Question Regarding Cross Drilled Automobile Hitch Pin

[sethro5hc]

I'm designing a stabilizing hitch pin for trailer and hitch applications, everything is good to go except my supervisor would like it to be cross drilled near the end so that a locking pin may be used. My problem with this is that I think somebody could just come by with a hammer and knock the end off fairly easily. I've attached a picture to allow better understanding. The diameter of the hole you see is 5mm. there is another hole through the center of the top view, which is illustrated with the red lines shown. It also has a diameter of 5 mm. The rest of the dimensions of outer and inner screw thread diameter and distances should be in the picture I've given. From my understanding and just an estimate, there will basically be four sectors connecting the main body of the pin, to the part after the 2 cross drilled holes. The centers won't be connected at all. Each sector is estimated at having an area of around 9-12 mm^2. Could somebody please help me determine which force acting downward in the Y-direction from the end would break off this end piece? Any help is greatly appreciated.

 

[ivymike]

um, a really big one. Try hitting one with a hammer sometime, and see if it's as easy to break as you think. That looks to me like a fairly short&fat thing to try to break with a hammer.

 

[sethro5hc]

If that sector arrangement is correct, it'd be roughly the same as hitting the back 5 mm section of a tube with radius around 3.5-3.8mm. Made of steel and I don't think some random is going to come up and just lop the end off. Thanks for the help.

 

[Keith1029]

If I understand correctly the “Locking Pin” will be used as a security feature, presumably key locked, that would prevent unauthorized persons from removing the hitch pin? From a “gut feel”, I agree with you, the design seems like it could be revised for strength. That being said, without material specifications, or loading requirements it is impossible to say if the objective is met. If the locking pin is just to prevent the pin from backing out, however, perhaps designing for a hammer strike on the end is overkill. In either case though, in order to do a proper evaluation you would need material assumptions and it would be helpful if you were able to explicitly quantify the maximum loading requirements.

 

[sethro5hc]

Well the material is stell, but we're just going with a stock option like CRS 1020 or something. The loading on the pin won't effect this section much, as the shearing it would put on the pin near the holes of the hitch receiver would be relatively far away from this cross drill section in comparison to the size of this hole. I'm just wondering in a very generalized sense, you have a threaded cylinder with D(o) of almost 16mm and D(i) of almost 14mm. A 5mm diameter hole is drilled in from the top and the right side, creating sort of a "plus sign" shaped hole in the middle. The 4 quadrants outside the cutout add up to around 35-45 square mm of total connecting material, and the material is steel (just say 1020 cold rolled). The center of the 5 mm holes are drilled just 7 mm from the end of the piece, so there isn't a long enough piece to create a large moment, so would the force produced by a normal sized person swinging a hammer on that roughly 4.5 mm piece hanging on the end, be enough to break it off, which would let the hitch lock fall off and they could take whatever it was protecting? My thinking is no, but it's always best to have some sort of calculations going, unfortunately, most of my relevant textbooks are not with me

 

[patprimmer]

If you bevel the end of the bolt from the holes down, the hammer will not be able to get a direct hit.

2 X 5mm holes certainly removes a major portion of the metal. Do you really need 2 holes and do they need to be 5mm.even 4mm holes would substantially increase the area left.

an end view with both holesmarked would have been easier to analyze, especially if it was overlayed with a fine grid.

Regards
Pat
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[patprimmer]

Opps

I read 6.91 dia not rad. With 13.8 minor ia, there is plenty left. putting a 45 deg bevel on the end will still help

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[GregLocock]

I don't know about the strength, I do know that if that locking pin is ever hit by anything it will damage the thread and your name will be mud.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Not if the thread is turned off in the bevel process.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[mrpi]

Is the hole actually for theft deterrence, or more for nut retention?

If you intend to put a cotter in the hole to keep the nut from falling off, then I would source a castle nut first then size the cotter pin hole based on the available castle nut slot width.

If its actually to keep people from stealing the trailer, I think a locking hitch is probably good enough for keeping the honest man honest. Besides, if somebody wants the trailer bad enough they'll take it regardless of what type of pin you put through that hole.

Parenthetically, I bet hitting the end of a grade 8 bolt till it broke off at the cross hole would deform the threads bad enough that the nut would be hard to remove anyways.

Beat to fit, paint to match.

 

[mrpi]

I just re-read the question and follow-up post. If I understand this correctly, the 5mm holes are there to allow a hitch lock to be used on the end of a 5/8" bolt?

If that's the case, you probably dont need two holes through the bolt. And what happens if they were to hit the bolt with a hammer and remove the lock? Would they then have to unscrew the bolt from something in order to finish stealing it?

Using a Nylock nut torqued down will itself provide theft deterrence and vibration-loosening resistance.

Beat to fit, paint to match.

 

[MikeHalloran]

Must the cross-drilled holes intersect each other?



Mike Halloran
Pembroke Pines, FL, USA

 

[sethro5hc]

So I've done a little more checking, and the holes are actually not intersecting. They are in perpendicular directions of one another, but the centers are separated by nearly 10 mm.

The purpose is so that a locking hitch can be used for added theft deterrance. Obviously if somebody really wants whatever is on the hitch, they're going to get it. I took a hitch to the shop, cross drilled 2 holes and beat the crap out of it yesterday and it held fine. Like somebody pointed out and I suspected though, doing this will obviously damage the threads then the nut won't be coming off anyways without some serious assisstance.

Thanks for the help guys, I think the design is ok for it's intended purpose, it's really not a whole lot more than just sticking up the equivalent of a "brinks home security" sign in your front yard. If people want something, they'll still get it but the idea that it's there should help deter the lazy ones.