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Most of the joints in the assembly consist of a single bearing, 1/4" machine screw, and 1/4" locknut, with washers if necessary. This joint design allows the bolt (the joint pin) to freely rotate while still preventing the pin from falling out. The bearing reduces friction even further, but it is not strictly necessary - the hole in the second link could simply be made smaller.
The main tower rotation uses a 4-inch turntable with a built-in ball bearing, purchased from McMaster. Because of the weight on this joint, it was critical that it have low friction, which the ball bearings provided.