Muncaster's Grasshopper

[kvom]

Recently Julius De Waal posted a set of plans for this engine, which to my mind would be an attractive project.  Although the plans are metric, that poses no issues for CNC machines or manual machines with DROs.  I would use imperial fasteners to the nearest size as I don't own metric taps and metric fasteners are less available here.

In that light, I spent the past two days starting to model the engine in Solidworks.  I have encountered an issue that needs resolution before I commit to a build.  The attached picture shows most of the structural components, and I am able to animate the model successfully.  However the plans show a pair of swinging links connecting the end of the beam with the holes shown near the heads of the columns on the right.  If i add these to the assembly, the mechanism locks into a single position.  When removed and I move the crank to various positions the center distance between the two holes varies by a couple of mm out of a total length of the link of 106mm.

Some of the grasshoppers I've seen do not have the left link that attaches to the beam just to the right of the conrod.  I am wondering if the two links are a variation of the Watts linkage on regular beam.    That said, the Elmer grasshopper, the Eaton and Anderson, and The Potty do have both links, with both having the rocking link on the end of the much longer than on the Muncaster.

Any thoughts on the geometry issues would be appreciated.

[crueby]

I have a Stuart Turner grasshopper, the difference in the geometry between it and yours is that the link on the left has its upper pivot in the same plane as the piston rod, where yours is to the left of the rod. Try moving that over to line up with the piston.

[Jasonb]

I suppose this is the problem with third hand drawings of engines that have not been built.

Julius got them off the web and as far as I know none of his Muncaster engines have been built in metal only on the screen. Julius used the illustration from Westbury's 1957 series of articles which was only a general arrangement drawing with a few dimensions on it. Westbury based his GA on one that Muncaster did in 1934 which I have not seen and don't know if it had any more detail on it, but based on the others in Westbury's series and Muncasters original sketches in his book I doubt the 1934 one was fully detailed and may never have been built.

You can find the 1957 series on John Tom.

[Jasonb]

Might have found it.

Julius part Number, Julius dimension, Westbury dimension, julius /westbury

1-12, 90mm, 3.75", 24
2-06 102mm, 4.25, 24
2.08, 126mm, 5.125, 24.58

From this we can see that the first two links have been scaled by the same factor but the third has not. Working back using the 24 constant and westburys dimension gives the CtoC for part 2.08 of 103mm not 106mm.

Try that on your simulation and see what happens

[kvom]

I had a nice email from Julius where he explained that he's also using SW and had the same issue.  His solution was to use a tangent mate between the end links with the hole a bit larger than the cross rod.  So his automation worked, basically with a loose joint.  He also offered to send me his SW part files, and that will save me a lot of work in recreating the entire model.

I'll give the other dimension a try tomorrow.

The fact that this engine has never been built is actually a good part of the challenge.

[crueby]

My old school double check would be a few sticks, screws, and mock it up. 

[JackPick]

Might have found it.

Julius part Number, Julius dimension, Westbury dimension, julius /westbury

1-12, 90mm, 3.75", 24
2-06 102mm, 4.25, 24
2.08, 126mm, 5.125, 24.58

From this we can see that the first two links have been scaled by the same factor but the third has not. Working back using the 24 constant and westburys dimension gives the CtoC for part 2.08 of 103mm not 106mm.

Try that on your simulation and see what happens

I think Jason meant 123mm

[JULIUS]

The calculation of JackPick  is correct if you use the theory of Jason.

Julius

[JULIUS]

Grasshopper fans

I have done some more extensive calculations of the parts in question.
Part 2-08 : the center dimension will stay as 126 mm. Calculation turned out to be 125.93 mm
I have made 2 alterations.
1) Part 2-08 only one of the holes is enlarged to 7 mm round. This hole to be put onto shaft/axle/rod 2-10
2) Part 2-07.3 the hole in the bush is now reduced to reamed 6 mm round.
In my Solidworks animation this engine works fine.
I have attached the update drawing.

Julius
 

[Jasonb]

Won't oversize holes give a bad knock as the engine actually runs? You can't hear that on the screen

Yes meant to type 123mm which would be a constant 24mm to the inch scaling factor

You should have some sideways (arc actually) movement at the far end of the beam where you have not shown the link

Also if you want to keep it true to the Muncaster design then it should have a two part beam much like the Easton and Anderson that I built rather than a single one with it's unusual bushes that Julius has drawn.

[kvom]

From my understanding of the beam drawing in ME, Julius' model seems correct.

[Jasonb]

The reasons I said it is a twin beam are as follows.

The ME elevation which does not show anything sectioned has the top of the conrod and top of the pistion rod obscured by "the beam" which means they must both be behind it. The only way that would be true is if the beam or cylinder were offset from the engine ctr line which they are not. You can see the pump rod is infront of the beam but that is correct as the plan view shows the pumps ram offset from the engines ctr line.

The other give away are the three other unused "bosses" shown on the beam, these are the bosses for the spacers that keep the two beams apart.

You can also see that the rods all have split bearings with a means of adjustment for wear as they would on full size, pressed in bearings in the beam would never have been used on a model let alone full size. Same goes for the big end which is shown in ME as being of the marine type and totally prototypical unlike the simple pressed in bearing on Julius' version and this is the joint that has the most rotation.

[kvom]

That makes a lot of sense.

[Gas_mantle]

Is there a way I can see the fully assembled Grasshopper as I'm struggling to visualise what the extra link is that people refer to ?

Cheers 

[Jo]

Don't you hate it when he is always right   

Jo