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Thread: ZRA Set Screw FAIL

  1. #1
    Senior Member WolfeMan's Avatar
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    ZRA Set Screw FAIL

    I was rolling through a bed of rocks last night at the neighbors and my front tires dug in and all of a sudden there's a load 'POP' and my chair leaned down on the front left. The left front fork was laying cockeyed. So, myself and 4 other inebriated guys looked at it with the use of Tonja's light on her scooter and at the time none of use knew what happened. The nut was still on, but when they were pulling me backwards home, I heard something fall. The nut on the set screw fell off. I'm even more baffled at that point. So after sleeping off a Miller/Captain Morgan drunk, I got up this morning and disassembled the set screw that determines the angle of the forks. The nut threads and screw threads were sheared off. I had some spare parts and it was an easy fix, but that doesn't seem acceptable. Anyone else have this issue?
    Larry
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  2. #2
    Senior Member tooley's Avatar
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    I assume you are talking about the Series1 ZRa.

    I had an issue with one of these fasteners. When I finally finished tweaking my dump/COG on my first chair (ZRa) I had to correct a caster wobble. I neglected to clean out the gunk in the allan head and stripped it while adjusted the caster angle. I had to drill off the head of the fastener. These bolts are soft, which can be a good thing - they'll bend before breaking. But with you being a bigger guy I'm sure you just finally fatigued it enough that it sheared. Chalk it up to lower-quality hardware, I knew something wasn't right by how easy the head stripped.

    And yeah I'm a quad, but I turned wrenches professionally pre-SCI, I know my hardware.

  3. #3
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    Sooo many questions...

    I don't have the same chair, but if it's similar to the ones I have used, the threads are in shear?

    Like if you have a hole through two parts then, ideally, the clearance drill hole size (Barely Larger than the unthreaded screw shank) would extend (counterbored) into the threaded part, and the screw shank would be long enough to extend into the counterbore.

    Then the full smooth diameter of the shank is taking the load, rather than the much smaller base diameter of the v-shaped threads which have a built-in crack starter at the root of the thread.

    This is the strongest possible joint assembly. Many times stronger than having the threads in shear!!!

    (I have messed with turning wrenches, engineering, and machining, too!)

    Naturally, this costs more, which is why you won't see it done this way except on spacecraft and top-level racin' cars an' such...

    But you could roll into a machine shop and they could set you up.
    Last edited by a la carte; 08-13-2011 at 01:17 PM.

  4. #4
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    Of course, you could also get a nice set of Frog Legs with big fat soft roll tires and reduce the shock load on all the threads by about a billion times.*

    *Very big SWAG. But right on!

  5. #5
    Senior Member tooley's Avatar
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    Quote Originally Posted by a la carte View Post
    Like if you have a hole through two parts then, ideally, the clearance drill hole size (Barely Larger than the unthreaded screw shank) would extend (counterbored) into the threaded part, and the screw shank would be long enough to extend into the counterbore.
    excellent explanation and theory, but I think the length of the bolt must be threaded in this case to provide proper adjustability of the caster fork angle. Figure 11-7 on page 11-4 of http://tilite.com/assets/pdf/RIGID_OWNERS_Z_Series.pdf shows the parts in detail.

    fwiw, I replaced mine with a short length of grade-8 threaded-rod with nuts on both ends. Not as pretty, but no risk of corrosion in an allan head recess.

  6. #6
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    Quote Originally Posted by tooley View Post
    fwiw, I replaced mine with a short length of grade-8 threaded-rod with nuts on both ends. Not as pretty, but no risk of corrosion in an allan head recess.
    Thanks for the link, tooley! Like I said, there are sooo many questions , and my first one was axing whether the screw is in shear (which it appears it is not).



    Just a guess, but it looks like the screw goes through the hole in the dowel (the hole must be oversized to allow adjustment?) and through the "notch" in the bullet housing.

    So, when you tighten the screw, only friction between the bored hole of the frame and the OD of the Bullet Caster Housing maintains the caster angle. Naturally, the screw must be "Very Tight" to provide such friction.

    Is that an accurate understanding? (If so, I really don't understand why the dowel is even there, so I could very well be off base...)

    If so, the screw is purely in tension with no fatigue load applied and your solution of a stronger (Grade 8) screw is excellent.

  7. #7
    You could replace them with Grade 8 chrome bolts like these from Fastenal:
    Attachment 41560
    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>

  8. #8
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    chas,

    You'd be much better off with stainless if corrosion resistance is a big concern and the reason for your chrome plating thought. Note: Some "stainless" is more corrosion-resistant than others! When looking at an engineered screw application like this, ALWAYS go through a reputable screw manufacturer so you KNOW what you're getting!!!! I HIGHLY recommend Unbrako You will pay more, of course...

    Chrome plating causes hydrogen embrittlement that will cause premature failure at the root of the thread in this application.

    Unless you can find the spec strength of the screws that are used on the ZRA, it could be assumed that they are made of Grade 5 ("typical" for screws) material. Even the lowest grade stainless screws are higher strength than that (approx like Grade 8, as a rough approximation). You can get much stronger, depending on how much you spend, of course.
    Last edited by a la carte; 08-13-2011 at 02:27 PM.

  9. #9
    Quote Originally Posted by a la carte View Post
    my first one was axing whether the screw is in shear (which it appears it is not).

    . . .

    So, when you tighten the screw, only friction between the bored hole of the frame and the OD of the Bullet Caster Housing maintains the caster angle. Naturally, the screw must be "Very Tight" to provide such friction.

    Is that an accurate understanding? (If so, I really don't understand why the dowel is even there, so I could very well be off base...)

    If so, the screw is purely in tension with no fatigue load applied and your solution of a stronger (Grade 8) screw is excellent.
    The tension forces close the gap in the housing around the caster mount, which provides the friction component of keeping the caster at the proper angle. When significant rotational forces are applied to the caster mount-housing connection (e.g., rolling over rocks), the frictional component is not enough to hold the angle, and the extra force is transferred to shear on the bolt. Of course the looser the bolt, the less friction, and the more shear.

    I, too, don't immediately see the purpose of the dowel.
    Last edited by chasmengr; 08-13-2011 at 02:39 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>

  10. #10
    Quote Originally Posted by a la carte View Post
    chas,

    You'd be much better off with stainless if corrosion resistance is a big concern and the reason for your chrome plating thought.

    Chrome plating causes hydrogen embrittlement that will cause premature failure at the root of the thread in this application.

    Unless you can find the spec strength of the screws that are used on the ZRA, it could be assumed that they are made of Grade 5 ("typical" for screws) material. Even the lowest grade stainless screws are higher strength than that (approx like Grade 8, as a rough approximation). You can get much stronger, depending on how much you spend, of course.
    My belief is the stainless is considerably softer than grade 5. I'll look for documentation. Chrome is for corrosion resistance. (BTW, I've changed almost all the fasteners on my AeroZ. When I called TiLite about it, they said don't use stainless because it's too soft. I used stainless anyway at low stress points (e.g., solid seat pan), and chromed Grade 8 at high stress points (e.g., axle mounts). Total fastener cost ~ $225. Chromed Grade 8 is pricey)).

    PS - I found documentation. (18-8 is about the same as Grade 5 in 1/4", but it can vary a lot.

    Attachment 41562
    Last edited by chasmengr; 08-13-2011 at 02:31 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>

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