Often on construction drawings or in articles I come across the term `fit`, usually something like `push fit`, unning fit or `press fit`. What does this mean?
Well the descriptions speak for themselves – a running fit will enable a pulley to rotate freely on a shaft, for example. A press fit needs considerable force, at least a vice but usually some sort of hydraulic or mechanical press will be needed. A shrink fit means that you heat up the pulley or wheel to expand it, drop it onto the shaft and let it cool down, contracting tightly onto the shaft. There are also some special fits, such as for ball bearings. There are special tables giving the tolerances, that is how much clearance or interference you need, to achieve these fits if the draughtsman hasn’t specified it.
