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Air tool question


Anderzander

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This may be a daft question - but I bought an air cut off tool and I'm wondering why it seems to stall so easily. My compressor is 14cfm - almost twice the figure quoted as needed.

There is an adjustment (pressure?) on there that I haven't fiddled with - so is this that I may have the pressure set wrong or are they all just like that?

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It will probably not be enough air being delivered to it. My air cut off needs 14CFM! My compressor has a free air delivery of 14CFM. Most compressors will quote the max displacement which will be a higher figure, mine being 18CFM.

So check what the free air delivery is of your compressor, you'll probably find that its lower than the requirements of the Air cut off. That will mean you empty the tank pretty quickly meaning it will slow it right down.

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As a test put a pressure gauge in line right next to the air tool, if the pressure drops significantly during use as soon as you start using it either your pipework / valves / water traps / pressure regulators / lubricators / fittings along the way are too restrictive, if the pressure dies off a little more gradually either your receiver or pump are too small. Remember unless you have a nice variable speed compressor your pump won't start until the pressure in the reservoir falls below around 6 bar so don't let that fool you.

I have seen people tap off the inspection bung before to bypass all the valve work to improve flow.

However, generally air tools work on the basis of high speed, they have no low down torque generally so as soon as you start to stall them they can lose power very quickly.

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Is there any way to have the pump coming on before 6 bar to keep the pressure up?

I have heard of people having the air line straight out of the tank, not sure about that myself.

I have a 3hp 150 liter tank, I can get it to run everything air I have plus it's not that old and doesn't see much action really.

Now my mates comp 3hp 200 liter tank will run my tools better than mine and he has had his from when the Roman army left. Weird.

Must look up variable speed compressor.

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The 3hp is only the motor power.... It could be running a pump with a 1"^3 displacement or a 20"^3 displacement. One is going to shift far more than the other!

I believe there is a way to change the pressures that the pump kicks in on.... But not sure myself. The biggest issue you'll have if the pump is running too much is the air being heated and then more moisture in the air.

The same air cut off tool on my old 25L (can't remember the cfm or HP) used to run out of puff about 30secs after pulling the trigger. More recently with my 3hp, 200L, 18cfm displacement (14cfm free air) it will run indefinitely.

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Test the pressure at the tool, the regulators and pipes etc that come with domestic compressors often have rubbish flow rates.

The usual pressure switches do have an adjustment screw in them but from memory it's more for the full pressure than empty.

I guess if you binned it you could buy a 7 bar and 8 bar switch and set them up for on an off. Obviously it's going to cycle more often. I don't know why 6 and 8 bar are chosen although most air tools I have recommend 6 bar max so generally what's happening tank side of the pressure regulator is less important.

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Yes mine stalls all the time too – I have the Lidl 3hp 50 l compressor. However, when I worked at a company with a ‘proper’ compressor, all my air tools worked like a hot damn.


The biggest single improvement I’ve made so far, however, is a proper air hose with a decent bore. Screwfix had 10mm one on special a while ago. I bought that and suddenly most air tools perform better.


The cut-off tool and die-grinder still stall out reasonably quickly though, so next step is to bypass the stupid restrictor in the compressor and plumb some copper lines round the workshop directly to the inspection hole (via an anti-vibe hose and filter-drier-regulator). Yes I know, duty cycle of the compressor might be a bit questionable, but frankly for home-gamer use, once a week it should cope.


The inspection fitting seems to have a really odd metric thread though (M18 x 1 from memory), so I will have to turn a custom adaptor, as soon as I figure out metric screw cutting on my old lathe that is!


Incidentally, whilst waiting for the rush-hour traffic to die down yesterday I called into the local Machine Mart. Whilst admiring the pressure-pot sandblaster, the manager wondered over and said something along this lines of “Don’t even think about it unless you have a 150 l or bigger compressor”.


Completely off-topic side-note: Actually I bet you could build a pressure-pot blaster for <£20 in fittings and an old gas cylinder. The hardest part would be welding on bosses for a relief valve and media supply at the bottom.

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I don't think it will make much of a difference going straight out from the tank to be honest... you'll just end up emptying the tank quicker. You'll still get the slow down before the tank gets low enough to recharge. I'd also watch the duty cycle.... my old compressor shat the bed when the conrod in the pump let go quite spectacularly! :D

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Heres a pic of my compressor not giving much away and I can't find the instructions.

Rand 150l 3 hp runs of a 13 amp household plug

No markings on the pump. So i don't know the stats other than it was supposed to put out 14 cfm.

I'm not really that well versed on compressors its just for my own use but I would like to tweak it or maybe twerk it who knows.

Any thoughts.

Pics not loading not that well versed in computers either it looks.

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I don't think it will make much of a difference going straight out from the tank to be honest... you'll just end up emptying the tank quicker. You'll still get the slow down before the tank gets low enough to recharge. I'd also watch the duty cycle.... my old compressor shat the bed when the conrod in the pump let go quite spectacularly! :D

The point is to remove the deliberately-installed restriction in the compressor's own regulator. In hobby-type compressors this is quite substantial, so that the tank doesn't empty itself too quickly, thus causing the motor+compressor to go over its rated duty cycle. By removing the deliberate hobbling and fitting a regulator of decent bore, I'm assured you will see a marked improvement, for substantially less outlay than the man-size compressor that I remember seeing in your build thread Ross!

As for over-duty cycling the compressor, whilst it is possible, I must admit to being a bit laissez faire on this. My previous compressor, a 'Wolf' from the back of a sunday paper glossy magazine (a reject too) once accidentally ran for 24hrs to no ill effect. Other than my electricity bill! It happened because on exiting the garage for the night, I'd diligently undone the drain tap to vent it, but neglected to turn it off!!

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A friend of mine has one of those machine mart sand blasters. It's quite good although it takes a bit of fiddling with the valves to get it to flow nicely without clogging up. The tips wear away super quick and your face masks frosts over quite quick too so I would recommend good PPE. He bought it to restore a load of cast iron radiators so supports a tarp with a few step ladders and blasts in that. I did one radiator for him, it was fun for about 10 minutes then quite tedious, however the most painful bit is sweeping up the media, sieving it and pouring it back in. It uses a hack of a lot of air and he runs it off one of those clarke compressors with the two single phase pumps.

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Heres a pic of my compressor not giving much away and I can't find the instructions.

Rand 150l 3 hp runs of a 13 amp household plug

No markings on the pump. So i don't know the stats other than it was supposed to put out 14 cfm.

I'm not really that well versed on compressors its just for my own use but I would like to tweak it or maybe twerk it who knows.

Any thoughts.

Pics not loading not that well versed in computers either it looks.

Probably they are over quoting the horse power of the motor as you cannot really run a 3hp motor from a 13Amp Plug.

http://www.engineeringtoolbox.com/elctrical-motor-full-load-current-d_1499.html quotes more like 17Amp is required.

I tried running my 3HP compressor from a 13Amp plug and it would just about run, after a couple of starts the 13Amp fuse would blow (as start load is a bit higher)

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Probably they are over quoting the horse power of the motor as you cannot really run a 3hp motor from a 13Amp Plug.

http://www.engineeringtoolbox.com/elctrical-motor-full-load-current-d_1499.html quotes more like 17Amp is required.

I tried running my 3HP compressor from a 13Amp plug and it would just about run, after a couple of starts the 13Amp fuse would blow (as start load is a bit higher)

I rely on Engineering toolbox far more than I should, but I'm not sure where they're getting that figure from?

3 * 745.7 /230 = 9.72 Amps

Obviously startup load will be higher, but the motor should have startup capacitor to assist this, and in any case the compressor's unloader valve should ensure the motor is started off-load.

But yes, I take the point that 3 hp motor-rating is about the limit of a 13 amp supply. And equally, I'm not disputing what the rating plate says might be, ummm, optimistic.

Matt

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That equation works well for DC current calculations but not well for AC current calculations where you have to factor in the Power Factor for the inductive load.
http://electrical.theiet.org/wiring-matters/18/power-factor.cfm?type=pdf

A quick search shows a average single phase motor will have a Power factor of around 0.7
But a note that the power factor can be improved with the correctly sized run capacitor (if fitted)

So really it is a complicated calculation based on many factors, so the straight listing of HP to Amps probably is on the conservative side and probably assumes no run capacitor.

As a side note, a neighbour had on very old 2hp compressor and it started blowing a 13Amp fuse as it built up pressure.
I suggested replacing the 2 capacitors (one run and one start) as it was a cheap attempt of a fix instead of replacing the motor / whole compressor.
Replacing the very old capacitors did solve the problem and it would run on a 13Amp fuse (which you would expect for a 2HP motor)

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