Zoom lenses with manual settings are adjusted to change their focal length using either a rotating ring or a tube. To allow the lens to extend or retract, the moving parts slide along plain bearings in guide slots. 

**These plain bearings**

are often made of plastic, which can break after decades of use. The broken pieces can then get stuck in the guide slots and block the zoom mechanism. 

**The only solution is**

to disassemble the zoom lens and replace the plain bearings. To do this, their metal retaining screws must be unscrewed. If force is applied, these screws can also break, which complicates the repair. 

**Unfortunately**

there is no standard size for plain bearings, meaning that different sizes are used in zoom lenses. Sometimes, however, you get lucky, and plain bearings from other zoom lenses can be used, as is the case here:

[Tokina RMC 25-50/4: Zoom blockage fixed, plain bearings replaced, helicoids greased](https://www.reddit.com/r/AnalogCommunity/comments/1p118az/tokina_rmc_25504_zoom_blockage_fixed_plain/)

**Buying used zoom lenses**

I therefore recommend checking that the zoom mechanism runs smoothly when buying used. 

Jerky or stuttering movements indicate that the plain bearings are already damaged. However, the problem can also arise suddenly without warning. Heavily used zoom lenses are particularly prone to this wear and tear.

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All information provided without guarantee and use at your own risk.

Broken plain bearings: When a zoom lens suddenly stops moving