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+--- Thread: Eva-Ballscrews? Feasibility study! (/thread-2292.html)
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Eva-Ballscrews? Feasibility study! - Hopefuldave - 09-17-2014
Hi all, this is just a thought I want to throw out, trying to find ways around a requirement for something I'm planning to build (at the Crap-O-CAD stage at the moment)...
I'd like to build a linear actuator but with some constraints - it needs to be compact, have about a 3/4" linear movement and do it with about 90-degrees of shaft rotation - yes, that's about a 3" lead if it were threads! Also, I want to make it "bidirectional", so that it can be actuated by the shaft rotation, or the shaft can be rotated by linear action. I guess that for this I'd ideally want a helix angle close to 45 degrees?
A regular multi-start thread would work, but the 3" pitch is going to be hard to machine conventionally by screwcutting (my lathe only goes down to 2tpi), and I'm pretty convinced that friction is going to make it a non-starter. I could thread mill it, by messing with change gears and mounting a milling spindle on the carriage (at a suitable angle for the thread), but I'd still be worried by the friction in the actuator...
An alternative that I thought of was a short ballscrew, minimal friction, multi-start and it should be able to accommodate the needed pitch and helix angle, but I've not found any online so far, which brings me to a Daft Idea:
If I were to thread mill the shaft, using for example a 1/4" endmill tangential to the shaft at 45 degrees, I could make the shaft fairly easily (for certain values of easy involving indexing the starts, fiddling with change gears) with half-circular ball tracks;
a number of precision rods the same diameter as the intended bearing balls could be tightly wrapped around the shaft in the ball tracks, ends turned out at 90 degrees to mimic the path of the recirculating balls and brought out through an outer shell;
the ends of the shell could be blanked off (with the shaft passing through at each end) and a Suitable Plastic or resin poured/injected in to fill the remaining space;
the shaft could then be unscrewed, the rods removed carefully into the space formerly occupied by it, and I should be left with a ball-nut, just needing balls and return channels to be added to complete it.
I've left out things like boring (relieving) the nut for clearance, mould release, choice of plastic/resin (DIY Moglice-alike?), but so far I can't see any impossibilities in the process - so I must have missed something! This doesn't need leadscrew accuracy, it's simply an actuator moving the end of a lever through 3/4" or so remotely via the shaft (and the shaft has to rotate, I can't just use a pushrod, I need axial movement to do something else!), repeatability matters but it doesn't need to be to better than maybe a 16th or 32nd of an inch.
OK, what have I missed?
RE: Eva-Ballscrews? Feasibility study! - stevec - 09-17-2014
There's a somewhat similar thread on another forum (Home Shop Machinist) .
Is there any possibility of gearing the drive up to give you the required speed of linear motion with a finer pitch leadscrew?
RE: Eva-Ballscrews? Feasibility study! - Hopefuldave - 09-17-2014
Not really, there isn't room to add a gear-train, pulleys or whatever as it has to fit within an existing piece of equipment! I have a total space about 2" x 2" narrowing to 1&1/2" x 2", by 6" long along the axis of the operating shaft, and have to fit a few other bits in there with it...
RE: Eva-Ballscrews? Feasibility study! - ScrapMetal - 09-17-2014
There is such a thing as an "internal throttle" used on motorcycles that has a similar function. Here's a link to a home build of one...
Internal throttle build
It might give you some design ideas.
Hope that helps,
-Ron
RE: Eva-Ballscrews? Feasibility study! - PixMan - 09-17-2014
(09-17-2014, 04:24 PM)ScrapMetal Wrote: There is such a thing as an "internal throttle" used on motorcycles that has a similar function. Here's a link to a home build of one...
Internal throttle build
It might give you some design ideas.
Hope that helps,
-Ron
I think a helix like that would be the way to do it. I wish my Walter dividing head had the gear train to hook up to a milling machine's-axis to do something like that, but alas, it doesn't. :(
I could, however, mill a groove in a flat piece of sheet metal then roll it to give the movement required.
Thoughts?
RE: Eva-Ballscrews? Feasibility study! - Hopefuldave - 09-17-2014
Interesting...
Pix, I can see that working, you'd need decent rollers though as the helically slotted tube would have to be fairly thick, and the interrupted bend (?) might tend to open the slot out - at least I can picture that happening! Have you thought about using a timing belt and pulleys on your mill's X-axis screw/handle to provide a drive to a rotary table / dividing head? an added shaft on the DH worm with a pillow block at the far end might be all you'd need to get drive to it for spiral milling, if you could gear down enough?
That helix and slot arrangement could work, but I don't have a Proper Milling Machine nor a hefty sheet roller, and I'd have to translate it so the outer sleeve moved back and forth with rotation of the inner - I suspect I'd want to make the helix angle around 45 degrees to make it "bidirectional" so the linear movement of the sleeve could rotate the angular movement of the shaft as well as vice versa...
It may be simpler for me to try the ballscrew idea, rather than have to make up the roller-bearing carriers though, at least with the equipment I have available at home (I'm sure the CNC kit at work would be capable of machining the lot, but that would mean producing a CAM file AND persuading the workshop manager that someone should run the job at lunchtime - last home job I had done cost a box of Krispy Kremes!).
This may be something I'll try once my house move is done and I've set up the tools in a bigger space, assuming the bikes don't fill the bigger garage completely!
SO... back on track, anyone see any problems with the ballscrew, at least in theory?
RE: Eva-Ballscrews? Feasibility study! - dalee - 09-21-2014
Hi,
Been awhile since I've wander in. Mind if I pull up a chair again?
Couple questions. How fast does the move need to be? How accurate? Repeatable? There are other ways to make this move that may be as good or better. That do not need a screw.
Dalee
RE: Eva-Ballscrews? Feasibility study! - Hopefuldave - 09-21-2014
Good questions...
Speed - hand operating a selector fork to engage/disengage dogs on a shaft running at a few hundred RPM, accuracy and fine repeatability are secondary to the selector fork being able to return the operating shaft (rotation through 90 degrees) and vice versa. Think actuator, not positioner or leadscrew! A degree of 'feel' would be a Big benefit, so I.want to minimise friction and backlash as much as possible.
The selector fork needs to move 3/8 - 1/2 inch in each direction along the shaft axis or parallel to it for a 45 degree shaft rotation in each direction from a midpoint, there will be ball detents at start, middle and end of the shaft rotation (not on the.fork) and the shaft itself needs to have end float (1/8" or so each side of midpoint) relative to the actuator as there are some other bits and pieces built along with it.
The really difficult bit seems to be.fitting it in the available space (about 1" radius around the shaft axis, perhaps 6" long, which precludes using cranks, face cams etc... Using a long-lead ballscrew seems most apt, IF I can make one and IF it will work...
RE: Eva-Ballscrews? Feasibility study! - dalee - 09-23-2014
Hi,
A pnuematic cylinder would be my first thought. Small diameter, compact, and speed of motion can be easily controlled. Bimba is cheap and easy to get.
Dalee
RE: Eva-Ballscrews? Feasibility study! - Hopefuldave - 09-24-2014
And requires compressed air... I'm looking for.something purely mechanical, not requirementmy a compressor, hydraulic pump, auxiliary motor etc. IF the ballscrew"s practical, the rest of the mechanism sort of designs itself, rotation of the shaft and ball-spring detents, collar with the ball sockets that can move.along the shaft axis and pop the.balls out to allow a sprung return to move the ball-nut back to mid.position... nicely simple and mechanical!
IF I can make.a.long-lead ballscrew, of course.