What do I need to know about BK/BK (both ends fixed supports) setup on ballscrew. How to calculate right distance, does one must have some offset to make right tension?
The problem with both ends fixed is that thermal expansion will probably change your preload. If the screw heats up more quickly or more than the bed then the expansion of the screw will change the preload thus causing backlash.
You could preload one end and then use springs, bellville washers, on the other end to compensate?
Ok, I will search about bellville washers. At the moment I want this setup: x axis about 1500mm(2x) and I want to use servos 2x750w direct fit. Ballscrew diameter 20 to 25mm and stroke 5mm. Gantry weight about 120kg. All this gives me wanted speed with double fixed support.
Yes, this is VERY common to pre load the ballscrew. IT won't change your preload, as one end is fixed preload, and the other is floating with ballscrew tension. You will need to find your operating temperature of the ballscrew, length between bearings, and coefficient of expansion. this is a simple calculator that may help https://www.engineeringtoolbox.com/l...on-d_1379.html
This was the old (and arguably the best) way to control thermal growth on axis.
Mori Seiki uses ball screws that are fixed at both ends. You grind the main side outer ring clamp to bring the preload to what it's supposed to be, then the screw is physically stretched into tension with a couple well reasoned taps on the mounting nut on what would normally be the free end. Yes the tension is transferred from the screw to the inner then outer rings into the housing. It is a very touchy operation needing 0.0001" or 0.001mm resolution minimum indicators reading off a bearing ball greased into the center hole of the screw. The amount of stretch is calculated according to screw length or maybe screw travel. Can't remember which. It is a three bearing arrangement (BDBT) on the main/motor end and two bearing arrangement (BDT) on the other. I did it on three screws I replaced on a Mori mill. It was a bit nerve racking getting to the point of being absolutely confident in what you were reading on the indicator was stretch and nothing else. All this while working inside a machine, though at the time all the sheet metal and way covers were off, which really helped get to the work area.
BK bearing blocks use a double set of angular contact bearings setup with opposing angles. When you put an axial load on them by tightening the nut on the ball screw, it pulls the bearings together such that any axial movement is eliminated. This is bearing preload.
This is usually done at the drive end of the screw. Now at the other end of the screw you need a bearing block to keep the screw from flapping or drooping. Usually this end will have a bearing that is not locked into an axial position but it can float with the end of the screw. As the screw changes temperature, warms up, it gets longer. With a 1500mm screw, a change in temp from 70F to 100F will cause a change of about 0.010". If this end of the screw is also fixed, then there must be a way to accommodate a slight change in the length of the screw.
If both ends of the screw are threaded then you can use the BK block but put a bellville washer under the nut. This will apply a tension to the screw, and when the screw cools and shrinks, the washer is compressed more.
Is this the right way: Servo - Nut - BK support with angular bearings - ballscrew length - bellville washers <> - BK support with angular bearings - bellville washers <> - Nut ?
And last BK support bolt holes must be regulated on length, because chinese ballscrews are not cut and machined precisely?
Almost:
I would eliminate the washers in bold below. If you have them, they will put the screw into compression. Tension is better. Where you were going to put these washers, leave a space of 10-20 thousands if you do the assembly when cold. You could try these wavy washers instead of the belville. https://www.amazon.com/Glarks-Compre...4958687&sr=8-3
Is this the right way: Servo - Nut - BK support with angular bearings - ballscrew length - bellville washers <> - BK support with angular bearings - bellville washers <> - Nut ?
And last BK support bolt holes must be regulated on length, because chinese ballscrews are not cut and machined precisely?
Do you plan to turn the ends of the screws yourself, or order them special?
Double fixed support on ballscrew
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