Single start vs double start ballscrew.

[darik]

Hi all,
I bought two ballscrews, both 15mm diameter with 10mm leads.
Now I have to decide which one to keep.
The first one is Issoku ground C5, the second one - Issoku ground double start C3. Well, that's what the seller said when I bought them.

So, what do you think guys? Which one is better and if easy to explain, why?

Thanks

[Oldmanandhistoy]

What are you planning to use the screw on; router, mill or other?

John

[Zumba]

The 2-start screw can support a greater axial load due to the two ball circuits, at the expense of added friction.

I'd choose the one in better condition.

[ger21]

So your saying a 2 start ballscrew is less efficient than a single start screw? If so, then why would it be the opposite of acme screws, where multiple start acmes are much more efficient than single start?

[walter]

Originally Posted by ger21 So your saying a 2 start ballscrew is less efficient than a single start screw? If so, then why would it be the opposite of acme screws, where multiple start acmes are much more efficient than single start?

Isn't the efficiency lead dependent?

Multiple start screws could visually appear similar to single start screws and have the same pitch.
The difference in efficiency would come from a higher lead (?)

Example:

1/4" lead two start screw vs
1/8" lead single start screw

- Both have the same 1/8" pitch
- Two start screw is more efficient

?

[Zumba]

Ger, if multiple starts add efficiency to a leadscrew, then I've learned something new.

One thing that can confuse things is the terminology used. ACMEs are always classified by diameter x TPI, e.g. 3/4 x 8. Ballscews are always classified by diameter x LEAD, e.g. 0.750 x .250.

A 3/4 x 8 ACME, single start, has a lead of .125.
A 3/4 x 8 ACME, double start, has a lead of .250.

The higher lead of the double-start ACME is what makes it more efficient, not the extra start. Compare this to ballscrews:

A .750 x .500 single start screw has .500 lead obviously
A .750 x .500 double start screw also has .500 lead.

Same lead, but the double start screw's ballnut has twice as many rolling elements, so even if the loads are the same, there is more friction with the double start.

The extra friction is negligible though. The real reason why you'd want to buy a single start vs. a double or quad with a ballscrew is to save money. They're drastically cheaper.

[Zumba]

Walter, I type too slowly. You beat me by 4 minutes.

Oh well, thanks for supporting my theory! Or rather, vise versa!

By the way, here's something interesting... a 1.000 x 1.000 rolled ballscrew typically has four starts, but each ballnut only has two circuits. I guess if you use a double anti-backlash nut (four circuits total), you can set it up so that all four circuits are running on separate starts, minimizing wear.

[ger21]

Thanks guys, typing without thinking is dangerous.

[walter]

I saw you in there - I knew I had to make it quick!

BTW, what's the difference between these single and double ballnut assemblies:

- Single ballnut is 7" long four circuits
- Double has two nuts with 2 circuits each, same total length of 7"
- Same preload
- Same screw shaft
- Same lead
?

I'll need a long life "super duty" C5 ballscrew and I'm trying to learn the details..

Any comment will be appreciated!

[Verfur]

could some one difine efficiency? Is this describing how much force it takes to turn the screw vers. how much weight it can lift?

I would think single start would have less drag and that double start of same size would have a greater "MAX" strength do to it having twice the bearing surface for the same lead.

If a 1" screw had a 5 tpi single start (lead of .2) and ran in a 1" long nut then it would have 5 threads worth of engagment vrs. a double start would have same lead of (.2") and a thread count of 10 so ten threads would be in the same nut lenghth so twice the bearing surface and most likly at least 2 times the drag.

Would this not be right?


Just thoughts

[Zumba]

Originally Posted by walter I saw you in there - I knew I had to make it quick! BTW, what's the difference between these single and double ballnut assemblies: - Single ballnut is 7" long four circuits - Double has two nuts with 2 circuits each, same total length of 7" - Same preload - Same screw shaft - Same lead ? I'll need a long life "super duty" C5 ballscrew and I'm trying to learn the details.. Any comment will be appreciated!

A double nut (aka adjustable preload nut) usually consists of two non-preloaded nuts. You can tell that they're not preloaded by looking at the ball return tubes, which should be screwed on. Preload is obtained by placing some sort of disc spring between the two nuts, and can be adjusted.

On a single preloaded nut, the ball return tubes are attached by some sort of banding, which applies the preload. The preload is not adjustable. However, this type of nut is typically made to higher tolerances than the double-nuts.

I'm not sure about other brands, but with Nook, the double adjustable-preload nut is usually meant for "SRT" rolled screws and the single preloaded is meant for "SGT" ground or "XPR" high precision rolled screws and aren't interchangeable.

[Zumba]

Originally Posted by Verfur could some one difine efficiency? Is this describing how much force it takes to turn the screw...

Yes, that is correct. Efficiency is described in percentages. So a 50% efficient screw will transmit half of the motor power to linear motion, while the other half will be eaten up by friction and turn into heat.

Originally Posted by Verfur I would think single start would have less drag and that double start of same size would have a greater "MAX" strength do to it having twice the bearing surface for the same lead.

Agreed.