To change or not to change ...

In July, my Powerspout will have run for 4 years on the same set of bearings; in that time I've not greased them once; they've run only on their factory fill of grease and, having run continuously, the total hours will be just over 35,000.

EcoInnovation's recommendation is to grease regularly and to change the bearings every year (8,760 hours); I've not followed that advice, not because it's bad advice, but because I wanted to see how long I could get bearings to last.

Mind you, ...the bearings I'm using are a premium type from SKF, designated E2 Energy efficient and more expensive than their standard Explorer series; the claims for them are: "longer service life", "longer grease life", "reduced frictional loss" and "lower cost of ownership";

...and I've modified the sealing arrangement at the 'wet end' of the shaft to reduce the risk of water getting in and causing the grease there to be degraded; see here.

So the title of this post: "To change or not to change..." looks at whether after 4 years, the time is ripe for a new set.

According to the SKF leaflet, bearing service life is almost always limited by grease life; for their E2 bearings, SKF give the following chart to estimate how many hours the grease in E2 bearings should last; the estimate is based on an L10 grease life, and that is defined as the period of time at the end of which 90% of a sufficiently large group of seemingly identical bearings are still reliably lubricated.

The red arrow on the chart is placed to indicate the 'life curve' for an A value of 40,000 and that 'life curve' can be seen to intersect the logarithmic y axis at around 80,000 to 100,000 hours, the same as if the A value was 100,000. This means the L10 grease life should be at least 80,000 hours, - or 9 years of continuous running.

This being the case, I am not going to change the bearings this year; I'll hold out at least for another 12 months and think again when the next anniversary comes around in July 2022.

For the technically minded: 

The diagram shows 'grease life curves' for various values of A, under varying bearing operating temperatures, and for bearings working with a load value P = 0.05 * C; this value of P signifies the lowest loading and was assumed to be the value most likely to be applicable for the load experienced by the bearings operating in a Powerspout; the leaflet gives de-rating factors to lower the L10 hours for bearings subject to a higher load value.

the operating temperature of bearings in a Powerspout is almost certainly less than the lowest value given in the diagram (50 deg C); as the dashed lines of the life curves show, this will mean an arbitrary reduction in the value of L10 hours for any given A value.

the A value for the size of bearing used and the particular conditions under which the bearing is operating is calculated from:

A = n * dm

where: n   = rotational speed in revolutions / min (for my turbine 1000 r/m)

            dm = mean diameter, mm, of smallest bearing, 6005 size, ( [25 + 47] * 0.5 = 36 )

Thus: A = 1000 * 36 = 36,000 (which is near enough 40,000)

Post script added 11 September 2022.

One year on from when the above was written, the bearings are still sounding perfectly OK, so I have again decided not to change them and wait another 12 months.