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View Full Version : Why do crankshafts use plain bearings instead of roller bearings?


Howard
05-06-2010, 01:49 PM
as above

DivideBYZero
05-06-2010, 02:19 PM
Probably due to load. I don't expect a roller bearing could take the huge stresses of a crankshaft for very long.

Howard
05-06-2010, 02:22 PM
300 lb-ft of torque on a 4" stroke/crank is 900 lbs of force. I'm sure they make roller bearings that can handle that much radial load.

Gillbot
05-06-2010, 02:25 PM
How would you mount a roller bearing? You'd have to use a split style and those are inherently weaker. Plain bearings are better for the types of forces the crank journal will be subjected to.

exdeath
05-06-2010, 02:30 PM
A crank doesn't really have or need "bearings" in the sense you are thinking of, it actually rides on the oil that fills in the space between the crank and bearing. Even on the power stroke when all the liquid oil is pressed out, boundary additives (moly, zinc, etc) deposited by the oil are still in effect. The bearing shell is just a serviceable wear component that shims up the clearances to spec and allows you to reuse the block and crank indefinitely.

The whole purpose of that bearing shell is to be something softer than either the block or crank that will wear first, and is cheap and easy to replace (as opposed to machining the block and crank, and even then the gap would get bigger and need to be shimmed, eg: with a thicker bearing).

jaha2000
05-06-2010, 02:30 PM
What Gillbot said...
Split bearings suck...

Howard
05-06-2010, 02:32 PM
A crank doesn't really have or need "bearings" in the sense you are thinking of, it actually rides on the oil that fills in the space between the crank and bearing. Even on the power stroke when all the liquid oil is pressed out, boundary additives (moly, zinc, etc) deposited by the oil are still in effect. The bearing shell is just a serviceable wear component that shims up the clearances to spec and allows you to reuse the block and crank indefinately.
Well, yes, plain bearings can ride on a fluid film, but the dynamic friction is still greater than for a bearing that uses a rolling element, no?

Gillbot
05-06-2010, 02:43 PM
Well, yes, plain bearings can ride on a fluid film, but the dynamic friction is still greater than for a bearing that uses a rolling element, no?

no, plain bearings can have less friction due to there only being an oil wedge supporting the component.

Plus, the manufacturing costs and complexity are less for plain bearings.

exdeath
05-06-2010, 02:48 PM
Well, yes, plain bearings can ride on a fluid film, but the dynamic friction is still greater than for a bearing that uses a rolling element, no?

The friction of the crank bearings is meaningless next to cylinder compression and ring friction. You can turn it by hand rather easily before you install the pistons/rods.

Bearing with a crank is more about something that takes a high pressure load than it is about spinning freely with minimal friction like a wheel.

You calculation with the force is a little off as well. We aren't talking about 900 lbs of force, or the torque you see at the flywheel at 6,000 RPM. It's the instantaneous and constant thousands of psi at pushing the piston into the rod, into the crank, and the crank into the block. Peak cylinder pressure occurs at TDC when the rod is straight up and the forces are pushing straight down. It's the bearings that have to withstand that pressure as the crank is forced to turn rather than go straight out the bottom of the block.

I'm not a bearing engineer, but I'm going to say that roller elements would lack the surface area and contact in the working space of a crank journal to handle that kind of force without unavoidable deformation and galling.

Meghan54
05-06-2010, 03:03 PM
I'm not a bearing engineer, but I'm going to say that roller elements would lack the surface area and contact in the working space of a crank journal to handle that kind of force without unavoidable deformation and galling.



Completely roller bearing automobile crankshaft, both mains and rod journals:

http://www.motortrend.com/features/auto_news/2008/112_0804_2008_sae_show/photo_18.html


Of course, motorcycles and marine engines have used roller bearings on their crankshaft main and rod bearings for decades.

But, it's expensive to do so. BMW, for instance, used all roller/caged ball bearings on their crank in the R series in the '60's.

http://www.bmwriderparts.com/showAssembly.aspx?ukey_product=1024348&ukey_assembly=195218&assembly=38059

But, you can see from that exploded diagram, each journal was separate and had to be inserted into each rod. Only a twin, but in theory could work for cars, but would probably be prohibitively expensive.


And, yes, exdeath, roller bearings can be designed and engineered and built to withstand any loads you want to place upon them. Just how much do you want to spend. (This from asking a couple of bearing engineers at work....Torrington Bearings.)

exdeath
05-06-2010, 03:09 PM
Completely roller bearing crankshaft, both mains and rod journals:

http://www.motortrend.com/features/auto_news/2008/112_0804_2008_sae_show/photo_18.html

Interesting.

Any tangible benefits worth the complexity? I would think not, and this is just showing off their engineering feat. The majority of resistance in an engine isn't in the crank turning, it's in the valve train, rings, and compression. Also will these last 200,000+ miles like an oil film solid bearing shell? Have they ever been put into production use?

As often as roller bearings fail in idler pullies and things like that I'd hate to hear how a crank bearing would sound if one little particle got in there and cause pitting/scoring on the races. A normal crank bearing can be lightly damaged or worn and not cause problems, but the slightest pit in the race of a hardened roller bearing = hell breaking loose.

PottedMeat
05-06-2010, 03:12 PM
Interesting.

Any tangible benefits worth the complexty? I would think not, and this is just showing off their engineernig feat. The majority of resistance in an engine isn't in the crank turning, it's in the valve train, rings, and compression.

here's their description:
Timken Roller Crank
The journey to carbon reduction and 35-mpg economy starts with many small steps, and Timken offers several small but significant steps, by way of reduced friction. The company claims that by switching to its split-cage roller-bearings for the crankshaft and connecting-rods it can reduce internal engine friction by 25 percent, which translates to a five-percent fuel-economy improvement. Furthermore, a swap to all roller bearings significantly reduces the power required to start a cold engine, which could permit downsizing of the battery and starter, and with drip oiling of all those bearings the oil pump might even be able to come down in size.

Bignate603
05-06-2010, 03:40 PM
I do bearing design for turbine engines (they are all high speed ball or roller bearings) at work so I can give a few insights...

1. Load is pretty high, you can design a roller bearing to withstand the load pretty easily but it would be relatively large, noticeably larger than a typical journal bearing. You might be able to fit it in the same axial space on the crankshaft but at the very least you'd have to greatly increase the size of the bottom of the connecting rod to fit in a roller bearing. If you need more axial room than a typical journal bearing you'll probably end up having to lengthen the crankshaft which would force you to lengthen the block. Packaging a correctly sized bearing could significantly affect the rest of the engine design in a bad way. You don't want your bearing design to drive the rest of the engine.

2. Because of reason 1 you're increasing the rotating mass of the engine which isn't a good thing. Also, the balance of the crankshaft will likely be thrown off. Trying to balance a shaft assembly that has 4, 6 or 8 roller bearings on it would be a complete pain, you'd have to make sure each bearing weighed a pretty exact weight or you'd have a crankshaft that was out of balance. I can tell you right now that even bearings for aerospace can differ in weight enough from one bearing to the next that this would be a problem. It's not insurmountable but it would be a concern. Requiring the mass of bearings to be within a certain spec would increase the cost.

3. Roller bearings have limited life. For turbines we can get bearing lives in the 10k hours, 20k hours, or even upwards of 100k hour range depending on the design. However, they pretty much all have a limited life. While they could design bearings beefy enough to effectively have a longer life than the car the size and cost of those bearings would be horrendous. The bearing life for realistically sized bearings would probably be quite low. On the other hand a normal journal bearing has effectively infinite life as long as it has a constant supply of good oil. They have no contacting parts during normal operation and only really wear during starting, stopping, operation with very cold oil, or if something is wrong with the engine. Most modern cars never need their bearings replaced.

4. The life mentioned above for roller bearings is only valid if the oil is extremely clean. The oil in car engines is relatively filthy compared to what is needed for roller or ball bearings running under high loads. There's hard particles all over the place. However, for a car engine the journal bearings have such a thick oil film that the typical containment particle size is smaller than the thickness of the oil film. That lets the particle flow right through without actually being crushed between the inner and outer parts of the journal bearing. On a roller bearing the oil film between the raceways and rollers is much smaller. A particle that flows right through the journal bearing with no problem would get pinched between the roller and raceway. This can create a little dent in the metal and create a point for a crack to begin forming. A large enough dent can take a bearing that has a 100k hour predicted life down to a few hundred hours. This is one of the reasons why transmissions which have rolling element bearings (ball or roller bearings) do not share oil with the engine, it would significantly lower the life of the transmission bearings.

5. As was mentioned before there's no good way to install the bearings without a split ring. That's a poor design for a highly loaded bearing. You could try to make the crank out of a material that would be a suitable raceway material (EXPENSIVE) and then thread the outer ring around the bends in the crank. That may not be possible because the outer ring really isn't that much larger than the other bearing features. You'd then have to install the rollers by hand and use a two piece cage. I'm not a huge fan of two piece cages and the amount of hand assembly would be very expensive and prone to assembly errors. It's not worth the trouble, you're more or less stuck with a split ring.

6. More parts, harder to assemble, and more things to break. None of these things are what car companies want. They'd also be more sensitive to how they're handled before assembly. Many bearing steels used for highly loaded bearings rust like you wouldn't believe. If you let it sit out in the open it will rust. If you touch it with bare hands you might have a rusty finger print form (the oil in your hands attracts water). If you let the vehicle sit for a few months the oil film left on the bearing can attract moisture. That will rust too. Car companies wants the parts to be easy to handle, assemble, and don't want to add any additional service to the car.

7. COST, COST, and did I mention COST? Journal bearings are cheap. I mean ridiculously cheap. I can guarantee that car companies are paying very little for your typical journal bearing. A roller bearing would be significantly more expensive to buy and more difficult to install. Our aerospace bearings can cost hundreds per bearing (I've got one on my desk for a development engine that cost over $3k per bearing to make :eek:), an automotive bearing would be cheaper but it could still be 10x or 100x more than it costs for a journal bearing.

PlasmaBomb
05-06-2010, 03:50 PM
here's their description:

Timken says a $50 to $80 premium over the cost of an engine using standard bearings.

Compared to some of the other "green" engine technologies that's pretty good (stop-start, regenerative braking, changing body panels on eco-models).

Demon-Xanth
05-06-2010, 03:50 PM
Good Information: This thread delivers.

Bignate603
05-06-2010, 04:10 PM
A crank doesn't really have or need "bearings" in the sense you are thinking of, it actually rides on the oil that fills in the space between the crank and bearing. Even on the power stroke when all the liquid oil is pressed out, boundary additives (moly, zinc, etc) deposited by the oil are still in effect. The bearing shell is just a serviceable wear component that shims up the clearances to spec and allows you to reuse the block and crank indefinitely.

The whole purpose of that bearing shell is to be something softer than either the block or crank that will wear first, and is cheap and easy to replace (as opposed to machining the block and crank, and even then the gap would get bigger and need to be shimmed, eg: with a thicker bearing).

This isn't quite right. The spinning crank builds a hydrodynamic film that can support even the power stroke. It actually relies on the spinning shaft for it to function, builds up this wedge of oil. If it is properly running the bearing should never rub during normal operation. It's one of the big advantages of journal bearings.

The soft material gives it a nice smooth surface to run on which is necessary for a journal bearing to work. Any spots that mess with the film will be worn off, any particles that get stuck in it will be compressed down into the material and still leave a nice surface without any deep scouring.

Compared to some of the other "green" engine technologies that's pretty good (stop-start, regenerative braking, changing body panels on eco-models).

I don't think that cost includes all the other issues that you'll run into. That number may not include the extra cost for assembly or any increased costs for maintenance. Because rollers are more sensitive to contamination it's likely that the maintenance will cost more. It's also more likely that the bearings would be need to be replaced during the life of the car.

Meghan54
05-06-2010, 04:28 PM
I do bearing design for turbine engines (they are all high speed ball or roller bearings) at work so I can give a few insights...




Timken? Just curious......we're Torrington, once independent, then bought by several companies wanting our design and manufacturing processes-> IR, Timken, and most recently Koyo.

Bignate603
05-06-2010, 04:40 PM
Timken? Just curious......we're Torrington, once independent, then bought by several companies wanting our design and manufacturing processes-> IR, Timken, and most recently Koyo.

Honeywell Engines. We do almost all of the design of our turbine engine bearings in house.

Meghan54
05-06-2010, 06:11 PM
Honeywell Engines. We do almost all of the design of our turbine engine bearings in house.


Cool....thx!

Howard
05-06-2010, 08:01 PM
Honeywell Engines. We do almost all of the design of our turbine engine bearings in house.
Hiring any mechanical engineers?

Bignate603
05-06-2010, 08:50 PM
Hiring any mechanical engineers?

They've been picking up a few guys straight out of school but not really much else. I think they've been recruiting those guys at schools that we have relationships with but they would still have to post those job postings on the Honeywell website.

I'm not familiar with any positions that are open so I can't point you in any particular direction. I assume you're just finishing up school? Engineer I positions are for new grads but if you see an Engineer II you like I'd consider applying. If you don't have the experience they won't hire you on as a II but if they really like you they may decide to change the position to fit you.

Howard
05-06-2010, 09:51 PM
They've been picking up a few guys straight out of school but not really much else. I think they've been recruiting those guys at schools that we have relationships with but they would still have to post those job postings on the Honeywell website.

I'm not familiar with any positions that are open so I can't point you in any particular direction. I assume you're just finishing up school? Engineer I positions are for new grads but if you see an Engineer II you like I'd consider applying. If you don't have the experience they won't hire you on as a II but if they really like you they may decide to change the position to fit you.
I was sort of being facetious. I'm not looking to relocate out of Toronto.

Bignate603
05-06-2010, 10:00 PM
I was sort of being facetious. I'm not looking to relocate out of Toronto.

Yeah, Honeywell has it's engines division in Phoenix. If you don't like it hot you're not going to like it here. It was in the mid 90's today.

Gillbot
05-06-2010, 10:12 PM
Timken? Just curious......we're Torrington, once independent, then bought by several companies wanting our design and manufacturing processes-> IR, Timken, and most recently Koyo.

What about Timken? :D

jlee
05-06-2010, 10:31 PM
What about Timken? :D

Timken actually has a place here, but I have no idea if they're hiring. Pretty much polar opposite of Phoenix.. :P

Meh I looked around a little and found electrical engineer...didn't see anything on mechanical, though.

http://www.manufacturingworkers.com/job.asp?id=23860121&aff=925A0F9A-7C85-4AAD-A3D2-5E5C81892E2F

Gillbot
05-07-2010, 05:10 PM
Timken actually has a place here, but I have no idea if they're hiring. Pretty much polar opposite of Phoenix.. :P

Meh I looked around a little and found electrical engineer...didn't see anything on mechanical, though.

http://www.manufacturingworkers.com/job.asp?id=23860121&aff=925A0F9A-7C85-4AAD-A3D2-5E5C81892E2F

We are not hiring, if we do it's very limited.

http://timken.com/en-us/careers/Pages/default.aspx