This article is an overview of the theory of hydraulic connectors, the three main types of connectors and the two key functions that all hydraulic connectors must fulfil.
Three Main Types of Hydraulic Connectors
It provides basic information about the three main classes of hydraulic connectors:
- Tapered Threads
- Metal seal connectors
- Soft seal connectors
Two Key Functions of Hydraulic Connectors
It’s important to understand that there are two functions that a hydraulic connector must fulfil:
Sometimes the holding and sealing functions are provided by the same mechanism, and sometimes by two different parts.
Hose connectors may seem very confusing because of the many different types available.
However, when looking objectively, connectors will always simply do two things. Hold, and seal under pressure.
Development of Hydraulic Connectors
Hydraulic connectors were developed in this order:
- Tapered threads
- Metal seal connectors
- Soft seal connectors
Tapered pipe threads were one of the first ways to connect pipes together.
They involve cutting the pipe thread in a tapered manner, so that the diameter of the pipe changes over the length of the fitting.
These kinds of connectors require some form of sealant, either a tape sealant, which can block the pipes, or a chemical sealant, which is preferred.
Tapered pipe threads can also be difficult to orient correctly, leading to poor sealing and leakage.
Tapered threads can also cause damage to other components, including deformation of aluminium and cracking of cast iron, due to the pressure of the steel pipe thread within the component.
They also can’t be connected to hydraulic tubes without soldering or welding of the tube to the part.
Metal Seal Connectors
Metal seal connectors were developed to address some of the limitations of the tapered pipe threads.
A metal seal connector is basically a tapered pipe thread with the addition of a 30 degree machined face, to form a seal.
This feature is generally only in hydraulic systems, because of the high pressures that need to be dealt with.
The addition of the “chamfer” on the face of the thread split the holding and sealing features of the connector.
The thread does the holding and the machined face does the sealing.
This modification also makes the connector easier to orient, better sealing, and eliminates the need for a sealant.
However, metal seal connectors still can’t accept hydraulic tube unless the tube is soldered or welded to it, and they can also have issues with vibration.
The JIC connector is a further development. It has a 37º metal seal and can accept hydraulic hose and tube. BSP Thread, BSPT, BSPP and NPT are common types of threads used in metal seal connectors.
Soft Seal Connectors
Soft seal connectors are the most recent development in the evolution of hydraulic connectors.
They make use of an elastomeric seal – natural or synthetic – to prevent leakage.
The soft seal also helps connectors resist the effects of vibration, as well as the high-pressures used in many hydraulic systems today.
The seal in a soft seal connector usually takes the form of an O-ring.
Another advantage of soft seal connectors is their rapid assembly, which can be important in high-volume production environments.
The main limitation of soft seal connectors comes from the fact that performance of the o-ring can be affected by temperature and the system fluids, if they’re not compatible with the elastic substance.
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What is force multiplication?
Pascal’s law states that pressure set up in a confined body of fluid, acts equally in all directions, and always at right angles to the containing surface. When a pressure is applied to a fluid trapped in a confined space, that pressure acts on each square millimeter of that surface. The force output of a hydraulic actuator is the result of the pressure applied, and the area to which that pressure is applied.
Force = Pressure x Area
If a given pressure is applied to two identical cylinders, then they will have an equal output force.
How does force multiplication work?
When the same pressure is applied to different sized cylinders, then the larger cylinder will have a greater force output. This is because the area which sees the hydraulic pressure is greater.
Different cylinder sizes create different pressures
The cylinder on the right in this example is twice the diameter of the cylinder on the left.
The area of the circle multiplies the force
The area of a circle is calculated by this formula:
The area of a circle = Pi x the radius squared
When a given pressure is applied to two different sized areas, the larger area will see a larger force output than the smaller area. This force differential is often significant. For example, while a 200mm diameter circle is twenty times larger than a 10mm circle in diameter, the area differential between the two is 400 to 1. When pressure is applied, the output force increase would also be 400 to 1.
A bottle jack piston uses force multiplication
A hydraulic bottle jack is a perfect example of force multiplication at work.
A reciprocating piston is moved with a hand lever. This piston applies pressure to the fluid, which is then fed through and applied to a larger piston. The output force is magnified due to the area differential of the two pistons, thus making it possible to lift a car by hand. The bottle jack piston moves slowly in relation to the pump piston because of the volume differential between the two chambers.
The weight of a load affects the hydraulic pressure
The hydraulic pressure required to lift a load is dictated by the mass of the load, and the area to which it is applied.
The weight of a load changes the hydraulic pressure required
The three cylinders on the left are all lifting the same weight, 20,000kg, (20T). This mass applies a force of 19,600N to the piston area of the cylinder. The hydraulic pressure that is created is called the load induced pressure. i.e. The pressure that is induced in the fluid by the load applied to it. When the same force applied to different cylinder areas, the load induced pressure will be highest in the smallest cylinder.
Load induced pressure vs system pressure
When the pressure applied to a cylinder is greater than the load induced pressure, the cylinder will begin to move forward and lift the load. As the load is lifted, the pressure seen within that area of the circuit is generally that of the load induced pressure, as long as there is no resistance to movement.
As the cylinder mechanically reaches the end of its stroke, the pressure will rise to the maximum allowed within that area of the system. This maximum pressure allowed is limited by valving such as pressure relief valves, pressure reducing valves or similar hydraulic fittings.
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