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extending hitch; anyone see an issue with my math?


steiny93

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Currently the 5th wheel hitch (Air Safe) is mounted on top of a pair of 4" x 6" rectangular tubes 1/2" walled 17" long sitting on the tall side (6").

The hitch and those tubes are bolted through a 1/2" plate which is attached to the top of the frame rails.

Four bolts are holding the hitch to the 1/2" plate through the 4x6 tubes which are 5/8" grade 5 bolts

 

[i apologize for no pics]

 

I need more clearance between the 5th wheel and the atv's I'm putting on the bed of the HDT, 28" would be perfect.

If I take the 17" long 4x6 tubes and re-do them at 45" long.

Doing so would have the first 20" fully supported by the 1/2" plate then the remaining 25" would be in overhang beyond the plate.

 

Those tubes would then be held on by four 5/8" grade 8 bolts; combined providing 588k lbs of proof load (grade 8 is 120k psi)

The tubes at 6"x4" by 1/2" walled in the length of 45" with 20" supported has a max bending movement of 32k lbs combined (both tubes, 30k psi steel and .55 yield).

 

The trailer we would be pulling in this config is light

2500lbs pin

16.5k gross

 

Does anything about this plan seem off to anyone?

 

 

 

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Those tubes are stronger than the chassis. 1/2" thick wall, wow. Just be sure to provide enough attachment strength to chassis. Wouldn't it be easier just to extend chassis?

 

I will rarely need the extra length (maybe 1 trip every 3 years type of a deal); rest of the it will be back in normal short mode. I'm trying to land a solution that lets me put on and remove as necessary.

 

yah, at this point i'm thinking about the attach; but I think I'm all good if I go grade 8.

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Looks like a sliding 5th wheel plate is needed. There are plenty of engineers on here that will have ideas. I even looked at doing this for a while then I was given a roll back deck. Maybe that would work build a sliding deck and mount the hitch to the deck. It doesn't have to be a tilting deck, just sliding.

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When I built the bed it was built with 2x5 box tubing stood 5" high. Then with a air pin box on the 5thwheel I mounted the Brinkley head on 2 1/4" of rubber for give on the hitch.

To build the sliding hitch I used the old sliding mechanism for the commercial hitch. Just torched the locking part off the sliding hitch very carefully. The new sliding part was built with 3x5 box tubing 1/4" wall with two tubes on each side.

This gave me enough room for the Brinkley head to fit in between the tubing rails with enough room for 3/4" rubber on all for sides of the new bigger base plate I welded on the bottom of the Brinkley head.

The sliding hitch will provide enough travel to put the centerline of the pin 27" behind the bed when needed.

To help with a 6000 lb pin weight I use two top links for on tractor equipment (cat 3 I believe) that attach to the bottom of the bed at the back.

The bed is only sixteen inches longer than the frame was originally. In the storage box that goes across the back of the bed there is support angled down for the top links to attach to.

The front of the sliding hitch bolts tight to the frame if the hitch is out or in. The latching mechanism is what really holds the hitch in place

 

Just what I did for clearance for a jeep

 

Vern

 

It has been 9 years since I built the hitch. I think the 3x5 box tubing maybe 3/8" wall.

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Those tubes would then be held on by four 5/8" grade 8 bolts; combined providing 588k lbs of proof load (grade 8 is 120k psi)

 

It doesn't work that way! You need to take the area or the 5/8" bolt cross section, then multiply that by the 120K "tensile strength" then because you have them in shear instead of tension multiply that by .5 then by 4 because you have 4 bolts.

 

So: .307 x 120,000 x .5 x 4 = 73,631 lbs shear strength "if" you drill and ream the hole so they share the load perfectly. (they don't) so divide that in half again 36,815 lbs shear strength. Simple answer... add 2 more bolts.

 

Sorry to bust your bubble but better that than your (insert choice here)

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Stekay, great feedback

 

 

It doesn't work that way! You need to take the area or the 5/8" bolt cross section, then multiply that by the 120K "tensile strength" then because you have them in shear instead of tension multiply that by .5 then by 4 because you have 4 bolts.

 

So: .307 x 120,000 x .5 x 4 = 73,631 lbs shear strength "if" you drill and ream the hole so they share the load perfectly. (they don't) so divide that in half again 36,815 lbs shear strength. Simple answer... add 2 more bolts.

 

Sorry to bust your bubble but better that than your (insert choice here)

 

thx for double checking my math; 588k did seem very large.

it does look like another 2 bolts would be a good thing using the above numbers. The load is only 16.5k; but being crazy over the number vs ~2k seems reasonable.

 

for my clarity, question on the tension vs. shear.

Since this scenario has the tubes acting as a lever wouldn't these bolts be both in shear and in tension? Wouldn't the lever force (up & down) be tension force and the fore / aft & side to side forces be in shear?

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Steve is right on the mark....of course you could take Steve's hint and match ream the holes and then raise him up a notch and then upset-hot rivet the plates then you would then have a hole-filling-fastened joint and.....then just fill out a ECO and send it to the Boeing Structures Geeks to get about 500 comments....uh my past is flashing back.....just add two bolts and have a cold adult beverage....

 

Drive on.....(Who is in shear and who has...tension....)

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for my clarity, question on the tension vs. shear.

Since this scenario has the tubes acting as a lever wouldn't these bolts be both in shear and in tension? Wouldn't the lever force (up & down) be tension force and the fore / aft & side to side forces be in shear?

Steiny, I don't have a perfect picture in my mind of your set-up, but yes you are generally correct under most conditions. It is a dynamic scenario with many variables. Let's not let one of those ruin your day however. BTW, friction is your friend so let's make sure we torque those bolts to 75% of their tensile strength. There are plenty of charts to look this up. Use either a lubricant or a threadlocker (it's also a lubricant) on the threads to get an honest torque. Safe travels my friend!

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  • 3 weeks later...

update

finished the project and went with 6 instead of 4 bolts :)

 

having the hitch 28" further back makes a crazy difference in the maneuverability in reverse as well as causing the trailer to track significantly closer to the tractor's wheel track (the hitch started just slightly rear of the axle center line)

 

all in all, worked out hyper well

thx for all the inputs

 

Leaving for a 3k mile 2 week trip today at noon!

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