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After getting the gearbox drained and cleaned of gear oil, I planned on how I could make a oil cooled bearing that would take the shear load of the power turbine shaft. The stout gearing of this gearbox has me assured that if anything melts down, it will not be this baby!
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My hopes are to use the lubrication system of the GR-1 to irrigate and cool the bearing that will be close to the hot turbine wheel. The oil will migrate down into the gearbox and be recirculated into the system through the oil cooler. I will have to figure out how deep I will need to submerge the gears in the new viscosity oil (5W-30) as the old gear oil was very thick. I figure that the friction of the partially submerged gear will be reduced if I lower the oil level to achieve higher RPM.
The next step in my quest was to find a suitable ball bearing that could handle the high RPM and temperature. After shopping my favorite tool supply McMaster-Carr , I found a ball bearing that fit the bill. I bought a few 5/8” ID steel ball bearings that are rated for 22,900 RPM at 1260 pounds! These bearings will hold up to 350 degrees Fahr. without being damaged and the best part is that they cost about $5.00.
I needed a 16” X 5/8” precision hardened steel shaft for my turbine wheel so I purchased one from McMaster for about $14.00. This will be cut to length later when I hammer out my drive shaft configuration.
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To make my bearing holder/housing for the GR-5, I purchased a rod of 8620 steel alloy (from McMaster) which is great for its machining and weldability properties. After drawing up a couple of ideas I settled on a bearing tube design that has a bearing at the end that is cooled and lubed by a stream of synthetic oil. In order to do this I have to machine a bearing holder that has a high temperature seal on the end. For the seal, I plan to use fiberglass or ceramic rope packing material held in a race on the bearing cap.
The fiberglass rope will not melt like a rubber seal and should survive the heat of the turbine housing during operation. The rope seal will not have to positively seal the shaft, only keep oil from splashing into the heat shield as my design will hopefully keep positive oil pressure away from the seal.
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The steel alloy cut very cleanly with the band saw and I was able to get started immediately. I chucked up a piece on the lathe and started turning a bearing holder that would later be welded into a piece of 1-1/2” EMT to complete my bearing tube. This stuff is awesome to machine!
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I machined a taper into the bearing holder so the cooling oil would run away from the seal on the end. Once I got the bearing to fit the holder snugly, I started machining a cap to fit the holder that would not only hold the bearing race but also house the fiberglass packing rope. I sliced off a piece of steel alloy and went to work.
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After I machined the bearing cap, I made a steel washer that would hold the bearing race snug by acting as a spring. Notice the raised center of the bearing cap, this will push the washer and in turn push on the bearing race. The washer had to be cut thinner in the center so the inner race would not rub the washer. You may also notice the rope seal race on the bearing cap, the washer will hold the seal in place. I now had to drill my holes for the cap screws so I pulled out my rotary table and mounted it up on my mill.
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With the bearing holder now ready, I can start building the bearing tube that this part will be welded into. I had to make a plate that would mount to the gearbox so I cut out a 1/4” piece and fabricated the plate.
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I cut a piece of 1-1/2” EMT to use for my bearing tube. I tacked the tube into place so I could fine tune my center. Well, this is my chance to become a millwright :o) After finding the sweet spot I welded both the flange plate and the bearing holder into place.
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With the bearing tube ready, I started making my oil jet that will spray oil on the turbine bearing. I used a steel pneumatic hose quick release for a 1/4” NPT bung that will feed the oil to the oil jet. I purchased a short piece of 3/16” steel brake line and welded it into the modified fitting.
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The oil jet is complete and ready to be welded into the bearing tube. I cut a hole into the bearing tube at a 45 deg. angle to vertical so the oil jet will not obstruct the flow of return oil down the bearing tube.
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The oil jet is set up to spray oil at the bearing retainer, keeping the bearing lubed and cool. I tested the jet with 50 PSI of oil pressure and it seems to make a good spray pattern. I needed to drill and tap a hole into the gearbox cover for the gearbox oil drain. I made an educated guess on how high the oil level should be and drilled the hole for the 1/2” NPT thread.
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There will most likely be a lot of oil spray inside the gearbox so the oil level can be lowered. As the oil drains out of the bearing tube, it will drain out of this fitting into a oil accumulator that will separate the air bubbles out of the oil. The oil will then return to the oil cooler of the GR-5. To mate the gearbox with the turbine housing, I figured on using the factory cover bolts to hold a bracket that will be welded to the turbine housing.
To make the housing bracket seat more accurately, I milled the rough casting flat so the housing could be bolted to three cover bolts. I am really happy with my investment in this tooling as I am learning a lot.
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The cover is essentially ready an I am set to install it on the gearbox so I can install the turbine shaft. The gearbox input gear has a shaft collar designed for a 5/8” keyed shaft as typically used on a electric motor. I do not wish to use a shaft with a keyway as it is not balanced and I could not use a seal with it. To lock the shaft in the collar, I used a set screw that I tapped into the shaft collar. The set screw is seated in a flat spot I ground in the turbine shaft.
To balance the void of the keyway that was cut into the shaft collar, I ground some of the opposing side off. This was just a guestimate but I feel that it is a good starting point.
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The hardened precision shaft that I purchased is a very hard material and needed to be ground instead cut with a saw. I used a grinder with an aluminum oxide wheel to shorten the shaft to my desired length. After I cut the shaft, I temporarily put the gearbox together to test for shaft wobble. With my dial indicator I measured 0.00025” runout which to me is good for a backyard contraption. I am now ready for the next step, mounting the exhaust diverter with a bracket system to the gearbox.
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My gearbox is done and I cant wait to get it coupled to my turbine housing so I can make my turbine wheel. I am really curious if my idea will work or just melt down in front of me. If anything, I can use half of the turbine housing for an afterburner housing for the GR-1 :o)
Till the next installment....
Don Giandomenico
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