Pop top scissor bolts snapped & will not rise

[RickL]

Hi

Here is a link to a bolt grade chart: http://www.boltdepot.com/fastener-infor ... Chart.aspx

Locate the broken bolt and see what grade it is. I'd recommend using the next highest grade. Stainless Steel is not a good choice in this application since it is more brittle than regular steel. A bent and/or sheared bolt in the PH is not a good sign. SMB should be using stronger bolts.

HTH

Rick L

[kallen]

So finally the Sportsmobile is whole again.

I replaced the scissor bolts and slider bolts with the grade 8 (highest strength) equivalents.

I replaced the mangled slider with an equivalent section of small diameter PEX tubing sliced down the axis of the tubing and then silicon-ed it in place. It was a snug fit w/o the silicon so I expect it will hold.

While I was in there, I took a link out of each chain so the top goes up a bit easier.

I also greased the rollers and bolts to reduce friction - the PH goes up and down like butter now.

I took some pics but they are to high-res for the site so I can't post.

Re-hash of steps for replacing scissor bolts and/or slider bolts and/or adjusting tensioning chain.

1. Support penthouse (PH) top with 2x4s and blocks at each end. Raise each end a little at a time w/ blocks until fully erected. Make sure supports distribute the load evenly and keep hands and fingers away least the supports fail and trap human appendages and digits. (major ouch if hands/fingers get caught in there). Jacks and/or multiple human beings could also be used to raise the PH.

2. Replace bolts with grade 8 (highest grade) equivalents (do not use stainless steel) . You can find these at Home Depot or Ace Hardware. It'll take a bit of work to line up the holes. I use a ratchet strap and a car jack to line up the holes.

3. Use ratchet tie downs to "unload" tensioning springs if chain needs adjusting or if roller bolts are bent. Bent roller bolts cause the mechanism to bind and will ultimately cause the scissor bolts to shear. Keep supports in place while doing this.

4. Grease rollers and sliders for smoother operation.

5. Smallest diameter PEX tubing seems to make a good slider guide replacement. Slice along the axis and replace plastic sliders if OEM sliders are damaged. Silicon in place and allow to dry. Grease sliders when done.

I would like to thank everyone who contributed. Thank you all very much.

[jage]

Originally Posted by kallen

I took some pics but they are to high-res for the site so I can't post.

You can provide a link to another larger format site or gallery, or use something like Picasa from Google to export them as 800x and upload them.

Glad you got it fixed.

[LenS]

Any close-up pics of the failed parts yet?

[kallen]

I took pics but the resolution was too high for the site. I was asked to decrease the rez but I'm headed to Africa in a couple/three weeks and I've got a ton of prep work to do (Carnet for moto, vaccinations, route planning, moto details etc etc) and so the pics are kinda low on the priority totem pole if ya know what I mean. Send me your e-mail address and I'd be happy to send them to you. Mine is keith underscore allen 53 at yahoo.

[LenS]

Got pics from Keith and the following message:

Hey Len, Thanks for taking the initiative.


First the bolts:


S1 (scissor 1): I don't know if you can see the slight bend in both the bolt. Notice that both scissor bolts sheared at approx the same location.
S2 (scissor 2):
R1 (roller 1): The roller bolt was bent but not broken (not sure if that is obvious when looking at the picture). After extracting the plastic slider, I had to cut the bolt to extract it. I may have lost a little piece of the bolt but there's enough there to get a good feeling for the stresses that were jamming the bolt in the guide way and thus transferring the forces to the scissor bolts causing them to snap. Pic 923 shows the new roller bolt assemply plus the small diameter PEX tubing that was sliced along its axis and siliconed in place.


Let me know if you have further questions, thanks Len.


Keith

I zoomed in on Keith's two scissor bolts...


Original pics supplied by Keith

[LenS]

Finally a picture to look at!!!

It would be interesting to know what SMB sites are having penthouse scissor bolt failures.

Kevin's bolts that failed were 2.5" bolts....THEY SHOULD HAVE USED 3" BOLTS AND CUT OFF THE EXTRA LENGTH. That would move the bolt thread starting point a half inch farther away from the maximum stress point in the bolt and put the start of the threads very close to the lock nut. Kevin's OEM bolts show no signs of being cut off at the nut end. My SMB-I installed penthouse has had the bolts cut off flush with the lock nuts. Can't really say without removing one that it started life as a 3" bolt but that would be my guess.

I just bought myself 2 1/4" x 3" grade 8 bolts or my SMB tool kit. If I could remove a bolt w/o to much difficulty to inspect it I would.

QED

[jage]

What are these, 1/4"? Grade 8 are 91,000 lb/in² so the loss in surface area at the threads is ... about 40%?

nominal cross section ~ ? * (.125 in)² = .0491 in²
thread cross ~ ? * (.098 in)² = .0301 in²

Which is the difference between 4,419 and 2,709 lbs shear.

It gets better as american standard bolts get larger:
5/8" is .625 in to .598 in is 27600 lb to 25300 lb

Well, better as a percentage anyway... being on the threads is probably more prone to fatigue and then it's whether these are under load when driving...

Guess who worked on winch mounting vs Aluminess fasteners today?

I do think the failure in the track put more shear on the pivot bolts, and threads or no, better to have those shear then the tubes fail. Have no clue what the numbers for THOSE are.

[LenS]

Not sure failure is pure shear. The scissor bolts appear to also be bent in the middle. So failure could be a combination of shear and low cycle bending fatigue.

By going to a 3 inch 5/16th grade 8 bolt the strength would significantly better. Fatter diameter [ (4/5)^2 = 36% less stress ] and moving the threads away from what appears to be the high stress area another 40% reduction in stress as you calculated.

Has any one who has had to replace the scissor bolts gone up a size (or two) in the scissor bolt?? It would take a LITTLE bit of metal away from the scissor tubes but not much.

[kallen]

I thought seriously about going up a size on the scissor bolts but my reticence was that would be making a tube failure a possibility and that would be much more difficult to replace than a sheared bolt.