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Posts tagged: Hydraulic Cylinders

Hydraulic Cylinders: When Hysteresis Isn’t Funny

There’s a common phenomenon in literally 100% of hydraulic systems that simply must be taken into account, and it has the misfortune of bearing a name that makes it sound like something we should all be grinning at: hysteresis.

 

What Is Hysteresis?

Simply put, every single kind of hydraulic cylinder, hydraulic valve, or hydraulic end-device (winch, servo arm, whatever it may be) that has at least two directions of movement suffers from the same problem. That being, the pressure required to move them from one position and the pressure required to move them back aren’t the same pressure.

 

Let’s take the simplest example: a valve designed to open when pressure gets too high — a ‘release valve.’  As the pressure behind the release valve builds, it will remain steady until that pressure reaches a certain point — say, 500 psi — at which point the valve opens. As the pressure decreases, however, the valve does not then close at 500 psi; it will only close when the pressure hits a lower point — say, 480 psi. The difference between those two pressures is called the ‘hysteresis’ of the valve, and it’s most often given as a percentage — 480 being 96% of 500, we would say the hysteresis of the valve is 4%.

 

Hysteresis is also rate-dependent, so for example, bringing a given hydraulic cylinder slowly from one pressure to another will result in less hysteresis than if you just crank the control all the way over to the other side in one swift action.

 

Why is Hysteresis a Problem?

Hysteresis is a problem primarily because of the way in which the human mind thinks — we expect, for example, that if we put all of the settings on a given hydraulic circuit to the same positions they were in last time, we’ll get the same result. But because of hysteresis, the result you get as you increase the pressure to get to Point X can differ significantly from the result you get if you reach Point X by decreasing the pressure — and the results can vary even more if you increase and/or decrease the pressure quickly rather than gently.

 

The end result is that confident operators can do everything they are ‘supposed to’ in order to achieve a specific end result, and end up missing that end result by enough to cause a disaster on the job site. Minimizing hysteresis — and constantly monitoring it — are critical goals for every job site that relies on hydraulic circuits, especially when lives could be endangered by an error.

 

Importance of Hydraulic Cylinders

Have you ever paid attention to the importance of hydraulic cylinders in modern day construction? Every single tube you find on or under a machines arm while staring at a building in process is most likely one of these. As you may have guessed by the description given before, they are what power these arms in a unidirectional stroke in order to move or lift something which requires a lot of force.

The Power Itself

A hydraulic cylinder is made up of many parts like barrels and pistons, but mainly the force will come from a pressurized hydraulic fluid which tends to be oil. Inside the cylinder, this fluid’s flow is regulated by a pump which will in turn generate a pull force.

You will also find that there are two types of cylinders: Single acting and double acting.

  1. The first ones are simpler in design and generally cheaper. The oil enters through a port at one end of the cylinder, extending the rod which later gravity will send back into the piston.
  2. Double acting cylinders, on the other hand, have a port at each end and require the fluid to flow in or out of both sides for both movements (extension and retraction).

Different Designs

In the industry of hydraulic cylinders you will find many designs available:

  • Welded Body: it doesn’t have a tie rod and the barrel is welded directly into the end of the caps as well as the ports.
  • Tie Rod:  mostly seen in industrial factory applications, these hold the end caps using threaded steel rods.
  • Compact: utilized when the mounting space is expensive or difficult to acquire and the application demands a high force.
  • Telescopic: also called multi stage rod, due to the fact that it is actually a succession of rods tied together for force or distance required

Which One Should You Buy?

As you can see, there are many kinds of hydraulic cylinders in the market so the best thing to do is actually to ask for a professional’s help to suit your manufacturing or building needs. That way you’ll find the proper tool for the proper task.