Livewire Unicycles has a 'technical guide on how to install square taper cranks' (http://tinyurl.com/blcbl). It explains a proven method for getting square taper cranks tight so they won't come loose. He says to go to 40 foot-pounds. I don't go above 35 foot-pounds.
The secret is the torque wrench and the use of Loctite. That combination will get the cranks tight and they will stay tight. You can get a suitable 'Craftsman torque wrench' (http://tinyurl.com/w8abn) for $25 or less if Sears is having a tool sale.
I have not had a crank come loose on me since using the torque wrench and Loctite method.
I've always used a rubber mallet to pound the cranks on before installing the nut (this is primarily on bikes). I just do it to seat the crank fully onto the hub's taper. Is this a bad idea? I've not needed loctite, either, but maybe that's due to luck.
I've always used a rubber mallet to pound the cranks on before installing the nut
One problem with using a hammer or mallet to do such an installation is that you're applying forces that are not parallel to the hub, possibly damaging the cranks (i.e. creating "slop" between crank and hub). This is why a press is the recommended method of installation.
Of course, using a rubber mallet is better than using a mallet, and using a mallet is better than nothing, but it's not the ideal solution to the problem.
Had that exact problem on my 36. Changing the cranks didn't help at all; riding about 5km was enough to bring the cranks from 'set on and torqued on hard at the LBS' to virtually falling off, on the second cranks, twice; the first one, it got loose at a gathering after a short bit of riding to get used to it, the cranks were set on and torqued on hard while I was there, and the crank fell off riding the 1k home. Me, UDC, and the LBS finally decided it was some flukish defect in the hub, UDC replaced the wheelset and i've had no issues since.
