Crankshaft for opposed twin 5cc CI aero engine

[Niels Abildgaard]

Hi Pat

In order to be able to produce and assemble the crankshaft in a straight line, I used an auxiliary mandrel.
(In German: "Hilfsdorn", google translater found only the words "auxiliary mandrel" for it.)

It is a steel bolt that has been precisely ground for a slight tight fit.

I didn't mark the connecting rods in the sketch, but they were of course installed.

For the assembly of the crankshaft, the bolt was used as a guide/alignment.
For the final machining (cylindrical grinding plain bearing seat) of the front and rear crankshaft diameters,
the crankshaft was clamped between centers on the cylindrical grinding machine.

Only when the grinding work was finished and the crankshaft was secured with pins,
the bolt was pulled out and the hole in the middle part closed.

Because the crankshaft was case-hardened,
I had to drill the holes in the assembled state with a solid carbide drill Ø 2 mm.
Then hammer in the pins and it was done.

Dieter
View attachment 155472
I used this type of locking pin (Ø 2 mm)

View attachment 155473
Edit:
I forgot to mention, the engine run about 11,000 revolutions per minute.
It depends on the propeller you use.

Your method will work just as well for a V2 crank.
I have tried to compare mass for Your pressed version with my screwed and difference for a 5ccm V2 will be less than half a gram.
ByeBye feeling a genious on that account but it was nice,free and unpoluting as long as it lasted.
Thank You for describing it so well.

 

[Peter Twissell]

When I made the crankshaft for my opposed twin, I machined the main journals (slightly oversize) and webs in one piece, with the webs connected by a continuation of the main journals between them.
I then bored the webs for the big end journals, which are hard dowel pins.
The pins were then pressed into place with the rods in place.
I secured the pins to the webs by TIG welding.
Finally, I cut out the sections of main journal from between the webs and finish turned the bearing surfaces.
This process guarantees alignment of the crankshaft assembly.
The rods can only be removed by cutting up the crankshaft, but I don't consider this to be a problem, as any event which requires replacement of the rods is also likely to require replacement of the crankshaft.

 

[Bentwings]

Have started on dimensions from Holly Buddy.
Significant change is 6mm instead 5mm crankpin diameter but only 8 long instead of 10mm.

View attachment 155443
Are there someone that has indicated a model Diesel (that is not a diesel)?
What is max pressure?

View attachment 155444

Have started on dimensions from Holly Buddy.
Significant change is 6mm instead 5mm crankpin diameter but only 8 long instead of 10mm.

View attachment 155443
Are there someone that has indicated a model Diesel (that is not a diesel)?
What is max pressure?

View attachment 155444

I might suggest you look at using a hub with three threaded holes this is current practice on our Rc engines and electric motors props are usually 6 mm holes with three short grub screws to prevent prop or spinner slippage long threaded bolts and studs at relatively old fashion to put it “ bluntly” sorry . P 516” spinners can get out of balance and make a real mess if they depart the aircraft. We have an electric belt reduction drive that makes up to 17 shaft hp at 7,000 rpm pulling a 4 blade paddle prop. We use a 6” dia aluminum or carbon fiber spinner. It easily out revs and out pulls any 5.8 cu in gas engine single or twin .

 

[Niels Abildgaard]

We are talking of different things.
OP wants to make a boxer crank for two 2.5 ccm opposed cylinders .
I try to calculate mass difference between making the crankshaft as a pressed up thing like mr Dieter has done or using a screwed construction like the one I posted from a V2 dream of mine.
Some 2.5 ccm diesels have a 5mm diameter,5 mm long journal for conrod big-end and I use 6mm dia 4 long to ease matters for the long ,thin screw.

 

[pat_pending]

My father and I built this OPS flat twin in the mid 70s. The rear support on the crank is now missing and was supported with a tiny ball type bearing. The crank pins were press fit and tack welded, rods are titanium. The engine showed amazing speed and rpm's but suffered from many failures. Cracked welds on the crank pins, rear support bearing failures and poor front cylinder charging due to only having one rear induction carb.
Great memories with dad though at Fleetwood marine lake.

lovely story. I bet it sounded great too! -P

 

[pat_pending]

Hi Pat

In order to be able to produce and assemble the crankshaft in a straight line, I used an auxiliary mandrel.
(In German: "Hilfsdorn", google translater found only the words "auxiliary mandrel" for it.)

It is a steel bolt that has been precisely ground for a slight tight fit.

I didn't mark the connecting rods in the sketch, but they were of course installed.

For the assembly of the crankshaft, the bolt was used as a guide/alignment.
For the final machining (cylindrical grinding plain bearing seat) of the front and rear crankshaft diameters,
the crankshaft was clamped between centers on the cylindrical grinding machine.

Only when the grinding work was finished and the crankshaft was secured with pins,
the bolt was pulled out and the hole in the middle part closed.

Because the crankshaft was case-hardened,
I had to drill the holes in the assembled state with a solid carbide drill Ø 2 mm.
Then hammer in the pins and it was done.

Dieter
View attachment 155472
I used this type of locking pin (Ø 2 mm)

View attachment 155473
Edit:
I forgot to mention, the engine run about 11,000 revolutions per minute.
It depends on the propeller you use.

Thanks for taking the time to post this Dieter. Very comprehensive explanation. My mind is already wandering to how this would work in practice with the machinery I have in my workshop.

While grinding as you describe is of course the 'proper' way to do it, with hardened crank pins e.g from needle roller bearings, I think performance for me would be adequate using unhardened high-tensile steel especially if the crankshaft were to run on ball-bearings.

Thanks again,

Patrick

 

[pat_pending]

When I made the crankshaft for my opposed twin, I machined the main journals (slightly oversize) and webs in one piece, with the webs connected by a continuation of the main journals between them.
I then bored the webs for the big end journals, which are hard dowel pins.
The pins were then pressed into place with the rods in place.
I secured the pins to the webs by TIG welding.
Finally, I cut out the sections of main journal from between the webs and finish turned the bearing surfaces.
This process guarantees alignment of the crankshaft assembly.
The rods can only be removed by cutting up the crankshaft, but I don't consider this to be a problem, as any event which requires replacement of the rods is also likely to require replacement of the crankshaft.

Thanks Peter. I'm liking this method too. With the tig i suppose there is no option for anything to move out of alignment as you have a light press fit for the dowel pins to be inserted and the main journal still in place at this point. Question: i assume a spot weld at e.g 50a at each side without need for filler?

Also, building the crankshaft this way, did it necessitate having a 2 (or more) pice construction of the crank case?

Thanks,
Patrick

 

[Niels Abildgaard]

A picture of a DLE crank made same way as Dieter describes
Boxer Crankshaft

 

[Peter Twissell]

Hi Patrick, the TIG weld was with no filler, just fusion. I don't remember the current, but probably less than 50A (My pins are 5mm diameter).
My twin has a two piece crankcase, in fact, each half is both crankcase and cylinder and the halves are identical.
It is possible to fit an assembled crank with both rods into a one piece crankcase. BMW boxer twin motorcycles do exactly that, as do Dneprs and Urals.

 

[GRAUBELE]

Hello everyone

Here I have a few more pictures of the plain bearings on the crankshaft.
The engine is a two-stroke engine that sucks in fuel through the crankshaft.
The windows in the crankshaft determine the time at which fuel is sucked in.

Dieter

 

[Basil]

Very nice strong design.

 

[pat_pending]

Thank you all for taking the time to reply. I will sketch up a design on CAD and then get to making some swarf. Hope it will be obvious in the 3D model whether I need to make a 2piece crank case or I can tweak the parameters enough that the can assemble the engine with a single piece machined piece. You’ve all got me ‘mental machining ‘ at times throughout the day which is normally correlated to when I actually get off my backside and get into the workshop