Strength of Cast iron

I think we're all getting a bit carried away. The figures quoted for the eyebolt will assume it is screwed into something strong enough that the eye bolt gives up first, how else could they test it.

​however I'm concerned about this offset shackle idea. If not careful it could increase the load on the M6 thread by a lever effect. It's still probably strong enough, but let's have a sketch.

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... On reflection, perhaps I can:

We have, I think, established beyond all reasonable doubt that a properly installed M6 eyebolt would comfortably support Peter's 17Kg load. BUT the local quality of the cast iron remains unknown.

Although unlikely, it is feasible that Peter has tapped into a casting flaw ...

Given the modest load under discussion, it should be a very simple matter to conduct a 'proof test' at [say] 80Kg.

MichaelG.

As to the potential non-linearity of a gas spring: I have not noticed this myself.

With the weight of the head sitting on the spring, it seems pretty linear in both the upwards and downwards directions on my mill, requiring about the same level of force to move throughout the range of motion, regardless of direction of travel. It can be wound up or down using one finger on the rack feed handle albeit with a small level of resistance due to the gas moving in or out of the cylinder on the spring.

This maybe down to my choice of spring - I am using a 15Kg gas spring with 210mm of possible movement.

The mill head will also stay put without needing to be clamped wherever I choose to stop, due to spring resistance and friction in the rack assembly (obviously I would not think of machining like this), so acting as a pretty good counterbalance I believe.

Also because the threads on my spring go in from the side of the mill, I have no worries about them ever pulling out because there is very little tension on the threads, all the force is at right angles to the threads and cast iron is very good at resisting shear forces.

Far be it from me to stop you from installing a counterweight and chain - whatever works for you, I am merely suggesting an alternative that was cheap (IMHO), takes up minimal space, took less than 30 minutes to install and would still allow the tilting head to be used on the original installation if required.

Edited By Zebethyal on 15/05/2018 10:16:25

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This may be of interest: **LINK**

http://www.stabilus.com/fileadmin/download/Vehicle_GB_12_12.pdf

MichaelG.

Thanks Michael,

That confirms what I was seeing - the document states that "Unlike mechanical springs, gas springs have an extremely flat, almost linear characteristic curve and therefore allow a uniform comfortable adjustment or pivoting movement", the graph also shows exactly the same force required for both compression and extension.

Just a thought, when I replaced my brake calipers I noted that they were attached with M8 screws into cast iron, I have cast iron chucks/faceplates fitted using M6 studs.

I think we are ALL guilty of overthinking this, the simple answer 'M6 will be fine' was all that was needed.

Neil

I would be very surprised if 19th and early 20th century cast iron poured by a reputable foundry would have a tensile strength below 20,000 psi. Even the lower grades if iron used today would have a tensile strength around 40,000 psi.

Cast steel is a much different metal having a carbon content probably less than 0.4% where cast iron would have 3.5 to 4.5% carbon. Cast iron contains flakes or nodules of graphite. Steel does not.

Apples and oranges I suspect!

20000psi is the breaking point of some well-made cast-irons. But you wouldn't run a machine made of cast iron at anything like that level. In practice there is always a hefty safety factor built in. How big it is depends on the material and conditions of service, but it's unlikely to be less than 4:1, and 8:1 and stronger is likely. As even top quality cast iron is poor in both tension and alternating stress Richard's 2000psi limit is very reasonable.

Edited By SillyOldDuffer on 15/05/2018 18:27:37

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

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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.

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

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.

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

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S.O.D. ... for info. ^^^

MichaelG.

Doh!!!

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

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