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Mechanical Seals Will Save You Money

Increasingly, mechanical seals are being used on fluid pumps for replacing lip seals and packed glands.  Pumps that have mechanical seals are more efficient and reliable performance over extended time periods.  Mechanical seals are used to prevent the leaking of pumped fluids on the drive shafts.

Between the two flat surfaces for the housing and rotating shaft is the path that controls the leakage.  The gap for the leakage path varies, as the faces on the mechanical seals are subjected to external loads that vary.  This tends to move the faces of the mechanical seals relative to one another.

Different shaft housing designs are needed for mechanical seals compared to other kinds due to the arrangement being more complicated and no support is provided to the shaft by the mechanical seals.

The faces of the mechanical seals are usually hydro-dynamically lubricated so that the mechanical seals will perform over extended periods of time with low friction.  The fluid film has to be able to carry substantial loads.

Introduction to Mechanical SealsIf a load does become too high, surface contact takes place on the film and bearing failure occurs.  Generally, the lubricating film is a maximum of 3 micrometers thick.  The thickness is crucial in order for the sealing function to work correctly.

Frequently, mechanical seals have one face with the right solid lubricant so that the mechanical seals are still able to operate without having fluid film for awhile.

The Use Of Pressure Balanced Mechanical Seals

The contact pressure of the mechanical seals can be reduced by using a mechanical seal design that is pressure balanced.  A portion of the force that the pump fluid pressure generates is off set.

Mechanical Seals Design Features

Usually, three static seals are included in mechanical seals.

  • Housing seal — usually a gasket o-ring.
  • Seal between the sleeve or shaft and moving seal member- frequently an o-ring.  However, it can be a vee or wedge seal.  This kind of seal can’t be used with a bellows kind of mechanical seal.
  • Sleeve seal- usually this is an o-ring.

These mechanical seals must need to be compatible with the associated environment and the fluid being contained.  These seals might limit high temperature application designs.  The best option in thee cases might be a metal bellows kind of mechanical seal:

Generally, the sealing faces are pressed together through the use of some kind of spring loading.  There are several different kinds of spring loading systems that can be used.

  • Magnetic
  • Metal bellows mechanical seal
  • Disc Springs mechanical seal
  • Multiple springs that are distributed round the body of a mechanical seal
  • Single spring mechanical seal

A single spring arrangement used with conventional mechanical seals.  When space is restricted, another kind of spring arrangement is used.

Introduction to Mechanical SealsUsually the seal faces are made out of dissimilar materials.  The softer face usually has the narrower surface.

Similar hard materials such as tungsten carbide get used for abrasive applications.  Mechanical seal surface need to have enough strength for withstanding hydrostatic fluid force.  Heat that is generated from the sliding action must also be removed.

Introduction to Mechanical SealsFrequently carbon is used against stainless steel, cast iron, bronze, silicon carbide and tungsten carbide.

The surface of the mechanical seals needs to be squared to the shaft, smooth and flat.  Normally, both surfaces are lapped into a finish of high quality.  The most important surface is the harder one.  The design of the softer surface allows it to run-in past the initial operating time.

Shaft design is crucial.  It needs to be rigid enough so that it can support the mechanical seals in their correct positions.  The finish of the shaft surface needs to ensure that the static seals have good sealing (a minimum of 04. micrometers CLA).

The shaft’s Total Indicted Run-out needs to not be more than 0.125mm.  Also, shaft vibration needs to be kept to a minimum.  The shaft might get subjected to fretting corrosion.  This results from seal micro-movements.  Using sleeves or having locally hardened surfaces is frequently desired.

Replacement Seals 4 these Pumps +

  • KSB ®
  • MONO ®
  • EBARA ®
  • APV PUMA ®


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