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Between the cases?
Why is silk thread used as gasket material
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DGlaeser
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The thread is not the gasket, it acts as a spacer to prevent the form-in-place gasket material from being totally squeezed out when the parts are bolted together.
Tom-D
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Got a reference for that ?
Skylane81E
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Yeah, I was taught that it was the other way around,
Silk is used because it works! It's not the only way however.
wabower
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Wayne
I was told that it was because the guy who was rebuilding the engine had spent his last dime on parts and was down to his skivvies. A string from the waistband was the only thing that would work.
jhausch
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jhausch
Whoda' thunk A&Ps wore fancy silk skivvies? Learn something new every day...
Skylane81E
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Strength and malleability, it works. Silk has many industrial uses even yet.
Jay Honeck
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Because the FAA has unintentionally succeeded in freezing engine technology in the year 1946.
Skylane81E
Final Approach
Lyc allows an alternate meathod that eliminates the tread if you wish. Doesnt work any better though
Jay Honeck
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Name me one automotive engine that still uses silk as a gasket.
If it's so great, why isn't BMW (for example) still using it?
DGlaeser
Pre-takeoff checklist
From an Internet search to back up my memory:
The latest revision of Textron Lycoming Service Instruction No. 1125 specifies POB No. 4 Perfect Seal and silk thread as the generally used items for sealing crankcase finished parting surfaces that do not employ gaskets. Two other products, RTV-102 or LOCTITE-515 applied as a very thin film without silk thread, may be used as alternate materials for sealing crankcase parting surfaces. Other sealants have not been tested and approved for this purpose, and NONE are approved for other uses in the assembly of Lycoming engines. Improper use of these and other sealing compounds can create serious problems. As an example, a red colored sealant was used by a mechanic to hold the pressure screen gasket in place. As the material solidified, pieces broke loose and eventually blocked the engine's small oil passages causing oil starvation and engine failure. The bottom line: only use those sealant materials approved by the Lycoming Overhaul manual and other service publications, and only use them for approved purposes.
On further searches, it explains that the sealant is to hold the thread in place, and that the thread is the gasket. So much for my memory
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Skylane81E
Final Approach
Does BMW seal the main bearing caps?
Old Geek
Pattern Altitude
I believe we have a winner.
They have no similar application, there are other industrial seals still using waxed silk thread for the primary seal, I promise you that. One reason that it is not used is that it allows for rougher cheaper machining tolerances. I don't particularly consider an application that requires a more depth forgiving filler the superior product to be holding up in comparison.
Skylane81E
Final Approach
So why do we have other ways to do it?
As I said, other methods allow for lower quality production standards with a commonly acceptable result.
Skylane81E
Final Approach
My point simply was that if the FAA was the reason that there would not be any other approved methods
More correctly it would be that nobody had applied to the administrator proposing a suitable alternative, ever wonder why?
The answer to why is that it cures a non existent problem.
Skylane81E
Final Approach
Reminds me of the question I was privy too asked of a Cessna engineer type.
"why not flush rivet the 182?"
"we tried that, 2-3kts faster and 20% more labor costs to build."
Dan Thomas
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Dan Thomas
Frozen in 1946? Even earlier than that when it comes to wheel bearing seals. I've bought a couple of sets of new Cleveland wheels and brakes for 172s and they're still using a felt seal. Automobile makers quit that sometime around 1920 and started using leather. That ran until 1950 or so. Leather was replaced by synthetic elastomers.
Felt has to be cleaned and greased or it will let water in. Even then one must be careful not to squirt water directly at the seal. And it does a rather poor job of keeping dirt out, anyway. It won't maintain tension on the seal shoulder of the axle.
Dan
Felt has to be cleaned and greased or it will let water in. Even then one must be careful not to squirt water directly at the seal. And it does a rather poor job of keeping dirt out, anyway. It won't maintain tension on the seal shoulder of the axle.
Dan
Ted
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iFlyNothing
Typical cork/rubber gaskets would not work in the crankcase halves. Remember that the crankcase halves are part of what hold the crankshaft together. Most main bearing caps don't have any gaskets whatsoever. The silk thread gasket is both very thin and pretty good. The other options just eliminate the silk, but are otherwise like any other liquid gasket.
1946 technology? Yeah, I suppose you could call it that - your new BMW probably has the same sort of liquid gasket technology in various places.
1946 technology? Yeah, I suppose you could call it that - your new BMW probably has the same sort of liquid gasket technology in various places.
Tom-D
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Your reference doesn't support your statement.From an Internet search to back up my memory:
The latest revision of Textron Lycoming Service Instruction No. 1125 specifies POB No. 4 Perfect Seal and silk thread as the generally used items for sealing crankcase finished parting surfaces that do not employ gaskets. Two other products, RTV-102 or LOCTITE-515 applied as a very thin film without silk thread, may be used as alternate materials for sealing crankcase parting surfaces. Other sealants have not been tested and approved for this purpose, and NONE are approved for other uses in the assembly of Lycoming engines. Improper use of these and other sealing compounds can create serious problems. As an example, a red colored sealant was used by a mechanic to hold the pressure screen gasket in place. As the material solidified, pieces broke loose and eventually blocked the engine's small oil passages causing oil starvation and engine failure. The bottom line: only use those sealant materials approved by the Lycoming Overhaul manual and other service publications, and only use them for approved purposes.
further searches, it explains that the sealant is to hold the thread in place, and that the thread is the gasket. So much for my memory
Tom-D
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Tom-D
OBTW, any manufacturer can up date their manuals with out FAA approval. They could change the seal any time they wanted.
Remember we overhaul by the manufacturers manuals. not type design.
You have it backwards, the felt absorbs the centrifugally spun off grease, the grease itself is what provides the seal. You clean and dry the felt of old grease in the parts washer tank. They are supposed to return clean and dry between the thin washers that act as the lip of the seal trapping the grease. If it is mine and I want to seal the felt and prep it for my plane, I clean it well, dry it with CRC Electramotive or similar chlorinated solvent then saturate it with a few good dosings of LPS3, maybe do the first still damp with the CRC. It ends up with an oxidized hard shell wax surface film that doesn't attract water or grime, the opposite in fact, and will have a very shallow border of mingling with the bearing grease. Before you repack your bearings with grease if you used a hydrocarbon type solvent to clean, you probably want to give the bearings and races a good spray with the CRC and then blast with air until very dry or you see King Ding Dong and he says "Bacaba". At that point you should stop and open the door and walk outside for air...
Tom-D
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The reason silk thread is used in the aluminum cases in all 3 major engine producers is very simple, it provides a dry seal that allows the two case halves to be bolted together metal to metal and not creep or fret during usage.
THe outer case flange is not a portion of the bearing clamp. the bearing clamp is done at the bearing by torquing the thru case studs not the outer case seams.
as shown below the 0-200 has 4 thru case studs at the front main, 2 at the center main and 2 at the rear main, that carry the main bearing torque, the outer case seam has nothing to do with bearing clamp.
THe outer case flange is not a portion of the bearing clamp. the bearing clamp is done at the bearing by torquing the thru case studs not the outer case seams.
as shown below the 0-200 has 4 thru case studs at the front main, 2 at the center main and 2 at the rear main, that carry the main bearing torque, the outer case seam has nothing to do with bearing clamp.
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Ted
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iFlyNothing
Correct, it isn't part of the clamp. However it is machined so that the outer flanges pretty much line right up with the inner flanges. If there was much room for additional tightening there that could lead to some extra stresses in the case and more cracking. I can't imagine a rubber or cork gasket working particularly well there over time.
Skylane81E
Final Approach
Yeah, if additional space were provided for a gasket with the bearings clamped tight there would be some interesting stresses as the perimeter bolts were torqued up, and the whole thing vibrated.
Funny thing, we have an industry dedicated to making synthetic fibers to replace silk since WWI, it's part of why we entered that war, over securing access to mid east oil fields. The Aramids like Kevlar are the what the synthetic industry came up with and they are 10xs the cost of silk.
Tom-D
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The for most engine case shop in America http://www.chuckneyent.com/neynozzle.asp has much to say about how the seal on the cases work. and does not allow any sealant to be applied at the bearing bosses.
When you really need a lesson on cases call Chuck, he will give you an ear full.
The main reason there is no gasket at the outer cases seam is the manufacturers do not want the cases moving in service so they clamp them dry. and the most often cause of a case being lost is fretting because the overhauler applies a sealant at the seam to hold the thread in place as the case is built up.
Dan Thomas
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Dan Thomas
You have it backwards, the felt absorbs the centrifugally spun off grease, the grease itself is what provides the seal. You clean and dry the felt of old grease in the parts washer tank. They are supposed to return clean and dry between the thin washers that act as the lip of the seal trapping the grease. If it is mine and I want to seal the felt and prep it for my plane, I clean it well, dry it with CRC Electramotive or similar chlorinated solvent then saturate it with a few good dosings of LPS3, maybe do the first still damp with the CRC. It ends up with an oxidized hard shell wax surface film that doesn't attract water or grime, the opposite in fact, and will have a very shallow border of mingling with the bearing grease. Before you repack your bearings with grease if you used a hydrocarbon type solvent to clean, you probably want to give the bearings and races a good spray with the CRC and then blast with air until very dry or you see King Ding Dong and he says "Bacaba". At that point you should stop and open the door and walk outside for air...
From Cleveland's maintenance manual http://www.parker.com/literature/Ai...B Product Catalog Static Files/AWBCMM0001.pdf
Quote:
"(c) If felt seals are used, lightly coat all surfaces of the felt with the wheel bearing grease (refer to paragraph 3.A. Bearing Grease). If rubber lip seals are used, lightly coat the rubber surfaces with bearing grease.
(d) Install the bearing cones, grease seals (felts and rings or rubber lip seals) and snap rings. Excess grease will squeeze out. Remove the excess grease with an inward rotating movement against the bearing cone ID. Disperse any small amounts of grease on the exterior surface of the grease seal and snap ring and remove any grease from the hub outside surface."
They want grease on the felts. Interesting to note that they also have rubber seals. Why don't they use them on all their wheels?
Dan
Light film is what they say and that is to provide the barrier coat I prefer to use LPS3 for. It's not 'greased' with a bunch, all that does is creates a mess and causes sand and grit to be attracted. If you put that felt in there clean and you have well packed the bearings, you will have a very good low drag seal formed before your T/O roll is complete.
Look how some front wheels work, they aren't on a spindle preloaded with an adjustable nut, they are preloaded by shims clamped between the forks. The outsides of the mains as well use no dust cap. The double plate felt sandwich with the circlip ring provide a nice clean seal.
Jay Honeck
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The use of felt in wheels is beyond quaint. It's absurd.
I remember the first time I greased the wheel bearings on my first plane, during my very first "owner-assisted" annual. When I had the wheel dismantled I actually laughed out loud. My A&P just rolled his eyes and said "I know!".
The state of certificated aircraft technology is laughably antiquated.
Of course the "hot setup" would be to use a CNC, ball mill and machine a slot for a O ring...
:wink2:
Tom-D
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Tom-D
And it has been working for nearly 100 years.
The new technology wheel seal on my Jeep are already leaking.
Ted
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iFlyNothing
And the reason you haven't gone out and built an experimental is...?
We're mostly flying airplanes that were built in the 70s and earlier. At that time, the technology was still better than what you find in most cars.
Of course the "hot setup" would be to use a CNC, ball mill and machine a slot for a O ring...
Definitely, that would be an improvement.
I suspect it's not been looked at because it would add cost to test, certify, and implement, and the improvement, while there, would be minimal.
Tom-D
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Tom-D
removing the tapered races and installing a double row, sealed roller bearing would be the best and simplest upgrade, and can be done on field approval.
Yes there are methods to up grade production aircraft.
Yes there are methods to up grade production aircraft.
Old Geek
Pattern Altitude
Of course the "hot setup" would be to use a CNC, ball mill and machine a slot for a O ring...
And 4 dowel pin locations for alignment and to reduce fretting.
BTW, a conventional O-ring installation uses a flat-bottomed groove. Sorry for being pedantic.
Tom-D
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Tom-D
That's already done with 4 close tolerance bolts in the case mating seam.