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Thread: HINGING and PIVOTING mechanisms: A PRIMER

  1. #1
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    HINGING and PIVOTING mechanisms: A PRIMER

    I thought it might be good to start a thread on hinging and pivoting mechanisms, like doors, hoods, wipers, access panels, etc. This could easily encompass suspension systems but I will leave that broad subject to others.
    This subject may include many posts on different aspects of these issues and all are welcome to participate. Some of the general principals and subjects worth discussing are below:

    The Hinge Line
    The Hinge Base
    Vertical displacement of panels
    Instantaneous Centres - so much cooler when you use the English spelling
    Line of action
    Multiple link mechanisms (four-bar, sliding, etc.)
    The Mockup - thumbtacks and erasers
    The Compass is your friend
    Counterbalancing
    The Overcenter concept
    Adjustability
    45% concept
    Goosenecks
    Seals and compressibility
    Force and velocity with mechanisms
    Differential in linkages & Mechanical advantage
    SLOP
    Etc.


    Let me start with “The Hinge Line”…
    Even a simple door hinge from the lumberyard has a Hinge Line (see the pix). The line of action through this device is obvious and critical, especially when it is just one of several hinges on the same panel. The OVERALL hinge line must line up & conform to the individual hinge lines or there will be a problem, notably BINDING. This tends to cause things to go bad over time. Things break sooner when subject to unnecessary stresses and load. The thing to do is to line things up so that there are no conflicts. This is simple enough with two hinges but gets a little more complex with three or more. Nonetheless, the idea of lining things up is basic.
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    The Hinge BASE is a simple concept although many people don’t appreciate its importance. As the pix show, there is a relationship between the distance between the hinges and the length of the panel being supported against gravity. The broader the BASE the more force the unit can handle. If it is too small, one could actually break the door off by leaning on it. The base is affected by the door shape and the section, which can really work against you if it is not fairly vertical and broad at the hinge area.
    Front door cuts which are not essentially vertically (in side view) cause the designer to either TILT the base hinge line or go with a smaller dimension to begin with. SECTIONS that are not fairly vertical invite the tilting to occur in the lateral view. Any Tilting of the hinge line to make it bigger creates other issues (which we will look at later).
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    It is important to look at the FORCES on the hinges caused by gravity. In the closed position, the weight of the door will create a rearward force on the top hinge and a forward one on the lower hinge. The latch and seals may help support the door in the closed position which is good because of the dynamic loads of driving.
    As the door opens, these forces are swung around sideways into and out of the car structure.
    Designing the body structure, these forces should be accounted for or there will be deformation over time and the door will no longer fit.

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    Senior Member ncrazyballa's Avatar
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    next, can you do a tutorial on how hammers work?

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    Let me finish this one first. Then I may think about it

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    Returning to the issue of TILTING the hinge lines, the issue of VERTICAL DISPLACEMENT comes into play. This is simply saying that a tilted hinge may cause the cg of the door to RISE or FALL as it is pivoted. On the one hand, this may be a good thing, making the door to lift up over a curb which it would otherwise hit. On the other hand, that door will tend to fall while closing, maybe hard onto your leg. A door that falls open is usually bad in that it gets away from you and will suddenly open against the car next to you or out into traffic. In general, a vertical displacement in either direction is a liability, with limited benefit.

    Specifically, as the lower hinge on our picture is moved forward, the hinge line pivots in a clockwise direction and the door will RISE when opened. In end view, as the lower hinge is moved outboard, the door will also RISE when opened.

    It is common for door designers, when faced with a front door cut that causes them to move the lower hinge rearward, to try to COMPENSATE by also moving it INWARD – so the total vertical displacement is reduced or eliminated. Likewise, when the section of the body forces the lower hinge inward or outward, the side view hinge line can be adjusted in compensation.
    Generally a little rise in the door when opened is better than having it fall open, for functional and safety reasons.
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    Last edited by aflo; 07-15-2012 at 10:04 AM. Reason: Correction

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    thanks aflo this a good topic, i think we can all use it sooner or later, i know i can i wiil be working on my K1 attack doors soon, keep up the good work

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    Great topic for all. Until you've had to adjust doors and hinges, you have no real idea what all is involved, and how a mm here makes a meter's worth of difference in the motion.

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    Before going to the issue of instantaneous centers, I need to show how to do a simple geometry task, finding the “perpendicular bisector” of a line. This is the line that is perpendicular to the line joining two points and exactly midway between them. One takes a compass, set to a size slightly larger than half the distance between the points, and swings an arc from each point. Where these arcs intersect, a line is drawn and it is the perpendicular bisector of the original line. See the drawing. Hopefully it makes sense to you.
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