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Scissor Car Jack

  1. Dec 14, 2008 #1
    What is the purpose of teeth in such jack ? Isn't the screw enough to provide the lift ? What will happen if these gear like teeth are not present in a scissor jack ?
  2. jcsd
  3. Dec 14, 2008 #2


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    I'm not sure what you mean by 'teeth', since I've never seen such a thing. My best guess without a visual aid is that it might be some kind of safety catch in case of the worm drive system stripping out.
    Can you post a picture?
  4. Dec 15, 2008 #3
    Check the attachment....Number 19 is what I was referring to...

    Moderator Note: In future, please resize your images to a maximum of 800 x 600 pixels.

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    Last edited by a moderator: Dec 20, 2008
  5. Dec 15, 2008 #4


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    Those teeth allow struts 12&14 and struts 13&15 to move past one another while remainng in in contact in such a way as to structurally support each other.
  6. Dec 15, 2008 #5
    And if these aren't there then what change will come to these struts while e.g. ascending and descending the car?
  7. Dec 16, 2008 #6
    Look at the construction at the bottom of the jack. There are actually to adjacent pivots, not a single pivot.

    Now imagine for a moment that the gear teeth are not there. The base plate, the two lower arms, and the screw are all connected by pin joints, so they act like a four bar linkage and can move laterally. This would be highly undesirable for the jack! The purpose for the gear teeth is to remove this degree of freedom and prevent the lateral motion of the jack.
  8. Dec 20, 2008 #7


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    Altairs: The scissor jack teeth (item 19) constrain both sides of the jack to rise or descend exactly the same amount, thus forcing bracket plate 16 to remain parallel to base plate 11 (and therefore parallel to the ground surface). If the teeth were not present, bracket plate 16 could rotate in response to grossly eccentric loading applied to plate 16. And, if bracket plate 16 (and base plate 11) were free to rotate, then the entire jack could translate/rotate laterally about base plate 11, as pointed out by Dr.D.
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