Tesla Large Drive Unit (LDU) Motor Teardown and maintenance

[SuperV8]

Lots of people are not fans of waterless coolants for that reason: with water being "1", glycols typically have a thermal capacity of around .68-.72. In the ICE world so very many people seem to be chasing cures for overheating, and waterless coolant just isn't the right fix for almost all those situations, so it's gotten a bad rep.

However, in the LDU I don't think we need a whole lot of heat-moving capacity; there's just not a lot of heat to be moved, and unlike a legacy ICE situation, the pump capacity is not limited by the idle speed of the ICE; Tesla's thermal management can push the fluid as fast as is needed regardless of other conditions.

I assume that most people are not going to go this route, but it's a valid idea for harm reduction of the expensive bits.

Ah yes, good point regarding the electric pump decoupling the flow from the motor rpm. I'm assuming the pump speed will be dependent on the temperature?
An extreme case - but on here someone reported running into thermal limiting issues in their track driven Cobra with LDU, just unable to remove heat from the rotor quick enough - so in that 'extreme' case waterless coolant would not be a good idea but on a road car could be valid.

As a thought experiment? how about the opposite - no coolant through the rotor? on a road car does the rotor actually need cooling? if you're not drag racing/high speed/towing etc..? there are no magnets to worry about - the copper rotor should be quite resilient to higher temps.?

[asavage]

I can't recall: On the Model S, does the A/C-assisted heat-exchanger operate with the LDU, or only with the battery cooling loop?

I would think the BTU-moving ability of the A/C system would be good, because it can push the temperature of the hot fluid (refrigerant and/or coolant) in the radiator to a higher temperature, yielding a better ΔT than perhaps the coolant alone could do, working with only the heat of the LDU. Put another way, the hotter the hot side, the more BTUs that can be moved into the air.

I really haven't looked at this, it's only a thought.

[later]
I had it wrong: the A/C system apparently only cools the batteries on the Tesla, and not the LDU.

[howardc64]

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[howardc64]

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[SuperV8]

- Rotor temp is hard to measure (no direct contact) This seems to be a huge research area including many tech papers and patents filed to estimate. Tesla's 2010 patent ( https://patents.google.com/patent/US877 ... 0120007532 actual invention might be a couple of years before patent filing effort that starts a bit after engineering) in rotor temp estimation claim while rotor temp can be very high, the bearings temp rating is usually far lower. This paragraph in the patent is particularly interesting

"For instance, while the components of an AC induction squirrel cage rotor can operate at high temperatures (i.e., shaft, magnetic steel, bars/aluminum), there are bearings that are thermally well-coupled to the rotor shaft and these bearings typically have a much lower operating temperature limit than the other components. In this AC induction motor, it is extremely hard to measure directly the temperature of the rotor or the bearings. Additionally, many methods of directed rotor and bearing cooling come with efficiency penalties or mechanical design challenges. Thus, in such a system where a primary thermal rejection path for the rotor is via convection across an air gap to the stator, it is critical to have some quantification of the temperature of the thermally limiting component (e.g., the bearing(s))."

Interesting links - yes Patents are very difficult to read!
Thinking more on this even cruising and say using 30kw you'd still need to dissipate 1.5kw if 95% efficient and most of this goes into the rotor on an induction motor, and presumably fitting the Large drive unit you want to use the power

[SuperV8]

True but the ICE drivetrain ratio is quickly reduced via transmission shifting as the car gets moving, So not staying at this ratio for long.

In any case, curious the source of LDU's gears mud production and weather its any worse than typically gearboxes which has clutch material wear as well (less metallic?)

By 'mud' do you mean the metallic particles stuck to the magnet?

These are just from the gears - when new they will be bedding in to each other on the micro level. I would expect if you were to clean off the magnet and check again in 20-30k there would be much less. Not something I would worry about. Manual gearboxes I have re-built had this metallic 'Christmas tree' on the magnet and is quite normal.

[howardc64]

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[howardc64]

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[asavage]

Link to someone else thinking around using non-conductive coolant (sparse on info, though):
https://www.evcreate.nl/shop/cooling-sy ... e-coolant/

[asavage]

Didja know that Nissan/Subaru O2 sensor connectors fit this Tesla Model S coolant pump. I just received one from Amazon, $11, and it snaps right in place.

https://smile.amazon.com/dp/B07Z5ZRBY5

Amazon: Nissan/Subaru O2 sensor connector/harness.

Tesla Coolant Pump w/Nissan/Subaru O2 connector/harness.

[howardc64]

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[SuperV8]

BTW, just discovered Audi e-tron has coolant cooled rotor. Many reports of coolant leaking into electronics. Not much details yet (fairly new car so mostly warrantee motor replacements) and dealers describe problem as a failed "gasket". More mechanically inclined are chiming in the rotary shaft seal is suspect. I was curious to see what seals others use to solve this problem.

https://electrichasgoneaudi.net/models/ ... ain/motor/

One diagram shows the motor (1 of 3, looks like not exact same design) uses 2 slip ring seal (presumably PTFE) on both sides of the rotor shaft coolant tube. Don't know if that has better longevity. The motor could also be unidirectional if gear box provides reverse. This would probably give the seal better chance at preventing leak.

Here is a paper reviewing all EV motor cooling methods. Table 1 listed e-tron and Mercedes EQC as coolant/water cooled rotor.

https://d-nb.info/1232812978/34

Interesting that the E-tron also used a cooled rotor induction motor. Can't see anywhere that this special 'laser' seal is available separately - many other seals are available but it looks like it would be a new drive unit if this seal ever failed! These are between £3,000-4,000.
Also interesting to note that Audi specifically say "however a small amount of leakage cannot be prevented"! and also designed a small reservoir in the resolver housing - with a drain plug!!
I have also found that this 'drip container' has a 24 month service requirement!

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[SuperV8]

The EQC also used induction motors.
seems they also have some issues with coolant sealing (only recalled in China though!)
https://www.autoevolution.com/news/merc ... 81994.html
https://www.laitimes.com/en/article/34nq8_3ld7t.html

surprised by the low RRP for a new ELECTRIC DRIVE - which includes the inverter and reduction box!:
Front (10):

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rear (200):

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[howardc64]

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[asavage]

I'll venture that the presence of aerosolized coolant is not a significant factor to lubricant washout on the end-cap rotor bearing. You can look up SKF's spec on the bearing's seal's capability, but that's my guess: unless there's a pressure differential, a bunch of axial movement, or even more likely enough bearing wear to render the bearing's seal ineffective, the presence of coolant just running down the seal's face isn't going to want to get into the bearing. The bearing's internal grease/seal interface is an effective incidental moisture barrier all by itself, until other forces act on it. IOW, look elsewhere for greater bearing longevity. That's my guess for now.

A bit of a rant . . . As I mentioned above, providing a weep hole between the pump seal and bearing was standard practice for lowly water pumps on pretty much everybody's automotive and industrial engines for a century. I don't know why any engineer would have reason to assume that a seal would operate with 100% efficacy for any significant length of time in this type of application, and I'm surprised that these engineering youngins with their fancy-pants 3D modelling haven't looked backward a bit to see how this type of engineering problem had been resolved by others.

Do they really think it's so disposable now that it just doesn't matter if the thing leaks and becomes the cause of a significant repair? IDK, I've worked in an Engineering department where we were trailing-edge and overbuilt everything, because everything we built was pretty much one-off, so trying to wring every penny out of a design wasn't worth hardly any engineering time, and after building one-off equipment for 50 years, the business had a model of engineering that worked very well, within those limitations (ie don't try too hard to reduce costs to build; stay within our engineering talents' lane).

---

For centuries, one reason that bearings of nearly all types were fitted with external lubrication mechanisms was to provide a way to remove excess contaminants from the bearing that had made their way in to the moving bits. "Sealed" bearings aren't; they're only sealed to a certain percentage of ingress, for a certain range of size of particulates or viscosity of liquids, and only within a certain pressure differential. In the quest to reduce costs, reduce drag (increase bearing efficiency), reduce service attention intervals, etc., this is where we've come: it's "good enough".

All anything has to do is make it out of warranty; the next owner be-damned.

Sorry, I'm having a grumpy old man moment; you get old and know things, and nobody wants to listen. Quoting Vonnegut, "Everyone wants to build, nobody wants to maintain."

[howardc64]

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[asavage]

You've typed lots of things I nodded along with.

wk057 says he can't reliably get multi-year service life from battery packs in which only modules were replaced (I think I have that right).

Gruber (the ones who have had two separate shop fires burn up many irreplaceable Roadsters) says wk057 just haven't figured out how to do (module? brick?) replacement right, and that Gruber have.

Rich Rebuilds is replacing modules (though not bricks) and thinks they're getting away with it.
https://www.youtube.com/watch?v=T7Q0nNkQTCo

This hasn't shaken out yet, and as you say even with all these people working on it, it can't scale, because prominently Tesla won't help (generalized and summarized). I'm hopeful that the obstacles (ie Tesla) will be worked around before I have to contemplate my own pack repair; I'm at 106k and on a "refurb" pack since Aug2019/73k, which might mean I have updated contactors; I assume ALL the RAV4 EV packs in existence were assembled before 2015. I don't think that Model S modules fit in RAV4 EV packs, they're the same tech but different physical form factor. So, I either find modules from another pre-2015 (ie OLD) RAV4 EV pack, try to determine if Model S bricks can be retrofitted into the RAV4 EV modules, or branch off into experimental land by replacing the OEM modules with someone else's cells (and therefore having to ditch pretty much ALL the Tesla<->Toyota integration, as the Tesla Gateway won't likely work with anybody else's BMS, etc.

Really, that makes a great case for scrapping the vehicle.

[SuperV8]

A bit of a rant . . . As I mentioned above, providing a weep hole between the pump seal and bearing was standard practice for lowly water pumps on pretty much everybody's automotive and industrial engines for a century. I don't know why any engineer would have reason to assume that a seal would operate with 100% efficacy for any significant length of time in this type of application, and I'm surprised that these engineering youngins with their fancy-pants 3D modelling haven't looked backward a bit to see how this type of engineering problem had been resolved by others.

Agreed, especially considering the consequences of any leak - if your coolant pump seal leaked no big deal, just fit a new water pump; If your coolant seal leaks on your induction motor that can result in total drive unit failure/replacement!

[SuperV8]

So far, haven't found/heard any info on failure modes. Is the leak during rotation? while sitting? reverse spin direction? This rebuilder claims its the axial play on the bearings (I presume then suggest the shaft axial movement) is causing the leak in the video's comments

This doesn't quite make sense since LDU has a pair of disc springs next to the outer rotor bearing. This suggest rotor axial movement is present and need to prevent the rotor from slamming side to side.

Very high speed bearings (like on the LDU rotor) have more clearance than standard bearings required due to thermal & centrifugal forces.
From one of Prof John Kelly's videos - one of the bearings is constrained - while the other is allowed to float axially to compensate for rotor thermal expansion.
The belleville or wave washers are often used in high speed machines fitted with these high speed bearings (which have more clearance) to 'gently' pre-load the bearings. This helps with running accuracy, and prevent skidding.

[asavage]

"skidding" in this instance meaning the balls do not rotate, but slide on the races; I want to differentiate this from the effect Howard thinks he sees, with the outer race's OD fretting the gear case's counterbore.

[howardc64]

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[howardc64]

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[asavage]

Well, there are no new low-mileage packs for my RAV4 EV

Interesting about the OTAU reducing your 240v at-home charge rate. I've got a 40A EVSE at home and no DCFC on the car at all, so when I charge, it bulk charges at ~38.2A AC input to the OBC.

Off the top of my head, I recall the BB1-3793 being sold by various sources ($150 (counterfeit?) - $450 ea.) as the brg to use for both ends of the rotor. I have no idea how appropriate that is.

[dzolotnyuk]

thought I would add one more 2013 tesla model SP85 to the list of dead rear drive units my car has 137K miles when seal leaked again on a unit that was replaced some time in april of 2019 under warranty when car had 65K miles by the way my unit that failed is an R version had a single lip seal. I have been looking for a solution lake everybody else. I should be receiving a seal form https://ahpseals.com/product/sb20/ $175 is what i paid for it. it was about 3 weeks wait time now i need to machine an adapter plate hopefully some time later this week or next week. I am not an engineer of any kind just desperate to get my car back on the road so i thought i would get an opinion of a professional on the forum about that seal if it fail i will be modifying coolant manifold and adding water to oil cooler

[howardc64]

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