My advice is first measure how much torque and speed you need - these are fundamental to
choosing a motor system. Measure torque in the most adverse conditions then double the value.
This is a starting point.
Power = angular velocity x torque
So once you know the torque needed you can get an idea of the power needed too. You might
have to compromise on speed (remember gearing trades torque for speed, but lots of gears
means lots of losses too).
isbora:
First of all speed is not important for me. Fast or slow not important.
Wrong thinking - you need to ask what actual speed you need - this is a number
you need to work out, "fast", "slow" are meaningless in the context, you need to
know how slow you are prepared to go. 10rpm? 1rpm? 0.0001 rpm?
"how much torque" is very important, but how can I measure it. It soo hard for me, I trial and error.
You need to measure a force and a distance - not so hard.
Read up about estimating torque, there are many ways to do it.
I need 20 cm dig pit using with iron bracket. In this case needed torque depend soil type.
Yes, that's why you have to measure the torque in the most adverse conditions.
Don't underestimate the amount of power required to dig a hole in the ground. When you put your foot on a shovel and give it a shove you are applying more than your body weight down.
Now take the same shovel and slowly put your body weight on it and it might just sit there on the surface not even cutting through the sod. Going straight down is not the best approach.
All the torque in the world is not going to help unless you have some heavy weight behind it.
An excavator takes this force and translates it horizontally mostly. Digging with bucket curl. The weight of the machine and the friction between the tracks and the ground allow the bucket to curl through the ground. The machine is the anchor. A 15 horsepower excavator (very small) needs about 2 tons weight to be useful.
You will probably find that even a small machine might need few hundred pounds of weight to be able to dig properly. All depends on the soil though.