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What is the technical difference between a hydraulic cylinder and a spring? For sure, I understand their principles but I wonder about physical-technical differences. For example, the most obvious aspect (for me), is that they both can deflect. A spring, on the contrary, can store potential energy, is this also valid for the cylinder? It is also able to oscillate. Are there more differences?

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  • $\begingroup$ I think you question needs more focus. You need to describe more clearly what is the setup of the hydraulic cylinder. In some cases you can have storage of energy (potential energy), but if there are check valves in place, then because most fluids used in hydraulic cylinders are not compressible then they do not. As it is your question, has a wide range of valid answers. $\endgroup$
    – NMech
    Jun 7 at 9:17
  • $\begingroup$ @NMech Thanks for the comment. Tbh, I have no specific setup of the cylinder in mind. Is there any version that resembles a spring? $\endgroup$
    – Ben
    Jun 7 at 9:36
  • $\begingroup$ you can have different configuration of a hydraulic cylinder that behave as a spring yes (see gravity acting, or dashpot with a fluid and Nitrogen) etc $\endgroup$
    – NMech
    Jun 7 at 11:03
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    $\begingroup$ Perhaps you meant a pneumatic cylinder. Some bicycle suspensions use air as cushion / springiness. $\endgroup$
    – AJN
    Jun 7 at 12:00
  • $\begingroup$ Isn't the only difference between a pneumatic and a hydraulic cylinder that gas is replaced with a fluid? $\endgroup$
    – Ben
    Jun 7 at 12:41
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A hydraulic cylinder filled with hydraulic fluid cannot deflect unless the fluid has somewhere to go. It cannot act as a spring unless the reservoir has the capability to absorb the fluid and the ability to maintain a restorative pressure during compression. In a simple system this would be a spring and at that point you are asking, "Would a spring act like a spring?".

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Figure 1. A compact hydraulic power unit. Image source: ResearchGate.

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  • $\begingroup$ Thank you! Do I need an inlet/outlet for y cylinder? I thought it would basically embody a hydraulic press? $\endgroup$
    – Ben
    Jun 7 at 10:31
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    $\begingroup$ I don't understand your comment question. This is the power unit. The hydraulic actuator ("cylinder" in your question) would be located elsewhere and connected to the "inlet and outlet" in the diagram. Pressurised hydraulic fluid would fill the left chamber and the spring chamber may be vented to atmosphere. $\endgroup$
    – Transistor
    Jun 7 at 10:33
  • $\begingroup$ This is not the cylinder? But it has "everything" like the cylinder and a piston. I thought a fluid would displace/press the piston to act as a force. I even understand your explanation as such? $\endgroup$
    – Ben
    Jun 7 at 12:43
  • $\begingroup$ Yes a fluid would displace the power unit piston but the fluid would have to come from somewhere else and that's the actuator. Think of it as a bit like a car's hydraulic brake cylinder (actuator) and the master cylinder (the power unit). I've only shown the power unit. $\endgroup$
    – Transistor
    Jun 7 at 14:22
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@Ben This is to answer the comment whether there are configurations that a hydraulic cylinder acts as a spring.

Please Note that with the term hydraulic cylinder, I consider any cylinder with a hydraulic fluid that can act as an actuator.

Automotive shock absorber:

The following is an automotive shock absorbed. Notice the use of oil and gas (usually nitrogen). This type of shock absorved acts like a spring (the gas compresses) and a dashpot (the piston in the oil).

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Figure 1: This is an automotive shock absorber (source: aresmotorsports)

Connected hydraulic cylinders

The following image are two connected hydraulic cylinders.

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Figure (2): connected hydraulic cylinders (source: University of Alaska Fairbanks

In this configuration you can place a dead weight on one of the two cylinders. Then by applying a force on the other end, the vertical motion of the cylinder effectively stores potential energy.

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