yang686526

circular runout
i am looking at a drawing of a brake disk.
datum b appears to be the axis of rotation.
both faces of the disk have a circular runout tolerance referenced to datum b.
i am having difficulty imagining how to interpret this.
i suspect that the intent is to control perpendicularity.
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this normally is used to control "wobble".
i understand the intent.
all of the examples of runout in my books apply runout in the radial direction wrt the axis of rotation.
is the use of runout perpendicular to the axis of rotation a valid use?
if so, wouldn't total runout be more appropriate anyway?
never mind, i found a reference for this usage.
do you? if the sole desire is to control squareness to the datunm axis than a perpendicularity callout is preferred. if the desire was to control orientation and form than total runout would have been used. but since it is specified circular runout than someone wants to control the circular elements of the surface while in rotation and not the surface orientation.
i suspect the intent is to control the "wobble" only.
the runout should be referenced from the surface that contacts the hub as primary and the hub pilot diameter secondary (according to all the common disk rotors that i have seen). it is common although for specifications to lack complete datum references when the features of a part either physically limit or are functionally apparent in the choices for orientation.
perpendicularity applied to each surface separately would be the same as declaring total runout on each surface (since perpendicularity of a surface constrains all points of a surface between two parallel surfaces spaced the tolerance value apart).
i think that the circular runout is immediately more apparent to the customer in the form of shudder from the brakes but if there is conicity in the surfaces there is a chance that the caliper piston will cock and sieze before the pads wear in.
most mechanics routinely check runout of the installed rotor surfaces. i would say that either total runout or perpendicularity (take your choice) would be appropriate for the design but circular runout would be more suitable for repair manuals.
paul
most
do i what?
yes, i understand what i think the designer wanted to accomplish. (sounds dangerous doesn't it?)
no, i don't understand why he elected to use runout rather than total runout or perpendicularity.
i agree with paul's set up with the primary and secondary datum.
circular runout confirms, in this case, perpendicularity to the axis and is sometimes called "lateral runout". it does confirm the wobble.
i would still stick the circular runout rather than total runout which is difficult to confirm but would cover the complete surface.
if the part was quite large, i would suggest perpendicular rather than either runout but here it may be appropriate.
dave d.
i disagree,
circular runout does not, in this case, determine perpendicularity of the surface to the axis! both surfaces could be coned (making the size thin outboard and thick inboard) and if the conicity was symetrical on both surfaces.. all runout measurements could be zero while the perpendicularity would be half the difference in outboard vs. inboard thickness! if perpenpendicularity was applied to the size (thickness) in this case, then it could zero the same as runout because it would be controlling the "center plane".
the point is that total runout and perpendicularity (when applied to each surface separately) are equivalent.
circular runout (lateral in this case) or perpendicularity when applied to the thickness (centerplane) does not control uniform taper or coning of the surface.
paul


circular runout would also allow dishing or grooving as long as the defect is concentric.
this is a "cheek" disk. it's bolted through the plate of a wheel, so there is no feature on the disk that actually contacts the axle.
circular runout does not include flatness of the surface while perpendicularity and total runout certainly would include flatness. circular runout would not catch any cone or groove.
lateral circular runout will control the tilt of the overall surface that should be on 90 degrees relative to the axis. some might call it a wobble or perpendicularity.

dave d.