roll pin application
roll pin application
(OP)
Full disclosure: The device in question is more hobby than work related. But as engineers, safety is paramount is all aspects of our lives. Since my question is safety related, and I'm thinking of making a report to the relevant government agency, I believe my question should be allowed.
Please see the connection between the threaded rod and collar in the photo. The threads don't end at the bottom of the collar, but extend to the top of the rod above the pin connection. The inside of the collar is cylindrical, not threaded. This means the pin crosses a bit of a gap, unlike the upper pin which joins a cylindrical shaft and hole. I believe the gap causes some of the stress on the pin to be bending moment instead of straight shear like in the upper pin. Is this EE correct on this mechanical issue? Is this a proper application? I had a pin like this one (not my photo) fail at a time when it caused no safety issue, but at another time it might have caused a motor vehicle accident. Thanks.

Please see the connection between the threaded rod and collar in the photo. The threads don't end at the bottom of the collar, but extend to the top of the rod above the pin connection. The inside of the collar is cylindrical, not threaded. This means the pin crosses a bit of a gap, unlike the upper pin which joins a cylindrical shaft and hole. I believe the gap causes some of the stress on the pin to be bending moment instead of straight shear like in the upper pin. Is this EE correct on this mechanical issue? Is this a proper application? I had a pin like this one (not my photo) fail at a time when it caused no safety issue, but at another time it might have caused a motor vehicle accident. Thanks.





RE: roll pin application
Ted
RE: roll pin application
RE: roll pin application
In my review of spring roll pins specifications on-line, I have found shear strength information but no bending moment specs, leading me to believe this might be a misapplication.
RE: roll pin application
Ted
RE: roll pin application
The weight would tend to push the assembly together against the stop, so no vertical load to speak of. The freewheeling ball screw would tend to transmit some torque, but even with the motor assembly removed there is enough friction in the right angle gear above to prevent the screw from backing down. Instructions advise only letting the camper sit on the jacks long enough to load/unload. Failure in this manner is not my main concern.
"Can you tell if the pin failed due to vertical load or twisting load?"
The crumbs left gouges to the side inside the collar, so I would say twisting load. I'm guessing that stopping/starting/reversing the motor took it's toll, since I was careful to avoid letting the clutch slip when fully retracting it.
"Is the pin the only element transmitting torque from the collar to the screw?" Yes.
"You will probably have to due the calculations to determine pin bending capacity." And compare to what? It's the lack of published specs on bending moment and the brittle nature of the pin that lead me to believe that it may be a misapplication to use where any significant bending moment is involved.
Jack is rated 1900 lbs and four of them are used on a 1900 lb dry weight camper.
RE: roll pin application
Ted
RE: roll pin application
Withoutut testing or calculating, can the group state anything regarding whether they believe this is a proper application? Thanks.
RE: roll pin application
I would say that putting a roll pin through a thread was a normal application, the pin is used sometimes to prevent rotation of a threaded joint loosening under vibration or if it is subjected to a load it should be sized and selected to take the required service load see this link
http://www.engineersedge.com/roll_pin.htm
Now if over a period of time the pin as worn the hole that was first drilled to accept it, it's possible that the pin would see a slight impact load every time the device is used in which case it may fail due to fatigue.
RE: roll pin application
RE: roll pin application
I.e., I wouldn't do it that way.
If forced to do so, I'd oversize the pin.
If I had merely purchased the jack, and it had failed once already, I might try filling the cup cavity with JB Weld or other filled/hard epoxy during assembly, in an attempt to place the pin in pure-ish shear.
If that didn't work, I might replace the rolpin with a solid hardened dowel, and retain it with an external sleeve or circlip around the cup.
Mike Halloran
Pembroke Pines, FL, USA
RE: roll pin application
If you think about it, the pin would tolerate higher loading in bending than shear. Bending failure is result of exceeding yield or ultimate tensile strength which are both higher than shear strength. How much clearance is there that the pin must span? Less than a pin diameter?
I would guess some other failure mechanism is at work here. Perhaps impact.
Ted
RE: roll pin application
RE: roll pin application
In the link below on page 39 it discusses the gap between mating components for pins and recommends either a solid or spirol pin depending on load type.
http://www.technifast.co.uk/downloads/Techinfast%2...
RE: roll pin application
I've visited the Happijac website and saw the piece on there that matches your photograph to get a better understanding of the problem.
Rereading your original post and the information I found on the link in my last post I can see that the pin would have to transmit the torque from the motor or hand crank operation and I don't think that's a brilliant idea.
The gap would possibly cause some bending on the pin and not just pure shear and also the pin would be subject forward and reverse torque during lifting and lowering.
How old is your jack? as I can see the pin being subject to impact loading particularly if the holes in the adjoining components start to wear, in addition there is the possibility of fatigue failure also.
To get some idea of the torque the pin would have to transmit you could use the relationship:-
P (lifting capacity) = Torque/(0.02*d) where d = ball screw dia and 0.02 = friction factor
So transpose the formula to get torque, use the lift capacity of the jack and diameter of the ball screw to get the torque. Once you have the torque we can look at the size of pin and the related shear stress/ bending.
I think you might have a good point in your original post and I suppose its whether the manufacturer can demonstrate that the pin was designed fit for the purpose.
RE: roll pin application
RE: roll pin application
Have you any pictures of the failed pin itself? if so looking at it might also give a clue to its failure.
In addition look at the hole the pin goes through and see if there is any elongation of the hole ie should look like a very small slot.
good luck
desertfox
RE: roll pin application
RE: roll pin application
Is the piece of pin sat on the base from the centre of the bore or is it part of the piece through the hole on the right hand side of the main piece.
Also looking at the piece of pin the face looks as though its failed at a slight angle?
Any chance of a real close up on the fractured spirol pin face, trying to look for clues of failure but I can see it was a brittle failure looking at it.
A little trick to test for brittle failure is if you have both half's of the failed pin then try putting them together, if they fit together quite easy a bit like a jigsaw puzzle then its usually a brittle fracture, if its a ductile fracture there is usually a lot of distortion and the failed bits don't quite fit together.
desertfox
RE: roll pin application
RE: roll pin application
RE: roll pin application
Thanks for the update.
The pin appears to be failing under a maximum tensile stresses on the principle planes ie at an angle to the cross section of the pin.
Desertfox