Ohrndorf 5 Cylinder Radial

[petertha]

With the setup established, the crankpin was turned down to diameter as well as the rear face of the counterweight profile and a thin boss profile for the master rod bushing. I tried a different style of lapping tool which was kind of a squeeze clamp affair.

 

[petertha]

Counterweight Profiling
Back to the mill. With the holding fixture still on reference surface & presented to the vise, the counterweight profile was cut out. Then the roundover profile was milled & hand filed away using a slip on guide bushing.

 

[petertha]

Crankshaft Counterweight
The design calls for an additional counterweight mass which is bolted to the matching crankshaft profile. One of those 5 minute jobs that took 3 hours. It has a relief arc cut to accommodate the master rod, but its center occurs at a different center than the OD, so required 2nd setup in the 4 jaw. I integrated that registration point in the same fixture used to hold the crankshaft for crank pin turning. Brass face mills really nice with the sharp uncoated inserts used on aluminum.

 

[petertha]

Some partial assembly pics of the crankshaft components at this point

 

[Peter Twissell]

Nice work.
I'm always amazed at how spindly some of these crank designs appear, although they are clearly up to the job.
The crank for my radial is a monster by comparison.

 

[Shopgeezer]

The O5 crankshaft was turned from a bar of 1144 SP (stress proof) steel. This is the first time I've machined this material & I don’t have much comparative experience to similar tougher alloys like 4xxx series, but I was pleased with the results.

When I took machining classes oh so long ago the instructor told us to use stressproof whenever possible but lamented that it was getting hard to find. It seems to have all but disappeared these days. I would love to find some but all the suppliers I know don’t list it. It is even getting hard to find leaded steel. The gnarly cold rolled crap I end up with is hard to finish to any degree of accuracy. I’m sticking to aluminum brass and bronze these days.

 

[Peter Twissell]

I'm not sure what is meant by "stressproof" in this context.
Proof stress is a measure of the materials yield strength.
When buying steel for anything with specific requirements for strength, workability, surface finish, hardening etc. it is worth looking up the available grades and their properties, then specifying exactly what you need.
My radials crankshaft is made from EN24T (817M40T), with counterweights made from much cheaper and easier to machine EN3B (070M20).

 

[mu38&Bg#]

Stressproof is a trade name for SAE 1144 equivalent to ETG100.

 

[Peter Twissell]

Thanks dieselpilot, I had wondered whether it was a trade name.

 

[petertha]

I've also heard it called 'stress relieved' which is probably a better description, but 'SP' is what you commonly find it listed under.
This isn't the link I was looking for but if you scroll down has a pdf spec sheet (and some of the metrics seem to vary +/- by source I've noticed).
https://www.aedmotorsport.com/store/materials/1144-stressproof-cf-round-bar
Anyways, it seemed like a good choice based on what I'd had read of others experience on more complicated crankshafts, like multi throw Vee or opposed where there was more non-symmetric material removal & they struggled with post machining distortion. I suspect this radial with simpler mostly axial layout could have just as well been made from 4140 but the price was going to be the same to me regardless. Another possible option to reduce the volume of waste material left in the swarf bin is buy rectangular bar so that the crankpin & some stub of counterweight was encompassed in the width & the thickness is just a bit more than main OD. But I think 1144-SP only comes in rounds, so it would be back to 4xxx alloy available in bars. I've also read where people have made cylinder liners from 1144-SP. I didn't myself, but thought it would be good to try with the extra slug. I think it would be good for tooling shanks & such as well.

 

[petertha]

I forgot to include a cad section showing the crankshaft & various mated components.

 

[petertha]

Sorry for the long lapse again. I will now continue on with the cam drive assembly. The O5 is similar to other radial engine layouts where the crankshaft drives a planetary gear reduction assembly, the output of which is connected to two separate cam plates. One plate is dedicated to intake, the other to exhaust where cylindrical cam lifters ride along the cam profile. As the cam lobe raises the lifter, the connected pushrod act on the rocker arms to open the valves.

The O5 planetary gear is a 4:1 reduction ratio. A 15-tooth module-1 crankshaft gear drives a 15-tooth intermediary gear which is sandwiched against a 10-tooth gear, which drives a 40-tooth internal (ring) gear. The intermediary 15/10 tooth gear cluster rotate together on an idler shaft. Because of 4:1 ratio, each cam plate has 2 identical lobes 180-deg apart for 2 complete (suck/squeeze/bang/blow) events per single cam revolution. The intake & exhaust cam plates are phased angularly to each to achieve timing relative to TDC. Both plates are attached to a cup which contains the ring gear. Here are some overview sketches. Hopefully this will make more sense as the parts & assembly are shown in real life. I will have more to say about intake/exhaust timing later on.

 

[petertha]

The gear plate is made from 2024 aluminum. It houses one of the 4 crankshaft bearings on the rear side & also holds the idler gear shaft on the front, nose case side. The plate attaches flush to the crankcase front face retained with M3 screws & also snugly fits the crankcase ID with a matching boss. The plans called for 1.5mm OD O-ring seal on the internal boss & another on the lip OD to seal the nose case.

I mentioned earlier that the original O5 design called for the nose case chamber to be partially filled with oil to splash lubricate the gear & cam assembly. The O-rings are to seal this bath oil from the crankcase & the outside world. But I was becoming less comfortable with potential oil migration issues & dragged my feet on this matter for as long as possible. For example, even though the rear bearing was presumably left shielded I thought oil would eventually get in behind the shield, dilute the grease & ultimately leak into the crankcase. Then the risk becomes hydraulic lock on lower cylinders. Also, because the lower cam lifter bushings would always be submerged in oil, it seemed like another potential migration path out.

So, after a lot of deliberation, I finally decided to abandon the oil bath mode. Rather, I made a series of modifications to my existing parts & this gear plate was one of them. Therefore, you will see a mashup of old & new pictures, hopefully it’s not too confusing. I decided to subsequently drill an array of passage holes in the front plate to allow intake mist charge originating from the rear via the carb & crankcase, into the nose case & lubricate the gears & cams that way. This is actually the established lubrication method of other commercial & shop made radial methanol glow engines, so hopefully will prove to be the right decision. For added insurance I will make a threaded/capped port hole on the nose case to squirt lubrication oil in prior to running. I'll be careful during break-in runs to see how wet things are. The O-rings are already done & will still serve their intended purpose.

 

[petertha]

The basic gear plate profile was turned from solid bar. The main diameters & bearing counterbore were done in one setting. The O-ring groove dimensions also need to be done at this point before removing the part. The grooves were a bit fiddly to obtain the right fit to the matching components. I’ve seen some O-ring groove formulas that get you pretty close, but in the end, it was a progressive trial & error thing. I used 70 durometer Viton O-ring cord & spliced a custom ring using CA glue. That part went amazingly well. If you ever need oddball O-ring diameters to make, I can recommend this as a cost-effective alternative.

 

[petertha]

Pictures showing the stock gear plate design confirming fit to crankcase.

 

[petertha]

Once the plate was machined, it was set up in rotary table for hole drilling. I trusted the CAD pitch circle calculation to drill & ream the idler shaft hole using mill DRO.

 

[petertha]

Then I made an MDF fixture to hold the plate, return to lathe & counterbore the relief for idler bearing spur gear.

 

[petertha]

Here are some pictures of the subsequent plate modifications to drill the array of oil mist holes. Of course, that required a new fixture to reestablish the geometry which would have been SO much easier the first go-round. I managed to get the 2 lower apertures slightly intersecting the gear cluster so hoping it will get directly misted.

 

[petertha]

I purchased the steel module-1 spur gears from Maedler, the same company I got the internal gear from MÄDLER - your expert for power transmission elements
Originally, I contemplated making the spur gears myself. But these were reasonably priced, high quality & each gear requires quite a bit of modification. At my snail’s pace construction, this was probably a good decision in hindsight.

For the crankshaft gear, I first machined a closefitting aluminum pot chuck in the lathe. Then swabbed the ID surface with acetone. Then without disturbing this setting, inserted the gear blank which was as a tight push fit on the teeth & back face. Checked the bore with a DTI, all good. Then I spotted some CA in glue among the teeth to prevent it rotating loose & a spritz of kicker. Then bored out the ID to fit the crankshaft diameter & faced/profiled to length. The steel was reasonably hard but machined well. The glue held things firm, even during interrupted tooth cuts.

Once the assembly was removed, my plan was to heat the assembly with light torch heat, expand the aluminum more, break down the glue & gear would drop out. It put up a bit more resistance than I expected but eventually parted ways with slight persuasion from a rod. The crankshaft OD was just a hair over diameter near the rear stop so a bit of lapping compound got the two parts fitting snug. The gear will be permanently bonded with Loctite retainer.

 

[petertha]

The 15T idler gear blank was held in a 5C collet for opening up the bore & machining to length. The 10T gear was positioned on an axle fixture to turn down a portion of the gear which then fits inside the 15T bore. The inner gear bore rides on an idler shaft. The gear assembly will be bonded together with Loctite retainer.