What’s Up Doug? Ten questions about a coal mine’s belt conveyor
A mine site had a host of questions about its conveyor belt, ranging from the life expectancy of the bearings, to contamination of the bearings upon installation and the optimal frequency recommended for pillow block inspections. Following are ten questions about conveyor belts that are applicable in many contexts:
1) How would an increased load on the overland conveyor belt affect the life expectancy of the bearings? From my knowledge there has been an increase of approximately 800 tonnes per hour (300tph – 1100tph) on the belt over the 30 years that it has been running. However, the conveyor was originally designed for 970 tph.
The relationship between life and load is (close to) a cube:
Life = (bearing capacity/applied load)^(10/3)
However, this is only a prediction of fatigue failure. Your bearings are not as likely to be failing from fatigue as opposed to contamination and wear.
> contamination resulting from ingress of abrasive particles such as dust
> wear from lubrication issues and specifically since the bearings are only rotating at 30 rpm, you are always getting boundary lubrication, there is always some degree of metal to metal contact causing wear
2) It has been speculated that our upper and lower drive pulleys are different sizes and in turn operate at different speeds ~0.2RPM seen at the pulley. However, if they are the same size but there is secondary reason for this speed difference, would this affect the life of the bearings?
There may be a slight difference in the calculated life, however, as noted in Q1 it is not likely significant.
3) The majority of our bearing installations occur in the field. Due to the nature of coal processing, there can be small amounts of contamination in the bearing housing once installed. I know that this would cause decreased life, but is there a known value for how contamination affects the life expectancy of a bearing?
Yes, when we do the life calculation, we can put an adjustment factor for the degree of contamination.
4) With these bearings being slow rotating bearings, how would over-greasing and under-greasing affect the life expectancy of the bearing? Which one is worse?
As far as I am concerned, there is no such thing as over-greasing. The issue is the inability of the bearing housing to purge the excess grease and allow the rolling elements to rotate freely. So if your seals are getting plugged up and not allowing the grease to purge (which also could be caused by a lack of grease, that is, if there is no flow through the seals, the grease in the seals cakes up and forms a dam) this can cause an excess amount of grease to accumulate in the housing. Perhaps the existing seal and housing design is not allowing grease to purge? At any rate, the bottom line is that the seal/housing design is the limiting factor in the amount and frequency for grease insertion and this should not limit the appropriate amount of grease being put in to lubricate the bearing and to purge contaminants (remember, grease has two purposes – lubrication and sealing/purging).
In summary, under-greasing is worse, while over-greasing is a misnomer and is really caused by poor seal/housing design not allowing excess grease to escape.
5) In some cases, we have had to reuse parts, that is, taper locks. How would this affect the life expectancy of a bearing?
Reusing tapered sleeves is acceptable if you are able to get an appropriate contact area between the bearing bore and sleeve and the sleeve and shaft. A minimum of 80 per cent contact is recommended. If you inspect the sleeve and shaft upon removal, you should be able to assess whether the surfaces have been compromised which means not able to achieve the appropriate contact.
6) Are there tools available to aid in installation and setting of tolerances on these large bearings?
In terms of installation, there is a method for installation that uses pressure applied to a hydraulic nut that defines the starting point from which a precisely measured axial drive up is applied.
7) Are hydraulic nuts available for bearings of this size?
Yes, they come in a variety of sizes.
8) What is the best form of NDT testing for slow speed bearings?
Regarding low speed applications we use standard data acquisition systems (portables and online) and software. The only specifics is that sometimes we use a 500 mV/g sensor (instead of std 100 mV/g)
9) Is there a frequency recommended for pillow block inspections?
The frequency of inspections would be calculated based on the application (as opposed to the type of bearing). In Reliability Centred Maintenance theory there is a concept called the P-F interval from which you determine your inspection interval.
10) What style of loading is hardest on a slow-speed dual roller bearing?
The type of loading that would be hardest would be shock loading in which the load exceeds the static capacity of the bearing.
Douglas Martin is a heavy industry engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Reach him by email at firstname.lastname@example.org.
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