Just How Fast Can You Get a Cylinder to Move?

Mar 24, 2014 2:19:32 PM LEAVE A COMMENT

ID-100215461Well, if that isn't a loaded question. Over the years more people than you would expect have proposed this puzzle and while there may be no single answer to this complex query, there are many variables to consider in attempting to answer it -- so let's start there.

When starting out on the quest to satisfy a particular need for high cylinder velocity; there are several things that must be examined -- and not just the superficial concept of developing lots of fluid or compressed air capacity; then throwing it at the ports to see what happens.

In an attempt to answer this question, however, for your specific application -- we propose: identifying, weighing and even studying the following ten items relevant to your particular project and design.

10 Items to Identify, Weigh and Study to Determine Just How Fast Your Cylinder Can Move

1. Of course it matters that you have enough volume appearing at the cylinder port; so availability of supply is one thing

2. But there is then the delicate task of the fluid velocity and how big and/or how many ports might be needed to transport that particular volume.

3. This then needs balancing against the pressure drop which can be tolerated. In the standard NFPT cylinder package, internal space at both ends is somewhat restricted; so the cylinder designer has to look carefully at both the flow areas as well as the twists and turns the fluid has to take, before coming to a view on, effectively, what might be termed the Cv (Kv if you think in metric) for that particular flow path.

4. Viscosity also creeps into the calculations if you are dealing with fluids. What is sometimes not fully appreciated is that it might well be the outgoing fluid from the opposite port which is likely to be the more critical feature to consider where ‘piston speed / back-pressure’ is concerned!

5. Seals and bearings are an obvious consideration; and the design / material used for each individual component will have a very big affect upon whether the expected velocity can be achieved. Even if the required velocity is reached, meeting anticipated ‘life’ of the product has also to be factored in before success can be acclaimed 

6. Getting a cylinder piston / rod moving fast is one thing – but stopping can be a whole different ball game! Firstly, it is necessary to know the curve of the velocity over the entire stoke - or at the very least, is the velocity requested expressed as a ‘peak’ or a ‘total stroke average’.

8. The mass of the piston and rod, together with whatever is attached to the ‘customer-thread’, has a big bearing upon how fast the assembly can be accelerated and also how quickly it can be brought to a stop or the direction of travel reversed. Most cylinder design engineers, when faced with high velocity requests, like to hear that the process is to be controlled by a servo or proportional valve and that the piston will be reversed within the available stroke, thus never hitting the Head or Cap.

9. If control of the fluid cannot be relied upon to avoid the piston hitting the Head or Cap; then an external mechanical stop might be the next bit of slightly better news – but if it really is a solid, instantaneous, ‘rock hitting a hard place’, stop scenario; with incoming pressurized fluid still in full flow; then high frequency peak pressures may well start to become a factor to worry about. There are, of course, safety factors built into the strength of all of the components which comprise the ‘pressure vessel shell’ but it has been proved on countless occasions that excessively high pressure ‘spikes’ can all too easily be generated by the seemingly most innocent of system design.

10. Each case will need individual attention and other factors will no doubt emerge which also need considering – but many things are possible – so it is always worth asking "Can we make it happen?"

What other pieces of advice might you give someone proposing the same kind of question(s)?

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Topics: Application, Innovation