The steps you must take for motor sizing:
(1) determine open / close cycle time & distance moved
(2) leadscrew pitch versus motor RPM to achieve the open/close cycle time
(3) strength calculation of the leadscrew determines diameter
(4) motion profile (acceleration, traverse speed, deceleration) to achieve the cycle time
(5) identify all torques required of the motor during motion (involves rotational mass moment of inertia of all rotary masses, torques necessary to translate masses)
(6) sum all torques to get peak torque, apply suitable factor of safety for friction/stiction, dirt, gummy bearings, duty cycle & heating issues, and a 1001 other things
(7) select your motor

Most vendors of linear leadscrew stages will have an Engineering Section of their catalog that is very useful. You can also websearch & download the Smart Motion Cheat Sheet for a summary of equations used for sizing exercises like this.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

Thanks for the tip. I've found good worked examples that have helped.
One of my questions regards ball screws. I've been looking on the THK website, I'm not sure about the length of the ball screw itself. I thought they came in standard lengths. However when I go to the download section of the website I get to specify the length myself. Doe this mean that the ball screw is made to order to the length I require? Or do I have to purchase a longer screw and have one end machined down to fit in the bearing? Or do I have to choose a standard length that is longer than I need? These questions might be specific to THK but any help would be appreciated.

Ballscrews are typically custom parts, so you can specify what you want, and pay for it.

They get cheaper in mass production, so you should consider buying a complete standard actuator assembly with a ball screw in it.
... or maybe a semicustom derivative from the actuator manufacturer.

Mike Halloran
Pembroke Pines, FL, USA

In many cases the manufacturing process for rolled and ground threads on the screws is semi-continuous: stock lengths will be long. They are cut to order, and also then ends can typically be machined to suit with flats, keyways, threads,...many options. Depending on from whom you order. Or you can do it yourself. I once outfitted an entire assembly line conveyor with each section having adjustable widths using stepper-motor driven rolled ball screws. I purchased 8' stock pieces, cut them to length and machined ends on them in our tool room.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

Check out misumi.. They have TONS of linear components and allow you to specify the exact length and what end machining is required,etc...
They are one of my "go-to" suppliers for all things mechanical/power transmission.

Thanks for the tips. For the application in mind size and packaging will probably be a constraint so having a ball screw made to a custom size would be preferable.

Hello,
Thanks for the tips. I've found that THK include a very useful guide on how to select a ballscrew, this is in the back of the one of their ball screw catalogues and includes an easy to follow worked example.

One quick question I've got that I'm hoping people can answer.
If I were to change from a ball screw to a lead screw. What do I have to change when calculating the required torque?

I'm assuming it is the efficiency value, but I'd like someone to confirm.
I've calculated the torque needed for the external load (guide surface or external force) using:

T1=(Fa x lead) / (2pi x efficiency)

Fa being calculated by the mass of component multiplied by the coefficient of friction of the guide surface multiplied by gravity acceleration. This is for a load being moved in the horizontal plane.

The torque required for the acceleration is calculated by:

T2= Inertial moment x angular acceleration

The intertial moment is calculated by:

mass x (lead / 2pi)^2 + inertial moments of shaft, gears etc.

Am I correct in thinking that I only need to change the efficiency value (and change any other terms such as inertia of shaft and the lead) to change the formula from a ball screw to a lead screw?
In my case the torque for the external load is very small compared to the torque for accelation. As the efficiency term only appears in the torque for an external load, by changing from a 90% efficient ball screw to a 30% efficiency lead screw as an example, the effect is very small.
Thanks in advance.

Efficiency. That usually accounts for the difference in friction components.
And backlash if you are concerned about reciprocating positioning accuracy.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com