Opposed Bearings & Stacked Plate Turbine Design
In Figure 46a we see a six-inch turbine with a typical ring case and end plates. What's radically different from last month's design is the use of opposed bearings rather than an overhung rotor. Overhung rotor designs group the two bearings into a common bearing housing, whereas opposed bearing designs place the bearings at opposite sides of the rotor case.
There are pros and cons for both designs.
With a common bearing case design, we end up with only two oil seals instead of three - and it is much easier to align the bearings. It is also much easier to isolate the heat of the hot rotor from the bearings with the overhung design.
Advantages of the opposed bearing design include fewer parts and a slightly lower cost of construction.
In building the opposed bearing design, one of the endplates must be solidly welded to the base plate, or both endplates may be bolted to the base plate with angle brackets.
Any number of plates may be stacked to form a rotor case. In our example we use a stack of four plates, with bearing bodies welded to the two outer plates.
This design is both low-cost and easy to build. The case may be fabricated using 1/4-inch to 1/2-inch 6061 aluminum sheet stock. The inlet port may be drilled or slotted with an end mill.
An advantage of the opposed bearing design is the smaller diameter shaft. Overhung designs must use a thicker shaft due to strong unsupported gyroscopic forces beyond the bearings. The opposed bearing design supports the rotor-induced forces between the bearings, allowing for smaller diameter shafts.
For more information on stacked plate designs, visit Robert O'Brien's website at www.obilaser.com Robert is supplying a small turbine for organic rankine cycle work at SVSU (Saginaw Valley State University) as a follow-up to a custom-built 10-inch turbine we supplied them last May.
During the course of engineering a turbine for closed-loop operations we used three (3) industry standard nitrile shaft seals to contain bearing lubricant and CFC refrigerant. We discovered that the amount of drag associated with the lip-seal acted as a brake and prevented low power spool-up of the rotor. Since the 10-inch turbine is rated for about 20 horsepower, it will take at least several horsepower of gas to initiate spool-up.
Presently we are working around this problem with new seal concepts that will effectively contain pressurized bearing lube. Once this problem is solved we will resume offering turbine products to our Club members.
Next month we will cover yet another solar collector design for study. Until then, keep those turbine experiments rolling.
Last updated: January 12, 2005 10:16 PM
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