Determining Keyway Length

Just a bit longer than the key!

Key should normally engage the whole length of the hub. So length of keyway should be just a bit more than the length of hub + the corner radiuses (x2)

Length of the hub is decided by the torque to be transmitted.

You may also refer ANSI/AGMA 9002 for some guidance on taper hubs. Though that as usual is not the final say on the subject.

I think what you might be asking is a standard measurement for the key Din 6885 specifies for 150mm to 170mm dia shaft a 40mm x 22mm key should be used

But what about the length of the key, which is the OP's question? [See #4.]

Do some homework.

Why troubling CR4 experts, without refering any handbook or design books?

My slide chart says same as 'Grochy' 40mm wide x 22mm high for diameter range of 150 to 170mm. Surprisingly the same chart does not show the length part.

However, for the next lower diameter range of 130 to 150mm (Key size 36x20mm) the length is indicated as Minimum 100mm and Maximum 400mm. Lower than that for diameters between 110 and 130 (Key size 32x18mm), the lengths are Minimum-90mm and Max-360mm.

So you can take a cue from this and make your assumptions. I assume that Minimum length is >100 and also max is not less than 400mm.

Capri - the lengths listed are probably shortest length that does not twist, immediately shear, or has a tendency to NOT immediately damage the shaft in some form or another. The longest key listed is probably the length that results in "standard" (non-hardened) shaft damage should the torque rating exceed that key's ability to shear. These are length recommendations at the extremes, and should be taken with a grain of salt (and engineering know-how). This is purely for your informational purposes, NOT an attempt at chastisement or anything. Good chart, but it needs to be figured for the length, rather than arbitrarily pick a number between these points.

The actual answer to the original question is Tornado's up above. You have to figure for the load if you expect the key to do any protection of the system. A key should shear off before anything else gets damaged. A maintenance guy can replace a key in 15 minutes (or much less) for about 40 cents and his labor cost, whereas a sprocket, gear, or pulley may be two days out of hand and hours of labor to track it down, expedite it, and get it installed. In the mean time the plant/equipment/vehicle is down and out, making no one any profit, and possibly putting the owner out of business.

If you don't care about any of the above, the key should be the length of whatever it is tying together and no longer. Extra length beyond the hub is not only extremely dangerous (clothes/skin catcher), it also has no economic value, since it is just there attached to the shaft and nothing else.

That is what those of us who paid attention in our first year of mechanical engineering learned...