life expectency of hydraulic cylinder and piston seals

[greycloud]

Hi

I notice that seals on hydraulic cylinder fail quite often which gives you the impression they can't live for long; however when you look at piston seals used in tranmissions or brakes you find that they can live for 30 years or more without servicing!

why is that the case? it really has me confused.

 

[3DDave]

travel distance. Pistons on brakes travel a few dozen millimeters over their life. On a hydraulic cylinder they will travel that distance during assembly. Sliding back and forth grinds off material a tiny amount every time.

 

[TugboatEng]

Temperature is a key factor. Brakes can use EPDM rubber seals which are resistant to temperature related aging. Mineral oil based hydraulic systems are stuck with buna-n seals which age rapidly at temperature above 150°F.

 

[greycloud]

travel distance of transmission clutch pistons can be between 5 to 10 mms per stroke; so its travel during its life is far more than a dozen millimeters for sure

temperature is sure a contributing factor but why are hydraulics limited to Buna-n seals? why can't they use EPDM or other seal typess?

 

[3DDave]

Oh goody. You have a claim about clutch pistons. I'm sure it's comparable to the kilometers of travel a hydraulic cylinder seal might see. But go on and be like that. Solve your own problems.

 

[greycloud]

I'm just discussing what you pointed out as this is a discussion, I hope ou don't get upset if I don't agree with some of your points.

a transmission clutch can be actuated hundreds of thousands of times so its total travel will be in the range of kilometers as well.

You may be right about travel being a factor but I'm not sure if it's the major cause here.

 

[TugboatEng]

Buna-N has good wear properties vs more stable rubbers such as silicone and Viton. EPDM is not compatible with mineral oil based hydraulic fluids.

 

[Compositepro]

My understanding is that brake caliper seals don't even slide during normal actuation. The rubber only sees a slight shear deformation and then returns the pad to its original position, just out of contact with the brake disc. Sliding only occurs as the pad wears, and it is always the clean surface that enters the seal. Quite a different application than a hydraulic cylinder, so no comparison.

 

[TugboatEng]

You are correct. In fact, modern calipers have a non-square cross section of the groove to promote pad retraction as the seal bounces back.

https://www.jstor.org/stable/44719307

 

[3DDave]

greycloud, Ahh - you are looking for the shelf with the 100MPG carburetors and the engines that run on distilled water.

edit because Tugboat slipped a comment in ahead of mine and thought this was about him.

The belief that hydraulic cylinder makers are refusing to use an obviously superior product for an application that isn't even close to being the same is very much the same as the magic thinking that superior products are purposely not used.

I'd love it if the first comment was "I used a <name> seal material on a fleet of hydraulic cylinders and I'm getting 10,000 hours when I previously got only 1,000 hours."

Instead it sounds more like "Why don't gerbils live as long as tortoises?"

 

[TugboatEng]

Don't be so facetious. Rubber is an underappreciated material that is regularly misapplied. Caterpillar only figured out in 2018 to replace the buna-n o-rings in their ORFS fittings with EPDM when used for engine coolant service. A huge improvement in reliability at zero extra cost.

HNBR is taking over in many other applications and would probably greatly improve the life of hydraulic cylinders if someone would just use it.

 

[BrianPetersen]

Transmission seals for internal valves and actuators don't have to be absolutely perfect ... they only need to leak slower than the ability of the oil pump to maintain supply to the actuator. The whole innards operate in an oil bath, any minor amount leaking past valve spools and actuator seals still stays inside the transmission. (The input and output shaft seals don't have to resist much pressure.)

I'm not saying they leak by design when brand new ... but you probably won't notice internal leakage as it develops over time until it gets bad enough that the oil pump can't keep up and it starts slipping.

In a normal hydraulic cylinder, leaking piston seals will result in the cylinder moving despite valves being closed (inability to hold a load from falling by gravity, or inability of a clamp to stay put, for example), and leaking piston rod seals will result in fluid drips on the floor.

 

[hydtools]

Hydraulic cylinders will not normally drift if the piston seal fails, assuming fluid cannot exit the cylinder. If the load is tending to retract the cylinder, the fluid volume tending to be displaced in the head end is greater than the volume tending to increase at the rod end resulting in no flow, no drift. If the load is tending to extend the cylinder, a pressure-balancing will occur and lock the movement. You can search for experiments that demonstrate this.

Ted

 

[zeusfaber]

What Ted says works for a single cylinder, but not necessarily in applications where you have a pair of cross-connected cylinders working in opposition. We're drifting off-topic, but that's one of those rules of thumb that's only true when it's true.

A.

 

[greycloud]

I get what Brian is saying, but in many cases you open transmission after a 10 year use to find piston seals are in perfect condition!

I was wondering if it has to be about transmission enviroment being more controlled than in the case of hydraulic systems and cylinders.

 

[hydtools]

What is meant by "fail quite often"?
Failure causes: poor material choice for operating conditions, gland or cylinder failure, debris or fluid contamination. We would have seals last for several years in reciprocating tools: paving breakers, chipping hammers, mounted breakers.

Ted

 

[greycloud]

I mean like it starts to leak within two year period.I'm talking from experience with construction equipment.like excavators, wheel loaders, etc.
Transmisdions on other hand may live for 10+ years without issues

 

[TugboatEng]

With construction equipment you have a few extra factors, especially dirt. There are improved cartridge seal designs for hydraulic cylinder rod ends as well as coatings that will prolong the life in dirty environments.

You also have to deal with abuse. Construction operators love to overload equipment and this can overpressurize systems or wear or damage rod bushings which causes misalignment of the seal.

I suggest doing a tribological analysis of the failed seals to identify the causes before continuing further. Temperature, fluid incompatibility, abrasion, misalignment, and overpressure are all factors that need to be considered.

 

[greycloud]

I'm not talking about any specific case right now but about my experience in general.

all the factors you mentioned above show one thing, that transmissions can last longer because it's a more controlled environment where temp, pressure, actuation length can be more uniformly which allows you more freedom with seal types. this in addition to it being isolated completely from contaminants.

one more factor could be that seals used in transmissions are usually of higher quality