i was always told not needed , this from somewhere else
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here is a major misunderstanding (by some) on this thread regarding the relative lateral motion of two solid surfaces in contact.
There are two main components and non-lubrication mostly affects just one.
Static ('stiction') is that between non-moving surfaces. Kinetic friction is that between moving surfaces.
Stiction (dry friction) arises from the interaction of surface features, known as asperities. Seemingly smooth surfaces such as a tow ball, are far from smooth. At a close to atomic scale, they are rough (they have 'asperities').
What is intended is that stiction locks the tow ball during minor sway forces - such that they are damped by the tow vehicle's tyres. It does so effectively. Lubricating destroys that intended damping.
The friction between a tow ball and its coupling (once moving) is close to zero. Greasing it makes no odds (it does not need it - the friction is negligable!) - that is why after many years there is next no wear. It will be less than 50 mm for the simple reason that a 50 mm peg does not fit into a 50 mm hole! Some clearance is required.
If you grease that ball you decrease its intended yaw damping.
AL KO has a stabilising version that uses sprung loaded friction pads (and some even grease those!).
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