FORCE and TORQUE CALCULATION

[bretthydra]

Hi,
Not done this sort of things since college days.
Some input would be much appreciated.

Please refer to the attached .pdf. The diagram is a schematic drawing only.

In order to determine the weld size, I need to calculate the force (N) at the diameter of the welds (888mm).

The inner flange is driven by a 250 kW motor which has a rotational speed of 42 rpm.
Given that motor power Kw=(T*rpm )/9552.4 where T=Torque (N m).

 T=(kW*9552.4)/rpm  T=(250*9552.4)/42  T=56859.5 N m

I calculate that the force at the weld diameter to be:-

force at weld=56859.5*((888/2)/1000)

 force at weld=128062 N

I deduce that the welds will be in shear.

 

[SandCounter]

bretthydra, please check your kW to T*RPM conversion factor. I suspect you're off by a factor of 1000.

Also, please consider whether you have any thrust or acceleration in the application.

I used to count sand. Now I don't count at all.

 

[LittleInch]

No that what I get going back to basics. That is a powerful motor to only do 0.7 reV per sec.

If the view you show is a section the force on a circular weld or weld's will be more like a shear stress not a force.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jlnsol]

Shear stress yes. Over the thumb calc: if you take the 128062N as shear force it will act on a circular surface of app. 888*PI*8 = 22318 mm2 when the smallest weld width is 8 mm.
So shear stress is app. 5,7 N/mm2 which is a low value.

 

[bretthydra]

Thanks everyone,
As suggested by SANDCOUNTER, I am doubting the size of the force 128062N.
Take it a stage further from jlnsol:-
Circumference at the weld = 888*PI = 2789.7
Shear strength of weld material is 285N/mm2
Bearing in mind there are 2 welds.
So, area of weld required = 128062/285/2 = 224.7mm2
Width of weld = 224.7/2789.7 =0.08mm

All this of course is assuming that the motor is stalled by a force resisting rotation and no safety factors applied to the weld size.

 

[jlnsol]

bretthydra, the force you calculated is correct, no doubt about that.
omega = 2*PI*n/60 = 2*PI * 42/60 = 4,398 rad/s
power is 250.000 Watt
torque is P / omega = 250.000 / 4,398 = 56844 Nm
force is torque / radius = 56844/0,444= 128027 N = 128 kN

 

[LittleInch]

No. It's assuming that the 250kW motor is geared down to run at 42 rpm. A stalled motor torque might be a lot higher.

But yes you've discovered that welding is very strong.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SnTMan]

Aside from all the rest, you want to be looking at the throat dimension, not the leg.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[jlnsol]

Stalled motor torquw might be higher..how much higher do you want it? times 2, no problem, times 4, no problem.