Murray’s Hypocycloidal Engine – Antony Mount

[Mick B1Participant @mickb1]

[Mick B1Participant @mickb1]

I found the action of this engine interesting – it's pretty early for a standalone engine – 1802. The hypocycloidal method seems an overelaborate way of transforming reciprocal to rotary motion, as if the crosshead-and-connrod idea hadn't quite caught on by that time for some reason.

So far I've only turned and bored the cylinder, and flycut the face for the steam chest.

There are supposed to be 19 drawings, but actually there seem only to be 18, and I suspect the sheet with bolts for the steam chest is missing, 'cos I haven't found a detail for them, and the cylinder drawing doesn't tell me how deep to tap the 8 BA holes – have to work that out.

If anybody else has built this, I'd be interested in whether it's practical to use bigger screws than 8 BA. I want to use hex nuts & bolts – even if they're strictly incorrect for period – because I don't fancy making as many square ones as the drawings state! I've got some 7 BA hex left over from a previous project and can see no reason not to use them.

I'm also not well-versed in silver soldering, so I might try and bolt the feet to the frames instead of following the drawing.

I'll post updates as I go on, especially if I make any spectacular cockups… 😀

[JasonBModerator @jasonb]

Some of the more elaborate drive methods were to try and get round patents taken out by the well known engine makers which tended to restrict competition unless you paid them royalties.

Studs and hex nuts would not look too out of place and better than hex bolts/screws. Also has the advantage that you can easily put a 7BA thread onto standard 3/32" Stock so studs are quick to make.

You could have a go at soldering the feet and if they do happen to drop off it won't be much work to clean up the frames and drill for fixings then make the feet from bits of angle that can bolt on.

[michael pottsParticipant @michaelpotts88182]

Mick.

This engine was one of the first small rotative engines, apart from the Bell crank engine of Boulton and Watt. Even Richard Trevithick had barely started in earnest. No one knew how to guide the piston rod without bending it rather too much for its own good. The cross head had not been invented by 1802, but when it was used it swept all else away.

Matthew Murray must have designed this engine about 1800, just as the Boulton and Watt patents were lapsing. Two examples exist, one in the Thinktank museum at Millenium Point in Birmingham, the other is in America, probably in the Ford Museum. Both came from the same site in the West Midlands.

Mike Potts

[Mick B1Participant @mickb1]

Thanks, Jason for the advice on 7 BA studs, and Mike for the bit of circumstantial history.

[Neil WyattModerator @neilwyatt]

In a cunning move a chap called Pickard patented the use of a crank in a steam engine in 1780 putting the kybosh on everyone else (the crank principle was well known) and sparking a frenzy of ingenuity and over-complexity.

Watt must have been unhappy – he'd already got to using a crank after abandoning the use of a ratchet and pawl to generate rotary motion!

Neil

(Thanks to my copy of Watt's Perfect Engine for reminding me of the details)

[Mick B1Participant @mickb1]

OK, so I've made my first mistake.

Cutting the valve ports on the steam chest face of the cylinder with a 3/32" slot drill, I've placed the outside edge of one of the inlet ports about 15 thou too close to the other inlet port.

I think that if I enlarge the port to bring the edge to the right place I'll unbalance the piston strokes because that port will be too big, and I might also get too much overlap on the stroke.

So I think the way to fix it is to reduce the eccentric offset by half the error, ie. about 7 1/2 thou.

Ah, and reduce the size of the pocket in the valve slide by the error.

Anybody got any comments? Thanks in advance.

 

Edited By Mick B1 on 01/12/2017 16:29:00

[JasonBModerator @jasonb]

So are you saying you have 0.094" between one inlet slot and the exhaust and about 0.70" between the other and the exhaust

[Mick B1Participant @mickb1]

Posted by JasonB on 01/12/2017 20:53:15:

So are you saying you have 0.094" between one inlet slot and the exhaust and about 0.70" between the other and the exhaust

If I'd cut the exhaust port, that would be so; but I haven't yet, so I can still aim to position it to split the gap between the 2 inlet ports evenly, and reduce the valve stroke and flow pocket correspondingly – if that makes sense.

[JasonBModerator @jasonb]

That would be the best Idea, If I get a chance later I'll draw it out to check. Recess in valve will want reducing to match.

[Mick B1Participant @mickb1]

Posted by JasonB on 02/12/2017 07:20:28:

That would be the best Idea, If I get a chance later I'll draw it out to check. Recess in valve will want reducing to match.

Thanks. That's what I was calling the 'flow pocket'. Probably reduce the overall length of the valve too to keep the lands the same width?

[JasonBModerator @jasonb]

Looks like if you place the 0.125" exhaust slot centrally between the two inlet slots, reduce height of valve recess bu 0.015" and also overall height of valve by the same 0.015" you will be OK, eccentric throw stays the same.

Midway

Full travel

[Mick B1Participant @mickb1]

Thanks very much for that! 😀

I'll post further as I go.

Edited By Mick B1 on 02/12/2017 18:47:31

[Neil WyattModerator @neilwyatt]

Nice one Dr. B.

[geoff walker 1Participant @geoffwalker1]

Hi Mick

Could I ask, what is the the throw on this engine and how has it been calculated?

I assume from Jasons drawings the throw is the width of an inlet port plus the lap.

So 3/32" + ?

What I'm trying to say is how much is the lap and do you know why it is that amount?

Just curious Mick I am making a similar engine with a similar size valve and trying to understand how the size of the lap is determined.

best Geoff

[JasonBModerator @jasonb]

5/32" throw

exhaust 1/8", inlets 3/32", between ports 3/32" as designed

5/16" recess in valve, 5/8" overall valve

Therefore 1/16" lap

 

Edited By JasonB on 02/12/2017 20:51:22

[Mick B1Participant @mickb1]

Posted by JasonB on 02/12/2017 20:42:32:

5/32" throw

exhaust 1/8", inlets 3/32", between ports 3/32" as designed

5/16" recess in valve, 5/8" overall valve

Therefore 1/16" lap

 

Edited By JasonB on 02/12/2017 20:51:22

Looks as if Jason has the drawings, or built the AM model!

I measured the inlet ports I cut again today in daylight, more carefully, and I think the error is about twice what I previously thought. I dialled off the distance from the end face datum correctly, and then somehow managed to move it, 'cos the intial plunge cut for the incorrect port is in the right place. Next time I'll lock the crosslide like I b100dy should've done.

It may be Jason's solution will still work, with the new error substituted in, but I dunno about risk of leakage in the arc where the plunge hole reaches the correct drawing dimension.

I'm wondering whether it's practical to fill the 'ole wi' soft solder and cut it again – the casting's LG2 or similar, and the steam passages aren't yet drilled, but maybe it's too tricky to get it to soldering point around the slot without distortion or local melting of the casting? Might do a trial run with a bit of scrap LG2 bar.

 

 

Edited By Mick B1 on 02/12/2017 21:17:22

[JasonBModerator @jasonb]

Post the actual sizes and a photo of the port face, only takes a few secs to alter the dims on the sketch.

Not made it but have AM's book which it is in.

[Mick B1Participant @mickb1]

Posted by JasonB on 03/12/2017 07:24:43:

Post the actual sizes and a photo of the port face, only takes a few secs to alter the dims on the sketch.

Not made it but have AM's book which it is in.

Here's a brutally frank picture of the current condition:-

You can see where the initial RH slotdrill plunge nearly touches the height-gauge-scribed line. Dimension across both slots into the arc on the RH one is .495" , 5 thou short of nominal. OAL of slots is .323" (L) and .313" (R) – both undersize but I can't see that mattering.

Experiments with soft solder and a bit of scrap LG2 look promising, though I think I'd run a couple more trials before doing it for real. The engine will only run for tens of hours in my lifetime – maybe a lot less – and on air, so I doubt that soft solder would erode seriously.

[Mick B1Participant @mickb1]

I completed the porting and passages, and have now more-or-less completed the steam chest and cover.

I needed 11 studs 8 BA x about 1 1/16" to secure the steam chest to the cylinder, and I'd bought 2 12" lengths of alleged 8 BA studding.

It measured OK on o/d, but wouldn't screw into 8 BA nuts bought from the same source, into holes tapped 8 BA, or by finger into an 8 BA die with the centre screw tightened so as to open it to maximum. I could only conclude it's not been cut to full depth, so I ran an 8 BA die adjusted as per normal down the full length of both pieces at low revs with a bit of 3-in-1. Seems to've worked OK.

One of those jobs where you work for a couple of hours to produce half a teaspoonful of fine swarf!

[Mick B1Participant @mickb1]

I've now made almost all the piece parts, but I'm stuck for a method to make the slot through the piston rod end for the key and wedge that hold on the bigend strap.

In the bigend strap, I could cut the 1/16 x 5/32 slot using the pilot of a centre drill, which cuts on the flutes as well as the lips. The strap is U-shaped and thin, so the pilot was long enough.

But the piston rod end has a concave radius to fit the lower half of the split-bush bearing, and another 1/16 wide x 5/32 long slot right through the 1/4" thick flat, and the little centre drill pilots are nothing like 1/8 long so won't do.

I haven't any spark-erosion kit. I'm wondering about drilling a line of contiguous 1/16 through holes, milling between them as much as I can from both sides with the centre drill pilot, then grind up some sort of punch with an angled face to try to chisel/scrape/broach away the hole segments the pilot can't get at in the middle.

But that has every prospect of ending up looking like it was carved from an old potato by a drunken sailor.

Anybody know of a simple way to do this?

I'd post a scrap view of the drawing, but have seen that Anthony Mount posts on here and don't want to provoke any copyright issues…

Edited By Mick B1 on 14/05/2018 19:55:07

[JasonBModerator @jasonb]

The way I do these is to use a 1/16" drill to first drill out each end and then get as many holes in between as I can. I Then change to a 1/16" milling cutter and make a series of plunge cuts working along at about 0.010" for each plunge, I think Anthony suggested this method in more than one of his engine builds and it seems to work better than milling along the slot. If the slot is deep you may need to mill from both sides.

This will leave you with a round ended slot and to make this square I have a warding (flat pointed) needle file that I have ground the teeth off of in the flat faces so I just have the edge teeth which means you don't make the slot wider while filing the ends.

J

[Mick B1Participant @mickb1]

Thank you for that, Jason.

Looks like I'll have to splash out on that 1/16 milling cutter (presumably 2-lip slotdrill?), then.

I wasn't sure it was worth squaring off the ends – it might be easier to file radii on the key & wedge, but I'll jump off that bridge when I come to it…

I had been thinking about modding a flat Swiss file to make the rectangular punch affair I was proposing, and taking off the face teeth to stop it opening the slot is a good one – ta.

[JasonBModerator @jasonb]

2 or 3 flute will be OK

[Mick B1Participant @mickb1]

OK, so I bought a 3-flute 1/16 cutter – and, as I suspected I might, broke it on the second pass. Probably should've wound the speed up higher than the 1000-odd I was running at.

So I went back to plan A*, drilled 3 'oles and milled between them with the pilot of a BS2 centre drill. Broke the pilot on one end too, but carried on with t'other. Then poked out the tiny bit of remaining metal with a Swiss file and cleaned up.

The other mistake I made was misread the drawing of the crankpin, so the capped journal for the split-bushed bearing was too short. I could've, maybe should've, remade it, but I cheated and tapped it 5BA and turned the fixed cap down to journal size (3/16) and interposed a stepped washer so's it wouldn't bind.

Here's the resulting subassembly:-

As a macro shot it looks rather agricultural, but it's nice enough to normal vision.

Lotsa work to make 1/4 teaspoon of swarf…

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