## F1 in schools bearings

Post here information about your own engineering projects, including but not limited to building your own car or designing a virtual car through CAD.
0
richard_leeds wrote:Are we agreed that the bearing is subject to relative movement of the two rings? The direction of movement relative to outside observers is irrelevant? Agreed?

So that brings us onto Tok's experiment , it is theoretically not possible. Is it repeatable? What secondary effects are going on to result in different movements?

I'm still due repeating it. But I'd guess the balls are slipping. The fact that they are being spun with no load at all is one good reason for it. And we don't know in which shape that beaing is to begin with, but we can see the rusty spots.
rjsa
12

Joined: 2 Mar 2007

0
Try Toks experiment with a geared epicyclic and the results will be the same as for a bearing less the slip.

More important to the project, is the fact that with the outer race of the bearing in the wheel, the balls will rotate in the same rotational direction as the wheel, 'forward' in the direction of vehicle movement.

If the outer race is in the chassis, the balls will be forced to rotate 'backwards, creating higher friction.

I doubt I would choose bearings with any separate rotating elements if there is a choice.
Depends on the side loads expected.
autogyro
33

Joined: 4 Oct 2009

Why is the friction different? The balls only input is the relative movement of the rings, its all about rpm. There is no input in relation to the direction of travel of the vehicle?
richard_leeds
Moderator

Joined: 15 Apr 2009
Location: UK

0
autogyro wrote:Try Toks experiment with a geared epicyclic and the results will be the same as for a bearing less the slip.

More important to the project, is the fact that with the outer race of the bearing in the wheel, the balls will rotate in the same rotational direction as the wheel, 'forward' in the direction of vehicle movement.

If the outer race is in the chassis, the balls will be forced to rotate 'backwards, creating higher friction.

I doubt I would choose bearings with any separate rotating elements if there is a choice.
Depends on the side loads expected.

See if I get you correct gyro, you're saying the opposite to Tok, that if the outer race is stationary, the friction is higher?

Oboy, so much engineerish creativity, I have to consider all this for a little while.
"Bernoulli is a nine-letter name"
xpensive
12

Joined: 22 Nov 2008

1
Right let get back to some basic geometry

I rpm of the inner race has a shorter circumerance than 1 rpm of the outer race. Thus the ball will travel less distance

A little bit about bearing friction

Bearing friction is dependant on the pressure created by the tolerance fit bettween the bore and the shaft the bearing is used to support. These tolerances are use to gurauntee that the races don't slip on the bore or the shaft when the parts expand and contract.They are also there to stop premature bearing failure due to axial radial movement and vibration.

For F1 i schools you need to use a finger press fit which very gently holds the bearing in place. please look at 1/12th RC race car front wheel.

An live axle supported by a bearing will require some method of controlling the axial movement of the shaft in bearing a nut or circlips. Which is stronger but longer. Than using a a dead axle with the bearings in the wheels. which shorter and slightly lighter.

To Prepare bearings for RC competions use. First make sure everthing is clean any dust of drit will ruin the prep work
Remove the bearing seals and a soak the bearings in brake cleaner to remove the OEM lubricant.

Then let the bearings dry out as brake cleaner is a solvent. Then use a lubricant I found, finish line dry chain lube the best. Then place the bearing on the axle and spin the excess lube out, by spinning the outer race. Wipe the bearing dry and put them in the parts. Do not refit the seals as they are a source of friction

You will find some bearing are better than others due transportating damage manufacturing tolerances etc. So i would advise you to buy say 20 bearings and pick the best of the bunch i.e the ones that spin the longest.

For sizing bearing it is usally based on the radial load. Using the smallest size possible will reduce the rotating mass and hence reduce the amount of energy used to turn the wheels though this is still dependant on load. idealy you should use two bearings per an axle to minimise radial movement
Smokes
1

Joined: 30 Mar 2010

0
Smokes wrote:Right let get back to some basic geometry

I rpm of the inner race has a shorter circumerance than 1 rpm of the outer race. Thus the ball will travel less distance

A little bit about bearing friction

Bearing friction is dependant on the pressure created by the tolerance fit bettween the bore and the shaft the bearing is used to support. These tolerances are use to gurauntee that the races don't slip on the bore or the shaft when the parts expand and contract.They are also there to stop premature bearing failure due to axial radial movement and vibration.

rjsa
12

Joined: 2 Mar 2007

0
Smokes wrote:...
Using the smallest size possible will reduce the rotating mass and hence reduce the amount of energy used to turn the wheels though this is still dependant on load.
...

This thread is breaking some serious ground now!

But seriously, this link is a good start to get things straight, it even has an on-line calculator, just take it from the top;

http://www.skf.com/portal/skf/home/prod ... ink=1_0_35
"Bernoulli is a nine-letter name"
xpensive
12

Joined: 22 Nov 2008

0
SKF is great for industrial applications you will find these bearing are packed with grease and do not have the low friction that you require. this because they are designed for very long lifecycles and contant running at a steady speed and the grease is designed to drop vicosity as the bearings warm up.

For F1 in schools the best ant cheapest option is to use ceramic bearings for a R/C race cars.

The other reason why mounting the bearing in the wheels is basic physics essentaly the axle is pushing the bearing to generate a turning moment at the bearing. turning moment = Force x Distance or in this case Force x Radius if you assume the mass on the bearing x the friction cofficent of the bearing will give you the intial force required to over come the friction of static bearing. If you divide that by the radius you will find that you need less force to over come the static friction on the outer bearing race than on the inner bearing race. This means you need to use less energy to over come static friction hence get of the line quicker.
Smokes
1

Joined: 30 Mar 2010

2
I am now writing about bearing fatigue life. It is what really determines what size bearing is required in most cases since that is what normally causes a bearing to fail (excluding unusulal causes such as lube failure, crashes etc).

As I recall when I was at university the mechanical design office had a SKF bearing manual where there was the formula for calculating the effective load on a bearing subject to both axial & radial load. In fact my recollection was that it also applied for simple radial loads too. That formula included a factor to account for whether it was the inner or outer race that was stationary. We were also issued with personal FAG bearing catalogues which I still have. It also includes the bearing life calulation method. I was surprised that it makes no mention about which race is rotating. So I went to Expensive's SKF link expecting to find the formula as i recall from my varsity days - but the on-line formula is the same as the FAG formula. http://www.skf.com/portal/skf/home/prod ... ink=1_0_25
Radial bearings are often subjected to simultaneously acting radial and axial loads. If the resultant load is constant in magnitude and direction, the equivalent dynamic bearing load P can be obtained from the general equation

P = XFr + YFa
where
P = equivalent dynamic bearing load [kN]
Fa = actual axial bearing load [kN]
Y = axial load factor for the bearing

An additional axial load only influences the equivalent dynamic load P for a single row radial bearing if the ratio Fa/Fr exceeds a certain limiting factor e. With double row bearings even light axial loads are generally significant.

So I did a bit of Googling and found the formaul that i recall from my varsity days at manchester University. http://cfd.mace.manchester.ac.uk/twiki/ ... ngs%29.pdf
P = V X R + Y Ft
Ft = actual thrust load (lb)
X = radial factor (usually 0.56)
V = 1.0 for inner race rotating
= 1.2 for outer race rotating

Thrust factor Y is obtained from Table 20.2

I stress that that is the fatigue life calculation but it does include a factor which accounts for the extra fatigue caused by external race rotation causing the balls to travel further per rev thus causing more fatigue loads.

I am very surprised that this seems to be news here & it is even doubted that the distance travelled by the balls is defferent depending on which race rotates.
Last edited by tok-tokkie on Fri Jun 01, 2012 4:26 pm, edited 1 time in total.
tok-tokkie
9

Joined: 8 Jun 2009
Location: Cape Town

0
tok-tokkie wrote:I am now writing about bearing fatigue life. It is what really determines what size bearing is required in most cases since that is what normally causes a bearing to fail (excluding unusulal causes such as lube failure, crashes etc).

Not needed in this case as the cars only run for a short period of time.
Smokes
1

Joined: 30 Mar 2010

0
richard_leeds wrote:Why is the friction different? The balls only input is the relative movement of the rings, its all about rpm. There is no input in relation to the direction of travel of the vehicle?

It is the rotational direction of the balls and their relative movement radialy to the axle and the result this has on the lubricant used.
autogyro
33

Joined: 4 Oct 2009

0
Smokes wrote:SKF is great for industrial applications you will find these bearing are packed with grease and do not have the low friction that you require. this because they are designed for very long lifecycles and contant running at a steady speed and the grease is designed to drop vicosity as the bearings warm up.
...

When you obviously have limited xperience with modern lubrication, you should perhaps spend some time at skf.com?

As for the novel idea that the bearing's friction would differ depending on which race you turn, I have given up as I only have
10 points of credibility here anyway, but I have asked for a statement from my business associates at FAG in Schweinfurt.
"Bernoulli is a nine-letter name"
xpensive
12

Joined: 22 Nov 2008

0
I have skf and FAG and NSK ball and needle bearings from 30 to 70 mm od at work the still have too much friction for this application. I know about modern lubrication but most of the bearings I use are the sealed for for life varety. Hence packed with grease. Even bike hub bearing have to much friction for this application. ABEC5 is the stand ard is best for really low friction brearings.

http://www.zxsq.com.cn/ these are the guys that make Boca bearings Ithink.
I need to have another look at work though.
Smokes
1

Joined: 30 Mar 2010

0
cerobear ceramic bearings maybe no need for lubricant at all...and damn light compared to steel.You might need to sell your mother... though

http://www.cerobear.de/index.php?id=3
marcush.
102

Joined: 9 Mar 2004

0
ive been trying to get my head around this bearing rotation thing, i took apart a bearing and marked it like tok-tokkie did. he's totally right. i have a feeling the ball travels two revolutions farther when comparing rotation directions. it reminds me of something i learned in high school physics or astronomy: when a planet is rotating around the sun, say clockwise, and rotating about its own axis counter-clockwise, the planet "gains" a day. even if the planet does not rotate about its axis at all, one day will elapse during the course of a year... so that's where the extra bearing rotation, and extra rolling distance, comes from. the extra distance is either one or two times the circumference. that's all i got.
thisisatest
3

Joined: 16 Oct 2010

PreviousNext