Hub Motor / Direct Drive Axle Stress Safety Discussion

[hummieee]

@hummieee

if you have a force at the wheel the same amount of force is applied to the hanger,somewhere. It’s the same force applied to hanger regardless and adding a gearing isn’t decreasing it.

that’s all that matters

[professor_shartsis]

@hummieee

that’s all that matters

@professor_shartsis

total force at wheel edge (pounds force thrust, 84mm tire diameter): 81.4lbs force = 363.57n * 0.224 pound force per newton

total force at axle (pounds force): 855lbs force = 3817.5n * 0.224 pound force per newton

u saying the force on the axle doesn’t matter?

[hummieee]

@professor_shartsis

the length of the “lever” is often different (shorter) with direct drives, which can multiply the force.

@professor_shartsis

the same amount of force can be generated at the edge of the wheel while different amounts of stresses and forces are developed at different parts of the drivetrain depending on the leverage at that point.

im saying what youre saying isn’t right. regardless of what drive system it is, whatever force is at the road the same force is applied to that hanger. its that simple. forget the math. aluminum hangers where motor mounts attach aren’t ripping off the aluminum these days.

[professor_shartsis]

@hummieee

im saying what youre saying isn’t right. regardless of what drive system it is, whatever force is at the road the same force is applied to that hanger. its that simple. forget the math

well if a stator is mounted by 4 bolts at the edge of the motor (as in a gear drive) versus a single bolt in the middle (some hub motor designs), then it is a different amount of leverage and force on the motor mounts, even if the motor produces the same torque. much more leverage and force is applied to the motor mounts in the latter scenario.

[professor_shartsis]

if anyone disagrees, prove it, just remove 3 out of 4 of your motor mounting bolts, and then put the remaining one close to the center of the stator, and secure it with glue and see what happens…

[hummieee]

every motor has all the torque going through a motor shaft. a single shaft in the center have you seen the grub screws or keyways on high torque outrunner motors? they’re tiny. if you find another pic of an axle that broke that’s relevant and has happened not 3 years ago…then we’d have a point. youre talking about a non-existent thing. no one is running torque through 8mm axles that have a large load on them, way larger than 50lbs when you hit a rock at 30mph, …theyre running through like 5mm motor shafts or something.

[professor_shartsis]

@hummieee

every motor has all the torque going through a motor shaft.

then if its true motors don’t need mounting bolts, and there aren’t reactions for every action as well.

also because of the low kv (75kv) you use for your motors, they generate much more torque per amp than a 190kv motor, but spin slower which is appropiate for direct drive.

[hummieee]

you were saying motor mounts don’t experience the same force on the hanger as direct drive that’s what I disagreed with. a thin shaft will have more stress than a big shaft.

[professor_shartsis]

@hummieee

the force at the wheel is the same as on the hanger regardless of the kv.

the torque is the same in newton meters, but since the radius is different at the wheel edge and at the axle, the force is different. you appear to be confusing force with torque.

[dani]

Happy afternoon everyone

If you remember just one thing from my reply, let it be this:
Loads by themselves have no meaning, the resulting stresses from loads have.

Elevated loads don’t automatically translate to larger stresses or structure failure, to make conclusions first calculate the stresses.

@professor_shartsis

total torque: 15.24nm

For 12mm axle, the stresses resulting from this moment will be ~45MPa, which is less than 10% of yield stress of any nice axle steel (or even Titanium)

@torqueboards

I don’t see how the torque is applied directly to the axle considering that the motor sits on multiple bearings. That would be a horrible way to try and apply torque to an axle by using bearings.

Exactly, this isn’t even go entirely to the hanger/axle.
To make it closer, you can degrease your bearings and load them with sand and shitloads of red loctite, then test your torque by all means.

I don’t really sure what was the purpose of this thread to begin with, but any serious consideration of this topic have to include stresses and not just loads.

PS
I asked the OP nicely to stop this “elevated load” apocalypse of hub motors in another thread, but I guess he didn’t listen.

PPS
I can argue that standard motors/mounts apply larger loads on the hanger than hubs. The hangers see additional bending moment.

PPPS
I want to start a thread discussing loads and fatigue loads on hangers to demistify the issue and to supply some kind of design criteria for new hangers/motors/whatever. (But this thread was out earlier)

Happy Sunday,
Dani

[professor_shartsis]

@dani

I want to start a thread discussing loads and fatigue loads on hangers to demistify the issue and to supply some kind of design criteria for new hangers/motors/whatever. (But this thread was out earlier)

I wish you would simply post the information here since we are already having the conversation. In your opinion, what minimum design features are necessary to prevent all future wheel separations with direct drive?

[torqueboards]

First get your definition right as far as hub motor and direct drive lol. They are designed and setup differently.

[mishrasubhransu]

@professor_shartsis please use some annotated figures to explain your case, otherwise whatever you write on this forum will generally be ignored.

I didn’t go into the details discussed above, but if the stator is directly mounted on the axle, then axial stresses beyond what the material is able to handle will lead to failure. And like @dani said, the torque doesn’t matter but the stress. If you use a larger dia axle, the stresses go down.

In case of direct drive( I am talking about eLofty) the stator applies torque on the 14 mm square shaft at the very back using a square hole.

eLofty Direct Drive Discussions-read before buying

Finally received the replacement axle to fix my DD. While I’m at it, I opened up the motor as much as I’m comfortable with. I think this is as much as anyone has disassembled this Lofty DirectDrive so I will document all the parts for your future maintenance. Replacement Axle. Not sure if there’s a difference. Kenna told me the replacement is stronger but she still haven’t tell me how it’s stronger yet. [received_341255030148049] [IMG_20190807_153748] The hanger: 14mm with a squared end fo…

[professor_shartsis]

@mishrasubhransu i think you went over them once in a different thread:

@mishrasubhransu

I went through the equations. They seem right. I have 3 questions though.

1. Under what assumptions is motor_current == battery_current_per_motor/duty
2. Is KV = 1/KT always true?
3. L v^3 +v K = J has three solutions. How did you choose the right one?

[mishrasubhransu]

Irrelevant

[malJohann]

@professor_shartsis

it doesn’t apply any torque to the truck or axle, what does it apply it to then?

I would think an acceleration force, in the direction of movement.

[torqueboards]

他妈的你在说什么

[Sebi]

I’m just disappointed there were no animated graphs.

[Bobby]

[Jacobee]

That’s what I thought reading when I was through this. And in the case the acceleration isn’t moving the board forward because of say a wall I’d assume the wheels would loose traction and spin before the force would be great enough to bend or break the axel. Of course I have no idea about any of the math on this and am no expert on physics but that’s just what made sense to me.