This can't be real life...

[frickenbored]

The cutting of the Key left what is called a stress risers in the Gear. The shaft and gear left just round would not have any. When they put cuts in cement they create stress risers in it. Then it will probably crack in those cuts. On some of the high Quality keyway cuts on usually big machines the corners on the keyway will be round to prevent cracks. I believe someone over tighten the nut on the shaft. Lots of force pushing outward on the gear hub. The hub is weaker the crack starts there but will likely continue out to the edge of the gear. If you end up wanting to use this one. I recommend taking a 1/16" drill to stop drill this crack. I would guess you can't see the crack yet on the back side. I use stuff that Welders use to find cracks. Use good light like a good sunny day at noon outside. Make a cross at the end and center of the crack. This is when I First spray a Dye Penetrant. Use a die or small grinder to make a flat spot at the center of the cross. If I had a Dye Penetrant Spray again. Wipe off then Spray Developer, it will reveal the end of the crack. If you don't have those goodies reconnect the cross. Just outside the cross Center punch it. Use a Drill Press and Drill All The Way Through the Gear.
Best of Luck

I've actually thought of doing this, I've had very good luck stopping cracks in car windshields by drilling a tiny hole at the end of the crack just past the first layer of glass with a diamond bit. I also thought about drilling a small hole at the end of the crack and grinding a v-groove along the crack then filling it with weld, but I'm guessing that gear is cast.

 

[Cat977]

I would avoid welding you may distort the gear. On the hub opposite the key way. Use a 4 flute center cutting end mill in a mill or drill press. Have everything locked/bolted in place. In a drill press use the lever to lock the spindle to put a tightening force on the spindle to control chattering. Cut perpendicular to the crack use say ⅛ endmill spot face or cut most of a circle. Then drill and tap for UC ⅛” set screw. You Want Run the Threads trough both sides you aren't clamping, just stabilizing. Totally Clean Treads of All OIL. Use red Loctite 272 271 red or 290 or 690 green or any high strength one. Between the stop drilling and the set screw you should be good to Goooooooo!

 

[frickenbored]

Well she lives again, I found a new injection pump drive gear for cheap on ebay and I replaced the thrust spring. The old girl runs great again, hopefully it stays that way.

 

[petepilot]

Well she lives again, I found a new injection pump drive gear for cheap on ebay and I replaced the thrust spring. The old girl runs great again, hopefully it stays that way.View attachment 238080

been kinda watching your thread `glad to hear she lives