Hi everybody,

Sadly I'm unfamiliar with lead screws and need some advice. I need a linear actuator that when free wheeling offers very little resistance.

Current parts:
NEMA 8 lead screw
Rod: 3.5mm dia, 1mm lead
Length of rod: 100mm
(Parts exactly as shown in the link above)

I have already ordered parts as a test, but they're still in shipping.

USE:
While the stepper motor is off/freewheeling, the user should be able to slide the lead screw back and forward quickly with very little resistance (ideally with one finger). Assume a high quality bearing at the end of the rod. Assume no other outside resistance in the mechanics.

From my research, ball screws provide more noise and higher resistance compared to lead screws - am I correct about this?

If I'm wrong and the parts above would provide medium to high resistance, do you see another scenario that uses a lead screw style linear actuator?

Thank you for your time.

 

You cannot push a lead screw actuator along without turning the screw.

 

Th

You cannot push a lead screw actuator along without turning the screw.

Thank you dendad!

 

As you approach the ability to back drive a lead screw you lesson the force the nut can produce. Lubricated high helix screws will back drive. Recirculating ball nuts will also and still provide force.

 

That is true! But there are all varieties of lead-screws with different pitches, and those that advance more than half a shaft diameter per revolution may actually be back driven.
AS for friction, BALL screws in good condition have much less friction than lead screws, but if they wear a bit certainly they can be noisy. But they still have much less friction because they are rolling, not sliding.
So while a plastic bearing fast lead-screw in a printer may be back driven because of it's fast lead, a finer adjustment lead-screw can not be back driven. Ball-screws with greater lead pitches may be back driven depending on the application.
But none of these move freely with the drive motor not driven, unless they have a very fast lead angle.

BUT if you want to have it be close to free-wheeling then I suggest a belt-type linear actuator.
But to select a linear actuator you need to know the required force , speed, acceleration, bearing load, and required accuracy. (I have had to specify them for a couple of machines.)
What is the application for this linear actuator? The application also has a great effect on the choice.

 

Thank you all for the incredibly helpful replies and taking the time to read.

@dendad - I misread your post sorry. Yes I understand the screw has to turn, I was curious if it required much force to turn the rod by moving the screw linearly.

@ulms - Thank you, very helpful

@MisterBill2 - This is very helpful.

This part of the application is essentially the same as a 100mm linear potentiometer.

I understand that the belt-type actuator would be good, however along with backdriving/freewheeling I am also applying load to the motor (at different times to the freewheeling of course). My concerns for a belt-type actuator were both wear over time and accuracy.

Normal motorised linear potentiometers have a DC motor and are belt driven - but in this situation a stepper motor with lead/ball screw of some sort seemed the best solution (except for the hurdle I'm currently facing).

Provided it can freewheel freely, a rack and pinion was another idea - where the rack stays fixed and the pinion moves - though if I'm not mistaken that sounds more complex as it requires the motor to be moving with the pinion. Probably not the best idea for a small device.

Any advice is gladly received!

 

Just for an experiment. pick up an old printer or scanner. These are often tossed out. Then you will have a linear actuator, that is, the print head mechanism.

Some printer mechanisms come with an optical sensor reading a strip scale for accurate positioning.



There are 2 linear actuators here, one from the scanner (top of picture) and the other from the printer part.

 

Just for an experiment. pick up an old printer or scanner. These are often tossed out. Then you will have a linear actuator, that is, the print head mechanism.
View attachment 320506
Some printer mechanisms come with an optical sensor reading a strip scale for accurate positioning.

View attachment 320507

There are 2 linear actuators here, one from the scanner (top of picture) and the other from the printer part.
View attachment 320508View attachment 320514

Love this idea! Thank you @dendad

 

Many of the stepper motors that I am most familiar with do not really "freewheel" when the power is off. There is some residual magnetism and so there are "torque bumps" that remain after the drive power is removed. They are quite obvious when turning the motor by hand. But those are the larger motors that weigh more than a pound. Probably it will not be an issue with the much smaller motors like those used in printers.

Using a stepper to move a linear pot is an interesting application. I have worked on stereo receivers that used motorized controls to provide remote adjustment of the volume setting,those used brush type DC motors with gear trains to operate the pot. It may not have been the best idea because it did not seem to catch on. But certainly that could be another concept to think about.

 

Thanks for your input @MisterBill2

I've got another one coming - a small stepper motor with an 8mm lead length. If that doesn't work I might try a ball screw, and then possibly give up on attempting a threaded rod style actuator. The problem is the smaller you go with the thread and motor, the less likely it is to find a longer lead length. And ball screws are expensive!

The data sheet for this one says the "power off self-locking force (N) < 1" so I'm hopeful.

 

Don't know if anyone else caught this - a stepper motor does not "free-wheel". They move from step to step. Also, they have at least four wires going to the motor. If the motor you have has two wires it's not a stepper motor.

You showed the cartridge drive motor and the optical strip. That's not a stepper motor, it's simply a motor that is controlled by the computer. It moves from side to side at a given rate, and the distance it travels is also controlled by a microprocessor.

On the subject of lead screws - I'm out-a-here. I know nothing more than they're simply a screw - or worm drive, and you can't force them to turn. I read a comment someone said if the screw is steep you can, and I'll accept that as being true. As for ball screws - again, I know nothing about them.

I think this is all I have on this topic.

 

From my research, ball screws provide more noise and higher resistance compared to lead screws - am I correct about this?

Not from my experience with CNC retrofits etc, !
The precision ball screw offers very little back-feed resistance and are silent compared to the rougher lead screw !

 

You showed the cartridge drive motor and the optical strip. That's not a stepper motor, it's simply a motor that is controlled by the computer. It moves from side to side at a given rate, and the distance it travels is also controlled by a microprocessor.

Yes, I know. But the point of those posts is to say that old printers and scanners are a good source of parts to experiment with. Some printers do use steppers, but that one I trashed for the photos does not.
Even so, a DC motor driving a dual slider pot, one side for the audio and the other for position feedback, may be a good way to go.

 

Yes, I know. But the point of those posts is to say that old printers and scanners are a good source of parts to experiment with. Some printers do use steppers, but that one I trashed for the photos does not.
Even so, a DC motor driving a dual slider pot, one side for the audio and the other for position feedback, may be a good way to go.

Interesting concept, Max. I would use a simple encoder and count pulses to determine the position. Of course, we do not know what the linear pot controls. If it is a volume control then mostly , folks adjust the volume by listening. At least I do it that way.

 

The print head is driven back and forth without precision. The drive motor and the optical strip tell the MPU where the head is and it decides when to print a letter. Motor precision is not necessary. Only needs to know where the head is at any given moment in time.

Yes, printers are good sources of motors and gears.

As for the question: if you have a motor with a lead screw it's unlikely you'll be able to turn it by forcing the final output wheel to turn. The gear ratio is likely much too high to achieve what you're asking for. Or more direct an answer - you're not going to turn a motor with a lead screw. Not unless you turn the motor shaft directly.

 

Not from my experience with CNC retrofits etc, !
The precision ball screw offers very little back-feed resistance and are silent compared to the rougher lead screw !

Thank you @MaxHeadRoom - very useful!

 

The print head is driven back and forth without precision. The drive motor and the optical strip tell the MPU where the head is and it decides when to print a letter. Motor precision is not necessary. Only needs to know where the head is at any given moment in time.

Yes, printers are good sources of motors and gears.

As for the question: if you have a motor with a lead screw it's unlikely you'll be able to turn it by forcing the final output wheel to turn. The gear ratio is likely much too high to achieve what you're asking for. Or more direct an answer - you're not going to turn a motor with a lead screw. Not unless you turn the motor shaft directly.

Thank you @ThePanMan. I understand that's one of the key qualities of a lead screw - it's ability to hold tension. The information is niche, but there is people/companies sharing that under the right conditions (lead length, etc) it may be possible. It's likely the only way forward for me is trial and error on some hardware.

 

You might consider using halfnuts to disengage the slide from the lead screw.
I could be spring loaded to disengage with a lever to slide and reengage when released.
https://www.answers.com/Q/What_is_half-nut_mechanism
https://thehabitofwoodworking.com/2024/what-is-a-half-nut-on-a-lathe/

 

Thank you @MaxHeadRoom - very useful!

I should add, the quality versions have a built in preload in the ball nut to reduce any backlash down to zero as close as possible.

 

Certainly many CNC lead-screws can be back-driven without much effort. Those are the ones with a much greater lead distance. And certainly CNC lead-screws are quite high quality devices. BUT without exception, they are neither small nor cheap.
While many printers do use belt drive for the traverse function, I have dismantled at least one that used a lead-screw and a stepper motor. And quite a few printers do use some stepper motors. Steppers are more likely to be used in the older and much more expensive printers than in the more current models, though.