Torque Margin for a Lead Screw

[Form1011]

Hi and thanks for taking a look at my question.

I'm designing a lead screw mechanism and I'm seeking to show that I have sufficient torque margin (I know this is vague, but that's the crux of my question as you will soon see). I'm using splines on the lead nut to keep the nut fixed rotationally with respect to ground, so that the nut can only translate on the lead screw. Friction is produced on the splines/ground interface as the splines react the input torque, and therefore produce drag in the translation direction. This is overcome by by adding additional torque to the leadscrew.

So as you can see, there is a feedback loop here. Iterating a few times yields the necessary input torque, no issues there.

However, the question I have comes from the addition of margin in the system. There are 2 locations I can think to add it, after the feedback loop, or within it. I'll demonstrate here.

System 1: feedback out of the loop

[pre]
Desired linear force of the leadscrew ---> torque needed ---> margin ---> actuator capability
/ \ |
|--- drag --|
[/pre]

System 2: feedback in loop on drag (this system is the more conservative of the two)
[pre]
Desired linear force of the leadscrew ---> torque needed ---> margin ---> actuator capability
/ \ |
|--- drag ---------------------|
[/pre]

In words, I'd describe these systems as
1: I have torque margin over the worst case torque required to produce the required force
2: I have torque margin to overcome the leadscrew's needs and friction produced by the maximum actuator torque

Thank you in advance for any insight as to which method is preferable.

 

[drawoh]

Form1011,

You seem to have completely deformatted your post Each of your paragraphs is a single, long line. Did you use the code tag on everything?

Perhaps you should red-flat this post, and try again, without the formatting.

--
JHG

 

[Form1011]

Thanks! I've now applied the tags just at the diagrams.

 

[drawoh]

Form1011,

That is a huge improvement.

--
JHG

 

[drawoh]

Form1011,

Now, for a technical question...

What are you trying to do, specify a motor? A motor sees all your torque. It was no way of "knowing" which component is responsible for the torque. You need enough torque and power. You don't want to use excessive electric[ ](?) current and voltage, or an excessively big, expensive motor. You don't want the motor to break your lead screw. You don't want to mess up. What are your priorities?

--
JHG

 

[Form1011]

Thanks drawoh. I think this really boils down to a question of conventions and the appropriately way to determine torque margin. My ultimate purpose is to select an actuator (specifically the output planetary gearhead, since speed and power are not important here, only torque). I need the actuator to produce the torque needed to overcome losses and drive the lead screw, while still retaining proper margin on actuator output torque. The force with which the lead screw needs to push and pull is known, so that's my staring point. Then it's a matter of following the correct block diagram to size the actuator.

For my specific case, system 2 requires about 10% more torque at the actuator than does system 1, purely as a consequence of the margining style. This is because some of the torque in system 2 is lost while pushing against the spline friction on the lead nut in the presence of the full, margined actuator torque. In system 1, this spline friction is based only on the unmargined torque needed to drive the lead screw.

I agree that a motor sees all of the torque. What makes this confusing is that I'm trying to determine margin, so that's torque that ideally won't even be created. Once built, the friction in the mechanism and the lead screw torque needed to operate it are going to be what they're going to be regardless of how I define margin.

I appreciate your willingness to look at this!

 

[drawoh]

Form1011,

I see no point applying margin to one or two components of your drive. Your motor is driving the whole thing. Also, unless your frictions and other loads are very well understood, and controlled, I would apply a very large margin. Efficiency only matters if the motor runs continuously. If you need this thing to be lightweight and/or cheap, plan on lots of testing.

Is this an actuator with feedback? Read up on stepper motors. Usually, they are sized by rotor inertia, not torque. The next time I select a DC[ ]actuator, I will size that by rotor inertia. Make sure your lead screw is strong enough to take the resulting torque.

--
JHG

 

[Form1011]

Ok there's some great advice in there, thank you. I'm starting to think maybe just using a margin on the leadscrew force is enough, and then determining torque from there based on geometry, friction, etc. This will be a brushless motor with plenty of feedback control, but I can also see if stepper motors may be right for the job. And then test a lot.