I'll add a few things. You won't be able to find the exact type of grease required for these anywhere in any hardware or automotive store.
It's too specialized, there's too much variation between grinders, requirements vary based on things as small as the percentage of copper in the alloy of a gear of a specific model, and there's no demand. These are industrial greases not available to a consumer. Theyre one-offs, made to order, customized for each model and manufacturer. There's engineers and chemists involved, lawyers to draw up contracts. These greases are sold by the fiscal year or by the ton, not by the tube or by the pound, and supplied to factory authorized dealers in 55 gallon drums. So there's no point in arguing what grease is best. If you can, just leave what's in there in there. The gears have a much longer service life than the motor or the bearings.
Not saying you cant find a grease that'll make your grinder run quieter or smoother, because you can. And you can find some greases that are pretty close to OEM, but you're not extending its usable life if this is the only service you're doing. Best case scenario you're only doing minimal damage. But that might be a worthwhile tradeoff to you, and that's fine, personal preference.
A few things to think about when selecting a grease. Many (not all) angle grinders use a metal alloyed with brass on the circular gear and hardened steel for the pinion. There's less friction with this setup than with steel to steel. If the gear is a yellow metal you have one of these setups. If you do, don't use an EP grease. They typically have sulpur additives which attack yellow-metals, eating away at them. If both your gears are steel use an EP.
Yes, grease gets flung off, but there are spots in the grinder where it's always there. Usually a well at the base of the pinion and multiple wells between the top bearing and circular gear. When it gets hot small amounts do melt from these pockets and lubricate the gears. The whole thing isn't supposed to be immersed in grease at all times, not even partially. That would be impossible in a gearbox that isn't fully enclosed. Don't worry, there are solid lubricants already on the surface of the gears like lead or moly or various others that are there even when you can't see them. These lubricate the gears when it's cold, and oil flows from the wells when it's hot. The gears are supposed to look clean when you open it up, it's fine and it's normal.
If you do replace the grease and clean off these solid additives make sure you select a grease that has some more, moly grease being the obvious choice. You also want a grease that is thick and sticks, at least nlgi 1, 2 or 3 is fine too if its not impervious to temperature so it can melt easily with a little heat, like a compounded grease or a good polyurea, this is especially important if you don't have a yellow-metal gear.
Lastly, 90% of wear and tear is at engine start up. Its not a huge deal if some grease gets flung off at speed. Some grease flows back into the wells after you shut off a hot grinder that stays in place for engine startup, and there are still solid lubricants in place on the surface, that's enough to lubricate through the worst of it.
If you're fine with shortening the life of your grinder, you can get them to run quiet enough that you can carry on conversation while its spinning. Maybe experiment on a cheap one first though. You can also buy a new set of gears for like $10.
It's too specialized, there's too much variation between grinders, requirements vary based on things as small as the percentage of copper in the alloy of a gear of a specific model, and there's no demand. These are industrial greases not available to a consumer. Theyre one-offs, made to order, customized for each model and manufacturer. There's engineers and chemists involved, lawyers to draw up contracts. These greases are sold by the fiscal year or by the ton, not by the tube or by the pound, and supplied to factory authorized dealers in 55 gallon drums. So there's no point in arguing what grease is best. If you can, just leave what's in there in there. The gears have a much longer service life than the motor or the bearings.
Not saying you cant find a grease that'll make your grinder run quieter or smoother, because you can. And you can find some greases that are pretty close to OEM, but you're not extending its usable life if this is the only service you're doing. Best case scenario you're only doing minimal damage. But that might be a worthwhile tradeoff to you, and that's fine, personal preference.
A few things to think about when selecting a grease. Many (not all) angle grinders use a metal alloyed with brass on the circular gear and hardened steel for the pinion. There's less friction with this setup than with steel to steel. If the gear is a yellow metal you have one of these setups. If you do, don't use an EP grease. They typically have sulpur additives which attack yellow-metals, eating away at them. If both your gears are steel use an EP.
Yes, grease gets flung off, but there are spots in the grinder where it's always there. Usually a well at the base of the pinion and multiple wells between the top bearing and circular gear. When it gets hot small amounts do melt from these pockets and lubricate the gears. The whole thing isn't supposed to be immersed in grease at all times, not even partially. That would be impossible in a gearbox that isn't fully enclosed. Don't worry, there are solid lubricants already on the surface of the gears like lead or moly or various others that are there even when you can't see them. These lubricate the gears when it's cold, and oil flows from the wells when it's hot. The gears are supposed to look clean when you open it up, it's fine and it's normal.
If you do replace the grease and clean off these solid additives make sure you select a grease that has some more, moly grease being the obvious choice. You also want a grease that is thick and sticks, at least nlgi 1, 2 or 3 is fine too if its not impervious to temperature so it can melt easily with a little heat, like a compounded grease or a good polyurea, this is especially important if you don't have a yellow-metal gear.
Lastly, 90% of wear and tear is at engine start up. Its not a huge deal if some grease gets flung off at speed. Some grease flows back into the wells after you shut off a hot grinder that stays in place for engine startup, and there are still solid lubricants in place on the surface, that's enough to lubricate through the worst of it.
If you're fine with shortening the life of your grinder, you can get them to run quiet enough that you can carry on conversation while its spinning. Maybe experiment on a cheap one first though. You can also buy a new set of gears for like $10.
