# Bearing Replacement Advice



## Zak650 (Sep 20, 2008)

When you replace the bearings make sure you go with sealed bearings not shielded bearings. Shielded bearing are ok when there is a seal elsewhere to contain lubrication and keep out water but a terrible idea where water is liable to find it's way to the bearing.


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## TooQik (May 4, 2013)

I posted on this forum a little while back regarding this subject as I had some doubts/questions myself about the bearing being able to withstand the clutch forces. The conclusion from that discussion was that the motor bearings will generally withstand conventional clutch forces.

Since then I've done some more digging. The motor I'm going to use for my conversion uses 6309 bearings on both the driven and non driven ends of the motor. Reading the specifications from the SKF bearing website for this bearing shows a static load rating of 31,5 kN. Halving this to determine axial load rating give you 15,75 kN (or 15750 N). Now reading the specifications for the motor, it states a maximum axial load of only 700 N, which is well below the bearing ratings from the SKF website. Given that I want to run a heavy duty clutch which requires a release load over 700 N means that I will be using a motor adapter which contains a floating hub design with bearings to take the axial load of the clutch directly rather than passing the forces onto the motor shaft.

Some people will probably say this is overkill, given that the clutch forces will only act on the shaft for a very short time, but for me I'd rather remove any extra stresses on the motor bearings outside the manufacturers suggested limits for peace of mind.

As for which bearing manufacturer to go with, as long as they're a genuine bearing from a reputable supplier I don't think you'll have any issues as long as you pick the right rated bearing ie. size, limiting RPM, loads and seal type (as Zak650 pointed out, you want sealed bearings for an electric motor).


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## puddleglum (Oct 22, 2008)

Mad Professor said:


> Good day all.
> With my EV conversion I was looking to go clutch-less.
> 
> But I am still driving about in my donor car, I have tried many times now to try and change gear slowly without using the clutch and it's very rare I can get a smooth gear change without crunching the gears.
> ...


I can't speak from my experience but lots of people have been very happy with there clutch-less conversions. I don't think you can draw any conclusions about how a clutch-less EV will shift by trying it with the ICE. Your electric motor has less rotating mass and is not under power when you are shifting like a ICE is. It's your car though, so you should build it like you want. If you ask the brg. supplier for motor grade sealed brgs. you should be fine.


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## Mad Professor (Dec 18, 2010)

Thanks for your input.

I have just been informed that the my stock clutch pressure is around 485-525 Newtons (109-118 lbs).

I will indeed replace the current shielded bearings with sealed bearings.

Thanks again for your time and advice.


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## dillond666 (Dec 27, 2010)

"I have just been informed that the my stock clutch pressure is around 485-525 Newtons (109-118 lbs)."

The actual axial thrust the bearings will be subjected to when the clutch is actuated is far less than that. The leverage ratio across the clutch diaphragm makes it so.


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## shortbus (Sep 27, 2011)

The pedal linkage doesn't reduce the clutch plate pressure. It only reduces the pressure needed to release the clutch. The old lever and fulcrum mechanism.


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## shortbus (Sep 27, 2011)

No matter what replacement bearings you use, unless you use a thrust bearing you are still not going to eliminate end pressure on the shaft. A thrust bearing between your flywheel adapter and the motor end bell would put the end pressure on a larger surface.


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## dillond666 (Dec 27, 2010)

"The pedal linkage doesn't reduce the clutch plate pressure. It only reduces the pressure needed to release the clutch."

I agree, but that's not what I was referring to. The clutch plate pressure acts between the flywheel and the pressure plate. 
The pressure required to release the clutch is much less than this because there is leverage across the clutch diaphragm. The bearings see the "equal and opposite" of this smaller force.

In summary, the only time there will be axial load on the shaft is when you apply (disengage) the clutch. The axial load will be equal to the load on the clutch release bearing(throwout bearing) and much less than the force clamping the clutch disc. This is due to the leverage gained across the clutch diaphragm.


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## TooQik (May 4, 2013)

When I did my follow up I put the question of clutch axial load to an engineer here in Australia who specialises in clutches. He stated that the release load acting on the motor shaft will in fact be equal to the clamping load of the pressure plate.

He also stated that most ICE engines have a main bearing on the crankshaft which acts as a thrust bearing. This bearing is designed to take the axial forces of the clutch and in cases where clutches with a high clamping force are used the thrust bearing does wear out quicker or can fail if appropriate bearings are not used.


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## dillond666 (Dec 27, 2010)

"He stated that the release load acting on the motor shaft will in fact be equal to the clamping load of the pressure plate."

This is just not true. Even the experts get confused sometimes.

The point about high clamping force clutches wearing the crankshaft thrust bearings is valid as the leverage ratio of the diaphragm is probably the same but the spring is stronger.

The simple graphic on this site might help people who are unfamiliar with the operation of the automotive clutch to see the point I am making. http://www.howstuffworks.com/clutch1.htm

Notice on the diaphragm clutch animation the fulcrum on the diaphragm is offset thus giving mechanical advantage. In real life this offset is more pronounced as it is less stressful to the clutch release bearing and the crankshaft bearing.

The clamping force exceeds the bearing force. 

Do the names Newton and Archimedes mean anything on this forum?


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## shortbus (Sep 27, 2011)

I can tell you've never worked on a clutch. If you have access to a diaphragm clutch, even one setting on a bench with no pressure plate load on it, try pushing the spring in by hand. Unless your superman, you won't move it too far. Even with the ~10:1 ratio of the clutch pedal, your still talking around 20-30 pounds of foot pressure in the car, to disengage the clutch.

The same leverage your talking about in that drawing in the link, is what is putting the pressure on the clutch disc. If the clutch plate has 200 pounds of holding force, some where the system(clutch plate + foot pedal + crankshaft/motor thrust bearing) HAS to with stand the 200 pounds it will take to disengage it. No free lunch or magic in the equation.

The bearings in an electric motor are not made to take that much end force.


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## TooQik (May 4, 2013)

dillond666 said:


> The clamping force exceeds the bearing force.


Totally agree if you're talking about the clutch release bearing, but the bearing force alone is not the entire force acting against the clamping force. Why?



dillond666 said:


> leverage ratio of the diaphragm


As you yourself point out, the bearing force is multiplied through the lever action of the diaphragm. It's this force which is acting against the clamping force and in turn being exerted on the motor shaft.


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## dillond666 (Dec 27, 2010)

Ok let me try again, perhaps I am not articulating myself in a way that is clear to all.
The attached diagram should hopefully clear the issue up. The blue bit represents a coil spring for an easier representation of the diaphragm spring.

Because the fulcrum is attached to the clutch cover, the equal and opposite force to the 500lbs clamp force is contained between the fulcrum and the cover when the clutch is disengaged and between the flywheel and the cover when the clutch is engaged.

When the clutch is engaged there is no thrust on the crankshaft bearing.
When the clutch is disengaged the 100lbs force is present on the crankshaft bearing and the clutch release bearing, obviously in opposite directions otherwise the thing would be moving through space forever



shortbus said:


> The same leverage your talking about in that drawing in the link, is what is putting the pressure on the clutch disc. If the clutch plate has 200 pounds of holding force, some where the system(clutch plate + foot pedal + crankshaft/motor thrust bearing) HAS to with stand the 200 pounds it will take to disengage it. No free lunch or magic in the equation.


Very good Shortbus, that would be the clutch cover, not the bearings.



TooQik said:


> As you yourself point out, the bearing force is multiplied through the lever action of the diaphragm. It's this force which is acting against the clamping force and in turn being exerted on the motor shaft.


Not quite TooQik, the multiplied force is contained between the fulcrum and the clutch cover. The only force that acts on the motor bearings is the force on the clutch release bearing.


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## shortbus (Sep 27, 2011)

Not so fast there, _ dillond666, look at you picture again. The clutch cover, and fulcrum point are one and the same. The clutch cover is bolted to the flywheel and the flywheel to the shaft, whether crankshaft or electric motor shaft doesn't matter.

Any force that is needed to move the clutch spring is transferred into this whole assembly. Your ratio of the spring at 5:1 is not what they are in real life. It is 1:1 or maybe at the most 1.25:1 on a diaphragm clutch.

I've rebuilt enough IC engines and always could tell when taking one apart whether they were from a automatic or manual transmission car by the wear on the thrust bearing surface on the main bearings. Without knowing what car they came from.
_


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## dillond666 (Dec 27, 2010)

shortbus said:


> Not so fast there, _ dillond666, look at you picture again. The clutch cover, and fulcrum point are one and the same. The clutch cover is bolted to the flywheel and the flywheel to the shaft, whether crankshaft or electric motor shaft doesn't matter.
> 
> Any force that is needed to move the clutch spring is transferred into this whole assembly. Your ratio of the spring at 5:1 is not what they are in real life. It is 1:1 or maybe at the most 1.25:1 on a diaphragm clutch.
> .
> _


To address your last point first, I have exaggerated the leverage ratio for the purpose of this discussion in the name of clarity. The ratio is less in most real life situations, I've never seen a 1:1 though.

Using my previous drawing look at the length of the flywheel to the fulcrum, it's the same as the diaphragm to the fulcrum, the leverage works opposite to the applied force meaning the flywheel gets the same thrust as the clutch release bearing.

If I might beg your indulgence I'd like to try an abstraction of my other drawing to see if that makes things clearer for you.
I have attempted to draw a pair of common pliers in black, I added a bit of grey to show the flywheel friction surface. The red is the clutch disc and the blue as before is the spring, the fulcrum is white.

Notice the relationship of flywheel, cover, spring and diaphragm lever is consistent with my previous drawing, just the appearance differs.
To compress the spring (disengage the clutch disc) I apply force on the handles of my pliers. The force I put in the right one(diaphragm lever) is the same as that required to oppose it on the other handle (crankshaft).


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## dillond666 (Dec 27, 2010)

Here's a random clutch cover assembly, the diaphragm ratio is easily observable.


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## shortbus (Sep 27, 2011)

OK, you win, not correct but, you win. Won't argue with you any more. I do though know from experience that a clutch in no way act like the 'pliers' example. Not worth the time to try and change your mind.


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## PStechPaul (May 1, 2012)

I've been following this "argument" with some interest, and I think there is relatively little pressure on the motor shaft when the clutch is disengaged, because the fulcrum of the actuating lever is attached to the motor/transmission assembly, and the forces cancel. I may be wrong, but in any case, there is a considerable difference in force due to the leverage ratio. And it should be possible to design a clutch where the release fork is actually like a bicycle caliper, which only exerts a squeezing force. It's been a while since I replaced a clutch and it's easier to conceptualize with the actual parts in hand. I found some other articles that may help make this more clear:

http://www.edmunds.com/car-technology/when-fact-meets-friction-the-basics-of-clutch-operation.html
http://thecartech.com/subjects/design/Automobile_clutchs.htm


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## TooQik (May 4, 2013)

dillond666 said:


> TooQik said:
> 
> 
> > "He stated that the release load acting on the motor shaft will in fact be equal to the clamping load of the pressure plate."
> ...


I've gone back and re-read my correspondence with the clutch engineer I spoke to and can confirm that I've misquoted what he said. He said that the full bearing load will act on the motor shaft, not the release load as I stated, so both dillond666 and he agree, my apologies. 

So to determine whether your motor bearings can withstand the clutch axial load now comes back to calculating the total bearing load required to overcome the clamping load, which requires knowing the lever ratio of the clutch pressure plate.

For my application where I'll be using a heavy duty clutch with a clamping force of over 8000N means I still need to design a floating hub to take the bearing load, as a lever ratio of 12:1  would be required to not exceed the 700N axial load rating of my motor.


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## PStechPaul (May 1, 2012)

I found an old thread which discusses this in detail. In essence, I think, you can add a bearing to the motor shaft which transfers the axial force to the frame of the motor (or the transmission body). It might even be possible to use a spline coupling (or other type) between the motor and the clutch/transmission housing so that no axial force will be exerted on the motor shaft itself.

http://www.diyelectriccar.com/forums/showthread.php?t=31470&goto=nextoldest


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## dillond666 (Dec 27, 2010)

TooQik said:


> For my application where I'll be using a heavy duty clutch with a clamping force of over 8000N means I still need to design a floating hub to take the bearing load, as a lever ratio of 12:1  would be required to not exceed the 700N axial load rating of my motor.


Not sure what motor you're running but if it's a warp 9 or something with a tailshaft you could fit an angular contact ball bearing to the tailshaft end. This would require a little housing to be machined up and depending how you did it, might interfere with your speed sensor. You would also need to incorporate a grease seal. The setting up would need to be "just right" so that you didn't put too much or too little preload on the motor deep groove ball bearings. It would be an interesting solution if you happened to be a machinist/engineer though.


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## TooQik (May 4, 2013)

I'm going to be running a Siemens 1PV-5138 motor. I have some design ideas on paper for the floating hub but if I struggle to get something together myself then I'll farm it out and pay someone else to do it.


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