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Mechanical seal with embedded lubrication

Mechanical seal with embedded lubrication

Mechanical seal for providing a seal around the drive shaft of a variable displacement swash plate type compressor used in an air conditioning system for a vehicle comprising a rotating bearing surface, an associated stationary bearing surface and a lubricant embedded in the stationary bearing surface.

During operation of the compressor, the rotating bearing surface is caused to move against the stationary bearing surface causing lubricant to be released between the bearing surfaces, thereby providing efficient lubrication of the bearing surfaces.

The present invention relates to a mechanical seal, and more particularly to a mechanical seal disposed between two relatively moving bearing surfaces such as, for example, between a rotating bearing surface and an associated stationary bearing surface for providing a seal around the drive shaft of a variable displacement swash plate type compressor used in an air conditioning system for a vehicle.

A mechanical face seal is frequently used in an automotive cooling pump or refrigeration compressor. Generally, such seals include a stationary annular bearing surface integral with the compressor housing and an associated sealing ring disposed on a rotating drive shaft. A means are provided for urging the facing surfaces of the stationary bearing surface and the sealing ring together. The rotating surface of the sealing ring contacts the stationary bearing surface to form a sealing face which is perpendicular to the shaft. The stationary sealing surface is typically formed of cast iron, stainless steel, ceramic, hard chromium-plated steel or hardened bearing steel; and the associated rotating ring is formed of sintered carbon-graphite, resin-bonded carbon-graphite, resin impregnated carbon-graphite or ceramic.

Refrigeration compressors are used to compress refrigerants, such as carbon dioxide, as part of a standard vapor-compression refrigeration cycle. Typically, a gaseous refrigerant is mixed with a liquid lubricating medium, such as oil, before entering the compressor. The oil is employed to lubricate the compressor components, such as bearings and seals, to reduce component wear. Refrigeration compressors typically include a shaft rotatably supported by bearings within a compressor housing. Mechanical seals are typically employed in such refrigeration compressors to inhibit leakage of lubricating oil between the compressor housing and the shaft.

When a mechanical seal is mounted in a conventional variable displacement swash plate type compressor of an air-conditioner for a vehicle in which carbon dioxide refrigerant is used, the operational conditions of the mechanical seal become severe. The pressure within such a compressor is greater than within a compressor using a different refrigerant, resulting in a greater axial sealing force on the mechanical seal. Additionally, conventional lubricating oil is not soluble in carbon dioxide and therefore the lubricating oil cannot be efficiently distributed within the compressor. Such inefficient distribution of lubricating oil can cause the sealing face of the mechanical seal to receive an insufficient amount of lubricating oil. Insufficient lubrication will cause excessive friction in the sealing face, resulting in over-heating and failure of the mechanical seal.

The mechanical seals of the prior art rely primarily on the flow of oil mixed with refrigerant gas to effect proper lubrication. Therefore, ineffective lubrication of the sealing face occurs due the lack of consistent flow of refrigerant gas within the compressor.

It would be desirable to produce a mechanical seal wherein a constant supply of lubrication is released into the sealing face to result in improved lubrication of the mechanical seal.

Consistent and consonant with the present invention, a mechanical seal wherein a constant supply of lubrication is released into the sealing face to result in improved lubrication of the mechanical seal has surprisingly been discovered.

John Crane Type 3710 Split Seal Installation Video

John Crane Type 3710 Split Seal Installation Video. John Crane is a global leader in the design, development and manufacture of a wide range of products and services for the world’s process and industrial markets including the oil and gas, power generation, chemical, pharmaceutical, pulp and paper and mining sectors. With a workforce of 6700 people in 50 + countries, John Crane provides an unrivaled global presence combined with constantly expanding local service and support. John Crane offers end to end solutions with an extensive range of engineered mechanical seals and seal support systems, mechanical packing, power transmission couplings, advanced hydrodynamic bearings and filtration systems under the brand names John Crane, Sealol, Safematic, Flexibox, Metastream, Inovit, Orion and Indufil.

Mechanical Seals

Mechanical Seals

In general a mechanical seal is a piece of equipment which enables you to connect the systems or mechanisms collectively to stop the outflow or the leakage in a plumbing structure which contains pressure.

In the fluid business, the meaning is much narrower. By a mechanical seal we mean a device which is used to seal shafts of pumps, mixers or agitators.

The older technology, in many cases still used now, is mechanical packing. Mechanical packing is compressed in a stuffing box by a gland so it effectively fills the space between the shaft or stem and the casing of the equipment. Certain contact pressure on the shaft or stem is needed to effect the sealing function. We could call it a radial sealing device. While mechanical packing is indispensable to seal valve stems where the speed is very low, it is not as good to seal pump shafts due to a number of problems associated with shaft rotation:

Friction consumes power;
Frictional heat requires cooling by allowing certain leakage of the sealed product, which may be expensive or unsafe;
Friction leads to shaft sleeve wear;
Friction leads to mechanical packing wear which calls for frequent packing adjustments.

While waste of power is an issue in every industry, in many industries use of mechanical packing is prohibited where even small, controlled amount of leakage from a pump is not acceptable due to environmental or fire safety reasons.

In mechanical seals main friction happens in the axial direction, between faces (seal rings) lapped to a high precision. Wear is compensated in axial direction by springs or bellows. Many mechanical seals do not wear shaft at all. Contact pressure and thus the friction force is kept to a minimum by a balanced design. A balanced seal consumes much less power and it does not require product leakage to cool the seal faces. Actually when a mechanical seal leaks (visually seen leakage) usually it means that the seal needs to be repaired. Sometimes a perfectly normal mechanical seal leak. This is true if the sealed pressure is high. Good seals do not have visual leakage up to 50 bar and above.

The technology of mechanical seals is by no means new. It has been accepted in industry for decades. In most cases it has proven to be effective and reliable. The range of designs and materials extends from home heating circulating pumps to 380C hot hydrocarbons pumps at oil refineries or to 100 bar crude oil transfer pumps, costing from 10 dollars to 20,000 dollars and even more, depending on the industry and country.

Construction:

Mechanical seals consist of the following components:

Seal rings or faces, rotating and stationary;
Secondary seals such as o-rings, PTFE V-Rings or wedges, rubber boots, PTFE or flexible graphite gaskets;
Spring action elements such as springs, metal bellows;
Drive elements such as pins, set screws, etc.;
Metal parts such as shaft sleeves, glands, collars, set plates or clips, holders, etc.
Additional elements such as safety bushings which, if a seal malfunctions, will direct leakage to a specialized drain.

Mechanical Seals can be classified by two ways.

1: Classification on the basis of Arrangement

Single – inside and outside;

Multiple (usually, dual) – tandem or double. Such a seal will require a seal support system to supply fluid or gas between the inboard and outboard faces. If the pressure of the supplied fluid is lower than the pressure in the seal chamber, it is called buffer fluid or gas, and the seal is called a tandem mechanical seal. And if the pressure of the supplied fluid is higher than the pressure in the seal chamber, it is called barrier fluid or gas, and the seal is called a double mechanical seal.

2: Classification on the basis of Design

Balanced and Unbalanced. Unbalanced seals are the cheapest and used only for low linear speed and pressure;

Pusher type – O-Ring, V-Ring and Wedge Ring, single spring or multiple spring;

Non-Pusher type such as a metal bellows seal.

There are competent companies that provide mechanical seals for process pumps with high dependability and good value for money. Some mechanical seal manufacturers are strong in providing seals in huge quantities for automotive industry or home heating at very low cost, others are best in pulp and paper industry, third ones are very strong in making mechanical seals for the oil and gas industry.

Seals for each industry will have certain commonalities driven from field experience. Some industries may even accept a standard which will specify minimum requirements for seal design, testing and documentation. The API 682 Standard for making seals for petroleum industry is a good example. End-users specifying such standards in their purchasing requirements greatly benefit from this as they are protected from low quality or inexperienced manufacturers.

Mechanical Seals Introduction

Mechanical Seals Introduction

Mechanical seal is being used increasingly on fluid pumps to replace packed glands and lip seals.Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time.

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.

In order for the mechanical seal to perform over an extended time period with low frictional the faces are generally hydrodynamically lubricated.The fluid film will need to carry substantial load.If the load becomes to high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick , or less. This thickness is critical to the required sealing function.Mechanical seals often have one face of a suitable solid lubricant such that the seal can still operate for a period without the fluid film.

Pressure Balance Mechanical Seals
It is possible to reduce the seal contact pressure by using a pressure balanced seal design of off-set a proportion of the force generated by the pumped fluid pressure.This principle is illustrated in the sketch below.

Design Features
The mechanical seal generally includes a three static seals.

The sleeve seal – this is usually an O-Ring
The seal between the moving seal member and the shaft or sleeve.- This is often an o-ring but can be a wedge or vee seal. This seal may not be used for bellows type mechanical seals
The housing seal is generally an o-ring of a gasket.
All of these seal must be compatible with the fluid being contained and the associated environment.These seals may limit the design for high temperature applications. In this case the bellows type alternative may be the best option.

The sealing faces are generally pressed together using some form of spring loading. Several different spring loading systems are available.

Single spring
Multiple springs distributed around seal body
Disc Springs
Disc Springs
Bellows
Magnetic
For conventional mechanical seals the single spring arrangements is used.The other spring arrangements are used in the space is restricted.

It is vitally important that the sealing surfaces perfectly flat and are parallel.

The seal faces are usually dissimilar materials with the softer face being the narrower surface. For abrasive applications similar hard materials are used e.g tungsten carbide. The seal surfaces must have sufficient strength to withstand the hydrostatic fluid forces and must be able to remove the heat generated by sliding action.Carbon is often used against bronze, cast iron, stainless steel etc.

The seal surface must be flat, smooth and square to the shaft.  Both surfaces a normally lapped to a high quality finish. The harder surface is most important because the softer surface is designed to run-in over the initial operating period.

The shaft design is critical. It must be rigid enough to support the seal in the correct position and the shaft surface finish must be suitable to ensure good sealing on the static seals (0.4 micrometers CLA or better). The shaft Total Indicated Runout (TIR) should not exceed 0.125mm.There should be minimum shaft vibration.The shaft may be subject to fretting corrosion as a result of micro-movements of the seal and is is often desireable to have locally hardened surfaces or to use sleeves.

Assembly Options
There are a number of mechanical seal options

External Seal.. This design is installed on the outside of the stuffing box with the sealed pressure inside. This provides good access allowing the seal components to be be cleaned.
Internal Seal.. Generally mechanical seals are mounted inside the stuffing box with the sealed pressure outside the seal.
Double Seals.. Mechanical seals mounted in pairs are used for sealing hazardous, toxic or abrasiv fluids and are often provided with clean flushing fluid between the seals.Double seals also provide an additional degree of safety were the pressure differentials are likely to reverse and/or there is a high risk of the sealing failing.  There are a number of double seal assembly options as listed below
In Series – Used primarily to overcome the risk of failure of a single seal.

Face to Face – Used when a cooling fluid interface is required . One seal is used for the process fluid the other seal is used for the coolant.

Back to Back – Used when an abrasive fluid is being contained and both seals are flushed with a clean buffer fluid.The flushing fluid is introduced at a higher pressure the process fluid

The are a large number of variant mechanical seals e.g split seals.Improved systems are constantly being introduced onto the market

Additional Equipment
The use of mechanical seal generally involve the use of additional equipment primarily for the flushing /coolant systems.This includes pumps, coolers, strainers, filters etc.

Self-contained rotary mechanical seals

Self-contained rotary mechanical seals

A self-contained rotary mechanical seal is provided for mounting on a rotating shaft to form a seal between the shaft and structure on a housing through which the shaft extends. The seal includes an annular lug holder to be secured to the shaft as by set screws. A plurality of lugs extend from the lug holder parallel to the shaft.

Tines extend circumferentially from the ends of the lugs concentric to the axis of the lug holder. A belleville washer assembly is disposed within the lugs. An annular carbon seal washer also is disposed at least partially within the lugs. The seal washer includes radially extending shoulders dimensioned to engage the lugs and tines. In assembled form, the shoulders 15 of the carbon seal washer are urged by the spring force of the belleville washers against the tines. The tines retain the carbon seal washer in a position partially within the lugs.

In a rotary mechanical seal of the type having an annular base adapted for mounting on a rotating shaft, a seal washer, and a spring means located between the base and the seal washer for urging said seal washer against a housing through which the shaft extends to form a seal, the improvement comprising:a plurality of longitudinal lugs extending from said annular base in circumferentially and radially spaced-apart relationship with said shaft such that said spring means and seal washer can be received between said lugs and shaft;circumferential projections extending away from said lugs on the ends of said lugs opposite said base; and a plurality of circumferential projections extending radially outward from said seal washer in circumferential spaced-apart relationship sufficient for receiving said lugs there between so that said seal washer and base can be engaged for rotation and secured against longitudinal separation under the action of the spring means by inserting said seal washer under said lugs and twisting said seal washer until the seal washer projections are locked behind the lug projections.

Mechanical seals have long been known and used for forming a seal between a rotating shaft and a housing through which the shaft extends. Typically, a carbon seal washer is mounted to rotate on the rotating shaft and to slidably engage a seal seat mounted in the housing. The sliding engagement between the carbon seal washer and the seat forms a fluid-tight seal that, in conjunction with additional stationary seals, is operable to complete a fluid-tight seal between the shaft and the housing.

Often a plurality of coil springs are provided to force the carbon seal washer against the seal seat on the housing. In this assembly, the carbon seal washer is continuously urged along the shaft toward the seal seat so that the washer and the seat are maintained in a firm engagement even after both have been used for long periods of time.

However, problems have been encountered with the use of mechanical seals having coil spring forced carbon seal washers. Conventional mechanical seals utilizing a multiple coil spring arrangement often utilize coil springs mounted in a recess in the seal housing. The coil springs located in such recesses are susceptible to fouling and clogging by suspended particles and the like. The fouling of the springs may cause the seal to malfunction.

Further, in the assembly of a mechanical seal utilizing a multiple coil spring arrangement, usually, each individual spring must be properly positioned in the mechanical seal housing, and then the carbon seal washer must be forced into the seal against the spring force. Finally, some measure must be taken to retain the carbon seal washer within the seal housing resisting the spring force. This assembly procedure has proved difficult, and installation of such seal was often cumbersome. Mechanical seals of this type are generally expensive to manufacture.

The present invention overcomes the foregoing problems and others long associated with mechanical seals by using an anti-clog, self-cleaning, belleville spring arrangement and by using an annular seal housing designed to facilitate assembly. In the present invention, a mechanical seal includes an annular base for mounting on a rotating shaft within a housing. A plurality of lugs extend from the base and circumferential projections extend from the ends of the lugs. A seal washer is provided for being mounted at least partially within the lugs and is dimensioned for snuggly encompassing the shaft. The seal washer includes radial projections for engaging the circumferential projections. A spring is provided within the lugs between the base and the seal washer for urging the radial projections of the seal washer against the circumferential projections extending from the lugs. In this manner, the seal washer is retained partially within the lugs and mounted on the mechanical seal. When the mechanical seal is mounted on the shaft, the spring structure is operable to urge the seal washer against housing structure to form a seal. Also, the lugs will engage the radial projections to force the seal washer to rotate with the lugs and shaft.

In accordance with a particular aspect of the present invention, a mechanical seal is provided for use on a rotating shaft extending through a housing. An annular lug holder is dimensioned to snuggly engage the shaft and includes a plurality off asteners for mounting the lug holder on the shaft. The lug holder includes a plurality of slots, and a plurality of lugs are mounted in the slots and extend from the lug holder in a longitudinal direction parallel to the axis of the lug holder. A plurality of tines are provided with two tines extending circumferentially from the end of each lug in a direction perpendicular to the lugs and concentric to the axis of the lug holder.

A plurality of belleville washers are dimensioned to snuggly encompass the shaft and for being disposed between the lugs and the shaft adjacent to and engaging the lug holder. The belleville washers have sufficient elasticity to resilientlyoppose compression. A contact washer is provided for encompassing the shaft and for being disposed adjacent the belleville washers. An annular carbon seal washer is center-bored for snuggly encompassing the shaft and is disposed partially within the lugs adjacent the contact washer. The seal washer includes an annular recess adjacent the contact washer and an annular nose surface facing away from the lug holder. An O-ring and a back-up ring encompass the shaft and are disposed within the recess in the seal washer to form a seal between the shaft and the seal washer.

A plurality of shoulders extend radially outward from the seal washer for engaging the tines to retain the seal washer partially within the lugs. The shoulders have a sufficiently small width to pass between the confronting ends of the tines extending from adjacent lugs and have a sufficient radial extension to engage the tines and lugs. The mechanical seal may be assembled by inserting the seal washer partially within the lugs to engage the contact washer and to compress the belleville washers. The seal washer is then rotated until the shoulders engage the lugs. Then, when the seal washer is released, the belleville washers will urge the seal washer forward until the shoulders engage the tines to retain the washer on the mechanical seal.

A seal seat is mounted in the housing and includes a face surface for slidably engaging the annular nose surface of the seal washer to form a seal. The mechanical seal is positioned on the shaft relative to the housing such that the seal washer is urged against the seat by the spring force of the belleville washers. In this position, the seal washer is displaced rearwardly toward the lug holder such that the shoulders extending from the seal washer do not engage the tines.