Al-Ko Trailer Tongue failure

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 Thursday, July 21, 2016 at 6:42:04 PM UTC-7, AS wrote:
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

Hi Uli,

I can absolutely see why clamping force is king. Lots of other things to consider as well I'm sure. I remember being fascinated by the use of tapered studs to control shear lag / bond stresses in the bonded steel fasteners for the FRP blades. Interesting industry.

Cheers,
Craig
7Q

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

As an ignorant non-engineer why not reduce the cause of the problem
rather than use a technical solution to it.
As I understand it the problem seams to be caused by excessive
pitching of the trailer exceeding the design loads on the tongue.
What caused this pitching? One thing mentioned it is large overhangs
between the rear axle and the hitch on the tow vehicle. If this is
sufficient to cause problems with the integrity of the tongue what is it
doing to the glider inside?
Another problem is loading the trailer so there is too much weight on
the tongue. The pitching causes a loading + or _ around that weight.
i.e if the tongue weight is 30lb and pitching causes a variation of +or -
100lb the load would vary between -70 and +130. If the tongue weight
was 100lb plus as I have seen recommended on this site it would vary
between 0 and 200lb. Insert your own figures but you get the gist of
my argument.
As a matter of interest I havnt heard of this problem on this side of the
pond where we usually tow with much more compact, lighter vehicles.

Nigel Pocock wrote on 7/22/2016 8:32 AM:
As an ignorant non-engineer why not reduce the cause of the problem
rather than use a technical solution to it.
As I understand it the problem seams to be caused by excessive
pitching of the trailer exceeding the design loads on the tongue.
What caused this pitching? One thing mentioned it is large overhangs
between the rear axle and the hitch on the tow vehicle. If this is
sufficient to cause problems with the integrity of the tongue what is it
doing to the glider inside?
Another problem is loading the trailer so there is too much weight on
the tongue. The pitching causes a loading + or _ around that weight.
i.e if the tongue weight is 30lb and pitching causes a variation of +or -
100lb the load would vary between -70 and +130. If the tongue weight
was 100lb plus as I have seen recommended on this site it would vary
between 0 and 200lb. Insert your own figures but you get the gist of
my argument.
As a matter of interest I havnt heard of this problem on this side of the
pond where we usually tow with much more compact, lighter vehicles.

Changing to a more trailer-friendly towing vehicle than a motorhome is a
non-starter if I want my wife to travel with me; I also prefer
traveling (particularly to glider events) in a motorhome.


It does happen on your side of the pond when towing with motorhomes, but
likely isn't as common due to fewer towing miles and smaller motorhomes.
I've towed my ASH 26 E more than 170,000 miles in it's Cobra trailer,
and it's in fine shape, so the pitching doesn't seem to be a problem for
the glider. I did have to replace the original, smaller tongue about 15
years ago, which developed a crack at the weld for the tongue wheel mount.

--
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