Sanjay’s Banjo Engine

[JasonBModerator @jasonb]

Sanjay (Redhouseluv) posted some images of a small unknown engine in this thread on MEM Forum at the beginning of April. It will now always be known as “Sanjay’s Banjo Engine”.

https://www.modelenginemaker.com/index.php/topic,12258.0.html

I was quite taken with its looks and although I had made a pump with the banjo type arrangement in the past I had not done an engine with the crankshaft in this form which is not unlike his recent Trapizoidal engine. A couple of evenings on and off with Alibre making use of “trace” to pick sizes off of a screen shot of the laid out parts and I had something that looked the part and while designing it gave some thought as to how I would go about making it. As usual I did the design in metric and it has a bore and stroke of 13mm x 13mm.

It was a toss up between using iron or bronze for the cylinder but in the end a piece of bronze bar won the day. After facing the end I turned a 2mm length to the diameter of the top flange before drilling and finally boring to the final 13mm diameter. it was then reversed in the chuck and the bore clocked true before facing to final length and forming a similar flange diameter as the other end.

I roughly squared up 3 faces of the mid section so it was easier to hold and then took the port face to it’s final height.

The end cover screw holes were drilled and tapped M2 and two 1.5mm holes drilled down to where the inlet ports would be, doing the holes first meant that the drill would not break out into the side of the port and risk snatching or worse. Repeat for the other end as well as milling a notch so the end cover spigots do not restrict steam/air flow.

Over to the CNC which made a relaxing job of cutting the three ports slots. The outer ones are 1.5 x 4mm and the middle is 2 x 4mm. Back on the manual mill and the valve chest stud holes were drilled and tapped M1.6

A quick hole in the side to link up to the central exhaust port tapped m4 x 0.5 was the final drilling operation. I then supported the cylinder on a 12mm ground bar resting across the tops of the vice jaws and using a 10mm R1 corner radius cutter machined the mid secton down to it’s final shape and finished by blending out the resulting facets with a fit of filing.

The two end covers were fairly straight forward, if small turning jobs from some offcuts of cast iron bar. held in a 5C collet block to drill the screw hole son their PCD.

 

[renardiere7Participant @renardiere7]

Very interesting, I assume “Banjo” from the valve slide/eccentric link?  Thanks also due for reminding me about MEM, not been there for donkeys!

[DiogenesParticipant @diogenes]

Certainly different, especially the valve train – though I see his is quite a nice little runner..

Don’t give yourself eye-strain..

[JasonBModerator @jasonb]

The ” Banjo” is used to describe the conrod in this case where it is a loop shape to clear both sides of the cylinder. It can also apply to part of the valve linkage as you might see on say a Stuart Sirius where it loops around the top of the engine.

Valve train is fairly standard although very small slide valve operated by an eccentric, the only unusual part is the top horizontal link.

My feeling is that the engine was intended to sit low down in the Vee of a boat hull and to be as low as possible to fit below deck level hence the banjo and the valve rod entering the chest from above. When it is finished (just painting) I will take a photo of it next to a similar capacity engine of the more usual uprigt layout to compare heights.

 

[JasonBModerator @jasonb]

The piston, its rod, the rod end and pin were all fairly straight forward turning jobs. I used aluminium for the piston, 2.5mm stainless 303 for the rod and some Colphos 90 for the end, silver steel for the pin.

Being quite small the valve chest and cover allowed me to use up some off cuts of cast iron which were squared up and the two ends of the valve chest turned.

Small offcuts also got used up for the valve and nut. Not sure if going for the usual threaded nut was the best choice as it does have a tendency to want to unscrew at high speed as the valve rod needs to be a loose fit in the top link. The original just had a reduced diameter to the rod which allowed it to fit a narrow vertical slot in the valve.

The base plate was from 2mm thick steel, I drilled the 4 mounting holes on the manual mill and then used those to hold the plate down while the profile and important holes for cylinder and bearings were done on the CNC

The two crank shaft bearings/supports were CNC cut on the ends of a piece of bronze bar before being sawn off, milled to final thickness and two 2mm fixing holes added

Coming together now. The coil is approx 7/8″ or 22mm diameter

[JasonB]

On JasonB Said:

..The original just had a reduced diameter to the rod which allowed it to fit a narrow vertical slot in the valve..

I guess it stops them undoing but fiddly to adjust?

[JasonBModerator @jasonb]

Luckily there is adjustment at the top of the valve rod as the nuts either side of the top link can be moved but the screw thread and valve nut does make it easier.

[JasonBModerator @jasonb]

The eccentric was next, first turning to overall diameter and cutting the 1mm x 1mm groove for the end of the rod to locate into. I then held it in a collet block to drill and ream the crankshaft hole followed by laying the collet block on its side to drill & tap for a grub screw. Back to the lathe and using the grub screw to hold it to a piece of 4mm rod the spigot was turned down to size.

For the strap I started by boring a hole in the end of a piece of bar then used the CNC to cut the lollypop shape which was parted off and a M1.6 hole tapped for the rod

A piece of stainless steel rod completed that group of parts

Finally the part that gives the engine the other half of its name. Two discs were machined to suit the internal radii of the banjo halves and these were screwed to some scrap angle iron at the correct ctr distance and offsets and some 2mm dia steel rod bent around them.

The top and bottom blocks were fairly straightforward milling and drilling. I used another bit of scrap bar as a jig to hold them in place while I soft soldered the two halves of the banjo into place and that completed the build. Well I did make a few screws and a couple of pivot pins as well.

A bit of orange paint and time to play test the engine.

 

This photo shows how much lower the banjo arrangement gets the engine compared with my Bassett Lowke replica which has a similar bore & stroke be that to get it below decks or just lower the ctr of gravity.

[renardiere7Participant @renardiere7]

First class!

[DiogenesParticipant @diogenes]

Nice little runner.

Shiny-lookin’ fingers, not so much cast iron in this one then?

[JasonBModerator @jasonb]

I wash my hands once they go to paint.

Might get a bit grubby with these that arrived today, I’m led to believe there is a flywheel inside every one

[DiogenesParticipant @diogenes]

..ugh bath night – the worst night of the year..

..that looks nice, can’t wait to see it get chipped up..

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