This gear size is the most common gear to split in consumer items due to its size. Gears split because the hole is slightly smaller than the shaft so it's a tight fit so it won't slip. But that tight fit puts stress on the gear's plastic, it cracks over time, and then slips. I drill out my gears with precision drills to a smooth fit. I scruff up the shaft with a hobby file and put some scratches inside the gear with a needle. Then add a tiny drop of super glue as I slip the gear onto the shaft. The gears last a lot longer this way, but it takes practice to get just a tiny drop of super glue on the shaft and slip the gear quickly into position. If not, simply cut off the gear with a hobby knife, scrape the glue off the shaft and try again with a fresh gear.