Flex Wind-Up

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • FloatDaBoat
    Bare Bones Fabricator
    • Sep 2009
    • 368

    #1

    Flex Wind-Up

    I’ve read several posts about how much a flex cable shortens under load, but the specific cable length was never addressed in any of them.
    So, here’s my question: Utilizing .130” flex, is 3/32” (2.38 mm) enough clearance between the drive dog & strut, to allow for the torsional compression in an eight-and-a-half inch length of cable (measured from the motor coupler to the front of the prop shaft)? I’ve provided a photo for visual reference. The prop is an Octura X440. Trying to avoid a Tug-O-War situation here. My wife’s tired of hearing that I broke it & need more funds to fix it again.
    Attached Files
  • domwilson
    Moderator
    • Apr 2007
    • 4408

    #2
    Looks about right to me. Maybe a little more like 1/8" or 5/32". But try it out and see.
    Government Moto:
    "Why fix it? Blame someone else for breaking it."

    Comment

    • wolf IV
      Senior Member
      • May 2008
      • 961

      #3
      Originally posted by floatdaboat
      i’ve read several posts about how much a flex cable shortens under load, but the specific cable length was never addressed in any of them.
      So, here’s my question: Utilizing .130” flex, is 3/32” (2.38 mm) enough clearance between the drive dog & strut, to allow for the torsional compression in an eight-and-a-half inch length of cable (measured from the motor coupler to the front of the prop shaft)? I’ve provided a photo for visual reference. The prop is an octura x440. Trying to avoid a tug-o-war situation here. My wife’s tired of hearing that i broke it & need more funds to fix it again.
      I RUN ABOUT 3/16'' ON MINE .. BUT THEIR .187'' DRIVES...NOT SURE THE DIF. IN SHRINKAGE? great job on the hardware!!
      http://www.youtube.com/watch?v=e-npTLBHefY

      Comment

      • Gerwin Brommer
        Senior Member
        • Dec 2008
        • 918

        #4
        I also think 3/16th is far better..............

        Comment

        • paulwilliams
          Member
          • May 2007
          • 82

          #5
          Without consulting the manufacturers' documentation, I would *assume* that there is a directly proportional relationship between flex length at rest, and length under a given load.

          In plain English, a long cable will shorten more under load than a short cable.

          I have no doubt that the operating conditions we subject our flexi shafts to exceed by some margin the limits of maximum rpm and torque as defined by the cable manufacture(s). If you're regularly breaking flexis, then you either need a bigger flexi or a wiredrive.

          Paul
          www.fastelectrics.net

          Comment

          • bustitup
            Fast Electric Addict!
            • Aug 2008
            • 3071

            #6
            Originally posted by FloatDaBoat
            I’ve read several posts about how much a flex cable shortens under load, but the specific cable length was never addressed in any of them.
            So, here’s my question: Utilizing .130” flex, is 3/32” (2.38 mm) enough clearance between the drive dog & strut, to allow for the torsional compression in an eight-and-a-half inch length of cable (measured from the motor coupler to the front of the prop shaft)? I’ve provided a photo for visual reference. The prop is an Octura X440. Trying to avoid a Tug-O-War situation here. My wife’s tired of hearing that I broke it & need more funds to fix it again.
            In my opinion go about 1/16" more than the pic you posted and take a piece of fuel line about 1/8" long and feed it onto the flex shaft before you feed it into the strut so that it ends up in between the drive dog and the strut....works as a seal and cushion

            jmho......
            SPRINT CAT 40.........BOOGIMAN 25" MONO 8xl
            EX President of the Offshore FE Vultures Society

            Comment

            • Grimracer
              Senior Member
              • Nov 2008
              • 662

              #7
              rule of thumb

              Distance back = one cable dia.

              Grim

              Comment

              • FloatDaBoat
                Bare Bones Fabricator
                • Sep 2009
                • 368

                #8
                Thank you all. Your answers are very much appreciated!!!

                Guess I’ll go with the median suggestion - - 5/32”.
                If someone had sufficient data on all the variables involved & adequate testing facilities, I’m sure some sort of answer could be arrived at. Even then, static test results often differ from actual dynamic realities.
                I like the idea of having a sealed strut to keep water out. I don’t think a piece of silicone tubing is the answer, though. Something along the lines of an O-ring seal might be better; O-rings probably wouldn’t be able to withstand the high shaft RPM’s. Oh. Well . . . .

                Comment

                • Fluid
                  Fast and Furious
                  • Apr 2007
                  • 8012

                  #9
                  The clearance necessary depends. Depends on the length and diameter of the cable, the prop diameter and power level, the inside diameter of the stuffing tube, the weight of the boat and how much cable is exposed between the stuffing tube and the coupler. Two separate forces are operating together to shorten the cable: torque and compression. The twisting action coils up the cable - then releases it twice per revolution. The push from the prop is transfering all the thrust to the motor (and then through the motor mount to the hull) which shortens the cable proportionate to the acceleration/speed of the model. There is no perfect answer, except that using Grim's suggestion above has worked well for generations of R/C boaters.


                  /
                  ERROR 403 - This is not the page you are looking for

                  Comment

                  • TRUCKPULL
                    Fast Electric Addict!
                    • Apr 2007
                    • 2971

                    #10
                    I was told by Ed. Hughey at the 05 Nat's in MI.

                    Distance back = one cable dia. for each 10" of length.

                    Larry
                    Past NAMBA- P Mono -1 Mile Race Record holder
                    Past NAMBA- P Sport -1 Mile Race Record holder
                    Bump & Grind Racing Props -We Like Em Smooth & Wet

                    Comment

                    • Drag Boat Bob
                      Senior Member
                      • Apr 2009
                      • 304

                      #11
                      Originally posted by Fluid
                      The clearance necessary depends. Depends on the length and diameter of the cable, the prop diameter and power level, the inside diameter of the stuffing tube, the weight of the boat and how much cable is exposed between the stuffing tube and the coupler. Two separate forces are operating together to shorten the cable: torque and compression. The twisting action coils up the cable - then releases it twice per revolution. The push from the prop is transfering all the thrust to the motor (and then through the motor mount to the hull) which shortens the cable proportionate to the acceleration/speed of the model. There is no perfect answer, except that using Grim's suggestion above has worked well for generations of R/C boaters.


                      /
                      Twice per revolution?...

                      Wouldn't that depend on the number of poles a motor has?

                      Of course I am new to this sport and have little experience, but it would seem logical given the how electrical motors work.

                      I am not disagreeing with your analogy, just trying to understand the physics involved.

                      Comment

                      • TRUCKPULL
                        Fast Electric Addict!
                        • Apr 2007
                        • 2971

                        #12
                        Twice per revolution?...

                        I beleive he is refering to a 2 blade prop NOT a 2 pole motor.

                        Larry
                        Past NAMBA- P Mono -1 Mile Race Record holder
                        Past NAMBA- P Sport -1 Mile Race Record holder
                        Bump & Grind Racing Props -We Like Em Smooth & Wet

                        Comment

                        • Drag Boat Bob
                          Senior Member
                          • Apr 2009
                          • 304

                          #13
                          Originally posted by TRUCKPULL
                          Twice per revolution?...

                          I beleive he is refering to a 2 blade prop NOT a 2 pole motor.

                          Larry
                          Now why would a propeller cause any change in twisting compression unless it was in and out of the water? Even then, why twice per revolution?

                          Comment

                          • TRUCKPULL
                            Fast Electric Addict!
                            • Apr 2007
                            • 2971

                            #14
                            Two blades X(Times) one revolution = twice
                            2X1 =2

                            We do run surface props so it is only one blade in the water at a time.

                            Larry
                            Past NAMBA- P Mono -1 Mile Race Record holder
                            Past NAMBA- P Sport -1 Mile Race Record holder
                            Bump & Grind Racing Props -We Like Em Smooth & Wet

                            Comment

                            Working...