The torque (and stalling torque) of a servo motor

[Owen Ransen]

TL;DR Summary

How to interpret torque claims for servo motors which seem to me to be ridiculous.

I'm using a servo to lift a pen. The pen and a bit of mechanism weights 35g.

The servo I use to bring the pen up and down claims to have a torsional moment of 1.5kg/cm. Sometimes they call it torque in the specs. But surely that does not mean they are claiming that if I hung a 1.5kg weight a 1 centimeter arm then the servo would be able to move?

The tiny 9g servo is shown here:

https://www.amazon.com/dp/B01MXPTCHO/?tag=pfamazon01-20

My servos fail a lot (die) and I'm asking because I've been told that the pen is too heavy for the servo. But if the servo can do 1.5kg.cm sure it can do 0.035*2 kg.cm? What am I missing?

(I realize it should be in Newtons per meter, but servos seem to always be specified in kg.cm (or oz.in even!).)

 

[trurle]

Typical stepper motors capable of 0.15 N*m (1.5 kgf*cm) are typically in form factor NEMA42, weighing about 200g.
Servos are lighter, but it still should be realistically about 30g.

Your Kuman SG-90 is geared servo, giving it higher torque for small weight. Your main mistake is in mechanical setup. Your lever-like setup multiply required force by approximately (length of pen/(1.5*length of arm). Not sure if your balancing load works properly. Also, small geared servos are weak against high radial loads as in your setup. I would recommend to switch to larger, non-geared servo or stepper and have a flexible coupling at output shaft like below:
https://www.amazon.com/dp/B00SMO7B8A/?tag=pfamazon01-20

 

[Owen Ransen]

Typical stepper motors capable of 0.15 N*m (1.5 kgf*cm) are typically in form factor NEMA42, weighing about 200g.
Servos are lighter, but it still should be realistically about 30g.

Your Kuman SG-90 is geared servo, giving it higher torque for small weight. Your main mistake is in mechanical setup. Your lever-like setup multiply required force by approximately (length of pen/(1.5*length of arm). Not sure if your balancing load works properly. Also, small geared servos are weak against high radial loads as in your setup. I would recommend to switch to larger, non-geared servo or stepper and have a flexible coupling at output shaft like below:
https://www.amazon.com/dp/B00SMO7B8A/?tag=pfamazon01-20

Thanks, you've given me something to think about.

However, does that mean that the specs of the servo on amazon I posted are completely wrong?

 

[Owen Ransen]

The photo does not show well the arrangement, I don't think the length of the pen comes into, here's a clearer image:

 

[Tom.G]

Something to consider:
On commercially available Pen Plotters, the pen lift is generally a simple electromagnet, with gravity supplying the downward force.

 

[Owen Ransen]

Thanks for the idea, but I moved to this system because simply dropping the pen splatters the ink and damages the nib (on this home made system).

 

[Dullard]

Did you read the reviews in the Amazon link? I don't think that your problem is Physics.

 

[Owen Ransen]

I did read the reviews but took a chance. Some worked well, but not for very long. Then I got one which cost 10x the price and it did not last much longer than the best cheap one.

 

[Tom.G]

The comment about pens being destroyed when using gravity for pen-down, along with your sketch, makes me suspect you are using a fountain pen or a straight pen. If a fountain pen or similar is required for some reason, try adding a small dashpot (essentially an air piston shock absorber) to control the downward speed.

Alternative approaches would be use a felt tip-style pen (actually nylon fiber), or a drafting pen. Drafting Pen nibs (also called Technical Pens) are a hollow tube with a thin wire protruding from the end. When they contact the writing surface, the wire is pushed in and opens a valve to allow ink flow. They can be found in drafting supply stores, or sometimes in art supply stores.

Since it seems you are using the pen on a plotter, you may need to use a pen specifically designe for plotter use Plotter pens have a stronger nib to resist bending from the higher lateral force encountered. See if you can find a style of plotter replacement pen that can be adapted to your pen holder.

Cheers,
Tom

 

[Owen Ransen]

Thanks Tom for the suggestions.

The actual system works very very well, and is very flexible about speed, angle etc. It is just the motors which break. What I'm going to do now is rejig mechanism to reduce the torque by 50% by using a shorter arm...