Author Topic: Whats wrong with the coolant pump, and how to fix it..... maybe?  (Read 2289 times)

Offline John Hill

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OK, lets look at these coolant pumps that come with our Chinese machine tools.  Mine works, more or less, OK but if I try to adjust the nozzle for a trickle flow all I get is a few dribbles that it stops or maybe dribbles a bit then stops again.

These little centrifugal pumps are very simple machines and should be ideal for a job like this but they have a few quirks that need to be allowed for. They can have a good flow and a reasonable head but if the flow is restricted the pump will start to cavitate and the head (i.e. the pressure) will decrease to almost nothing.

Those small plastic tubes in the chromed metal flexible pipes they put on the machines may look the part but they have barely the bore  required for enough flow to keep the pump operating within its effective range, add just a bit more restriction and the flow stops all together.

One solution would be to mount a header tank above the machine and use a large bore (say 1" plastic hose) for the pump to keep the tank charged then use gravity to feed the coolant nozzles.  The tank could be mounted as high as the pump will pump, which will not be very high but the higher it is the more 'head' there will be on the coolant nozzles.  Your header tank would require an overflow pipe back to the sump or I suppose you could use a float switch in the header tank to turn the pump on and off.

Perhaps the idea of a header tank is not to your liking, too unsightly, too expensive, too lazy, take your pick.

An alternative could be done with all basic plumbing bits...  put a tee joint on the output of the pump with one leg going to the nozzles using the existing pipes and the other going to a length of larger bore hose which is routed up towards the roof, then another tee a couple of feet above the level of the nozzles, we will call this the 'wall tee'.  One leg of the wall tee goes via a large bore hose back to the coolant sump and the third leg (which could be a smaller bore hose) goes up towards the roof.

This how it works, with the pump on and the nozzles closed, coolant will flow up to the wall tee and return via the large bore return hose. At this point the static pressure at the nozzle will be the difference between the height of the nozzles and the height of the wall tee.  When the nozzles are opened the coolant will flow.

The purpose of the third hose on the wall tee is to provide an air breather so that a siphon is not set up.

Now the point is that the pressure at the nozzles will remain constant (except for the loss of pressure due to those small diameter hoses) and the pump will always be operating at full flow.

From the den of The Artful Bodger

Offline Pete.

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Re: Whats wrong with the coolant pump, and how to fix it..... maybe?
« Reply #1 on: August 23, 2011, 05:48:39 AM »
If you want a nice steady flow, pipe up the pump to re-circulate back to the tank all the time it's running and take a tap off that recirculating system for your delivery hose. That way the pump is always pumping a good volume and the pressure stays fairly consistant.