MB22 Launch Engine

[Sekim]

Hi.  Since downloading the MB22 drawings and construction notes I have made some progress and now have a virtually complete engine which has run satisfactorily on air.  It follows closely Malcom Beak's excellent drawings and notes, other than for some minor mods and a different slide valve detail.  The minor mods are 1) crank made up as illustrated, but "loctited" and pinned rather than silver soldered. 2). Bore increased to 10mm, but only because I had a new reamer of this size. 3). 1.5 mm cross section "O" rings fitted to the pistons, whether this is an advantage or not time will tell, but it runs very sweetly on air at  5-10 psi.  4). Cylinder width inc to 0.75" as I don't intend to it timber cladding.  5). Set screws used throughout rather than studs/nuts, but this could change.
The biggest change is the use of a 0.280" dia circular slide valve with a 0.160" dia circular exhaust cavity in its face.  These are very simple to make, plain turning of the outside and a home made 0.160" dia "D" bit to form the cavity.  They are used in small scale garden rail locos successfully and seem to work well with small dia inlet ports.  They are obviously NBG for larger circular or rectangular ports.
The work has all been carried out on a Myford ML10 and a Wabeco mill with DRO, the latter saving a lot of setting out.  Throughout I have made sure each stage rotates freely as sorting out odd binding on completion can be problematic.
Altogether a very satisfying if slightly fiddly project.  Thanks again is due to Malcolm Beak for sharing his drawings and experience.  On now to the reversing gearbox and Boiler.
Regards  Mike
 

[DamienG]

Very nicely done  

[malcolmbeak]

Mike
Nice to see another of these engines. I've made several of them, the first had the crank soldered with Comsol - a"high temp" soft solder. All the later ones have been machined from solid. I originally used soft packing on the oistons, then some PTFE rings, but now always use silicone O rings. I'm interested in the slide valve and ports. I assume the exhaust is just a 2mm drilled hole and the inlet a single 1mm hole. I realise that the method I used to hold the valves in place was far too fiddly and now use a much simpler method.
Recently I've been playing with a couple of very short stroke engines - see the photos. They are both 1/2" bore and 1/8" stroke. The twin incorporates a fwd - neutral - rev gearbox.
Malcolm

[steam up]

Very interesting engine Malcolm can you explain the thinking behind the new engine design?

[Sekim]

Malcolm - The ports are as you specified them 2mm ex and 1mm inlet.  Attached are pics of the valve.  Its made from a short length of 5/16" round with a 3/16 deep slot 1/8" wide in one direction and 1/16" in the other.  After turning end for end in a collet, the OD was reduced to .280" and the cavity formed 1/16" deep with a .160" D bit made for the job.  The drive nut is 3/16" x 1/8" with a 10 BA grub screw securing it to the valve rod.
The piston O rings are viton from exactrubber.co.uk who have a vast range at sensible prices, 10 No.(10 OD x 7ID ) were about £3.40 inc postage.
Ive just made a start on the gearbox but have some way to go with this, as you can see.

I too was intrigued by the reasoning for such a short stroke engine, relative to the bore size.  Are you looking for high speeds?

Regards  Mike

[malcolmbeak]

Mark and Mike
A few years ago I made a small V twin oscillator for a Vap 50 boat I had. The cylinders were made from ¼” OD thin wall brass tube. This had a bore of about 0.22” and the stroke was 7/16”. It worked very nicely and several people saw it and asked if I would make an engine for them. I resisted for a while, but finally gave in last year. Disaster! Try as I might, I could not get a decent seal on the pistons. I prefer to use O rings on my pistons and there were none of a suitable size. I tried soft packing and making ptfe rings but nothing worked well enough. So I gave up and they didn’t get their engines.
  I still wanted to make engines with small capacities, so why not go for a reasonably large bore using O rings that I knew I could make work and a short stroke. I had no idea if this would work well enough so went for a single cylinder version.
  Oscillators have to have a relatively long stroke to get enough movement of the cylinders for anything like a reasonable port size, so it was going to be a slide valve design. In the past I have always kept the con rod length fairly short (no more than twice the stroke) to produce a reasonably compact engine. This has meant that there is quite an angular thrust at the crosshead therefore needing a crosshead guide.
  In this latest engine, the con rod is 7/8” between centres (7 times the stroke) which produces very small angularity. So I have done away with the crosshead guide, but included a relatively long piston rod bearing and gone for a rather larger diameter piston rod than is actually needed. The big end bearing is a ball race as I happened to have some handy and the valve ports and timing are the same as for the MB22.
  Although it runs fairly high speed (7000 on 30 lb of air) it can also work slowly driving a large prop for this size engine.
  The twin is basically a pair of the singles but with the output shafts geared to a central drive shaft.
  I have to say that I’m rather pleased with the result although there a few mods I need to make.
  Hope this answers your question

Malcolm

[proflooney]

Nice will be following. I am currently trying to cad up this engine in solidworks for machining etc. but since I am new to this stuff having a couple probs understanding from the drawings.I am doing the modified version with the 1/2" bore and stroke.

right now I am trying to figure out the main bearings. they added 1/8" deeper to it but doesnt mention whether they adjusted the holes to center them can anyone tell me if the locations re the same or if they were adjusted?

[proflooney]

here is the problem I am having. in the first I have the extra height added and then the ream but when i flip to top view the 3/8" width dont work cause only 1/4" square or is it some european way of showing top side and front. here front is the bottom then top above and right on the right but the way he has them drawn here doesnt seem the same

[proflooney]

ok got it figured out lol tired from work misread it