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#1
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Epoxy Bonding to Aluminum and Magnesium
I'm working on a Hummel Aviation Ultracruiser Plus. The way the
engine mounts is quite different from the mounts on most VW powered homebuilts. The Conventional Wisdom bolts the thing to the firewall using the clucth end bellhousing, often with an accessory case in between. The Ultracruiser Plus is different. Two aluminum angles are bolted to the sides of the magnesium case, in the sump area, and, suitablely reinforced, are used to bolt Berry mounts to "bed" type engine bearers built up out of aluminum, that extend from the forward fuselage. These angles are bolted and epoxied to the side of the magnesium case. My point: what good is the epoxy? It's probably not carrying any loads. A basic tenet of structural design is that the stiffest load path carries the load, and the bolts through the angle and into the case (secured with nuts and washers inside the sump, before the engine is assembled) seem a bunch stiffer that the epoxy. It's not a sure stop against leaks. Epoxy is a wonderful material, but it doesn't bond particularly well or reliably to metals. Plus, it's mechanical properties, from it's modulus of expansion, to it's ductility, are far different that the aluminum, steel, and magnesium sandwich is it the Mayonnaise of. My concern is the epoxy will eventually crack. I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase: "The epoxy is to keep the angles from working and hogging out the holes. The epoxy I use is a real good sealer, you've got to grind it off." My thoughts are this: LAP the angles to the side of the case (instead of sanding with 80 grit), but use Permatex or Curil T to seal things. Use close tolerance bolts in reamed holes in the side of the case and the appropriate Loctite product to seal the bolts. And I'm inclined to put the bolt heads INSIDE the engine. Comments? To see what this installation looks like: http://flyhummel.com/forums/album_pic.php?pic_id=170 Ultracruiser (with 1/2 VW) is the same deal |
#2
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To be honest this all sounds like a bit of a mess.
There are a number of issues here ... however it is difficult to comment in any detail until I know what sort of epoxy is specified and without further details of the surface preparation and details of the design. 1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx 90 deg C. If the epoxy is taken above that temperature then two things happen ... first it softens and the stiffness and strength reduce dramatically. Second irreveraible damage is done to the epoxy and it will never be the same again - even when cooled. Unless you are using one of a small number of epoxies that are designed for high temperature operation (some of these have Tg of approx 400 F, 200 C I think without reaching for a calculator) then it is certain I think that if used on an engine the 90 deg C limit will be exceeded. 2. Next there is the issue of surface preparation. If the plans say prepare by running with 80 grid paper then it is fairly clear that the guy who wrote the plans knows little about what he is doing ! Epoxy metal bonding is reliable if the surfaces are prepared properly but from what you have said I doubt that is the case. 3. Bolted and bonded joints should be avoided because it is difficult to predict the load transfer etc ... I won't go into detail on this - perhaps later. 4. What are the differences in the thermal expansion coefficients of the aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an issue. 5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! 6. I think galling of the metal is a possibility but if appropriate tolerances are used for the bolts and holes than this would be less of a problem. Hard to say without seeing the drawings etc. Sounds to me like if the holes are flogging out then the design has some fundamental problems and that one should not be relying on epoxy that probably cannot withstand the temperatures to fix it. My gut feel is that you need something between the alumnium and the magnesium for corrosion protection and possibly the help the galling issue. I would assume that this is sufficiently ductile an rubbery that it will not pick up any load and that the fasteners will transfer all the load. I would use a rudder like sealant compound that can take the temperature ... not epoxy. When you use sealant of this type in a joint with fasteners extra largers of safety should be allowed because of the extra flexibility that this produces in the joint (typically an extra factor of 1.5). "Ryan Young" wrote in message om... I'm working on a Hummel Aviation Ultracruiser Plus. The way the engine mounts is quite different from the mounts on most VW powered homebuilts. The Conventional Wisdom bolts the thing to the firewall using the clucth end bellhousing, often with an accessory case in between. The Ultracruiser Plus is different. Two aluminum angles are bolted to the sides of the magnesium case, in the sump area, and, suitablely reinforced, are used to bolt Berry mounts to "bed" type engine bearers built up out of aluminum, that extend from the forward fuselage. These angles are bolted and epoxied to the side of the magnesium case. My point: what good is the epoxy? It's probably not carrying any loads. A basic tenet of structural design is that the stiffest load path carries the load, and the bolts through the angle and into the case (secured with nuts and washers inside the sump, before the engine is assembled) seem a bunch stiffer that the epoxy. It's not a sure stop against leaks. Epoxy is a wonderful material, but it doesn't bond particularly well or reliably to metals. Plus, it's mechanical properties, from it's modulus of expansion, to it's ductility, are far different that the aluminum, steel, and magnesium sandwich is it the Mayonnaise of. My concern is the epoxy will eventually crack. I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase: "The epoxy is to keep the angles from working and hogging out the holes. The epoxy I use is a real good sealer, you've got to grind it off." My thoughts are this: LAP the angles to the side of the case (instead of sanding with 80 grit), but use Permatex or Curil T to seal things. Use close tolerance bolts in reamed holes in the side of the case and the appropriate Loctite product to seal the bolts. And I'm inclined to put the bolt heads INSIDE the engine. Comments? To see what this installation looks like: http://flyhummel.com/forums/album_pic.php?pic_id=170 Ultracruiser (with 1/2 VW) is the same deal |
#3
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"smjmitchell" wrote in message . au...
5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! Interesting. Where might I information like this for making decisions on what metals can be safely bolted together w/r/t galvanic corrosion? At issue right now is whether it'll be OK to thread my brass fuel-system finger strainers and fuel drains into aluminum blocks, but other such questions will certainly come up in future. Thanks, Greg |
#4
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See below for data as requested:
Also see MIL-STD-899 for further info (you can download that from the web) at http://stinet.dtic.mil/str/index.html Metal Potential Magnesium and its alloys -1.60 Zinc die-casting alloy -1.10 Zinc plating on steel -1.10 Zinc plating on steel, chromate passivated -1.05 Galvanised iron -1.05 Tin/Zinc (80/20) plating in steel -1.05 Cadmium-Zinc solder -1.05 Wrought Al-clad Al Alloys -0.90 Cadmium plating on steel -0.80 Aluminium alloy castings -0.75 Wrought aluminium -0.75 Non-stainless steel & grey cast iron -0.70 Duralumin type un-clad alloys -0.60 Lead -0.55 Lead-silver solder -0.50 Tin-lead solder -0.50 Tinned Steel -0.50 Cr plating (0.0005") on steel -0.50 Stainless 12% Cr -0.45 Tin plating on steel -0.45 Cr plating (0.00003") on plating steel -0.45 Chromium (99%) -0.45 Stainless High Cr (18/2) -0.35 Copper and its alloys (Brass, bronze, etc) -0.25 Nickel-Copper alloys -0.25 Stainless, Austenitic (18/8) -0.20 Silver solder -0.20 Monel metal -0.15 Nickel plating on steel Titanium -0.15 Titanium -0.15 Silver and silver plating on copper 0 Rhodium plating on silver plated Copper +0.05 Carbon +0.10 Platinum +0.15 Gold +0.15 Condition Max. Potential Difference Marine and outdoor environment 0.3 Volts Indoor environment 0.5 Volts Interior assemblies, hermetically sealed No restriction "Greg Reid" wrote in message om... "smjmitchell" wrote in message . au... 5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! Interesting. Where might I information like this for making decisions on what metals can be safely bolted together w/r/t galvanic corrosion? At issue right now is whether it'll be OK to thread my brass fuel-system finger strainers and fuel drains into aluminum blocks, but other such questions will certainly come up in future. Thanks, Greg |
#5
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Thanx for yourlearned and informed reply.
"smjmitchell" wrote in message . au... To be honest this all sounds like a bit of a mess. It does to me too, but on the other hand, there are a fair number of flying installations like this, in spite of my "desk engineer" concerns. 1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx 90 deg C. If the epoxy is taken above that temperature then two things happen ... first it softens and the stiffness and strength reduce dramatically. Second irreveraible damage is done to the epoxy and it will never be the same again - even when cooled. Unless you are using one of a small number of epoxies that are designed for high temperature operation (some of these have Tg of approx 400 F, 200 C I think without reaching for a calculator) then it is certain I think that if used on an engine the 90 deg C limit will be exceeded. That's about 190 deg F, which is a pretty comfortable oil temerature, and this arrangement attaches to the side of the oil sump. But oil temps up to 250 deg F need to be thought about. And that's higher than most epoxy resins and adhesives, even post-cured. I don't know what epoxy is used, I'll find out. 2. Next there is the issue of surface preparation. If the plans say prepare by running with 80 grid paper then it is fairly clear that the guy who wrote the plans knows little about what he is doing ! Epoxy metal bonding is reliable if the surfaces are prepared properly but from what you have said I doubt that is the case. That's actually not outside the "standard of Care" for metal bonding in low-stress operations. Gougeon brothers, makers of WEST and PRO-SET epoxy, suggest sanding and etching for aluminum. The side of the magnesium case is fairly rough, and not particularly flat. Sanding it flattens it, removes the oxide coating, and still leaves some "tooth". 3. Bolted and bonded joints should be avoided because it is difficult to predict the load transfer etc ... I won't go into detail on this - perhaps later. My point exactly. 4. What are the differences in the thermal expansion coefficients of the aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an issue. There is also the steel bolts to consider, but their lower coefficient of expansion actually tightens up the joint. Perhaps to the point of fracturing the epoxy! I can't find good numbers on cured resin alone, but cured composites have pretty low coefficients, like 2.0e-6. 5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of cars, with WATER running through their engines, have cast iron blocks and aluminum heads. In this situation, there is no ready source of electrolyte. I don't see this as a huge concern. I'm more bothered by oil leaks. 6. I think galling of the metal is a possibility but if appropriate tolerances are used for the bolts and holes than this would be less of a problem. Hard to say without seeing the drawings etc. Sounds to me like if the holes are flogging out then the design has some fundamental problems and that one should not be relying on epoxy that probably cannot withstand the temperatures to fix it. Amen! Albeit, there are those pesky flying examples to be explained away.... My gut feel is that you need something between the alumnium and the magnesium for corrosion protection and possibly the help the galling issue. I would assume that this is sufficiently ductile an rubbery that it will not pick up any load and that the fasteners will transfer all the load. I would use a rudder like sealant compound that can take the temperature ... not epoxy. When you use sealant of this type in a joint with fasteners extra largers of safety should be allowed because of the extra flexibility that this produces in the joint (typically an extra factor of 1.5). This aligns with my thinking. High Temperature Room Temperature Vulcanizing Silicone rubber looks good for this. The security of the joint would be in the close fit of the mating parts, not in the epoxy. The bolts would be a tight fit in reamed holes, backed up with Loctite Red. "Ryan Young" wrote in message om... I'm working on a Hummel Aviation Ultracruiser Plus. The way the engine mounts is quite different from the mounts on most VW powered homebuilts. The Conventional Wisdom bolts the thing to the firewall using the clucth end bellhousing, often with an accessory case in between. The Ultracruiser Plus is different. Two aluminum angles are bolted to the sides of the magnesium case, in the sump area, and, suitablely reinforced, are used to bolt Berry mounts to "bed" type engine bearers built up out of aluminum, that extend from the forward fuselage. These angles are bolted and epoxied to the side of the magnesium case. My point: what good is the epoxy? It's probably not carrying any loads. A basic tenet of structural design is that the stiffest load path carries the load, and the bolts through the angle and into the case (secured with nuts and washers inside the sump, before the engine is assembled) seem a bunch stiffer that the epoxy. It's not a sure stop against leaks. Epoxy is a wonderful material, but it doesn't bond particularly well or reliably to metals. Plus, it's mechanical properties, from it's modulus of expansion, to it's ductility, are far different that the aluminum, steel, and magnesium sandwich is it the Mayonnaise of. My concern is the epoxy will eventually crack. I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase: "The epoxy is to keep the angles from working and hogging out the holes. The epoxy I use is a real good sealer, you've got to grind it off." My thoughts are this: LAP the angles to the side of the case (instead of sanding with 80 grit), but use Permatex or Curil T to seal things. Use close tolerance bolts in reamed holes in the side of the case and the appropriate Loctite product to seal the bolts. And I'm inclined to put the bolt heads INSIDE the engine. Comments? To see what this installation looks like: http://flyhummel.com/forums/album_pic.php?pic_id=170 Ultracruiser (with 1/2 VW) is the same deal |
#6
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5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of cars, with WATER running through their engines, have cast iron blocks and aluminum heads. In this situation, there is no ready source of electrolyte. I don't see this as a huge concern. I'm more bothered by oil leaks. Auto engines have head gaskets between the electro different metals, plus if you don't run an inhibitor is a mutli-metal water cooled engine you will corrode the aluminum away, period. Yours Vern |
#7
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Vernon Klukas wrote in message .. .
5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of cars, with WATER running through their engines, have cast iron blocks and aluminum heads. In this situation, there is no ready source of electrolyte. I don't see this as a huge concern. I'm more bothered by oil leaks. Auto engines have head gaskets between the electro different metals, plus if you don't run an inhibitor is a mutli-metal water cooled engine you will corrode the aluminum away, period. Yours Vern Vern is 100% correct. I have repaired several automotive engines where this has been a problem. I used Belzona Super Metal (1111) to repair the problem. Also have used it to fix a multitude of diesel engines. These were do to electroylisis and cavitation. This is caused because the owners did not change or maintain the proper chemical balance of the antifreeze. Toyota engines require the Toyota antifreeze. Dexcool is not approved and according to a Toyota engineer, it will eventually damage the engine. He also told me not to mix Dexcool with glycol antifreeze. To make a long story short, follow the engine manufactures recommendations. |
#8
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Vernon Klukas wrote:
5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! Hmm, perhaps. I get that aluminum alloy is about -1.05. But look at the difference between Cast Iron (-.5) and aluminum (-1.05). Lots of cars, with WATER running through their engines, have cast iron blocks and aluminum heads. In this situation, there is no ready source of electrolyte. I don't see this as a huge concern. I'm more bothered by oil leaks. Auto engines have head gaskets between the electro different metals, plus if you don't run an inhibitor is a mutli-metal water cooled engine you will corrode the aluminum away, period. Yours Vern Rotary engines don't. Some people are running straight H20. Never heard of that particular problem...period. -- http://www.ernest.isa-geek.org/ "Ignorance is mankinds normal state, alleviated by information and experience." Veeduber |
#9
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I was thinking ProSeal or similar for the interface...
"smjmitchell" wrote in message u... To be honest this all sounds like a bit of a mess. There are a number of issues here ... however it is difficult to comment in any detail until I know what sort of epoxy is specified and without further details of the surface preparation and details of the design. 1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx 90 deg C. If the epoxy is taken above that temperature then two things happen ... first it softens and the stiffness and strength reduce dramatically. Second irreveraible damage is done to the epoxy and it will never be the same again - even when cooled. Unless you are using one of a small number of epoxies that are designed for high temperature operation (some of these have Tg of approx 400 F, 200 C I think without reaching for a calculator) then it is certain I think that if used on an engine the 90 deg C limit will be exceeded. 2. Next there is the issue of surface preparation. If the plans say prepare by running with 80 grid paper then it is fairly clear that the guy who wrote the plans knows little about what he is doing ! Epoxy metal bonding is reliable if the surfaces are prepared properly but from what you have said I doubt that is the case. 3. Bolted and bonded joints should be avoided because it is difficult to predict the load transfer etc ... I won't go into detail on this - perhaps later. 4. What are the differences in the thermal expansion coefficients of the aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an issue. 5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on the galvanic table. That is a big different. You definitely need something to separate the two or the magnesium is going to get gobbled up !! 6. I think galling of the metal is a possibility but if appropriate tolerances are used for the bolts and holes than this would be less of a problem. Hard to say without seeing the drawings etc. Sounds to me like if the holes are flogging out then the design has some fundamental problems and that one should not be relying on epoxy that probably cannot withstand the temperatures to fix it. My gut feel is that you need something between the alumnium and the magnesium for corrosion protection and possibly the help the galling issue. I would assume that this is sufficiently ductile an rubbery that it will not pick up any load and that the fasteners will transfer all the load. I would use a rudder like sealant compound that can take the temperature ... not epoxy. When you use sealant of this type in a joint with fasteners extra largers of safety should be allowed because of the extra flexibility that this produces in the joint (typically an extra factor of 1.5). "Ryan Young" wrote in message om... I'm working on a Hummel Aviation Ultracruiser Plus. The way the engine mounts is quite different from the mounts on most VW powered homebuilts. The Conventional Wisdom bolts the thing to the firewall using the clucth end bellhousing, often with an accessory case in between. The Ultracruiser Plus is different. Two aluminum angles are bolted to the sides of the magnesium case, in the sump area, and, suitablely reinforced, are used to bolt Berry mounts to "bed" type engine bearers built up out of aluminum, that extend from the forward fuselage. These angles are bolted and epoxied to the side of the magnesium case. My point: what good is the epoxy? It's probably not carrying any loads. A basic tenet of structural design is that the stiffest load path carries the load, and the bolts through the angle and into the case (secured with nuts and washers inside the sump, before the engine is assembled) seem a bunch stiffer that the epoxy. It's not a sure stop against leaks. Epoxy is a wonderful material, but it doesn't bond particularly well or reliably to metals. Plus, it's mechanical properties, from it's modulus of expansion, to it's ductility, are far different that the aluminum, steel, and magnesium sandwich is it the Mayonnaise of. My concern is the epoxy will eventually crack. I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase: "The epoxy is to keep the angles from working and hogging out the holes. The epoxy I use is a real good sealer, you've got to grind it off." My thoughts are this: LAP the angles to the side of the case (instead of sanding with 80 grit), but use Permatex or Curil T to seal things. Use close tolerance bolts in reamed holes in the side of the case and the appropriate Loctite product to seal the bolts. And I'm inclined to put the bolt heads INSIDE the engine. Comments? To see what this installation looks like: http://flyhummel.com/forums/album_pic.php?pic_id=170 Ultracruiser (with 1/2 VW) is the same deal |
#10
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"Blueskies" wrote in message m...
I was thinking ProSeal or similar for the interface... I was too, until I looked at the temperature resistance of polysulfides (ProSeal being that). It's pretty low, it releases pretty completely at about 250 deg F, whereas the cheapest RTV starts from there, and they have readily available compounds that go up to 700 deg F or higher. I've always looked down my nose at RTV, and I've seen many jobs bodgered up using it, but in this case, I'm beginning to believe it's the right goop for the deal. |
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