Hydraulic systems problem, ly

[kuleke]

Hydraulic systems problem, need help urgently

I need to get an idea of what kind of forces one can get with real world hydraulic systems. I need to be able to deliver a huge force very quickly. The magnitude of the force should be enough to accelerate a 50kg cylinder to 22m/s over a distance of 1.5m. Thank you

 

[JaredJames]

So 0 to 22m/s over 1.5m gives and acceleration of ~162m/s² - that's huge.

With an acceleration of 162m/s² and a mass of 50kg that gives a force of 8100N.

Just to give you a comparison, something with a weight of 8100N on the surface of the Earth would have a mass of ~827kg, which is just over three quarters of a metric tonne.

I don't think you'll get a hydraulic system that will work for you in this application. You'd need something like a compressed air jet or steam ram.

To understand just how massive your required acceleration is, a rocket has an acceleration of about 20 to 30m/s².

 

[AlephZero]

A hydraulic ram that will provide 8KN is no big deal. You can find bigger ones than that in any car repair shop - they are used to operate the car lifts.

Making a ram that will extend 1.5m with an acceleration of 17G is more of a big deal, but not impossible. If I remember correctly, a university that did some testing for us had a ram that would deliver 50KN at 10G acceleration and a stroke of about 1m. So your specification isn't crazy, but it won't be a cheap piece of kit to buy.

 

[JaredJames]

A hydraulic ram that will provide 8KN is no big deal. You can find bigger ones than that in any car repair shop - they are used to operate the car lifts.

For the record, I have no problem with the forces involved - my local car repair shop had a manual hydraulic bench press capable of providing 3 tonnes of force. I also know other industries use presses capable of significantly higher values (steel mills etc). My issue comes with the next bit as you point out.

Making a ram that will extend 1.5m with an acceleration of 17G is more of a big deal, but not impossible. If I remember correctly, a university that did some testing for us had a ram that would deliver 50KN at 10G acceleration and a stroke of about 1m. So your specification isn't crazy, but it won't be a cheap piece of kit to buy.

It is the required acceleration that is the problem. No standard hydraulic ram is going to achieve it - you'd need it bespoke made for your application and we're talking big bucks for it.

Hence my recommendation to go with compressed air or steam (steam being used on the old launchers on aircraft carriers).

You could do the compressed air calcs quite simply for the required cylinder pressure and work out if you could rig something up. If not, go for steam.

 

[kuleke]

For the record, I have no problem with the forces involved - my local car repair shop had a manual hydraulic bench press capable of providing 3 tonnes of force. I also know other industries use presses capable of significantly higher values (steel mills etc). My issue comes with the next bit as you point out.


It is the required acceleration that is the problem. No standard hydraulic ram is going to achieve it - you'd need it bespoke made for your application and we're talking big bucks for it.

Hence my recommendation to go with compressed air or steam (steam being used on the old launchers on aircraft carriers).

You could do the compressed air calcs quite simply for the required cylinder pressure and work out if you could rig something up. If not, go for steam.

Please tell me more about these calculations. I'm not familiar with the compressed air system at all. Also, I don't need to make any of it, I only need to design it and do the calculations. Thanks

 

[kuleke]

Thank you everyone, I think I'm going with the compressed air idea now so I'm open to ideas regarding that. Please bear in mind that I won't have to actually purchase any of this stuff, I only have to design it.

 

[AlephZero]

No standard hydraulic ram is going to achieve it - you'd need it bespoke made for your application and we're talking big bucks for it.

Hence my recommendation to go with compressed air or steam

The lab ram I mentioned did use hydraulic fluid, because it was designed to apply controlled dynamic loads, e.g. loads that accurately ramped up linearly with time, or even sine-wave loading at frequencies up to about 10 Hz.

If you are only interested in "firing something from a gun" with a given final velocity, a compressed air system would be much easier to design and construct, as Jared said.

 

[xxChrisxx]

Need a hard, fast push? Use a pneumatic system.

EDIT: You already have decided to... well done!

 

[kuleke]

So its definitely decided then, pneumatic systsms are the way to go. I'm going to need to put a spring in there cause i need the 'ram' to return to its initial position as quickly as possible after 'firing'. I'm not sure what materials make great springs but here is an idea of how stiff it would have to be: before firing, it will have to hold a mass of 60kg up vertically with minimal extension. It would also need to be able to recoil as soon as the cylinder hits the workpiece so it doesn't affect any measurements of strain. Also, I'm not sure how you would damp one of these but oscillations after firing are definitely not welcome. Thanks again

 

[timcanfly]

You can purchase single acting pneumatic actuators, some with spring return and others are mechanical return. However you will also find available double acting pneumatic cylinders. Point is, to apply the same principals of the dual acting cylinder and use air for your return cycle. The return times will be much quicker than any mechanical methods. In regard to dampening, once again I would recommend you utilize a similar method as is currently being used, in pneumatic cylinders. Control the air exhaust at the end of the stroke, creating an air cushion.