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01. Types of hydraulic cylinder oil ports
The oil port hole is the direct channel for the pressure oil to enter and exit the hydraulic cylinder. The hydraulic system is inseparable from the pipeline connection, and the pipeline connection is inseparable from the selection of the oil port.
According to the different connection methods, the hydraulic cylinder inlet and outlet oil ports are divided into threaded connection and flange connection. Usually, when the hole diameter is less than 25mm, choose the threaded oil port, and when the hole diameter is greater than 25mm, choose the flange oil port.
This does not mean that there are no large-sized threaded joints, but large-sized threaded joints require a large wrench to tighten and require a large operating space. In order to save space, flange connections are generally used at this time.
02. Oil port standard
1. Threaded oil port
The three most commonly used standards for threaded oil ports are:
Metric thread, profile angle of 60°, thread feature code M;
British thread, profile angle of 55°, thread feature code G;
American thread, profile angle of 60°, thread feature code UNF.
If you look at the real thing directly, these three thread features are almost the same. It is really difficult to tell which one they are with the naked eye. However, their tooth shape, pitch and diameter specifications are different, and they need to be matched with thread tools to measure accurately, so they cannot be used interchangeably.
The national standard GB/T2878-93 specifies the oil port size series of the threaded connection of the hydraulic cylinder inlet oil port, see the table below:
Naming rules for threaded oil ports:
What is annoying is that the naming method for each standard thread is different, which can easily lead to confusion.
The naming rule for Metric thread is to multiply the major diameter of the thread by the pitch, such as M20x1.5, where 20 means the major diameter of the thread is 20mm, and 1.5 means the pitch of the thread is 1.5mm.
The naming rule for Imperial thread is expressed by nominal pipe diameter, such as G1/2, which means the nominal pipe diameter is 1/2 inch, but the actual major diameter of the thread is 20.64mm, which means you can’t use 1/2×25.4 to directly calculate the major diameter of G1/2 thread.
The naming rule for American thread, although American thread is also in inches, it is actually expressed by the major diameter of the thread, which is different from British thread. For example, 3/4″ 16unf, where 3/4 means the major diameter of the thread is 19.05, which is composed of 3/4×25.4 (1 inch = 25.4mm), and 16 means that 1 inch in length contains 16 teeth.
2. Flange oil port
The most commonly used flange oil port is the American SAE flange. The same specification of American SAE flange is divided into light series (3000psi) and heavy series (6000PSI).
03.How to choose the oil port size?
In the process of using hydraulic cylinders, in order to ensure its working effect, it is necessary to pay attention to the selection of oil port size. The determination of the inlet and outlet ports of the hydraulic cylinder mainly needs to match the pipe fitting interface in design, because it is mainly connected to the pipe fitting. In addition, it must also be combined with the use occasion and environment in order to select the appropriate structure.
1. Oil port diameter
In general, the main reference indicator for oil port size is the flow rate of the hydraulic system. If the oil port is small, it will not only cause the flow rate to be insufficient when the oil is inlet, affecting the piston movement speed of the hydraulic cylinder, but also cause obstruction when returning the oil, forming back pressure, affecting the piston’s retraction speed and reducing the load capacity of the hydraulic cylinder. For designs with strict requirements on the reciprocating speed of the hydraulic cylinder, the size of the aperture must be calculated.
In order to improve the working efficiency of the equipment and reduce vibration and noise at the same time, the effective oil diameter of the oil pipe should meet the oil flow rate below 2-4.5m/s.
2. Oil port position
The oil port position of the hydraulic cylinder is usually set on the cylinder barrel, cylinder head or piston rod. Specifically, for a hydraulic cylinder with a fixed piston rod, the inlet and outlet oil ports can be set at the end of the piston rod; if the hydraulic cylinder does not have a dedicated exhaust device, the inlet and outlet oil ports should be set at the highest point of the hydraulic cylinder so that the air can be discharged from the hydraulic cylinder first.
3. Oil port connection form
When installing the hydraulic cylinder, the characteristics of the oil port must also be considered.
Although the threaded connection is simple to manufacture and easy to install, due to the poor installation direction, especially the right-angle joint, the direction may not be right after tightening. The impact resistance of the threaded connection is slightly poor, and if it is tightened too much, the wedge effect will occur, which will squeeze the oil port.
Generally speaking, the sealing performance of 60° tapered pipe threads is better than that of 55° tapered pipe threads. The former is mostly used in high-pressure systems, while the latter is mostly used in low-pressure systems. Generally speaking, hydraulic cylinders have high requirements for the sealing of equipment, which is also an indicator of whether they can work normally. Therefore, it is often necessary to improve the sealing performance of tapered pipe threads. Therefore, it is necessary to use them in conjunction with polytetrafluoroethylene film or sealant.
Cylindrical pipe threads are generally used in conjunction with sealing rings or gaskets. At present, ordinary fine pitch threads are widely used, and there is a trend of gradually replacing imperial threads.
