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Under pressure – or not so much? ben lee

Designers working on hydraulic systems often opt straight for high working pressures, often due to the availability of components that are tried and tested as working well together.  There may also be a requirement to use a certain manufacturer or part in the design, specified due to cost, stock and other business reasons.  Designers often also like to use high pressures to reduce the cylinder diameters for aesthetic reasons, as well as design specific reasons; if a system would not fit in the desired place with a wide cylinder then a narrower one must be used, necessitating a high pressure system.

It seems then, that designers of hydraulic systems opt for high pressure because of familiar components, familiar design and because increasing the pressure results in smaller cylinders that may fit better in the space and which certainly look nicer.  Very often, the designer will not be working with costs at the forefront of their mind, but if they were to use the monetary factor as their starting point when designing a new system, they may look more favourably on low pressure systems, as the materials are cheaper and just as reliable.  When working at pressures of 600 bar plus, carbon steel is the standard option for component manufacture, as the material needs to be incredibly strong to withstand the forces occurring within.  Low pressure systems, operating at around 60 bar or so, can be fabricated in alloy or composite metals, even plastics in some cases, bringing costs down.  The other benefit to not using carbon steel is the weight of the finished product; where weight is an issue, it is a good idea to start the design process by looking at how a low pressure hydraulic system can achieve the desired result.

The main differences between low and high pressure hydraulic systems is the reservoir size, as filling a large diameter cylinder requires more fluid than a smaller one.  This may be a problem when creating space-saving or space-limited equipment, but if space is not such an issue then the reservoir size need not be a problem.  The input power is not much more than with a high pressure system; the same force can be achieved by exerting a high pressure on a small area as by exerting a low pressure over a large area, so overall the input is the same.  Of course, balancing these forces exactly may result in a bulky system to operate at low pressure, but very similar forces can be achieved without going to extremes.

The alternative to low pressure hydraulic systems is electric actuators, but in systems that require multiple cylinders the execution of the design becomes more complex, as each one needs to be powered individually, requiring more wiring and by extension, more components that could fail.  By remaining with hydraulic technology the system can be kept simple, with only one hydraulic pump (which can be sited remotely, another plus for systems where the input is physically separated from the output) needed to effect motion in all cylinders.  Manual override is also a lot simpler in hydraulic systems and they tend to be more reliable and sturdy than the electrical alternatives.

The final benefits of low pressure hydraulic systems are the reliability, and the flexibility of material choice for piping.  Although seals can fail on any hydraulic equipment, they are less likely to occur when the seal is not constantly under high pressure and any leak will not be as catastrophic as in a high pressure system, more akin to the pressure from a kitchen tap than a fire hose.  Some hydraulic hoses can be bulky and inflexible, but if the system is operating at a low pressure then nylon tubing can be used instead.  This is flexible, narrower and much easier to work with than most hydraulic hoses and is therefore, a great option when installing hydraulic systems in places that are hard to access or where the output is sited far away from the input and there is a lot of complex and immoveable machinery in the way.  Nylon tubing is also cheap, further reducing the costs.

A good example of a low pressure hydraulic system that works incredibly well is the BFT hydraulic gate and door opener.  On the market for over 30 years, this system is still the benchmark for gate opening systems and proof that low pressure hydraulic systems can work very well; sometimes they are even the best option.




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