I was in charge of the design of a high-speed rotor that was subjected to an overspeed test in a spin pit before final assembly. The rotor was balanced before the test, after assembly with the test fixture, and after the test. I was approached by a couple of spin pit technicians who told me my rotor was the worst one of all to run. They said it was subject to being dropped in the spin pit, which ruins the rotor. They also said it was difficult to accelerate through a rotordynamics critical mode to reach the required test speed.
Not having had a lot of exposure to the world of rotordynamics, I investigated the literature and found that experts recommended avoiding any ratio of 0.8-1.2 between the axial and transverse moments of inertia for rotors in similar situations. I arranged torsional pendulum tests to measure the two parameters and found that the rotor/test fixture combination was at a ratio of 0.96. It could have been worse, but not by much. Since the rotor was the product, the test fixture had to be changed.
The fixture was pretty minimal in its mass, and it clamped to the rotor with a Belleville washer that my analysis showed was being operated in the plastic deformation range but treated as a reusable item. At high rotational speeds, rotors lose length due to the effect of Poisson's ratio, so a spin pit fixture must maintain a clamp to maintain the balance of the assembly.
The rotor/fixture combination was subject to tumbling from a rotordynamics point of view. Since the test fixture was looked at as a permanent fixture, the rotor was being unbalanced and then rebalanced to avoid damaging the test fixture, and the fixture's clamp on the rotor (and balance of the assembly) was being lost due to an unfortunate clamp design.
My recommendations were incorporated into a fixture design that:
- Lowered the spring rate of the fixture, so that its elastic deformation would be more than enough to accommodate the shrinkage of the rotor without a separate spring, with its implied need for a follower, and a clearance to make the motion of the follower possible
- Added mass to the ends of the test fixture to make the transverse moment of inertia more than 1.2 times the axial
- Added polar arrays of threaded holes to the fixture at each end, so that set screws in pairs in the same hole could be used to adjust the balance of the assembly without metal removal, unbalancing, and then rebalancing the rotor
Satisfaction comes from increasing the probability that your product makes it to final assembly, reducing the cost of your product, and having a group of technicians thank you for turning the most difficult part into the easiest part to run.
This entry was submitted by Oscar Carlson and edited by Rob Spiegel.
Oscar Carlson is a retired mechanical engineer.
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