Lusus Naturae – Drawing Revisions

[JasonBModerator @jasonb]

[JasonBModerator @jasonb]

Good to see that the missing dimensions have been added to the web site.

 

Unfortunately these revisions are wrong, the Valve cranks are made from two pieces of 1/16″ material and  you have now changed the overall thickness to 1/2″ not the 1/8″ as printed in the magmust be used to working in metric as the 1.5mm and 3.0mm sizes are right.

 

There is still no vertical centre distance for the holes, though it looks like its the same as the horizontal at 3/8″

 

And for consistancy should the 3/8″ hole centers also have a nominal 10mm as all the other imperial sizes have a metric equivalent shown.

 

Jason

Edited By JasonB on 09/02/2010 19:42:34

Edited By JasonB on 09/02/2010 19:49:05

[David Clark 13Participant @davidclark13]

Hi There

I only added the missing dimension.

However, I used the original drawing, not the one in the original magazine.

This must have had the incorrect 1/2in. dimension.

I have forwarded corrected drawing to web editor to upload.

regards David

 

[RichmondParticipant @richmond]

Hi,

 

Tthe new drawing has been uploaded as per David’s request

 

Rgds

 

[JasonBModerator @jasonb]

Thanks guys but the vertical centres of the holes are still missing.

 

Jason

[Nick Clarke 1Participant @nickclarke1]

Hi there, Where on the website can I find the baseplate drawing and correctly dimensioned valve gear drawing for Lusus Naturae? They were there last time I looked, but I can no longer see them.

Many Thanks,

Nick

[JasonBModerator @jasonb]

Right at the bottom of the home page, just keep scrolling down

 

J

[WearyParticipant @weary]

Is anyone building or attempting this engine?  I would be interested to hear of your endeavours as I am considering having a go at this engine.  Whilst having a look at this as a project ………

 

I have had a quick look at the geometry of this engine as drawn and have the following inital observations, which I throw in for any comment.  Please note they are all as a result of calculation and drawing and have not been proven by construction:

 

1) Beam Support Arms are too long as drawn (page 12).   The 41mm centre to centre length needs to be reduced to  38mm

The engine will not work with them 41mm apart.  

Stan Bray almost certainly arrived at the 41mm  figure by drawing rather than calculation, the introductory photo’ at the top of page 8 shows that he had to shorten his Beam Support Arms by bending in an attempt to get the engine to run.

 

2) I would suggest that the pivot hole in the Beam Support (drawing page 11) would be better arranged to be on a level with the horizontal Beam center-line at mid stroke. 

This could be arranged either by making the Beam Support taller – base to pivot centre 33mm; or by making the beam connecting rod pivot to pivot dimension 41mm and similarly shortening the Swinging Arm to be 89mm pivot to pivot. (All dimensions allow for gasket in cylinder end).  The second option would be my preference, I have sketched it out and it looks OK.

 

This will reduce variations around a radius which need to be taken up at the Swinging Arm to the minimum.  The further the Beam Support pivot is from the Beam center-line at mid stroke the greater the variation around a radius of the Swinging Arm lower pivot at that end of the Beam.  As drawn the upper Swinging Arm pivot holes would need ‘easing’ to form an elongated slot along the axis of the arm. 

 

From the text I am guessing that Stan Bray never got his engine to work smoothly – if at all – despite his attempts to fix the Beam Support Arms conflict as the Swinging Arm movement is still in conflict.

[WearyParticipant @weary]

Addendum to my post above:

 

Re: Point 2)  The dimensions in the post above point 2 are incorrect as they do not apply to the plan as drawn in the magazine but refer to my personal calculations and scaling.  The principle of having the beam horizontal at mid’ stroke is correct, and I am suggesting that the dimensions should be adjusted to achieve this.

(Sorry about the confusion but I am considering making the engine to 2x the plan and making it double acting.)

 

The suggested dimensions to the scale and plan as drawn should be:

Beam Support to be 25mm from base to pivot center,

or,

Swinging Arm to be shortened to 45mm from pivot to pivot.

Either will place the Beam horizontal at mid’ stroke, the Beam Connecting Rod should in both of these suggestions be shortened to 38mm.

 

(But you know now to check my work!!)

[CirclipParticipant @circlip]

Come on guys, where are all the howls of derision at Weary”s mistake??? What it does prove to all those that constantly witter about drawing errors is that even when there is no pressure, how easy it is to get one or two sums wrong?  24 Hrs. from publishing to correction, well done Weary, Once the presses roll it’s a public hue and cry.

 

  Regards  Ian.

[BobParticipant @bob17059]

I am building this engine at 1.667 times the original size (to suit a piece of material I had to hand).

 

I have just completed the cylinder and cylinder head and fitted a piston. I am concerned at the amount of compression/vacuum at the top end of the cylinder. Have I missed something? Should there be an air vent in the top of the cylinder head?

 

Bob

[WearyParticipant @weary]

Built to your dimension there may be problem, particularly if you make the piston rod a good fit in the piston gland.  By inference, in the original design Mr Bray made the gland and piston rod hole a loose fit to allow air to pass.  You could always drill a passageway somewhere discrete, or fit a relief valve if you prefer.

 

…..”By virtue of the design, there is no need for the piston rod hole to be steam tight since there will be no steam in this area.  However if desired a dummy gland can be fitted…….” The best of Model Engineer, Vol.2, page 10, third column, first new para.

 

Regards,

Phil

[BobParticipant @bob17059]

Hi Phil

 

Situation regarding vacuum/compression above piston now eased by 2 mm hole in top pf cylinder head.

 

Regarding the layout of grasshopper engines in general, looking at Antony Mount’s drawing  (Historic Engines Worth Modelling Vol 2 General Arrangement Page 30) it shows the beam horizontal and the centres of the end holes equidistant from the centre of the end of the radius rod. The radius rod is also horizontal and in this position the fixed centre of the radius rod coincides with the centre of the end hole of the beam. The swinging arm is then perpendicular to the beam. This means that the centre of the cylinder must be the same distance from the centre of the swinging arm bearing as the centres of the end holes in the beam.

 

Using the drawing of the base for Lusus published on this website and the Imperial dimensions from the article in The Best of Model Engineering Vol 2,  the distance between the centre of the cylinder to the centre of the swinging arm bearing is 2 31/32″ but the end holes on the beam centres are 3″ apart.

 

Further, the radius rods (aka Beam Support Arms) are dimensioned at 1 5/8″ but made to suit the job. Is the 1 5/8″ dimension the undeveloped length before the jog is put in? I would have expected these centres to be 1 1/2″ apart.

 

The relationship between the length of the radius rod and the main centres on the beam should be:

 

AB = Length of radius rod

BP = Distance between mid hole centre and piston rod eye centre

CB = Distance between mid hole centre and swinging arm centre

 

AB x AB = CB x BP

 

I am sure that if CB = BP them AB = CB = BP

 

I wonder if the loose fit between the gland and piston rod may be important in more ways than just allowing the escape of pressure above the piston, could it also easy any lack of true parallel motion of the piston rod eye?

 

(The drawings show both the piston rod and the bores in the cylinder head and dummy gland to be 3/32″)

 

Bob

[WearyParticipant @weary]

…… the radius rods (aka Beam Support Arms) are dimensioned at 1 5/8″ but made to suit the job. Is the 1 5/8″ dimension the undeveloped length before the jog is put in? I would have expected these centres to be 1 1/2″ apart.

 

Yes, I agree, the 1 5/8″ dimension is prior to setting in the offset (jog).  The length of the Beam Support Arms should be 1 1/2″, but note that (I seem to recall) Mr Bray confused the geometry a little more by making the beam non-horizontal at piston mid-stroke.  In part this is why he had to kink the arms in his original construction to get the engine to run, and why the actual centre lengths will not be exactly 1 1/2″ if built using the dimensions as printed.  I guess it is also why he is vague about the ‘kinking’ in the text.  If you look at the photo at the head of the article you can see that the arms are kinked in the vertical plane so shortening them without affecting the horizontal offset that he mentions.

 

…..I am sure that if CB = BP them AB = CB = BP.

 

Yes – here is a nice explanation also mentioning that ……… small horizontal movements of C give rise to linear movements of P – to a very good approximation…… Hence you are almost certainly right when you write…..

 

……..I wonder if the loose fit between the gland and piston rod may be important in more ways than just allowing the escape of pressure above the piston, could it also easy any lack of true parallel motion of the piston rod eye?

 

Regards,

 

Phil

Edited By Weary on 09/08/2010 19:42:41

Edited By Weary on 09/08/2010 19:48:19

[BobParticipant @bob17059]

Hi Phil

 

I am looking at moving the radius rods to the inside of the supports. I don’t think there is any prototypical president for doing so, but there seems to be room if the rather ungainly nuts are omitted from the beam end pivots. I am planing to use tiny circlips to retain the pivot pins and as these will not be visible no one will be any the wiser! This will also allow me to make the radius rods straight.

 

Bob

[WearyParticipant @weary]

Like this one in Silk Museum at Derby Museum of Industry and History?

 

Or, this one, at Lound Waterworks?

 

Scroll down this site to second pic down, for yet another example.

 

Or even this one in Manchester Museum of Science and Industry. 

 

Another one here at Wrotham Park Estates.

 

Even a giant grasshopper in Birkenhead

 

My guess from what I have seen on-line is that in fact your ‘design improvement’ makes the model more prototypical, simply because it is better engineering!  I cannot find any pics of a real engine arranged as Lusus Naturae. 

But….. ‘prototype for everything’, I bet that there is at least one!  The contemporary illustration on this page appears to show a machine arranged as Lusus Naturae, however I am not terribly convinced that this is how it was actually constructed .

 

Phil.

Edited By Weary on 11/08/2010 22:39:56

Edited By Weary on 11/08/2010 22:45:07

Edited By Weary on 11/08/2010 22:58:45

[BobParticipant @bob17059]

Just a quick update on my version of Lusus. I am building it at 1.667 times the original size to suit material I had to hand.

 

Point 1   I made the swinging arm exactly to to the drawing. I then measured the centre height of the top bearing with the arm set vertical. I used this height to mark the centre positions on the supports for the radius rods on top of the cylinder. I fitted the material to the cylinderhead to do this. Once this was established I marked out the remaider of the supports and drilled and filed to shape. This ensured that at mid stroke I could get everything in line and level.

 

Point 2 I have changed the design to make the radius rods fit inside the supports on top of the cylinder head. I have fitted counterbored bronze bushes to the radius rods to save some space as it is a bit tight here. The radius rods are 2.5″ between centres, exactly the same distance as between the end eye of the beam and the mid- centre of the beam.

 

Point 3 My piston rod is a nice sliding fit in the cylinder head. ( 5/32 ” reamed hole and commercial 5/32 ” stainless rod lightly polished.

 

Point 4 Con rod made of two pieces of steel. The top fork has a tapped M4 thread inside. The rest of the rod is turned (between centres) and left slightly over length. Once installed it can be adjusted by turning a bit off the threaded portion and extending the thread. This process is repeated until the engine can turn over without the top of the piston fouling the cylinder head. Once the length has been established it is taken apart, degreased and then threadlocked in place with Loctite. Put two pieces of rod through the con rod eyes and set the assembly on a pair of parallels to cure. This ensures that the axes of both ends are parallel.

 

So the good news is that once assembled everything runs very smoothly!

 

Next challenge is to sort out the valve gear.

 

Bob

Edited By Bob on 19/08/2010 19:30:45

[Author]

Posts