OS.61 SF - strange recess on crankshaft..?
04-05-2011, 07:18 AM
#1
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OS.61 SF - strange recess on crankshaft..?
Hi out there, changed bearings on an old OS.61 SF the other day and found a strange "recess"(correct word..??) on the crankshaft as shown on pics. Have never seen anything like it before and wonder why this could be.
... Anyone out there with an idea of the thought behind this "recess"....???....Cheers/Harald
... Anyone out there with an idea of the thought behind this "recess"....???....Cheers/Harald
04-05-2011, 07:33 AM
#3
Thread Starter
RE: OS.61 SF - strange recess on crankshaft..?
Hi jeffie, I thought so too first but since only the center portion of the counterweight is "grooved" the outer portions would still hit the liner..!!
A groove for the piston then..??...but why on earth go to the length of "grooving" the very hard counterbalance when it would be so much easier to notch the piston skirt instead..??...Cheers/Harald
A groove for the piston then..??...but why on earth go to the length of "grooving" the very hard counterbalance when it would be so much easier to notch the piston skirt instead..??...Cheers/Harald
04-05-2011, 07:48 AM
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RE: OS.61 SF - strange recess on crankshaft..?
Clearance for the piston skirt, would be my guess. Before you put the back plate on, rotate the crank and see.
04-05-2011, 07:11 PM
#8
RE: OS.61 SF - strange recess on crankshaft..?
I'll go with the material removed for balance too. It looks like they were unable to remove enough material from the crankpin side, so they needed to cut a little off the heavy side.
04-05-2011, 10:09 PM
#9
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RE: OS.61 SF - strange recess on crankshaft..?
This recess is clearance for the descending piston's skirt.
The OS.61SF is an under-square engine; i.e. a long-stroke engine. It has a bore of 23 mm and a stroke of 24 mm
The piston at BDC (Bottom Dead Center) descends slightly further than it does, in over-square engines; such as the FSR and FX (both have a 24 mm bore and a 22 mm stroke).
Many engines have a cut-away in the front of their piston's skirt, to clear the crank-web...
In this engine, either this was insufficient, or OS preferred to 'reduce' the counterweight, rather than the length of the piston.
The OS.61SF is an under-square engine; i.e. a long-stroke engine. It has a bore of 23 mm and a stroke of 24 mm
The piston at BDC (Bottom Dead Center) descends slightly further than it does, in over-square engines; such as the FSR and FX (both have a 24 mm bore and a 22 mm stroke).
Many engines have a cut-away in the front of their piston's skirt, to clear the crank-web...
In this engine, either this was insufficient, or OS preferred to 'reduce' the counterweight, rather than the length of the piston.
04-06-2011, 05:28 AM
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RE: OS.61 SF - strange recess on crankshaft..?
Dar is correct and I am thinking that if the material was removed off the piston to clear it, It would leave an open port or I should say the small edge of the exhaust port and it would loose some of the new fuel charge during the cycle.
04-06-2011, 06:54 AM
#11
RE: OS.61 SF - strange recess on crankshaft..?
ORIGINAL: DarZeelon
This recess is clearance for the descending piston's skirt.
The OS.61SF is an under-square engine; i.e. a long-stroke engine. It has a bore of 23 mm and a stroke of 24 mm
The piston at BDC (Bottom Dead Center) descends slightly further than it does, in over-square engines; such as the FSR and FX (both have a 24 mm bore and a 22 mm stroke).
Many engines have a cut-away in the front of their piston's skirt, to clear the crank-web...
In this engine, either this was insufficient, or OS preferred to 'reduce' the counterweight, rather than the length of the piston.
This recess is clearance for the descending piston's skirt.
The OS.61SF is an under-square engine; i.e. a long-stroke engine. It has a bore of 23 mm and a stroke of 24 mm
The piston at BDC (Bottom Dead Center) descends slightly further than it does, in over-square engines; such as the FSR and FX (both have a 24 mm bore and a 22 mm stroke).
Many engines have a cut-away in the front of their piston's skirt, to clear the crank-web...
In this engine, either this was insufficient, or OS preferred to 'reduce' the counterweight, rather than the length of the piston.
Thanks.
Richard
04-06-2011, 08:50 AM
#12
Thread Starter
RE: OS.61 SF - strange recess on crankshaft..?
WOW, thanks all for a very educational thread, there seems to be more to it than I first thought. Unfortunately the engine is not mine and the guy I helped has already mounted it in the plane so opening the rear cover and looking (again) is not so easy. I have one more idea about the rationale for the cutaway but need to make a drawing before presenting it. Keep info flowing meanwhile.....
....Cheers/Harald
....Cheers/Harald
04-06-2011, 08:54 AM
#13
RE: OS.61 SF - strange recess on crankshaft..?
Hello, have someone forgotten the length of connecting rod? To make the engine more compact, the connecting rod is shortened then the height of cylinder is reduced at engine block and the piston can be shortened or make recess on crankshaft to give place for the piston skirt in case the engine is a two stroke engine who need piston skirt to cover the ports in cylinder under two stroke timing period.
04-06-2011, 09:02 AM
#14
RE: OS.61 SF - strange recess on crankshaft..?
Hi,
The discussion has so far been about cutting in the crankshaft balance or cutting in the piston skirt in order to avoid contact between them in this long-stroke engine. Can it instead be so, that the cut in the counterbalance is to avoid the balance from blocking one of the schneurle ports? Maximum gas transfer, from crankcase to cylinder, shall be possible when the counterbalance is in its top position.
That's my suggestion,
Lars
The discussion has so far been about cutting in the crankshaft balance or cutting in the piston skirt in order to avoid contact between them in this long-stroke engine. Can it instead be so, that the cut in the counterbalance is to avoid the balance from blocking one of the schneurle ports? Maximum gas transfer, from crankcase to cylinder, shall be possible when the counterbalance is in its top position.
That's my suggestion,
Lars
04-06-2011, 09:06 AM
#15
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RE: OS.61 SF - strange recess on crankshaft..?
ORIGINAL: spaceworm
Since it appears that the cutaway in the crank counterbalance is towards the bearing or to the outside of the piston, does this mean that the the uncut part of the counterbalance actually passes inside the piston skirt at BDC?
Since it appears that the cutaway in the crank counterbalance is towards the bearing or to the outside of the piston, does this mean that the the uncut part of the counterbalance actually passes inside the piston skirt at BDC?
Strange, but I was thinking of the same thing...
Yes, that's what actually happens.
The piston's skirt is not very thick, but it is curved; so OS even took that into account.
If you look at Harald's photos, you would see the 'bevel' between the cutaway and the 'proud' part of the crank-web...
It is the curvature of the piston skirt, that made this angled cut necessary.
OS cut away from the crank-web, the absolute minimum amount of steel; to retain this engine's more difficult to achieve partial balance.
You probably remember that balancing a single-piston engine, is still only a compromise; between up-down vibrational forces and side-to side vibrational forces...
04-06-2011, 09:19 AM
#16
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RE: OS.61 SF - strange recess on crankshaft..?
ORIGINAL: abachiwing
Hi,
The discussion has so far been about cutting in the crankshaft balance, or cutting in the piston skirt in order to avoid contact between them in this long-stroke engine. Can it instead be so, that the cut in the counterbalance is to avoid the balance from blocking one of the schneurle ports? Maximum gas transfer, from crankcase to cylinder, shall be possible when the counterbalance is in its top position.
That's my suggestion,
Lars
Hi,
The discussion has so far been about cutting in the crankshaft balance, or cutting in the piston skirt in order to avoid contact between them in this long-stroke engine. Can it instead be so, that the cut in the counterbalance is to avoid the balance from blocking one of the schneurle ports? Maximum gas transfer, from crankcase to cylinder, shall be possible when the counterbalance is in its top position.
That's my suggestion,
Lars
Maximum transfer speed through the bypasses, will likely not occur; when the piston is dwelling at a stand-still at BDC; and the exhaust port is fully open and a still positive pressure residing in the cylinder.
I don't think the rear Schnürle (or Schnuerle - note spelling) port would benefit very much in that state, from giving it that extra 'clearance'.
04-06-2011, 12:47 PM
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RE: OS.61 SF - strange recess on crankshaft..?
Dar
It's the same spelling
. We both know that "ue" ( e added to umlaut vowels) is only used by us that don't have keyboards with umlaut letters.[&o]
Denis
It's the same spelling
. We both know that "ue" ( e added to umlaut vowels) is only used by us that don't have keyboards with umlaut letters.[&o]Denis
04-06-2011, 01:11 PM
#19
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RE: OS.61 SF - strange recess on crankshaft..?
ORIGINAL: djlyon
Dar
It's the same spelling. We both know that ''ue'' ( e added to umlaut vowels) is only used by us that don't have keyboards with umlaut letters.[&o]
Denis
Dar
It's the same spelling
. We both know that ''ue'' ( e added to umlaut vowels) is only used by us that don't have keyboards with umlaut letters.[&o]Denis
My keyboard doesn't have the umlaut letters either...
But when it is necessary, I just to to the ASCII code ([link=http://webdesign.about.com/od/localization/l/blhtmlcodes-de.htm]this site[/link] for instance) and copy-paste the specific character 'ü', or any other.
I remarked regarding the spelling, because Lars spelled 'eu'; instead of 'ue'...
See [link=http://en.wikipedia.org/wiki/Schnuerle_porting]the Wiki page for Schnürle porting[/link]. In the part about Adolf Schnürle, it specifically says the 'eu' is a misspelling...
04-06-2011, 01:28 PM
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RE: OS.61 SF - strange recess on crankshaft..?
AHHH
I missed that Lars had misspelled Schnürle Schneurle
Interesting discussion though. My knee jerk reaction was balance. But after reading more careful thoughts I guess we would need to talk to the guy that made the decision, and he probably can't remember.
Denis
I missed that Lars had misspelled Schnürle Schneurle
Interesting discussion though. My knee jerk reaction was balance. But after reading more careful thoughts I guess we would need to talk to the guy that made the decision, and he probably can't remember.
Denis
