Linear Bearing Problem

[grenfell]

We have a problem with linear bearings failing on one of our projects.

From 4 - 1" Open Linear Bearings (Ball bearing style)I have supported a structure which weighs approx 500lbs. From this frame I have a long set of forks (aluminium extrusion) that extend approx 8' beyond the frame the bearings are mounted to. On these forks a very light load is applied (sheet of fabric).

Bearings are mounted open side up ie suspended from rail.Bearings are rated at 2800 lbs each (static)

This frame is manually pushed by the operator as part of the process.

PROBLEM: even though we have cushioned limit stops, and the operators are trained to be "gentle" the shock to the bearings as the backstops are encountered cause bearing failures almost weekly. Plastic housing which holds the Bearing balls in place are being subjected to repeated hammering by the balls, ultimately punching through and breaking down.

I have spent quite a bit of time trying to locate a "super" bearing that can withstand these forces. We have also looked at better stops & cushions, but feel that the answer lies in the bearings.

Hoping that one of you guys out there can recommend a bearing manufacturer, or provide useful tips to solving my prob.

THANKS in advance!!!!

 

[willdo]

You may want to look at

http://www.pacific-bearing.com

and see what they have to offer. This may be an option for you. Their catalog also has very good examples of proper bearing spacing and loading.

How does running a linear bearing open side up as you describe change the bearing's capabilities? I'm not saying you did anything wrong, just curious as I have not seen this before!

 

[grenfell]

Thanks for the reply willdo,

Yep, I did look at Pacific - their bearings are pretty much generic, in fact the 1st set I used were Pacific, and then went on to use Thompson. It is documented that there is no load capacity difference up or down.

My main problem is the catilevered load, and the associated "leverage" on the bearings combined with the shock effect, when the back stop is contacted.

I am now looking at...
a) methods/techniques to protect the bearing from the shock loads, rather than beefing up the bearings. ANY idea appreciated
b) Frelon type bearings - Unfortunately, although coefficient of friction is similar to ball bearings, the initial inertia required to move from start is much greater. - so some ideas to overcome that problem would also be MUCH appreciated.

Great group - keep up the good work!

 

[sreid]

Could you use air or hydrolic dampers/snubbers to minimize the shock loads at the end of travel?

 

[Philrock]

Linear bearings such as the LM series by THK have much higher load capacity than the Thomson style you are using - they have line contact rather than point contact. They are also considerably more expensive. ABBA is a much less expensive Taiwanese alternative to THK.

http://www.abbatech.com.tw

 

[brgguy22]

Have you tried Thomson XR (Extra Rigid series) or IKO has preload versions available.

 

[blfarrar]

A company called ACE (USA) make dampers that can be integrated into many assemblies to dampen shocks at end of strokes.
Most of these are adjustable.

Can the bearing assembly be modisfied to mount the assembly on disc springs (Schnorr / Belleville / Ringfeder) or rubber mounts.

 

[diamondjim]

When you extend the forks, aren't
you also getting a moment load
which compounds the loading?

 

[diskullman]

You could also look at Lee Controls. They have a pillow block that utilizes cam followers instead of the standard linear ball bushing.

http://www.linearmotion.com

Russell Giuliano
Unique Technologies Associates - Cobra Solid Lubricant

http://www.uniquetechnologies.com

 

[gotglu]

The plastic inner housing is cracking/chipping away from ball bearing vibration most likely. You will see bearings coming loose, or popping out, which will crack the outter metal housing. Danaher Motion makes an all metal inner/outer housing for likely 50% more cost. The inner metal housing will not chip/spall/crack during impact and should not need replacing. Go to

http://www.danahermotion.com

and see their 143 page pdf file for bearings. Page 72 & 74 to choose the offset.

Danaher Motion brand is Thompson, while IKO brand in Tompson. The number of rows allows for better centering, however for your use likely not critical, as more rows or higher alloy stainless steel for chemical resistance adds cost to make. 5-6 row all metal will work for ~$20 more.