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Technical Engineering question, a serious one

Discussion in 'The Hokey Ass Message Board' started by 31Vicky with a hemi, Mar 7, 2018.

  1. I have a situation where I need to build an off set brake pedal, about 3".
    It needs to have the ability be assembled on either side and come apart too.
    The 3/4 shaft will ride on Oilites in a DOM tube welded thru the frame rail.

    My question is on the torsional strength of the shaft ends and couplings being used in a critical connection in a brake pedal.

    The couplings proposed are high quality common steering shaft units - 1.25 OD with 0.250 wall. Splined ends 3/4-36 and the other will be 3/4-DD. These couplings will be welded to the pedal end and master cyl pushrod link end..

    I tried looking the information up and could not find what I was expecting, a simple torque load rating on a shaft with those ends.
    I found some information but I clearly do not understand how to interpret it at all.

    I figure the torque on the shaft to be normally 100 ft lbs. and an estimated 500 ft lbs in a shock load panic stop. I don't know if the 3/4 shaft and those ends can take that so I'm asking for help, advice, or a direction on where to look for what I need.
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 40,548


    Ask the parts supplier?
  3. bct
    Joined: Apr 4, 2005
    Posts: 2,776


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  4. BJR
    Joined: Mar 11, 2005
    Posts: 4,639

    from Minnesota

    To me the shaft if hollow, is the weak link. Can you put it together and test it with a solid link instead of going to the master cylinder?
  5. If I could get to the design and engineering desk I would.
    Talking to the front line gate keepers is not fruitful.

    3/4-36 spline and DD shaft end design is not proprietary to automotive manufacturing, the auto industry assimilated it.
  6. Just a thought, you might try a local college, Mechanical Engineering dept.
    Looks like a good practical exercise.
    Frankie47, RacingRoger and pitman like this.
  7. My only real worry with the diagram you provided is the roll pin. In this arrangement, the roll pin is the weak link due to shear. If you could utilize a double-set screw connection on the DD end, I would be less worried. And if you could find a female DD coupling, that would be optimal (obviously with a set screw as well).

    The steering shaft material should be plenty strong enough to handle the torsional load, but as Squirrel said, it wont hurt to ask Borgeson or others that supply these components., I didn't stay at a Holiday Inn last night!! ;)
  8. ^^Yep^^
    The shaft is solid and DD on the master cylinder end and 3/4-36 on the pedal end.
  9. squirrel
    Joined: Sep 23, 2004
    Posts: 40,548


    As a practical way to analyze it, you could assume that the steering shaft parts you intend to use would be able to withstand the maximum torque a person could apply to a steering shaft.

    How does that torque compare to what a person could apply to the brake pedal shaft?
    Frankie47 and 73RR like this.
  10. I honestly believe you have a solid looking setup,the DD shaft is designed for steering and can handle the forces,have you thought of using a longer set screw instead of the roll pin? HRP

  11. DIYGUY
    Joined: Sep 8, 2015
    Posts: 488

    from West, TX

    Why not splines on both ends? Thinking of splined axle shafts. More splines=stronger.
  12. Rickybop
    Joined: May 23, 2008
    Posts: 5,036

    from Michigan

    Hey 31... internet search terms such as "shear strength solid steel shaft" will get you some info.

    Sent from my VS835 using The H.A.M.B. mobile app
  13. Rickybop
    Joined: May 23, 2008
    Posts: 5,036

    from Michigan

  14. My gut and common sense and experience tells me the foot and leg can apply much greater forces. How much more, I don't know that. Percentage wise, don't know that either. Where's the failure point of the splined connection, that's the think I need to know.

    I'm more than reasonably certain someone has done destructive testing on the connection, specifically 3/4" 36 and 3/4 DD and wrote that info down for others to be able to work from.
    I can't find it.
    pitman likes this.
  15. squirrel
    Joined: Sep 23, 2004
    Posts: 40,548


    Maybe you ought to look into using a larger diameter shaft?

    The splined connection will be the strongest part, if it's long enough.
  16. Ever wonder how much force is on a suspension control arm of any type and how that sleeve doesn't rip open? KIS or KISS?
  17. pitman
    Joined: May 14, 2006
    Posts: 4,424

    from Hampsha

    Unless you make a lot of hard-stops, your design looks first at catastrophic, ultimate str. failures.
    Fatigue is a whole different ball game.
    Last edited: Mar 7, 2018
  18. Gerrys
    Joined: May 1, 2009
    Posts: 298



    something like this should work.
  19. Gerrys
    Joined: May 1, 2009
    Posts: 298



    something like this should work.
  20. bonzo-1
    Joined: Oct 13, 2010
    Posts: 311


  21. Here's what I find. And other stuff like it when I look.
    This is above my comfort zone and I know it. That's why I asked for help. The good thing is I know when I'm out of my zone.

    Shaft 3 (Diameter=3/4”)

    • Material: 1045 Steel, Yield Strength= 530 MPa, Ultimate Strength= 625MPa
    • Max Stress
    o The shaft is keyed for a 3/16” key, thus the actual yield strength can be equated to ¾ the materials yield strength (Keyed Yield Strength=398 MPa)
    o Loading is comprised of three components
    ▪ Moment-Based on the axle length between bearings and the force exerted by the weight of the system (21.53 N-m)
    ▪ Force- Based on axial load exerted on the shaft from turning forces (235.44 N)
    ▪ Torque- Exerted by the stall torque of the motor, through a gear ratio of 8:1 (38.56 N-m)
    o Stress Calculation-
    [​IMG]= 59.0MPa
    [​IMG]= 32.7 MPa
    o Factors of Safety-
    [​IMG]= 6.7
    [​IMG]= 6.1
    • Fatigue Life
    o Infinite Life- 500RPM=8.34 cycles/second
    ▪ 5 year life @ 1 hour operating time (2 hr per week)-apprx 1,908,000 seconds of use
    ▪ 8.34*1,908,000=1.6E7 cycles to failure for infinite life
    o The endurance strength can be calculated using the stress concentration factors from the keyway (197 MPa)
    o ’F=Sut+345MPa= 970 MPa
    o [​IMG]=-0.109915548
    o [​IMG]=.673
    o [​IMG]=900 MPa
    o Loads are based on typical operating conditions, not max conditions
    ▪ Moment-Based on the axle length between bearings and the force exerted by the weight of the system (21.53 N-m)
    ▪ Force- Based on axial load exerted on the shaft from turning forces (235.4 N)
    ▪ Torque- Exerted by the operating torque of the motor, through a gear ratio of 8:1 (11.28 N-m)
    o [​IMG]= 35.6 MPa
    o [​IMG]= 5.7E12 cycles to failure
  22. Engine man
    Joined: Jan 30, 2011
    Posts: 3,455

    Engine man
    from Wisconsin

    Have you looked at a heavy truck clutch setup? You can find them in salvage yards.

    You could set up a test for the items you are using by connecting the output end to something that won't move and press as hard as you can on the pedal.
    alanp561 likes this.
  23. DDDenny
    Joined: Feb 6, 2015
    Posts: 10,149

    from oregon

    I realize your concern being a commercial business and the letigeonous society we live in.
    But bonzo is right those splined shafts are more than up to the task, it's those DD ends I don't like, if for no other reason than they are ugly, hate those shafts with the full length flats, the only reason the aftermarket adopted them is the ease of mfg and assembly.

    I'd be more concerned about the stress riser issue with the sharp corners on the DD flats if it is to be cut with a standard end mill.
    Why not make both ends splined, unless I've missed something there should be no difference assembly wise.
    Not that they are lending any overall strength but I think the retaining bolts could be at least 5/16", it would be awful easy to overtighten a 1/4" bolt.
    May need to give more detailed drawing but I just don't see why that won't work.
  24. dreracecar
    Joined: Aug 27, 2009
    Posts: 2,569

    from so-cal

    The torsional load is taken buy the DD and the roll pin is just to hold posission?? If the fitment snug on the DD, there is no issue, but would not use a roll pin if the fitment is a little loose. instead use a ream for the hole and a AN bolt. AN bolts, the shank is more presise then a hardware store bought bolt
    Joined: Dec 14, 2009
    Posts: 1,127


    Find a company around you that does equipment engineering.forklifts man lifts repairs etc or a fabrication shop that deals with these repairs and modifications

    Show them your drawing and ask them.

    They would know 100%
    Nice thing as well considering it’s brakes and a critical component they could sign off on the engineering tag it and stamp it with an engineering number so everyone knows it’s safe and will perform in fatigue stress and extreme situations

    You would be surprised what they charge, use them at work a lot and if it’s a straight forward meeting, see the drawings, sign off or make adjustments then a visit to inspect the finished parts they have done it for less then $200 bucks
    I can get you in contact with a few reputable companies that I deal with but your kind of far away.
  26. DDDenny
    Joined: Feb 6, 2015
    Posts: 10,149

    from oregon

    Forgot to add and I'm sure 31 Vicky knows this but as with securing couplers on steering systems with set screws; I always recommend dimple drilling each set screw location and putting a drop of small screw threadlocker (Loctite) on the threads.
    The37Kid likes this.
  27. indyjps
    Joined: Feb 21, 2007
    Posts: 3,463


    If you want a reference, consider a bmx crank set up, this is pro level stuff where guys are jumping off buildings etc and landing with full body weight on pedals, it definately will withstand full leg force and shock force from landing.
    19mm spindle, 48 spline spindle to crank, tubular arm.

    Your shaft and joint design exceeds this, to be able withstand full leg power and shock force. As long as the pedal arm will support the proposed force, theres no concern.
    caseywheels, j hansen, j-jock and 5 others like this.
  28. Happydaze
    Joined: Aug 21, 2009
    Posts: 630


    I'm no expert or engineer, but I'm not immediately following this. The force needs to be multiplied by the pedal ratio I'd have thought? Isn't the 'standard' braking effort on the pedal for calculation purposes 100lbs but more like 150 in a panic (requiring to then be multiplied up by the pedal ratio) ?

    My best guess is that what you're proposing is more than adequate but will follow with interest in the hope of the scientific solution.

  29. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,284

    V8 Bob

    The 150 lb. pedal force value was the maximum effort allowed during normal FMVSS 105 vehicle stopping distance testing, but 200 lbs. minimum in less than .08 seconds was required for final spike apply testing. (Surprised I have a right knee that still works OK. :)) I would make sure the pedal and linkage can withstand 150-200 lbs pedal effort before any twisting, bending or deformation, imo.

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