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Single
Mechanical Seals

Single Mechanical Seals - Know how they work!

Single shaft seals are by far the most common configuration to be found in industry.

They consist of a rotating unit with a mating face which butts onto a stationary unit.


The rotating unit seals in two places, at the interface between the rotating face and stationary and between the shaft and the seal body. The stationary seals against the pump body.



Mechanical Seal

Single Mechanical Seal

The seal body is sprung by either a metal or rubber bellows, a series of small springs, or by a single coil spring. The purpose of the springs is to hold the seal shut when the pump is stopped. In the case of the many less expensive seals the spring is also used to drive the rotating head of the seal. These seals must be fitted with the correct coil spring for the rotation of the shaft. The major cause of failure of this type of seal is a broken drive spring. This is due to the use of 316 SS stainless steel as the coil material. The spring has to maintain the drive of the seal head and to flex with the seal head as it rotates. Any mis-alignment in the stationary face will cause the seal head to flex twice in every rotation. Stainless steel suffers from chloride stress corrosion therefore as the spring is under constant stress and probably exposed to chlorides, cracking will occur in the spring, leading to complete failure.


The same effect can occur when the seal is fitted with a series of small springs if they are constructed from 316 SS. A better material to use is Hastelloy C which does not suffer from chloride stress corrosion.


The purpose of the spring is to keep the seal shut when the pump is stopped. When the pump is running the pressure in the seal chamber, being higher than the outside pressure, acts upon the seal surfaces to increase the closing force. Higher closing force means that any lubricating effect between the seal faces is marginalised leading to higher friction, more heat generation, and increased wear. Seal design can mitigate this increase in closing force. The design of a balanced seal incorporates surfaces to reduce the load imposed on the seal by the chamber pressure. By this means a manufacturer can reduce the pressure between the seal faces and produce longer life of the seal.


The seal body has to make an effective seal to the shaft. This is achieved in a number of ways. The single drive spring seal is a variant of the rubber bellows seal. The rubber bellows allows the seal head to flex and to become vulcanized to the shaft producing the shaft seal required. This arrangement is often found on small water pumps. It should not be used on abrasive water service as the abrasives will wear the bellows causing failure.

Mechanical Seal

O'Rings

O-rings seal by deforming under pressure to effect a seal inside their operating grooves. The pressure also acts to squeeze the ring onto the shaft. Have you noticed that where the o-ring seats a shiny, polished area can be seen when you remove the seal body? This area is very important to you, it is known as a fret ring and represents the damage that the o-ring has caused on your shaft or shaft sleeve. As mentioned above, the head of the seal has to flex with any mis-alignment. The shaft may experience harmonic vibration, transient vibrations, the stationary unit may not be true to the axis of the pump, the shaft may bend slightly, and in order to maintain a proper seal the head has to accommodate all this movement. Additionally the seal faces are constantly wearing so the seal head has to be able to move down the shaft towards the stationary unit. The consequence of all this movement is that the O-ring too has to flex. It moves back and forth polishing the surface of the metal.

The oxides present on the surface of the metal become embedded in the O-ring. Gradually there is a build up until the O-ring becomes a very effective polishing tool covered in oxide and grinding away at the surface of your shaft sleeve. Metal removed from this area will prejudice the replacement seal you fit because a leak has developed, unless you change the sleeve. It is a good idea to fit a seal which includes a disposable shaft sleeve in its design, or be prepared to replace the whole shaft.


The O-ring account serves to illustrate exactly what is going on with all of the other seal elastomers. The Teflon wedge, the U-cup or the chevron set all produce the same or similar effects. I have worked mainly with O-ring seals.

There may be cases where alternative elastomers are effective, they may well work for you but my experience is that O-rings are:


simple and easy to fit.

give a high degree of flexibility in material compatibility.

survive well in a wide range of temperatures.

provide the long term flexibility required by the seal head.

do not cause damage to pump shafts if properly fitted.


The rule is always check the shaft or shaft sleeve for damage under the seal's shaft seal. You'll find it there, I have no doubt, leave it, save a few pennies, and have to change out the new seal in hours. If, like me you hate having to do the same job twice in the same shift, do it right first time around and replace all worn parts.


To avoid fretting corrosion from damaging your pump ensure that the seal you fit has a disposable sleeve and change that sleeve whenever you change the seal. Some single seals incorporate a static O-ring together with a dynamic O-ring mounted onto a sleeve made as an integral part of the seal body, the Chesterton 880 series seal is one example of this type.

Seal Face Combinations

When you have got the backup materials and configuration of the ideal seal for your application its time to look at the seal face combination. The most common face combination is carbon / ceramic. However there are two types of ceramic and many different types of carbon. For applications beyond water pumps different material combinations or variations on the basic carbon and ceramic should be considered if your aim is to extend the life of your seals.

Specific pump environments cause readily identifiable seal problems. Coking, chipping on seal ID or OD, hang-up, heat check, chemical attack, all related to the inter-action between the seal materials and the environment.

Look at the seals you remove from your pump units, keep the parts together and take time out to think of the conditions which could cause the damage / wear / failure that you are seeing. At that point seal face selection can begin, the seals fitted to your plant in the first instance were an informed choice, but if that combination has failed in service for any reason, your job is to fix it.

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