Hydraulic pressure (gauge) question.

Hi All,

Just checking some stuff with the panel.

I've had a Machine Mart 12 tonne - floor standing hydraulic press for some years now and a while back the pointer fell off the pressure gauge. The gauge is filled with Glycerine so I don't think I can just pull the thing to bits to re-fit the pointer?

I spoke to MM CS and they wanted the best part of the cost of a new press, just for a new gauge. ;-(

So, I can buy a pressure gauge easy and cheaply enough but I don't think I can get one that would be calibrated in tonnes (and kg/cm^2) like my existing ones is. I think that because I'm guessing the force the ram can exert in tonnes and the pressure seen behind the ram will be a function of the (size of the) surface area of the end of the ram and therefore there is no direct correlation of the two?

However, 12 tonnes on my existing gauge relates to 750 kg/cm^2 and using an online converter suggests that's also ~10668 psi?

So, am I simply looking for a replacement gauge that can cope with say

11,000 psi (or 14500 psi as the existing gauge goes to 16 tonnes) please?

If so, I could then have a simple chart stuck on the press giving me the conversion from psi / bar to what it should be in tonnes exerted by the ram and kg/cm^2 etc?

Cheers, T i m

Reply to
T i m
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I don't think it will be as high as that. You need to know the area of the piston delivering the 12 tons. One source of cheap high pressure gauges is from gas welding sets (the cylinder pressure) Normally 2000psi.

Reply to
harry

Ok. I agree it did sound fairly high.

That was the bit I was trying to get my head around. ie, if the pressure / force exerted by the ram was '12 tonne' (the load / area would be a function of the area of the bit applying the load etc) and that equates to 750 kg/cm^2 (as indicated by the existing gauge) we are now talking two different things aren't we? The '12 tonnes' could be seen as applying a static load of 12 tonnes onto an object (like putting a 12 tonne load on a lorry) with out any concern with the point loading (12 tone sitting on 1 sq meter or 10 sq cm's etc) versus something that was actually stipulating the specific area over which the 'load' was being applied. So, according to my workshop press gauge, 12 tonnes = 750 kg / cm^2 and I can't see how that can be?

I can't see 750 kg (force)/ cm^2 relates to anything other than .75 tonnes (force) / cm^2? Could that also be seen as 12 tonnes force / 16 cm^2? Maybe that answers the question about the area of the end of the ram (4.48 cm diameter. Pi * r^2 - 2.24^2 x 3.14)?

So is the gauge actually displaying the pressure in the system and the

12 tonnes is equal to the equivalent force seem throughout but can only be applied when you know the contact area of the ram on the job?

I'm confused ... ;-(

Cheers, T i m

Reply to
T i m

Yes, definitely. :)

Maybe I'm wrong about this, but it may be that the press has a head (if that's the right term - I mean the bit that applies the load to the work) with an area of 16 cm2. In that case, a total load of 12 tonnes would be equivalent to 750kg per cm2. Personally, I think that's a daft way of looking at it, but I assume that's what they mean.

The pressure gauge presumably just measures the pressure in the hydraulic fluid? Say the piston has a X-sectional area of 20cm2 and the pressure in the fluid is 250Kg/cm2. Then the total load on the work would be 5000Kg. If the piston has double the cross sectional area, then the load would be double.

Reply to
GB

These days, I would have thought that you could buy a pressure gauge with metric calibration without to much difficulty.

If you look at:

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I presume that you can easily get kg/cm^2

If the existing gauge also has a tonnes scale you might find it easier to keep a conversion chart on the wall, as the conversion from kg/cm^2 to tonnes depends on the area of the piston in the press which you could perhaps back calculate from the scale on you old gauge.

Reply to
Michael Chare

Thought so. ;-)

This might just help with the visualisation bit (and yes, I'll go with your terms). ;-)

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Understood.

Yup, that seems to be what the dual scale ('tonnes' and kg / cm^2) seems to suggest. Again, a picture speaks 1000 words ..

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(Sorry for the flash glare but it was dark in my workshop).

Agreed.

Agreed, so, if we take the scales on the existing meter that seems to be related to something tangible to it's role in and at that location, (hydraulic fluid pressure), then the kg/cm^2 scale with a value of 750 at the '12 tonnes' position (given the theoretical area of the piston) would actually sound plausible?

eg, pump the system up till we read a 'pressure' in the hydraulic fluid of 750 kg / cm^2 and apply that to a piston with an area of ~ 16 cm^ then you will have a 'force'(?) of 12 tonnes from the piston.

So, assuming I can't easily find another gauge calibrated in the same way, to find a replacement I need to know the maximum pressure of the system, in this case, 1000 kg/cm^2.

According to this online converter:

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1000 kg/cm^2 = 14,223 psi.

And it seems you can get such things from eBay: Item 261050190029 for example.

So, whilst it sounds like a very high pressure, it seems the maths supports the theory? Either that or I'm missing something?

Cheers, T i m

Reply to
T i m

Agreed, even off eBay. ;-)

Nice site, thanks Michael. ;-)

Well, it seems psi / bar are the most common scales and it doesn't really matter what it reads as long as I've got an idea what is going on. Basically I just need a read mark on the glass at what equates to

12 tonnes and that should be enough. In practice, you generally only use 'enough' force on stuff to say press a bearing in or out and as soon as the pressure climbs rapidly then the chances are you are done.

Yes, that was the idea. The big question is still the one of 'am I on the right track here'. ;-)

Cheers, T i m

Reply to
T i m

get a quote for repair before you buy something that may not work as intended on that application

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Reply to
Mark

This is DIY, just pull it to pieces, replace needle,refill with glycerine,simples.

Reply to
F Murtz

First, are you sure it's a 12 ton press?

If it's a 12 ton press and the piston is say 20 squ inches in area, the pressure will be 12 divided by 20 = 0.6 tons/squ inch.

0.6 tons =0.6 x 2240= 1344psi

You can measure the diameter of the piston where it emerges from the cylinder. Area = 3.14 x radius squared.

Reply to
harry

I am.

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But if it's only ~5cm in diameter (and it is) it will have to be nearer our 10,000 psi.

I will when I go down there but my calculations so far, and the markings on the existing gauge seem to bear out.

What I think it is is the 'pressure' in the gauge would be acutely 750 kg/cm^2 (science) for the press to be exerting a force of 12 tonnes (maths). That was why I suggested at the top that the gauge may have been calibrated specifically for this (and all similarly sized piston) applications.

Cheers, T i m

p.s. Lately I've not been seeing some of my posts (and potentially replies) on whatever server VM use now but have seen them on ES.

Reply to
T i m

Will do, good point Mark and thanks for the heads up (and done). ;-)

Cheers, T i m

Reply to
T i m

Hmm, I don't think it is that 'simples'. I looked at this when it first happened and I think the case is formed around the glass and so you would have to peel that back to be able to release the glass to be able to get to the needle. Then, if the needle fell off untouched by any external forces, how would I fit it back on so that it wouldn't come off again? How would I know that the needle was back in the right place and hence the gauge calibrated (even roughly? Ok, if I fit the needle back on at the low end stop that would probably be close enough).

Q, am I bothered to do all that when a new replacement might only be less than a tenner delivered ... ?

There are things I will attempt a d-i-y repair on, even when those who know say it's impossible (like the sealed tub washing machine I repaired 7 years ago and was working till just the other day), but this ... ;-(

Cheers, T i m

Reply to
T i m

Are we *sure* it is full of glycerine? That's something I have never come across after a fair amount of experience with pressure gauges. You might expect the hydraulic fluid to make its way there, and I suppose it could be a glycol but I would have thought it is more likely that an MM press would have mineral oil as the working fluid.

Anyway, what's the need to refill? Air is a fluid, its compressibility is not relevent. The gauge should still indicate the pressure in the system

Reply to
newshound

Well it is (mostly) full of something and glycerine seems to be commonly used in these gauges?

If the mechanism leaks you mean? In most cases I'd expect (hope) whatever was being measured for pressure *not* to make it's way out anywhere, including into the meter casing. ;-)

Yup, I believe the working fluid is completely separate from the fluid surrounding the gauge mechanism.

Quite, except I don't believe the fluid surrounding the gauge mechanism has any bearing on the 'pressures' or the reading, it's simply there to do other things, like dampening (and possibly lubricate?) the mechanics.

I believe the way this type of pressure gauge works is via the use of a sealed (at the far end) curved or coiled 'Bourdon tube' that uncoils slightly as the pressure inside said tube increases. That is connected to the pointer via some gears or a linkage and the pointer moves accordingly.

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Cheers, T i m

Reply to
T i m

My mistake. When you said "the gauge is filled with glycerine" I thought you meant the bourdon tube was filled with gycerine, not the whole gauge!

It's still something I don't recognise at all. I can see in some applications that a damping fluid might be useful, but surely not in a manual hydraulic jack? If it were mine, I think I would either drain the glycerine and fit the needle back on, or bin the gauge and replace it with something with an appropriate range, judged by dividing 12 tons by the ram area. And I would be surprised if that was 11,000 psi. OK that is not quite diesel injector pressures, but still pretty nasty if you get in the way of a leak.

Reply to
newshound

To be fair, it isn't something I've seen much either, outside this particular application. Like, none of my compressors have any 'wet' gauges.

I think it's one of those things where it is a design typical in that sort of game and they used it on these presses, even when it isn't strictly pertinent.

Easy (there is a rubber plug at the top)

Unfortunately, it's not as easy as that. The case appears to be 'crimped' over the glass so it isn't simply a matter of unscrewing a bezel or similar.

I think that is what I'm going to have to do and most of why I posted the question here re 'what' I should get.

Agreed. Now I've talked it though with people here I think I have now realised what it is all about. The kg/cm^2 scale is the std pressure scale and the 'Tons' are simply put on there by the makers of the press to equate to the actual pressure being applied by all rams of this diameter.

Seems like it is though.

Quite. That said, I'm not sure there is likely to be any residual / pump pressure behind any of the hoses, unless say a pin-prick type leak could cause harm if a jet of fluid hit you skin and before you stopped pumping? I mean that as soon as there was a leak the pressure would drop off very fast as there isn't a lot of flexible hose etc.

I'm still waiting on the cost of a professional repair but with a new gauge potentially less than a tenner delivered, I'm guessing the postage both ways would be more than that. ;-(

Cheers, T i m

Reply to
T i m

In message , T i m writes

Another snip.

I've forgotten what the question was:-)

FYI the pressure gauge on my crop sprayer is part filled with some liquid. Max pressure around 10 bar. The pump is a triple diaphragm type so the gauge flickers rapidly.

As has been said, fluid damping for the Bourdon tube and needle mechanism perhaps.

Reply to
Tim Lamb

Mainly ... 'what pressure range is required for a replacement gauge. ;-)

It was the fact that it was marked in kg/cm^2 (a 'pressure') and 'Tons', (a mass / force) I couldn't really see any correlation between those two. This was because there isn't any of course. ;-)

The only time there 'is' a correlation is when it is used on something like a press and when the ram area exposed equates to a force of 12 Tonnes when there is 750 kg/cm^2 in the system. ;-)

I have an enquiry out to see how much it would cost to get the existing gauge repaired and will compare that with a new meter calibrated in psi / bar. [ £38 for a new meter but no mention of Vat or p+p.] I'll have to check back with MachineMart as then I can at least have a gauge marked with both scales ... if it wasn't silly money (as I remember from my enquiry years ago) and was more reliable than the first one. ;-(

If I am going to get a new one anyway there is nothing stopping me maybe actually draining the fluid and cutting the bezel off to remove the glass and re-fitting the needle and 3d printing a new bezel to hold the glass back on and not bother with the fluid (as I don't think it is required in this instance (hand pumped)).

Ok. Probably glycerine or similar.

145 psi. A long way short of the 10,667 I'm going to need. ;-)

I wonder how much more it might flicker or how long the linkage would last without the liquid?

Agreed (and lubrication possibly).

Cheers, T i m

Reply to
T i m

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