GR-7 Turbojet Engine Project 5/28/06

Posted on May 28, 2006

       The backplate assembly of the gearbox was next to be fabricated. The backplate will hold the mainshaft tube and also support the engaging solenoid. The 12 volt solenoid I am using will mount directly into the backplate which should save weight and space. The completed gearbox will help serve as a template to make the new backplate from which in turn will save a lot of time.

       To get started I cut out a 1/8” plywood template to be used as a guide for the plasma cutter (just like the combustor elbow fabrication). Using the template I cut out a 10 gauge 304 stainless piece to be used for the backplate. I then used the mill to cut the various holes needed for the mainshaft tube, solenoid and mounting hardware.

       Once complete, the backplate was installed to the gearbox for a fit check.

       The next step was to build the mainshaft tube which will support the starter shaft. For this design to work I needed to employ some high speed bearings. Theses bearings will be supported by the mainshaft tube at either end. This will require that the tube is bored precisely to the outer diameter of these bearings. I chose to use a piece of 1/2” schedule 40 304 stainless steel pipe for the mainshaft tube. The diameter of the pipe is perfect for housing the bearings and tough enough to withstand heavy forces.

       I mounted the stainless pipe into the lathe chuck and installed a “steady rest” onto the bed way. The steady rest helps support the tube at the far end to aid in proper machining. I then proceeded to bore out one end of the tube to receive one of the mainshaft bearings. I used a “step drill” to bore the hole as it was the perfect size I needed for the bearing (.750”). The step drill or Unibit also helps reduce “chattering” as it cuts in layered “steps” reducing vibration.

       This end will be the turbine-end of the mainshaft tube. For this reason I needed to install a “snap ring” retainer to keep the end bearing in place. I used a “parting” tool to carefully cut a groove into the inside face of the tube which will receive the 3/4” retaining ring.

       The next step was to weld the tube to the backplate. I carefully aligned the tube into it’s respective backplate hole and proceeded to TIG weld the tube in place. This procedure had to be done very carefully as the welding process could severely warp the backplate and tube. Using very low heat I was able to join the two successfully.

       The backplate end of the tube could now be bored for the second mainshaft bearing. I hesitated to bore the hole until the welding process was complete as the heat would of probably warped the weakened end of the tube. I mounted the tube on the lathe and used the steady rest to support the backplate end. The Unibit was then used to bore out the hole.

       Unlike the turbine-end bearing, the gearbox-end bearing is held in place by the main spur gear and does not require a snap ring retainer.

       The next step was to install an “Oilite” bushing into the gearbox to properly align the main spur gear within the gearbox. The bushing will hold the main spur gear against the gearbox-end bearing, keeping it from moving around. I had ordered a bushing that was slightly larger than the mainshaft to prevent contact with the shaft itself. The bushing should only touch the spur gear as to prevent unnecessary friction.
       I proceeded to bore out a hole for the bearing in the gearbox. The bearing was then press fit into the hole allowing for a .010” clearance to the main gear when assembled.................      So far, so good!!!

       I now needed to build the hex-drive adaptor for the turbine-end of the mainshaft. The adaptor will hold a special hex-drive coupling, joining it to the mainshaft. I cut a small piece of 6061 aluminum rod for this piece and mounted it to the lathe. I used a drill bit to bore a pilot hole into the center of the adaptor. A 5/16-18 tap was then used to tap one end of the adaptor.
       The use of the lathe for tapping out the center of the adaptor helped make the threads axial. I hand-fed the tap as to not harm the motor drive of the lathe.

       I trimmed the adaptor to the size needed and readied it for the set screw holes.

       Using the mill I was able to drill out four holes, two on either side. These will be used as set screw holes. The set screws will hold the adaptor to the mainshaft as well as the hex-drive coupling to the adaptor. The set screw holes were tapped with a 6-32 hand tap.

       To prevent the adaptor from spinning on the mainshaft, small “flats” were ground on the end of the mainshaft. The set screws can bite into these flat spots providing a positive connection.

       The adaptor was installed onto the mainshaft with blue Permatex threadlocker (Loctite) and and then tightened down.

       The next step was to create the engaging selector fork which will push the mainshaft in and out to engage and disengage the hex-drive. The selector fork will ride against two thrust bearings on the mainshaft allowing it to move the shaft with minimal friction. I prepared a piece of 6061 aluminum bar stock for the needed part. Having a rough idea of what I wanted I started milling the part.
       The first cut was used to create the “fork” end of the selector arm. This portion will ride over the mainshaft itself. I used the rotary table to cut the rounded profile of the fork that will be in contact with the thrust bearings. Notice the registration mark on the fork’s side. This was used to help align the part on the rotary table.

       The next cut was used to create the solenoid linkage point. This is where the solenoid connecting rod will pull on the arm to engage the mainshaft.

       I am really enjoying what I am able to do with my “budget” metalworking machinery. The possibilities are apparently endless :0)

       I now needed to add a pivot point in the mainshaft tube for the selector fork. I cut out a couple of stainless plates to be used as a pivot point and carefully welded them to the side of the mainshaft tube. Notice the spacer between the plates. This helped align the two plates perfectly.

       With the plates in place I was able to mill out the hole in which the selector fork will pass. The pivot hole was also drilled at this time.

       I could now put the finished parts together for a fit check. Notice the stainless “shouldered” pivot bolt. The non threaded portion of the bolt will help prevent friction when in use.

       The parts fit into the mainshaft tube very nicely, only requiring a little fine tuning with a file.

       The starter system will eventually be electronically controlled by the ECU (Electronic Control Unit). For the ECU to know what state the starter is in (engaged or disengaged) there must be an indicator switch. To fill this requirement I chose to use a microswitch mounted on an aluminum bracket. When the shaft is fully engaged the switch will be activated by the selector fork linkage an in turn signal the ECU of it’s state.

       The last part to fabricate was the solenoid connecting rod. Once complete, the parts were assembled and the solenoid was tested. Notice the return spring on the solenoid armature. This spring will disengage the starter when the engine is running.


       I am fairly pleased with the progress so far despite the intense work it has proven to be. A leaf blower would of been a much easier choice in starting systems :0P  Nevertheless the project will forge-on so check back to see the continuation of the GR-7 Turbojet project!!!

Thanks for dropping by............

Don Giandomenico

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