Cheap Ballscrews update

Pictures and descriptions of the internal parts of a satellite dish ballscrew actuator

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WB

Reply to
Wild Bill
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Wild Bill, I have purchased a satellite dish ballscrew actuator from a Tractor parts supply house that is setup for use to lift a snowblower or plowblade or mower. They cost a lot but were closed out for $50 when the tractor changed designs greatly! I haven't installed it yet to give a analysis but prelim look it is a 12 volt motor or 24 v that when powered will screw out or in about

6" but when hooked to the heavy brackets with extensions on them they can go further than that. They come with all hardware and switches, but I am still using winches to do the same job. The screw time is rather long, maybe because its like you say a 36v motor being run on 12v! Supposedly it will lift 300 lbs.

Jim xprtec

Reply to
XPRTEC

I note the following comment in your web page:

====================================================================== The ballnut block is a little loose, and I wonder if .025" (1/8") balls would fit, and how much difference this would make. The 3-D math is beyond my comprehension and measuring abilities, so I'd have to try the larger ones to see. ======================================================================

And my reply to that is to point out a difference between this and a ball nut as used on CNC equipment:

This one has a single ball nut, as obvious from the single feedback tube.

CNC ballnuts are actually *two* nuts, either with provisions for adjusting the spacing or for rotating one relative to the other, so one is pulling in one direction, and one in the other, taking out the remaining slack in the races.

Take two of those nuts, and thread them into a collar so you can rotate one relative to the other, and you should be able to tune out all of the measurable backlash.

And -- for another comment:

====================================================================== Not clearly visible in the pics images C & D are two spiral spring clutches, made out of closely wound square spring wire, that restrict each thrust bearing to only turn in one (but opposite) directions. I don't know the reasoning behind this, but it's assembled so that one thrust bearing rotates as the telescoping section extends, and the other thrust bearing rotates as it retracts. ======================================================================

I think that the reason for this is that a ball bearing which is used only over a short amount of rotation most of the time tends to wear in that spot. If you assure that the bearing goes multiple turns, the wear is more evenly distributed. This way does assure that each bearing rotates only in one direction, so even if you're hunting back and forth over a small distance, the wear is properly distributed.

Reply to
DoN. Nichols

Yep, I agree. One low accuracy use might be to couple it with a rapidly-reversing drive to use as a guide traverse for spooling/level winding.

One of the reasons I wanted to disassemble the unit was to see if the ballnut was just plastic, but I'm glad that it isn't.

WB ..............

Reply to
Wild Bill

I don't believe that is true. While CNC ballscrews may have multiple feedback tubes they do not eliminate backlash by adjusting the spacing between two ballscrew nuts. CNC ballscrews are precisely made to not have any backlash. During manufacture the size of the ball can be adjusted to produce the proper preload. New ballscrews must have the proper friction, not too much and not too little. This friction represents the preload.

Pete.

Reply to
Peter Reilley

[ ... ]

Some use the two loops slightly out of line, to set up a preload which is adjusted by the size of the balls. But as far as I know,

*none* of them have a single continuous ball run.

I know that the ball nuts on my Emco-Maier Compact-5/CNC have provisions to rotate one nut within the housing of the other for turning out backlash.

From page 12.21 on the manual for the Compact-5/CNC lathe and the F1 CNC mill (this one happens to be a page for the mill):

====================================================================== Setting the ball circulation bearing pretension of the ntu-mount:

By clockwise rotation of the setting ring (2), the ball circulation bearings are given stronger pretension.

To adjust the balll circulation bearings, complete removal of the spindle unit is required.

However, this setting will hardly be required, since no wear occurs, and the pretension is set in the works.

Procedure:

+ Unscrew the tapped stud (3) with screwdriver size 3 (1) + Screw in the tapped stud (4) with screwdriver size 3 (1). This will turn the setting ring in the direction of the arrow. + Retighten the tapped stud (1) (Position locking of the setting ring).

Attention: With excessive resetting of the setting ring, breakage of the balls in the ball circulation bearings can occur. ======================================================================

Of course -- this is German translated to English, and having the drawing (to which the numbers refer) would help. But, what is happening is that a pair of opposed screws are being used to rotate one ball nut in the end of the housing containing the other one. There is a pin extending from a flange on the nut, and the two screws bear on either side of it -- one rotating it in the direction to loosen it, the other to tighten it.

The ones shown in the manual for the Bridgeport BOSS-3 do not have separate adjustments, but I feel sure that the do have two opposed screw, tensioned by the size of the balls. The true test for this would be to remove the recirculation tube from one of the two loops, and measure the backlash to compare with the condition of both loops fully population.

Enjoy, DoN.

Reply to
DoN. Nichols

I am not sure what you mean by "out of line". Having multiple runs does not mean that there is an adjustment between the two runs.

I have an Emco-Maier Compact-5/CNC but no service manual.

I have a Bridgeport conversion mill with ballscrews. There are multiple circuits but no adjustment for backlash. The nut is one solid block.

I suspect that the Emco uses cheap ballscrews and takes up the backlash with an adjustment. In a production machine the ballscrews are precise enough that no adjustment is needed.

Pete.

Reply to
Peter Reilley

I've seen another scheme for preloading that does use a single ball circuit. There is a very small difference between the pitch of the nut and the screw. Tweaking the diameter of the balls adjusts the preload, just as in the dual circuit nut. Presumably the capacity of such a nut would be somewhat reduced beacause the balls are not loaded uniformly.

Ned Simmons

Reply to
Ned Simmons

[ ... ]

By "out of line" I mean that the two do not act as a perfect continuation of the thread, but that one is slightly ahead of or behind the other, so that when both are loaded with balls, one set of balls is pressing on the + side of the groove, and the other set of balls is pressing on the - side of the groove -- so slightly larger balls will press harder ahead and behind, without bottoming harder in the bottom of the groove, and thus wearing that much faster to no purpose.

Some of these days, I intend to scan the service manual (as I have already done with the programming manual for the last firmware version), and put them both onto a single CD-ROM. For the moment, I have only the hardcopy versions. (Note that I expect problems with the scanning in certain parts, as the schematics are fold-outs, too big for my scanner to do at a single pass.

[ ... ]

[ ... ]

This is not a conversion -- it is a mill built by Bridgeport as a CNC mill from the ground up. This includes a large hollow ballscrew around the quill, and a corresponding ballnut rotating in a pair of opposed tapered roller bearings, with a timing belt gear on the bottom of the rotating part of the ballnut.

But -- if you check the circuits, you will discover that there is a slight discontinuity between one and the next, which is what allows the tuning of the ball sizes to preload it axially, without increasing the lateral preload and leading to early failure.

You have already mentioned the adjustment method -- changing the size of the balls slightly. I would like to draw the setup, but that is outside my capabilities with ASCII graphics. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

That is an interesting feature -- and I agree that it would lose capacity over one which had two circuits with constant pitch but longitudinally offest threads. In that one style, the middle balls in the circuit are essentially doing nothing -- unless the load gets great enough to temporarily deform the threads.

Thanks, DoN.

Reply to
DoN. Nichols

I had assumed that the balls in precision ballscrews were designed to contact the races at 4 points. Two points on the inner race and

2 points on the outer race. Just like regular ball bearings that are designed for high axial loading. I know that there is a name for them but I can't recall it.

There is no reason why ballscrews with two circuits cannot be made as you describe, it just seems unnecessary. A ballscrew with 4 point contact races would have double the load carrying capacity without being any larger.

I would be very interested if you do come out with a CD.

Pete.

Reply to
Peter Reilley

Short comment only. Most plough blades use mechanical force to "lift" only, and gravity to drop. For example, a cable system, which then allows some undesirable obstruction to "lift" the blade if struck, to prevent damage and hopefully injury. If a ball screw actuator is used, I would think that it should not be a "direct" connect, as it won't allow this inherent feature.

Take care.

Brian Laws>From: "Wild Bill" snipped-for-privacy@usachoice.net

Reply to
Brian Lawson

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