Re: Power Transmission grade ballscrews? was: Does this make sense?
Posted by
ballendo
on 2003-08-13 03:05:18 UTC
Vince,
That appears to be a hi-grade ground screw.
Transmission grade ballscrews are typically used in "Actuators",
which you've probably seen on a larger satellite dish. They are also
found steering large boat motors, opening supermarket doors
automatically, and in jacks used in semi heavy lifting. If you go to
ballscrews.com, and look at actuators, you'll see what I mean...
The point is that like most other "screws", BALLscrews come in MANY
different grades. Since many of us try to save a buck, we are often
buying and using the ballscrew equivalent of "hardware store" grade 2-
3 screws and nuts. Just as the hardware store has grade 5 and grade 8
bolts (well, they used to anyway!<G>), ballscrews intended for
precision machinery are of a higher grade.
This higher grade shows itself in many areas. First the material
itself may have a smaller grain size, and/or better structural
properties. Just as the steel used to make grade2 bolts is of poor
quality compared to that used for grade 8's. Next there will be
greater attention paid to the geometric properties of the screw and
nut. Things like straightness, lead accuracy, circularity, and
threadform all fall into this category. Often this is achieved by
using a grinding mfg. process, instead of the rolling process typical
of "transmission grade" screws. Finally the higher grade parts may be
hardened to a greater degree.; which is possible due to the
difference in material and mfg. technique.
Ballscrews have one thing going for them; they can move heavier loads
using less force than other thread types. If you think about this,
you can see that there are lots of places where THIS quality is
desirable, WITHOUT the need for great ACCURACY... However, in the
world of CNC we would like to benefit from the lower power needs, but
we ALSO want (or truly NEED) the most accuracy we can afford...
So we have to be careful we know what we're buying, because that
first type, which Jon calls "power transmission" grade (referring to
its ONE reason for being) is lying in wait all over the place...
We ALSO need to be aware that some of the higher grade screws and
linear bearings REQUIRE a higher grade structure to be fastened to!
It is quite possible to have problems by bolting a high grade motion
component to a flimsy (relative to the bearing /screw needs)
structure! It wont run right, and you sit there cussin', cuz you know
you used the hi-dollar stuff...
Hope this helps,
Ballendo
P.S. Serviceable CNC machines CAN be made from transmission grade
ballscrews. MANY on this list have done so. But sometimes it pays to
remember that the screw may have a huge lead error (as Dan Mauch
recently pointed out) because we chose this less expensive
(initially???) route. And IF we remember, we can do things like put
mating parts on our machines such that they both "use" the
same "part" of the screw. So while they may not be ACCURATE, they
WILL fit together.
As hobbyists (and for a whole lot of commercial jobs, IMO) we can get
away with this. But in a job shop, making parts to someone elses
spec's, we'd better be careful to ensure that the parts we make meet
the specs required, or expected.
And lastly I'll say something I've posted often here on CCED, which
is: Most people I've seen new to CNC "think" they need (and actually
BELIEVE they can get!) more ACCURACY than they will...
Resolution is NOT accuracy, though it's easiest to achieve; and is
often "talked" about AS IF it WAS accuracy... Repeatability is what
we can hope to get. The good news is that repeatability can
be "nurtured" into accuracy, and most often is enough by itself.
Which is why we can get away with the transmission grade ballscrews
we order from Mcmaster. Just don't kid yourself. True accuracy is
Damn hard to achieve; and harder to maintain. so be glad you prob'ly
don't need it, cus if ya do, ya better get out yer billfold<G>
--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Vince Negrete"
<vnegrete@r...> wrote:
That appears to be a hi-grade ground screw.
Transmission grade ballscrews are typically used in "Actuators",
which you've probably seen on a larger satellite dish. They are also
found steering large boat motors, opening supermarket doors
automatically, and in jacks used in semi heavy lifting. If you go to
ballscrews.com, and look at actuators, you'll see what I mean...
The point is that like most other "screws", BALLscrews come in MANY
different grades. Since many of us try to save a buck, we are often
buying and using the ballscrew equivalent of "hardware store" grade 2-
3 screws and nuts. Just as the hardware store has grade 5 and grade 8
bolts (well, they used to anyway!<G>), ballscrews intended for
precision machinery are of a higher grade.
This higher grade shows itself in many areas. First the material
itself may have a smaller grain size, and/or better structural
properties. Just as the steel used to make grade2 bolts is of poor
quality compared to that used for grade 8's. Next there will be
greater attention paid to the geometric properties of the screw and
nut. Things like straightness, lead accuracy, circularity, and
threadform all fall into this category. Often this is achieved by
using a grinding mfg. process, instead of the rolling process typical
of "transmission grade" screws. Finally the higher grade parts may be
hardened to a greater degree.; which is possible due to the
difference in material and mfg. technique.
Ballscrews have one thing going for them; they can move heavier loads
using less force than other thread types. If you think about this,
you can see that there are lots of places where THIS quality is
desirable, WITHOUT the need for great ACCURACY... However, in the
world of CNC we would like to benefit from the lower power needs, but
we ALSO want (or truly NEED) the most accuracy we can afford...
So we have to be careful we know what we're buying, because that
first type, which Jon calls "power transmission" grade (referring to
its ONE reason for being) is lying in wait all over the place...
We ALSO need to be aware that some of the higher grade screws and
linear bearings REQUIRE a higher grade structure to be fastened to!
It is quite possible to have problems by bolting a high grade motion
component to a flimsy (relative to the bearing /screw needs)
structure! It wont run right, and you sit there cussin', cuz you know
you used the hi-dollar stuff...
Hope this helps,
Ballendo
P.S. Serviceable CNC machines CAN be made from transmission grade
ballscrews. MANY on this list have done so. But sometimes it pays to
remember that the screw may have a huge lead error (as Dan Mauch
recently pointed out) because we chose this less expensive
(initially???) route. And IF we remember, we can do things like put
mating parts on our machines such that they both "use" the
same "part" of the screw. So while they may not be ACCURATE, they
WILL fit together.
As hobbyists (and for a whole lot of commercial jobs, IMO) we can get
away with this. But in a job shop, making parts to someone elses
spec's, we'd better be careful to ensure that the parts we make meet
the specs required, or expected.
And lastly I'll say something I've posted often here on CCED, which
is: Most people I've seen new to CNC "think" they need (and actually
BELIEVE they can get!) more ACCURACY than they will...
Resolution is NOT accuracy, though it's easiest to achieve; and is
often "talked" about AS IF it WAS accuracy... Repeatability is what
we can hope to get. The good news is that repeatability can
be "nurtured" into accuracy, and most often is enough by itself.
Which is why we can get away with the transmission grade ballscrews
we order from Mcmaster. Just don't kid yourself. True accuracy is
Damn hard to achieve; and harder to maintain. so be glad you prob'ly
don't need it, cus if ya do, ya better get out yer billfold<G>
--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Vince Negrete"
<vnegrete@r...> wrote:
> Jon,transmission
>
> I'm having trouble understanding what you mean by "power
> grade".ViewItem&item=2549803136&category=12577
>
> I have something similar to this:
> http://cgi.ebay.com/ws/eBayISAPI.dll?
>actually
> Can you send a link to an example transmission grade one that I can
> see?ballscrews
>
>
> Vince
>
>
>
>
> ----- Original Message -----
> From: "Jon Elson" <elson@p...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Tuesday, August 12, 2003 11:01 AM
> Subject: Re: [CAD_CAM_EDM_DRO] Does this make sense? was Re: Bent
> Nookballscrews ?????
>
>
> > But, a number of people are using "power transmission grade"
> > (with bends, bows or even KINKS in them) and 2 separate nuts tomake
> > a homemade anti-backlash nut system. I think you HAVE to havecompliance
> > in these, especially if "tight spots" are discovered in thetravel.
> > You don't want a bunch of screen-door springs, you need somethingwith a
> > real high spring rate, like thick Bellville washers or somethinglike
> that.backlash in
> > If the pitch variation is so slight that you can tolerate the
> > some areas by setting the fixed preload to just not bind on thetightest
> > part, that is best.between two
> >
> > But, I am totally convinced that putting a heavy preload force
> > ball nuts by some spring-like mechanism will not cause a torqueload
> > on the screw of the same magnitude as delivering that same forceto
> > an external axial load. If Mr. Lux would care to drop by myshop, I
> > am pretty sure I could throw together a demonstration of this ina couple
> > of minutes that he would be absolutely unable to refute.wanliker@a...,
> >
> > Jon
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
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> > http://groups.yahoo.com/group/jobshophomeshop I consider this
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> subjects, that are not allowed on the CCED list.POSTING THEM.
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Discussion Thread
Vince Negrete
2003-08-12 11:36:03 UTC
Re: [CAD_CAM_EDM_DRO] Power Transmission? was: Does this make sense?
ballendo
2003-08-13 03:05:18 UTC
Re: Power Transmission grade ballscrews? was: Does this make sense?