# Failed Trailer Jack Coupler



## YYCHM (Jul 15, 2019)

This coupler failed on my electric travel trailer tong jack.  It couples the motor gears to the jack screw.
The hex head part allows the jack screw to be turned manually with a crank handle.





















Apart from the hex head end it's pretty darn simple and well with in the size metrics of my mini-lathe.

The hex part I should be able to do on my mini-mill using the RT.

I have a piece of round stock that's the perfect size but haven't a clue what grade it is.

Should I be looking at obtaining a specific grade of round stock to use for this project?

Thanks


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## johnnielsen (Jul 16, 2019)

If I were making it, I would use 4140 annealed just to be on the safe side. I would also ensure a shear pin weaker than the two pieces it joins is used.


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## YYCHM (Jul 16, 2019)

Getting close.  One more hole to drill.






I ended up having to file the hex head.  My x-axis DRO scale precludes mounting my RT in the vertical.






I did use the RT in the horizontal to mark the centers of the six hex flats.  For a hand filed solution it turned out better than I thought it would.






This is how the drive gear is tied to the coupler shaft.  I'm thinking of reducing the dia of that pin by 1/4 or 1/3.  Leave more meat in the shaft.  If the drive pin fails it's fairly easy to get at to replace.

Do you think I should attempt to harden the hex head portion of the coupler?


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## johnnielsen (Jul 17, 2019)

If you used 4140 there should be no need for heat treating in this application. Reducing the shear pin hole size is a good idea. The shear pin should be a snug fit so shearing, not bending, is the primary force on the connection. What are you going to use as a shear pin, brass stock or a notched steel pin?


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## YYCHM (Jul 17, 2019)

johnnielsen said:


> If you used 4140 there should be no need for heat treating in this application. Reducing the shear pin hole size is a good idea. The shear pin should be a snug fit so shearing, not bending, is the primary force on the connection. What are you going to use as a shear pin, brass stock or a notched steel pin?



I used the round stock I had on hand as I wasn't sure I could pull off making a duplicate in the first place.  The hex head portion was easy to file (as in soft) so the hardening aspect came to mind.  Probably not necessary.  The manual crank would only be used when 12v is not available or the motor dies.  I've never had need to use it.

As for the drive pin.  That pin not only drives the jack screw but is the only thing supporting the weight of the entire jack screw assembly and foot.  10 lbs worth.  Imagine 10 lbs bouncing up and down on a bumpy gravel road and crossing cattle gates.  Good design eh!  Power Fist stuff.  The original drive pin is 0.310", and the coupler shaft is 0.614" where the pin hole resides.   I'm thinking of using a 1/2" standard grade bolt turned down to 0.247" as a replacement with no failure knotch.


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## RobinHood (Jul 17, 2019)

YYCHobbyMachinist said:


> This is how the drive gear is tied to the coupler shaft. I'm thinking of reducing the dia of that pin by 1/4 or 1/3. Leave more meat in the shaft. If the drive pin fails it's fairly easy to get at to replace.


I think that would be a good idea. Whatever size you decide on, make a spare pin while you’re at it.


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## YYCHM (Jul 18, 2019)

Finished the new drive pin and reassembled all the major pieces.






I made the drive pin from a 1/2" grade 8 bolt but am not happy with the fact the pin does not extend through the coupler all that much.  The pin is long enough, I located the cap screw hole poorly.  Still have to make a backup drive pin so I'll be more diligent with that one.

I have to say, my mill DROs really came to the rescue in terms of locating and drilling holes.

Sunday I'll re-install the jack on my trailer and next weeks trailer outing will be the test.

If this actually flys, it will be the first real world useful thing I have made with my lathe and mill!


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## kevin.decelles (Jul 20, 2019)

I find I make 5:1 things for me then the missus. I find it encourages the support for more tool purchases

Here’s hoping it is her camper


Sent from my iPad using Tapatalk


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## DPittman (Jul 20, 2019)

YYCHobbyMachinist said:


> Finished the new drive pin and reassembled all the major pieces.
> 
> View attachment 5616
> 
> ...


Looks like a good fix. Well done!


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## YYCHM (Jul 20, 2019)

Second go at the drive pin.... This is what I had in mind in the first place.  

It's a strange arrangement.  I can picture the shear forces on the pin due rotation, but, since the pin is only supported by the cap screw there has to be a bending moment involved.


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## YYCHM (Jul 21, 2019)

kevin.decelles said:


> I find I make 5:1 things for me then the missus. I find it encourages the support for more tool purchases
> 
> Here’s hoping it is her camper
> 
> ...




Brilliant!  I never thought about leveraging on the fact that the trailer is her baby.  Mind you the power tongue jack is more for me than her, as I'm the one hitching/unhitching, pulling and leveling the sucker.  The original manual crank jack is probably just fine in her view LOL.


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