Project Log 37: Calculating electric motors because why not.

A project log for DIY Human-sized Mech

Mechs are not viable, nor cheap, so I will try to design and build one alone anyway.

FulanoDetailFulanoDetail 03/25/2023 at 13:370 Comments
Saturday, 25/03/2023, 10:25


I kinda have a confession: I'm kinda procrastinating too much with this project, and I think I know why.

You know how much I tell in these project logs that Mechs and Exosuits aren't viable and all that stuff?

Well, they are cool as heck, but I'm not in the mood for building this thing right now... In fact, I'm not in the mood for anything right now, but that is more of a "me-problem".

I mean, if this isn't viable, then why build it in the first place? I kept trying to convince myself to just keep going no matter what, but right now I don't know anymore...

Also, you know that I said that "if I made a tank-treaded-mech, it wouldn't need as much power"?

I'm also starting to really doubt that.

I mean, a 10hp engine can make a vehicle reach around 80 kilometer per hour (or 50 MPH)... But that's the exactly same amount of horsepower need to make the exosuit work.

I could just have a clutch that changes between wheels on the feet and the hydraulic pump.

Of course, that depends a lot on the kind of vehicle, weight and horsepower.

I could find some motorized bycicles with 3 horsepower that can achieve 50 kph (or 30 mph), but can only carry 100 kg of weight including the driver.

But basically, If I only had the top half of the body, I would need 3 horsepower in order to pump 500 liters per minute at 8 bar of pressure, I know I wouldn't need to keep pumping this exactly value all day, but what is a machine if you don't use it constantly?

Plus 3 or 6 hp to move around, thus 6-9 hp in total, which varies between 4500 and 7500 watts.

A car/truck battery with 3000-4000 watts hour would only be able to fuel this thing for more or less 1 hour. And these cost around 1500 Reais (285 dollars). And I would need around 10 of those for more work time.

But I don't know anything that comes close to a tank-mech in the day-to-day life to compare these values to.

It is normal for a heavy duty machine work for just 1 to 2 hours before recharging?

It is common for a machine to just be useful for half a day?

I really don't know...

I don't know why, but I just feel boomed out these days...

I can't tell if all of this trouble I went through has any usefulness on the real world.

And that's the thing:
after investing so much money I could be spending on useful stuff for myself, why spend on this project that I may or may not never use?

It is like buying a toy for adults (not the +18 kind)...

I was thinking on kinda using the mech to help people around my town, collecting trash from the streets or just help people moving heavy weight around you know?

But reallistically speaking, it is like getting outside and trying to find someone in need to help. And of all things, someone that needs help with something that you know how to help.

I'm no medic, no engineer, no electrician, no plumber...

But even if I actually found people and used the bio-methane extracted from trees and/or trash to power the exosuit or mech, I would basically be chopping down half the city per day just to keep this crappy thing working...


I started this project log because I made a terrible math mistake and thought that a cheap brushed electric motor could lift 10 kg at 20 thousand rpm.

So I freaked out and was filled with hope thinking the mech was finally going to be real, but...

But in fact, the cheap electric motor could ift just 1 kg.

... And the final results were closer to the amount of energy used for the previous design. :/

And thus, I got stuck rewriting the entire project log. :|

So, I was avoiding using electric motors because of a simple reason:

I searched for "horsepower calculators", and every horsepower calculator, you need to input the RPM of the engine/electric motor and the torque.

The big brain me thought: "well, I doubt there is something wrong with this logic, the reduction gear takes too much time to rotate anyway, lol"

And inserted "180 rpm" (the speed in which the human body moves, 1/3 of a second) and the torque, around 300 kg, and thus, the calculator said that I would need around 565.5 watts.
And since I have 26 or more axis of rotation/muscles, I would need in total 14703 watts of power... Which is 20 horsepower in total.

The problem is that there is something called "gear reduction", and it doesn't take minutes to happen, it is instantaneous. :|

And thus I started my adventure on the online gear reduction calculators.

Meaning that if I take a common 200 watts brushed electric motor that can achieve 20,000 rpm, and use a reduction gear of around 0.009:1 (around 110 times reduction), thus, giving 180 rpm on the output...

And 222 kgfcm on the output.

Meaning that if I were to make the exosuit with this, I would need around 5200 watts in total, which is around 7 horsepower.

Of course, the bottom half needs to be 3 times stronger than the top half, so I would need around 2400 watts on the top half, which is around 12 electric motors of these and 8400 watts on the bottom half, giving 10800 watts in total.

Which is around 14-15 horsepower.

Well, I didn't calculate the stuff for a mech yet, but, I recalculated the hydraulic piston pump.

Assuming that I need a 116 kg torque at 50mm of distance (which is the "lever" in a piston/crankshaft travel) with a piston 100mm wide, I would need a torque of 500 kg.

And since our little 20,000 rpm, 1-2kgfcm torque and 200 watts electric motor will need a gear reduction of 0.02:1, which would give 40 rpm, which would be 31.44 liters per minute of flow rate at 8 bars of pressure...

And thus, I would need around 47 of these pumps and 9500 watts of power in total to pump 1500 liters per minute at 8 bars of pressure. Which is around 13 horsepower.

If I were to calculate that multistage centrifugal pump I talked about in previous project logs...

Well, every pump has 200 watts of power and 3600 rpm, which means I need 5 kgfcm of torque to drive those, since I have 10 of those stacked up, I would need 50 kgfcm per pump.

If I were to put just 20k rpm and 1kgfcm of torque in the gear reduction calculator, I would only get 80rpm and 50kgfcm on the output.

And thus, I would need 10 of those electric motors to achieve 10 kgfcm, increasing the power output fo 2000 watts.

And since I need 3-4 of those centrifugal pumps to achieve 1500 liters per minute at 8 bar of pressure, I would need 6000 to 8000 watts of power in total.

Which is around 8 to 10.6 horsepower in total.

Not much different from what I previously calculated for the exosuit.

I know I said that "I was looking for a 15hp combustion engine" although I only need 10hp.

Well, I "asked around" like I always do, and some people said I should have some few extra horsepower left to spare so I wouldn't be forcing the engine to its limit all the time.

Besides, I do plan to have some extra stuff attached to the engine, like a stator or whatever.

Anyway, now calculating the mech:

Well, if I wanted to achieve 3 tons of torque in order to lift 1 ton, I would need considerably more power than previously calculated.

I would need an electric motor with at least 500 watts of power (or 0.7 hp) in order to achieve 3000kg at 180 rpm, some electric motor with 180kgfcm of torque and 1800 rpm and a gear reduction of 10:1.

of course, assuming that I find some kind of electric motor that fits perfectly in these standards, even if it is just a tiny electric motor that rotates really fast, I would need around 6000 watts on the top half of the body and 21000 watts in the bottom half.

Meaning I would need 36 hp of potency to make this thing move.

Not much different from what I previously calculated on Project Log 35 I believe... 

PS: I talked something along the lines that those brushless drone electric motors use kilovolts of electricity back in the first project logs, but that information is incorrect.

When those electric motors have some number before "KV", it is actually somekind of measurement.

There are online calculators that asks how many KV and Amps an electric motor needs in order to achieve a certain amount of RPM.