Low friction rotary shaft seal for middrive ebike motor 1

[raficzeqq]

Hello,

I'm currently working on fixing my Bosch Gen 4 mid-drive motor and need a new rotary shaft seal with very low friction that doesn’t require grease. Standard materials like NBR or FKM aren’t suitable because they have a relatively high friction coefficient when running dry.

There is an e-bike service shop that uses custom seals that work well, but they only offer repair services and don’t sell the seals separately. Their seals seem effective, so I’d like to replicate something similar. Based on my research, the material they use is likely PTFE. I’ve already contacted Chinese manufacturers on Alibaba to see if they can produce a small batch for testing. However, one manufacturer mentioned that the current design may not be ideal for a PTFE seal due to the dimensions, and they suggested an alternative design.

Here are the key details of the seal I’m looking for:

• Shaft diameter: 21.5 mm
• Torque tube diameter: 25.5 mm
• Seal width: approximately 4 mm
• Design: single lip facing outwards
• Material: likely PTFE for low-friction performance without lubrication

I’m quite invested in this project as I’m studying automation and robotics, and currently taking a course on electric motors and motor control. This has become more about the challenge and enjoyment of fixing the motor than cost considerations. Additionally, the e-bike service shop doesn’t have any partnerships in my country, so obtaining these custom seals isn’t an option locally.

I have some pictures of both the seals they use and the alternative designs recommended by the Chinese manufacturer. I’d appreciate any advice on which type of seal would work best, along with some explanation if possible.

Thank you!

 

[BEMPE16524]

if you are looking for complete set of PTFE made oil seal might be a bit challenging. but having a stainless steel seal with PTFE lip may not be an issue. they are a lot in the market. just need a little bit of search effort.

 

[raficzeqq]

I was thinking about modding ptfe lip seal like this one. I think they are used in air compressors. I did a lot of effort trying to figure out what would work best. Unfortunately I don't have knowledge about seals specially in automotive applications when minimal friction and no grease is needed specially about the design of the seal itself.

 

[BEMPE16524]

looking at the size again, I notice it is quite difficult to find what you need.
is the motor oil filled? I assume not.
what is the problem if you put grease on? do you think it will burn?
can you draw a cross section of your mechanism?

 

[raficzeqq]

No, it is not oil-filled. There are two major issues with applying grease. First, the location of this part within the motor makes it very difficult to service, which is why a maintenance-free solution is needed. The other issue is the presence of self-lubricating bushings between the shaft and the torque tube. Applying grease to these parts can cause drag, similar to the effect of seals made from high-friction materials running dry.

 

[A8yssUK]

I wouldn't use a straight PTFE as they have poor wear resistance, a filled PTFE is what you want.
Freudenberg has what you want. https://www.fst.com/products/dynamic-seals/radial-shaft-seals/

I consider them the best in the business

Also a point, dry running PTFE seals require a run in period.. the friction at the beginning will be good, then it'll rise, then it'll drop again and level out. This is as the tribofilm is created on the mating surface. Just so you know if you are measuring anything

 

[raficzeqq]

Chinese manufacturer suggested adding graphite. Thats why I created this post. To gather information about materials and the design.

 

[BEMPE16524]

this guy already give solutions. nothing wrong to use grease actually.

also from this video:

not too difficult to maintain, just need proper tool. also with better grease, you could protect your inner part of the shaft and seal much better.
high performance grease e.g for marine application will give better performance.

 

[raficzeqq]

It’s not the same issue, though. In the problem I described, water isn’t entering through the bearings but rather through a shaft seal that I need to replace. I’ve already taken the necessary steps to protect the bearings; I even replaced the old ones with improved versions that have rubber covers on both sides. The drag issues only occur when freewheeling or pedaling backward. I've posted another picture here that should clearly illustrate what I mean.

One more thing: while applying grease to the seal itself could be an option (though it’s strongly discouraged by ebike experts), I still need a new seal because the old one is completely destroyed.

I also gathered some information on seals from Reddit. One user mentioned that a spring-energized PTFE seal design should work well but recommended using a cantilever spring with graphite-filled PTFE.