Rotary Engine Con Rod Crank Pin Assy

[NickG]

As Bernd says, there will be a moment about the big / little ends on the slave rods but it can't really be helped, surely that is the case with any cranked rod and was so on the original cygnet design. Machining could be difficult but I know you'll find a way Stew. This may be one for the tilting head on your mill. Maybe use a bit of square stock, tilt the head over and mill the angle then you still have the datum faces for the bearing holes. Put head back square, mill down to thickness on small and big end to give you desired offset - it should work out as long as the angle is right! Might even be better to drill the holes for the bearings before milling to thickness?

Nick

[kvom]

My general impressions is that you are overthinking this, esp. for a part that won't be visible.  On my Halo build the master rod is only 1/16" thick, but all the force on it is linear  If you want more beef you could just make the master and slave rods thicker.   

[sbwhart]

Thanks for youre input chaps some good comments their, getting me back on the road.

As some of you guys said the way I was going was too complicated. So I went back to the begining and decided to see how things would look with a 1/8" thick master con rod saring the same 1/4" dia crank pin with the slave con rods to the sizes of the cygnet design.

I carfully drew up the con rods then assembled them onto a common piston to see how they would fit this showed that they interfeared with each other by about 1mm, decided to redraw taking the interference out, by reducing the thickness of the big end.

This is what I got.



Some of the sizes are a bit odd this is because I've converted from imperial to metric, when I make i'll round down to the nearest 0.1 mm with a tolereance of about 0.2. The straight webs will be quite easy to make I've worked out a what i think will be a easy method.

For the big end I've got some nice oillite bearing that i'll fit for the little ends I'll fit some phos bronze bearings.

First I'll make the crank shaft, fly wheel and pistons so that i can cut and fit to see how it all goes together.

Cheers

Stew

[madjackghengis]

Hi Stew,  as you can probably guess, I've spent some significant time on this issue before getting started on my radial engine project, and what I came up with is that for practical purposes in a scale engine, the offset of actual angular timing is insignificant, completely, and the easiest method is master rod, slave rod as most aircraft engines.  As an aside, most all are odd numbered to avoid harmonic vibrations which are multitudinous in a radial engine, and an even number multiplies them by at least twice.  One thing I've seen and looked at, is a set up pretty much as your first model shown, the one which appears to have a ring with all the cylinders free.  I have seen this, but was not aware one rod was attached, and thus it acts like a master rod set up.  I have also seen the same setup however the "ring" was substantial, with a full bushing in it, all the rods were free, and the ring was connected to the inside of the crank case with two or three straps, so it articulates with the crank while never changing its angular relationship with any of the rods.  It was among the earliest full size aircraft engines, and did fly, so it is a working design.  I think Pratt and Whitney worked out a maximum of about six degrees of difference between the two cylinders furthest apart in ignition timing, using the master rod set up, and compensated for it in both electrical ignitions and in magnetos by slight changes in the ignition cams and their angles.  Again, something that scaled down, is too small to be an impact on design.  I'd like to pursue the "sleeve on the crank with nine slave rod" idea when I get my current radial engine done, just because I haven't seen it duplicated anywhere and have only read about it.  I have seen working hydrostatic drive units that use the same concept as they need dead even power in all cylinders when working with the high pressure of serious hydraulics, and the "leashed collar" system equalizes all the angular movements for all the rods and cylinders exactly.  I just thought I'd toss that into the mix, since it is Stew, and I am building a radial, so I can get away with such a comment.  Mad Jack

[sbwhart]

Hi Jack

Thanks for your reply and insite to radial engines, I went on and built the engine using the layout in my last post and it turned out a nice runner.

Its interesting what you say about scale effects reducing significance of features, I,ve part designed OHC V4 engine, reading up on cam profiles for it the same point is made.

Cheers

Stew