Gear alignment is automatic, you just have to drag and drop a gear onto another with some overlap and the gears will be placed together nicely. There are three levels so you can have a small gear on the same shaft as a big gear and change speeds very easily.
The game is low point scoring. Score is based on the number of gears used to complete a level times the time it took. So the trick is to use as few gears as quickly as possible.
As an example, a motor turns at 3 RPM and one of the levels requires you to turn a target gear at over 100 RPM. It's not just a matter of stringing gears together, but to get ratios right to get things moving fast.
It works on phones and tablets (2.1+), though with the phone you will have to zoom in a bit more so your finger isn't blocking the gear you want to view.
Google Play Link: