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Epoxy Bonding to Aluminum and Magnesium



 
 
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  #1  
Old September 13th 04, 06:40 AM
Ryan Young
external usenet poster
 
Posts: n/a
Default 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
Ads
  #2  
Old September 13th 04, 11:30 AM
smjmitchell
external usenet poster
 
Posts: n/a
Default

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  
Old September 13th 04, 02:14 PM
Clay
external usenet poster
 
Posts: n/a
Default

(Ryan Young) wrote in message . com...
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:


I use a lot of Belzona products for a variety of industrial repairs
and find them to be very strong and reliable.
www.belzona.com
You may want to use Black Beauty, Flint, Apache Blast, Alumimum Oxide
or some other angular abrasive. Sand, shot, beads, or sperical
abrasives do a good job of polishing but do not give a good profile
for an epoxy to bond properly.
A word of CAUTION, practice on a piece of scrap material before grit
blasting your project. Alumimum is soft and you could cause great
damage if you use too much pressure or dwell in the same place too
long.

http://flyhummel.com/forums/album_pic.php?pic_id=170

Ultracruiser (with 1/2 VW) is the same deal

  #4  
Old September 13th 04, 05:27 PM
Greg Reid
external usenet poster
 
Posts: n/a
Default

"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  
Old September 13th 04, 08:19 PM
Ryan Young
external usenet poster
 
Posts: n/a
Default

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  
Old September 13th 04, 10:15 PM
Blueskies
external usenet poster
 
Posts: n/a
Default

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





  #7  
Old September 13th 04, 11:32 PM
Bill Hale
external usenet poster
 
Posts: n/a
Default

Where can U get Belzona products in the US? Bill Hale
  #8  
Old September 13th 04, 11:40 PM
Ryan Young
external usenet poster
 
Posts: n/a
Default

(Ryan Young) wrote in message . com...

These angles are bolted and epoxied to the side of the magnesium case.
My point: what good is the epoxy?


I had further conversation with Scott Casler.
================================================== ===
Hi

I use a epoxy that is similar to JB Weld.

Scott ( Hummel Engines)
----- Original Message -----
From: [email protected]
To: [email protected]
Sent: Monday, September 13, 2004 3:24 PM
Subject: What Epoxy do you use?


To bond the aluminum angles to the side of the magnesium case?

Thanx for your previous reply to my inquiries.

Ryan Young
Ultracruiser Plus Builder


================================================== ==

From the JB Weld web site:

Properties (lbs/psi)
Tensile Strength: 3960
Adhesion: 1800
Flex Strength: 7320
Tensile Lap Shear: 1040
Shrinkage: 0.0%
Resistant to 500 F

I'm not sure what "Resistant" means in this context. JB Weld is a
steel-filled epoxy adhesive. Belzona is much the same stuff, and I'm
intimately familar with both of them. Devcon is another brand, and
they have both the widest variety and the best technical information:

http://www.devcon.com/techinfo/107.pdf Aluminum filled epoxy.
Temperature Resistance Wet: 120F; Dry: 250F
Adhesive Tensile Shear 2,600 psi
Compressive Strength 8,420 psi
Modulus of Elasticity 8.0 psi x 10(5) in.
Flexural Strength 6,760 psi
Coefficient of Thermal Expansion 29 [(in.)/(in). x F)] x 10(-6)

Wowee Kazowee, look how much it expands! Twice as much as aluminum or
magnesium!

http://www.devcon.com/techinfo/101.pdf Steel filled epoxy putty, more
like JB Weld
Adhesive Tensile Shear 2800 psi
Compressive Strength 8260 psi
Modulus of Elasticity 8.5 x 10(5) psi
Flexural Strength 5600 psi
Coefficient of Thermal Expansion 48 [(in)/(in) x F)] x 10(-6)
Even more expansive, and not much stronger.

Application instructions which cut pretty close to Hummel's plans:
1. Thoroughly clean the surface with Devcon Cleaner Blend 300 to
remove all oil, grease, and dirt. This is probably a detergent - RRY
2. Grit blast surface area with 8-40 mesh grit, or grind with a coarse
wheel or abrasive disc pad, to create increased surface area for
better adhesion
3. Clean surface again with Cleaner Blend 300 to remove all traces of
oil, grease, dust, or other foreign substances from the grit blasting.
4. Repair surface as soon as possible to eliminate any changes or
surface contaminants.
  #9  
Old September 14th 04, 01:40 AM
Ron Webb
external usenet poster
 
Posts: n/a
Default


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.


I recently did some testing with West Systems and aluminum. I used 3 test
strips, side by side. 1" wide fiberglass tape, 3 inches of bonding surface
on 6061.

The first strip was applied with no surface impression at all.
The second strip was applied after sanding with 80 Grit
The third was sanded and etched with an etch (Duramix 4925)

The first strip came off with maybe a pound of force, pulling on the end of
the fiberglass tape.
The second strip (sanded) took maybe twice that - it still came right off
The third (etched) strip never did come off, I tore the fiberglass tape It
stood up to at least 30 pounds in shear!

Surface preparation makes a BIG difference!.


 




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