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I am designing a gearbox and wanted to know whether the key on the shaft and gear must fit exactly with the bore in the gear and keyway in the shaft.

Ex. If there is a key that is 3x3 mm, and does the shaft have to have a 1.5mm keyway and must the bore in the gear be 1.5mm? This is with respect to the key face not length, but also should the length be spec'd so that the measurements for the key and key way are the same?

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There are recommended fits for keyed shafts and hubs in Machinery's Handbook as well as the actual keyway dimensions but you need to know your specific application for sizing. If you're replacing a part with a worn keyway, you may want to go to an oversized key and keyway to accomodate any clearance that has been worn. If the part is going to be disassembled frequently you'll want more clearance in the keyway.

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As a machine builder, we purchase oversize key stock and always hand grind them to exact fits for the application, so from our standpoint, we don't care how the key is called out on the drawing. Were simply given the parts and make it work as needed for the application.

As the machinist, we care a little bit more. The Machinist hand book has charts that call out exact dimensions for milling straight female key slots on shafts. Most of the time the other part is broached so dimensions are not super critical if you have the correct shims/bushing for the particular broach. But if your slotting that keyway, then you'll need the charts.

As the engineer, we should care the most, but usually we don't. Here's where having a solid support group of machinists and builders helps. If you don't, then you'll have to call out exact dimensions WITH tolerances for the shaft slot, and part slot.

So to answer your question, "yes a 3mm key would theoretically sit half way inside both parts", but there is a little more to that. The machinery handbook is your bible.

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Some gears are made as part of the shaft which makes them stronger, others need to rotate independent of the shaft they are on at times and are connected by intermediate gears.

The size (length) and depth of the key has to be sufficient for the force that has to be transmitted. Not only that force which might be easy to calculate but also any shock loads as most breakages tend to be from shock loading.

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