53,760 hours.

 53,670 hours is the number of hours in just over 6 years, and this week I decided after this length of time it was time for the bearings to be changed; this blog illustrates what everything looked like when I took the bearing-housing apart having never disturbed it in all that time.

I installed the bearings on 5th July 2017 and they were SKF E2 Energy Efficient bearings; they have run continuously ever since bar one period of 48 hours in October 2018 when there was insufficient water; the only other times when the turbine shaft has not been turning is when a nozzle is changed, or the stator is changed, and such stoppages are typically for only a matter of minutes.

Of particular note is that the bearings have not been greased at all; neither was grease preloaded into the bearing housing when they were first installed; they have operated only on the grease put in by SKF at manufacture.

I made the decision to change them only because 6 years seemed a long enough interval; there were no warning signs of impending failure that prompted the change; literature from SKF suggests that the bearings can be expected to be serviceable for up to 9 years; after this length of time the grease will have come to the end of its ability to lubricate and 10% of a batch of apparently identical bearings will fail; this metric for predicting the likelihood of a bearing failing is called the T10 life expectancy; it is very much dependent on the conditions under which the bearing is operating, especially the load it is carrying, the temperature it is operating at, and most especially whether water and other contaminants can get to the rolling parts.

6 years is thus within the T10 life expectancy for the bearings - except the big unknown is the conditions under which they are actually operating.

So here is a pictorial account with captions of what the seals and bearings looked like: -

on my turbine, I have a specially made cover to help prevent water ingress; its purpose is to give a metallic face for the V-lip seal to rub on; it fits over the plastic 'Top-hat' and is held in place only by being a tight fit. 

a puller was needed to get it off.

another modification I have made on my turbine is this deflector to discourage water from entering the drainage hole of the 'Top-hat'

the first glimpse of the condition of the shaft indicated that little moisture was getting to it

for comparison, this is a picture from the previous bearing replacement in 2017, which was done after just 14,448 hours of operation and before steps were taken to prevent water ingress; it shows limescale encrusting the shaft, indicating that quite a bit of water was getting in.

detail of, and explanation of, the marks on the shaft; the brown colouring was of silt-like consistency and rubbed off very easily with wire wool; the surface of the shaft was not scored where it had been polished by the seal.

I was surprised by how much the stainless steel of the cap had been worn by the rubbing of the V-lip seal; it was almost as bad as the wear on the plastic of the Top-hat in the next picture, except that the plastic wore to be like this in a matter of months.

when I had removed the dust shields of the two bearings, the grease around the balls still looked pretty good

a close-up of the balls shows they were still well lubricated

SKF's E2 bearings have a very different looking ball cage; this is what the reverse side looks like.

the radial shaft seal on the inner end of the Top hat was a bit mucky; it has two lips and the second picture is a close up of the space between the two lips showing it to be full of the silt like material present on the shaft.

In summary, the bearings looked good enough to do another few years, but the seals were in need of being changed. The feel of the shaft rotating in the housing was of very free rotation as if the bearings were well 'run-in'. By comparison, when I had put in new bearings, of the same sort, the rotation felt rather stiff and not so free.

Unsurprising then that when I powered up the turbine with its new seals and bearings, power output was 16 watts down on what it had been, - that's a 4% loss of efficiency, - down from 44% to 40%, taking efficiency here to mean whole system efficiency, ie water-to-wire.

Ah well, - better a small drop in generation now than having the bearings fail later, - possibly in winter and having to do the job of changing them on a cold, wet day, as an urgent rather than an elective undertaking !

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.

14,448 hours

A couple of days ago I noticed the Powerspout wasn't sounding right. This is the sound of it coming to a standstill:

The bearings have not been changed since 10th November 2015 and that's 14,448 hours ago. The turbine has run continuously for all of that time bar stoppages for nozzle changes and one de-silting of the header tank.  Clearly the time to change them had arrived and in this post I want to tell how things looked after such a protracted period of operation.

The bearings at each end of the bearing housing looked completely unremarkable; a small amount of the SKF Lesa 2 grease I have been using was evident at both ends, more or less equally distributed between the two ends, and was only a little grey in colour at the pelton end:

My greasing regime has been 4 pumps from a small grease gun once a month (0.8 grams / 1.4 ml per month).

From the encrusting on the length of shaft that sits in the 'top hat water flinger', it was evident that quite a bit of water penetrates into the 'top hat'. The picture below of the wet side of the SOG seal shows a tide mark to half way up it suggesting that water enters more quickly than it can quickly exit from the drain hole: 

Crucially however, there was no evidence whatsoever of water getting past the lip of the SOG seal, and had it done so, the notch cut in the bearing housing to allow it to drain downwards was fully patent and not blocked with grease which had passed through the pelton-end bearing.

The bulkhead showed where water had been striking it and it was pleasing to see this was a mirror image of the splash pattern seen on the front glazing, - indicating that nozzle alignment directing the jet onto the splitter ridges of the cups was pretty good:

The old bearings were taken out and thoroughly examined by dismantling them.  At first there seemed to be little damage to explain the rumble that had indicated the end of their working life, - but on examination with an eye glass there was spalling in the groove of the outer race and this pit was evident at one point in the groove of the inner race:

Spalling* in the groove of the outer race: 

These bearings were SKF units (made in China) as supplied by EcoInnovation. What I have installed in their place are SKF E2 energy efficient bearings which I do not intend to grease at all, - relying on their factory fill of grease to last for their entire working life.

EcoInnovation encourage Powerspout owners to change the bearings every year and this is undoubtedly the right advice: 14,448 hours is 4 months short of 2 years so it is better to stick to an easily remembered 12 month regime.  The exercise described here of pushing the boundary to see how long I could get bearings to last was just an experiment - and it has been an instructive one too.

Here is how new bearings SHOULD sound as the turbine comes to a standstill:

*Spalling is when tiny flakes of material are broken off from the wearing surfaces of a bearing and become deposited in the grooves where the balls run making them no longer smooth.

Managing moisture

It has been a busy few days with the Powerspout.  With autumn well on the way, the weather has been warm and wet.  The flow available to the turbine has been gradually increasing so I have been able to change from the reduced core stator to the 42 pole, full core one. This will now see the turbine set up for its winter period of peak output.

Whilst doing the change of stator, I also changed the bearing block.  I had to deliver on a promise to Michael Lawley that once winter flows arrived, I would run another trial of ceramic bearings.  The first trial was done in June and the bearings lasted just 4 weeks.

This second trial started yesterday, - and finished yesterday ! Just 8 hours !  Inspection of the failed bearings showed much the same as the first trial: pitted balls causing locking up of the race, but this time there was also abrasion of the PTFE spacer ring, leaving particles of PTFE everywhere.  I think this has to be the end of the road for the ceramic bearing dream !

In the course of this repeated delving into the turbine, I have progressed the idea of trying to run the Smart Drive compartment as dry as possible.  This I've done by stopping the ingress of moist outside air by blocking off the ventilation ports and de-humidifying the captive air inside with bags of silica gel.

The idea is to create a warm, dry environment for the electrical side which will inhibit corrosion and promote insulation.

It seems easily possible to obtain very low levels of relative humidity.  Ten bags of silica gel, each of 100g, brings the humidity down to just 10% within an hour of closing the housing, and this is with an outside ambient humidity of 98%.

The rise in temperature is to about 6 ℃ above ambient when the power output to grid is 300 W. At the reading showing this morning, 22.4 ℃, I am happy with this, although as power output increases in coming weeks, and with it greater heat output from the alternator, the rise in temperature will have to be watched.

To touch briefly on the theory of humidity and its relationship to dew point, if the relative humidity inside the housing can be kept at this 10% level and the temperature in the housing does not rise above 36 ℃, it will completely prevent any condensation (which is dew) forming on the bulkhead, even when the temperature of the bulkhead gets down to 0 ℃.  Since it is condensation forming on the bulkhead and then dribbling down over electrical components which probably causes most of the issues with insulation breakdown, this would be a significant advance.

The unknown will be how often the silica gel bags will need 're-charging', and the answer to that will be how often the compartment is opened allowing an air change with moist outside air.  I'll be keeping my inspections there to a minimum.

For those interested, 25 silica gel bags of 100 g cost £18.59 and a useful dew point calculator can be found here.

Post mortem on ceramic bearings

You can learn a lot from a careful inspection of a failed bearing. Yesterday I learned the story of why the trial of ceramic bearings came to an end after just 28 days: - one ball in the bearing at the Smart Drive end of the shaft appeared to have a manufacturing defect:

As the bearing rotated, this defective ball intermittently caused the inner race to 'lock up'.  When this happened, the inner race started to turn on its journal on the shaft.  Being harder than the stainless steel of the shaft, the journal got ground down:

I had noticed there was a slight 'catch' to smooth rotation of the shaft even before I had installed the housing with the new bearings, so there is reasonable certainty that this defective ball was indeed present from the outset, and did not become damaged during the 28 days of operation.

It's satisfying to have reached an explanation for this premature failure.  Just possibly, if it was a manufacturing defect, it might not mean it's totally the end of the road for using ceramics in a Powerspout.

For the moment however, the turbine continues to generate at lower flows than were possible in its first year of operation: presently putting out 223W on 0.96 litres per sec.  Nice to see the benefits being realised of the changes made to extend generation into these drier months.

The question now is: how low a flow can I usefully go to ?   All will be revealed as the dryness of summer progresses.

Another set of bearings on trial

Some things just don't work out as you hope: - today has seen the end of the ceramic bearing trial after just 28 days of continuous running.

Over the past week I had begun to notice a rumble coming from the Powerspout rear end and last night it became a more unhappy noise. As the turbine came to rest after shutting down, there was a clearly audible clicking as the shaft came to a standstill.  On inspection this morning, there was palpable free play in the shaft, - not much, but after so short a time, it was not worthwhile continuing the trial and risking a major bearing failure with damage to the Smart Drive rotor and core.

Michael Lawley commented when I told him about it that he had always had concerns about ceramic bearings, - "the only people who seem to use them are skate-boarders who would be lucky to do 100 hours".

So this morning was spent putting in a new bearing block, this time with SKF E2 Energy Efficient bearings.  I plan to run these with just their factory fill of grease, not adding any extra grease via the grease nipple.

Here are a few pics of doing the replacement to illustrate: 

• how tightening the runner retaining bolt can be a one-man job 
• a homemade tool for accurately aligning the runner to the jets:

Home-made tool to lock S-D rotor 

Tapping lugs into place in the holes in the rotor

Tightening to 38 ft lbs with the shaft locked

Piece of pipe which is a tight sliding fit in the nozzle holder,
with two wood inserts each end, bored dead centre
to take a 4 mm knitting needle

Device in place in bottom nozzle holder after removing nozzle

Close up of needle point on splitter ridge with just the
right number of packing washers behind the runner
to have the jet divided equally in two by the
splitter ridge. 

On trial - ceramic bearings.

This week I have started a long term investigation for EcoInnovation into the use of ceramic bearings in a Powerspout.

These are ceramic versions of the standard issue 6205 and 6005 SKF deep groove ball bearings, but both the balls and the races are made of silicon nitride, Si3N4 (sometimes zirconium oxide, ZrO2) instead of steel.

According to Wikipedia: "since silicon nitride ball bearings are harder than metal, this reduces contact with the bearing track. This results in 80% less friction, 3 to 10 times longer lifetime, 80% higher speed, 60% less weight, the ability to operate with lubrication starvation, higher corrosion resistance and higher operation temperature, as compared to traditional metal bearings".

The desirable attributes out of this list which apply particularly to their use in a Powerspout are their resistance to corrosion, their reduced friction and their ability to operate with 'lubrication starvation'.

Their lubrication amounts only to a small amount of thin oil applied at the time of installation and being free therefore of the grease which surrounds the shaft when metal bearings are used, we had the expectation, having previously demonstrated that grease causes drag on the shaft, of seeing a significant improvement in power output.  

And indeed that is exactly what we saw.  The new bearing block was installed following a protracted run at constant flow with standard bearings, during which power output was measured to be 403 watts.  As I write, a 48 hour spell of running under identical conditions with the new bearings is just being completed, and the power level is now 412 watts.  So a gain of 9 watts.

OK, 9 watts isn't a huge amount but the thing to remember is that the power lost to frictional losses through shaft bearings and seals will be a fixed loss whatever level of power is being generated.  At the power level I am getting at the moment which is 412 watts, a 9 watt gain represents an improvement of just 2.2% over 403 watts.  But when flow diminishes and power yield falls to, say 175 watts, gaining 9 watts will be an improvement of 5%.

With the other improvements mentioned in earlier posts, all designed to boost output at low flow / low power times of year:

• operating on one jet rather that two
• changing to a reduced core stator
• operating with a de-finned rotor

... there is the hope of greatly improving system efficiency in these drier months.

Already, the plot of flow vs efficiency suggests this might be so: the latest data points, which are indicated on the plot below, both lie well above the 'best fit line' for the other data points, all of which did not incorporate these improvements. 

If the left hand side of the above plot can just be flattened out a bit, which is to say if efficiency at low flow can be kept above the rather dismal levels it falls to below 1.5 litre per second, then this would amount to a real benefit.

Let's hope ! 

Grease ...the story !

End of the month again and not a bad month generation-wise: 437 kWh clocked up which, for a month with only 28 days in it, isn't too bad. It makes the capacity factor for the month 87% whereas the corresponding month last year saw 100%.  Ah well, you can't have a bonanza February every year !

End of the month is greasing time and this morning, just after 8am, I gave the Powerspout its usual 2 squeezes of the grease gun which delivers a meagre 0.7 mls ( 0.4g) of grease.

In its first life, the shaft and bearing housing of a Powerspout did service in a Whirlpool washing machine.  In such an application, the bearings didn't get the same 24/7 operational use so there was no need to re-grease them. When they failed, that was the signal for the washing machine to go to the scrap heap. But, if it was a lucky washing machine, its bearing housing, shaft and motor might get salvaged for a second life in a Powerspout.

In this second life, re-greasing becomes essential, not only because of the continuous use but because of the damp environment the bearings find themselves in.  So a greasing point has been added to the bearing housing which connects with a grease nipple on the turbine casing.

The point of relating all this is that when grease is pumped in, it has to envelop the rotating shaft and to travel left and right along the shaft to reach the bearings at each end.  New grease contributes significant drag on the shaft which can be seen as a reduction in power output immediately after greasing. After a few hours, the effect disappears. 

Here is the record of power out to the grid immediately after my greasing at 8am this morning, showing a drop of about 5 w.

Another point arising from this arrangement of delivering grease centrally between the two bearings is that significant hydraulic pressure can be produced by a grease gun especially if it is operated forcefully.  The dust shields, whether metal or rubber, are not designed to have grease forced against them and could conceivably be deformed by the pressure achieved.  (This paragraph was amended 14/4/2016)

To prevent this, I operate the grease gun very slowly. Also, I have removed the inner dust shields of the two bearings so they are more open to let grease enter the ball race, pass through it and out the other side.  It is quite easy to remove the shield by gently bending inward the tabs of the shield where it locates in a groove in the outer race.

So that's Grease...the story !  I don't think it's likely to be shown in a cinema near you anytime soon !