There seems to be a trope going around that modern KitchenAid mixers are specifically inferior to vintage ones because their “gears are made of plastic” (or a similarly phrased complaint).
This is categorically not true.
All of the gears in full-sized tilt-head KitchenAid mixers, from a 60s-era K45, the 90s KSM90 “Ultra Power” or the modern KSM150 Artisan (and all the variants in between, but not the Artisan Mini), are the same. This is also the same gearing used in the older bowl-lift models: K5A, K5SS, and KSM5 (but not the “Pro” series).
In these mixers, the main gear, accessory hub gear, planetary pinion gear (which makes the beater spin on its axis), planetary ring gear, and the drive pinion gear (which turns the main gear) are all metal. With maintenance and barring accident, they’ll last just about forever.
The worm follower gear is a “sacrificial” plastic gear which is designed to fail and protect the motor (and the other gears) if something jams the beater (or an attachment connected to the drive hub). This has always been the case, although the worm follower gears in very old mixers are made out of a different material.
In a mixer of sufficient age (or mileage), eventually the teeth on the worm follower will wear down enough that it starts to slip. However, a mixer needs to be *very* old or *very* hard used to get to that point. Still, it’s a good reason to inspect the plastic gear whenever you have the machine open for grease service (which you should do every couple of years, or more often if you use the mixer a lot or for heavy work).
In the early days, the metal gears in the drive train were machined from blanks; they’re beautifully crafted shiny bits of metal. At some point in the early 80s (late-series Hobart production) they went to a cast gear which isn’t nearly as nice but has no significant practical effect on the quality or performance of the mixer. But I digress.
The “Pro” series mixers use a different setup, in which the motor is a completely enclosed unit within the body of the mixer, and the motor shaft turns an enclosed transmission. The gears in the transmission are metal as well; and the worm gear follower is made of brass. This allows the Pro series mixers to handle heavier loads; but the brass gear is the “mechanical fuse”, and it too will wear and can fail.
(Side note: using a metal worm follower does not make the Pro series more reliable. The worm shaft can separate from its drive gear; and the upper center shaft bearing is pressed into the transmission housing and frequently drops out of its socket and slides down the center shaft to land on top of the accessory drive gear.)
Early generations of the Pro series used a plastic housing for the transmission, which was prone to failure. There was a recall for this; and modern production (after 2005 or so) uses a metal housing for the transmission. If you have a Pro series that old, it’s due for maintenance anyway, and at that time you can replace the transmission housing with the upgraded part.