40mA is a stress rating not an operational rating, do you understand the difference
Yes, I do. Didn't though it's so important. Thanks, won't do like this any more.
200-220mA without load. With load.. 1700mA.
So that is why your transistor is getting hot. Yes it is normal for a motor to draw more current when it is under load. The ultimate load is a load that is enough to stop the motor, then it draws what is known as the stall current.
Yes, I've already learnt about this stall current. But I still cannot understand something.. Firstly, the transistor is estimated on 6A. Lets assume even on 3A. Secondly, I've put 1.8V on PWM specially to reduce the base-emitter current, so that the collector-emitter current is also low. with 1.8V on pin, I have 20mA base-emitter current and 500mA (as my multimetr shows..) collector-emitter current. So with the motor loaded I only let 500mA run through the transistor.. Not 6A or even 3A. Where am I wrong? 
P.S.
1700mA is the current in a simple curcuit, not limited by transistors or other stuff.