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914 Ext. Alt Belt Tension

in 912 / 914 Technical Questions 5 years ago
Hi,

The maintenance manual (diagram attached) shows how the proper belt tension is measured. But, it shows 50 N (which I assume is Newtons) is 37. ft.lb., which is wrong in both number and dimensions. 50 Newtons is 11.24 Pounds (not Foot Pounds).

My kit company (The Airplane Factory) says they just twist the belt to 90 degrees, which not only seems inaccurate, but I can't twist it past 30 degrees and that's when it's loose.

Someone have a recommended tool and the right number to test to?

Thanks,

Craig
BeltSpec.JPG (You do not have access to download this file.)

  • Re: 914 Ext. Alt Belt Tension

    by » 5 years ago

    I think the "foot pounds" comes from converting 50 newton meters (equals 37 foot pounds) but in this instance we are just measuring force, not torque, so probably the 50 newtons is correct.

    I bought an automotive type belt tension tester, but even though it is not large, it is too large to work on the 914. I haven't started my engine yet (getting very close!) but at this point my plan is to just use common sense, and err on the side of it being too loose rather than too tight. If it is too loose there will be symptoms but if it is too tight the first symptom might be alternator bearing burnout.

    I too would like a good accurate way to measure this.
      
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  • Re: 914 Ext. Alt Belt Tension

    by » 5 years ago

    Ah, yeah, they should fix the manual to read 50Nm....

    But, isn't that a torque value?.... I'm so confused....
      
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  • Re: 914 Ext. Alt Belt Tension

    by » 5 years ago

    The Newton∙Meters is a unit of Torque. But that is NOT what we want.
    50 Newtons is a unit of Force equivalent to about 12 lbs. (Not 37ft∙lbs)
    6mm is a distance of roughly a 1/4 inch.
    The idea is to push on the belt and see how far it deflects.

    For a rough redneck calibration and measurement...
    Push on a bathroom scale with your thumb to get an idea of what 12 lbs feels like.
    Then push that hard on the belt at the midpoint and see if the belt deflects about 1/4 of an inch.
    Ken's Common sense approach.
    Or loop a piece of cord around the belt and pull on it with a Fish-Scale.
    Push, Pull, the Same idea.

    Remember that the spec is 50N and 6mm. It is Not 50±0.001N and 6±0.01mm.
    Don't overthink it!
    it will loosen up over time as the belt stretches and wears.
    If/when it starts to slip tighten it again.

    Note:
    A Belt-Tension Tester measures Tension. It will not be useful for this measurement.
    We do not know what the tension should be, only the deflection under a side force.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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  • Re: 914 Ext. Alt Belt Tension

    by » 5 years ago

    Sorry, maybe tension tester isn't the correct word for it, but it operates exactly the same as measuring the force of your thumb. The problem was that the distance between the pulleys is short enough that the tool doesn't really function as intended.
      
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  • Re: 914 Ext. Alt Belt Tension

    by » 5 years ago

    Thanks Bill, I won't overthink this and just do it the "redneck" way. ;-)

    Craig
      
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