Oil Tank


Early in the turboprop project, I decided on a modular, railed design which would have minimal external connections with the "outside world". Mike's effort in one of his prototypes is superb. In viewing what photos I could find, I suspect this was one of the designs with a gear-driven oil pump rather than an electric pump. Gear driven designs usually have a scavenge pump, meaning that the oil need not gravity feed into an external reservoir, but is actively scavenged from the gear case. Since this design makes use of gravity to drain the oil, I needed a low-profile tank which would mount beneath the gearbox. My choice of mounting makes this relatively simple.

  The oil tank starts as an accurate block of 6061 aluminum. I looked for suitable poly containers as well as prepared aluminum vessels, but none had the dimensions I was looking for. In the end, I did it the hard way. The tank measures 3" X 1.625" X 1" high.
I determined 6 screws would be adequate to hold the lid, and planned for the hogging of the interior. A long roughing cutter was used in the mill. Note that the bulk of the metal was drilled out first, as there is no place for the chips to go, and the operation must be halted periodically to clean out the swarf.
In the end, I had the tank body as shown. The six holes are drilled and tapped 4-40. It was pretty drab-looking, a simple aluminum box. I decided to dress it up a bit!
First, the front of the tank was bored for a sight glass. This is a poor location for it, being right in the prop blast, but was the only visible location, the other three sides being blocked by mounting hardware.
Next, the edges were rounded with a special end mill.
Finally, cooling fins were slotted in the body of the tank. This was a tedious process, but looks nice when done.

The lid was engineered. The drain barb was fitted with a splash bracket containing a pair of small magnets to collect and hold ferrous swarf. The other fitting in the corner is for the breather tube.

The front of the finished oil tank, without lid. The sight glass is in place, and a 1/4" hex brass drain plug graces the lower right corner.

In the rear is the takeoff for the oil pump.

And finally all together on the bench.

Here is how the oil tank will look when finally mounted. Note the location of the breather is not to print. I elected to place it using a commercial banjo fitting on the upper right side of the gear box. A stub of viton tubing connects the oil tank to the gear box, isolating the tank a bit from vibrations and allowing an easy disconnection when required.

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