Author Topic: A Wallaby of my own.  (Read 252231 times)

Offline steamer

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Re: A Wallaby of my own.
« Reply #990 on: January 13, 2019, 10:20:32 PM »
Key way cut    #4-40 tapped holes next
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Offline Tennessee Whiskey

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Re: A Wallaby of my own.
« Reply #991 on: January 14, 2019, 12:11:01 AM »
Looks sweet Tatah Tot  :ThumbsUp:. Just hold a steady course.

Cletus

Offline Admiral_dk

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Re: A Wallaby of my own.
« Reply #992 on: January 14, 2019, 11:08:40 AM »
The hollow big-ends do not make much sense to me on this kind of engine- a lot of extra work without much difference on the end result  :shrug:

The rest looks very nicely done  :ThumbsUp:

Offline steamer

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Re: A Wallaby of my own.
« Reply #993 on: January 14, 2019, 12:09:59 PM »
The hollow big-ends do not make much sense to me on this kind of engine- a lot of extra work without much difference on the end result  :shrug:

The rest looks very nicely done  :ThumbsUp:

Yeah im sitting here thinking the same thing    im trying to understand their purpose though before dismissing them.

Dave
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Offline Roger B

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Re: A Wallaby of my own.
« Reply #994 on: January 14, 2019, 12:45:19 PM »
The hollow big ends may be for weight reduction and the balance weights are calculated on that basis  :headscratch:

The end plugs as drawn appear to be at risk of blocking the oilway  ::)

Glad to see more progress  :ThumbsUp:  :ThumbsUp:  :wine1:
Best regards

Roger

Online Jo

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Re: A Wallaby of my own.
« Reply #995 on: January 14, 2019, 01:19:51 PM »
 :thinking: There does seem to be a problem with the balance weights being a bit lightweight. If you use steel instead of bronze the original article suggests making them thicker - 8mm instead of 6.35mm thick.

But if balance was a problem why did they originally have bronze connecting rods  :noidea: Ali ones would save weight. I'll go and ask if I am allowed to see Surus's set to find out what his connecting rods are made of.

Edit: He is now feeling cheated as he has found he doesn't have any connecting rod castings in his set  :facepalm:

Jo
« Last Edit: January 14, 2019, 01:26:09 PM by Jo »
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Offline steamer

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Re: A Wallaby of my own.
« Reply #996 on: January 14, 2019, 01:36:25 PM »
Sounds like I need to do a bit of math and see what all the bother is about....that slug is 5.13 grams or so
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Offline steamer

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Re: A Wallaby of my own.
« Reply #997 on: January 14, 2019, 01:37:47 PM »
Thats before you put the brass plugs in.  Each plug is a gram each.   So a 3 gram savings in weight.....im wondering if the intent was to provide for a temporary reservoir of oil for ths con rod.
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Offline Roger B

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Re: A Wallaby of my own.
« Reply #998 on: January 14, 2019, 02:10:05 PM »
I wondered the same but seeing as there is one intermittent feed by the center main bearing and two outlets it is unlikely there will be much oil left in the big end space after a run.
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Roger

Offline steamer

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Re: A Wallaby of my own.
« Reply #999 on: January 14, 2019, 02:32:18 PM »
I wondered the same but seeing as there is one intermittent feed by the center main bearing and two outlets it is unlikely there will be much oil left in the big end space after a run.

Thats not how it works.  The pump is engine driven and continuous.   And feeds via an annular groove in the center bearing.   My comment only applies if the pump fails to pump.
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Offline Roger B

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Re: A Wallaby of my own.
« Reply #1000 on: January 14, 2019, 05:16:12 PM »
Ah  :facepalm: I missed the groove in the centre main, I know that it is continuously pumped.
Best regards

Roger

Offline steamer

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Re: A Wallaby of my own.
« Reply #1001 on: January 15, 2019, 03:07:58 AM »
Alright Crankshaft is done! It was a fun challenge. On to the next part!!! Counter weights and connecting rods.

I'm going to noodle on the hollow crank pins.    It's a simple engine..   Two single cylinder engines with individual counterweights.    So the counterweights should be equal to all the rotating mass and half the reciprocating mass...so I'll noodle that out on CAD tomorrow and come up with some expected weights of these parts and see what adds up.   For the record, the print calls for hollow crank pins, bronze counterweights, bronze of Aluminum rods and aluminum pistons.     Now the rods really have me wondering...there will be a pretty big difference in weight between the two materials....but we'll let the math do the talking.

Dave
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Offline b.lindsey

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Re: A Wallaby of my own.
« Reply #1002 on: January 15, 2019, 03:18:58 AM »
Looks good Dave! Nice finishes too. Not sure how big the rods are but betting there will be a significant difference in weight depending on material choice.

Bill

Offline steamer

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Re: A Wallaby of my own.
« Reply #1003 on: January 15, 2019, 11:29:39 PM »
OK...so for some math.    Here is Westbury's method for computing the counter weights with actual engine parts to hand.

http://www.modelenginenews.org/etw/etw_bal/p2.html

I'll be doing this more on paper as I want to size some of these parts, and compare different materials....but the process is really the same...stay tuned

Solidworks says...

aluminum rod     11.12 grams

Bronze rod      34.2 grams

 :insane:
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Offline Tennessee Whiskey

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Re: A Wallaby of my own.
« Reply #1004 on: January 15, 2019, 11:47:36 PM »
I’m kinda lost on this crank pin thing. I just can’t see how at this scale and range of rpm running, that hollowing the crank pins would affect balance that much. Now maybe at a several thousand rpm  :shrug:

Cletus

 

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