revving designs, they want them locked down?

But I'm curious now with your tooling comment. You want the same sliding fit between piston/pin and pin/rod? What advantage does a floating wristpin within the mating piston hole provide?

10-28-2016 0003.jpg
 
Peter,
From a functional view, it would probably be better to have a slip fit on the rod and a light press fit on the piston, but doing it twelve times requires consistency I'm likely not capable of. In order to be certain the pin doesn't work itself sideways when the piston heats up, the fit would need to start out on the tight side. Then there's the issue of assembly/disassembly. I'll likely separate the two parts several times for one reason or another before they're installed for the final time, and so there's risk of damage if the fit is too tight. A 'slip fit' is a little more forgiving machining-wise than a 'light press fit'. - Terry
 
I remember a post on another site where it was claimed that most drill rod is not perfectly round but likely to be tri-radial, a relic of the manufacturing process. If that is true, you'd likely want to polish off a few tenths regardless.

I can see the first-run date coming soon. :)
 
...where it was claimed that most drill rod is not perfectly round but likely to be tri-radial

Interesting. That's consistent with what I discovered on some O1 drill rod. Maybe it varies by manufacturer, but I could make out 3 shiny spots after the slightest of lapping on new rod. I guess the typical +/- specs refers to equivalent largest diameter maybe?

Anyway, it made me think hard about fits like this. You have to start somewhere & decide where the adjustment will occur. If the drilled/reamed hole is the standard based on tooling, then pin has to compensate. If the pin is the standard, then the opposite is true. I'm not convinced that lapping the piston hole in my case might not be easier, but like Terry was mentioning, every situation is different.
 

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