torque calculation for electric motor selection

[ME.Gio]

Good morning to all,
I introduce myself, they're young, master student of mechanical engineering. I'm starting an internship and I have the first design challenges. in particular dimensioning and approach to problems.
Specifically what I have to do and so I am here to ask you a hand is the estimate of the "resistant" torque to be won by a mixer and then the subsequent sizing of the electric motor.
the problem is so composed: a mixer formed by scrapers (as in figure) is set in motion by a reducing motion and must turn about 120-150 rpm to perform its work within a r cylinder, the same scrapers push against the cylinder surfaces. These palettes need to rotate a viscous fluid to mix it.
for this reason, I thought to proceed in this way, considering initially all the forces and resistances to win:
- friction of the palettes on the cylinder;
-mass of the liquid to be put in motion that presses on the palette;
- inertia of the parts to be moved
How can I proceed?
thank you in advance of your time, waiting for your kind help I wish you a good day.
Thank you.
g

 

[meccanicamg]

hello and welcome to the forum.
This topic has already been discussed in several aspects on the forum. If you do a search, you'll find something.
the real problem is how to estimate friction of the fluid, or better to quantify the viscous friction of the mix.
Some users have to do every day and probably have tabulated standards according to tests performed.
Let's see what they'll answer.

 

[ME.Gio]

Good morning, thank you for the prompt answer.
Yes, exactly, as I chose the title I saw similar suggested topics on which I was fluttered. However, I have not yet found something that helps me, I will continue to look.
I found some posts where the notes were recommended on the mtu, being also I of the polytechnic I have the notes but I can not model the problem. I have a motor, a reducer and then a user who has rotating parts and a fluid to move, it seems so trivial that I am going crazy.

As for viscose force I would need an opinion because the fluid is not properly Newtonian but for simplicity I could consider it such. Moreover, as I imagined the problem the palette shifts the fluid at a constant speed after a certain time, so I do not have a speed gradient and therefore the viscose force, which in the notes that I have is df=ηs*(dv/dy), would be 0, condidering that the derivative is nothing. But maybe I have to consider that I have to win the friction since the speed is 0 to when it comes to regime.
I also tried to make a diagram of the forces, but compared to the typical example of the sphere that moves in a fluid here I can not schematize it. I'll still be right.
you say that the viscous friction is more influential than the inertias? (We talk about a 5lt of liquid to be put in motion)

I thank you again for your time and waiting for some other reply if you have to recommend a topic or a specific keyword to find some interesting threads on the forum I gladly accept.

Thank you.

 

[meccanicamg]

the problem arises in the first phase of start-up. therefore the transient requires a lot of couple. at regime you no longer have inertias for sure, being at constant speed.
the problem of the viscous fluid is that it crosses it with the blades. the larger and not ergonomic and the more you struggle to mix.

 

[TETRASTORE]

undoubtedly the agitator is one of the most complex applications influenced by numerous construction and product variants.
at the indicative level you can use the document: introduction to the selection of agitators.
if you want to deepen the themes of this sector, I suggest this thesis (unfortunately some formulas are not represented correctly).

 

[ME.Gio]

undoubtedly the agitator is one of the most complex applications influenced by numerous construction and product variants.
at the indicative level you can use the document: introduction to the selection of agitators.
if you want to deepen the themes of this sector, I suggest this thesis (unfortunately some formulas are not represented correctly).

Thanks for the snacks!
In these last days of research I came across the agitators myself and I believe it is really one of the most similar cases to mine.
For now I'm going to give up material on agitators to study and understand how I can apply it to my case.

at the moment I am firm in trying to evaluate the viscous friction of the fluid that presses on the "balls" in the correct way. for now I find only the example of a sphere moving in a fluid but always under the action of gravity! here is a little different.
I found exercises where they face the calculation of the pair of a cylinder that must turn immersed in a viscous fluid. But I'm still far from my model. there placed diagram and calculations, however, they are considered a small layer of fluid in which the disk acts, laminar flow and Newtonian. This possible contribution, more agile and inertia to win in the start stage should allow me to estimate the total torque needed, right?

while still thank you and good day.