A 15cc sidevalve opposed twin

[Peter Twissell]

The engine ran for the first time on Saturday morning.
And I broke it after just a few seconds.
Nothing serious, the timing gears were contacting the inside of the timing gear cover and generated enough aluminium debris the jam the gears.
I shall make a new cover with more clearance. I wanted to make a new cover anyway, with an oil drain / crankcase breather feature.
I had to make my own glow plug driver.
I had read that the common glow drivers which use a 12V source should drive two plugs in series, as they are current control devices.
My normal driver won't do it.
Fortunately, among my collection of electronics, i have a couple of boost/buck modules which can be configured to operate as current control.
This allows me to set a current and the device will compensate for variables such as long leads and keep the plugs glowing nicely.

 

[Arild]

The exhaust pipes were reasonably successful. After cleaning up, there is sufficient thickness of copper all over, but there are a number of pits and grooves in the surface. These will need filled with silver solder when I attach the end flanges, then the assembly will be nickel plated.
Yesterday I started plating the more complex induction manifold.
I made a shaped anode, in an attempt to even out the plating thickness.
After about 20 hours, the part looked like an octopus tentacle, with extensive "bobbles" over most of the surface.
I tried to file the bobbles down, to get back to a smooth surface, but found that the smooth plating was very thin and came away from the pattern.
I have now peeled all the plating off and I will start again.
I suspect the bobbling may be either due to the close proximity of the anode, or die to the anode being too large.

It looks likes arms from a starfish!

 

[Steamchick]

Brilliant work Peter. You have also shown us how much "time and perseverance" can achieve!
K2

 

[Peter Twissell]

Really?
I would like to have the time (and patience) for a lot more perseverance!
To my mind, this has been a fairly rapid design and build.

 

[Peter Twissell]

The engine has run again.
With a new timing cover, with clearance for the gears, it fired up easily and ran at about 1/4 throttle with the first guess needle setting.
The engine was turning a 12 X 4 prop, which I had guessed on the basis that it is likely to achieve power comparable with commercial 10cc four strokes.
When all seemed well after a couple of minutes running, I ventured to open the throttle to about half way.
This resulted in a rapid increase in rpm, followed by slowing quickly to a stop and feeling very stiff to turn.
Back on the bench and dismantled, I have broken the crankshaft.
So, back to the CAD to design a more robust crank.
On the plus side, the engine appears to be developing more power than expected.
I could probably reduce the compression a bit and make another crank to the original design, but I've tasted the power now!

 

[Steamchick]

Hi Peter,
From experience with crank design for High Voltage Power circuit breakers (single shot, but highly stressed! Max 120 g acceleration of the piston!), the most significant design improvement was to maximise the radius at the end of crank journals and main-shaft journals. A change from 1mm to 3mm in corner radius achieved in excess of 5000 operations instead of less than 20.... Just what we needed!
Check all the stress raisers, (sharp corners) and look-up stress concentration factors for shaft diameters and corner radii.... You'll soon work it out.
K2

 

[Steamchick]

Hi Peter, Just found the crank in Post #36. I was looking for the crank journal and main diameters... It looks like you have 10mmdia mains, for the bearings. But I was unable to find the crank pin diameter?
I have the book on building the Maltese Falcon - 260cc flat four engine:
Mains: 25mm dia,
Big-Ends: 0.660".
I am not sure how this relates to your engine, but may give you some idea of the design "balance" between components. The corner radius on journals is 1mm (0.040"). Not bad for stress reduction. maybe ~1.8? on the main bearing journal. Of course, your crank pins are not so easily calculated with being pressed pins, but which failed? Main or crank-pin?
K2

 

[Peter Twissell]

Thanks Ken.
My crank is a pressed assembly, because my rods are one piece.
The big end pins are 6mm hard dowels, pressed in to the crank webs and secured with a spot of Tig welding.
The welds failed, allowing the crank to twist.
I will make a new crank with more interference in the press fits. I was a bit over cautious with the fits at the first go, concerned about distorting the webs.
I will also add a weld prep so I can get more penetration.
I don't have space to make anything bigger, so I have to make the best of the original design.

 

[Vietti]

You are obviously a very accomplished builder, but just a thought, when I make built up cranks I put a deep chamfer in the web and a big taper on ends of the crank pin then either mig weld or silver solder. The deep resulting depression allows a generous weld, cleaned up with an end mill. I usually put in a pin but drilling a dowell pin might be tough.

I mig welded my last crank pin, drill rod, so I could put a needle bearing on the con rod journal. Did this because silver solder would heat up the bearing too much. Pinned also. So far working OK and I like the free spinning on a hit and miss.

John

 

[Peter Twissell]

Thanks John,
I am always happy to hear how others have done things.
I would be a fool if I thought I knew how to do everything.
I think you have described what I am intending to do with the weld prep.
I will Tig weld it, because I feel I have more control of the heat and filler than with MIG.

 

[Steamchick]

I understand - and think you know your job better than I! Thanks for the further explanation. I now appreciate why you avoided silver soldering. (Which is how I make cranks for steam engines with split bigends.). I don't know the calculations for the stress in the crank web when the interference fitted pin is pressed home, but I would simply use my old book of "standard fits". Maybe the calculations would be "hoop stress" plus a stress concentration factor? Then stress added for the pin loading at max revs and bmep on the piston?
Have fun with the numbers!
K2

 

[Peter Twissell]

Hi Ken,
Standard fits are applicable when the bore is in a relatively large section of material. My crank webs are of irregular section, so I calculate on the basis that the thick bits are essentially rigid and all the strain occurs in the thin sections.
This is, of course, an approximation, so I shall make a few test samples so I can measure strain and adjust the fit so as to maximise interference while avoiding yield in the thinnest sections.

 

[Steamchick]

Sensible. Thanks Peter. I would have calculated the hoop stress based on the thinnest section, but with a pre-stress equivalent to twice the expansion required by the interference fit. But I am just an amateur, and I don't have the detail shape and dimensions of your webs.
I'm pretty sure what you plan will work. Thanks for your time.
K2

 

[Steamchick]

Hi Again Peter, Strange what I dream about... But this morning I had a thought. Perhaps the TIG is "heat affecting" the material, either to some embrittlement that caused the failure, or to annealing the material - thus relaxing any interference? So does the finished assembled crank effectively need re-normalising and tempering? - NOT so practical with the races built in from the attached rods.
BUT perhaps in 2 stages: As there will be (on each crank pin) and highly stressed side and a less stressed side, is is possible to partly build the crank (without rods) and (say) Silver solder the pins on one side only? Then (on the "less stressed" side) after assembly and alignment secure the pressed pin with some anti-rotation means - such as a pin - as a key? It need not go all the way through the web, I would guess half to 2/3rds depth? - Drilled after assembly and trueing of the assembly? I have even heard of a threaded pin/bolt being used as such a key-pin, as it is easier to remove for service.
Such stupidity disturbs my sleep.
K2

 

[Peter Twissell]

Hi Ken,
My timing gears are pinned exactly as you show, but the crankpins are hard dowels and drilling is not practical.
The TIG weld was only a tack on the ends of the dowel, at their least stressed location.
It was the weld itself which failed, so my proposal is to add as much material as practical to the crank webs around the dowels, increase the interference fit and add a prep for a more substantial weld.

 

[awake]

Hi Ken,
My timing gears are pinned exactly as you show, but the crankpins are hard dowels and drilling is not practical.
The TIG weld was only a tack on the ends of the dowel, at their least stressed location.
It was the weld itself which failed, so my proposal is to add as much material as practical to the crank webs around the dowels, increase the interference fit and add a prep for a more substantial weld.

Are you taking any measures to reduce the cracking that comes with welding hardened steel?

 

[Peter Twissell]

Hi Awake,
I'm short, no.
I am using the weld to lock the dowel pins against rotation to prevent the assembly from twisting.
The primary strength of the assembly should be in the interference fits, so the weld should only be loaded in torsion, which spreads stress around the entire circumference of the weld in shear.

 

[awake]

I should have noted that I was asking out of a desire to learn - I have done a fair bit of welding as a hobbyist, but have never attempted to weld anything hardened. At least, not unless you count TIG brazing cast iron ...

 

[Peter Twissell]

I will also be learning!
I'll report how I get on with test parts.

 

[Peter Twissell]

The new crank went together without problems. I increased the press fit of the big end dowels to the point of yield in the webs (which are made from EN24T!). Welding brought the area up to red heat, so some of the stress will have relieved. The connecting rods were clamped in aluminium heat sinks during welding and the operation performed as quickly as possible to minimise heat transfer.
With the engine back together and a 12x10 prop purchased, I have now run it for several short bursts of a minute or so mostly at low throttle settings.
It responds well to the throttle up to around half open, then there is no change in rpm at higher settings. I suspect the carb is too big.