Single deep groove ball bearing, radial load offset from bearing centerline

[ddowns46]

I have an application where a roller chain sprocket hub is supported by a single deep groove ball bearing that is supported by a 20mm shaft. In order to get the sprocket tooth count to a low enough number to support the application, the sprocket is situated adjacent to the ball bearing (to clarify, the OD of the bearing is larger than the required ID of the sprocket, creating the need for the offset). This obviously creates a moment/tilting load in addition to the radial load that is applied by the chain. I would like to hear your input on the feasibility of using the following bearing based on the following speed and loads:

Bearing: 6804-2RS
OD: 32mm
ID: 20mm
Width: 7mm
Dynamic Load Capacity: 900 pounds
Static Load Capacity: 594 pounds

Maximum shaft speed: 4000 RPM
Maximum Radial Load: 288 pounds
Maximum Tilting Load: 14.4 foot pounds

There is already a product on the market that uses this exact same setup so I know that it works. I am simply trying to understand what the margin of safety is.

It is not possible to add a second bearing and it is not possible to increase the bearing size.

 

[Tmoose]

Is this an idler sprocket or part of a tensioner?

 

[ddowns46]

Its actually a driving sprocket connected to a gas engine that produces about 15 foot pounds of torque. Based on this value and the diameter and offset of the speocket i was able to provide the radial force and moment.

 

[MikeHalloran]

It depends on the groove.
Not just deep, but also shaped like a Gothic arch, so that the balls have four points of contact.
Kaydon calls them 'X' type ball bearings, and provides rating info and tables.
The ratings differ some from Conrad type ball bearings.

I have used them successfully to carry timing belt sprockets that were driven by a shaft not capable of carrying a radial load. That situation may or may not be analogous to the problem you face; a decent sketch of a cross section would clarify your problem considerably.







Mike Halloran
Pembroke Pines, FL, USA

 

[ddowns46]

Mike, thanks for the feedback. I've attached a picture showing a section view of the sprocket and bearing. The sprocket connects to a clutch that is driven by the keyed shaft that the bearing rides on. The clutch is not shown for proprietary reasons.

i know for sure that my competitors product uses a standard deep groove ball bearing, although it sounds like a 4 point bearing would probably be the best option.

 

[MikeHalloran]

Of course, when the clutch is disengaged, there's no chain pull to load the bearing,
and when the clutch is engaged, there's no relative motion between the shaft and the sprocket,
so they can contact each other without major damage, so maybe you don't need a real fancy bearing.



Mike Halloran
Pembroke Pines, FL, USA

 

[ddowns46]

Both of those statements are correct. But since this clutch is a friction clutch, there will be times where the clutch is 'slipping' and the bearing is slowing down until it is spinning 1-to-1 with the shaft.

 

[MikeHalloran]

How long must the bearing last?
Does the clutch get hot enough to burn off the bearing grease? to melt the seals?
How many cycles in an hour? In the warranty period? In the expected lifetime?
How many rotations between the crankshaft and the sprocket bore at what radial load and speed before a serious complication appears?
Does your clutch, or the other guy's, have an internal radial bearing between its moving parts?

I personally don't need to know, but it's stuff you have to evaluate as part of what appears to now be a not so simple problem.



Mike Halloran
Pembroke Pines, FL, USA

 

[ddowns46]

How long does the bearing last?
- Hard to say, I have about 30 hours of testing completed on one bearing with no issues so far.

Does the clutch get hot enough to burn off the bearing grease, to melt the seals?
-No. Bearing maybe gets to 180 degrees at most

How many cycles in an hour? In the warranty period? In the expected lifetime?
-Challenging question to answer!

How many rotations between the crankshaft and the sprocket bore at what radial load and speed before a serious complication appears?
-Thats a question I am looking for an answer for.

Does your clutch, or the other guy's, have an internal radial bearing between its moving parts?
-No.


The real question I would like to have answered is what the margins are with the loads I am predicting and a constant 4000 RPM speed differential.

 

[MikeHalloran]

I suppose you could measure/compute/estimate the ball travel radius, and use that to decompose the actual moment into a faux axial load. Combine that with the radial load in the typical equations to compute a life.

Recognize that the actual life testing from which the equations derive, most likely used a true axial load distributed equally over all the balls, rather than an axial load at one ball center radius on one face of the races, and an opposing axial load at one ball center radius on the other face.
I.e., normal ball bearings are not rated for moment loads, so there may be no test data or equations for that load case, and any life number you get as outlined above is likely to be egregiously optimistic.

If the life test you are conducting now is to be statistically valid, you need at least nine other test articles.

Check out Kaydon's ratings for X-bearings sized and loaded similarly to yours, and apply a large safety factor.

See if maybe anyone has published data for ordinary bearings with moment loads applied.




Mike Halloran
Pembroke Pines, FL, USA

 

[ddowns46]

Mike,

Your comment as to whether my competitor might have some sort of radial bearing has got me thinking...
A clutch basket is secured to the sprocket and that clutch basket is essentially splined to friction plates. Those friction plates that are clamped concentrically to a flywheel that is keyed to the shaft that the whole assembly rides on. I believe that as the clutch is engaged, the forces applied to the sprocket increases (bad), but the clutch basket starts getting supported by the driver via the friction plates (good). In effect, the moment load is not completely taken by the ball bearing.