Are 20mm Linear Bearings strong enough?

[WarrenW]

I found some 20mm linear shaft and bearings that are 60 inches long on vxb.com. Would these be strong enough at this length to hold a gantry for the x axis motion? I'm not familiar with how string these are. I would use one 20mm linear shaft on each side and a leadscrew down the middle under the table to move the gantry for its X axis.

I know most people would prefer rails instead - I'm just interested in what people say about the shaft design.

Thanks!

Warren

[mxtras]

At 60" long, I would imagine that you would have intolerable deflection and vibration during use if the shafts were unsupported.

Scott

[WarrenW]

I was going to support them on each end with brackets like this unit in the picture below. This machine below uses up to 36 inch linear shafts on each side the web site stated. I really wanted to stretch it to 40 inches for the X axis instead of using the whole 60 inches. What do you (or anyone else) think about that?

Thanks

http://www.3drouters.com/specs2.htm

[fyffe555]

Scott's right. Too much deflection with a 20mm shaft and trying to span 40 inches with any load.

A few things to note;

The link you've given describes sizes and accuracy for a machine with Bishop Wise Carver bearings and that is not what is in the picture - different machine.

From the machine in the picture and guesstimating from the size of the router (3.5" dia) the X axis is ~36" but the shafts are over an inch dia, say 1 1/2". Even then, if the gantry/router weighs 50lbs and with 1 1/2" shafts you're still going to get between 0.002" and 0.006" deflection due to weight alone.

IF you try to span 20mm shaft for 40" in the same design then the 50lb gantry will cause deflection of the X axis of between 0.03" and 0.12" Yes that's almost an eigth of an inch.

Also is this 40" for length over all or X axis travel? - if it's travel its going to be a lot worse.

The range of deflection is the difference between a perfectly fixed end point and a simply supported end. Actual deflection figure will be somewhere in the middle with the design linked.

Andrew

[WarrenW]

Thanks for the info. I needed to know that before I ordered the shafts and bearings. I suppose the best solution for the X axis is rails huh? If the rails are mounted down on something that will not move any then they should be fine I imagine. I keep looking on all these sites at rails and cannot find ones long enough or find the pricing. Maybe I'll make a seperate post asking for a direct source to find this. I looked on ebay a bunch but only find shorter ones. I want to move the gantry 4 feet for the X and Y axis. Of course, I could always load up on a bunch of roller blade bearings!

Thanks!

Warren

[WarrenW]

Just now I found this setup on this site. He is using two shafts per axis for X and Y. What size do you think those shafts are? I emailed him but may not hear back since he has not posted again since last year. I would guess thats about 3 feet on those shafts?

[davidmb]

There's a freeware application called 'BeamBoy' sorry I can't remember where I downloaded it from but it allows you to calculate the deflection on loaded beams, both hollow and tube of differing materials, if anybody wants this I can trawl back through my backups.

[fyffe555]

CNC machines are a bit like boats. Add a foot in length and double the price...

Rails are good but not just because they are rails but mostly because they are supported along their length. The problem transfers from support to alignment as you have to align the entire rail *and* support rather than just the ends with a shaft.

The last design you linked is probably trying to half the deflection by spreading the load across two shafts on the X, and this should work. However but the end fixing of the shafts isn't doing much to stiffen or locate the ends of each shaft and you've doubled your alignment problem.

There are ways to reduce deflection of a shaft; Physically support it along some portion of it's length, that would then require open bearings of course, but it becomes something like a rail arrangement.

Alternatively just use a larger diameter shaft and bearing but that costs more.

Finally you can support the ends of the shaft properly. the longer the span or the larger the load the stronger and more complex the end support needs to be. For example Shafting with two clamp supports at either end spaced, say 4" apart with significantly reduce the deflection. Thats pretty much the defelection calculated as the lowest deflection in my post above. It will still deflect, just a lot less than one support. Four end supports stiffens the shaft considerably by forcing the deflection to bend the shaft in a recurve - or three bends, one each end downwards and one at the point of load upwards. The worst thing to do is just drill a hole through a piece of 1/2" mdf or AL. and either just bolt through the ends or just clamp it in place. This means the deflection only has to bend one curve and the ends are effectively hinged - this is the largest deflection case in the note above.

I've attached a very rough spreadsheet I did a while back to calculate deflections in this stuff. Simple supports are like the second linked machine. Fixed supports are mathematically similar to the first machine, just with two clamps at each end of each shaft, or approaching a rail where it's supported along its length where you can add the shaft *and* the support into the bending calculation.

Whatever, a 4 foot travel with small deflections will be expensive however you do it.

[WarrenW]

Thanks for your info fyffe555! Its funny you mentioned about supporting it along the shaft and using open end bearings. I found a shaft (only 16mm though) that is 41" long but it has 9 tapped holes it in on one side and includes 2 open end bearings. So the shaft could be mounted down all along and include supports on each end. This would work just like a rail also. They were $89 each which I don't think is bad including 2 bearings per shaft.

They are all the way at the end of this web page.

http://store.yahoo.com/cnclinx/linearslide.html

[mxtras]

Good write-ups, fyffe. I like your style.

Kudos.

Scott

[fyffe555]

Warren,

A fixed shaft would be better and the smaller diameter would be ok as long as its load capacity met your needs.

I forgot something which Scott ( thanks Scott!) picked up first time round - vibration. You're way better off with fixed rails or shafts rather than end supported shafts when you consider the vibration caused by actually moving and cutting.

When working out the deflections under the weight of the axis its easy to forget the real loads are the forces of moving the cutter through the piece. A 1/2" bit in a 3hp router can produce *way* more than 50lbs of load and it will do so in all directions. If a Shaft is just end supported then it can and will be deflected under cutting side to side as well as just up and down.

Have you looked at hardened shafting at McMaster or MSC? their prices for the real stuff are not much more than drill rod and I think cheaper than the site you linked to. You could get the 20mm open cars from vxd and shafting from McMaster? For a 48" travel you need some pretty hefty rails..

Misumi has some good and fairly economical stuff too.

Andrew

[WarrenW]

Thanks, I'm satisfied that I either need to use rails or shafts that are tapped and can be mounted all along its length to prevent movement and vibration.

Can I take a drilling rod and tap threads into it myself along the bottom to mount down? I can get a 12 foot drill rod from ToolAndDie.com for $75 and cut it to length.

Or cnclinx.com has one already tapped every 9 inched but its not long enough for my needs. Wonder if I cut them and mount them evenly together to make a longer rod if that will work?

Warren