asavage wrote: ↑Tue Jan 03, 2023 10:36 pm
Opinion from what I've read thus far, regarding the triple-lip PTFE seal . . .
I feel that the excluder lip is non-funtional: it doesn't do anything, doesn't add any value. It's not designed to seal anything smaller than a threshold that is much larger than liquid, its place is to exclude larger particles than we need to be concerned with in this cooling system.
The non-wetted lip on the other end of the triple-lip seal is also useless: the pics you've shown of examples removed from service show a non-functional seal (gunk, wear, overheat, whatever). I think it has a place in a proper application, but not this one.
This thought make sense. But we just have so little actual data and close up inspection. QC Charge swears by triple lip but I think they really don't know the longevity of their triple lip install until they leak. Tesla's concentric groove cut bending aid single lip does seems to leak quite quickly. But again, limited stats, a lot of hearsay, don't really know the cause.
Univ of Stuttgart PTFE lip seal lab's papers (
viewtopic.php?p=49627#p49627 ) does show test data of their single lip seal with pump aid designs benchmarked against a single lip no pump aid design which leaks 1 g(~1mL?)/hr in with oil media at lower surface speeds than LDU application. Every tesla seal we have seen do not have pumping aid design. If this is true, then I'd expect we see leak immediately with a thinner media like coolant. We see LDUs that have 20k+ miles with no leak evidence on the speed sensor.
Pumping aid design is another topic entirely, Seems quite state of the art from Stuttgard's research. Most manufacturers have only shown classical solutions like unidirectional helix. Some have shown wavy patterns and shallow V patterns (I'll avoid going into detail here)
Ideally, proper analysis requires bench top test jig with properly prepped shaft spinning at 20m/s fully balanced. With matched media, temperature, contaminates (including crystallization over time from coolant filling/exiting/drying in the seal chamber) simulating actual usage conditions (including long term static leaking analysis) Not exactly easy to do. Univ of Stuttgart's PTFE seal research lab showed such a jig. PTFE seal test facilities have the equipment ($10k fee to test the seal) Short of all this, we would need to wait until our installed PTFE seals leak and carefully inspect the seal and shaft. If mine doesn't leak for say 5k miles, I'm debating on pulling the seal just for inspection. But this probably stops the leak longevity test. Once a seal is pulled, probably prone to failure if reinstalled... maybe okay if really really careful... not sure. Proper engineering all takes $$, $$ that Tesla has but we are probably getting summer interns at best and tier1/tier2 suppliers cost cutting efforts at worst.
From my research, multi lipped PTFE seal for heavy industrial machines usually have additional guards between each lip. They are often filled with gas/liquid to prevent original media from leaking through as well as circulation channels between the lip chambers to cycle the filler/contamination from media. Seal makers often even make seals that can be disassembled and replace individual lips. Perhaps this is the proper mult-lipped PTFE seals you've seen.
What is interesting is experienced quality seal producers such as Saint Gobain, SKF, and Parker all have had multiple lip PTFE seal profiles similar to Tesla's triple lip and Ceimin listed in their custom seal design guides (no filler between lip cavities, no circulation channel). So there must be some theory of how these work. But I've not been able to engage with their engineering to gain any technical understanding. Standard design guide literature offers no insight. Nor does any seal design books or research papers have shed any light on these type of multi lipped seals and how they function.
asavage wrote: ↑Tue Jan 03, 2023 10:36 pm
In fitting Teflon (PTFE) seals, it has been common practice to use a resizing tool during installation (
random link for such a toolset). From memory, and from the handful of times I've had to handle and install Teflon seals, using the resizing tool to maintain a temporary larger diameter whilst installing is preferred, but it's also possible to use the tool and then rapidly remove it and perform the assembly, without the tool in place -- which would be necessary for our LDUs.
Below is a representation of the type of tool I'm thinking would be appropriate to reduce single-lip PTFE seal damage during assembly.
Tesla_Teflon_Seal_Resizing_Tool_01.png
I'm not aware of a cone install tool of these dimensions being available off-the-shelf -- and I'm not convinced the dimensions I've picked are ones I'd settle on -- but it would certainly be easy to fabricate. Certainly for less than $200 for a prototype, probably less then $100 ea. for qty. (50) or so (rough guess using eMachineshop for fab and based on a lot of stuff I've had them prototype for me over the past 16 years). A local facility could possibly be more cost effective, as EM typically has a cost-effecdtive place for only a small range of quantities.
Temporarily changing (deforming) the seal lip reduces reliance on chamfer angle and surface finish during installation.
Resizing tool would make it more install error proof. Having test installed a couple of seals, I can conclude this is the single biggest area for mistake and proper shaft surface preparation is necessary. Definitely much more precision and experience than elastomer seals for oil based media that most people are familiar with.
One note is the install tool can not be too much larger than 30mm depending on the PTFE seal's construction. Ceimin seal for example has no bending aid. It appears to just be lathe cut with very thin tip but gradually thicker base on each lip. PTFE blended material is made via sintered process (melt a bunch of dust particles together) which can have internal voids as source to start cracks (Lots of literature on this for Silicon Nitride ball bearing's sintered manufacturing process in our "ceramic" bearings)
Attaching my latest version of seal design request document. Page 12 contains installation technique. For anyone installing these seals for the first time, read/understand and have 2 seals (1 for practice, 1 for real) Its quite easy to get it onto a known good shaft once experienced. My 2nd one went on easily after figuring out the technique. Just to state what that is again in detail
- Test fit the seal onto a good shaft first. If have excluder lip, tilt seal at slight angle and press about 60 degree region of excluder lip against the shaft to stretch it. Rotate the seal 60 degrees, repeat. Do so all the way around a couple of rotations. Then, with slight tilt and pressure on one side of the lip (usually the bottom where you can't see) the other side lip will just clear the chamfered edge. The excluder lip then goes onto the shaft without any damage. Probably need to apply proper pressure to stretch the PTFE lips to avoid damage from too much pressure or folding the lips from too little pressure while trying to fit it on.
- Minimize time between stretching/test fit PTFE seal lip to install into manifold and mounting manifold onto the motor to ensure seal lips don't shrink. For my rebuild, I simply did all these back to back in about 20min to avoid any chance to damage the seal install.
- On the multi lipped seal like Ceimin, on first test fit, place finger on the reluctor wheel bevel when pushing the coolant facing lips to clear the chamfer, Amount of force required will slide the seal's excluder lip to hop over the reluctor wheel bevel after the last lip clears the chamfer.
Anyway, avoid treat this PTFE seal install like elastomer seals. Its a totally different beast.