About Seals- Chapter two some seals

Mackintosh

About Back-up Rings
% i. |6 U( q# g: aBack-up rings are washer-like devices that are used to prevent a seal from extruding through any gaps while under pressure. These seals are installed in the downstream side of the gland.  Back-up rings are not seals themselves, but are usually used in conjunction with O-rings in high-pressure applications (over 1500 psi) for support or as spacers.  Typical applications include hydraulic cylinders and high-pressure hydraulic systems, and high-pressure valves. In many applications it is best to use two back-up rings; one above the o-ring and one below, to provide maximum support.
Back-up rings are available in two distinct face design configurations – a flat, washer style, and contoured.  The simplest form has two flat, parallel surfaces, much like a washer.  In contoured back-up rings, one face is straight and the opposite face is curved or contoured.  The contoured face is the one that comes in contact with the o-ring, providing better grip and support.
% g9 C. W! ]# S" K) P( nBack-up rings may be of solid or spiral construction.  Spiral back-up rings are of overlapping, coiled construction.  As the o-ring exerts pressure upon them, the overlapping rings provide a number of contact levels against the shaft providing extra support.  
When selecting between the numerous types of back-up rings available, the three most important specifications to be aware of are the inner dimension of the ring, or the outer dimension of the shaft on which the back-up rings will be applied; the radial cross section of the seal; and the ring thickness, or the axial width of the cross section of the ring, calculated as (ring O.D. - ring I.D.)/2.
Back-up rings are available with a number of material compositions, either rubber or polymer, metallic, or natural materials, each of which have specific advantages and disadvantages depending upon the application in which the back-up rings will be used.  In some cases, seal manufacturers use their own proprietary materials in the manufacture of back-up rings.  When in doubt, it is best to check with the back up rings’ manufacturer as to which material would be best for the application at hand
) ^  U! F- v, q8 \# Z( \  G9 eAbout Bearing Isolators
' n' B' h1 c& w& T& BBearing isolators are dynamic seals designed to protect bearings from outside contaminants.  They are comprised of a rotor (rotating) and stator (stationary) member.  Some bearing isolators are of labyrinth construction while others use O-rings or other positive seals.   The most important specifications to consider when searching for bearing isolators are the dimensions.  These include shaft outer diameter or seal inner diameter, housing bore diameter or seal outer diameter, and axial cross section or thickness.  The shaft outer diameter or seal inner diameter refers to the size of the seal inside diameter or the shaft or piston outside diameter.    The housing bore diameter or seal out diameter refers to the size of the housing bore diameter or the seal outside diameter.  The axial cross section or thickness refers to the axial cross section, or thickness, of the seal.
/ [" |" m2 H" `% R# `- V& LImportant service limit parameters to consider for bearing isolators include maximum operating speed, maximum operating pressure, operating temperature, and vacuum rating.  The maximum operating speed is generally given as ft/min or m/sec.  The maximum pressure specifies the maximum pressure that the seal is rated for without failure.  The operating temperature refers to the full-required range of ambient operating temperature.  Some seals or bearing isolators are rated to operate in a vacuum.  Common seal features found on bearing isolators include spring loaded, integral wiper or scraper element, and split seal.  Spring loaded seals are also called spring-energized or having a garter spring. The spring helps keep the sealing lip in contact with the shaft.  Bearing isolators can have a wiper or scraper (exclusion) element in addition to primary seal.  Frequently an oil or hydraulic seal will also have a wiper or scraper element to keep contaminants from the sealing area.  The split in an elastomeric seal facilitates assembly.  A split rigid (metallic) seal is similar in functionality to a piston ring.
Common materials of construction for the sealing element or lip material include none or clearance or labyrinth construction, ethylene acrylic, fluoroelastomer, fluorosilicone, nitrile, highly saturated nitrile, nylon or polyamide, polyacrylate, polychloroprene, polyetheretherkeytone, polyoxymethylene, polyurethane or urethane, natural rubber, sintered bronze, cast iron, stainless steel, felt, or leather.  A polymer is a solid, nonmetallic compound of high molecular weight the structure of which is composed of small repeat units (mers). Plastics and elastomers are polymers.  Many seal manufacturers use their own proprietary material. Where possible, these materials have been categorized into the generic equivalent or material family. Consult with manufacturer for proprietary material specifications
Bolt Seals - Bolt seals are used under the head of a bolt to prevent the ingress of fluids or dirt.
Brush Seals - Brush seals are as a barrier against flying debris and liquid. They are also used to reduce or block the infiltration of light or air flow
About Caps, Grips and Closures
Caps, grips and closures are protective devices for the ends of a shaft, tube, pipe, fitting, connector, screw, bar, or other shape. They can also be used to provide a hand grip or hanger tip. Caps are available in many different shapes and materials.  Common shapes include round, square, rectangular, round handle or hand grip, flat handle or hand grip, hex cap, “U” shape cap, “I” shape cap, 90° angle cap, screw cap or plug, and washer cap.  Common construction features include threaded, tapered, and anti-roll.  Threaded caps have a female thread.  The thread can be straight or tapered (NPT).  Caps have a female thread and plugs have a male thread.  Tapered caps and plugs have tapered sides, providing a snug fit.  Some can be used as either a cap or a plug.  Tapered caps and plugs are versatile and can be used for straight tubes and bars, male and female NPT threads, and male straight threads.  Anti-roll caps are round caps with a square end to prevent tube or shaft from rolling.
# r5 N' s& G+ r; [' LImportant specifications to consider when searching for round caps, grips, and closures include NPT size, BSP size, and diameter.  The NPT size option applies to round caps only where the object to be capped has a male NPT (National Pipe Thread) thread.  The BSP option applies to round caps only where the object to be capped has a male BSP (British Standard Pipe) thread.  The diameter dimension applies to round caps and plugs only.  For straight threads the major thread diameter is the most important (for example: .25 for a ¼ straight thread).  This dimension is not considered where cap/plug or object being capped/plugged has a tapered (NPT or BSP) thread.  Common round cap features include flange, knurled or faceted head, slotted head, retaining head, and tear tab.  A flange is included for easy removal or additional protection of bearing surfaces.  They also prevent accidentally pushing the plug all of the way through the opening.  Knurled or faceted heads are usually found on threaded caps and plugs only, knurls or facets allow easier installation and removal, either by finger or wrench.  Slotted caps and plugs can be installed or removed with a screwdriver.  Retaining rings fit over the object to be capped and function to keep the cap in place during servicing.  Caps with a tear-tab or strip are easily removed.  When the tab is pulled the cap tears away, once removed they cannot be reused.  Important dimensions to consider for square or rectangular caps, grips, and closures include width and length.  The width is the smaller cross sectional dimension for rectangular caps or plugs.  If the section is a square, the width and length is the same dimension.  The length is the larger cross sectional dimension for rectangular caps or plugs.  If the section is a square, the width and length is the same dimension.
; S, m5 k7 [* t" ~, zCommon materials of construction for caps, grips, and closures include aluminum, Delrin, EVA, Monel, nylon or polyamide, paper, polyetheretherkeytone (PEEK), polyethylene, polypropylene, silicone rubber, styrene butadiene rubber (SBR), stainless steel, and vinyl.  Additional features to consider when searching for caps, grips and closures include hanger tips, lift or pull tabs, and venting.  A hanger tip allow for the hanging of tools or other items.  Hanger tips are sometimes integrated with handles or grips.  Caps can have a tab or ring for easy removal or plug is of pull plug construction.  Pull plugs are designed for "through-hole" applications.  They use a smaller diameter "pull section" to pull the plug until it is snug in the hole.  Vented caps are commonly used in the medical industry.  The vent allows airflow during gas sterilization and prevents the cap from blowing off during extreme heat conditions.