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I have a 15 inch (381 mm) shaft with 24.95 mm diameter. That shaft is attached with a gearbox (ratio 5:1). The gearbox is driven by a motor (rpm 2750). There is one UC bearing and mechanical seal and a agitator (propeller) attached with that shaft. Whenever we try to set the shaft with the whole setup, the alignment is not remains perfect. In dial gauge there is around 5° deflection in the top of the shaft. The shaft is installed at the bottom of a vessel where load is around 300 kg. The problem is, when it is installed with the misalignment, solution is dripping from inside. On running condition there is around 3 bar pressure. I have two specific question:

  1. How can I align that shaft with the whole setup?
  2. I have checked the shaft, prepared it carefully in the workshop. But found that it is not aligned perfectly. So, is there anyway to make perfectly aligned shaft with the said setup?
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    $\begingroup$ I assume the issue is you have insufficient references surfaces to check the alignment? Or are you unable to tap things into place under indicator measurements? How you go about things really depends on the setup and surrounding shapes so photos would help. In some cases it's not possible to get sufficient alignment because the setup was not made with the appropriate facilities to allow alignment. And are you sure angle alignment is the problem? Because it sounds like being off-center from the seal could just as easily cause a leak. $\endgroup$
    – DKNguyen
    Commented Dec 7, 2022 at 21:34

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There are several ways to connect the two shafts.

Use couplings which can tolerate some misalignment. There is a rubber joint called rotoflex used to couple driveshafts to the gearbox on a Hillman imp - b1tch to fit but capable of flexing, power capacity may be a consideration.

There are other joints like Constant Velocity joints which may be preferable but they don't have the cushioning effect of rubber.

As for alignment, you will need to set a reference and get suitable measuring tooling to line things up. Even using lasers may be a method to consider.

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Propeller loads can generate very large unbalanced forces if the fluid inflow conditions aren't perfectly symmetrical with the prop axis. There is simply no way this will work with a single bearing on a short, thick shaft. Given the size of the shaft, and, as I understand it, the shaft enters the tank below the fluid line, you are probably best off using some sort of cutlass bearing inside the tank. I guess it depends what you are agitating. Another option is to install a heavy "shaft log" into the tank so that the prop is no more than one shaft diameter out from the bearings in the end of the shaft log. Also check that the agitator doesn't generate more thrust that the gearbox and other components can handle. There are radial, low thrust agitator designs.

So summing things up-

Your short, fat shaft needs two bearings. The inboard one needs to be fixed to the gearbox with a frame, with just enough room for the shaft coupler. That frame needs to be stout, made from steel at least as thick as the gearbox casting's mounting flange.

The outboard shaft bearing needs to be no more than one diameter from the end of the agitator hub, with a fluid seal system outboard of that. If the material is noncorrosive and nonabrasive and not too viscous, consider a cutlass bearing.

Shaft logs can be pressure equalized depending on which way leakage is the bigger problem. This requires an inboard seal on the shaft log and a tank of (typically) oil to gravity feed the shaft log interior and lubricate and protect the bearings from contamination from the tank. This also lets you know when the seals are failing.

The shaft log is mounted only to the tank, not the gearbox. Shaft axes should align +- 0.002 inches and angles should be no more than 25% of whatever your shaft coupler system says it can accommodate. If that doesn't sound like a one hour job, consider a belt or chain drive. That simplifies the sealing problems considerably by isolating most of the drivetrain mass from the shaft. But now you need the propshaft bearings to handle the thrust, so double ball or conical roller bearings would be the norm.

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