Al-Ko Trailer Tongue failure

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.

Thanks F1 and company--love the shared knowledge here.

I've had one nose cone bolt and one front handle/top bolt fail, and carry all spares. Just back from Utah, and I'm again amazed and dismayed at some of our roads in the U.S. The road-bridge-road junction must be very hard to engineer--ugh.

Anyone successfully upgrade the Cobra suspension to help with the sharp jar and also big dips? I run 13" trailer tires at high psi, original steel rims. Does Cobra offer a heavy duty option for insert-state-here?

Thanks in advance.

On Monday, July 11, 2016 at 1:43:56 PM UTC-7, Auxvache wrote:
Thanks F1 and company--love the shared knowledge here.

I've had one nose cone bolt and one front handle/top bolt fail, and carry all spares. Just back from Utah, and I'm again amazed and dismayed at some of our roads in the U.S. The road-bridge-road junction must be very hard to engineer--ugh.

Anyone successfully upgrade the Cobra suspension to help with the sharp jar and also big dips? I run 13" trailer tires at high psi, original steel rims. Does Cobra offer a heavy duty option for insert-state-here?

Thanks in advance.

That bolt did not fail because it was under-specified for the application - there is something else going on here. Adding a bolt with greater strength will not resolve the issue. Just think about it: what forces are being applied to the bolt by towing that would have caused it to fail? As an engineer, I have seen so many times when people become fixated on the wrong cause of the problem (I have done it myself). You may have just have had a nut that came loose because of vibration.

Tom

Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.

On Monday, July 11, 2016 at 4:43:56 PM UTC-4, Auxvache wrote:
Just back from Utah, and I'm again amazed and dismayed at some of our roads in the U.S. The road-bridge-road junction must be very hard to engineer--ugh.


I can only second that, 'At the cows'! I just returned from Moriarty on I-40. West Memphis has a long bridge or elevated highway build with concrete sections which are all sagging in the middle. That excited my van and two-axle trailer to the point where I felt the front wheels coming off the ground.. The whole rig was galloping and the only way to make it stop was to slow down to about 35mph, which did not please the truckers behind me. I did inspect the Cobra trailer, ALKO tow bar assembly and the contents of the trailer afterwards and did still find everything being tight and in place.
Road/Highway maintenance does not seem to be important in certain states.

Uli
AS

A question for the mechanical engineers on this group:

Isn't it true that the load rating of a bolt is an indication of the
tensile strength of the bolt and not its shear strength? Is there any
indication of the shear strength of a bolt? Can anything be inferred
about shear strength from tensile strength?

On 7/20/2016 8:31 PM, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.


--
Dan, 5J

On Thursday, July 21, 2016 at 10:43:44 AM UTC-4, Dan Marotta wrote:
A question for the mechanical engineers on this group:

Isn't it true that the load rating of a bolt is an indication of the
tensile strength of the bolt and not its shear strength? Is there any
indication of the shear strength of a bolt? Can anything be inferred
about shear strength from tensile strength?

On 7/20/2016 8:31 PM, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.


--
Dan, 5J

Dan - bolting theory 101: Never design a bolted joint where the bolt(s) are going into shear! A bolt is supposed to clamp two or more members together and the whole thing is supposed to hold together by friction. If the joint moves, the axial pre-stress or clamping force provided by the bolt(s) was not high enough! Short, stubby bolts will not hold their clamping force for long. That's why long, slender bolts that can be elongated up to their yield point and act as axial springs are preferred over short, stubby ones. Going up in bolt diameter does in most cases not solve the problem.
If you really want to improve the ALKO design, use stand-off bushings (NOT stacks of washers!) and longer bolts. Figure out what the torque rating for that bolt grade is and precisely torque it to that value. Use a good torque wrench and NOT Bubba on a 3ft cheater pipe!

Uli
AS

On Thursday, July 21, 2016 at 7:43:44 AM UTC-7, Dan Marotta wrote:
A question for the mechanical engineers on this group:

Isn't it true that the load rating of a bolt is an indication of the
tensile strength of the bolt and not its shear strength? Is there any
indication of the shear strength of a bolt? Can anything be inferred
about shear strength from tensile strength?

On 7/20/2016 8:31 PM, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.


--
Dan, 5J

For typical steels used in bolt fabrication, shear strength correlates pretty well with tensile strength. Current guidance from the American Institute of Steel Construction use a shear strength equal to 60% of the tensile strength (ultimate values, not yield). This is for a bearing type (not friction) connection with threads included in the shear plane. High strength bolts are allowed shears up to 75% of tensile if the threads are excluded from the shear plane.

Craig
7Q

On Thursday, July 21, 2016 at 9:12:54 AM UTC-7, AS wrote:
On Thursday, July 21, 2016 at 10:43:44 AM UTC-4, Dan Marotta wrote:
A question for the mechanical engineers on this group:

Isn't it true that the load rating of a bolt is an indication of the
tensile strength of the bolt and not its shear strength? Is there any
indication of the shear strength of a bolt? Can anything be inferred
about shear strength from tensile strength?

On 7/20/2016 8:31 PM, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.


--
Dan, 5J

Dan - bolting theory 101: Never design a bolted joint where the bolt(s) are going into shear! A bolt is supposed to clamp two or more members together and the whole thing is supposed to hold together by friction. If the joint moves, the axial pre-stress or clamping force provided by the bolt(s) was not high enough! Short, stubby bolts will not hold their clamping force for long. That's why long, slender bolts that can be elongated up to their yield point and act as axial springs are preferred over short, stubby ones. Going up in bolt diameter does in most cases not solve the problem.
If you really want to improve the ALKO design, use stand-off bushings (NOT stacks of washers!) and longer bolts. Figure out what the torque rating for that bolt grade is and precisely torque it to that value. Use a good torque wrench and NOT Bubba on a 3ft cheater pipe!

Uli
AS

Uli,

Use of bearing vs. friction for bolts in shear depends on which industry you're operating in. For building structures it's quite common to use bolts in bearing since often the loads are relatively static. Friction bolting is advantageous in applications where the loads are cyclic and fatigue is a concern. It's also advantageous for applications that require accommodation of loose field tolerances since the bolts can be tightened to clamping friction and the bolt holes can be oversize or slotted.

I agree the ALKO tongue connection is subject to cyclic loads and fatigue and would benefit from a properly installed friction bolted connection. A compression bushing between the inside walls of the square tube is needed to develop the proper clamping force. Drilling a larger hole for a bushing and pushing it through the square tube won't accomplish the goal.

Craig
7Q

Very informative, thanks!

How about this for the use in question: Drill the holes in the trailer
tongue oversize and use a steel bushing the exact length of the outer
tube of the tongue. Then use a high strength bolt to hold it together.
Or simply a trailer hitch pin with a spring clip or lock to hold it in
place.

On 7/21/2016 10:23 AM, Craig Funston wrote:
On Thursday, July 21, 2016 at 7:43:44 AM UTC-7, Dan Marotta wrote:
A question for the mechanical engineers on this group:

Isn't it true that the load rating of a bolt is an indication of the
tensile strength of the bolt and not its shear strength? Is there any
indication of the shear strength of a bolt? Can anything be inferred
about shear strength from tensile strength?

On 7/20/2016 8:31 PM, Dave Springford wrote:
Hi Tom,

Very doubtful that the nut came off from vibration. Nylock nuts are used and they are not prone to this. My trailer is towed behind a motorhome with a large axle to tow ball distance resulting a bending moment on the tongue and shear load on the (front) bolt (in particular).

After my post, another motorhome owner checked his trailer and found that the same bolt on his trailer was broken - shear plane through the threads, bad design - but fortunately the rest of the bolt was still in the tongue so he was able to take pictures of it. It was certainly a failure due to shear.

I question why AL-KO switched from 1000 MPa bolts to 800 MPa bolts at some point. I checked several other older trailers than mine and they all had 10.9 bolts. Mine had 8.8, as did the other trailer that failed.

--
Dan, 5J
For typical steels used in bolt fabrication, shear strength correlates pretty well with tensile strength. Current guidance from the American Institute of Steel Construction use a shear strength equal to 60% of the tensile strength (ultimate values, not yield). This is for a bearing type (not friction) connection with threads included in the shear plane. High strength bolts are allowed shears up to 75% of tensile if the threads are excluded from the shear plane.

Craig
7Q

--
Dan, 5J