Al-Ko Trailer Tongue failure

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.

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 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

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

Hi Craig - point taken! I didn't even think about static applications. I designed mining and tunneling machines and now I engineer around wind turbines. Think high vibrations and dynamics in both cases, so in my world, stuff is constantly trying to fall apart while I desperately try to keep it together - with bolts as long and tightly torqued as the materials allow ;-)

Uli
AS

On 21/07/2016 18:35, Craig Funston wrote:
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.


From my understanding of bolted joints, cyclic shear loads can induce
fatigue failure and using a higher strength steel is not going to
prevent fatigue. I suspect the designers underestimated the number and
magnitude of the fatigue cycles these trailers are subject to when towed
behind motor homes.

Note I have not looked at one of these in person to know if these
suggestions are practical. But I would look for a simpler fix:

- Drill out the 12mm bolts and fit 16mm bolts, and make the problem "go
away" for a long time, hopefully forever.

or

- Drill additional holes for 2 extra 12 mm bolts, one adjacent to each
of the existing ones. Then fit two extra bolts, but with one head of
each pair of bolts on opposite sides of the trailer. Biggest advantage
of this is there are now four bolts and any 3 will easily carry the
load. So if you check it once in a while and find a bolt missing or
damaged, you will have ample chance to replace it and check the other 3,
before things deteriorate to a point of potential catastrophic failure.

In reality the redundant bolts will probably not take any load, but
hopefully with pairs of bolts fitted from opposing sides, their will be
an un-threaded portion bolt to take the load in each major shear plane.

Ian

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

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