Frictional Loss in Roller Bearing

[crunchie12268]

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

 

[3DDave]

The biggest part is this - the amount of aerodynamic drag is the same for a given velocity and shape. For a given shape and speed, energy loss rate to drag will be the same, no matter how heavy the car is. A heavier car will have greater potential energy to put into drag loss and will have more left for kinetic energy in the form of speed.

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.

 

[tbuelna]

Regardless, the friction is tiny, and it is unlikely that something else is not orders of magnitude more important.

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

 

[tbuelna]

crunchie12268-

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.

 

[Tmoose]

Another calculator

http://www.fag.de/content.fag.de/en..._online_easyfriction/bearinx_easyfriction.jsp