How to Replace the Planetary Gears on a KitchenAid Mixer
Back when Hobart owned KitchenAid and made solid-state stand mixers with numbers like K5SS, K45SS, and KSM90, they named the part of the mixer that turns the beater the "Planetary Gears. It is the major gear mechanism of KitchenAid stand mixers, and it's quite easy to fix yourself.

Taking the KitchenAid Stand Mixer Apart

1. Wrap the mixer's wall plug in a plastic bag and secure it with a rubber band. This is a safety measure to ensure that no grease gets on the plug.
2. Remover the drip cup. The drip cup is the silver ring that encircles the planetary housing. Use a small flat screwdriver on the top lip of the drip cup to loosen and remove it.
3. Lay the mixer on its side. It is a good idea to put a towel or cloth underneath the mixer.Remove the groove pin that holds the planetary gear assembly to the center shaft. Use a 5/32" punch to do this. Removing the groove pin allows the planetary assembly cover to be removed.
4. Pry the planetary gear cover down and off the shaft using two flat screwdrivers.Unscrew and remove the five screws on the perimeter of the gear assembly. These screws hold the internal gear assembly to the gear case.
5. Unscrew and remove the four large screws that secure the base to the top portion of the mixer. Use a large Philips screwdriver.
6. Remove the attachment hub bevel gear from the gear case and motor housing. This can be done by pulling it out of the attachment hub.
7. Clean out the gear case. Remove any and all old grease from the gear case. Pack the gear case with six ounces of new grease before reassembling the mixer.
8. Remove the three Phillips head screws from the worm gear assembly that secure it to the top housing. Lift out the worm gear assembly.
9. Check the worm gear, the shaft and pinion. The shaft and pinion has 11 teeth on the K45SS; on all other models, it has ten teeth.Drive out the groove pin with a punch. This allows the gears to be replaced.
10. Reassemble the gear assembly, being sure to replace the fiber washers where they originally were located.Reassemble the worm gear, shaft and pinion.Place the worm gear assembly and bearing bracket on the gear case bottom cover, making sure the two dowels fit into base of the bearing bracket. Securely tighten the three screws that hold the bearing bracket.
11. Clean the gasket cover of the gear case. Replace the gasket cover by securing it over the dowel and pressing the gasket down.Reattach the top and bottom mixer sections, being careful to line up the shaft in the upper gear housing. Do not force the pieces together; they should assemble easily.
12. Replace the internal cycloidal gear by prying it loose from the gear case cover. Replace it by aligning notches with the holes in the gear case, and screwing in the five screws to secure it.Fill the teeth of the internal gear with new grease.Reattach the top and bottom mixer sections by using the four large Phillips screws.
13. Reassemble the planetary gear housing, being sure to lightly grease the shaft and all gears. Attach the reassembled planetary housing to the mixer by driving in the groove pin that secures it and replacing the drip cup on the Planetary Gears case.