Conveyor Pulley Bearings Need to Breathe Part 4
Gary James
Bulk Solids Handling Specialist, MechEng Hons, FIEAust,CPEng, NER, RPEQ, RPEV. ASBSH, Ex-Bechtel Fellow,
If you are looking at the pressures due to a temperature change with a constant volume, or the expansion of air and hence flow, see https://www.periodni.com/gas_laws_calculator.html
This calculator provides a number of options re volume, temperature etc. It can be applied to the air in gear reducers, bearing housings etc. If there is lubricant you will need to look at its expansion separately. Air expansion is approx. 5.4 x the lubricant.
Here is an example. It is interesting to note that the pressure change is independent of volume. Small bearings may generate a lower temperature and hence lower pressure. However, any pressure is not recommended across the seals. Ultimately, the seals fail to hold back the pressure and the equipment fails.
If the equipment is completely filled with lubricant, expansion contraction will still be a problem. Work in Canada has shown grease nipples becoming projectiles due to grease temperature changes. See https://www.hsac.ca/images/posters8B.pdf. Hydraulic Safety Authority of Canada HSAC has advised that, "one degree of heat applied to confined hydraulic fluid, pressure can increase by 50 PSI."
The tool to calculate the pressure change is Gay-Lussac's law. The above shows a 17.5kPa change for a temperature decrease of 52 C. This is 1.75m head of water. Imagine the equipment running at the bottom of a deep swimming pool. How long would is run?
Withstanding this pressure would require pressurised oil seals used on ship propeller shafts see example https://www.kemel.com/product/pdf/SNAMEAirSealFinal07_30_03.pdf, even then there are bilge pumps.
Having the seals withstand plus minus pressures of say 1 to 1.5m of head for years is not a good idea. The seals will eventually fail, allowing contaminants and moisture in.
The max temperature, can be measured in the field. The temperature at the bearing or local lubricant, may be hotter. The temperature, can be calculated, allowing for solar radiation, bearing and seal heating, gear churn, surface cooling etc.
If you measure pressures and they do not correspond with the gas law, there is a leak. This could be at the bearing joints or the seals.
The web page explains the various laws below. If you are looking at flow through a breather, at constant pressure, use Charles' law.
State Manager & Industry Sector Manager - Paper & Steel
4 年Hi Gary, these are interesting findings. I'm interested to find out how you measured the pressure differential across the bearing housing seal. Can you share any of the field test data?
Principal Mechanical Engineer (Director) | Rolling Element Bearings Expert
4 年Hi Gary, we are specialists in impartial and independent bearing failure analysis. In our failure data files we have in excess of 200 conveyor pulley bearing reports (over 400 bearings inspected) and as a result have quite detailed statistics. We've also completed workshop investigations with a test rig simulating conveyor pulley applications that included a White Paper and this did not indicate any influence from pressure differences. Although your technical information is impressive, I can assure you that in reality 'water being sucked in' as a consequence of bearings 'needing to breathe' does not align with the failure data we have. I understand that you have good intentions of improving bearing reliability, but I do appreciate that there isn't any practical evidence to support your case. I would like to share that it is a little disappointing readers of this information will be now 'fishing' for 'Red Herrings' in their failure analysis. We're keen to share any technical information on this if you're interested.
Senior Mechanical Engineer
4 年Hi Gary, interesting articles. I'm keen to know more about the statistics of failure modes in conveyor pulley bearings, is there any data you can share?
Head of Sales | Flender Australia
4 年Nice looking drives there Gary?? Bobby looks good too.