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# Thread: Faster to and fro Linear Motion design

1. ## Faster to and fro Linear Motion design

I'm new to machine designing and linear motion devices.
I want to create a linear motion device that is capable of reaching the speed of 750 Inch per second. Which would translate to 68kmh.

I wanted to use 8mm (Inch Per Revolution 0.75) self reversing ball screw attached to a dc motor that rotates at 60,000RPM, that would give me 1000rpm per second, since TPI is 0.75, I would get 750 inch per second. That was my initial idea as a beginner.

I found out that ball screws have critical speed limitation and since the length of my screw is 630mm and 8mm in diameter, my critical speed limit is around 5800rpm according to (http://www.nookindustries.com/Engine.../CriticalSpeed). Which means; I would have to create a ball screw of 8mm diameter with IPR greater than 7, to achieve my speed of about 750inch per second.

I found these two links with design of reversible screws that can deliver high speed

The video below shoes a video of self-reversing device which I think travels far per revolution: https://www.youtube.com/watch?v=rIE4frV3SBg

This second video shoes another video of fast reverse but slow forward: https://youtube.com/watch?v=O_-391UUHm0

The third video suggest about using support to act like anti-whipping so as to achieve high speed without whipping of the screw: https://www.youtube.com/watch?v=kkcV...ature=youtu.be

My Questions:

1. Is it possible to achieve that speed with screw of 8mm diameter or less?
2. What do you think about the videos?
4. Should I use a belt system?

2. ## Re: Faster to and fro Linear Motion design

Disclaimer: I am not a mechanical or motion-systems engineer... so take this with appropriate grains of salt... (in other words... I could be totally wrong - hopefully somebody with more experience and/or schooling in the subject than I will chime in here)

I would be worried about whipping the ballscrew at 8mm diameter. I can flex that with my hands visibly... yes, it takes quite a bit of pressure to do it, but, well, you get the idea.

For what you are going to be doing... is a belt drive feasible? How much load does it need to move? How often? Is it continuous duty, or intermittent? What about the acceleration profile? Does it need to synchronize with anything else? Do you have a computer, PLC, or microcontroller available to manage it? And what's cost profile look like - is this a one-off; a prototype with a reasonable budget, or are you looking for something that can directly translate into a mass-produced product?

3. ## Re: Faster to and fro Linear Motion design

Details matter! Is all you want to do is move fast? How much mass do you want to move at that speed and how long of a distance? Also how fast do you want to accelerate and decelerate? How accurate you need to position to when you stop? The big question is what cost are you willing to spend to accomplish this? Fanuc makes a linear motor that requires no ball screw to run and can obtain fast speeds 4 meters per second, and how long does this have to last mechanically? Sounds like an interesting project, good luck with it?

4. A belt system is feasible, but will require extra room for the load that it will carry, since it will have to rotate.
The load: Max 1kg, mostly 300g
Continuous duty not stopping
Once it starts, no stopping unless switched off.
Doesn't synchronise with anything else
micro-controller to keep it at constant speed
A prototype that can directly translate into mass-produced product.

5. Originally Posted by drdos
Details matter! Is all you want to do is move fast? How much mass do you want to move at that speed and how long of a distance? Also how fast do you want to accelerate and decelerate? How accurate you need to position to when you stop? The big question is what cost are you willing to spend to accomplish this? Fanuc makes a linear motor that requires no ball screw to run and can obtain fast speeds 4 meters per second, and how long does this have to last mechanically? Sounds like an interesting project, good luck with it?

The load: Max 1kg, mostly 300g
I really just want to move fast
Distance 24inch, 48inch
Since the system is setup for continues operation, accurate positioning is not much of a deal.
Thanks for Fanuc suggestion, I will check them out, even though my speed requirement is around 20 meters per second.
Should last two to three years.
Continuous duty not stopping
Once it starts, no stopping unless switched off.
Doesn't synchronise with anything else
micro-controller to keep it at constant speed
A prototype that can directly translate into mass-produced product.
Well, if it's feasible and we found a solution money wouldn't be an issue since we are at prototype stage; so far that we can reduce the cost when we want to mass produce.

6. ## Re: Faster to and fro Linear Motion design

Look at Bell-Everman Servobelt linear stages. They might be able to do what you want.

7. ## Re: Faster to and fro Linear Motion design

Em..
Nothing commercial will work as-is, if they have any contact surfaces.

So ball bearings, linear slides, etc. are all out as they wear out very fast at speeds below yours.

But..
hydrodynamic bearings, such as every car engine with an oil pump, last potentially forever and can do such speeds easily.
Old water-pump mechanisms that inject water between the surfaces have lasted hundreds of years.

If you want to accelerate a potential 1 kg mass to 20m/sec, in less than half of 48 inches travel, and then stop it before the end, and then reverse it, you will need lots of power and a very hefty mechanism.

TLAR says you need maybe 5-10 kW of power, steady state, to have a typical 100% reserve meant for mechanisms running 24/7 for 2-3 years.
And that TLAR is for both the payload and the mechanism, if you can make a very strong light mechanism You trust to run such loads long-term.

A 750 W servo motor (10 Nm peak, 3.3 Nm cont) pushes about 1000 kgf peak, 300 kgf cont.

You need a 1 kg mass to have 20 m/s in less than 500 mm, before decelerating to zero before the end of travel in 1000 mm.
Avg speed is half of desired, 10 m/sec, 1 g.
Time is 1 / 10 / 0.5 (m) = 50 ms, or 1/20 of second.
To get to 20 m/s, in 50 ms, is 20 g of acceleration needed for just 1 kg.

1. In a frictionless system, back and forth,
3. with perfect efficiency.

1., 2., and 3. are all impossible, and while getting near each goal on it´s own is fairly easy with common engineering stuff, it is expensive and more expensive the better You want.

1.
You can easily reduce the friction to near zero, effectively zero, by running the payload on a linear air bearing of 1000 mm effective travel length.
Lots of losses from air friction, and pumps, so it starts to cost some real money.
1-3000\$ in costs to make the stuff.
Assumes the payload can be held in a rectangular air-bearing or magnetics on the sides.

2.
Acceleration is critical.
How do you accelerate it, and stop it, without a mechnical arm ?
2.1
A mechanical arm, with a single-line contact (linear rod) can easily do the motion but must be strong and quite long to deliver 20 g of acceleration on a 1 m travel length.
It will be heavy, tens of kg.
Huge kinetic energy, huge power needed, many kW and a brushless servo motor is essential.
2.2
Air-bursts could accelerate and stop the payload, but would need a huge pump, power, lots of heat and noise, and would need 2 pumps and an accumulator for reliability in 24/7 use.
.. and so on..

Modern lasers accelerate about 5 g, and pay about 100.000 \$ extra just for the structure/stuff to be able to do so (vs 1 g for example).
20 g is extremely hard for a large big mechanical payload (vs a galvanic coil for example).

If one just needs high speed in 2 directions, 2 payloads (or more) in different circular (or vertical) tracks could be dirt cheap and easy and reliable.

Off-hand I cannot envision anything except maybe ships main engine cams doing such motion long term, and on much shorter travels.
Oil derrick ends are slower, afaik.

The desired solution essentially wants 4x ferrari acceleration-stop-acceleration 24x7, 3 years, cheap.
Hard to do.

8. ## Re: Faster to and fro Linear Motion design

Think internal combustion engine piston.

A connecting rod attached to a flywheel will provide the motion you need. Sealed bearings and lubrication of the sliding assembly, and it will last forever.