I've been working on a design for my new CNC machine. I've already secured a number of high quality Bosch Rexroth size 25 linear rails and bearing blocks.
I think it is most likely within my remaining budget to use the ballscrews and ball nuts that are made in china and available on ebay. My reasoning for this is that from what I've seen they have acceptable quality and good reviews.
The purpose of this post is to present to you four different ideas to add preload to the readily available 2505 made in china ballscrew and ballnut using a double nut configuration.
I've heard much talk about using a double nut configuration to eliminate backlash but have seen very few pictures or videos of actual working examples. Please feel free to post a link if you have a good example to share. I will explain my thought process for each of the four designs. Please do voice your opinion for which design you believe to be the best and why. If you believe a different design would be better then please post a link to one that has been already made or provide design feedback that is relevant. For example, if you believe that I should use a different size of spring then please choose one that actually exists and is easily and economically obtainable as opposed to a fictitious spring that is not in production.
A few notes about my CAD work. I didn't draw each spring exactly. I borrowed a model of a spring and scaled it so that the overall length was correct and the outside diameter was correct. The drawings of the springs are representative and may have more or less coils than the actual spring they are based on.
From what I've read so far, I am under the impression that appropriate prelaod ranges from 100 - 200 lbs.
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Design #1: The spring used in this design has an OD of 40.13mm and an ID of 29.62mm. The diameter between the edges of the socket head cap screws is 41mm. The cap screws keep the spring in place and from contacting the ballscrew. The free length of the spring is 2.5 inches with a rate of 210 lbs per inch and a suggested max load of 223 lbs.
Advantages: This is the simplest design. No extra parts are required.
Disadvantages: It is longer than some of the other designs. The preload is adjustable in 5mm increments only (the distance travelled per revolution of a 2505 ballscrew) and once assembled it requires dissassembly to change the preload on the ball nuts.
Attachment 275904
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Design #2: The ball nut brackets for this design are made of steel and are made in China. It will require modification of these brackets for this design however this is doable with my manual mill. The spring has an ID of 40.26mm which fits over two ball nuts of 40mm OD. The spring has a free length of 1.38 in. with a rate of 703lbs/in, a suggested max deflection of 0.260 in. and a suggested max. load of 186 lbs. One half will be bolted as per the other designs while the other half is free to move. A plate was not show for clarity of the design.
Advantages: Reduced length. One spring vs. two.
Disadvantages: The preload is adjustable in 5mm increments only. In this case only one revolution of the second nut can be used for preload with a stiff rate of 703 lbs/in.
Attachment 275906
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Design #3: The springs used in this design have an ID of 6.3mm and an OD of 12.5mm. The cap screws (with an OD ~6mm) that go through the center of the springs serve only to align the springs. This reduces the number of screws that connect the ballnut to the ballnut housing from 6 to 4. There are several readily available springs that can serve this design. I'd put some washers on each side of the springs, these aren't shown in my drawing.
Advantages: Simplicity.
Disadvantages: The preload is adjustable in 5mm increments only and once assembled it requires dissassembly to change the preload on the ball nuts. Compressing and installing the springs might be a pain.
Attachment 275908
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Design #4: This requires me to fabricate some brackets and bolt them onto readily available ball nut housings. It will take some effort but it is completely doable with my manual mill. The washers shown are 4mm thick with an OD of 21mm and are readily available. The tensioning bolt is an M8. The spring has an ID of 8mm and an OD of 16mm with a constant of 118 N/mm. In contrast to the previous three designs, this design pulls the ball nuts in towards each other. I have not heard of any design that pulls the ballnuts together instead of pushing them apart. Does it matter which way the force is applied?
Advantages: The overall length is reduced. The preload can be adjusted in any increment with no dissassembly required.
Disadvantages: Complexity. Extra part fabrication is required. Both springs will need to be adjusted equally. Does it matter which way the force is applied?
Attachment 275910
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Which one is best or is there another alternative that is better? Please vote in the pole and add comments below.
Attachment 275912