O.S. FS20 Crank Play
03-17-2009, 01:40 AM
#1
O.S. FS20 Crank Play
I've got an O.S. FS20 I bought new in Japan when they had just come out. I flew it back in the late 80s in only one plane and then put it away full of Marvel Mystery Oil. It was never crashed. I took it out tonight to check it out because I am thinking of putting it in an Early RC kit of the Andrews H-Ray. Just seems to be a perfect engine for this plane. Well, I took the backplate off to get a look inside to see if there was any rust, congealed castor oil, etc. It is absolutely clean inside. Just love this little engine. My question is, when the prop is at TDC I can wiggle the prop back and forth with no resistance about an 1/8th of an inch with any movement of the piston. I don't see any play in the rod to crank journal. I have not taken it apart any further. Do you think this is the bearing or the piston pin/connecting rod? Or, am I worrying for nothing. Any ideas?
Mike MacLean
Mike MacLean
03-17-2009, 03:18 AM
#4
Senior Member
RE: O.S. FS20 Crank Play
Mike,
If anything, I would consider this 'play' as insufficient!
There are three joints that require lubrication (besides the ball-bearings):
1. The con-rod bottom-end on the crank-pin
2. The con-rod top-end on the wrist-pin
3. The wrist-pin in the piston
Lubrication for all three must come from small amounts of oil in the blow-by gas that passed by the piston ring.
There must be some 'oil clearance' in all three, for lubricant to enter and protect the sliding bearing from wear.
Four-stroke engines have less play than two-stroke engines, since in a two-stroke engine running normally the con-rod is always under compression.
Let's assume that each has just 0.02 mm of play; the total vertical movement (crank-pin descent from TDC without movement of the piston, is 0.06 mm...
It may seem like virtually nothing to you, but it goes by the arc-sine value of degrees, with half the engine's stroke as the radius.
Half the stroke for the OS.20FS is about 0.8 cm, or 8 mm. This is the 'r'.
Do the calculation yourself! You should be able to 'wiggle' the prop 7° either way of TDC; and 14° in total, without the piston moving even one ångström (Å - one tenth of one nanometer).
If all three bearings are tighter, at 0.01 mm (ten microns) of 'oil clearance', the total play would still amount to 0.03 mm. This amount seems way too tight for comfort!
In crankshaft degrees, this would be 5° either way of TDC and 10° in total.
The 1/8" you are seeing (assuming a 10" diameter prop) is ~1.43° means the oil clearance is insufficient, or there is congealed castor oil in all bearings, which should loosen up when the engine is run.
If anything, I would consider this 'play' as insufficient!
There are three joints that require lubrication (besides the ball-bearings):
1. The con-rod bottom-end on the crank-pin
2. The con-rod top-end on the wrist-pin
3. The wrist-pin in the piston
Lubrication for all three must come from small amounts of oil in the blow-by gas that passed by the piston ring.
There must be some 'oil clearance' in all three, for lubricant to enter and protect the sliding bearing from wear.
Four-stroke engines have less play than two-stroke engines, since in a two-stroke engine running normally the con-rod is always under compression.
Let's assume that each has just 0.02 mm of play; the total vertical movement (crank-pin descent from TDC without movement of the piston, is 0.06 mm...
It may seem like virtually nothing to you, but it goes by the arc-sine value of degrees, with half the engine's stroke as the radius.
Half the stroke for the OS.20FS is about 0.8 cm, or 8 mm. This is the 'r'.
Do the calculation yourself! You should be able to 'wiggle' the prop 7° either way of TDC; and 14° in total, without the piston moving even one ångström (Å - one tenth of one nanometer).
If all three bearings are tighter, at 0.01 mm (ten microns) of 'oil clearance', the total play would still amount to 0.03 mm. This amount seems way too tight for comfort!
In crankshaft degrees, this would be 5° either way of TDC and 10° in total.
The 1/8" you are seeing (assuming a 10" diameter prop) is ~1.43° means the oil clearance is insufficient, or there is congealed castor oil in all bearings, which should loosen up when the engine is run.
03-17-2009, 03:54 AM
#5
RE: O.S. FS20 Crank Play
Dar Zeelon,
Holy Cow! What an in-depth explanation that was. Thanks. This engine really does not have that much time on it, (maybe 25-50 flights) so I would not suspect abnormal wear. I always used the correct oil content in the fuel. I measured the play with a 9-6 prop. O.K., just put it in the plane and go have fun as w8ye says!
Mike MacLean
Holy Cow! What an in-depth explanation that was. Thanks. This engine really does not have that much time on it, (maybe 25-50 flights) so I would not suspect abnormal wear. I always used the correct oil content in the fuel. I measured the play with a 9-6 prop. O.K., just put it in the plane and go have fun as w8ye says!
Mike MacLean
03-17-2009, 09:30 AM
#6
My Feedback: (11)
RE: O.S. FS20 Crank Play
Also, something that wasn't considered is the fact than when the piston is at Top Dead Center or Bottom Dead Center, you can have some prop movement with no apparent piston movement. That's because there's much more sideways movement of the crank than up/down movement at those points, so the piston moves very, very little, while the prop tip moves through a significant arc.
Some people see this as "play".
Some people see this as "play".
03-17-2009, 09:47 AM
#7
Senior Member
RE: O.S. FS20 Crank Play
ORIGINAL: rrengineer
"Holy Cow!"
"Holy Cow!"
The Hebrew Bible had the holy red cow.
Just run the engine, Mike!
03-17-2009, 10:15 AM
#8
Senior Member
RE: O.S. FS20 Crank Play
ORIGINAL: Bax
Some people see this as "play".
Some people see this as "play".
However, all sliding (and rolling) bearings are engineered with a degree of freedom in them...
I.e. any shaft is slightly smaller than the hole it fits into, to allow for lubrication between the moving parts.
When the parts are static, the small relative movement between them can be felt as 'play', without it being a sign of incorrect machining practice, or of wear.
When rapid relative movement sets in, a stable film of oil forms between the parts and this 'play' becomes imperceptible.
