DAILY TIPS

• The term "hydrogen embrittlement" refers to a process that is destructive to bearings. Here is the process:

  Water molecules pass through the load zone on a bearing. High temperatures break the water down to hydrogen and oxygen. Hydrogen ions are forced into microcracks in bearing surfaces, which expands subsurface cracks.

• When determining the filter size of a hydraulic system, a good rule of thumb to consider is that the minimum number of hours of operation that should be expected from the filter is 800 hours.
  
  
• Some of the dissadvantages of using grease instead of oil are:
  • Grease does not carry away heat
  • Grease needs larger bearing clearances
  • Grease is not easily circulated or filtered
  • Grease is difficult to purge completely

MORE GREAT TIPS

Today we are going to continue the series from "The Lubricating Grease Guide" by the National Lubricating Grease Institute. This passage is from the section on Applying Grease Lubricants.

Consistency

"Some discussion of consistency seems appropriate. First, the question of hard versus soft greases. Penetrations are determined at 77 degrees F(25 degrees C). If temperature falls, say to 32 degrees F(0 degrees C), the grease will be firmer by one or two NLGI consistency numbers. If temperature rises to around 110 degrees F(43 degrees C), the grease will be at least one consistency number softer than at 77oF(25oC). Grease which is stored at ambient temperature will feel soft in summer and hard in winter.

"The same variation is found in different climates. In hot areas, NLGI Consistency Numbers 2 and 3 are usual; in cold locations, NLGI Consistency Numbers 1 and 0 are more common. "Where grease is handled in a dispensing system, the grade chosen may be related more to the capabilities of the system than to the requirements of the application. In service, consistency differences are important. Consider, for example, the use of grease in a gear case. The grease should be carried into the meshing gear teeth, which are thus kept covered with lubricant. If the grease is firm, the gear teeth may cut a channel through the grease and run dry, causing damage to the teeth. Unless operating temperature is high, therefore, soft-to- semifluid greases are ordinarily recommended.

"In ball or roller bearings, carrying grease to the rolling elements is considered undesirable, since that grease will churn, soften, heat up, and work its way out through the seals. Thus a common recommendation for ball bearing is to have the bearing space only about one-third full of grease of firm consistency - often NLGI No. 2. If the bearings support a vertical shaft, the grease had best be still firmer - probably NLGI No. 3.

"For a grease to perform well in service, its consistency must be chosen with regard to operating conditions, the mechanism to be lubricated, and the means of lubrication - hand, gun, system, or whatever. Consistency is the characteristic which often makes grease the lubricant of choice.

"Greases may soften on working, then firm up on standing. This characteristic, thixotropy, is shown to different extents by many greases. Products of high thixotropy may become so firm in storage as to fall out of specification range. However, in terms of performance, the same behavior leads to lower torque
when the bearing is turning and better sealing when the bearing is standing still."

Q & A

"How do I check to see if my aged hydraulic and/or lubricating oils can still shed water effectively?"

This is an issue of demulsibility, which is the measure of an oil's ability to release water. Of course, if you have time, your lab can run a specific test (ASTM D1401) that can be performed to measure the remaining demulsibility potential.

For a quick Pass/Fail test, measure an equal amount (1 pint) new lubricant and water, and fill a blender. Heat to 100 degrees F (38 degrees Celsius), or whatever temperature closely resembles the operating temperature of the sump. Thoroughly mix the oil with the water. Measure the amount of time it takes to separate. Also measure how much of the mixture does not separate (the emulsion is called the 'cuff'). Now repeat the test with the aged lubricant.

If it takes more than 20 percent longer to separate then consider having a lab run the test according to the lab procedure. If the results indicate that the demulsibility capacity is diminished then consider how to best address the problem, particularly if the sump is prone to moisture contamination.

Today's Tip

When converting a gearbox from a PolyAlkylene Glycol (PAG) synthetic that is water soluble, it is advisable to purchase the manufacturers' flushing medium and flushing the unit prior to putting the final charge in the unit and restoring it to service.

Failure to do so may mean residual mixing of incompatible base fluids. This is likely to cause headaches and shortened lifecycle of the new fluid and possibly the gear drive. Consult the supplier of the PAG product for a good guideline and advice on the flushing fluid.

The latest issue of Machinery Lubrication magazine is online at: www.machinerylubrication.com

DAILY TIPS

• Water contamination can be harmful to engine and engine oils.
  Some common problems that occur are:
  • Causes corrosion/rust
  • Can impair the lubricant film
  • Forms sludge
  • Adversely effects additives
    
    
• When calcium is detected in an engine crankcase oil, the
  detergent additive (calcium sulfonate) is the most common
  reason.
  
  
• Some elastomers that are compatible with phosphate esters
  include:
  • Ethylene propylene (EPM, EPDM, EPR)
  • Polyamide (Nylon)
  • Polyethylene
    

MORE GREAT TIPS

Today we are going to look at another excerpt from Heinz Bloch's new book "Practical Lubrication for Industrial Facilities." This passage is about water contamination:

"For corrosion to occur, water must be present. Free water, in particular, will settle on machinery surfaces and will displace any protective surface oil film, finally corroding the surface. Emulsified water and dissolved water may vaporize due to frictional heat generated as the lube oil passes through bearings. Very often, though, the water vapors recondense in colder pockets of the lube oil systems. Once recondensed, the free water continues to work away at rusting or corroding the system.

"Larger particles generated by corrosion slough off the base metal surface and tend to grind down in the various components making up the lube system, i.e. pumps, bearings, control valves, and piping. The mixing of corrosion products with free and emulsified water in the system results in sludge formation which, in turn, can cause catastrophic machinery failures.
Suffice it to relate just one of many examples of water-related damage to major machinery.

"When a steam turbine at a medium-sized U.S. refinery failed catastrophically, the initial problem was attributed to coupling distress and severe unbalance vibration. When the coupling bolts sheared, the steam turbine was instantly unloaded and the resulting over-speed condition activated a solenoid dump valve.
Although the oil-pressurized side of the trip piston was thus rapidly depressurized, the piston stem refused to move and the turbine rotor sped up and disintegrated. The root cause of the failure to trip was found to be water contamination of the turbine control oil. Corrosion products had lodged in the trip cylinder and, although enveloped in control oil, the compression spring pushing on the trip piston had been weakened by the presence of
water."