Author Topic: Rotary Engine Con Rod Crank Pin Assy  (Read 21563 times)

Offline sbwhart

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Rotary Engine Con Rod Crank Pin Assy
« on: December 04, 2009, 12:44:52 PM »
I've been chewing arround the best way to configure the con rods on the Three cylinder Rotary I'm building.

I'm aware of three there may be more, please shout if you know of any others



1:- Is the method used by Elmer for his three cylinder rotary, what you've got is a spool that centres on the crank pin, with the con rod equaly spaced around the spool on their own crank pin.

2:- Is the method used in the Haylo, what you've got her is a master con rod attached to the crank pin, with the other two con rods attached to the master con rod.

3:- Is the method used in the Cygnet Royal what you have her is a master con rod with a extened bearing bush attached to the rod that roates around the crank pin, the other two rods run on the outside of the bearing bush with cranked con rods.


Like most things all these designs are comprimises 1:- Is compact but not very robust, and you don't have a stright action on the pistons 2:- again is compact but you don't have a straight action on two of the pistons3:- Is not as so compact but you have a straighter action.


What I've had in mind is to modiified version of option 3:-

4:- Instead of having the extended bearing bush, just have a short bush on the crank pin master con rod with the same on the cranked con rods all bearing round the crank pin:- like this




Any ideas, comments, criticism will be welcome.

Cheers

Stew
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Location:- Crewe Cheshire

Offline tinkerer

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #1 on: December 04, 2009, 01:37:51 PM »
Check this out. I don't know how you coould improve on this design concept.
http://upload.wikimedia.org/wikipedia/commons/a/ab/Radial_engine.gif
« Last Edit: December 04, 2009, 09:21:44 PM by tinkerer »
Tink

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bogstandard

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #2 on: December 04, 2009, 04:03:58 PM »
Stew,

As you know, I have built the Cygnet Royale, and the crank shown on the plans works very well.

You have to be very careful when going over to another crank system, especially with shorter rods. The whole geometry of the crank swept area can change.

The only real way to do it would be to put what you propose into a CAD package and when animated, see if any obvious problems show up.

The usual problems are that the conrods can hit the bottom edges of the cylinders because they are sweeping over a larger angle, and length of strokes can change, causing either too little or too much movement of the piston, which in turn could require a cylinder length change and maybe timing adjustments.


John

Offline kvom

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #3 on: December 04, 2009, 06:45:56 PM »
While a CAD emulation is nice to have, you can calculate most of the critical factors yourself, and using a spreadsheet lets you experiment with various dimensions.

The stroke will be just twice the distance from the rod end to the crank pin regardless of the configuration.  The angle the rod makes is at a maximum when the crank pin is at 90 degrees to the cylinder centerline and can be calculated with a bit of trig.

A nice thing about a radial like Halo is that the input and output ports are all at the top of the cylinder, so that as long as the piston stays within the cylinder boundaries it will run.

Russel

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #4 on: December 04, 2009, 07:49:06 PM »
Keep in mind that configuration number one, Elmers, the hub is soldered or brazed to one of the rods, so it also has a master rod. The halo version, number two, the hub and master rod are machined in one piece. 

I realize that you are probably considering the differences in machine methods required, more that the configuration of the designs. I just thought I would point out that the Elmers version has a master rod also because it isn't super apparent in your diagram. (I know it took me a little while to grasp how a radial engine could connect all the rods to one hub without destroying itself while it was running.)

Russ

Offline sbwhart

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #5 on: December 05, 2009, 01:29:00 AM »
Thanks for the input Guys

Russ:- I completely missed the fact the Elmer hub was fixed to the spool making it a master rod, thanks for pointing this out.

I can remember reading an article in model engineer about radial engines regarding, connecting the rods to the master and the effect on the engine timing, the article ran over a couple of issues, apparently with a full size engines the slave rods wern't spaced equally around the master rod to compensate for the geometry effects on the timing, it concluded that for scale models however the small differences involved were not significant to good running of the engine, it was these series of articles that got me interested in radials.

Thanks again

Stew

A little bit of clearance never got in the road
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Location:- Crewe Cheshire

Offline John Hill

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #6 on: December 05, 2009, 03:01:40 AM »
I think the master rod (aka 'king rod') is the technique used in real life aero engines.
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bogstandard

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #7 on: December 05, 2009, 03:24:01 AM »
This is what I was on about, have a look at the conrod sweep on the lower left cylinder.

I don't know whether it is a drawing error or design error.

This Gif image came from the F.A.M.E site


John
« Last Edit: December 05, 2009, 03:25:39 AM by bogstandard »

Offline John Hill

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #8 on: December 05, 2009, 03:31:20 AM »
This is what I was on about, have a look at the conrod sweep on the lower left cylinder.

I don't know whether it is a drawing error or design error.

This Gif image came from the F.A.M.E site


John

There has to be some variation as the pins on the king rod do not trace a true circle and this is worse for those furtherest from the king rod.
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bogstandard

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #9 on: December 05, 2009, 03:55:03 AM »
John,

What I am trying to get over, on the original plans, it calls for something similar to a knife and fork arrangement on the big end, but in this case, two conrods are offset to each side of the main rod, using full length rods with a common crank pin, whereas when you go for this arrangement, with shorter rods and pivot point distances using a king rod and plate arrangement, the angles of operation of the rods change, and has to be proved before going any further as it might require modification to other components to prevents mechanical locks.

One thing I do know, having built it, the rods on this engine are very close to the cylinder bases as they are, and as I suspect, by changing the arrangement, problems will be found.

It is OK to modify things, but only if it benefits the original design, and even then, all related components have to be made compatible with the changes made.


John

Offline sbwhart

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #10 on: December 05, 2009, 05:20:34 AM »
John,

What I am trying to get over, on the original plans, it calls for something similar to a knife and fork arrangement on the big end, but in this case, two conrods are offset to each side of the main rod, using full length rods with a common crank pin,

Thats what proposal at 4 is.:- all con rods will rotate around a common crank pin centre line as original design.

Cheers

Stew
A little bit of clearance never got in the road
 :wave:

Location:- Crewe Cheshire

Offline John Hill

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #11 on: December 06, 2009, 03:38:12 AM »
Stew, I think your cranked rods are a good way to go as it is clear from John B. Standard's GIF that the geometry is warped a bit by using the king rod approach.  Obviously Messrs Pratt and Whitney know how to get around that! :coffee:

Your cranked rods will become a challenge when you go on to your 9 cylinder version! :lol:
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Offline sbwhart

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #12 on: December 06, 2009, 04:30:33 AM »

Your cranked rods will become a challenge when you go on to your 9 cylinder version! :lol:

Thanks John

I was thinking of a 16 cylinder Job  Like this. http://www.enginehistory.org/bristol_hydra.htm

I've watched this engine grow at model expos over the last few years.

this engine is a labour of love

Enjoy

Stew
A little bit of clearance never got in the road
 :wave:

Location:- Crewe Cheshire

Offline NickG

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #13 on: December 06, 2009, 05:19:44 AM »
Stew,

I think yours is the best concept, everything on the one crank pin an no bush because you'll be having 3 ball races the same size. They'll just have to be small ones so there is no interference due to increased big end size.

Nick
Location: County Durham (North East England)

Offline sbwhart

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #14 on: December 06, 2009, 08:06:55 AM »
Thanks for your input chaps.

I'm going to try option 4 as this is only a three cylinder engine the cranked con rod should be OK with any more cylinders the degree of crank would become unworkable, I've got some small needle roller bearing that just mite do the job, but I've got to measure them up first and do the drawing to see what space envelope I've got to play with.

Cheers

Stew
A little bit of clearance never got in the road
 :wave:

Location:- Crewe Cheshire

Offline dsquire

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #15 on: December 06, 2009, 01:37:34 PM »

Your cranked rods will become a challenge when you go on to your 9 cylinder version! :lol:

Thanks John

I was thinking of a 16 cylinder Job  Like this. http://www.enginehistory.org/bristol_hydra.htm

I've watched this engine grow at model expos over the last few years.

this engine is a labour of love

Enjoy

Stew

I just had a look at this site and wow. Pay particular attention to the layout of the connecting rod. 5 cylinders on 1/2 of center and 3 cylinders on the other 1/2. I have attached a link to the drawing so you can see what I mean.

http://www.enginehistory.org/ModelEngines/Perkins/Hydra/HydraDwgs/HydraRods.jpg     Connecting rods for16 cylinder radial

Cheers  :beer:

Don
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Offline sbwhart

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #16 on: December 06, 2009, 02:20:15 PM »
Hi Don thanks for the drawing link its mindbogling.

Every time I look at the site I see something new look at this for a double Rotary Table set up.

http://www.enginehistory.org/ModelEngines/Perkins/Hydra/DiffuserVanes_1.jpg

Cheers

Stew
A little bit of clearance never got in the road
 :wave:

Location:- Crewe Cheshire

Offline tinkerer

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #17 on: December 06, 2009, 02:29:24 PM »

[/quote]

I just had a look at this site and wow. Pay particular attention to the layout of the connecting rod. 5 cylinders on 1/2 of center and 3 cylinders on the other 1/2. I have attached a link to the drawing so you can see what I mean.

http://www.enginehistory.org/ModelEngines/Perkins/Hydra/HydraDwgs/HydraRods.jpg     Connecting rods for16 cylinder radial

Cheers  :beer:

Don
[/quote]
I am not seeing this. I see 8 cylinders evenly spaced at 34.8 deg all on the same plane.
Tink

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Prov 13:19

Offline dsquire

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #18 on: December 06, 2009, 03:53:29 PM »


I just had a look at this site and wow. Pay particular attention to the layout of the connecting rod. 5 cylinders on 1/2 of center and 3 cylinders on the other 1/2. I have attached a link to the drawing so you can see what I mean.

http://www.enginehistory.org/ModelEngines/Perkins/Hydra/HydraDwgs/HydraRods.jpg     Connecting rods for16 cylinder radial

Cheers  :beer:

Don
[/quote]
I am not seeing this. I see 8 cylinders evenly spaced at 34.8 deg all on the same plane.
[/quote]

Tinkerer

Look at the spacings of the connection points on the main connecting rod. This is for 8 cylinders so there are 2 of them for 16 cylinders. The 34.8 deg dimension is to a center line not the next cylinder. 5 of the secondary connecting rods are on the right side of this center line and 2 on the left of the center line. The 8th cylinder is the main connecting rod.

Hope this makes sense Tinkerer

Cheers  :beer:

Don
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Offline tinkerer

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #19 on: December 06, 2009, 07:54:21 PM »
Aha, I see the 34.8 deg is to the center of the slot. It depends on which center line you look at. If you look at the line from wrist pin to crank, they are equally divided, but at perpendicular center line they are divided 5 & 3. I think the other 8 cylinders would be just another bank of 8 but I could be wrong about that too.
Tink

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Prov 13:19

Offline 75Plus

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #20 on: December 06, 2009, 08:43:19 PM »
Does anyone else find it odd that the Hydra has an even number of cylinders? I cannot remember ever seeing a radial engine that had not either an odd number of cylinders or a multiple there of. The P-47 of WWII fame had two rows of 9 cylinders while the B-36, which never dropped a bomb in anger, had 6 engines each of which had 4 rows of 7 cylinders.

All the WWI rotary engines in the Air Force Museum have an odd number cylinders also.

Joe

Offline tinkerer

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #21 on: December 07, 2009, 12:10:54 AM »
Does anyone else find it odd that the Hydra has an even number of cylinders? I cannot remember ever seeing a radial engine that had not either an odd number of cylinders or a multiple there of. The P-47 of WWII fame had two rows of 9 cylinders while the B-36, which never dropped a bomb in anger, had 6 engines each of which had 4 rows of 7 cylinders.

All the WWI rotary engines in the Air Force Museum have an odd number cylinders also.

Joe
I did find that odd also. I have worked on several and they were either 5 or 9 cylinder. Never considered that even numbered even existed.
Tink

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Offline sbwhart

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #22 on: December 18, 2009, 04:34:20 PM »
Hi Chaps

Domestic duties kept me out of the shed today, that and its bloody cold, well for us soft limies it is.

Spent a bit of time in the nice warm house doing a bit of CAD on the crank shaft design, until I'd drawn it out I thought it would just be a simple off set of the con rod, but no on the origional cygnet design the master rod is quite thin 1/8 but in this design this doesn't matter because of the way the bearing is orientated, but the new design requires a thicker rod to give the bearing a bit of width so you have to give the slave rods a bit more crank. Anyway after bit of fidling and head scratching  :scratch: came up with this design.



I'll have to machine the con rods up with an angled web, this goes against my origonal objective of keeping the maching simple, and I've not yet worked out how to machine them.

So what do you think mmmmmmmmmmmmmmmmmmmmmmm

Stew

A little bit of clearance never got in the road
 :wave:

Location:- Crewe Cheshire

Offline Stilldrillin

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #23 on: December 19, 2009, 06:31:58 AM »
I'll have to machine the con rods up with an angled web, this goes against my origonal objective of keeping the maching simple, and I've not yet worked out how to machine them.

So what do you think mmmmmmmmmmmmmmmmmmmmmmm

Stew

Hhhmmmmmmmmmmmmm.............  :scratch:  ::)

David D

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Still modifying bits of metal... Occasionally, making an improvement!

Offline Bernd

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Re: Rotary Engine Con Rod Crank Pin Assy
« Reply #24 on: December 19, 2009, 10:04:24 AM »
Stew,

I'm thinking side forces on the whole assembly.  :med:

Bernd
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