Selection of the guide rail for jig

[hubihubi2]

Hi All,

I am constructing small device. Please refer to the picture attached.
It must be equipped with two guide rails along which the jig (red colour on the drawing) will be moving up and down on distance of approx. 250mm.
The jig will be driven manually, the weight of the jig is 5 kg and it will be balanced with counter weight.
The device will be working in ~20degC and in dusty environment.
I am on the stage of selecting the guide rail type for this device.
I consider three types of guide rails, please refer to the links below.


link 1 -

http://rhd.en.alibaba.com/product/6...r_guide_rail_linear_rail_for_cnc_machine.html

link 2 -

http://rhd.en.alibaba.com/product/1..._rail_SBR12_linear_block_bearing_SBR12UU.html

link 3 -

http://store.freertoolandsupply.com/danlyiem_adgps32x110.aspx

I would like to ask kindly for some guidance/opinion which guide rail would be most suitable for my device.
I think that guide rail from link number 1 would suit the best as it is not expensive and suitable for the load of 5kg+.


Thank you.

Regards

 

[MikeHalloran]

I wouldn't use alibaba.com for sourcing decisions. It's not a manufacturing company, but a web portal that represents a bazillion Chinese companies, in a perhaps overly enthusiastic way.

None of the guide rail systems you have proposed will work properly, given the kinematics you have shown.

Two nominally parallel bearings, rigidly connected to each other and riding on two nominally parallel shafts which are in turn connected to each other by a rigid frame, will chatter and bind and not provide enough guidance for any practical purpose.

You need to use three bearings on two shafts, and allow the odd bearing, or its support structure, to flex a little laterally in order to deal with variations in rail spacing.


Mike Halloran
Pembroke Pines, FL, USA

 

[dvd]

Try using a die set. It is already engineered. Just mount your components.

http://www.danly.com/PDFs/DanlyIEM-CatalogDieSets.pdf

 

[asimpson]

The ball bushings you looked at would suffer in a dusty environment. Plane bushings would be better.

http://www.igus.com/drylin

http://www.pbclinear.com/Gliding-Surface-Technology--Plain-Bearing-Linear-Motion

In your arrangement getting two columns parallel would very difficult. Especially with precision rails. As MikeHalloran pointed out. Perhaps a double rail on one side taking most of the load and a single on other side with some measure of compliance.

Plane bearings would also be a bit more forgiving on issues of binding.

 

[hubihubi2]

@MikeHalloran, dvd, asimpson,
Thank you for your opinions.
I had myself some doubts regarding to alignment issue of the guiding bars in the construction arrangment presented on the drawing sent earlier.
I have modified the construction, where there are two guiding rails on one side of the movable part of the jig (cantilever will be used).
New selected support system -

http://smi4motion.com/smi2/atuators/supported-systems/end-supported-systems-stage4a.html

The drawing of modified jig is attached to this post (orange part in it represents selected support system).

Please allow me to ask for your opinion about the re-design concept.

Thank you again for your replies.

Regards

 

[MintJulep]

The bearings don't appear to have any special provisions to deal with the "dusty environment".

The presumably steel shafts appear to be mounted on continuous aluminum supports. You say the thing needs to work at -20C. Unless it is always at -20C you should think about thermal movement. Aluminum and steel have different coefficients of thermal expansion.

In your sketch the orange thing appears to need to fit perfectly between the top and bottom plates of the green thing. Can you make the green thing perfectly?

 

[hubihubi2]

@MintJulep

The jig will be working in the temperature of approx. (~)20degC.
Fitting of the orange part- this is only concept drawing which will be converted into construction version later.

 

[MikeHalloran]

The dual guide assembly transfers the alignment responsibility to someone else, who can probably do it better in their factory than you can do it at your site. I'd call that a good thing.

As for dust, some ball bushing assemblies can be had with wiper seals or brushes.
That's one option that you can explore with Smi4motion.
Because those seals don't comprise a full circle, I wouldn't expect them to be effective against dust.

Another option is to fit a rectangular bellows over the entire assembly.
Said bellows are typically fitted to machine tools, are semi-custom, and cheaper than rolling your own.
Because of required modifications to mount the bellows, Smi4motion could probably do it better and still extend a warranty on the basic assembly.
They would have to add flanges to the end blocks, and build a box covering most of the carriage, except for the mounting surface for your stuff, and include the bellows in their final assembly.
Maybe they already do it; ask them.






Mike Halloran
Pembroke Pines, FL, USA

 

[SpecialtyMotionsInc]

I would just like to add a couple of miscellaneous facts that should be considered for this application or others...

For continuously supported shafting (using a support rail), a third pillow block is not needed. There will not be deflection in the shafting and with the pillow blocks properly adjusted / preloaded it will deliver smooth motion. In some instances, a second adjacent block is not required, but that is best defined by the size and weight of the load. A system with properly aligned shafting on each side, 4 pillow blocks in a 2x2 array with a rigidly attached load in between, will move linearly quite well.

Be careful of plane bushings. (We sell those, too, so we know their benefits and detractors.) They definitely have their place, but if they are mounted and separated by any real distance, they will bind. Typical rule of thumb is the "4:1 rule." The distance between two sets of plane bushings cannot exceed 4x the distance between 2 adjacent bushings. In short, if you have plane bushings that are not in close proximity to each other, be wary of binding.

Ball bushings are very good for keeping motion linear, independent of separation distance. Additionally, as was stated earlier, they are very commonly shipped with seals and they work quite well in dirty applications. Yes, bellows can be added and the system(s) fully covered; however, the amount of cost added for bellows is typically significant and for this type of application, not needed. Lastly, with the use of self-aligning ball bushings, any distortion within +/- 1 degree (thermal or otherwise), will be handled within the bearing itself without anything that the user will ever notice...

I hope this helps. If anybody has any questions at any time, look us up...

http://www.smi4motion.com

or give us a call (800) 283-3411.

Scott Cohen
Engineering Manager, Specialty Motions.

 

[MikeHalloran]

The exact nature of the dust has not been revealed.
Sawdust should not be a huge problem.
Ceramic, metal, or other abrasive dust would be a problem, and in that case the bellows might be justifiable.

Of course, if Scott gives you a really special price, you could treat the slide assembly as a consumable and just throw it away when it jams or gets wobbly. ;-)


Mike Halloran
Pembroke Pines, FL, USA