# SureStep Stepper Motors – How to Choose and Use (Part II)

Welcome Back. In part one we saw it was pretty easy to calculate

the number of pulses needed, the step resolution and the motor speed. In this video we’ll learn how to calculate

the torque required and then use that and the speed to select a motor. We’ll also take a look at how to use that

information to so the same thing for other types of mechanisms. Here we go. Step 4: How much Torque do we need to move

the carriage on the linear slide? The total torque we need to worry about is

how much Torque it takes to keep things moving and how much torque it takes to accelerate

the load. And acceleration torque is inertia times the

change in speed over the change in time. What exactly is inertia? It’s just a measure of how much an object

resists being moved. And it’s solely dependent on the mass of

the object. For example, which is harder to get moving

– a clay brick or a Styrofoam brick? The clay brick of course. Why? Because it has more mass. And because it has more mass it takes more

force to get it moving. That force has to be large enough to overcome

what? The inertia of the object. So inertia is just a measure of how much an

object doesn’t want to be moved. And since we are rotating stuff here, we need

torque to get it moving. Get what moving? Well, for a linear slide: The motor has to

rotate, the gearbox has to rotate, the coupler has to rotate, the screw has to rotate, and

the carriage has to move. And all of those things have mass so they

all have inertia – which means they are all going to work against us when we try to

get things moving. So we need to sum all of those inertia’s

to get the total inertia. The gearbox impacts the inertia of everything

behind it by the square of the gear ratio. In our example we don’t have a gear box

so there is no inertia and the gear ratio is 1 to 1 so that gets rid of this term. If you look in the SureStep user manual, Appendix

C there’s a whole bunch of scary looking equations that show you how to calculate all

of this stuff. And while you CAN do all that math that if

you want to, there is a MUCH easier way to do it. For the motor inertia, we just go to the spec

sheet and see the inertia of the rotor is 0.56 oz.-inches squared. If you don’t know what motor you are using

yet, just leave it blank. You can add it in later once you have selected

one to make sure it doesn’t affect anything. And if we go to the lead screw spec sheet

– look at this! It’s already calculated the inertia of everything

associated with the linear slide AND it gives us a factor we multiply the payload mass by

to give us that inertia. Be careful here – is this the weight in

pounds? No, it’s the mass. And since force is mass times acceleration,

we just divide the payload weight by the acceleration – which is gravity in this case, 32.2 feet

per second per second, or since we need inches in this example that would be 386.4 inches

per second per second. This slide can handle 110 pounds, so let’s

divide that by the acceleration to get our mass number, multiply that by the handy inertia

factor and we now have the inertia for this payload and the system inertia of the carriage

and coupler. Let’s add those to our chart. If we had a gear box, same thing – just

get the inertia from that datasheet. We don’t have a gear box but if we did,

we would put it’s here and divide the downstream inertias by the square of the gear ratio. This is really important: We need for all

of our inertia values to be in the same units. For this demo we want ounce inch seconds squared

so when we’re done we can just read the answer right off the motor curves which are

in ounce inches of torque. The only problem is all of these units are

different and how in the world do you convert these units with inches squared to these units

with seconds squared – that doesn’t seem right does it? Here’s the trick. Go out on the web and search for an inertia

units converter. Put the units you have here, the units you

want here and there’s the answer. Easy. Here’s all of our inertia’s converted

to the correct units. The bottom line is we didn’t have to do

any math did we? AutomationDirect provides all the inertias

we need in the data sheets and we used an on line calculator to do the units conversions. So to get the acceleration torque, we just

sum those, multiply by the change in speed in revolutions per second – which we found

back in step one of this video series – and divide that by the change in speed in seconds

– which is our ramp time. To get rid of this revolutions term we just

multiply by 2 pi radians per revolution. We now know the Torque needed to overcome

acceleration inertia. The other Torque term is just the torque needed

to overcome friction, gravity and any other external influences. Gravity is important if you are lifting something

– we aren’t so we’ll set that to zero. Friction is if there is any additional drag

on the system – that was built into the numbers we got from the datasheet, so we can

set that to zero. The External Toruqe is to account for any

other additional drag that might be placed on the system. Maybe you’re pulling something out from

a stack and the weight of the stuff above it is adding drag for example – we don’t

have any of that so we’ll set that one to zero. So we just looked up the inertia numbers in

the datasheets, summed them, and then multiplied by the change in speed and divided by the

change in time. That’s it. We now have the torque required by the motor

and the max speed, so we can flip over to the motor curves and see how we stack up. Our max level is at 720 RPM and .35 oz-in

of torque. As you can see, we are not even coming close

to stressing this motor – even with the full 110 lbs of load on the slide! And that’s not really a surprise because

all of the weight is being supported by the slide – not the motor – and the 0.2 pitch

screw has a lot of mechanical advantage. Which also explains why we didn’t need a

gear box for this application. For applications where more torque IS required,

we recommend that you leave yourself 50% head room on these charts – treat these curves

as the absolute maximum that you do not want to exceed. What if you wanted to use belts and pulleys

instead of a lead screw. Well, it’s the same thing right? You just gather up the inertias calculate

the torque and lookup which motor you need to do the job. The only difference is you’re not going

to find a lot of the inertia values for pulleys and indexing tables, so you will have to fall

back to the equations in the user manual to calculate those. The good news is there are step by step examples

in the user manual showing you exactly how to do it for all three – linear slides,

belts and pulleys and indexing tables. By the way, did you know that AutomationDirect

has a free tool that helps you with all of this? It’s the Visual Sizer and it’s a free

download from here. It’s intended for servo systems, but it

works perfectly for Steppers too. You just build your system, enter each devices

specs, setup your velocity profile, specify your motor and you instantly see how much

torque you will need. And it even handles S-Curves and triangular

and trapezoidal profiles. You can then view your system performance. Here’s what the motor is capable of peak

and rated and what your system requires, peak and RMS. If you do a lot of this kind of stuff it’s

worth spending the time to learn how to use this really cool tool. Regardless of how you calculate the inertia,

please remember that these calculated numbers are just a good starting point. You still need to apply some experience and

common sense. So, how close are these calculated numbers

to the actual measured numbers? Well, check out the next video where we compare

these calculated numbers with real live results …

If you need any help with selecting an AutomationDirect Stepper Motor please contact AutomationDirect’s

free, award winning support team during regular business hours – they will be happy to help. And don’t forget the forums. There are lots of folks there that love to

share their years of experience. Just don’t post and questions directed at

AutomationDirect’s support staff there, they don’t monitor the forums on a regular

basis.

## 12 Replies to “SureStep Stepper Motors – How to Choose and Use (Part II)”

Great video. Thanks

could you please share the link to download the Simulation tool

You made some great videos! Thanks!

Brilliant video very useful!!

Do you have PDFs of the slides for download?

Where can I have the user manual? Thanks

awesome info, thanks a lot

Could you tell me how to find out the torque inertia required to rotate the fly wheel

Exactly what I was looking after thanks

If I wanted to lift an object against gravity, how do I find Tgravity?

brilliant video…it helps me a lot to understand….thanks

Sir can i get your email..??