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Linear rails & Preload
What is preload in a linear rail?
Preload in a linear rail refers to the elimination of internal clearance between the rail and the carriage. This is achieved by using ball bearings or roller bearings that are slightly larger than the distance between the raceways of the linear guide and the rail.
By eliminating this clearance, preload increases the rigidity of the system and reduces deflection when external forces are applied (you don't want your carriage to "wobble" on the rail). This makes the linear guide more stable and precise, which is crucial for applications requiring high accuracy and minimal vibration.
Linear rails can come in different levels of pre-load (typically indicated on the carriage of the rail with a "Zx" or "Vx" label. ZC/VC meaning "clearance", Z0/V0 meaning "light preload", Z1/V1 "medium pre-load" and Z2/V2 "high pre-load".
Source: kiwimotion.co.uk
Typically linear rails for 3D printer are delivered with V1/Z1 pre-load. That means the carriage will resist movement on the rail slightly (the rail doesn't feel as "smooth" as an unloaded carriage. You can test this by holding the rail vertically. The carriage should not just not move (or at least barely move) under its own weight.
Rails showing this behaviour are ok and not defective! You don't want a "smooth" carriage just flying over the rail, as this will increase the deflection of the carriage on the rail and will lead to inaccuracies in movements guided by the rail.
A good explanation can be found here.
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