Al-Ko Trailer Tongue failure

On Friday, July 8, 2016 at 4:31:04 AM UTC+2, wrote:
Remember that "tensile strength" measures a bolt under a longitudinal load. What you are seeing in this instance is a failure under "shear load." Two entirely different situations, and one of the main reasons that aircraft bolts (AN) are not the same as "Grade 8" or the European (DIN) equivalent. High tensile strength bolts often exhibit less than desirable brittleness under shear load.

I think Mark is close to the root cause of the failing bolts. It is not the strength of the bolts itself that makes in brake (sounds funny, I know).
In the bolt-configuration in the initial post is the function of the bolts to hold the Al-Ko tongue tight to the square bar, the friction between the tongue and the bar makes the strength.
If the friction coefficient is low (fat, grease, dirt) or bolts is not tight (loosen by vibrations, deformations etc) is the friction between the tongue/bar low and a shear stress occurs on the bolts. The bolts are not dimensioned for this and will brake by fatigue.
This is a common problem on Cobra trailers(the nose-cone / spare wheel holder), the bolts a not tighten enough from the factory and / or they vibrate loose on the road.

Per Carlin wrote on 7/8/2016 12:43 AM:
On Friday, July 8, 2016 at 4:31:04 AM UTC+2,
wrote:
Remember that "tensile strength" measures a bolt under a
longitudinal load. What you are seeing in this instance is a
failure under "shear load." Two entirely different situations, and
one of the main reasons that aircraft bolts (AN) are not the same
as "Grade 8" or the European (DIN) equivalent. High tensile
strength bolts often exhibit less than desirable brittleness under
shear load.

I think Mark is close to the root cause of the failing bolts. It is
not the strength of the bolts itself that makes in brake (sounds
funny, I know). In the bolt-configuration in the initial post is the
function of the bolts to hold the Al-Ko tongue tight to the square
bar, the friction between the tongue and the bar makes the strength.
If the friction coefficient is low (fat, grease, dirt) or bolts is
not tight (loosen by vibrations, deformations etc) is the friction
between the tongue/bar low and a shear stress occurs on the bolts.
The bolts are not dimensioned for this and will brake by fatigue.
This is a common problem on Cobra trailers(the nose-cone / spare
wheel holder), the bolts a not tighten enough from the factory and /
or they vibrate loose on the road.

Can we be certain the missing bolt broke? I would expect the bolt hole
to show noticeable elongation or other damage from shearing the bolt,
especially on the side opposite the break (it would stay in the hole for
while, taking all the pounding, until that side also sheared, or the
bolt simply fell out), but the hole looks undamaged in the photo. Can't
see the other hole, however.

There is vertical bolt in the picture (about where Karl's seems to be),
just ahead of the missing bolt: what is it's function?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

I've often thought that the source of many trailer failures here in the U.S.. is partly that we just drive a lot more miles/kms than the typical European customer. I just got back from a nearly 5,000 mi. round trip to Nephi. And I've made numerous trips west with this trailer. How common is that in Europe?

The nose cone/spare tire holder on my Cobra failed last year and fell over (fortunately without damage) not because of a failed or loose bolt but because of a failure of the weld at the base of the supporting post. I had a failure many years ago of an Al-Ko tongue on a Komet trailer when the welds that secure the front tongue bracket inside the trailer broke.

Cobra/Komet designs have continued to improve each year since we first saw their appearance here (in the 70s?). But it seems like many of those improvements are in response to "destruction testing" performed by helpful U.S. owners. Is our experience shared by owners in similarly wide-open regions such as, say, Australia?

Chip Bearden
ASW 24 "JB"

In response to Eric's comments about whether or not the bolt broke, I can not say for sure, but the bolts are secured with ny-lock nuts that should prevent an inadvertent loosening of the nut.

The vertical bolt in the picture attaches to a plate on the bottom side of the upper (inverted) U-shaped piece of the tongue. This plate then butts up against the edge of the square tube that forms the inner piece of the tongue and it appears to be there to prevent the two pieces of the tongue from moving fore and aft against each other, or perhaps for alignment during installation.

On Friday, July 8, 2016 at 7:55:50 AM UTC-7, Dave Springford wrote:
In response to Eric's comments about whether or not the bolt broke, I can not say for sure, but the bolts are secured with ny-lock nuts that should prevent an inadvertent loosening of the nut.

The vertical bolt in the picture attaches to a plate on the bottom side of the upper (inverted) U-shaped piece of the tongue. This plate then butts up against the edge of the square tube that forms the inner piece of the tongue and it appears to be there to prevent the two pieces of the tongue from moving fore and aft against each other, or perhaps for alignment during installation.

Dave,

Just saw your comment on the vertical bolt. Please ignore my prior note.
Craig

On Friday, July 8, 2016 at 2:43:03 AM UTC-5, Per Carlin wrote:
On Friday, July 8, 2016 at 4:31:04 AM UTC+2, wrote:
Remember that "tensile strength" measures a bolt under a longitudinal load. What you are seeing in this instance is a failure under "shear load." Two entirely different situations, and one of the main reasons that aircraft bolts (AN) are not the same as "Grade 8" or the European (DIN) equivalent.. High tensile strength bolts often exhibit less than desirable brittleness under shear load.

I think Mark is close to the root cause of the failing bolts. It is not the strength of the bolts itself that makes in brake (sounds funny, I know).
In the bolt-configuration in the initial post is the function of the bolts to hold the Al-Ko tongue tight to the square bar, the friction between the tongue and the bar makes the strength.
If the friction coefficient is low (fat, grease, dirt) or bolts is not tight (loosen by vibrations, deformations etc) is the friction between the tongue/bar low and a shear stress occurs on the bolts. The bolts are not dimensioned for this and will brake by fatigue.
This is a common problem on Cobra trailers(the nose-cone / spare wheel holder), the bolts a not tighten enough from the factory and / or they vibrate loose on the road.

Per is right, a properly engineered bolted connection is never meant to hold shear loads. The longitudinal tension in the bolted connection should press the pieces together with a force that causes friction sufficient to withstand the bending moment.

wrote on 7/8/2016 7:36 AM:
Per is right, a properly engineered bolted connection is never meant
to hold shear loads. The longitudinal tension in the bolted
connection should press the pieces together with a force that causes
friction sufficient to withstand the bending moment.

Bolts are routinely used in shear, from toys to airliners. On my glider,
the tail wheel, landing gear, and propeller are fastened with bolts in
shear, along with many other items.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

On Friday, July 8, 2016 at 10:26:09 AM UTC-5, Eric Greenwell wrote:
wrote on 7/8/2016 7:36 AM:
Per is right, a properly engineered bolted connection is never meant
to hold shear loads. The longitudinal tension in the bolted
connection should press the pieces together with a force that causes
friction sufficient to withstand the bending moment.

Bolts are routinely used in shear, from toys to airliners. On my glider,
the tail wheel, landing gear, and propeller are fastened with bolts in
shear, along with many other items.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

Sorry Eric, you are wrong. Pretension (torque) on the bolt results in parts being pressed together. That force with a coefficient of friction normally being assumed at 0.1 results in the parts staying together without slippage. Shear can only occur if the parts slip, a properly designed connection never does.

On Friday, July 8, 2016 at 9:05:24 AM UTC-7, wrote:
On Friday, July 8, 2016 at 10:26:09 AM UTC-5, Eric Greenwell wrote:
wrote on 7/8/2016 7:36 AM:
Per is right, a properly engineered bolted connection is never meant
to hold shear loads. The longitudinal tension in the bolted
connection should press the pieces together with a force that causes
friction sufficient to withstand the bending moment.

Bolts are routinely used in shear, from toys to airliners. On my glider,
the tail wheel, landing gear, and propeller are fastened with bolts in
shear, along with many other items.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

Sorry Eric, you are wrong. Pretension (torque) on the bolt results in parts being pressed together. That force with a coefficient of friction normally being assumed at 0.1 results in the parts staying together without slippage. Shear can only occur if the parts slip, a properly designed connection never does.

Herb,

There are some cases where friction is what is relied on for a bolted shear connection, but these are very special cases and require specific installation procedures. In building structures, these bolts are designated ASTM A325 SC or ASTM A490 SC. The "SC" designating slip critical. In order to perform as slip critical connections there needs to be enough capacity in the bolted sandwich to allow the bolt to yield slightly in tension during tightening. This provides a consistent preload to develop the friction. This type of connection is impossible to achieve when bolting though a hollow tube unless there is a bearing sleeve through the tube to allow development of full tension in the bolt. Without the bearing sleeve the bolt simply crushes the tube and sufficient clamping force is never achieved. Many buildings and other structures are designed and constructed using bolts in shear without relying on friction between the faying surfaces. Slip critical bolting (friction bolting) is a nice solution when cyclic loads and fatigue are a consideration because it eliminates the movement that happens when bearing bolts (bolts in plain shear) are cycled from one load direction to another. Cyclic loading can also be accommodated by interference fit bolts, but the degree of accuracy required in fabrication is too expensive for all but specialty applications.

Best regards,
Craig Funston, P.E. , S.E. , P.Eng.

On Friday, July 8, 2016 at 9:29:13 AM UTC-7, Craig Funston wrote:
On Friday, July 8, 2016 at 9:05:24 AM UTC-7, wrote:
On Friday, July 8, 2016 at 10:26:09 AM UTC-5, Eric Greenwell wrote:
wrote on 7/8/2016 7:36 AM:
Per is right, a properly engineered bolted connection is never meant
to hold shear loads. The longitudinal tension in the bolted
connection should press the pieces together with a force that causes
friction sufficient to withstand the bending moment.

Bolts are routinely used in shear, from toys to airliners. On my glider,
the tail wheel, landing gear, and propeller are fastened with bolts in
shear, along with many other items.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

Sorry Eric, you are wrong. Pretension (torque) on the bolt results in parts being pressed together. That force with a coefficient of friction normally being assumed at 0.1 results in the parts staying together without slippage. Shear can only occur if the parts slip, a properly designed connection never does.

Herb,

There are some cases where friction is what is relied on for a bolted shear connection, but these are very special cases and require specific installation procedures. In building structures, these bolts are designated ASTM A325 SC or ASTM A490 SC. The "SC" designating slip critical. In order to perform as slip critical connections there needs to be enough capacity in the bolted sandwich to allow the bolt to yield slightly in tension during tightening. This provides a consistent preload to develop the friction. This type of connection is impossible to achieve when bolting though a hollow tube unless there is a bearing sleeve through the tube to allow development of full tension in the bolt. Without the bearing sleeve the bolt simply crushes the tube and sufficient clamping force is never achieved. Many buildings and other structures are designed and constructed using bolts in shear without relying on friction between the faying surfaces. Slip critical bolting (friction bolting) is a nice solution when cyclic loads and fatigue are a consideration because it eliminates the movement that happens when bearing bolts (bolts in plain shear) are cycled from one load direction to another. Cyclic loading can also be accommodated by interference fit bolts, but the degree of accuracy required in fabrication is too expensive for all but specialty applications.

Best regards,
Craig Funston, P.E. , S.E. , P.Eng.

I was about to start in with the shear versus friction discussion on bolted joints when I saw Craig's discussion. He is absolutely correct. I have been in the crane design industry for many decades and use the friction joint technique when using many bolts in a connection but when one bolt is used, shear is usually the design mode. Lynn Wyman, P.E.