A Flat Twin version of the Holly Buddy.

[edholly]

Ok guys - here we go again on the Holly Buddy train.

This time it will be a Flat Twin (hopefully). Hopefully as it does have an arrangement that may not work. I want to make it with solid conrods, and therefor need to have a made-up crankshaft. To this end I have made the front and rear crankshafts (the rear is to stabilize this type of arrangement) and the 2 will be joined by an interlink disc . I am aware that in engineering practice this is not a good idea and may not work, but I cannot see why it won't - so much so that I am willing to invest maybe 100 hours to see if does or does not. The interlink disc will not be fixed to either front or rear shaft's bigends, but will float there and stopped from going elsewhere by the bigend width of the conrods either side.

The way I see it is the front and rear shafts are captured and stabilized by their respective bearing surfaces - and a ball bearing is used immediately adjacent to the bigend disc front and rear, then when one rotates with the interlink disc it causes the other to follow, yes there will be major forces through the crankpin, and I have chosen 41/40 steel and made the bigend bigger than the Holly Buddy single, so my theory is that it should work, as I said, only one way to find out - build it and see. If it fails I'll have the makings of one and a half Holly Buddies so all will not be lost.

The original Holly Buddy build was awarded "Project of the Month" back in May 2017 and the build can be found at https://www.homemodelenginemachinist.com/threads/new-2-5-cc-model-diesel-design-and-build.26084/ The engine was also tested by Maris Dislers and written up over 4 pages in the January 2019 issue of Aeromodeller.

I will update the build as it progresses - here is a photo of the 2 shafts against a rough drawing of the crancase - the next thing to make.

 

[edholly]

About 6 hours work here to create the crankcase outer and the crankshaft tunnel. Off-set of the cylinders is 10 mm centre of conrod to centre of conrod = 2+2+6 the 2's are 1/2 rod thickness and the 6 is the interlink disc.

Next step is to bore the cylinder holes and tap the hold down threads each end. Then to machine the front and rear housings, make the disc and see if it works.

Getting the 2 bigends exact is very difficult I have to admit as you are trying to place a dimple in the 4 jaw and line it up with a probe from the tailstock. As best I can tell I am 3 thou out, might not sound much but it will be enough to cause the components to bind badly or even seize if forced together if the interlink disc is not a perfect match centre to centre, so a couple of discs might need to be made with a couple of thou spacing different - I think it will be one of those fit and try gently maneuvers.

I have corresponded with Steve Jenkinson who did such a great job with the original Holly Buddy plans and he has agreed to do this one - but will wait to see if it actually works first.

 

[edholly]

Now about 24 hours in, have made the main crankcase, the front housing, the front and rear crankshafts although the front one needs to be made pretty with a front taper and overhangs removed. Next job is the rear housing and the interlink disc and trial assemble and see if the theory works, albeit just turning the front shaft by hand ...

 

[KenC]

If the centres of your inter-link piece are greater that the stroke of the crankshaft you will not be able to assemble the engine. If the centres are slightly less than the crank stroke, you will be able to assemble the engine and depending upon the difference, the front crank may be able to rotate the rear crank. If the centres are very much less than the crank stoke, the front crank will turn the rear crank. If the centre distance of the centre piece is identical to the throw of the two cranks, as you have already been advised, it is not possible for the front crank to turn the rear crank, unless the two cranks and the centre piece are positively locked together as one rigid item.
Ken

 

[edholly]

It is now running. The central bobbin around a split and screwed together central bearing which is both Loctited and grub-screwed in place works well. The thinner than normal Holly Buddy conrods also seem to cope ok although the engine has only had about 5 minutes running. It starts easily enough and after some "catchiness" on initial assembly, that has gone and the engine now has a nice bounce to it. Will have to think about whether to ask Steve Jenkinson to do the plans for it. I think the cylinders could do with about 4 more mm of stagger and that way the central bearing support could be 2 mm wider and the conrods back to normal 4 mm width. I think then it would be quite a long-lived engine, whereas as it is now I feel it might has quite some wear after a shortish amount of time.

Without tweaking it it swung the 11 x 6 prop pretty well. There is a bit more of a write up at https://www.rcgroups.com/forums/showthread.php?1054975-Model-Diesel-Engines/page1125 with a link to the short video of its first run.

 

[Tomcat]

Ok guys - here we go again on the Holly Buddy train.

This time it will be a Flat Twin (hopefully). Hopefully as it does have an arrangement that may not work. I want to make it with solid conrods, and therefor need to have a made-up crankshaft. To this end I have made the front and rear crankshafts (the rear is to stabilize this type of arrangement) and the 2 will be joined by an interlink disc . I am aware that in engineering practice this is not a good idea and may not work, but I cannot see why it won't - so much so that I am willing to invest maybe 100 hours to see if does or does not. The interlink disc will not be fixed to either front or rear shaft's bigends, but will float there and stopped from going elsewhere by the bigend width of the conrods either side.

The way I see it is the front and rear shafts are captured and stabilized by their respective bearing surfaces - and a ball bearing is used immediately adjacent to the bigend disc front and rear, then when one rotates with the interlink disc it causes the other to follow, yes there will be major forces through the crankpin, and I have chosen 41/40 steel and made the bigend bigger than the Holly Buddy single, so my theory is that it should work, as I said, only one way to find out - build it and see. If it fails I'll have the makings of one and a half Holly Buddies so all will not be lost.

The original Holly Buddy build was awarded "Project of the Month" back in May 2017 and the build can be found at New 2.5 cc Model Diesel design and build. The engine was also tested by Maris Dislers and written up over 4 pages in the January 2019 issue of Aeromodeller.

I will update the build as it progresses - here is a photo of the 2 shafts against a rough drawing of the crancase - the next thing to make.

Hello Ed, Any chance of getting the plans for this? I use Solidworks

 

[edholly]

Hi Tomcat, I didn't publish the plans as there is a problem with stabilizing the centre interlink's housing in the case - l know how to solve it but haven't got around to it yet .... maybe one day - a few projects before hand though ... thanks for the query .. regards ... Ed