How Mechanical Seals Work - stuffing box - packing

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The mechanical seal is a device that is capable of isolating two environments, where between the two is found a component (like a shaft) with a circular or axial motion.

Jaes, for over a decade has been providing its customers with the best solution for their supply chain and in its catalogue catalog every kind of mechanical seal from the leading manufacturers
Mechanical seals are used in almost every hydraulic machinery, to understand their operation we will take as an example a classic centrifugal pump, in which a shaft connects the motor to the impeller.

The mechanical seal is placed on the backplate in order to contain the fluids that are inside the pump, which otherwise would draw from the space between the backplate and the shaft, helped also from the internal pressure. In this case, the backplate acts as a stationary housing, while the shaft is the rotating part; the mechanical seal will have to be able to hold the pressure during the pumping and to resist the friction caused by the shaft rotation.

One of the old methods to guarantee the sealing that is still widely used, is to employ some stuffing boxes, a system that can keep tight around the shaft, a material similar to the packing (a twisted wires rope).
However, this method has some relevant flaws, in fact in order to work properly, it has to be pressed against the shaft. This method implies several problems: a huge wear of the packing, its periodical maintenance, the necessity to use great quantities of water to cool it down because, while making friction, its temperature, the consumption of the parts in contact with it and even the wasted energy from the motor to win the friction force, they all increase
That’s why mechanical seals are replacing old methods day by day, since they are designed more and more, which are projected to remedy these disadvantages.

The most common mechanical seal has three sealing points.
The stationary part is fixed to the backplate of the pump with a sealing gasket between them.

The rotating part is, instead, fixed to the shaft with a seal between them as well, in this way the rotating part rotates is integral with the shaft.
One of the two parts(in this case the rotating part) possesses one moving element that is firmly fixed and pushed from a spring; this one presses against the permanent part creating the last sealing point.
This moving element follows the shaft movements caused by the bearing “game”, the misalignments caused by the production tolerances, and from the thermal expansions during the functioning.

The seal between the rotating and the stationary part is the element that the mechanical seals have in common.
As you can guess the two parts are in contact with each other, and the rotating one frictions on the other during the rotation. For this reason the faces of the two parts that are in contact are made of flat surfaces worked with extreme precision and smoothness.
Besides, the pressure inside the pump, combined with the centrifugal effect obtained from the rotating part rotation, cause a fluid film to be created between the faces in contact, made from the same fluid pumped from the system, that while lubricates them also prevents direct contact. This lubricant foil can also be introduced from an external source.

In this way, the mechanical seals can isolate the internal pump room from the external environment, despite the presence of the rotating shaft.

If you are interested in discovering the many types of mechanical seals, watch the videos in our playlist.

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