sEmple not simple. bearing adjustment

[DetroiTug]

He's right he's just suggesting something that is irrelevant. For a circle, if the diameter is known, the circumference is known as well. In class of fits, it's always diameter. Working with the circumference, how could it be measured? The only way to accurately control it with standard measuring tools is by the diameter. If it's other than a circle, then a CAD program has to be used to determine the length of the perimeter. But those dimensions are rarely needed to be known.

-Ron

[fredrosse]

Looks like Ron was answering this question with better clarity while I was formulating my "long winded" answer here, but here goes the same answer with about 3.14 x the amount of verbage:


If you are talking about a shrink fit, that should consider the diameter difference, for example, for a 1" shaft, with a 0.001 inch diameter interference, machine the crank web to 0.999 diameter.

Yes, in this case the circumference of the shaft is PI X 1" = 3.141 inches, and the machined hole is having a circumference which is .003141 inches less, but so what? The diametrical interference is what we machine, and what matters. If you are heating the crank web so it expands for an easy press fit, heat up the crank web so the hole diameter expands 0.001, and the circumference will expand about three times as much, but again, so what?

The same for clearances, such as a bearing where the one inch shaft runs in babbit with 1.001 inch bore. We machine diameters, and that is what matters here, not circumference. The fact that the circumference numbers are about three times as much is perhaps interesting, but not of any technical value in bearing clearances or press (or shrink) fits.

The only place I know where circumference might come into play is for piston ring gaps. In that case the ring gap will change about 3.14 x the change in machined ring outside diameter. For example, if you have a 1 inch bore cylinder, and want a 0.031 inch ring gap, then machine the ring (while held in a fixture that holds the compressed ring to zero gap while machining) to 0.990 diameter, which is 0.010 undersize. Then when the ring is placed in the cylinder, it will spring out to 1 inch diameter bore, with a ring gap of 0.031

However the better way to machine the outside diameter of piston rings is to compress the ring to the gap you want, then have it clamped into the machining fixture with the desired gap. Then machine the ring (Outside Diameter) to the exact bore diameter. Better sealing and shorter break-in time with this method.

[Lopez Mike]

All very fine and lucid.

Now I know that when we shim rod caps and such, we are theoretically making things our of round. Not enough to matter, of course.

But if I want an extra .001" of clearance then the usual notion is to throw a shim under each side of the cap. .001' on each side? Or should we add .0005" each side? I can't remember if I've plastigaged anything after adding the shims.

It also drifts into my mind that in adding .001" to each side I have just added .002 to the circumference which would add .002/3.14 or .00064 to the diameter.

Thoughts to ponder as I digest my holiday meal.

[barts]

All very fine and lucid.

Now I know that when we shim rod caps and such, we are theoretically making things our of round. Not enough to matter, of course.

But if I want an extra .001" of clearance then the usual notion is to throw a shim under each side of the cap. .001' on each side? Or should we add .0005" each side? I can't remember if I've plastigaged anything after adding the shims.

It also drifts into my mind that in adding .001" to each side I have just added .002 to the circumference which would add .002/3.14 or .00064 to the diameter.

Thoughts to ponder as I digest my holiday meal.

3.14159 * D = C is valid for circles, less so for ellipses and other non-circular shapes.

- Bart

[Lopez Mike]

There is an old bar joke that goes this way:

There is a non-stretching wire wrapped around a smooth and spherical earth and laying on the ground.

It is desired to raise it on fence posts so that one can walk under it anywhere. Say, six feet high.

How much extra wire is needed?

The answer (often non-intuitive) is that the diameter doesn't matter. It will 3.14 x 6 or about 19 feet no matter what size circle.

Very steam related stuff.

[DetroiTug]

Quote: "It also drifts into my mind that in adding .001" to each side I have just added .002 to the circumference which would add .002/3.14 or .00064 to the diameter."

And therein lies the true nature of those types of joints - they aren't perfect. If the strap is .001" too tight, then .001" shim is added to both sides. It's two semicircles being displaced in one axis only. The strap is no longer a circle (but it should be in due time , so again the circumference is irrelevant to the diameter.

-Ron

[wsmcycle]

Both diameter and circumference (length) are dumbed-down from the actual three dimensional expansion we know is true. The simplified math is simpler when based on diameter because diameter is where the clearance or interference is applied. if the formulas were reduced to circumference terms, they would have to be further identified at a diameter. So, it is more applicable to dumb-down to the diametral change.