Significant variation in Grease Fill of Sealed Bearings.
There seems to be quite some discrepancy in knowledge of Sealed Bearings. Based on my previous post some say 50% to 60% fill volume, well straight from the SKF website Sealed Bearings should have a low viscosity grease filling approximately 15% of the Bearing cavity. See below.
The initial grease fill volume stated G (cm^3) and is correlated to total lubrication life, obviously has many factors that affect bearing life.
Speed, Load, Temperature, Humidity, and Bearing Currents all affect Lubrication Life.
Many of these environmental factors may not be controllable and some situations outright harsh eg Evaporator Pumps which run in a hot, humid, 2 pole speed, and pumping under negative suction pressure.
My experience is that these evaporator pump bearings were only getting approximately 10,000 hours forcing a time based schedule replacement annually.
The dominant failure mode was grease depletion picked up with Vibration Analysis.
This initiated a trial where Sealed Bearings were repacked by a bearing company with a polyurea based grease. The results showed a life extension of x5 to 50,000 hours or more.
This was great and it lowered our Maintenance costs and increase reliability.
WAIT WAIT WAIT!
Was it the grease type that achieved this amazing result? Or was it Initial Grease Fill.
Following on from my previous post, https://www.3phi-reliability.com/blog/infant-mortality-in-sealed-motor-bearings I’ve repeated the procedure of weighing bearings, Ultrasonic cleaning, and reweighing to obtain the Initial Grease Fill.
What I have noticed is the As Purchased weight doesn’t vary much and is within 1 gram, but once Ultrasonic cleaned and weighed without the grease this is where the problem is identified.
This last batch that I purchase shows a grease fill variation of 1 gram to 3 grams, that’s 300%. My previous post showed a 5 gram to 11 gram variation on a slightly larger bearing.
So was my trial of repacking sealed bearings an exercise of Grease Fill Control or a Lower Bleed rate grease? I will never know, but I think it may have been a combination of both.
I have learnt is that some OEM’s fill to a nominal weight with a tolerance which seems to be much more sensible. I’m open to independently conduct this exercise on all willing Manufacturers.
You should be asking before Purchasing, as a significant number will fail early (And you’ll never know why?)
Note: My intention of this exercise is not to upset any one manufacturer but to elevate the Reliability of industry’s Assets which is my passion.
Straight from the SKF Website:
Super-precision bearings operating at high speeds should have less than 30% of the free space in the bearings filled with grease.
Open angular contact thrust ball bearings for screw drives should be lubricated with a grease quantity that fills ~ 25 to 35% of the free space in the bearing.
Freshly greased bearings should be operated at low speeds during the running-in period. This enables excess grease to be displaced and the remainder to be evenly distributed within the bearing. If this running-in phase is neglected, there is a risk that temperature peaks can lead to premature bearing failure.
The initial grease fill depends on the bearing type, series and size as well as the speed factor A.
A = n dm
speed factor [mm/min]
bearing mean diameter [mm] = 0,5 (d + D)
rotational speed [r/min]
The initial grease fill for open bearings can be estimated using
G = K Gref
initial grease fill [cm3]
reference grease quantity [cm3] (→ data tables)
Sealed bearings are filled with a high grade, low viscosity grease that fills ~ 15% of the free space in the bearing. They are considered to be relubrication-free under normal operating conditions. The grease is characterized by:
- high-speed capability
- excellent resistance to ageing
- very good rust inhibiting properties
Update: I have been sent a research paper conducted by SKF that states the GLF (Grease Life Factor) for polyurea based grease over Lithium based grease is a factor of 5. This corresponds to my experience in the field.
So why isn't the WT version of sealed bearings the industry standard?
Polyrex EM 103 per tube is cheaper than Shell Gadus so that's not the reason.
My bearing supplier as of writing this update has 2136 pieces of 6206 2RS (Lithium based) in stock, but the WT version has no stock.
The current price for a 6206 2RS each is 8.14 euro, the WT version 32.86 euro.
That's 400% difference in price, and the approximate life extension as stated is 500% over lithium.
So why is a superior bearing (reliability metric) cheaper to produce not available to industry at a price similar to Lithium based equivalent?