Question about tandem lead screws

[spalm]

Not sure I know what he’s saying either. Anyway, he is using two nuts in a locknut fashion against the bearing. I don’t think you will find Acme 2 start nuts. Also I have been told that this style of mounting is all wrong. The following is a quote from HachnHew to a question I had asked about leadscrew mounting. (I don’t know how to point to a single post)

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Steve,
I can tell you about the bearings....what you need are angular contact bearings, preferably mounted at the same end as the motor is located. Not only will they bear a radial load, they can also tolerate an axial thrust load. It takes two bearings facing one another, separated by approx. 0.100" (think of a bearing block with a 0.750 thru hole, with 1.00" counterbores on each side that the bearings lightly press into but bottom out before butting up against each other.) The block is rigidly mounted to your frame; the screw passes through each bearing, and is retained by a nut/jam nut arrangement. Tighten down the first nut to remove any backlash, and then run the jam nut tight up against the first nut. This requires that the screw be machined to fit the bearings and to have a threaded end. Once this assembly is set up, the far end of the screw can ride in a simple radial ball bearing (the screw end again machined for fit). Here's a link to a source for angular contact bearings:http://bearingsdirect.com/products/...y&id=4&subid=78

For some reason I've only been able to find them in metric sizes, and the local machine shop guy says that's the only way he's ever known them to be sized.--------------------------------------------------------------------------

A document on lead screws I found called "Technical Introduction - PowerAc"
http://www.nookindustries.com/acme/AcmeCatalog.cfm

This is why I think I need to machine the ends unless I can figure out an easier solution.
Steve

[Chunky]

Steve,
I can tell you about the bearings....what you need are angular contact bearings, preferably mounted at the same end as the motor is located. Not only will they bear a radial load, they can also tolerate an axial thrust load. It takes two bearings facing one another, separated by approx. 0.100" (think of a bearing block with a 0.750 thru hole, with 1.00" counterbores on each side that the bearings lightly press into but bottom out before butting up against each other.) The block is rigidly mounted to your frame; the screw passes through each bearing, and is retained by a nut/jam nut arrangement. Tighten down the first nut to remove any backlash, and then run the jam nut tight up against the first nut. This requires that the screw be machined to fit the bearings and to have a threaded end. Once this assembly is set up, the far end of the screw can ride in a simple radial ball bearing (the screw end again machined for fit).

There's a lot to comment on in this post. But I'm not sure he's saying anything against the idea of using a non-machined shaft. It's hard to picture an image by reading words (it is for me anyhow). But I drew a pic based on what I think he was saying. I'd like to know if this is what he meant and whether running threadds through a bearing is illegal. One pic is with threads, the other without. What's the difference? Just asking.

[ger21]

Originally Posted by Chunky I don't know what he's saying here. Do you?

http://www.mcmaster.com page 1025, steel ball thrust bearings, shielded. What he's saying, is that unlike a normal bearing with an inner and outer race, the outer race wraps around 1 side, so the center race is only exposed on 1 side, the side with the 2 nuts. These bearings are $13 each and have a load capacity of 3800lbs.

[ger21]

Using angular contact bearings in a bearing block assembly is the preferred method. A good example is here. http://www.5bears.com/cnc04.htm Typically, these are mounted using screws with machined ends. And they are terribly expensive, usually $200+ each.

For a small homebuilt wood router, you can get away with much less. The easiest way would be the method linked above, using the Mcmaster-Carr thrust bearings. You can get 2 start nuts, but they probably won't lock on the threads. I'd use clamp on collars, using a single nut to pull them tight before tightening in place.

Another way is to build the bearing block assembly, but use cheaper bearings instead of angular contact. I've read that non-thrust bearings can usually handle thrust loads of close to 50% of their rated radial loads. A $5 1/2" ID bearing is rated around 1100lbs. It should have no trouble handling the 100-200lb or so thrust load you'll be putting on it. It probably won't last as long, but it's a cheap option that will probably hold up for a few years.
Another bearing option is tapere roller bearings.

[spalm]

Gerry, you da man! Thank you if you can help us figure out a way to use 2 start screws without machining the ends.

So we are still taking about the motor end here, right? The other end is just rolling in a regular bearing without any nuts.

So it we take Chunky’s picture with the threads through the bearings as a start. Assume that the motor is connected to the right side. Make a “securely attached to the router’s frame bearing holder” to hold two thrust bearings with a spacer between them. Then use a dremel cut-off wheel to create a groove in the rod in place of the left two nuts and install a C-clip which will be pressed against the left bearing (or this could be a dual nut if they hold up). On the right side, use a nut tightened against the bearings with a motor coupler set-screwed to keep it from loosening.

I still need to find a dual start nut.

Steve

[ger21]

Instead of nuts on each side of the bearings, use a clamp on collar with some epoxy for insurance on the inside of the bearings, and then use another collar on the outside. McMaster-Carr has 2 start brass leadscrew nuts. As for connections to the motors, you can get many different types of couplers with 1/2" bore on 1 side, and 1/4" or 3/8" on the other.

[Chunky]

ger21]Instead of nuts on each side of the bearings, use a clamp on collar with some epoxy for insurance on the inside of the bearings, and then use another collar on the outside. McMaster-Carr has 2 start brass leadscrew nuts. As for connections to the motors, you can get many different types of couplers with 1/2" bore on 1 side, and 1/4" or 3/8" on the other.

I haven't heard anyone say it but it seems to me that what we're talking about here is axial (lenthwise) backlash as compared to the backlash that we usually discuss (radial). If I can establish that I can better understand the goal here.

[ger21]

Axial IS what we are usually concerned about. When you turn the screw, the nut needs to move (axially), no matter how little you turn it. If you have backlash, you can turn the screw, and it may not move until the backlash is taken up.

[ger21]

Now that I think about it, I think I may just order some clamp on collars and those thrust washers and see how it works.

[Chunky]

ger21]Axial IS what we are usually concerned about. When you turn the screw, the nut needs to move (axially), no matter how little you turn it. If you have backlash, you can turn the screw, and it may not move until the backlash is taken up.

I cannot belive what stupid things I do and say. I need to be watched 24 hrs a day. I know full well that what we usually talk about (ab nuts, etc.) is axial. What I should've said is that this bearing mounting business deals with ANOTHER opportunity to have or remove axial backlash.

[ger21]

The leadscrew nuts (drive nuts) and the leadscrew mounting bearings are both equally important. Any backlash in either will manifest itself the same way.

Keep in mind that the leadscrews are not the only place you can have backlash. If any part of your machine has any play in it, it can be considered backlash. It's important that absolutely no part of you're machine has any play at all, Or you'll notice it when you start cutting.

[Chunky]

ger21]It's important that absolutely no part of you're machine has any play at all, Or you'll notice it when you start cutting.

Yep.