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Slide Rail Shapes

08/24/2012 5:04 PM

I've designed a number of machines using X-Y slide tables, but this time I need a long (1.5m, 60") travel on the X axis, so I'm looking at rails. The machine is for tack-welding turbine engine parts, where I need location within 1-3 thousandths of an inch, and vertical support at all points of X-travel. I don't think the U-channel guides I've seen have sufficient rigidity, but either the profile or the supported round bar types look good.

Can anyone guide me on choosing a rail style?

Thanks for your experience.

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#1

Re: Slide rail Shapes

08/24/2012 5:50 PM

I've been involved in the design/implementation of several linear carriages or positioners so I'll throw out some things I have seen work.

The support beam is a square or rectangular tube. Its cross section and thickness is determined by the load it must support at a given length. All four faces have a hardened surface added to them where the roller elements, cam rolls, run. The moving carriage has two sets of cam rolls at each end. Each set of cam rolls are positioned around the tube so that there are four in each set which make for one cam roll on each side of the tube. Two sets of four make a total of eight cam rolls.

I call the elements cam rolls because they are a bearing on an eccentric shaft so that proper contact can be adjusted. The bottom two cam rolls may not be required or may be even spring loaded if gravity does not keep the carriage flat due to acceleration of the carriage.

Another possibility are V shaped cam rolls that ride on a bar that has a V machined into the top and bottom. This cuts the cam roll count in half to four.

Another possibility is to go to one of the many "erector set" type motion structure companies out there like Thomson. Companies like this offer various methods including recirculating ball bearing "trucks" that offer some advantages/disadvantages.

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#2
In reply to #1

Re: Slide rail Shapes

08/25/2012 8:59 AM

Thanks for your input.

You are apparently referring to other, and I think lighter duty, systems than I was considering.

End views of the ones I had in mind were roughly these:

The first two are round bars, one with circulating balls, and one with rollers. the other three are what I referred to as profiles, again with balls and rollers. If you rotate 3 or 4 by 90°, I think we have your second style.

Actually, in preparing these drawings, I think I have answered my own question. Clearly the round bars have smaller contact area than the flat mating surfaces of the profiles, with the rollers providing the greatest area, and therefore the least deflection for a given force applied.

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#3

Re: Slide Rail Shapes

08/25/2012 2:14 PM

dk, have a look at Franke Linear Bearings. They're better suited to a not so pristine environment due to the roller design. We've used the typical Thompson or NSK rails/carriages setup in the past but they are not that forgiving when it comes to dirt between the rail and carraige. We switched to Franke bearings for certain applications and they work really well.

If you need a really heavy duty rail then check out Pacific Bearings Hevi Rail line. We use them for lifts, high capacity gantry type x,y positioning, etc. They are not very accurate by design but it depends how you set them up.

Cheers.

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#4
In reply to #3

Re: Slide Rail Shapes

08/25/2012 10:57 PM

Thanks.

I was not aware of Franke. They do look very interesting. Unfortunately, I could not see how to register as a new user, and was unable to use their application. I have sent them an email...

We need greater precision than appears to be provided with the Hevi Rail line.

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#5

Re: Slide Rail Shapes

08/26/2012 7:53 AM

I have been advised that http://www.schneeberger.com represents "state of the art" in precise rigid linear transport.

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#6

Re: Slide Rail Shapes

08/26/2012 7:54 AM

Take a look at THK linear bearings, they are expensive, but very strong and accurate.

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#7

Re: Slide Rail Shapes

08/26/2012 1:11 PM

Thanks everyone. It seems to be unanimous, since no one so far has recommended any brand of the round rail type (1&2 of my second post).

I decided that due to the intermittent forces involved, I don't want to risk the steel-wire-in-aluminum type. I prefer to use made-in-USA when possible, so of those recommended, I'm leaning toward THK. I Prefer the website structure of DelTron, but they apparently don't make the long rails I need. I'll contact THK tomorrow.

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#8

Re: Slide Rail Shapes

08/27/2012 9:25 AM

This is my area of expertise and I'd like to make some general comments for you to consider before you make your final decision. There are 3 prevailing linear motion technologies for welding machinery in the field today. They are wheeled (cam followers,DualVee wheels,Roller followers,etc) ,Plane bushings, and linear guides (THK,Thomson,etc)

Wheel technology is easiest to mount,forgiving to variations in the mounting surface,unaffected by weld splatter,and easily adjustable. Bishop-Wisecarver (www.bwc.com) is a market leader in this. Also Rollon (www.rollon.com) is a successful version of this technology. Typically wheeled parts don't require machined surfaces to work perfectly and hold up under the worst of conditions.They can move in excess of 15 ft/sec.

Plane bearings from companies such as LM76 and Pacific Bearing aren't bothered by weld splatter and run on round shafting. These are easy to mount on almost any surface but care must be taken to ensure the load center is not more than 2x the spacing of the bearings or severe binding will result.They wear over time and can't be adjusted. They move in excess of 15ft/sec.

Linear guides such as THK,Thomson,Schneeberger,etc. that use recirculating balls or rollers on a machined trackway are the most rigid,hardest to mount,and are susceptible to failure with weld splatter. To protect these bearings you will need scrapers,bellows,and in some cases special seal sets. They must have machined surfaces and be perfectly parallel. If you are able to meet these criteria they will provide you with the most precise movement of your welding head. They typically will require a light to moderate preload for your application.

I prefer wheels for these applications but every machine is different. Good Luck.

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#9
In reply to #8

Re: Slide Rail Shapes

08/27/2012 12:22 PM

Thanks for your input. By most people's standards, these are tiny welds. We are just tacking the edges of the foil to hold it in position while in a brazing furnace. The braze is the permanent attachment. It is a capacitive discharge weld of very low energy, so there is virtually no splatter, and the microscopic bits that do get thrown off freeze to powder almost instantly. We've been doing this on standard X-Y Tables for years, with no problems I'm aware of. The difference is that this time we need greater travel, hence the investigation of rail/guide types.

We do need a repeatability within 1-3 thousandths of an inch, so we must have machined surfaces. The motion along the rails is mostly during setup, so speed is not an issue. I'm now down to investigating different versions of rollers on profiled rails.

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#10
In reply to #9

Re: Slide Rail Shapes

08/27/2012 2:08 PM

That was great input from Randy, he obviously knows what he's talking about. But if you wish to continue investigating the THK linear ways, be sure to look at their ability to resist torque and their zero play peload. I was able to build a couple of different machines with just one over sized rail with two bearings holding a cantilevered work arm. That required only one straight machined groove in a heavy walled 6x6 tube. I had the tube stress relieved prior to machining and used the pleated rubber way guards. I'm not a salesman for THK, just have had good luck with their product.

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#11
In reply to #9

Re: Slide Rail Shapes

08/27/2012 4:19 PM

Capacitive discharge welding is our specialty. We build and sell our own patented welders that do not give off any molten particles (expulsion). This is done by having exceptionally low output impedance so the the machine acts essentially like a constant voltage source. This dynamically and automatically limits the weld current to eliminate expulsion. This is a great help in maintaining a clean environment. My guess is that you could use recirculating ball or roller technology without having to protect the bearings. Our welders are used to seal medical implants where expulsion would cause reliability problems.

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