Roger Hess, a sales engineer at Maxon Motors, illustrates the theory and operation of Maxon’s spindle drive. 
The drive matches a brush or brushless motor with a gearhead that has incorporated two thrust bearings, allowing it to handle higher linear loads.

http://www.youtube.com/Electromate#p/c/EE70E9534221DDB4/0/N6jquNbmGVs
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Maxon’s Spindle Drive Linear Actuator Product Family
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