Concentricity needed for stub axle?

[321GO]

Hello Guys,

i have a axle stub which seats two separate taper bearings "back to back" configuration for a trailer axle. The ID's of the two bearings are slightly different from each other. Please see attachment for the axle stub.

Maybe someone can help me with this:

1) how much concenctricity is generally allowed between the two seating diameters of the stub? What effect does this concentricity have on the load distribution in the bearings?
How "critical" is this? I'm not been able to find any usefull info regarding this though.

2) why are the two ID's of the bearings slightly different (0,5mm in diameter)? What is the idea behind this? Why not equal ID's for both bearings?

 

[Metalfixer]

Hi,
2) why are the two ID's of the bearings slightly different (0,5mm in diameter)? What is the idea behind this? Why not equal ID's for both bearings?
I think it might be for assembly , it's easier to install the hub without having to go over the smaller diameter first with equal size bearing I.D's
Just a thought.
Larry

 

[321GO]

Hi Metalfixer,

you're right, of coarse that makes sense.
It would be nearly impossible to slide such a fit over such a long distance (due to normal machining tolerances).

Thanks. Now, somebody any idea for number 1?

 

[flash3780]

If I'm not mistaken, stub axles (like those in my 1975 Chevy Nova) are often a clearance fit to the inner bearing raceway.

I'm sure you'll want the runout to be controlled quite tightly, as well as the diameters. I wouldn't specify a concentricity - it is difficult to measure and unnecessary for this application.

At the end of the day, your gap is going to come from your tolerance stackup. I'd work with a manufacturing engineer to determine what tolerances are achievable for your application. Turned shaft diameters are quite often controlled to +/-.001" dia. Since they're often turned on the same setup, the runout is often quite accurate as well.

 

[Compositepro]

Concentricity is rarely an issue because only an idiot would machine those two cylinders in two set-ups. They are machined in one set-up so the centers are the same. But there is always a tolerance. Assembly is far easier if you do not have to press the first bearing over the seat for the second, particularly if they are not concentric.

The outer races may not be the same either. It is difficult to get two bearings with this geometry to share load equally. One bearing is probably intended to carry much of the load while the other keeps the parts in alignment.

 

[bobrax]

The diameter differences are to allow sliding the first bearing race over the seat as already stated. I also agree with the comments that concentricity wouldn't be much of an issue due to setup. A concentricity tolerence of .020" should be reasonable and obtainable. The double tapered roller bearings are intended to compensate for small misalignment. I would suggest you look closer at how the stub is mounted to the axle itself. If this is a trailer intented to be on public roadways it is important for the axle to be square and capable of supporting the intended load to prevent dogtracking or instability.

 

[Tmoose]

"Accuracy of form"

It's in bearing catalogs over 1/4 inch thick, and online.

cylindrity (total radial runout)

rectangularity (total axial runout of shoulders, etc)
I Don't want the bearing bore fighting to hold the race abutment face off the shaft shoulder. Bearing Races are not usually capable of supporting themselves. They must rely on the housing and shaft.

The bearing manufacturers will likely specify less than 0.001" for "normal" bearings up to 2 inch. Yes, it is possible to get away with a bit more, but troubles with noise, poor load sharing and maybe even heat generation will emerge sooner or later.

 

[321GO]

Tmoose,

i agree with your view.

If you look at how precise the grinding is of (roller) bearings, how can one justify that in a double setup, the spindle geometry does not need to precise? That does make much sense, in all respect.

For instance: bobrax wrote:
"A concentricity tolerence of .020" should be reasonable and obtainable. The double tapered roller bearings are intended to compensate for small misalignment"

Now, that's a lot of a concentricity if you ask me. How can a taper roller bearing "cope" with such a misalignment, wouldn't that induce an unintended loading of the bearings, like Tmoose noted?

 

[gruntguru]

Absolutely! 0.020" eccentricity would kill the bearings pretty quickly. I know these are tapered rollers but think of cylindrical rollers or deep-groove ball bearings and how much radial load would be required to deflect each bearing 0.010" radially.

 

[tbuelna]

321GO,

Concentricity of the two stub axle shaft diameters is very important. A duplex pair of tapered roller bearings like yours is designed to take combined radial, axial and moment loading. In order to achieve the bearing system's design fatigue life, it must have precision mounting and be properly preloaded. If the mounted bearings do not have the required geometric alignment or specified preload, then the rollers will edge load and/or not load share properly.

The allowable shaft runnout and concentricity is usually determined by the crowning on the rollers. Regardless, .020" radial offset is probably excessive for your bearing set. .002" would even be a bit more than you would want to see.

Hope that helps.
Terry