I've often mounted a cheap laser pen and "dialed in" the unloaded bearing shaft condition. When loaded, the deflection of the laser dot on a bulls eye for example, would give you deflection from which you could square things up.
Cheap but effective, probably good for in field usage.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
One indicator mounted on one journal and swing the other journal. That is if they are separate from one another.
Like a motor shaft that is to connect to a pump shaft.
If it is a common shaft, then it can be located in a lathe or vee blocks or rollers or bearing halves, and while rotating it you would see a wobble if they are outa line, except in a lathe where you would just indicate each journal separately.
maybe I don't understand correctly, but how do I know that the axis of the first journal is perfectly lined up with the axis of the second journal. It is a common shaft.
Can you mount the shaft in its bearings and then sweep a face you know "should" be square to the axis? If the bearing bores are on the same axis then you should register "0" on both faces (need to do both. The only fudge factor will be if the shaft has a sag, then you will read a difference top to bottom on both bearings.
If the distance is short and you are using babbitt bearings then bluing on a known straight shaft will also give some indication of the relative alignment.
Good luck
Ralph
If the shaft is easy to remove, check it between centers.
You would have to know how to use an indicator and height gauge or surface gauge, and surface plate to check it on Vee blocks. If it is a very large shaft, you will have to use either laser or optical alinement of some sort. Ralf2's idea will work too. It may help if we know the size and use of the shaft.
The phrase 'in the same plane' had me more confused than usual. Now I think you're talking about, say, the first and last main journals of a crankshaft, and you want assurance that their axes are concentric within some tolerance in some arbitrary plane of measurement. GD&T guys use 'extended tolerance zones' to specify stuff like that.
I'm sure it's possible to automate the process, and I'm sure it's been done. I'll conjecture a labor- intensive process that could be done with tools that I might be able to get.
Set up four sensitive dial indicators, one at each end of each journal. Using a degree wheel, spin the shaft slowly on whatever axis can be arranged; between centers, journals on v-blocks, whatever. Every few degrees, record the reading of each indicator. Complete one revolution.
In a CAD program, draw four perfect circles in 3D space at the locations on the crank axis where the indicator measurements were made. Rotate your coordinate system, and for each data record, draw a radial line away from the perfect circle corresponding to each indicator's reading in the appropriate direction.
Then fit four perfect circles to the ends of the radial lines you've drawn. Or draw polylines enclosing those line ends, close the polylines, and find the centroids. Now you've got two points in space that define the ends of each journal's axis. Draw a line between them, extend it to whatever measuring plane you'd like to use, and find the difference in the intersection points of the two lines with the measuring plane.
Of course, you can do the same thing with numbers in a spreadsheet, but I thought the graphical description might be more illuminative.
The difficulity of making this kind of measurment is probably one of the reasons holes like this are line bored. One could certainly make the measurement on a CMM.
There is specialty gauging instruments for cylindrical form measurements. The company Mahr makes a MarForm machine that will measure this to sub-micron levels. If you need this done once, perhaps they could contact someone in your area that has this machine.
What're your tolerances?
Typical ball or roller or plain bearings probably don't care about 0.0005 inch or so.
2 narrow not-very-precise v-blocks, a test indicator, and a 2 inch micrometer in the hands of a decent imaginative inspector should indentify any problem.
Measuring between centers can be deceiving. An excellent shaft with a buggered center spins the indicator needle pretty good.
