Nut backed off ball caused accident
I was heading up to santa cruz ca. in my 40' american eagle I had my motorcycle in tow on a utility trailer. Before I left I had tighten the nut on the ball with an air impact driver the kind that you...
Forum Discussion
Not everyone will go to a pintle, nor weld, nor anything else...though there
will be some
Hope brought this down to a level for some here to understand what
the heck this is for an designer
Here is the formula to figure out the clamping force. Clamping force is
calculated according to the dynamic loads and shock loads the
designer worked out for the worst case...plus margin. Once they
have the clamping force needed, then the process of selecting a
bolt (shank) size, or bolts. Along with grade and another whole
discussion on the selection of grade...cascading failure is one
where you want the lower grade on the part you wish to fail first, etc
T / KD = P
P=clamping force
D=shank dia or nominal thread dia
K=Constant and is the co-efficient of friction on the bolt/nut
T=Torque
On the constant and is the friction between the mating surfaces that the torque
must over come *BEFORE* the fastener sees any clamping force... 0.16 is a very
smooth and greased mating surfaces...rusty and rough surfaces would be around 0.75, or more (these numbers are IIRC, as it has been decades since the last
time I've calculated a fastener for a design)
Chose 1" shank dia for ease of math
Two thoughts on this.
The ball instructions of 450 ft/lbs
Theory of 150 ft/lbs & cotter/castle will do
Just the numbers for now
450 ft/lb / 0.5 x 1 = 900 lbs of clamping force
150 ft/lb / 0.5 x 1 = 300 lbs of clamping force, along with a cotter or castle
But there is more than just the clamping force, though the main component.
There is the hole that the ball shank goes through. It has tolerance, meaning
bigger than the shank by some designed in margin to allow the shank to easily
pass through...but that is part of the problem...that tolerance
If the hole is sized as a press fit, it will still have some movement of the
shank in that 'tight' hole
Movement is a VERY bad thing for this application
So if the clamping force is 1/3 of the designers spec, then the shank will
most likely move around in that hole...even it it has a cotter and castle too
Banging and over time will work the metal of both the shank and hole.
Moving like that to bang will also allow the nut to back off, but okay...the
cotter will stop that via interference...but the castle *WILL* back off, unless
it too have a cotter in the castles
Here is the biggie over and above the previous...the shank will be allowed to
lean and/or bend because it does NOT have the preload spec'd in by the designer
Since not enough preload for that application (meaning the static and dynamic
loads the designer worked to)...the loading will pull/push/etc the ball that
will then lever the shank.
All of that movement *WILL* cause other things to wear faster. Like the coupling
and it's latch
If tightened to the spec of 450 ft/lbs, which creates 900 pounds of clamping
force...it should not move when exposed to expected forces
Must qualify that...always say things designed are designed not for the good
days out there, but for the worst day out there when Mr Murphy crosses your path
'Maybe' the cotter/castle will keep the nut on, but with all of the increased
wear and tear...will the coupler hang onto the ball? Will the ball bend over
enough to create a condition where the couple dome recess will allow the ball
to move out of?
The more I read on these forums...the more concerned get about the setup next
to me going 65MPH or more...
will be some
Hope brought this down to a level for some here to understand what
the heck this is for an designer
Here is the formula to figure out the clamping force. Clamping force is
calculated according to the dynamic loads and shock loads the
designer worked out for the worst case...plus margin. Once they
have the clamping force needed, then the process of selecting a
bolt (shank) size, or bolts. Along with grade and another whole
discussion on the selection of grade...cascading failure is one
where you want the lower grade on the part you wish to fail first, etc
T / KD = P
P=clamping force
D=shank dia or nominal thread dia
K=Constant and is the co-efficient of friction on the bolt/nut
T=Torque
On the constant and is the friction between the mating surfaces that the torque
must over come *BEFORE* the fastener sees any clamping force... 0.16 is a very
smooth and greased mating surfaces...rusty and rough surfaces would be around 0.75, or more (these numbers are IIRC, as it has been decades since the last
time I've calculated a fastener for a design)
Chose 1" shank dia for ease of math
Two thoughts on this.
The ball instructions of 450 ft/lbs
Theory of 150 ft/lbs & cotter/castle will do
Just the numbers for now
450 ft/lb / 0.5 x 1 = 900 lbs of clamping force
150 ft/lb / 0.5 x 1 = 300 lbs of clamping force, along with a cotter or castle
But there is more than just the clamping force, though the main component.
There is the hole that the ball shank goes through. It has tolerance, meaning
bigger than the shank by some designed in margin to allow the shank to easily
pass through...but that is part of the problem...that tolerance
If the hole is sized as a press fit, it will still have some movement of the
shank in that 'tight' hole
Movement is a VERY bad thing for this application
So if the clamping force is 1/3 of the designers spec, then the shank will
most likely move around in that hole...even it it has a cotter and castle too
Banging and over time will work the metal of both the shank and hole.
Moving like that to bang will also allow the nut to back off, but okay...the
cotter will stop that via interference...but the castle *WILL* back off, unless
it too have a cotter in the castles
Here is the biggie over and above the previous...the shank will be allowed to
lean and/or bend because it does NOT have the preload spec'd in by the designer
Since not enough preload for that application (meaning the static and dynamic
loads the designer worked to)...the loading will pull/push/etc the ball that
will then lever the shank.
All of that movement *WILL* cause other things to wear faster. Like the coupling
and it's latch
If tightened to the spec of 450 ft/lbs, which creates 900 pounds of clamping
force...it should not move when exposed to expected forces
Must qualify that...always say things designed are designed not for the good
days out there, but for the worst day out there when Mr Murphy crosses your path
'Maybe' the cotter/castle will keep the nut on, but with all of the increased
wear and tear...will the coupler hang onto the ball? Will the ball bend over
enough to create a condition where the couple dome recess will allow the ball
to move out of?
The more I read on these forums...the more concerned get about the setup next
to me going 65MPH or more...
