Buffer and Main seal Leaks

[NJAus1]

Hi,

I have a suspension cylinder (from a mining truck) that has a buffer seal, main seal and wiper seal. See below image (in real life this is orientated vertically). The cylinder will be subjected to pressure spikes due to the load the truck carries, ground roughness, cornering etc.

I have observed:

(1) No damage to the buffer seal, I have cut the seal to observe the cross section and there is no damage or deformation;
(2) Main seal has no damage; and
(3) Wiper seal has blow out from the groove

The key aspect to the observation is no damage to the buffer or main seal, but the cylinder is leaking.

I have the following questions:

(1) To blow out the wiper seal, this will require high pressure fluid? If so then does this mean that the buffer seal has become ineffective?
(2) In the event of a pressure spike, how can the fluid bypass the buffer seal without damaging it?
(3) Is the blow out due to low pressure oil that gets past the buffer seal and creates a pressure trap that eventually passes the main seal and then blows out the wiper?

Thanks

 

[hydtools]

I would not suspect a pressure blowout of the wiper. The wiper lip has little capacity to hold pressure applied behind it. Do you see excess fluid in the wiper cavity?

What is the quality of the rod surface?
Are there scratches or other marring in the seal grooves?
Is the wiper installed properly? Is the wiper contacted by the rod end when the rod is retracted?
Is the rod adequately supported when fully extended?

Ted

 

[73lafuite]

Completely agree with hydtools.
Often the scrapers with metal outer cage, therefore mounted in an open groove, move because there is not enough tightening during assembly. Tolerance problem.

 

[NJAus1]

The rod usually has some pitting on it that will require it to be re-chromed. The rod is orientated like the below image.

If I ignore the wiper seal failure as it's primary purpose is to prevent ingress into the cylinder, I am assuming whether the wiper is intact or not once oil passes the main seal there will be a leak. The wiper seal failure maybe just consequential.

So essentially how can I get a leak past the buffer and main seal without causing any damage? Does this mean the seals generally provide insufficient sealing (i.e. there is minimal overlap between the seal and the rod)?

 

[hydtools]

Are these the original seals or replacements?

There is normally a carryover film of fluid from a properly working seal in order to lubricate the sealing edges of the seals.

The outboard support bushing may be sufficiently worn to permit more rod movement than the seals can follow thereby allowing fluid to pass during vehicle travel.

Ted

 

[NJAus1]

They are replacement seals, the cylinders are supposed to get a planned overhaul but have premature oil leaks. Each time they are overhauled, new seals including sleeve bearings are put in. The bearings don't exhibit signs of unusual or accelerated wear. The cylinders use a combination of nitrogen and oil.

With all seals, they will by a hydrodynamic layer of oil between the rod and the seal, I believe this layer probably isn't even visible to the naked eye.

Is there a way to determine the maximum pressure a seal can withstand before oil passes?

 

[hydtools]

Apply enough pressure to make the seal fail. Ask the seal manufacturer. How much pressure is against the seal in normal operation?
The first failure mode would be extrusion into the rod to rod bore clearance. These are pressure energized seals and it is unlikely the sealing lip will lift and leak under pressure.
The leak may be a low pressure leak if there is not enough initial squeeze on the seal. What is the seal pressure cycle high to low during travel?
What are the seal materials?

Ted

 

[73lafuite]

In the photo it is not a standard cylinder.
What is certain is that a scraper seal lets the oil flow inwardly and outward with less than 1 bar. So the thrust on the joint section remains ridiculous.
On this application there are strong radial loads: How is this load taken up, bronze ring, PTFE carrier. Too much radial play leads to leaks.
There is no oil exchange, so there are certainly significant temperature increases, such as in car shock absorbers when riding on a wavy surface. In addition all particles by gravity go into the joints.
Then at night when the truck is stopped, will the joint not hit soil accumulated at the low fixing point?

 

[saplanti]

Did you check the viscosity of oil against the recommended? In case the viscosity is the recommended so the design allows the oil leak during the suspension motions with the cylinder pressure and heat generated to reduce or elimenate the nitrogen leak. But, if the viscosity is high enough the cylinder is not going the be in the fast motion as expected and/or the generated heat will be higher.

Buffer will provide passage for nitrogen and oil under upper pressure towards the the main seal. When the cylinder expanded with the release of the load or partial load it will cause vacuum in the same cavity of the trapped nitrogen and oil. Partial nitrogen will move back but oil will stay, in the long term the oil will accumulate in between buffer and main seals. Sometimes the vacuum will be causing pitting similar to pump inlets.

For wiper; it is difficult to say something without seeing the drawing. As it was said, it does not hold the pressure. Either ther must be something wrong in the keeper design or the installation is not in accordance with the manufacturer’s procedure. Hard to say something.

 

[saplanti]

I forgot to mention my assumption for my comment. I assumed that the distance is short enough between the buffer seal and the main seal.

 

[saplanti]

The following does not refer seals nd arrangements, however it give a good background;

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=3473&context=doctoral_dissertations

 

[NJAus1]

Seals are thermoplastic polyurethane elastomers (TPU), the main is a is TPU and rubber.

The wiper seal is spring energized.


There is an upper and lower sleeve bearing, but yes due to the high loads and road conditions there will be inevitable radial play.

The solution to this would be more overlap between the seals and the rod I would assume?


The recommended oil is being used with a recommended friction modifier. Next opportunity I plan on getting a sample and testing it.

With the conversations regarding the nitrogen, the cylinder is nitrogen over oil, technically nitrogen is immiscible in oil, however reality is that it is partially immiscible as the oil "foams" so nitrogen and oil are mixing together.

Below is a better picture, although simplified.

 

[hydtools]

"So essentially how can I get a leak past the buffer and main seal without causing any damage? Does this mean the seals generally provide insufficient sealing (i.e. there is minimal overlap between the seal and the rod)?"

In the picture of the suspension, there appears to be a rippled, circular texture on the rod finish. A spiral lay in the surface may result from creating a finish with a travel feed grind method. The spiral lay will prevent sealing and allow fluid to follow the spiral passed the seal. A better method would be a plunge grind resulting in a parallel circular lay in the finish. Imagine trying to seal against screw threads.

Ted

 

[saplanti]

It seems that the cylinder, from the suspension photograph, is under an axial force as well as the bending moment. The Bending moment is taken by the radial forces on the sleeve bearings. The friction under this radial force will cause wear on the bearing materials. So this will end up with leaks. In case a large bending moment under large axial force with sudden motion may cause large wear on the sleeve and jet oil discharge in that area. This jet oil may cause wiper relocate unless it is rigitly fixed in location.

 

[saplanti]

What is the material of sleeves? What is the allowable bearing pressure of the sleeve material?

Do you replace them at each overhaul? It seems that you probably need to check them at each opportunity.

The other is, of course, the shaft surface. The diameter tolerance, surface quality etc. You already mentioned the hard chrome coating on the pitted surface.

I wish we could see the real drawing, the sketch does not give the proper information for the adequate discussion, we always guess.