Grease Leakage Issue

[GauravD28]

We are seeing a grease leakage problem in our one of our electro-mechanical product while we ship it to our customer. This does not happen with the all the parts. We do a vibration test before we ship it (no leakage found after test) and yet while shipping there is leakage in some of our parts. After analyzing we found that the leakage occurs in parts that use the last remaining portion of the grease. We use a pressure grease dispenser for applying the grease to this assembly and when the bucket of grease is about to empty the grease content is not consistent as the oil from the grease starts to separate. We have currently gathered the data of the parts and saw that the issue occurred in assemblies that use the last remaining portion of grease, however the data is not 100% consistent. Currently we are trying to change the grease dispenser to address the issue.
Could there be an alternate solution/ reason for the leakage?

 

[tbuelna]

Would using a seal that does not allow leakage of the lubricant be out of the question?

If some grease leakage during shipping is not acceptable, I can't imagine that your customer would tolerate continued grease leakage from this product during service. Proper design of grease lubricated devices is not as simple as many people think. If there is lubricant leaking from the device, it would also mean some moisture could be making its way inside. Moisture trapped in the small spaces between the grease pack and steel bearing surfaces presents a corrosion problem.

 

[GauravD28]

We haven't seen any grease leakage during service in this part. Its very rare. We do a test for this assembly before shipping where we run this part. At the time we don't see any leakage. The gear assembly is tightly enclosed and so there is no need for a seal I guess. The parts in service have not reported any leakage so far.
Regarding the moisture trapped - we will have to look into that.

 

[Comcokid]

Sound like to the the last portion of the grease has some air bubbles in it. When you ship the product, it probably goes by air and the change in air pressure makes the bubbles expand and pushes the grease out. I have seen this happen with silicon dampening fluid in mechanical assemblies.

Another possibility is that when the grease dispenser is near empty, the grease is older has absorbed some moisture. When shipped, the parts are getting heated (hot semi-truck trailer?) and the moisture outgassed pushing grease. This is more of a guess on my part. I see it happen with circuit boards a absorbing moisture and bubbling the solder during reflow.

 

[WKTaylor]

GauravD28

In addition to tbuelna's comments...

Are there any leakage differences between ground shipment [ONLY] or air shipment [pressurized or unpressurized cargo hold]?

also... Speculation...

IF oil is separating from grease, then the grease may be:

(a) Defective. 'Lots/Batches' of grease can be defective. May need to check vendor or for recalls on the specific grease [lot/batch] You are using.

(b) Known-to separate in long-term storage. Yes, some grease is known to have oil separation in storage that is annoying to mechanics and is hard to 'deal with' [MIL-PRF-32014 example]. In these case mechanics are often told to 'stir the oil back into the grease' before use... not a warm/fuzzy solution.... and clean-off any seeping oil. All the application tend to be purely mechanical, working in severe environments.

(c) Inadequate/inappropriate for the application. Yes, grease MUST be adequate/appropriate for the application... especially when used in critical electrical/mechanical devices.

CAUTION. Non-metallic materials in Your device [plastics, rubbers, sealant, etc] may be inappropriately matched to the grease chemistry. SEALS and other non-metallic parts MUST be compatible with the oil [natural or synthetic] and thickening agent [synthetic/metallic or clay].

NOTE. My company has experts in virtually every aspect of engineering technology or science: including lubricants. IF Your company does NOT have lubricant experts, then perhaps experts at NLGI - National Lubricating Grease Institute - can assist You.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

GauravD28,

Where does the leakage occur? You noted this electro-mechanical product involved gears inside of an enclosure filled with grease. I assume there is at least one opening in the enclosure for an output shaft. If the product is shipped with this opening positioned at the low point of the housing, any liquid from the grease would naturally drain down to this location.

 

[GauravD28]

Thank you all for your inputs.
tbuelna...we have decided to include a groove for a O-ring and improve the sealing.

Thank You!!

 

[WKTaylor]

GauravD28

The following SAE documents maybe useful as a 'start-point' for Your seal design effort...

AIR786 Elastomer Compatibility Considerations Relative to Elastomeric Sealant Selection
ARP1231 Gland Design, Elastomeric O-Ring Seals, General Considerations
ARP1232 Gland Design, Elastomeric O-Ring Seals, Static Radial
ARP1233 Gland Design, Elastomeric O-Ring Seals, Dynamic Radial, 1500 psi Max
ARP1234 Gland Design, Elastomeric O-Ring Seals, Dynamic Radial, 1500 psi Max
AS4716 Gland Design, O-ring and Other Elastomeric Seals
AS5857 Gland Design, O-ring and Other Elastomeric Seals, Static Applications
AS6235 Face Seal Gland Design, Static, O-ring and Other Seals for Aerospace Hydraulic and Pneumatic Applications
AS568 Aerospace Size Standard for O-rings
AS5752 Visual Inspection Standard for Elastomeric Sealing Elements Other than O-Rings
AS5798 Aerospace Size Standard for Oversize O-rings
MIL-HDBK-629 A GUIDE TO THE SELECTION OF RUBBER O-RINGS

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]