Max load on a bearing for a machine skate?

[extropic]

For conventional radial ball bearings, the bearing load ratings ASSUME the bearing is mounted in a housing that will support the outer race.
When you use the bearing assembly as a wheel (with no housing) the outer race will deform (flatten) under substantial load.
Cam followers are designed to run without an outer housing so the ratings are useful for the machine dolly application.
If you're going to use radial ball bearings, be very conservative with bearing size/quantity and/or make heavy wall steel "tires" to suit.

 

[pontiac428]

I have never seen a bearing load specification that states how much support is required to use the bearing at its rated load. I have never seen an assumption about how much support I will use printed on a datasheet packed with a new bearing. Also, the ball array distributes load across the balls rather than focusing all load on one ball. I may be wrong, I'm not a bearing dealer, but I've never seen this claim written down along with minimum housing support specs. It seems a little hard to do.

This guy on ebay uses 1.6" diameter bearings and rates them for 2,000 lbs each x 6 bearings per skate. That may be a little heavy for reality, but the design works. Of course you need a smooth floor for these little wheels to track on.

 

[RJSakowski]

Keep in mind that there are situations where the majority of ther vertical load can be placed on a single skate and the majority of the load can be on one or two of the bearings on that skate.

However, it is unlikely that one would see a catastrophic failure of an overloaded bearing, given the typical construction of bearings. Bearing load ratings are based on an assumed mtbf. I have busted a good many bearings to salvage the balls and application of shear force isn't sufficient in most cases. I either have to remove ball tetainer to crowd the balls to one side or cut the outer race with an abrasive disk.

 

[nnam]

A small piece of rock or metal can put alot of stress on one side of the bottom of a bearing, which is very hard, but prone to cracks. So the load may not be on the bottom ball or two, but on a side of it, creating very large torque. That is why I also think a small wheel or ring is an improvement to mone at all.

If using multiple bearings, then a tube can be used to distribute the load.

A roller bearing is even better, not sure about the cost.

 

[Mitch Alsup]

The static load is when the bearing is rotating so slowly that an impact force can cause the 2 most heavily loaded balls to make indentations into the race.

The dynamic load is when the bearing is rotating fast enough that one ball moves the equivalent distance between balls during the impact application.

 

[extropic]

I have never seen a bearing load specification that states how much support is required to use the bearing at its rated load. I have never seen an assumption about how much support I will use printed on a datasheet packed with a new bearing. Also, the ball array distributes load across the balls rather than focusing all load on one ball. I may be wrong, I'm not a bearing dealer, but I've never seen this claim written down along with minimum housing support specs. It seems a little hard to do.

You probably never will see a specification, from a bearing manufacturer, for an appropriate housing. I have read the requirement stated as "proper ring support". What is "proper" is left to the device designer, so the bearing manufacturer isn't responsible for any failure. I assure you that the published load ratings are determined by testing with the bearings mounted with "proper ring support". Therefor, the published ratings are not applicable to bearings not mounted with "proper ring support".

For any device, I hope that the device designer has sufficient engineering knowledge to determine what the minimum "proper" is.

The picture you provided shows an excellent example of very substantial looking skates. Does the manufacturer specify a Safe Working Load limit?

 

[MrCrankyface]

Great discussion, tons of good insight!
On topic of bearings needing support around the other ring, this is something I've also heard/noticed from working with linear bearings.
They will generally not work as intended unless pressed into the correctly sized tube/pocket.

Discussion aside, I ended up being really cheap and doing a steel on steel bushing instead though, everything from the scrap-pile so almost free.

Should help me move this in on monday, around 5000lbs!