Lead Screw or Ball Screw – Which Should I Choose?

Lots of misconceptions out there about the type of screw to use to drive a linear axis.  And it’s only going to get worse as more and more companies outsource such decisions.  Steinmeyer manufactures both lead and ball screws so consider these comments as unbiased.  In fairly basic terms, lead screws are used when linear speeds are slow (slower than 50 mm/sec), loads are light (i.e. the automated machine axis is on the small side) and cost is the primary concern.  Ball screws will always be more expensive – why?  That’s easy – there are more parts.  A lead screw is merely a threaded shaft and simple nut.  The precision of such a device is based on the grind accuracy for the shaft and the resultant play when the nut is installed.  And there will always be play if the lead screw only has a single nut – otherwise there must be an interence fit which means very high friction.   Steinmeyer lead screws can be provided with play down to several microns but to completely eliminate play in a lead screw requires two nuts, with a spring in between.  Why the speed limitation?  That’s due to the friction which limits the efficiency to at best about 50%.  Compare that to a ball screw, which achieves efficiencies approaching and exceeding 90%.  But this high efficiency also means that ball screws can be back driven – which may or may not be desirable.  Lead screws typically cannot be back driven.  Ball screws are more susceptible than lead screws to contamination – however there are quite effective wipers available, especially from Steinmeyer.  Lead screws require less lubrication – some operate quite well without any oil or grease whatsoever.  Shaft manufacturing is comparable on both lead screws and ball screws – the thread can be rolled, whirled  or ground.  As for material, ball screw and lead screw shafts are typically manufactured from either tool steel or stainless steel.  Lead screw nuts are available in plastic material (hence the low cost) or bronze or similar softer materials with low coefficient of friction, while ball screw nuts, and the balls inside, require the same hardness as the shaft (generally at least HRC 58).  Finally, another good thing about lead screws – you can spin the nut off the shaft quite easily – and then re-install.  It’s a lot harder with ball screws – and without a proper tool, spinning the nut off the shaft results in a lot of balls on the floor and a very unhappy customer!

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