Announcement

Collapse
No announcement yet.

ZRA Set Screw FAIL

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    #16
    I always cringed when I had to adjust the fork angle on a Series 1. My thoughts, use a military spec stainless steel socket head cap screw that accepts a 3/16 Allen wrench (McMaster -Carr probably carries the right size).

    I also have concerns about the Series 2 adjustment as they elected to use a a socket head screw that accepts a 3/16 Allen in the center, but the outer socket screw accepts an 1/8" Allen.

    I believe both screws have the identical thread. What could possibly be the rationale for doing this? 1/8" screw heads are prone to strip, and 1/8" Allen wrenches don't stay in "good as new" condition for very long.

    I've already replaced them with socket heads that accept a 3/16" wrench on our demo Series 2.

    Comment


      #17
      Originally posted by tooley View Post
      The holes in the frame are large enough to account for this. I assume Wolfe's girth and drunken escapades have something to do with the fastener failing.
      Let's not blame anyone's personal habits or "hold my beer and watch this!" escapades...

      Even if the frame holes are oversized, the screw will be bent into an "S" when torqued because of the flats that the head and nut tighten against.

      Comment


        #18
        SCI, did/do you work for this wheelchair manufacturer?

        Originally posted by SCI_OTR View Post
        My thoughts, use a military spec stainless steel...
        The military and SAE are two big organizations who have had a need and the resources to develop specs for screw materials (among many other things).

        They both have complete specs for the Complete Range of screw materials. When saying "Mil Spec," you need to quote which spec! Like "aircraft grade aluminum," ALL grades are used! Just different grades for a file cabinet than a critically-stressed component on a fighter... These terms have become ubiquitous in marketing , but are meaningless without specifying the spec...

        Some Helpful Tips with removing Socket (allen) Heads...

        1) If one feels like it is about to strip out, try a Torx wrench! Often you can find one that fits "more snugglier" and will give you that little extra little bit of torque you need to remove the offending allen head.

        2) Take a cut-off wheel on a Dremel and cut a screwdriver slot across the head. This Never (never say never?) has failed me, though sometimes you can't get access to the head...

        3) Grind off the worn portion of your wrench! Restores it to new. This should just be done regularly, anyway. They are cheap and not made to last forever.

        4) If you get the restored (new) end bright red hot and quench it in oil (maybe water for some steels, but usually oil), it will never wear out. It may break, but then you just do it over! If you're really good, you could try drawing back the temper to make it tougher and not so brittle...
        Last edited by a la carte; 13 Aug 2011, 4:01 PM.

        Comment


          #19
          SCI,

          In the Series 2 pic you posted:


          It looks like you position the eccentric (non-centered) screw in one of the 6 holes to set the caster, right?

          If I'm seeing it right, that screw is in shear(?) and definitely would benefit from the modification I described for screws in shear, above (use a "shanked" screw and counterbore the threaded holes in the Caster Mount accordingly).

          Without said mod (if the thread is, indeed, in shear), you really should not tighten the screw fully (to the material's elastic limit), because then the additional shear stress starts failure immediately.

          Comment


            #20
            Wolfeman how old is the chair? MY ZRA1 is 7 years old and everything has been rock solid but I'm a lighweight probably around 140 lbs and my chair probably isn't stressed over time as it would be with a larger person.

            Comment


              #21
              Just fer Grins...

              Something like this could eliminate some cost and problems. Just a "Quickie" I whipped up, so it could/would need a bit of development, of course! Might not work at all?

              Comment


                #22
                And ANOTHER THING!!!!!!

                Pinch bolts.... Gawd they're TERRIBLE!!!!!!!

                Essentially, the Series 1 screw is a pinch bolt.

                When they are made, the screw hole (threaded or not) is perpendicular to the flat(s) that the head and nut tighten against.

                When you torque the screw, the gap is closed and the hole in one half is NO LONGER perpendicular to the flat on the head end!!!!

                So as you turn the screw under increasing tensile stress, it is being bent around, like you would do when you're trying to break off a piece!

                There are different ways to solve this, but they all cost more to manufacture.

                Every time I see images of those hyper-expensive designer chairs with unobtanium and carbon fiber everywhere, I wonder if they cut corners on these fastener applications, too, like all the other wheelchair manufacturers???

                Comment


                  #23
                  Originally posted by a la carte View Post
                  SCI, did/do you work for this wheelchair manufacturer?
                  Nope. If I did, I'd have done everything I could to prevent them from using those screws in the components being discussed here.

                  Originally posted by a la carte View Post
                  The military and SAE are two big organizations who have had a need and the resources to develop specs for screw materials (among many other things). When saying "Mil Spec," you need to quote which spec!
                  I would need to quote the spec if I were an engineer and knew better or if I was billing for my services. I've replaced many of my fasteners on my ZRc with fasteners comparable to the one in the drawing below...


                  Comment


                    #24
                    Doing things the "best way" (engineering-wise), as might be done on spacecraft and high-end racin' cars, is usually prohibitively expensive. So, if you find that whatever material MC's screws are made of is suitable for your needs at an affordable price, Go For It!

                    I feel fortunate for having the abilities to design and build my own solutions, which eliminates most of the cost of the "best ways," and then make my own unilateral decision as to whether I want/need to for any given project I'm working on.

                    I'm not a myopic propeller-head who insists that things can only get done one way! That's why I can't work with most other engineers (just like everyone else - LOL).

                    Cheers!

                    Comment


                      #25
                      Originally posted by SCI_OTR View Post
                      I've replaced many of my fasteners on my ZRc with fasteners comparable to the one in the drawing below...

                      Note the Mil Spec (MS) and Nat'l Aerospace Std. (NAS) specs in the drawing's title block.

                      Among the list of the screw's complete dimensions, properties, etc. in the referenced specs is the min. Yield Strength of 30,000 psi and ultimate Tensile Strength of 80,000 psi.

                      So this gives you numbers to compare against SAE Grade 5 or 8 specs, for example. FWIW to you.

                      By the way, the socket dimensions required are .188 to .190, which is why the (typically undersized and soft) 3/16" (.1875) wrenches strip out so easily.

                      Usually, the OEM screws, as you have found, are of a much lower grade (cheaper) material and OEM suppliers will almost never admit the specs of their materials - their sales and engineering folks just say, "It's Aerospace Grade" or "Military Spec", etc... I know this from working for a company that did just that! I left after 6 months because of it.

                      A few customers complained about the poor quality of fasteners, plating, etc., but the owners didn't care. Higher grade stuff would eat into their profit margin and it was very few complaints that they actually heard, after all. Who knows how many customers "jumped ship" because of the poor quality? No way to quantify that. They were and are in business for a very long time. Says a lot about what people really want to buy, despite what they complain about!

                      Comment


                        #26
                        Screw Sources & Prices...

                        I see McMaster Carr gets $9.51 for a pack of 5 ($1.90 each).

                        Check out Aircraft Spruce as an excellent alternative source!

                        Same screw is only 82 cents, and you can order any quantity.

                        Comment


                          #27
                          Sorry to dig up an old thread but having this problem and even more confused about what screw and nut I should get after reading the discussion! Can anyone clarify? Planning to try and replace all the dodgy fixings on my ZRA once I have my new Helium so I'm trying to learn but I'm no mechanic!

                          Comment


                            #28
                            I sent you a PM
                            Chas
                            TiLite TR3
                            Dual-Axle TR3 with RioMobility DragonFly
                            I am a person with mild/moderate hexaparesis (impaired movement in 4 limbs, head, & torso) caused by RRMS w/TM C7&T7 incomplete.

                            "I know you think you understand what you thought I said, but what I don't think you realize is that what you heard is not what I meant."
                            <
                            UNKNOWN AUTHOR>

                            Comment

                            Working...
                            X