Linear bearing capacity

[dozer]

I'm an igmo in this whole bearing thing so you'll have to excuse me. We hired a third party to design a door that will slide along a rail and our customer is asking questions that led me to find out, guess what? The guy we hired is an igmo too!

Anyway, he's using an open self-aligning linear bearing right out of McMaster-Carr. The table says that it has a dynamic load capacity of 3380 lbs. (or something like that, this is from memory). Our client is asking if this is the capacity with a factor of safety or the maximum it can hold period. In my world, anytime you say capacity in a context like that, you mean safe capacity (ie, with a factor of safety) but on the off chance you bearing guys operate different I'll ask, which is it?

What I'm really more concerned about is the designer we hired showed me a catalog cut from a vendor that may or may not be who actually makes the bearing and it gave a static capacity much less than a dynamic capacity. Now I'm really confused. Can somebody shed some light?

Maybe point me to a web site that explains static vs. dynamic bearing capacity. I tried googling it but didn't really come across anything very helpful. Thanks.

 

[volpegrigia]

The static capacity of a bearing (linear or otherwise)defines the load one can place on a bearing that would cause a defined contact stress at the most heavily lioaded contact or, as an alternate criteria, a permanent deformation (usually 0.0001xdw, where dw is the rolling element diameter). This is a physical load. The factor of safety is nothing other than the the static load rating divided by the actual load.

The dynamic load rating is a 'potential" and it is expressed in term of allowable load to reach a given durability (life). For conventianoal bearings, the dynamic load rating is defined as the load that would allow 90% of apparently identical bearings to reach or surpass the durability of 10^6 cycles (or revolutions). In linear bearings, the ratings means the same, but the durability is usually espressed in travelled length and not in revolutions. The static rating is purely a function of geometry and materials of the elements in contact. The dynamic rating is derived using statistical data of materials endurance to repeated stresses.

There are a couple of books you can get that are quite good in covering all of the issues related to bearing design, calculations etc.

The titles of these are:
Rolling bearing analysis - Tedric Harris - Wiley
Ball and roller bearings -Theory, design and applications -Herschmann - Wiley

 

[dozer]

Thanks Volpegrigia, what you are saying makes sense to me. Let me give you some specific information and perhaps you or someone out there can decipher it.

I'm looking at a Nook Industries catalog. For the 1 1/2" open single pillow blocks they give a dynamic load of 1307 lbs and a static load of 867 lbs.

So let's say that the travel life is very small, less than 1 million inches and the Rockwell C hardness of the shaft is at least 60. This way you don't have to take a reduction on the dynamic load capacity.

So here's my first question. Is the allowable 1307 lbs or 867 lbs. If it's 867 lbs then they do a very poor job of explaining it, though that is what I think it would be.

Now question two. In the McMaster-Carr catalog they have a bearing that looks identical to me. I know looks can be deceiving. Anyway, all they list is a dynamic load capacity of 3880 lbs. No mention of static load.

My question is how can there be two so different values of bearings that appear to be the same? While I'm at it, hasn't MC done a huge disservice by not mention bearing life?

 

[dimjim]

I too am mystified as to why the daynamic
would be greater than the static load, tho
I do not know much about linear bearings.

 

[volpegrigia]

The maximum allowable load is the 867 Lb. The 1307 Lb is not a load, it is only a number used to assess life. This is a shortcoming of the industry, though arguably, they would have to publish encyclopedias to include everything. If you place 1307 Lb on the bearing you will have plastic flow and a drastic reduction in life. I agree with you it is confusing. To confuse you further, I can tell you that load ratings (dynamic) are a function of the stroke length. This is how it works.
Since the load rating (dynamic) is derived by the probability of failure of the elelments in contact and such probability is a function of (among others) of the number of stress repetitions a portion of material is subjected to, then a long stroke will have one of the two elements (shaft) experiencing fewer stress cycle in a long stroke application hence the life would likely be greater. This is also a thing that many of these manufactureres do not tell you.

Also, MC is not a producer or an engineering house. It is a big "Hardware store" and they simply buy and resell items. They may have erroneously copied the load ratings. My suggestion would be to stick with the data provided by bearing producers (and use static ratings as the maximum allowable limit).

 

[volpegrigia]

dimjim,
The dynamic lad rating is a great variable. Many manuactures publish data for a life of 2x10^6 inches (US producers such as Nook, Thomson), some for 50km (far east producers) and some for 100km (european producers). The same bearing ought to have the same potential (i.e. dynamic load ratings) yet the published number is different for each life basis.

If you start with a 50km basis (100%) you will have a reduction of 21% for ball systems (1/2)^(1/3) and 19% for roller bearings (1/2)^(3/10). This means that ratings published by European producers would be 79% lower than the ratings published by a far east producer for the same ball bearing.

I was, and still am, an advocate to use static ratings to compare units and to size bearings. Dynamic load ratings should be used solely to assess the theoretical life.
Please remeber that the dynamic rating is a fatigue life predictor but less than 10% of all bearings fail due to fatigue.