Dynamic seals Flashcards
Dynamic seals are grouped into what two main categories? (2)
- Contact
- clearance
Dynamic seals can only be non positive, true or false?
false, can be positive and non positive
____ used as a non positive seal controls leakage between stationary and moving parts
packing
When sealing with packing, what are the four types of motion you can seal? (4)
- reciprocating
- rotary
- helical
- swinging rotary
Packing can be used as a positive seal to:
Prevent any leakage between stationary and moving parts
What are the three categories of packing? (3)
- Compression
- Automatic
- Floating
Packing installed in the moving unit is called:
Inside packed
Ex: Cylinder pistons, valve air spools
Packing installed in the stationary unit is called:
Outside packed
Ex: stuffing box
Packing must withstand: (4)
- Temperature
- Pressure
- various fluids
- shaft misalignment and movement
Hard packing withstands ____ better than soft packing
extrusion
Note: Hard packing also creates more friction
With compression packing, the amount of sealing or leakage is controlled by the ammount:
the packing is compressed
Compression packing is made of these 3 classes of material: (3)
- fabric
- Metallic
- plastic
____ compression packing material is made of strong, pliable materials such as braided asbestos, metal, graphite, cotton, flax, wool, aramid, carbon or PTFE
Fabric packing
____ compression packing material is made from materials such as lead, copper, or aluminum. Machined or pressed into rings.
Metallic packing
____ compression packing material is moulded to fit specific needs
Plastic
Compression packing cross sections may be: (3)
- Round
- Square
- Rectangular
Type used depends on stuffing box design, temperature, pressure, fluid type
Some packing is impregnated with lubricant, such as graphite or teflon. This makes the packing:
self lubricating at start up
Lantern rings are often used with compression packing. It is a channeled ring with a H cross section, and holes cut radially. The purpose of a lantern ring is to:
distribute sealing liquid under pressure to the packing
Sealing liquid is used to prevent air infiltration as well as provide: (3)
- Lubrication
- Cooling
- Cleaning
When sealing water is provided from the high pressure side of the pump, it is called:
internally supplied
Compression packing is removed periodically because:
it deteriorates from wear and dries out
Remove old packing using:
packing puller
A stuffing box code of 2-L-3-G means:
2 rings of packing, lantern ring, 3 rings of packing, gland
You can determine the correct amount of packing by:
measuring stuffing box depth, subtracting width of lantern ring, dividing remainder by the width of packing
You can determine the correct size of packing to use by:
Subtracting the shaft OD from the bore ID
Cutting packing is best done:
On a wooden mandrel
Packing can be cut as a square end cut or:
Skived cut
For rigid packing, use a _____ to place it over the shaft
s twist
If packing is slightly bulged, you can flatten it with a:
roller
Ensure packing joints are staggered at least ___° from each other
90°
After the last packing ring is installed, allow the gland to enter the stuffing box by ___” to ___”
1/8” to 3/16”
If packing is too tight, it can cause excessive heat build up and can:
burn the packing and damage the shaft
Which packing ring compresses the most?
the ring closest to the gland
____ packing uses system pressure to improve its sealing action
automatic packing
V ring and U ring packing and O rings are considered balanced packing because:
they seal on both the ID and OD. Pressure is balanced at both diameters.
Cup packing is considered unbalanced because:
It seals on one diameter only
V ring packing has a V shaped cross section and is used in sets of:
3 or more rings
Note: depending on the working pressure
V ring packing can be used as in inside or outside packed seal. V ring packing must be used with:
shaped internal and external adapter rings
With V ring packing, which adapter ring, internal or external, is closest to the gland?
Internal
The external adapter ring merely shapes the V ring and is not subject to wear, but the internal ring must be soft enough to compress slightly but hard enough to:
Prevent extrusion
V ring packing should, where possible, be installed as :
endless rings
At initial installation, V rings are tightened just enough to:
prevent leakage
Some V rings use ______ to maintain constant pressure
spring loaded external adapter rings
Note: leaves no chance of overloading the packing. Can be multiple springs or one large spring
V ring packing sets used on double acting pistons should
Face away from each other
Note: Internal adapter ring should be supported by a solid backing
The adapter ring can be made of a non metallic material and can act as a:
bearing
V ring sets can be made of the same or dissimilar materials. Dissimilar materials consist of hard and soft materials, the soft materials sealing and the hard materials act as:
spacers and bearing rings
U ring (U-cup) packing is a circular, one piece moulded ring with a U cross section, made from one of the:
Synthetic elastomers
U ring packing is designed to be used in set of ____ per seat
1
U ring packing can be inside or outside packed. Due to its low friction, U ring packing is generally used in systems under 1500 PSI. Above that, a ____ is required
support ring aka anti extrusion washer
Two U rings should not be placed:
against each other
At low pressures, cup packing may seal at the lip, but it doesn’t normally seal until:
The heel diameter has been expanded to the cylinder wall
Compared to cup packing, U ring packing:
requires less maintenance time
Cup packing with double acting pistons require a rigid ____ between the two cups
separator
Note: In some cases, the separator acts as a guide as well
Cup packing material can be: (5)
- leather
- synthetic
- rubber
- elastomer
- impregnated fabric
O rings are used as ____ on slow turning shafts or reciprocating parts
dynamic seals
The groove that sits an O ring usually has a width 135% to ___% of the O rings cross section
150%
The squeeze on an O ring required to produce a seal is at least ___ %
10%
Can be 10-30% or 10-40% for static seals
O rings are rated up to 1500 PSI with properly designed grooves, or up to ___ PSI with a backup washer or anti extrusion ring
3000 PSI
Backup washers are often made of: (3)
- Leather
- Teflon
- Moulded nylon
____ are used with O rings to eliminate extrusion, permit wider clearance, form a seal across a clearance gap, and act as a dirt wiper
back up washers
Back up washers require a:
wider groove
___ an O ring into its seat. Stretch it only as much as needed.
roll
Lubricate the O ring. Protect the O ring from cuts and from nicks from sharp metal corners
____ is a seal that is retained in a groove, but not held in a static position.
Floating packing
Automotive or steam piston rings are often used when rapid travel and frequent cycling are needed. Material in the rings can be cast iron or steel (plain finish or chrome plated). Piston rings are, or are not a positive seal?
Are not. They leak slightly
What are some advantages of floating packing? (3)
- Less friction
- High pressure applications
- Less affected by temp.
What are two disadvantages of floating packing? (2)
- Conforms to mating surfaces less
- Longer run in period
Lip seals are used in pressures of 2-__PSI
2-10PSI
Lip seals are primarily used on rotating shafts that are partially or totally immersed in lubricant. A _____ is formed at the point of contact
hydrodynamic film
Note: film is about 0.001” thick
To maintain the hydrodynamic film on a lip seal, the shaft surface must be controlled. For speeds up to ___m/s, the shaft should have a ground finish. For speeds above that, the shaft should be hardened and ground.
8m/s
Materials used for lip seals are: (3)
- Felt
- Leather
- Synthetic rubber
A lip seal has a lightweight, pressed ____ housing
metal
Lip seals rely on the elasticity of the material or a garter spring to maintain:
contact with the shaft surface
A lip seal positioned to keep lube in is called:
inclusion seal
A lip seal positioned to keep contaminants out is called:
exclusion seal
Double lip seals can be mounted in 2 ways: (2)
- Back to back
- same direction, tandem
If possible, use a _____ to install a seal in a housing
pressing device
When installing, lubricate the shaft, housing and sealing edges.
When installing a lip seal into a bore, ensure the bore is free of:
Sharp edges
When installing a lip seal on a shaft, ensure any sharp edges on the shaft are:
Covered, with plastic, shim stock, or even paper
Use a leader (mounting sleeve) or a light gauge shim stock to install a seal over a shaft, unless the shaft has a:
chamfer with a long taper
What are 4 emergency repairs you can make when dealing with a worn lip seal surfaces? (4)
- Change the seal to a wider seal (move lip contact)
- Reverse the seal (move lip contact)
- Install a spacer behind a replacement seal (move lip contact)
- Install a thin sleeve over the worn spot of the shaft
A ring seal is a non positive seal mounted in a groove of the housing. They can be made of: (4)
- Cork
- Felt
- Leather
- Synthetic rubber
Cork, felt and leather ring seals are _____ washers with a sealing edge broader than that of the lip seals. Over an extended period, they tent to polish the shaft
precision cut
____ ring seal material is used in slow speed, light duty applications up to 65°C
Cork
___ and ____ ring seal materials are used in slow, light duty applications up to 105°C
felt and leather
Note: these seals absorb some lubricant and redistribute it as a film between it and the shaft
____ ring seal materials are used in high speed applications and can withstand temperatures up to 250°C
synthetic
Note: this seal retains lube without absorbing it, and resists acids and alkalis
A ____ is a non-positive seal mounted on the stationary housing. It flexes against axially moving parts. Expel debris away from seals, packing, bearings
wiper/scraper
Note: the lip allows lubricant to remain on the rod or guide, but restricts entry from the atmospheric side
a synthetic rubber or plastic wiper protects against:
dust, fine materials
A metallic wiper protects against:
solid abrasive materials
____ are positive seals which have one end secured to the housing, and the other to the moving part.
Boots and bellows
A boot is used for:
short movements
A bellow is used for:
large movements
A ____ seal is a positive seal which has a flexible membrane which spans between the stationary and moving parts.
diaphragm seal
A _____ diaphragm seal is designed for a small amount of movement
flat diaphragm
A ____ diaphragm seal is a moulded membrane and rolls on the two mating surfaces as it moves
rolling diaphragm
A mechanical seal must always run in fluid. _____ feed fluid into the product side of the seal
flushing ports
If a mechanical seal has its flushing water supplied by the product discharge, it may need to go through a:
cyclone strainer
Note: May also go through a heat exchanger to remove heat
Quenching is done through _____ on the atmospheric side of the seal
quenching ports
Note: comes from an external source
A ____ seal is a non-positive seal which is mounted in the stationary housing. The minimal clearance between it and the rotating shaft restricts the amount of leakage from the high to low pressure sides. Considered the simplest of seals.
Bushing seal
A ____ seal is a non positive seal with a series of concentric or helical grooves in the housing bore. A drain hole is places at the bottom of the seal to make the seal effective. Concentric grooves fill with lubricant to prevent entry, helical grooves return the oil to the bearings.
Annulus seal
A _____ seal is a non positive seal which has a cupped washer-like flange mounted to the rotating shaft. It is placed just outside the seal housing to assist a ring seal in sealing. Relies on centrifugal force
Slinger (flinger) seal
A ____ seal is a non positive seal which has a tongue and groove design. A series of internal and external passages form a maze like path for debris. Can seal radially, axially, or a combination. Radial requires a split housing. Used in high speed applications. The passages can be lubricated to force out debris.
Labyrinth seal
