Research different types of seals available for rotary shafts
Determine operating conditions of seal
Determine main objective of seal
Choose seal type from those available that best fits our needs and can operate in defined operating conditions
There are generally 5 main types of seals used on rotating shafts 1. Labyrinth Seals
2. Felt Seals 3. Lip Seal
4. Ferro Fluid Seals 5. Mechanical seals
Labyrinth Seals:
This seal restricts the passage of solid, liquid and gas contaminants into the sealed area and also restricts the leakage of fluid out of the sealed container. Non-contact rotary and stationary elements provide a restricted flow path and make use of centrifugal force and gravity to prevent leakage.
The advantage of the labyrinth seal is that is unlike other rotating lip seals , it will not damage the shafts, has a virtually unlimited life, is frictionless, is largely unaffected by high or low temperatures and can be used for high shaft rotating speeds.
The obvious disadvantage of this type of seal is the presence of an engineering gap. Hence this seal does not work well under extreme pressure differences and for low shaft speeds.
This seal is somewhat excessive for our needs.
Felt Seals
Felt seals are mainly used as oil or grease seals for retaining lubrication while simultaneously preventing dirt or dust from entering the bearing.
The advantage of using felt is its wicking and oil absorption properties, its fine filtering and resilience, which allows the felt to maintain a constant sealing pressure and as the seal wears the felt surface remains unchanged.
Felt seals are usually pre saturated with lubricants of higher viscosity than the bearings. In the case that the seal does run dry it will tend to protect and polish the shaft rather than cause damage.
Under normal operating conditions the felt seal is highly economical, normally requiring replacement when the machine is overhauled.
Felt seals are not suitable for oils with low viscosity or if the lubricant is pressurised. The maximum running speed can be as high as 20 m/s if the rubbing surfaces are highly polished and lubricant is always present.1
This seal seems to be the most appropriate for our needs. It is simple and cheap and achieves all that we require from it, however they are unable to handle high running speeds, which we will need for our output shaft.
Lip Seals
This seal is comprised of a rubbing elastomer ring seal element, held in place by a spring. The seal friction is reduced by an oil film generated between the lip of the seal and the shaft.
Any damage to the shaft where the seal is located will cause leakage as the oil film thickness will be exceeded at that point. Hence the finish of the shaft is particularly important.
The lubricated rubbing provides the sealing action, which maybe difficult to maintain at high speeds if the shaft is not running perfectly true. To maintain the oil film thickness the seal must follow any shaft movement, which can become difficult if the shaft is subject to eccentricity, or vibration at high speeds. Typically these seals will operate in the region of 18m/s and the seals will be affected by friction. 1
This seal would be suitable for use in our gearbox. The friction generated, and its effect on the shaft is disadvantages, however we do not feel that this would have a significant effect.
Ferro Fluid Seals
This is a very particular rotary seal type which has huge benefits, but is only used in very specialised conditions.
The seal is comprised of a fluid ring which is retained in place between the rotating and fixed members under the action of magnetic forces.
Ferrofluidic sealing technology takes advantage of the action of a fluid, containing a uniform distribution of magnetic particles, to an applied magnetic field. It uses a magnet with magnetically permeable north and south pole pieces and a
magnetically permeable shaft to create a permanent magnetic circuit. The magnetic flux is concentrated in the gap under each pole and when ferrofluid is applied to this gap, it assumes the shape of a liquid o-ring and produces a hermetic seal.
Ferrofluidic seals offer hermetic sealing, long life, virtually frictionless sealing and smooth operation. They are non-contaminating, highly reliable and can operate at high speeds. This type of seal can be used over a wide temperature range, which can be increased by use of cooling, or heating circuits.
The seals have to be regularly maintained as the fluid properties deteriorate over time. 2
It is clear that the use of such a seal could not be justified for our gearbox housing.
Mechanical Seals
Used increasingly in fluid pumps to replace packed glands and lip seals. Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts. The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively. The leakage
path gap varies as the faces are subject to varying external loads which tend the move the faces relative to each other.
The mechanical seal requires a different shaft housing design arrangement compared to that for the other type of seals because the seal is a more
complicated arrangement and the mechanical seal does not provide any support to the shaft.
This seal would not be suitable for use in our gearbox as we do not have any pumped or pressurised fluids and could not justify the cost of such a seal.