Muncaster 2 Cylinder Engine

The linkage arms on the Muncaster engine below are supposed to keep the piston rod aligned with the axis of the cylinder but the left arm (one with a middle pivot point) has a slotted hole instead of a round hole, allowing the arm on the right to float in the slot.

This does not keep the piston rod aligned exactly with the cylinder axis because of the slop. Is this normal with this type of guide-linkage ?

Does anyone have geometric design data of how these linkages are supposed to work?

I'll have a look could it be another of Julius' features?

The bottom left-hand pic shows duplicate control arms to the one piston rod, presumably the other pic is simplified for clarity.

The right-hand pic showing the slot does make some sense, since the rod attached to the pillar has to describe an arc otherwise.

pgk

(Yes, the front arms in the detail views have been removed to see the slot in the rear arms)

I understand that each pivot arm describes an arc and assumed (should never do that) that both arms working together would cancel out the problem. If the slot is required, the rod will not be guided properly and will wobble as it moves. Not an ideal design but they look good when they move.

like you say the slot would allow some sideways movement, I have not been able to find where I thought I had details of the linkages but a couple of other examples, not the square tops to entablatures that I mentioned in your earlier thread on both examples.

Here and here

Thanks Jason

If it is anything like most versions of Watt's Parallel Motion Linkage on beam engines, it does not guide the rod end in a truly straight line, but in a very shallow S curve. Which worked ok on large engines for Mr Watt & co when they had very loose fitting pistons with leather piston rings and thick but loose packing around the gland. It allowed for a little bit of movement and misalignment.

So the slot may be to allow a more rigidly/tightly made modern model to function without the linkage binding, as the piston rod is held steadily vertical by the tight fitting piston and gland. So it has the "look" but is not really doing the guiding. Good enough for small model purposes.

If you Google Watts Linkage or Parallel Motion Linkage there is a lot of info. Wikipedia does a good summary, describing the path of the piston rod end as a shallow figure 8 rather than straight line. LINK

Edited By Hopper on 25/10/2022 06:47:45

I took a look at the drawings. That slot is given as 6x5mm with the linkage from it to the piston rod being quite short. It'd be interesting to sketch out parts of the arcs of movement from the flywheel at top & bottom because it looks like the 'wobble' will be quite small with the short length of linkage from there to the piston rod.
I also note that that bearing insert is given as brass - perhaps worth swapping to bronze??

I am not familiar with the drawings under discussion, but I did have a Watt's linkage [on the rear of the Scimitar] which prompted me, years ago, to find this:

**LINK** https://www.gutenberg.org/ebooks/25155

HOW TO DRAW A STRAIGHT LINE

A LECTURE ON LINKAGES

.

Essential reading, I would suggest, for anyone interested in more than just making the holes bigger.

MichaelG.

Edited By Michael Gilligan on 25/10/2022 08:06:32

Some good stuff there but could not obviously spot one like the engine and all are pivots not slots.

I'm 99.9% sure Julius took his from Westbury's artcles about Muncaters designs. What would be useful is if anyone has issue 1756 from Vol 72 that described these engines

.

Sorry, Jason … it was just a broad recommendation, not intended to be specific to this model.

The use of pivots, without things locking-up, is a good demonstration of ‘geometric purity’

MichaelG.

I just put a scan of the Illustration from Westbury's article into Alibre, scaled it to suit Julius' 73mm long link with the slot and then measured the other links.

The ctr of the slot on Julius' one is 26.5, mine 26.235.

The one on the left 90 for jules and 89.5 on mine

But the one anchored on the right has more of a difference as Jules has it at 63mm and it scales out almost 675mm on mine.

Needs the individual links drawing and simulating but small differences like this could make the difference between a round hole and a slot. If the geometric relationship could be found that may make things easier.

Link to Julius' drawings

Link To Westbury's article

But a General Assembly drawing is not necessarily drawn accurately to perfect scale.

I have just learned something ‘new to me’

[quote] unique for that time, these were designed as models, not scaled down industrial items. As such they were designed in their own scale [/quote]

… and, I regret, that ^^^ probably says a lot.

MichaelG.

.

Ref. __ **LINK**

https://modelengineeringwebsite.com/Henry_Muncaster.html

Edited By Michael Gilligan on 25/10/2022 09:08:05

Thanks for the Westbury link, Jason

I have just downloaded the document to read tonight.

MichaelG.

Indeed but it is what Julius used so reasonable to expect that he should get proportions the same as I got which he has in 3 of the 4 dimensions

Don't know if you saw the recent grasshopper thread Hopper but Julius is not averse to putting a 6mm pin in a 7mm hole to get things to move!

Ah ok. I was thinking he had worked off of more detailed drawings.

Yes I remember seeing mention of the pin and hole issue. Julius does seem to churn out an awful lot of these drawings, all of them incredibly detailed. I suppose he can't get all of it right all of the time. And I don't suppose he would build a working model of each project to "prove" his drawings.

Surely in this age of CAD, a virtual model of the parallel motion linkage could be drawn up at various positions and the track of the piston rod eye (or that slotted hole) determined, to see if it would be straight or the more usual shallow S or figure 8? In the bad old days we would have drawn it out in enlarged scale to do the same, or even made cardboard templates pinned to the drawing board and then mapped out with a pencil the track of the piston rod end as the cardboard model went through its full range of motion.

I'm not even sure if Muncaster's original had much more in the way of dimensions as it was only in a single issue and not noted as having a pull out plan like some of his but hopefully there was some text.

Thank you gentlemen for the reference data. You have given me something to play with and do as suggested and plot the linkage motion.

Paul

I suppose you could go straight to the source and ask Julius. His contact details are on the bottom of many of his drawings, including his business name and email address etc.

Edited By Hopper on 25/10/2022 12:52:27