api mechanical seal brands

Depending on your specific needs and application, Flexaseal can help you identify the ideal products. Contact us to tell us about your needs and request a quote.

To keep mechanical seal systems functioning as long as possible, we recommend using standardized seal piping plans. Detailed API seal piping plans ensure minimal seal face wear by maintaining the optimal seal chamber environment.

Since they were first formulated, seal piping plans have been maintained and remodeled by the American Petroleum Institute (API). Current plans are based on API 682 and are sorted numerically. In some cases, designated letters are also used to differentiate between plans.

... pressurized gas barrier metal bellows seal utilizing APGS non-contacting seal face technology. Welded metal bellows eliminate dynamic O-ring hang-up in a compact cartridge that fits ANSI and DIN standard ...

• Drive mechanisms external to the product;• Seal faces positioned for maximum protection;• A dynamic elastomer moves on a non-metallic surface, eliminating fretting defects;• Hydraulically balanced;• Cartridge easy to ...

• Static grafoil gaskets;• Temperatures up to 425 ºC;• Inconel bellows available;• Cartridge easy to install;• Metal bellows provide better faces alignment;• Does not have dynamic gaskets;• Self-cleaning;• Adapts to API standard pumps;• ...

Burgmann H74-D Mechanical Seal called as PC04 are specialized in mechanical seals products. This device gives double seal and it can be rotated in any ...

The MTM10-11 is a conical spring mechanical seal developed by Microtem. It is mainly used for general services machinery at low and medium pressure. This unbalanced mechanical seal ...

The MTM 25_26, manufactured by MICROTEM, is a conical spring mechanical seal that can compensate positioning errors and withstand stresses created by vibrations. The contact surface can be made with silicon ...

... agitators on sealed tanks will have a mechanical seal of some sort. For many sanitary process vessels, the mixer must have some type of sealing barrier to provide either a dust tight vapor seal, ...

mechanical seal for automotive engine cooling water pump, referred to as water seal, mainly composed of two parts: rotating ring and static ring. Static ring is installed in the pump ...

The 3-D Seal is designed to be the foremost solution for high radial misalignment and high run out applications. By combining Garlock’s proven P/S®-II and expansion joint technologies into ...

Aura™ reduces operational and transactional costs using a patented polymeric sealing device. Aura reduces leakage rates by up to 15 percent, lowering the total cost of operation while protecting the environment. An enhanced rotor design ...

The AESSEAL® API Type A, B and C single-seal range offers the user an unprecedented range of API engineered sealing solutions to suit all application ...

Mechnical seal type 5030 / 5031 »with rubber bellowssingle-actingnon-anisotropicAreas of usestandard pumpwaster water pumpsupply engineeringgeneral industrial usemass production seal

When purchasing the units, if left hand rotation, special seal, or a particular position of cartridge (shaft end or cover end) is needed, it should be ...

Cartridge Seals by CinchSeal are customized mechanical seals for rotary air locks in bulk handling equipment. They are designed to replace lip and packing seals in screw ...

Mechanical seals became the dominant sealing technology in refineries and chemical plants in the 1980s, causing the American Petroleum Institute (API) to establish a committee whose sole focus was to write standards for these components. The first edition, API 682 Shaft Sealing Systems for Centrifugal and Rotary Pumps was published in 1994 with this mission statement, “This standard is designed to default to the equipment types most commonly supplied that have a high probability of meeting the objective of at least three years of uninterrupted service while complying with emissions regulations."Currently in its fourth edition, API 682 continues to offer guidance based on process service for both mechanical seals and their support systems.

While much of the standard is focused on mechanical seals, a significant portion is devoted to seal support systems, as they are a critical component to the proper functioning of the seal and pump system. As a manufacturer of seal support systems, Swagelok Company and our sales and service centers have implemented the best practices of API 682 4th Edition. In this blog post, we will explain what some of those best practices are, and how implementing recommendations from the standard in the construction and design of your seal support systems can help you meet your goals of increasing reliability and safety while reducing costs.

Before we discuss best practices, let’s look at the functions of seal support systems. These systems are designed for a specific mechanical seal and set of process conditions. Typically, they supply either a gas or a liquid to the mechanical seal to regulate the environment in which the seal operates, protecting rotating equipment from damage.

Throughout API 682 4th Edition, there are references to reducing the number of connections in seal support systems. Whether welded pipe or tubing is selected for the system, threaded systems are discouraged. Every connection can be viewed as a potential leak point and possible reliability risk in hydrocarbon pumping applications. Leaks on seal support systems near pumps can cause asset damage, increased downtime, environmental issues, and safety risks.

In the past, many seal support systems were constructed out of pipe due to piping being historically preferred. More recently, seal manufacturers, end users, and pump OEMS have implemented tubing as a connection solution in seal support systems due to its long history of successful use in critical applications throughout the industrial world. As rotating equipment expert Heinz Bloch noted in a recent Hydrocarbon Processing article, “[the] American Petroleum Institute Standard 682 (API 682) began to endorse the use of tubing for some seal piping plans. Regrettably, tradition-bound purchasers still opt for hard pipe; we are asking them to reconsider. API 682 (4th Edition) now specifies seal support system connections almost interchangeably.”

Tubing can be utilized to reduce the number of connections by bending lines and appropriately using adapter fittings. Often, the only needed connections are those at the seal and the sealing system. Since tubing is annealed, bending the tubing work hardens the metal, increasing the strength of the tube at the bend. Innovative connection technologies such as flange adapters and extended male connectors further reduce the number of connections from threaded ports on the seal and seal pots by eliminating the need for multiple fittings. The use of tubing provides further financial benefit when we examine the MRO costs of the pump, seal, and support system. During maintenance operations where “piping” around pumps is reworked, the use of tubing eliminates the need for costly on-site welding and can be installed quickly to reduce downtime.

Seal support systems are critical to the proper operation of the seal and pump, and as such, require regular visual inspection. Making the job of visual inspection simple promotes system reliability and safety. When designing seal support systems, there are several best practice design principles to consider once the piping plan and general arrangement have been selected.

Mechanical seals are often damaged when pumps are started and stopped, sometimes as the result of improper seal support system operation. If the design of the seal support system facilitates proper operation, common mistakes when commissioning pumps can be avoided.

Additionally, API 682 supports these design considerations. It states: “All controls and instruments shall be located and arranged to permit easy visibility by the operators, as well as accessibility for tests, adjustments, and maintenance” (9.1.5)

Lastly, panels can include part numbering information, flow path indication, and operator instructions. These improvements help ensure safe and reliable startup and shutdown of pumps and seal support systems.

API 682 4th Edition also recommends block-bleed configurations for all gauges. If systems are not designed with this feature, it is likely that as gauges fail, operators will be left without critical information until the next turnaround or project when the pump and support system can be decommissioned and the gauge replaced.

Lastly, there are a wide variety of tubing connections and design options that allow for every serviceable component on a seal support system to be easily removed and replaced while continuing to operate the system. For seal pots, the 4thEdition stipulates “Local operation, venting, filling, and draining shall be accomplished from grade. Unless otherwise specified, systems that require the use of a ladder or step or that require climbing on the baseplate or piping are not acceptable” (8.1.8) .

Implementing these basic best practice design principles for mechanical seal support system increases reliability and reduces costs. To recap how you can realize better results with your systems:

Bringing pumps offline to fix minor instrumentation issues or fill seal pots should not be acceptable. Locating these systems on panels, with proper labeling and designing for easy maintenance reduces the chance for operator error which can damage seals.

Seal failures and the associated costs of seal replacement should be of great concern to rotating equipment groups at all plants. Ensuring that the best practices and design principals of API 682 4th Edition are followed helps prevent these costs and creates a safer and more reliable operation.

Swagelok provides design and assembly of seal support systems through our network of more than 200 authorized sales and service centers. We offer configurable, local, and reliable systems that are better by design to help you reduce costs, save time, and improve safety of your rotating equipment.

For additional advice on designing and installing your mechanical seal support systems, or to find the right API seal plan kits or assemblies for your applications, reach out to your local Swagelok team.

Pumping processes involving toxic or hazardous fluids that can’t risk leakage because of stringent environmental regulations require a double mechanical seal. Compared to a single mechanical seal, a double seal gives you significantly greater protection against leaks. With a double mechanical seal, you have an arrangement of two mechanical seals (a primary or inboard seal and a secondary or outboard seal) in series—back-to-back, tandem, or face-to-face. Each seal has a rotating (R) surface and a stationary (S) seal surface. These seals can be arranged in one of three patterns.

In a back-to-back arrangement, the stationary seal faces are positioned back-to-back with the rotating seal faces on the outside. The back-to-back arrangement is easy to install and used for many general pumping applications.

The tandem arrangement has the two pairs of seals mounted with the same orientation. This arrangement is preferred for toxic or hazardous applications because the outboard seal provides full pressure back-up, allowing the outboard seal to back up in the event of an inboard seal failure.

In the face-to-face arrangement, the rotating seal faces share a common stationary seal face. This arrangement is useful when equipment space is too constrained to permit back-to-back or tandem seal arrangements.

The American Petroleum Institute (API) Standard 682 classifies double mechanical seals into two configurations—pressurized and unpressurized. The pressurized arrangement has a barrier fluid delivered to the double mechanical seal by a seal support system. The barrier fluid is delivered at a higher pressure than the process fluid and must be chemically compatible with the process fluid as it will lubricate the inboard seal faces and mix with the process fluid. The unpressurized arrangement has a buffer fluid delivered to the double mechanical seal by a seal support system. The buffer fluid is delivered at a lower pressure than the process fluid.

The barrier and buffer fluids you use can be liquid or gas. They provide lubrication and help maintain the required operating temperature of the seal faces. The typical choices are water and water/glycol mixtures, low-viscosity petroleum or synthetic oils, kerosene, diesel, and nitrogen.

To gain a better understanding of the differences between the uses of barrier and buffer fluids, let’s look at two common API plans for double mechanical seals—API Plan 52 Buffer Fluid Seal Pot and API Plan 53A Barrier Fluid Seal Pot Pressurized by Nitrogen.

API Plan 52 takes buffer (unpressurized) fluid from a reservoir (seal pot), delivers it to the seal chamber, circulates it between the inboard and outboard seals using a pumping ring located driven by shaft rotation, then returns the fluid to the reservoir. In the event of an inboard seal failure, process fluid leaks into the seal chamber. When that occurs an increase in buffer fluid pressure and/or level alerts operators to the problem. The outboard seal, however, contains leakage until maintenance can replace the damaged seal.

This plan can include cooling coils in the reservoir to maintain the required buffer fluid temperature, visual or mechanical fluid level indicators, pressure and level transmitters, and connection to a collection system and buffer fluid replenishment source.

The overall design of this API plan for a double mechanical seal is relatively simple in comparison to other plans. Design decisions involving tubing size, length, geometry, type (carbon vs stainless steel), buffer fluid type, and volume of the buffer fluid reservoir are critical in maintaining the proper operating environment for the double seal. If you don’t have this expertise in-house, work with an experienced, local seal support system vendor to ensure the API Plan 52 is designed to meet your specific pumping requirements.

API Plan 53A is conceptually similar to API Plan 52 with the difference that the fluid being circulated between the double mechanical seals is under pressure. A pumping ring is used to circulate the fluid. The reservoir that contains the barrier fluid is pressurized by plant nitrogen. Reservoir pressure should be set a minimum of 20 to 25 psi (1.4 to 1.73 bar) above the maximum seal chamber pressure, allowing the barrier fluid to leak (and lubricate) across the inboard seal faces into the process fluid. For this reason, the barrier fluid must be chemically compatible with the process fluid.

Because barrier fluid is depleted as it moves across the inboard seal faces, it needs to be replenished. This can be done manually or automatically by way of a system that serves multiple pumps. API Plan 53A design options include reservoir type and volume, cooling coils, fluid level and pressure indicators, and transmitters to alert to level or pressure changes that indicate seal failure.

When you choose an API plan for a double mechanical seal, your primary decision is between a buffer or barrier plan. I’ve highlighted two of the API plans for double mechanical seals above to show the basic differences. There are multiple API plans for double mechanical seals to choose from—pressurization from bladder or piston accumulators, plant nitrogen delivered directly to the seal chamber, and custom-engineered external systems. Your choice will be determined by the process fluid and pumping conditions and the type of double mechanical seal your vendor recommends.

With this information in hand, it’s best to work with an experienced local seal support system vendor. They’ll be able to meet with you on-site to review the specifications for the pumping process, the pump, and the double mechanical seal. They’ll evaluate your existing infrastructure and its influence on seal support system design. Based on this information, they’ll then design the seal support system to meet the specific pumping requirements.

If you work with a global vendor like Swagelok, based on the design, we can quickly assemble and thoroughly test the API plan at our local facilities prior to delivery. We’re also conveniently available for follow-up consultations, on-site, remotely, or by way of a quick phone call.

For well over 50 years, Swagelok has worked closely with Northern California process industries to confidently choose the right API plans for pumping needs. Our locally based Field Engineers and certified technicians provide field verification of your seal support requirements, designs based on best practices gained from global experience.

To find out more about howSwagelok Northern California can help you choose the right API plan for double mechanical seals, as well as process and atmospheric side seals,contact our team today by calling

Morgan holds a B.S. in Mechanical Engineering from the University of California at Santa Barbara. He is certified in Section IX, Grab Sample Panel Configuration, and Mechanical Efficiency Program Specification (API 682). He is also well-versed in B31.3 Process Piping Code. Before joining Swagelok Northern California, he was a Manufacturing Engineer at Sierra Instruments, primarily focused on capillary thermal meters for the semiconductor industry (ASML).

API 682 Seal. Viking’s seal chamber will accept almost any customer-specified brands and types of API 682-compliant Category 1, 2 or 3 cartridge mechanical seals, and can provide API seal plans to meet application requirements. Standard seal is a category 2 single mechanical seal. Carbon vs. silicon carbide with API Plan 13.

3mm corrosion allowance above Maximum Allowable Working Pressure (MAWP). The XPD 676 has an additional 3mm corrosion allowance built into all pressure containing components over and above our standard steel Universal Seal series pumps.

Casing drain and seal chamber vent. XPD 676 has cast-in casing drain with ANSI Class 300 RF flange and seal chamber vent to completely drain the casing before maintenance.

Our Mechanical Seals are used in a wide range of pumps and rotating equipment worldwide to prevent liquids and gases escaping into the environment. We manufacture mechanical seal types to suit all industries and our investment in modular design means that we provide the best on-time delivery performance in the industry.

The AESSEAL® range of seals, seal support systems and bearing protectors are all designed to improve pump reliability and reduce maintenance costs. Our business is built around giving our customers such exceptional service that they need never consider alternative sources of supply.

AESSEAL® operates from 235 locations in 104 countries, including 9 manufacturing and 44 repair locations, and has more than 300 customer service representatives who visit industrial plants every day. Find Out More..

The scope of our mechanical seal product range far exceeds any other seal manufacturer. From small elastomer bellows seals used in millions of domestic water pumps to double mechanical seals that ensure maximum sealing safety and large, highly customized dry-running gas seals for mission critical high speed turbo compressors, John Crane has the right product for any application.

Our world-class rotating equipment technologies, paired with an unmatched breadth of applied engineering expertise, meet virtually all international standards including API 682 and help plants reduce maintenance costs, slash down time and improve reliability. When it comes to keeping your rotational equipment running 24/7, John Crane’s comprehensive range of mechanical seals and systems has you covered.

A range of seals for mission-critical applications, designed to solve the application-specific challenges of each industry. From API 682 compliance for the oil and gas industries, using gas seal technology on our innovative pump gas seals to eliminate fugitive emissions, dealing with slurry in the mining and minerals processing industries, to the difficulties associated with maintenance on large pumps and rotating equipment — we have a solution.

Dry-running, non-contacting gas seals have been the industry standard since the early 1980s for turbomachinery. John Crane gas seals, separation seals and support, monitoring, control and conditioning systems — the heart of any reliable sealing solution — are constantly evolving to meet the needs of customers. The product portfolio is supported by unrivaled global service capability providing repair, retrofit, gas seal storage and reliability expertise, delivering total solutions throughout the product lifecycle.

In industries like chemical, pharmaceutical, pulp and paper, and food and beverage, safeguarding and compliance with industry standards, avoiding contamination and efficiency are always top priorities. Our range of vessel and agitator seals optimize equipment performance, maintain product purity and conform to industry regulations, no matter where you are.

Our range of mechanical seals, packing and bearing isolators combines advanced, thoroughly proven technologies with extensive industry expertise to create a range of products characterized by innovative design concepts and outstanding manufacturing quality. Tried, tested and effective solutions for virtually any application that deliver robust performance, reduced installation times and lower maintenance costs.

Create the optimum operating environment that will ensure outstanding seal performance and reliability. Our comprehensive range of engineered pressure reservoirs, gas seal control panels, heat exchangers and abrasive separators can be combined to produce the perfect seal support system for any application.

Designed to overcome rigorous challenges, our comprehensive suite of seal face technologies combat limited seal face lubrication that adversely affects reliability, cost and durability. Our engineers designed these face treatments to extend rotating equipment life through advanced micro machined patterns and features improving seal face lubrication that optimizes equipment performance. We deliver the right face technology for the right application.

After more than five years of planning, the American Petroleum Institute (API) is preparing to release the 4th edition of API Standard 682 (ISO 21049:2011). The API 682 standard, which dates back to 1994 and is formally known as Shaft Sealing Systems for Centrifugal and Rotary Pumps, offers specifications and best practices for mechanical seals and systems to pump end users.

The standard’s latest edition began to take shape in 2006, when API formed a 4th edition task force to respond to end users’ questions and comments about previous editions. The task force soon realized that major changes, including reorganization and editing, would be necessary. While addressing every aspect of the resulting 4th edition (which is more than 250 pages long) would be impossible, this article summarizes the standard’s main points.

Those who use API 682 should understand the standard’s scope and remember that the standard does not include specifications for equipment outside that scope, such as engineered seals or mixers. Another important but often misunderstood point is that API 682’s figures are illustrative and not normative in their entirety.

For example, one of API 682’s figures shows a fixed throttle bushing combined with a rotating Type A seal, but seal manufacturers do not always have to combine these two components. The standard provides normative details in clauses and tables to help purchasers distinguish between requirements and suggestions.

The 4th edition continues to divide seals into three categories, three types and three arrangements. For all practical purposes, seal manufacturers can combine a seal’s component parts into nearly any orientation or configuration. Each orientation and configuration has advantages and disadvantages with respect to certain applications, performance and system disturbances.

Before the 4th edition, API 682 did not specify a minimum clearance between the inside diameter of a stationary seal part and the outside diameter of a rotating seal part. The 4th edition specifies this minimum clearance—typically the clearance between the sleeve and the mating ring. The specified clearances are representative of standard clearances that end users have used for decades. End users should not consider seal components to be “shaft catchers” to restrict shaft movement. The minimum clearance specified in API 682 also applies only to equipment within the standard’s scope. Equipment outside that scope, such as non-cartridge seals, older pumps, non-API 610 pumps and certain severe services, might benefit from larger clearances.

The new standard also updates the default bushings for the gland plate for the three seal categories. Fixed throttle bushings are now the default for Category 1 only, while floating bushings are the default for Categories 2 and 3.

While the 4th edition features the recommended seal selection procedure from the standard’s first three editions, it adds an alternative selection method in Annex A. Proposed by task force member Michael Goodrich, this alternative method recommends using material data sheet information to select a sealing arrangement.

Plans 66A and 66B are new to the standard, although end users have used them previously in pipeline applications. These plans detect and restrict excessive leakage rates in case of an Arrangement 1 seal failure.

The 4th edition has revised the data sheets in Annex C extensively to make them the same for all seal categories. Only two data sheets are included in the 4th edition—one in metric units and one in U.S. customary units. The new edition also folds Annex J into Annex E.

Previous editions of API 682 required metal plugs and anaerobic sealants when shipping new or repaired cartridges. After much debate, the task force decided that threaded connection points should be protected with plastic plugs for shipment. These plastic plugs should be red and have center tabs that operators can pull easily to distinguish the plugs from metal plugs. Shippers should also attach yellow warning tags to the plugs to indicate that end users need to remove the plugs before operation.

Although tutorial notes are scattered throughout API 682, this edition expands the tutorial section, Annex F, from seven pages to 42 pages. The expanded annex includes illustrative calculations. In particular, users interested in systems such as Plan 53B will find Annex F to be useful.

The 4th edition of API 682 is the product of more than 20 years of discussion, debate, usage and peer review. It includes a strong set of defaults and is by far the best and most logical starting point for mechanical seal and systems use. Equipment operators should take the time to familiarize themselves with API 682 to get the most out of this comprehensive standard.

One of PPC Mechanical Seals competitive advantages in the marketplace is our knowledge, experience, and facilities to repair and recondition all major brands of mixer seals. In addition, we can manufacture and design completely new seals for your units. PPC utilizes a dedicated mixer seal repair cell and is experienced in handling shaft diameters up to 9”+ inches. Our dedicated cell and experience mean that PPC has the fastest turn around in the industry for these types of seals.

A sealing system, consisting of a mechanical seal and an associated supply system that is balanced by individual applications, is the utmost guarantee for a reliable sealing point and uninterrupted pump service. The performance of the seal is greatly influenced by the environment around the seal faces, making the provision of suitable, clean fluids as well as a moderate temperature an essential topic.

This guiding booklet provides a condensed overview of all piping plans established by the API 682 4th edition guidelines. Each illustrated piping plan is briefly described, and a recommendation that considers the media characteristics in terms of the relevant application and corresponding configurations is given to help you reliably select your sealing system. Furthermore, the content of this booklet has been enriched by providing clues – so-called ‘remarks and checkpoints’ – where EagleBurgmann shares the experiences gained from multiple equipped plants.

Several factors play a major role when choosing the product, the product type, the materials used and how it is operated: process conditions at the sealing location, operating conditions and the medium to be sealed.

No matter what requirements our customers have, EagleBurgmann understands how these factors affect functionality and economic viability, and they translate this expertise into outstanding long-term, reliable sealing solutions. EagleBurgmann has all the expertise needed to manage and support the entire development, life and service cycle of its sealing solutions.

EagleBurgmann offers customers the widest product portfolio of seals and seal supply systems according to API 682 4th edition. The configurations listed for each individual piping plan are to be understood as recommendations including possible utilizations which may also be applied.

EagleBurgmann is one of the internationally leading companies for industrial sealing technology. Their products are used wherever safety and reliability are important: in the oil and gas industry, refining technology, the petrochemical, chemical and pharmaceutical industries, food processing, power, water, mining, pulp & paper and many others. More than 6,000 employees contribute their ideas, solutions and commitment towards ensuring that customers all over the world can rely on their seals and services. More than 21,000 EagleBurgmann API-seals and systems are installed world-wide.

Many users do not like the strong handed approach used by API 682. However, with a little study of API 682, the user can easily learn to specify his preferences in detail using the seal data sheet.

Some statements within API 682 are normative, that is, required, whereas others are informative, that is, descriptive but not required.  In particular, many of the illustrations are informative.  This distinction has not always been apparent to the reader.

In spite of its strong handed approach, API 682 encourages innovative or developing technology. However, non-standard alternatives should be carefully discussed between the purchaser and seal company.

It is important to realize that API 682 is a users’ standard; it was written by and for the end users of mechanical seals and these users wanted to force changes.  The result was an entirely new standard written around a limited set of seal types, arrangements and materials that were favored by the end users in refineries.  These new seals were also required to be proven through a series of rigidly prescribed tests.  Although everyone agreed that API 682 seals were robust and well suited to the best practices of the refining industry, cost quickly became a limiting factor to the specification.  Consequently, API 682 1stEdition was not applied as extensively as had been anticipated. Subsequent editions have had an increase in the scope and also are more flexible with respect to defaults and options.

It is important to know the background and development of API 682 in order to fully understand and apply the standard. The story of API 682 begins in the late 1980’s.

By the late 1980’s, mechanical seals had been accepted as the preferred method for sealing rotating pumps for many years. However, prior to API 682, mechanical seal standards were generally buried in other standards such as DIN 24960, ANSI B73 and API 610. All of these standards were primarily pump standards and any references to seals were directed at how mechanical seals would interact with pumps.

In the late 1980’s a group of refinery equipment engineers and managers began to compare sealing solutions in refinery applications. This group, led by V. R. Dodd of Chevron, came up with a general plan and the American Petroleum Institute (API) agreed to establish a standard for mechanical seals: API 682.   A Task Force was formed in 1990 and the first meeting was held in January 1991.  This Task Force was comprised of fourteen members from various refineries, seal and pump manufacturers.   API 682, First Edition, was published in October 1994.

Pump Projects is your go-to resource for advice, sales and service for mechanical seals for pumps. We have the internal resources to help you with this complex issue.

A mechanical seal is simply a method of containing fluid within a vessel (typically pumps, mixers, etc.) where a rotating shaft passes through a stationary housing or occasionally, where the housing rotates around the shaft.