Strength of Cast iron

[Neil WyattModerator @neilwyatt]

Posted by Tim Stevens on 15/05/2018 20:17:11:

Regarding gas springs and their non-linearity –

By the very nature of a gas spring it cannot be linear. The pressure doubles when the volume is halved, and this means that the load doubles when the length is halved. Halve it again (ie half as much further movement) and the load doubles again. The same extra load for half the extra movement = non linear.

The length in all this is of course the length of the gas chamber, not the total length (which is longer because of the piston rod, and the end fittings etc.) But the principle is sound – gas springs do not obey Hooke's law, or as you might say, they are non-linear.

Sorry

Cheers, Tim

Michael's link describes gas springs with progressive, linear and 'degressive' spring force. This appears to be achieved by a combination of partial liquid fill and the presence of pressurised gas on BOTH sides of the piston.

I'm sure we have all had the experience of opening a boot or bonnet where the spring force suddenly INCREASES near maximum extension, which is what I assume they mean by 'degressive'.

Here's a patent but I won't pretend to understand it after one look.

It would be interesting to read a simpler explanation of how this is done.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 15/05/2018 21:46:05:

Posted by Tim Stevens on 15/05/2018 20:17:11:

Regarding gas springs and their non-linearity –

By the very nature of a gas spring it cannot be linear. The pressure doubles when the volume is halved,

< etc >

… It would be interesting to read a simpler explanation of how this is done.

Neil

.

This is one of the better descriptions that I have seen: **LINK**

http://www.vapsint.com/en/technology-and-characteristics-of-gas-springs/

Sorry, Tim … I think your description might be a little over-simplistic.

MichaelG.

[Tim StevensParticipant @timstevens64731]

Simplistic – yes, that's me. I was referring to gas springs, though, not gas-hydraulic combinations, as if you complicate things you can get anything you like. Even then, linearity (which was the original concern I was addressing) is a very complex thing to achieve, and impossible with a gas spring properly so called.

Cheers, Tim

[ZebethyalParticipant @zebethyal]

If the gas spring reacted like you mention then it would be like attempting to compress a bicycle pump with the end closed off (thumb over the end) and I would agree with you entirely.

if such a spring were used on my mill, it would require increasing force to lower the head, constantly wanting to return to the uppermost position – this is most certainly not the case, it will in fact sit happily at any position throughout the travel without any locking of gibs.

The ones used for car boots/bonnets/cupboard doors most definitely do not react like that, they are smooth throught their operational range. Yes, they may require some effort to start compressing if there is no pre-load, but once that load is in place, they do not react like the above mentioned bicycle pump.

Maybe they are not true gas springs, but gas struts, or some other name, however that appears to be how they are marketed, and is most definitely the description on the item I bought.

[SillyOldDufferModerator @sillyoldduffer]

Not had time to absorb how the graphs work but this guidance should allow an reasonable estimate (of the bolt in cast iron problem, not gas struts!)

 

Edited By SillyOldDuffer on 16/05/2018 11:35:56

[Michael GilliganParticipant @michaelgilligan61133]

Posted by David Murray 1 on 11/05/2018 12:26:09:

Unbrako have a good PDF on the subject **LINK**

.

S.O.D. … for info. ^^^

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 16/05/2018 11:44:25:

Posted by David Murray 1 on 11/05/2018 12:26:09:

Unbrako have a good PDF on the subject **LINK**

.

S.O.D. … for info. ^^^

MichaelG.

Doh!!!

[Ian PParticipant @ianp]

Posted by Tim Stevens on 15/05/2018 20:17:11:

Regarding gas springs and their non-linearity –

By the very nature of a gas spring it cannot be linear. The pressure doubles when the volume is halved, and this means that the load doubles when the length is halved. Halve it again (ie half as much further movement) and the load doubles again. The same extra load for half the extra movement = non linear.

The length in all this is of course the length of the gas chamber, not the total length (which is longer because of the piston rod, and the end fittings etc.) But the principle is sound – gas springs do not obey Hooke's law, or as you might say, they are non-linear.

Sorry

Cheers, Tim

I dont profess to know the technicalities of how gas struts actually work but have a feeling that they are actually 'linear' (within a certain tolerance) over a specified operating range. As you mention, pressure would double if the volume was halved, but with a strut, how do we know the volume has been halved? It may be that there is a dead space in the volume so that the change in volume is only a small fraction of the total. The other big factor is the high initial pressurisation of the Nitrogen which means the pressure change due to volume reduction is only a fraction of the total static pressure.

The clever bit I think, are the seals. Judging by the 'Danger, do not dismantle' warnings I presume the internal pressures are quite high, certainly it has discouraged me from pulling one apart!

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Ian,

It's probably worth following the vapsint link that I posted yesterday.

… it gives a concise explanation of the principle.

MichaelG.

[Ian PParticipant @ianp]

Posted by Michael Gilligan on 16/05/2018 16:57:25:

Ian,

It's probably worth following the vapsint link that I posted yesterday.

… it gives a concise explanation of the principle.

MichaelG.

Thanks Michael

I had read all the posts on this thread and followed most of the links but missed that one.

Ian P

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