# Wobbly engine plan for critique.



## Thegsey (Jan 1, 2012)

Hello,

I originally intended to design and build a beam engine as my first home project but I decided I should learn to run before I can walk. Especially as I have now got a new (to me) mill and lathe as well various other equipment I thought a quick to start little project to get me going would be just right. 

I had a bit of time today so I put together some plans. They are a bit rough and definitely WIP. If anyone would like to cast their eye over them for some critique I would be very appreciative. I am interested in all comments especially about the material choice. I have gone mostly for aesthetics with my choices but I would like to know if they are also practicable. 

The drawings dont show some details including threads or dimensions at the moment so that may clear up some confusion. I am intending to make the plans available to all once they are all finished and dimensioned. Some suggestions as to which formats people would like the drawings in would be good. I can produce most.

The engine will most likely be run on compressed air but I would like it to be able to run on steam. Are there measures I need to take to protect for this option?

Thank you in advance.

Mike


View attachment Wobbler Drawings WIP_Sheet_1.pdf


View attachment Wobbler Drawings WIP_Sheet_2.pdf


View attachment Wobbler Drawings WIP_Sheet_3.pdf


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## Tin Falcon (Jan 1, 2012)

It seem odd and bold to me that one would want to try do design there own engine for there first build. there is so much to learn in this hobby that it makes more sense to us and existing design for a a first engine. then maybe combine some concepts of a couple of designed in a single engine. Do not get me wrong I have used the same set of plans for three different engines that do not look much alike. 

some comments on the design it looks nice but the details are what will make or break the design as a running engine. if the air passages and holes are not in the correct spot the engine will not run. Also the hole size is important. 
there is virtually no support for lateral forces on the con rod and no provision for a seal or packing . It will likely wear and leak in that area. 
tin


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## Thegsey (Jan 1, 2012)

Hi Tin,

Thank you for the reply. I guess I should have mentioned in my first post that this isn't the first engine I've built, just the first one at home. I learned to use the workshop at secondary school (equivalent to high school) and then again at university where we made wobbler engines running on compressed air. We made the engines to an existing design and then modified them to achieve maximum speed. 

A lot of my interest in engineering lies in design which is why I chose to design the engine myself. I am a design engineer by trade but do not get much opportunity to get into the oily bits. Learning how to design better engines is a big part of what I want to get out of this. Thank you for the comments on the design I will try to address those points. 

The air passages I think are all lined up correctly. I have run the model kinematically and the inlets are open during almost the entire power stroke and outlets during almost the entire exhaust stroke on both ends. From my knowledge it should "work" the way it is designed but I am interested in refining this so it works well! Do you have any pointers on how I should chose the hole sizes? 

I see your point about the lateral forces on the con-rod. I think I can address this by adding a protrusion onto the bottom of the cylinder to increase the amount of contact with the con-rod. This is my first thought though and I'm ready to be told why it may not be advisable. I have updated the drawing to show this. 

As for sealing or packing I am not sure what to do. It was not clear that this was accounted for on many of the designs I have been looking at and it is not an area I know much about. I will have to research further into this. Any ideas or a point in the right direction will be much appreciated. 

Thanks again for your feedback and any more from yourself or others is very much appreciated.

Cheers,

Mike 

View attachment Wobbler Drawings WIP2_Sheet_1.pdf


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## deverett (Jan 2, 2012)

Mike

To get an idea of how to guide the piston rod to reduce side thrust, have a look at Elmer's Twin wobbler.

http://www.john-tom.com/ElmersEngines/02_twinVwobbler.pdf

Of course this is just one solution; there are others which may be simpler to make.

Dave
The Emerald Isle


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## Tin Falcon (Jan 2, 2012)

The most important thing in a wobbler design is the hole locations in the standard. the distance between the pivot and crank and the stoke of the piston are the anchors and determine everything else. It is these that locate your port holes. 
I will find a drawing for you sounds like you may have this under control. 



> Do you have any pointers on how I should chose the hole sizes?


Imagine the two port holes in the standard and the one in the piston next to each other on the same plane. Draw your three holes. so the piston hole is centered between the other two. Make the holes too big and the air will pass leak between them. make them too small and air flow will be restricted. you can play with these a bit by changing the size of either the piston port or the standard ports. And the standard ports can be pushed a bit further apart so you only get partial lap or alignment. I think this was done on my lucy see though engine it runs great but the holes do non fully line up. 



> I see your point about the lateral forces on the con-rod. I think I can address this by adding a protrusion onto the bottom of the cylinder to increase the amount of contact with the con-rod. This is my first thought though and I'm ready to be told why it may not be advisable. I have updated the drawing to show this.



That would be the simplest approach and the way it is handled on the PM Research DA wobbler. It has no packing around the con rod. As it was suggested look at the Elmers engines plans on how to make an adjustable gland that incorporates packing. . 
Tin


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## bearcar1 (Jan 2, 2012)

Hello Mike, 

I admire anyone that can manipulate and navigate their way around a CAD program and you certainly have proven that you are capable of doing just that. If I may address one feature of your design that jumps off the page for me is the unsupported overhang of the flywheel. It just seems to me there should be a bit more support to the outboard end of the crankshaft. I like the two bearings in the support stand but to me they just seem too close together to adequately support the flywheel. Perhaps you might consider either an outboard bearing support with a bearing in it OR maybe think about counter boring the flywheel to accommodate a spigot or boss extending out from the frame with a second (third?)bearing in its outer end. This would lesson the fulcrum effect that you have there now. This very crude CoC drawing illustrates the two ideas perhaps a bit better. Otherwise, I think that your endeavors are coming along quite nicely and look forward to seeing the final drawings and better still, a running design. 

Cheers.

BC1
Jim


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## Ken I (Jan 2, 2012)

You've drawn your cylinder with an integral trunnion pin - that's going to be a PITB to make - you would be better off substituting a shoulder screw for detail 13.

You might want to then make the wall thickness greater at that side to accomodate the thread without breakthrough - but if you don't intend running a piston ring the hole can go through into the cylinder - as these did.




Don't lose too much sleep over hole size - you can get an awful lot of air through a small hole.

Agree with Bearcar - the flywheel doesn't look to be well enough supported - either put the bearings further apart or use an outrigger as suggested.
Ken


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## Thegsey (Jan 2, 2012)

Thank you all for all of the great and very sensible sounding advice. I will get back to the drawing board when I have a moment and see if I can make some improvements. I'll keep this updated with the changes.

Thanks!

Mike


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## Thegsey (Jan 3, 2012)

As promised, attached are the revised plans. I have tried to take in as much of your ideas as possible. Please let me know what you think.

Thank you very much!

Mike 

View attachment Wobbler Drawings WIP3_Sheet_1.pdf


View attachment Wobbler Drawings WIP3_Sheet_2.pdf


View attachment Wobbler Drawings WIP3_Sheet_3.pdf


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## Ken I (Jan 3, 2012)

Mike,
    That all looks about right. Good luck with the project.

You can save yourself a bit of trouble on the shoulder screw by making it a plain screw through a sleeve - no problem with the thread going all the way to the shoulder that way.

Ken 

View attachment DUMP Model (1).pdf


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## bearcar1 (Jan 3, 2012)

Say Mike, now it looks like you are cooking with gas. Do you have dimensions down yet? I ran across a packet brand new miniature ball bearings that I would be ideal for this engine. I'll have to find them again in the tool chest and verify what size they are. Keep up the good work. Thm:

Jim B.


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## bikerbob (Nov 16, 2016)

Thegsey said:


> As promised, attached are the revised plans. I have tried to take in as much of your ideas as possible. Please let me know what you think.
> 
> Thank you very much!
> 
> Mike



I am rather new to steam engines; but your design looks like something I could accomplish. I have downloaded the revised drawings but see that a number of the compounds lack dimensions. Is there a set of dimensioned drawings?
If so could you email a set to the address below
[email protected] Dot Ca
Thank you for your attention

Robert Wager


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## ssaxer (Jan 3, 2017)

We build a similar engine in the Intro to Manufacturing Operations class here at the university that I work at. We have found that hole placement is critical for best performance. As the spacing of the holes gets closer together the hole size has to get smaller but the cylinder has less distance to travel. If the holes are too close the pressure needs to increase (because the holes have gotten smaller) and you need a flywheel with a large moment of inertia (I) to get you over the dead spot at the top and bottom. As to flywheels a larger diameter, thinner one is better than fat small one as there are less issues with overhung support.You can calculate "I" this way:
Mass moment of inertia = (1/2) * m * r^2  
    m = density * (thickness * Pi * r^2 )  
So I = (Pi/2) * density * t * r^4

It is also important tho make sure that you guide the piston all the up and down in the cylinder. If yo let it come too far out of the cylinder it will rock and you will cause excess friction.


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