Clamshells & Transfer Shape

[LegDeLimber]

Is there any difference in the diameter of the crank weights?
Or where is more of the mass located?
Just for giggles. Have you tried to do some math on the balance percentage factor?
If you have a scale for it, did you weigh the two crank assemblies, shown in your pic?
How about piston weights?
Is it possibly tuning the mass to the anti-vibe?

just throwing some of the fragmented thoughts I'm having. maybe trigger someone elses analysis.

 

[LegDeLimber]

Yeah it's a little funny how you'll see that radiused groove touted as a cure for stress raisers, on one page of a book
Yet on another page they tell you to notch a broom handle, in a very similar fashion,
and then apply a side load to snap it at that notch.

 

[Chris-PA]

Well, OK, there may be other effects of having such wide weights, but I wanted to locus on the idea of the flat, wide front of the counterweights helping to push fresh charge up the transfers.

Basically, does it function as a crankcase supercharger? It is a spinning impeller in a case, with a valve that opens an outlet port and a valve that opens an inlet port at appropriate times (and the counterweight shape looks to be designed appropriately for that). OK, the impeller has got really poor sealing to the case and a lot of bypass, but does it help pump anything?

 

[woods works]

I'm thinking you might be right that the flat weight surface helps the transfer flow, but only at slow pull-start speeds. At running speed seems it would be like a boat propeller that's cavitating, just spinning in its own bubble. Then think of the crank stuffers on the 545/2252 vs 550/2253. They reduce case volume but also improve flow?

 

[hotshot]

So is the transfer flow laminar or turbulent, and have you calculated the Reynolds number yet?

Test on Friday.

 

[Chris-PA]

.

I'm thinking you might be right that the flat weight surface helps the transfer flow, but only at slow pull-start speeds. At running speed seems it would be like a boat propeller that's cavitating, just spinning in its own bubble. Then think of the crank stuffers on the 545/2252 vs 550/2253. They reduce case volume but also improve flow?

Yeah, it's clearly a different idea from crank stuffers (assuming that it's really anything like what I've described, which it may not be). I'm not sure it's quite the same as a cavitating prop - there is an outlet that opens up in one spot and an inlet that opens in another, so it should pump.

Not too different from the smog pumps on cars - except that those have shafts offset from the circumference, positive displacement, seals and fixed ports!

I dunno, I may be all wet - it still seems the configuration should help improve flow up the transfers and to pull mix into the case.

 

[Fubar]

when i build racing engines the crank receives special treatment , lightning , polishing balancing and i bevel the crank ends to a knife edge , this reduces turbulence and friction in the engine , especially at high rpm , i do not know if it will do anything for a 2 stroker or even hinder its ability to run at all , but i am going to give the next one i build the treatment to find out .

 

[LegDeLimber]

I always preferred just putting the radius leading from the journal diameter and sweeping into the shoulder face, not notching a shaft.
then specing whatever fits onto the shaft, to have a sufficient radius to fit the shaft.
Of course we're always concerned about tooling conditions, no matter how we do it.
gotta avoid those little machining marks/lines that become cracks.
I've looked at pics of pistons that sometimes get cutaways and nearly go apoplectic
at the file marks in them.
Used to see guys take a side grinder and open up the throat of a chain hook, a small bit,
So as to fit over a steel plate.
Nasty gouged cuts in the gullet! Always tried telling them, IF you're going to do that,
Do it with a die grinder so that any cut lines/marks are perpendicular to the load.
You could get hooks that had a that slightly wider gullet and were rated for that side load
vs a strait lift.
But they of course they had this huge set of side bulges on them ( and I know why) that wouldn't let you rotate them and remove them
after the workpiece was in the place you needed it to be.

 

[LegDeLimber]

But these days I'm more concerned with drug tested morons that wont stop and change a chipped insert.
And then there's the rough handling of workpieces.
Perhaps the big dent in the 2nd one explains why the fuzzy first one was cut so badly undersize.
These slip rings are dated at 4 yrs apart, I hate to think of this shop needing to turn something with a bit more load than a pair of brushes.

 

[Moparmyway]

That's a commutator ?
With that finish the brushes wont last too long

 

[LegDeLimber]

Yeah the first one [ img 2721] was returned. You see how thin the rings were cut?
Shoulda heard me ranting about it before I (politely) took it back.
The second one with big dent and displaced copper, I'm just going pull that one apart
and clean that set up.
Ignoring the dent for a sec, the surface and a few tearouts says slow speed and dry cut.
You should see the differences on the test spec stickers on them.
And i'll shut up about it here, so as to not further derail and clutter Chris-pa's thread.
Might go down and start a thread in the "off topic", perhaps "readers rides" section if anyone actually wants to see more off my little adventure.

If Chris or anyone else would prefer, I'll gladly delete these offtrack posts from here.

 

[joe25DA]

Micro poulans have 1 counterweight.

 

[Chris-PA]

If Chris or anyone else would prefer, I'll gladly delete these offtrack posts from here.

It's not a problem - I wish I knew a quarter of what you do about tooling and machining!

Micro poulans have 1 counterweight.

Yeah, and some little Dolmars too. But the only other saws I've seen so far with big fat counterweights with flat front edges are some newer cheap Homelites, and they've clearly been copying Poulan.

So if what I described is even a real effect, clearly it's not a commonly used one. Looking at the later cylinder and crankshaft designs of these saws, it would appear they tried to reduce the effect. Maybe it contributed to scavenging losses and made meeting emissions limits tougher?

 

[bikemike]

So to look at this further, I decided to make myself a new toy! In this image, the crankshaft is rotaing counterclockwise, the piston is coming down, and the counterweights are just about to cross into the transfers:

View attachment 329937

Notice how wide and flat the front of the counterweights are, and the rod is narrow as is the slot for it. The crank throw is also wide and flat. The volume inside the case is split into two separate regions by the counterweight assembly, with only the rod slot and the gaps around the outside:

View attachment 329938

Here is the situation just before the intake opens up - the piston is going up and the counterweights are moving away from the piston:

View attachment 329940

I have to believe that at 8000 rpm that spinning crank is helping to blow charge up the transfers as well as to pull mix into the case.

Also, these engines have a lot of squish, which is something I have expected. My suspicion is that they intentionally kept the compression down for easier starting by the inexperienced. I think it is why I just cannot get them to run like the other Poulan design used in my 2775 (and similar to the Husky 142).

View attachment 329939

Nice cut away job looks good

 

[bikemike]

.

Yeah, it's clearly a different idea from crank stuffers (assuming that it's really anything like what I've described, which it may not be). I'm not sure it's quite the same as a cavitating prop - there is an outlet that opens up in one spot and an inlet that opens in another, so it should pump.

Not too different from the smog pumps on cars - except that those have shafts offset from the circumference, positive displacement, seals and fixed ports!

I dunno, I may be all wet - it still seems the configuration should help improve flow up the transfers and to pull mix into the case.

Alm that cavitation talk is also working with dense mass. Air and fuel vapors are much lighter and can also be compressed unlike h20. The crank lobes spinning can cause a centrifugal force with the little mass that is accumulated in the case and send it in a outward direction. So that mass will need to find a cavity for escape. A turbo also is the good example of what im trying to say about this topic. You pull air into a cavity and you have a stationery rotating device that is slinging it out wards till it finds an escape

 

[Chris-PA]

Alm that cavitation talk is also working with dense mass. Air and fuel vapors are much lighter and can also be compressed unlike h20. The crank lobes spinning can cause a centrifugal force with the little mass that is accumulated in the case and send it in a outward direction. So that mass will need to find a cavity for escape. A turbo also is the good example of what im trying to say about this topic. You pull air into a cavity and you have a stationery rotating device that is slinging it out wards till it finds an escape

True, but there are some differences. The mix in the case goes around only once, and there are ports opening and closing and the case geometry is changing the whole time. At this position any gas flung toward the outside is meeting the descending piston:

Maybe just another way of looking at what I was getting at - the counterweights are contributing to increasing the local pressure right before the transfers open.

 

[Moparmyway]

At this position any gas flung toward the outside is meeting the descending piston:
Maybe just another way of looking at what I was getting at - the counterweights are contributing to increasing the local pressure right before the transfers open.

And meeting the pressure wave created by that descending piston, creating a higher pressure area right at that transfer opening, but also having the transfer flowing the mix up to the cylinder ..............

Very interesting way of seeing things, thanks !!