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What's wrong with this picture? — Parallax Forums

What's wrong with this picture?

W9GFOW9GFO Posts: 4,010
edited 2014-01-02 20:48 in General Discussion
From a company comparing the advantages of strain wave gearing (Harmonic Drive) over standard spur gears. http://www.hds.co.jp/english/company/business/strength/

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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2013-12-31 18:51
    The whole point of involute gearing is that it does not require "clearance" to operate. Even so, the errors introduced by backlash are non-cumulative.

    -Phil
  • W9GFOW9GFO Posts: 4,010
    edited 2013-12-31 19:03
    Yeah, hard to believe that they are making up disadvantages in order to make their product sound better. Harmonic drives shouldn't need deceptive marketing I would think. The thing is, aren't most of the people who would be looking at it already familiar with the non-accumulative nature of backlash?
  • WBA ConsultingWBA Consulting Posts: 2,934
    edited 2014-01-01 22:57
    Phil Pilgrim (PhiPi) wrote: »
    the errors introduced by backlash are non-cumulative.

    That was going to be one of my guesses. The other was that the tooth on the upper gear to the left of the red circle is wider than the rest of the teeth. Actually, for that matter, none of the teeth appear to be real symmetrical. Maybe that's just poor artwork.
  • whickerwhicker Posts: 749
    edited 2014-01-01 23:21
    Other than poor wording and not drawing an arc, I don't think the math is incorrect...

    Sector length of a 0.5 degree backlash over a radius of one meter:

    S = 2 * PI * r * (degrees) / 360

    S = 2 * PI * 1 m * 0.5 / 360

    S = 0.0087266 m

    S = 8.7 mm
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-01-02 11:23
    W9GFO wrote: »
    Yeah, hard to believe that they are making up disadvantages in order to make their product sound better. Harmonic drives shouldn't need deceptive marketing I would think. The thing is, aren't most of the people who would be looking at it already familiar with the non-accumulative nature of backlash?

    I am not sure that they are making up disadvantages. It may be that a new generation of engineers know no better, they may never have learned what was once common knowledge ... too busy looking for neat new technology.

    Over time with wear, backlash does creep into the situation. It is hard to retain zero clearance gear meshing without very solid bearings and hard faced gears.
  • TCTC Posts: 1,019
    edited 2014-01-02 12:34
    Loopy Byteloose wrote: »
    It may be that a new generation of engineers know no better, they may never have learned what was once common knowledge

    I would have to agree with you. Without going into detail (prototype part), We had a part we were testing for a new model car, and it would make a horrible grinding noise at about 1500 RPM. They had all of the designers and engineers trying to figure out the problem. I took one listen to it, and told them it was "gear slap" (large backlash). Someone did not design a way to set backlash, or pre-load.
  • W9GFOW9GFO Posts: 4,010
    edited 2014-01-02 20:36
    Loopy Byteloose wrote: »
    Over time with wear, backlash does creep into the situation. It is hard to retain zero clearance gear meshing without very solid bearings and hard faced gears.

    Backlash error does not accumulate but they have an illustration (and text) stating that it does. If they don't know any better, that means they do not know, so they are pulling the "disadvantage" of accumulating backlash out of their you-know-what.
  • ercoerco Posts: 20,256
    edited 2014-01-02 20:48
    W9GFO wrote: »
    Backlash error does not accumulate but they have an illustration (and text) stating that it does. If they don't know any better, that means they do not know, so they are pulling the "disadvantage" of accumulating backlash out of their you-know-what.

    +1. Backlash is not cumulative WRT distance or revolutions travelled. Further, any zero-backlash system transmitting reasonable power won't stay zero lash very long. Oil & grease fill the gap and lubricate in the process.
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