Perpindicularity of the trunnion gears is determined by the squareness of the tubes they are mounted in/on and the alignment of the trunnion horns. Page one solved the first half of this problem, now we will finish it.
When welding parts as critical as these it is absolutely necessary to have a sturdy jig capable of holding the parts in alignment. As a part is heated locally during welding it twists and bends as it's cross section shrinks and expands. Building this jig is as important as your plane and life maybe even more so. Imagine the possibilities of what might happen if your gear fails and you will catch my drift.(Slide underneath a gas truck OOPS!)
The trunnion "horns" can be finish cut by using the jig. I have thought of a simple way for builders to bend their horns with the tool on this page.
For those of you that have a sheet metal brake the horn forming tool is of course unnecessary. There are two reasons I'm suggesting this horn forming tool. Firstly it will allow you to make bends with the proper radius and placement. Secondly it will make positioning the 3/4" cross tube hole in the center of the horn easy. Finally you will only need to cut a rough shape from your 4130 plate. While this saves you from possibly wasting a fair sized piece of 4130. The downside is that you have to cut a second time after forming(finish cut). This tool can be made quite easily with basic metal working tools and a machinists ruler.
The air cutoff tool can be used in place of plasma cutter and torch. It costs $25.00 and the only pain about using it are cutting curves and waiting for the air compressor to catch up. I would not reccomend using the oxyfuel torch for cutting 4130. The jigs and forming tool are rough cut very quickly with the torch or plasma cutter. 4130 steel crystalizes near the cut when cut by a torch. Do not use a torch to cut your 4130 parts. If you have the parts laser cut DEBUR or sand smooth all edges to remove the .001” to .003” thick hard (dark) cutting edge left by the laser. THIS IS A MUST ON ALL PARTS MADE FROM .040” AND THICKER. Failure to do this may result in small cracks in the bends at the hard edges. Waterjet cutting is an ideal way of having your parts cut. The plasma cutter is a good, fast and cost effective way to cut your 4130. It would be worth it to mark your sheet 4130 with soapstone and take it to someone with one of these. A 4-1/2" angle grinder makes quick work of removing metal. The .090 plates for the main spar is one place where the grinder is handy. Slag removal is made easy with the grinder also.
Assorted jig and forming tool parts before welding. These parts have had certain sides machined but someone without a mill could make them with careful measurement, a square and a belt sander. A dial caliper should be used to ensure accuracy.
As soon as I find way to convert my CAD drawings to .jpg format I'll post the dimensions of these parts. It should be easy for someone with the plans to figure the dimensions and angles however.
There are two dimesions that are critical on the welding jig. The height of the 3/4" rod in relation to the base is what determines perpindicularity of the gears. One side higher than hte other is no good.(see quote from machinery's handbook below) The center line of the trunnion tube needs to be centered on the 3/4" rod especially fore and aft. Side to side centering of this part is not critical but as with all other things on the landing gear accuracy helps.
The horn forming tool is easy to make. Your goal is to end up with a 2.4" width across the inside of the ends of your horn. This tool should have one end squared so that you can bend the 13 - 15 degree bend in the top of the horn. (See picture above for a rough formed trunnion horn in the welding jig) Do a rough layout on your sheet for the first horn. After the first one is finshed you can trace it on the outside with paper and have a layout that is not as rough. This layout should have a little extra material in case the horn shifts while being bent.Although I used plate steel between my 3/4" rods three such rods could be laid side by side and welded.(This equals 2.25") Enco and MSC sell drill rod which is cheap and can be welded.(see metal advice suppliers on main page) The reason for the narrower width(2.25") is that 4130 doesn't conform readily to such a form but instead springs back a littlle. A piece of quarter inch steel is welded to the back of the tool to allow it to be held in a vice. Fortunately this tool does not need to be artwork and can be quickly slapped together.To help the 4130 bend without cracking I heated it a little with a torch. You don't want to bend 4130 this thick when it's cold as it might crack or be more prone to cracking.(so I've read)
Notice the flat plate was sandwiched between the tool and a flat plate while being bent. The trunnion horn needs to be held very securely while you hammer away. Bending this horn was relatively easy.After this step the 3/4 inch hole needs to be drilled. Find the centerline of the forming tool and scribe it on the tool continuing this line on the horn. Using the plans flat layout dimension measure up from the bottom of the horn and mark a line across the center line that you just marked. Drill the hole with an undersize drill and ream it. I'll explain why this hole needs to be as exact as possible later. To make a drill undersize the cheap way run it backwards while appying sandpaper to the diameter(not the tip). DO NOT WRAP YOUR HANDS AROUND THE DRILL! YOU MAY BE CUT.Hold the sanddpaper outstretched and run the drill. Bite the bullet and purchase a reamer. If you get a chucking reamer replace he drill bit with it to maintain the hole's center. If you are hand reaming try to use the drill press to help center your reamer if you have enough height. After the hole is made bend top of the horn 15 degrees and you are ready for trimming on the welding jig.
See the picture of the welding jig (four pictures above)for what comes next. Metal is trimmed off with the air cutoff tool until you have everything looking as it does in the plans. Holding the horn in the jig helps you to determine how much metal to remove. When making the welding jig center punch all critical points before welding it together. There is a 1/2" hole in the middle of the jig which holds a cylinder that helps to center the trunnion tube. This will also be used to hold the trunnion tube down during welding. You would be surprised how much things can move due to distortion during welding..
All this fuss about accuracy is due to these gears. Machinery's Handbook, the bible for machinists has a section devoted to bevel gears. I will give a synopsis of "Mountings for Bevel Gears", a paragraph in the book.To take full advantage of the miter gear's strengh it has to be mounted rigidly and run true and square in relation to the other gear. Testing has shown on miter gears of 6 to 15 inches in diameter that the following deflections are allowable. Neither the pinion or the gear should lift or depress more than .003". (That's three thousandths of an inch) Neither the pinion or the gear should yield axially more than .003" in either direction.
Our gears are smaller than six inches so I imagine the tolerances are slightly tighter. Shim stock should only be used to scoot the gears to a perfect mesh in the horizontal plane. Vertical plane alignment is covered in the next paragraph.
After making the 3/4" trunnion through hole perfectly square to the trunnion tube there are three critical parts. The 2.25" tube which the bottom miter gear sits in needs to be square.(use the lathe) It is extemely important to have the 1" tube on top of the trunnion machined square and exactly 1" tall. The last critical part is the ring under the trunnion tube. The ring needs to be welded on to the 2.25" tube with the trunnion tube and 1' tube in place. This makes it impossible for the trunnion tube to move up and down. The 1" tube and the miter gear needs to be installed before this lower ring is welded on of course.
This ends the most gruelling pursuit of accuracy on the landing gear. Breathe easy the rest is easy compared to this. You don't have to follow exactly what I've done to arrive at this point. You may find a better, easier way to produce the same accuracy in construction. At least you have an idea of how these important parts need to be fitted together.
To form your trunnion horns you could have a sheet metal shop bend a "U" section of 4130 from which you could cut your horns. You would also need to have a second wider "U" section made for your reinforcing gussets. This would be ideal since the radius of bend would have better control than the method below. Below is a homebrewed method creating the bends.
