Low pressure cylinders/high pressure systems

[tunna]

Some concern must be felt in the community ,after seeing Spawns cylinder explode ,I think there are a lot of machines out there using 10 bar cylinders
being powered by 70bar systems, in an arena without poly carb sides that situation could be a absolute disaster with bits of cylinder exploding
in all directions

[maddox]

Low pressure cylinders/high pressure systems

Tony I warned them at Worthing 2002 that the ram wont hold.

An event where Im in charge of CO2 machines overrunning low pressure rams without extensive rebuild wont get any CO2.

[tunna]

Low pressure cylinders/high pressure systems

Well done Mario,I have seen quite a few, and have expressed concern, but I am going to bring it up as a matter of urgency at the next FRA meeting

regards tony

[maddox]

Low pressure cylinders/high pressure systems

The tubing itself can withstand the pressure, but the endcaps cant. Especialy when the flipperarm is heavy.
There is a reason I developed the Gravity idea with the seperate flipperarm.

[tunna]

Low pressure cylinders/high pressure systems

I guess fastners/and or lack of tie bars on those type of cylinders come in to question, but whatever!I estimate a particle of cylinder being accelerated by 1000psi would hit speeds in excess
of 2000ft/sec before decelerating or hitting something!!!!!

[simon]

Low pressure cylinders/high pressure systems

Can LP rams be modded to take FP ? what is needed ?

[tunna]

Low pressure cylinders/high pressure systems

good quality tie bars,not just good engineering but common sense,

[m2xt]

Low pressure cylinders/high pressure systems

The issue of rams intended for use at low pressure being used at high pressure was raised at the last FRA meeting. Firstly, I would not recommend this approach unless the individual concerned can confidently ensure that necessary modifications are correctly undertaken.

As well as the pre-requisite pressure test, the ram has to be fit for purpose. It is unlikely that an un-modified LP ram at HP can be considered fit for purpose and history supports this view.

The FRA will allow the use of LP rams at HP subject to the team being able to satisfy the tech checker that they have taken the necessary steps to ensure the ram is fit to run at these elevated pressures.

Personally, I would look for some evidence to show that the cylinder tube can cope with the HP, that tie bars are of adequate size and grade of material and that the end caps are of sufficient strength (or some alternative means to ensure the mechanical shock at stroke end is absorbed sufficiently to remove shock from the end cap).

I would ask that tech checkers assume that the LP ram at HP is unsuitable until convinced otherwise by the team in question. This does not mean that the tech checker has to understand all of the theory of the mods but wants to be convinced that the team understands and can demonstrate what they did to ensure the ram is fit for purpose.

The FRA does not want to encourage the use of LP rams at HP but recognise that it can be achieved safely when approached in the right manner.

Paul
FRA Technical Team

[roger_plant]

Low pressure cylinders/high pressure systems

I see three main problems when using LP rams on HP.
1, the capability of the whole thing to withstand the pressure
2, the strength and type of the materials used for the ends of the ram.
3, decelerating the load at the end of the stroke.

1 and 2 can be taken care of by using a hydraulic ram BUT hydraulic rams are made for slow speed operation and do not have any cushioning at the ram ends.
1 and 2, most modern gas rams have cast alloy ends that will shatter easily when hammered hard by a high-speed load.
3. Can be taken care of by the use of Kevlar strops to decelerate the load at the end of its travel. However getting these strops exactly right is very difficult and even harder to prove that they are safe.

Good quality gas rams have an adjustable dynamic cushioning system at the end of the stroke. For this to work properly at high speed the ram ends need to be very strong as the gas pressures developed by the cushioning action can be a lot higher than the HP CO2 used to power the ram.
The use of rubber cushions, or €œdoughnuts€ within the ram is very dangerous, these can freeze to near solid at the low gas temperatures and become very brittle rather than springy.

Hydraulic cylinders may be able to withstand the pressures but they are also heavy in their construction, including the piston and rod. This substantially increases the dynamic loads at the end of a high-speed stroke.
Hydraulic ram seals rely on the oil to lubricate them. They generally have metal-on-metal sliding bearings that must be oil lubricated.
They are also not made to operate at the very low temperatures generated by CO2 expanding rapidly.
Gas rams may have Teflon sliding bearings that can withstand the high speeds without over heating.

The answer to the problems is not fully found in either normal gas or hydraulic rams.

For WBC I use old-fashioned gas rams with built in dynamic cushioning system at both ends.
They date back to the 1970€™s.
They are nearly impossible to find these days. Their modern replacements are nowhere near as well made.
The modern ones are cut to the bone in their construction to reduce costs.
I have replaced the cast ends, and other components that I considered too weak, with new ones made from 2014A aircraft grade aluminium alloy or Titanium alloy.
The seals are lubricated with aerospace grade grease.
This grease can withstand the very low temperatures of the expanding CO2 and is also HP Oxygen safe.
I have also done various other modifications to them and very substantial inspections and stress calculations on all their component parts.

I have been asked a few times by other roboteers how to make an HP gas system.
I have always declined to explain how to do it, working on the principle that if you have to ask how to do some thing potentially dangerous then you don€™t know enough about how to do it safely.
This may sound arrogant but it is better than giving someone some knowledge and then feeling guilty or responsible when they hurt themselves trying to copy what I have done.

I learnt my HP gas techniques from the aerospace and professional diving industry. I was taught by experts and only allowed to work on HP systems when those experts were sure that I was competent.
I was lucky to get that experience, it is not easily or commonly found in most industrial situations, or quickly learnt.

I don€™t know what the answer really is to using HP gas safely in hobby robots.
It requires highly skilled professional knowledge and supervision. Knowledge that is not common in industry or outside of military or aerospace engineering.

I can only suggest you look well and hard for the right people to help you design your system AND to check it is safe and properly maintained when completed.

The engineers who do know how to do it will also know how dangerous limited knowledge can be in the wrong hands and will be reluctant to help hobby builders who lack their professional experience.

Catch 22 perhaps?

The other answer is to use a 16 Bar system. These are much easier to build from standard HP air components.
They still require the use of correctly set and adequately sized pressure regulators AND safety relief valves.

If built well they are nearly as effective and far safer.

[Guest]

Low pressure cylinders/high pressure systems

Indeed, just look at M2.

[m2xt]

Low pressure cylinders/high pressure systems

Thank you Roger, your experienced input on this issue is appreciated.

We are trying to put off builders from this practice but not outlaw it as clearly there are builders who have, or have access to the expertise necessary.

Christian, even on M2, the ram was modded with new tie rods. The pneumatic cushioning is just about useless with the high velocities, so the next stage is to replace the top cap with a machined 6082 T6 block just like the base. Whilst it has yet to fail, continued use can only weaken the cast top end.

Paul

[Guest]

Low pressure cylinders/high pressure systems

Hmmm, I didnt know that...

[Guest]

Low pressure cylinders/high pressure systems

is there any cushioning that will not freeze up like rubber or something like that

[roger_plant]

Low pressure cylinders/high pressure systems

The dynamic cushioning on WBCs ram works very well, it is the only system that I would consider using, and it is standard on good quality cylinders.
But the end cap materials and tie rods must be strong enough to handle it.

[Guest]

Low pressure cylinders/high pressure systems

Our new HP- jack is constructed as a pulling cylinder ( tube from an old 100 mm bore FESTO LP cylinder, same quality as mentioned above from roger) on the end cap we use a hydraulic shock absorber for end cushioning. Tie rods are 12 mm 12.9 threaded rods, end caps machined from 7075 ali

regards
helmut