[NIC 77]

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

[alan_3301]

Here is how I would do it.

Attachment 275946Attachment 275948Attachment 275950

It looks like all your designs have the second ballnut floating, and secured by the flat resting on something. I don't know if this is good or bad, but I drew in bolts for it.
Instead of a spring I went with a steel dowel pin, and screws pushing it down, to separate the ballnut mounts.
In this case, there is no chance of overcoming spring pressure, the set preload is always going to be the same.
If my thinking is correct, the added preload could be very low, est 10-20lbs instead of the estimated 100-200lbs on other designs.
Now setting this preload would have to be calculated, and may be tricky to set consistently, but I think it would work well, and with very few extra pieces.
It also takes up very little extra space, around 0.25" give or take a little depending on dowel size.

[NIC 77]

Hi Alan,


That's an interesting an innovative approach. Thank you for sharing.

The way I see it there are a few problems with your approach.

There's a difference between backlash error (which preload will eliminate) and ballscrew error (which preloading will not eliminate) caused by variances in the distance between the machined grooves or the machining of the grooves in which the balls ride. Your design assumes that there will not be any ballscrew error so that a preload set at one end of the screw will be the same at the other end of the screw by the time the ball nuts reach that end. This could cause binding or at least a substantial preloading variance throughout the full travel of the screw. I believe the second ball nut needs to be able to slide independently for this reason and it needs to be loaded with some kind of a spring, whether that's a spring (belleville) washer, a rubber spring washer, or a conventional compression spring. I don't think a dowel is springy enough to allow for variances in ballscrew machining.

The preload in your design is not applied symmetrically or in line with the ballscrew. Is this a problem? I don't know.

I was thinking of some kind of belleville washer between the two ball nuts but I gave up on this idea because the ballnuts would have to align perfectly because the lbs/in of the belleville washer would be quite high and if it didn't match perfectly another half rotation of 2.5mm would be too much travel. Also, I couldn't find a washer of the correct dimensions.

How do you intend to make the two ball nuts line up so perfectly so that they both touch the dowel at exactly the correct spot so that you can apply the preload? With a 2505 ballscrew each rotation is 5mm and each half rotation is 2.5mm.

Have you seen a design like this is use before?

Also, the way I understand it is that 100 to 200 lbs of preload is required for optimum results. There's no point in setting a preload that is less than your cutting forces (ie: 10-20 lbs). If I am wrong on this point then please point out the errors in my reasoning.

This is just the way I see it and I could be wrong. I'm no expert. Thanks again for sharing, perhaps this will inspire you or I or someone else to come up with something a bit different that solves all the problems. I do like and appreciate your ideas, and if you disagree please let me know, I'd like to hear your thoughts on these discussion points. Really interesting and innovative approach, I just don't know if it would work.

[keebler303]

In designs 1-3 the 5mm adjustment increment could easily be changing using shims under the ballnut flange.

[alan_3301]

My design would have to be altered to accommodate the real world distance between bearing blocks, when they are aligned.
I would prefer a wedge instead of the dowel pin, but I was trying to find off the shelf parts for the design.
Yes It loads the ballscrew only at the top, but if the secondary ballnut is bolted to the flange it would hold it aligned. Obviously this wouldn't work if the preload was much greater than 10 or 20 lbs
An angled wedge would work much better at preloading in the axis of the ballscrew.
Even better I think would be A drilled through wedge at the top, and a tapped wedge at the bottom, to draw them together.

I think any of your designs will work though.shims are a good idea, so you're not stuck with 2.5mm increments