Total Runout - Long Shaft

[Cup909]

I'm looking to measure the total runout of a very long shaft along the entire length. A 1" diameter shaft, 5' long, with datums 3" from each end, and a total runout requirement of .005".

The traditional method would be to put the datums on Vee-blocks and roll it then drag the dial indicator up and down the shaft. A shaft of this length/diameter/support is going to sag more than the total runout. So how do you measure the total runout using traditional hand tools?

My strategy is as follows; First, constrain the diameter through measurement using snap/mic/etc. Then spin the shaft on rollers taking circular readings along the length of the shaft. A bend in the shaft will show up as circular runout at that spot. TLDR; total runout is circular runout as long as the form (diameter) of the part is constrained.

Does this strategy make sense?

 

[3DDave]

The sag continuously varies and the total runout zone doesn't. Because of the sag it immediately fails the requirement.

The only hope is in noting that the simply supported beam formula allows an angle at the supports, which isn't allowed by the callout; the datum feature simulators/true geometric counterpart simulators are supposed to clamp onto the datum features and hold them rigidly.

Look for a formula where the ends are fixed against rotation.

I'm not too hopeful as making such a clamping device without adding to the measurement error will be difficult. If the clamps are not aligned it could be just as bad and produce weird readings.

 

[drawoh]

The deflection I calculate (screenshot below) for a 1" round, 5ft long at 54" datums (3" from either end), is a deflection of 0.147", so a significant amount and well out of tolerance to just ignore. That will be lessened slightly by the 3" cantilever on either end, but not near enough to counter the runout.

Are you working to ASME Y14.5? Read up on flexible components. You need to specify how your part will be fixtured to control flexibility.

Is your shaft going to run at high speed? Will it be safe? I have not done a lot of power shaft design. You need a certain diameter to transmit the torque, but this not the only control on diameter. Perhaps you should work out vibration modes. If your shaft is stiff enough, you can measure run-out.

 

[jassco]

Hi, Cup909:

Do you have a print for this shaft? You generally don't measure something without prints.

Best regards,

Alex

 

[mfgenggear]

Op
This will not resolve this issue.. Re posting it will not fix it.
Hire a consultant with shaft experience. How to set up and machine shafts and how to inspect them.

 

[supergee]

Cup909

Technically speaking, you should inspect the part per drawing. What I worder is if engineering selected either the datum correctly of the tolerances correctly.

I came across long shaft (aerospace engine shaft) that needed precise runout. usually, the Datum should, when ever possible, reproduce condition of the part in function. Hence, engineeing should have selected the bearing location as datum, and should be aware of the sagging effect. (the key word here is "should").

If the shaft is to be used vertically, that is the way you should inspect the shaft. if it's not feasable, then maybe conforming the part on a horizontal fixture could be accepted but, personnaly, I would consult the client and get their signature before proceeding that way.

If using a CMM or a scan to compute the runout instead of actually checking it, it could be possible to support the part on V-blocks as ¼ of total length on each end. the theory is that the weight of the catiliver on each side will balance out the sag in the middle.

Again, check with your customer, expose your arguments. Reasonnable customers will want you to check their parts correctly : Quality is important to them.

 

[dgeesaman]

Yeah if that's a total runout, measured per the literal TIR definition (GD&T double-arrow symbol) where the indicator is moving axially as the shaft is rotated, you have a technical problem on your hands. It means every point on the part has to fall within two concentric cylinders, not a series of circles.

The first question is whether the shaft shape is axisymmetric. I assume so because otherwise the total runout condition would end where the diameter changes. The shaft will sag down in the vertical direction but it shouldn't sag at all side-to-side. Then I'd clamp down a long precision straightedge to be dead parallel to the v-blocks on one side and run my indicator down the length with the indicator point touching at 3:00 or 9:00 of the shaft surface. This is in some ways the same as running an indicator on the lathe tool post while the part is fixtured on the datum journals.

Odds are very good the customer doesn't fully understand the meaning of the symbol. They might have a past manufacturing practice of measuring this on the lathe/grinding machine before the part is removed. This is a good start but only conclusive if the lathe ways are dead straight and dead parallel to the rotating axis and the part is held without prestress and the runouts on the datum are simultaneously zero. They might be using the symbol with the assumption that it's ok to re-zero the indicator at each inspection point along the shaft, which I believe is only valid if the part has a single overall bow in one direction. If there is waviness or corkscrewing this method fails.

One other wildcard: I know that at my company, there were conversations using terms like "total indicator runout" to mean the full sweep of the indicator at one point (say, +.005 and -.003 = .008 "total" runout) vs. "radial runout" which is half of that value. These definitions had no relation to GD&T runout definitions and it took me longer than it should have to fully ground myself in both languages.

One last thing: parts with this level of straightness do not hold straightness. Temperature cycles, vibration, handling can cause changes. In my company we say "straightness is a perishable commodity". Will they inspect it again once you've shipped it to them, and will it be contractually binding? Unless you established other terms the answer is probably yes and a serious risk to you.