Frictional Loss in Roller Bearing
Frictional Loss in Roller Bearing
(OP)
Hello,
I am in the process of selecting a bearing for a tension control application and came across a question I would like to bounce of someone. Say I have a cylindrical roller bearing with a radial force of 4000lbf. Assuming everything else remains the same I increase the radial force to 5000lbf do I see a decrease in, what I am going to call, a friction moment? I guess I just expected to see an increase in friction but wasn't sure if bearings are designed to do this? Thanks
I am in the process of selecting a bearing for a tension control application and came across a question I would like to bounce of someone. Say I have a cylindrical roller bearing with a radial force of 4000lbf. Assuming everything else remains the same I increase the radial force to 5000lbf do I see a decrease in, what I am going to call, a friction moment? I guess I just expected to see an increase in friction but wasn't sure if bearings are designed to do this? Thanks





RE: Frictional Loss in Roller Bearing
Timken gives an equation. http://catalog.timken.com/WebProject.asp?CodeId=7.... page 45
Cheers
Greg Locock
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RE: Frictional Loss in Roller Bearing
RE: Frictional Loss in Roller Bearing
Google "1.2 Calculation of the Frictional Moment" and look at the first entry.
Or maybe this link will work (not sure):
http%3A%2F%2Fwww.m3.tuc.gr%2FANAGNWSTHRIO%2FSTOIXEIA%2520MHXANW...
It has spread out over 2 pages a detailed discussion of
M0 [N mm] load-independent component of the frictional moment
M1 [N mm] load-dependent component of the frictional moment
also variation of these components with speed.
A 3-d figure of these components and variation with speed... also a discussion of mixed component... makes me scratch my head.
To my simple thinking the sliding that results from deformation at the contact surfaces (non-ideal rolling) would be load dependent, and the sliding that results from interaction of the rollers and the cage would be load independent. But clearly there's a lot more to it.
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(2B)+(2B)' ?
RE: Frictional Loss in Roller Bearing
Attached is Electricpete's linked pdf
RE: Frictional Loss in Roller Bearing
That is simply what happens.
Otherwise folks would "design" them to have NO frictional moment, if it weren't for the kickbacks from the oil and power giants.
Looking at that FAG document Note even at middlin' loads the lubricant (oil) churning etc that make up the "load independent" part of the frictional moment exceed the bearing's EHD frictional moment.
Page 17 mentions Sealed, greased bearings friction can be 3X higher or greater (after even hotter run-in) and are best determined by "FAG measuring system R27". I'm //guessing// that is an actual test device.
As far as I could see no calculation to predict FM for sealed, greased bearings is offered.
I download all SKF and especially FAG (and others) bearing documents from the 60s thru 90s because as near as I can tell they are no longer available. The online calculators are convenient but fall short in their explanations and entertainment value.
RE: Frictional Loss in Roller Bearing
Oil lubed rolling element bearings, like your cylindrical roller bearing, incur two basic types of mechanical loss- frictional loss and viscous loss. With a high DN bearing, the viscous losses will predominate. With a lower DN bearing, the friction and viscous losses may be fairly similar. In your example you specifically asked about the difference in "friction moment" between two identical bearings operated under identical conditions with the exception that one had a 5000lb radial load and the other had a 4000lb radial load. But since you provided no other details of how the bearing is operated it is not possible to give you a precise answer.
With most bearing systems, what is usually of primary concern is power loss rather than friction torque. Friction torques are usually only a concern with bearing systems that oscillate or rotate very slowly. Thus, with bearing systems that constantly rotate, what matters are the combined friction and viscous losses. Based on your question about two identical bearings with one having a 25% higher radial load, the answer would be that the bearing with the higher load would have lower losses as a percentage of the power transferred through the system. Basically, the bearing with the lower load is oversized for the application.
RE: Frictional Loss in Roller Bearing
We compare effect of load for two identical bearings at same speed.
The higher loaded bearing has higher losses.
What basis do we have to infer anything about power transferred through the system (alternatively: why would we assume anything other than that the power transferred through the two systems is the same).
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(2B)+(2B)' ?
RE: Frictional Loss in Roller Bearing
No, the higher loaded bearing would not necessarily have higher losses as a percentage of the power transferred thru the system. Consider the case of a high-speed gearbox where bearing DN is high and loads are low. With such a case, bearing viscous losses can easily be many times friction losses, and viscous losses are independent of radial loads.
If we assume that the bearing radial loads from the gears are 25% higher for the same rpm, then we can assume that the power transferred is also 25% higher. In this case, the friction losses for the bearings with the higher radial loads might only be about 2% or 3% higher, while the bearing viscous losses would be similar. Yet the combined losses of the more highly loaded bearing system, as a percentage of the power transferred thru the system, would be lower.
In short, with regards to efficiency of high DN bearing systems, it is critical to optimize the pitch diameter of the bearing for a given load. If a cylindrical roller bearing has the capacity to accommodate a 25% increase in radial load for the same duty cycle, then it is substantially oversized and will not provide the best efficiency.
RE: Frictional Loss in Roller Bearing
With gears I can see the logic of your comment.
But the op said nothing about gears and your first comment said nothing about gears... so at that point it seemed kind of bizarre to me that you would invent a relationship between radial bearing load and transmitted load.
Thanks for explaining what you meant.
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(2B)+(2B)' ?
RE: Frictional Loss in Roller Bearing
My experience, admittedly with small bearings, is that bearings are sealed with rubber seals. These cause substantial friction, way in excess of the rolling resistance of the balls or rollers.
My college machine design textbook (V.M.Faires) has notes on bearing friction. Perhaps yours does! According to Faires, seals can increase bearing friction by several hundred percent. His reference is Shaw and Macks, Analysis and Lubrication of Bearings, McGraw Hill. This reference appears to be available online.
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JHG
RE: Frictional Loss in Roller Bearing
Your are correct that the OP did not specifically mention gears. There was only a vague reference to a "tension control application". I simply used the example of a gearshaft bearing set to illustrate that simple friction moment is often not the primary concern when optimizing the design of rolling element bearing systems.
Since you did not like my example of a gearshaft bearing system, how about considering the example where the bearing supports a belt drive pulley. As the power transferred by the belt drive increases, the belt "tension control" system applies a greater radial force thru the pulley bearings to prevent the belt from slipping. In this case, all other things being equal, wouldn't a 25% increase in radial force at the pulley bearings used to create tension in the belt basically equate to a 25% increase in transmitted power? Same pulley rpm, same pulley diameters, same margin of safety for friction between the pulley/belt, etc.
Section 9.2 of this NASA handbook gives a good explanation of all the factors involved in mechanical losses of rolling element bearings.
Regards,
Terry
RE: Frictional Loss in Roller Bearing
Thanks for the link. Kinda funny how some will jump your case. I followed along right away.
RE: Frictional Loss in Roller Bearing
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(2B)+(2B)' ?
RE: Frictional Loss in Roller Bearing
Thanks for the reply. My intent was to simply get the person asking the original question to think about the situation, and hopefully respond with more specific questions. Unfortunately, I'm not very good at expressing things in writing myself. So if someone "jumps on my case" a bit over one of my posts I probably somewhat deserve it.
RE: Frictional Loss in Roller Bearing
Transmitted power should not be significantly affected by the friction torque from the bearings. If it is, the bearings absolutely are not working.
Gears and pulleys apply side loads to shafts, but these are normal to the driving force. The only thing that affects the output power is the friction torque.
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JHG
RE: Frictional Loss in Roller Bearing
drawoh, It looked to me like the point was the reverse position, that friction torque can be significantly affected by transmitted power when the load at a given RPM is proportional to transmitted power.
RE: Frictional Loss in Roller Bearing
drawoh, your first comment about the impact of bearing friction on the mechanical efficiency of a rolling element bearing system not being significant is basically true. All other things being equal, an increase in radial load can affect friction losses slightly due to the influence of radial load on hysteresis and thickness of hydrodynamic fluid film contacts.
The OP did not really provide enough information to allow a definitive answer to the question. Unfortunately, in real life, the design of rolling element bearing systems can be quite complex, and there are often other factors that contribute to mechanical losses far more than friction. For example, with high DN roller bearings things like viscous/churning losses and skidding of the rollers in the unloaded race sector can be serious concerns.
The basic premise ("all other things being equal") behind the friction question asked in the OP ignores the reality of how rolling element bearing systems are designed. A well designed rolling element bearing system (ie. one that is fully optimized for fatigue life, efficiency, reliability, cost, weight, size, etc.) would not likely be able to accommodate a 25% increase in load without modifications. And the modifications needed to accommodate a 25% increase in radial load (typically an increase in roller complement PD) would likely increase friction losses in the bearing.
This is a very interesting topic of discussion to me. Maybe crunchie12268 can provide some additional details of the bearing system in question.
Regards,
Terry
RE: Frictional Loss in Roller Bearing
For the record, my reference, V.M.Faires, quotes equivalent friction coefficients, which means that the friction torque in the bearings is proportional to the side load. The friction coefficients quoted are between .0011 for deep groove ball bearings, to .0045, for needle bearings. The values are based on a load that allows one million revolutions, "and are suitable only for order of magnitude approximations".
Regardless, the friction is tiny, and it is unlikely that something else is not orders of magnitude more important.
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JHG
RE: Frictional Loss in Roller Bearing
The rules DO have a max 5 oz weight limit.
The best guess I can come up with is there is something about steel/plastic/graphite bearings' coefficient of friction that lowers with increasing radial load.
RE: Frictional Loss in Roller Bearing
Putting the CG to the rear gains a few inch-ounces of energy. Friction losses with the guide rails is another place for energ to go, but I don't know if that is weight independent or not.
If anyone ever finds the 'best' solution, then it won't be much of a race.
When my brother's kids were at it I suggested burying a 1/2a rocket motor (booster, so no ejection charge or locator smoke) in the body with a battery and a tilt-switch. Nowadays an Arduino or PIC watching an accelerometer so it could fire just as it gets to the level part of the track would be nice. Cheating? Heck yes. But it would be a real crowd pleaser; much better than the E-Bay polished axle and wheel sets.
RE: Frictional Loss in Roller Bearing
drawoh, yes the losses due to friction in the bearing design considered may be modest. But all we know for sure are that this is a roller bearing system, and the two cases involve a 4000lb and a 5000lb radial load.
Here are a pair of simplified equations that give approximate viscous and friction power losses of oil-lubed cylindrical roller bearings (power is HP, PD is inches, dyn cap is lbs):
viscous power loss = (4.76x10-9)(RPM^5/3)(bearing PD^3)
friction power loss = (7.14x10-10)(bearing dynamic capacity)(bearing PD)(RPM)
With the equation for friction loss, what you should note is that bearing dynamic capacity is indeed a factor. While the OP used the qualification "all other things being equal", in reality we should assume a 25% increase in radial load would require an increase in bearing dynamic capacity. If you accept that the friction power loss equation shown is valid, that would mean the friction power loss is proportional to the required increase in bearing dynamic capacity.
Looking at the equation for viscous losses, you'll note that the losses increase at bearing PD^3. So using a roller bearing with even a slightly larger PD can have a big effect on viscous losses. If the bearing system being considered operates at high RPMs or high DN, viscous losses can be quite significant.
Interesting discussion.
Terry
RE: Frictional Loss in Roller Bearing
If you're still out there, just for fun I ran a couple friction cases for your roller bearing using SKF's analysis tool. You did not provide details of the bearing size or operating rpm, so I used an NU306 roller bearing. Unfortunately, the analysis showed this bearing may be a bit undersized for the 5000lbf load case. I was too lazy to re-run the analysis with a larger bearing, but I don't think the basic trend with regards to friction losses would be any different.
I ran two cases where all input parameters were identical except that one used a 4000lbf radial load and the other used a 5000lbf radial load. This gives a comparison of the effect of radial load on friction losses.
I also ran three cases where every input parameter was identical except for inner race RPM. I used 1500 RPM, 4000 RPM and 8000 RPM, which would be a typical range of speeds for this particular bearing. This gives a comparison of the effect of RPM on friction losses.
The results are shown in the attachment. With the comparison between a 4000lbf and 5000lbf radial load, the total friction moment increased from 1.84 in-lb to 2.01 in-lb. The difference is less than 10%, and most would agree this 10% difference in bearing friction moment would not usually present a concern. But the results do clearly show that radial load does have an influence on bearing friction moment, all other things being equal.
RE: Frictional Loss in Roller Bearing
http://www.fag.de/content.fag.de/en/products_servi...