Which Ballscrew pitch / Dia / Length ?

Looking at options

what pitch / length diameter do you need for a driving 6dof

does it need to be that long ?

hoping to use 300w to 500w motors

is this suitable ?

https://www.ebay.co.uk/itm/Ball-Screw-SFU1605-700MM-16000RPM-End-Machined-Ballscrew-W-Single-Ballnut-CNC/302927292476?_trkparms=aid=111001&algo=REC.SEED&ao=1&asc=20160908105057&meid=3a2dc5b239ab4eeb8eac2d70cfa1fb6a&pid=100675&rk=1&rkt=15&sd=302927292476&itm=302927292476&_trksid=p2481888.c100675.m4236&_trkparms=pageci:f6160205-da01-11e8-a75a-74dbd180374e|parentrq:b644e09b1660a99bd8f965c1fffdf85b|iid:1


does the 1605 mean 16mm diameter and a pitch of 5mm ?

Yes,

SFU1605 indeed means 16mm diameter and 5mm lead. I am using those and found them to be quite good for the job. Plenty of other people have used them before with no problems.

SFU1610 (10mm lead) are OK as well, if you are looking for actuation speeds over ~400mm/s. But keep in mind you will need enough torque to drive them.

At RPMs over 4000 min^-1 and a length over ~700 mm, vibrations might become too strong, so you might want to consider a bigger diameter in that case, but for what we are doing here, I strongly doubt that to be necessary!

I made a small excel sheet (find it attached) for quick drafting. Fill out the orange cells, grey are intermediate results, green are results. Feel free to play around with it. In the end you won't come around actually understanding what you are doing in this hobby A great way to learn/grow/get better/ get smarter...

Enjoy it

Dirty

My (very personal) rules of thumb:

1. Let each motor have 3Watts of peak motor power for every KG of "MovingWeight". So, for a 50Kg platform and a 100Kg payload --> use 6 motors with ~450W each. 300W would be a little weak, 800W quite strong.
2. Use a ball screw with a length ~120mm longer than the travel you want to achieve. I use a 500mm ball screw to achieve ~380mm travel. The rest will be unusable. The bearing block, the ball-nut and the "buffer zones" will occupy those 120mm.
3. You will need each of your actuators to have a push force [N] of about 6 * MovingWeight[Kg]. So, if you want to move 120Kg (40kg platform + 80Kg payload) you will need each actuator to have 6 * 120 = 720N of push force minimum, if you want even the most unfavourable geometric constellations covered. Those are the ones with the platform all the way down.
4. You can expect your actuators to have a push force [in N] of about 6300 * Motor-Torque [NM] / Lead[mm]. So a 0.5NM motor on an SFU1605 will give you ~6300*0.5/5 = 630N. Note that the force generated does not depend on the ballscrew diameter nor the motor power!
5. 3 & 4 combined means, that you should make sure, that your motor has a Torque[NM] of at least Lead[mm] * MovingWeight[kg] / 1050. So if you want to move 120Kg around (40kg platform + 80Kg payload) with an SFU1605 (5mm Lead), you should have 5 * 120 / 1050 = 0,57NM of motor torque to stay well clear of situations in which your motor might become torque saturated.
6. You can expect your actuator to reach a max actuation speed [mm/s] of RPM * Lead[mm] /60. So, a motor with a speed of 4000RPM on an SFU1605 will max out at 4000 * 5 / 60 = 333 [mm/s]

All those rules of thumb are based on the premise that at any point on the "geometric envelope" the platform shall need only 50% of its force to hold the simple static load. That means, that no matter where you put the platform (statically, no motion!) your actuators will need at max 50% of their available force to hold the platform and will always have the other 50% available for acceleration. That means you can have your platform accelerate at 2G over the entire geometric envelope - in all directions!

Disclaimer:
While I have spend some brains on those formulae, they might be wrong! Hell,.. there might even be a stupid typo in there somewhere... or two. So please (!!!) don't tear me into pieces if there is. And also please don't rip me apart if there is an actual conceptual error in there.
That being said, I'd be the first to admit and correct my errors if there are clear and valid arguments against my positions here. I strongly encourage anyone with an engineering degree (@My.stAr help me out, will you?!?) on these forums to carefully check those formulae and the underlying premises for correctness or at least plausibility. I have intentionally kept them simple to be used by absolute beginners and I know, the mathematical notation is not according to any standard.

I hope I can empower beginners to pick this hobby by robbing them of their reasons of why they think they can't do these sort of things. I was one of them not too long ago!

Dirty

ups,... and I should probably add that those formulae are meant for a full-on 6DOF Stewart platform. 3DOF or other platforms can get away with a lot less

Platform accelerate with 2G must be 1G

Very nice Excel file, indeed!

Some interesting points there

How would a ballscrew with 1610 and a 1kw dc motor rated at 3500 rpm hold up ?

Would the mm/s be much faster ?

Also - is there a way to calc this information for a chain/belt drive actuator ?

Hi
I am thinking of build a 6DOF motion platform mostly to use with racing sim. I'am going to take much inspiration from Hoddems's liner actuator and of coursers from other to. I'am thinking of to going for sfu2005 600mm ball screw and place the motor on the side using a teeth belt. I belie by doing this I can keep the height down and moderate the speed with the teeth belt pulley to get the right speed. I believe most people use 1605 ball screw but using 2005 i may get some more margins and stability. Is this unnecessary? They cost almost twice as much but compare to the total cost it might be worth it?

That looks like a pretty sensible design to me

Your motor will have a torque of 2,7Nm


Your ballscrew will give you a transmission ratio of ~5 ( 5N of axial force for every N of tangential force applied at the 8mm radius)


Your actuator will reach an axial Speed of 580mm/s and almost 1700N of axial force.


Your platform...
(I estimated the weight and geometry!)


...shall be able to accelerate at 2G.


...which it does! Easily!!!


I wouldn't know why not to use those ballscrews & motors.

Dirty

btw. "WAHR" means "TRUE" in German

Do you mean, an actuator where the motor is not mounted in line with the ballscrew, but instead sits on top of the actuator in reverse and drives the ballscrew via a belt drive?

If the driving wheel and the driven wheel of the belt drive has the same number of teeth: No change! (maybe a small loss due to reduced transmission efficiency)
1. Count the teeth of the driving wheel.
2. Count the teeth of the driven wheel.
3. Divide 1. by 2.
4. Multiply the actuator speed with that quotient.
5. Divide the actuator axial force, G-load, by that quotient.

Remember, there is no such thing as a free lunch: If axial force goes up, speed must go down and vice versa.

Dirty

Hi dirty,

thank you for replying - do you think many people use the 5mm pitch ballscrew because everyone else uses it and its proven

the 10mm pitch makes a better sim from what i can see

---------------------------------------------------------------

the chain drive question was looking at using a motor like this to drive a chain up and down

https://www.motiondynamics.com.au/united-450w-24v-dc-brushed-gear-motor-440-rpm.html

Ahhh,... you mean something like Roland van Roy's design?

The specs of your motor say: 8Nm on the output sprocket @440 RPM.

To get force:
1. Measure the radius (not diameter!) of the sprocket (where the chain joints are). I use millimeters.
2. Then divide 1000 by that radius.
3. Multiply the torque (8Nm) with the quotient from 2 to get the pull force on the chain

To get speed:
1. Measure the radius (not diameter!) of the sprocket (where the chain joints are). I use millimeters.
2. Multiply that by 2π (~6.28) to get the effective circumference of the sprocket in mm.
3. Multiply that by 440 (RPM) to get the speed in mm/minute.
4. Divide that by 60 to get the speed in mm/second.


@30mm you can expect...
~266N of pull force
~1382mm/s speed

I'd say that's way too fast, and not enough force. At least for a linear actuator used on a Stewart platform.

Now that I look at the numbers, I can only say: I agree!
If your motors torque is high enough, then you can take advantage of the higher speeds of an SFU1610. Especially those racing addicts among you might want those high actuation speeds. With myself being a flightsim nerd, I was more interested in the smooth motion.

But the longer I think about it, be more appealing the SFU1610 appear to me...

Well, I can still replace the ballscrews if the 1605 are too slow. That is one advantage of having powerful motors: you still have the flexibility to adapt your system down the road.

Dirty

You can get the same speed from a 1610 ballscrew than a 1605 screw if you make a 2 to 1 belt reduction.
Youre making half te rpm from a 1605 screw with the same speed and force (there is a little loss from te belt)
But this got a lot off thechnical avantages( critical speed longer life time) and saves noice
Sorry for the bad english
Ad

That is a pretty good argument!

I have to admit, when I read the first two lines of your post I was already scratching my head over the question of "...why in the world would anyone wanna do this...?!?!?" ...but, yeah, a reduction of noise, vibration & increased longevity could most probably be achieved by such a setup, I guess.

There are a few reason not to do it as well . I did of course try a belt drive in my actuators design/test phase many years ago.

did you find the belts slips ?

I used a toothed belt and wheel, so no slip. It worked, but more noise and unwanted side loads on the motor and ballscrew shaft, as there ends up being allot of force on those belts. The direct drive just worked better for my single support bearing ballscrew actuator setup.

The 2 single row deep groove bearings in the motor can handle belt tension load easy, done milions of time without problems.
Belt tension load up on a single row deep groove bearingblock fitted on a ballscrew can become a problem over time.
When use a bk12/fk12 bearingblock on a ballscrew there are no problems
This will be taken by the double row angular contact bearing fitted inside bk12/fk12 bearingblock they can handle side loads easy. and are made fore it
Half the actuators in the world use belts this way without problems.

If i remember correctly you did a 1 to 1 ratio and not a 2 to 1 ratio, so youre screw rpm did not slow down by half.
The hunderds of actuators from different brands i fitted in machinery in the past become more sillent with lower rpm.
With a 1 to 1 ratio the rpm and noice stay the same, with a 2 to 1 ratio the rpm and noice drops
Mass interia builds up with rpm, with half the rpm the actuator will react faster.