Part 011: The Shaft Tunnel

The Shaft Tunnel is a very improtant part of the turbine and must be made with care and precision. It provides support for both front and rear ceramic cageless bearings, allowing the shaft, which carries both compressor and turbine, to run smoothly and true. Inside the tunnel is a pre-load spring for the bearing set, a tube to distribute the pre-load forces, and of course the shaft itself. Additionally, it provides a tight seal and support for mounting the diffuser, and also locates the NGV assembly during final assembly.

It is critical that the bearing recesses be truly axial, square, and parallel with each other, or forces induced in the shaft will ruin the expensive bearings in short order.

A blank piece of 6061 aluminum, 1.375" dia. is cut and faced to length. Held in the three-jaw chuck, it is then drilled through 1/2" and bored to the correct minor diameter of 14 mm. The recess for the turbine bearing is bored to be a light, sliding fit, and an o-ring groove is also machined. The o-ring lightly grips the rear (and front) bearings allowing them to flex and seat to optimum running positions.

After the aft portion of the tunnel is correctly bored, a mandrel needs to be used to finsish the compressor end as well as the exterior of the tunnel. I elected to use an expanding mandrel rather than a solid mandrel. These are extremely handy to have as a set, as they can be turned to whatever diameter or shape is needed, in situ, to be a very close sliding fit to the bore of the part. After a thorough polishing and deburring, the mandrel is cleaned and coated with oil. The shaft tunnel blank previously bored is then slipped carefully over the mandrel to avoid marring the rear bearing seat, and a couple turns of the hex wrench grips it securely.

Red marker is used to lay out the different flanges and diameters. The forward bearing recess and o-ring groove are machined to print. A heavy parting tool roughs out the external shape of the tunnel. Be careful not to heat the tunnel up too much, as this may cause it to expand and slip on the mandrel.

A form tool is inserted in the lathe, and the 2 radii which form the front and rear flanges are turned by jogging the cross and slide feed by hand as necessary.

The completed tunnel still in the lathe after polishing with steel wool. All that remains now is drilling and tapping the holes in the flanges for both the diffuser and the NGV.

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