Crankshaft/Piston/Xhead

Details of a 2 stroke marine diesel engines crankshaft,piston and cross head

1. Crankshaft
Function – to convert reciprocating motion of piston to that of rotary motion
at the output shaft.
Consists of journals, crank webs and crankpin (conn rod journal)
Two types – single piece (4 stroke)and shrunk fit type (2 stroke,large).
crank throw – distance from c/l of main journal to c/l of crank pin – equel to
half of engine stroke.
Counter weights – added to webs opp.to crankpins, improves the balancing
of engine and relieves the load on main bearing.
Tyes of Crankshaft
Fully built – webs are shrunk on to the main journal and crankpin – large
marine diesel engine.
Semibuilt – webs and crankpin as one unit shrunk on to journal – large and
medium speed marine diesel engine
Solid forged – one piece, either cast or forged – high speed diesel engine
Crankshaft- Important points

3. Stresses in Crankshaft
The crankpin is like a
builtin beam with a
distributed load along its
length that varies with
crank position. Each web is
like a cantilever beam
subjected to bending &
twisting. Journals would be
principally subjected to
twisting.
1.Bending causes tensile
& compressive stresses.
2.Twisting causes shear
stress.
3.Duto shrinkage of the
web onto the journals,
compressive stresses are
set up in journals &
tensile hoop stresses in
the webs.

4. MATERIALS.
In the case of large marine diesel engine the type of
shaft generally favored is the cast or forged steel
semi-built, a typical analysis, method of
construction and testing would be as follows:
Material analysis: Cast steel

Element. Percentage.
Carbon 0.2
Silicon 0.32.
Manganese 0.7
Phosphorus 0.01
Sulfur 0.015
Remainder iron.

5. Operational Information
The Two Stroke Crosshead Engine
The Crankshaft
The crankshafts on the large modern 2 stroke crosshead engines
can weigh over 300 tonnes.
They are too big to make as a single unit and so are constructed by
joining together individual forgings.
On older engines the so called fully built method was used.
This consisted of forging separate webs, crankpins and main
journals.
The crankpins and journals were machined and matching holes
bored in the webs, which were slightly smaller in diameter.

6. C/Shaft Details
The webs were heated up and the crankpins and journals
fitted into the holes (which due to the heat had
expanded in size). As the webs cooled down, so the
diameter of the bored holes would try and shrink back
to their original size. In doing so, the crankpins and
journals would be gripped tightly enough to stop them
being able to slip when the engine was being operated
normally.

7. C/Shaft Details
•Today, crankshafts for large 2 stroke crosshead engines
are of the semi built type. In this method of
construction the crankshaft "throws" consisting of two
webs and the crankpin are made from a single forging
of a 0.4% carbon steel. The webs are bored to take the
separately forged and machined main journals which
are fitted into the webs using the shrink fitting method
described above. The shrink fit allowance is between
1/570 and 1/660 of the diameter.
•The advantages of this method of construction is that
by making the two webs and crankpin from a single
forging the grain flow in the steel follows the web round
into the crankpin and back down the other web.

8. C/Shaft Details
•Because the crankpin and
webs are a single forging, the
webs can be reduced in
thickness and a hole is
sometimes bored through the
crankpin as shown, reducing
the weight without
compromising strength.

9. Here you can see individual crankthrows awaiting machining
Heating a crank web using gas flames before inserting the main journal.
Note the thickness of material around the hole for the journal.
Built up Crankshaft Manufacture

10. A crankshaft being assembled vertically
A semi built crankshaft in the lathe. The man gives an idea of the size!
Crankshaft views

11. THE WELDED CRANKSHAFT
•The welded crankshaft was developed in the 1980s. It was
made up of a series of forgings each comprising of half a
main journal, web, crankpin, second web, and half a main
journal. These forgings were then welded together using a
submerged arc welding process to form the crankshaft.
After welding the journals were stress relieved and
machined. As well as having the advantage of continuous
grain flow, the webs could be made thinner (no shrink fit to
accommodate), leading to a lighter shorter crankshaft.
•Why aren't all crankshafts produced by this method? Cost!
It was very expensive and only about twenty crankshafts
were produced by this method. They have performed very
well in service however.

12. All Welded C/shaft

13. Crankshaft and bearings

14. Operational Information
The Medium Speed 4 Stroke Trunk
Piston Engine
The Crankshaft
The Crankshaft for a medium speed 4 stroke diesel engine is made from a
one piece forging.

15. •First the billet of 0.4%
carbon steel is heated in a
furnace It is then moved to
the forging presses

16. •In the hydraulic forging press the crankshaft
throws and flanges are formed.

17. •The crankshaft is locally heated to a white
heat where the webs are desired to be
formed. The crankshaft is then compressed
axially to form the start of the webs

18. •Sets of hydraulic presses are then used to form the
crankpin journal ad webs.
•This method of forging gives the crankshaft
continuous grain flow. This is where the grain
structure follows a path parallel to and along the
journal, bends round along the line of the web,
round through the crankpin, and back down the
second web before turning again to follow the
journal. Continuous grain flow gives the
crankshaft better fatigue resistance.

20. •The forgings are then machined, stress
relieved, and the radii at the change of
section cold rolled.
•If the crankshafts are to be surface
hardened they are made of a steel alloy
known as nitralloy (a steel containing
1.5%Cr, 1% Al and 0.2% Mo)
•The crankshaft is heated to 500ºC in
ammonia gas for up to 4 days. The
nitrogen dissociates from the ammonia
gas and combines with the chromium
and aluminium to form hard nitrates at
the surface. The molybdenum refines
the grain structure at the still tough
core.

21. Fillet Radii
•At the change of section between journal and web
and web and crankpin, fillet radii are machined so
there is not a sharp corner to act as a stress raiser.
These radii are cold rolled to remove machining
marks, harden the surface and to induce a residual
compressive stress, again to increase fatigue
resistance.
•Re-entrant fillets are sometimes employed; This
allows for a shorter crankshaft without
compromising on bearing length.

22. Oil Holes in Crankshafts.
•Unlike the crankshafts for slow speed 2
stroke crosshead engines, which lubricate
the bottom ends by sending the oil DOWN
the con rod from the crosshead, the
crankshaft for the medium speed trunk
piston engine must have holes drilled in it
so that oil can travel from the main bearing
journals to the crankpin and then UP the
con rod to lubricate the piston pin and cool
the piston. If the surface finish of the holes
is not good, then cracks can start from the
flaws.
•At the exit points on the crankpin, the
holes must be smoothly radiused. So that
the crankshaft strength is not compromised
the holes should be positioned horizontally
when the crank is at TDC.

23. Crankweb Formation from Round Rod
•Web Formation
Crank Formation

24. Crankshaft

25. Con.rod Details (2 St)
•The Connecting Rod is fitted between the
crosshead and the crankshaft. It transmits the
firing force, and together with the crankshaft
converts the reciprocating motion to a rotary
motion.
•Made from drop forged steel. On the older
engines the bottom of the con rod terminates in
a flange known as a Marine Palm, which is
bolted to the split bottom end (Crankpin)
bearing, whilst at the top another flange is
formed on which is bolted the two crosshead
bearings.

26. The Two Stroke Crosshead Engine
Connecting Rod

27. Con.rod view
•Con.rod with brg.shell (New Design)
Bearings are made of Whitemetal. It is of two types, Lead based
whitemetal and Tin based whitemetal.Tin based white metal is an
alloy of Tin, Antimony & Copper, in which Tin is predominant
(SN -87.65%, Sb – 7.5%, Cu -4.5%, Pb – 0.35% ) . In Lead based
whitemetal, Lead will be predominant.

28. •Connecting Rods on the later engines are
produced as a single drop forging incorporating
the top half of the crankpin bearing housing and
the bottom half of the solid crosshead pin bearing
housing.
•On older engines the bearings were white metal
thick wall bearings, scraped to fit. Clearances
were adjusted by inserting or removing shims
between the bearing halves. Modern bearings are
of the "thinwall" type, where a thin layer of
white metal or a tin aluminium alloy is bonded to
a steel shell backing. The clearance on these
bearings is non adjustable; When the clearance
reaches a maximum the bearing is changed.
•Oil to lubricate the crankpin bearing is supplied
down a drilling in the con rod from the
crosshead. When inspecting the crankpin bearing
and journal it is good practise to check the
journal for ovality because if this is excessive, a
failure in the hydrodynamic lubrication can
occur.
Con rod Details

29. Trunkpiston Con.Rod (4 St)

30. Piston, Conn. rod Assly

31. Con.rod sketches. (4 St)

32. Connecting Rod
It is a bar or strut with a bearing at each end.
Main purpose is to transmit thrust to crankshaft, secondary to transport oil
between bottom and top end brgs.
large engine conn rod may have a separate brg box bolted to a foot on the
rod – shim between the foot and the box permits adjustment to compression
ratio.
Con.Rod

33. Connecting Rod brgs. & bolts.
The need for large crankpin bearings in highly loaded engines makes it difficult to keep
the crankpin end small enough to withdraw the rod through the cylinder. To make the
crankpin end more compact, while still retaining the same large bearing, the following
constructions are used.
1. Four bolts of smaller dia instead 2 large bolts.
2. Stud instead of bolts.
3. Splitting the crankpin box at an angle
• Bolts are made of heat treated alloy steel for greater strength.
• Provided with fine threads with close pitch for max strength and secure tightening.
• To prevent fatigue cracks developing in bolts threads cut carefully and shank smoothly
finished.
The inertia forces acting sidewise on conn rod tend to displace crankpin brg. In order
to keep the parts in line, the bolts are snugly fitted in the holes. Also to assist the bolts in
resisting these side forces, a step or tongue-and-groove is often used at the joint
between the bearing halves.
Con. Rod Brgs. & Bolts

34. Bottom End Bearing Bolts
•BOTTOM END BOLTS
•Because of the stress reversal mentioned above, bottom
end bolts have a limited life. This varies from engine to
engine, but is generally around 12-15000 hours. If a
bottom end bolt was to fail in operation, then the
results would be disastrous.
•Bottom end bolts should be treated with care when
removed from the engine during overhauls. They
should be inspected for any damage to the surface from
which a crack could start. This damage could be due to
corrosion (water in LO) or because of incorrect
handling.

35. 10. Compression shim
11. Upper part of lower conn rod bearing
12.shims for adjusting vertical bearing
clearence
13.bottom part of lower conn rod bearing
14.fixing screw for bearing shell
15.locking sleeve
16.nut to conn rod bolt
M.E. Connecting Rod

36. Gudgeon pin
• Link between the conn rod and
piston.
• Pin is usually steel and is made
hollow for lightness.
• arrangement is made in 3 ways…
-(a)Gudgeon pin is secured in piston,
bearing is held in the conn rod end .

-(b) Floating pin – pin is free and
bears against bearings in both the
piston and the rod – most popular.
- snap rings or end caps used to
prevent pins contacting the liner walls
-(c) “Saddle form bush” – pin is
fastened to the rod, bearing is part of
the piston.
• Receives lubrication from the
crankpin bearing through long holes
drilled through the conn rod.
Gudgeon Pin

37. Floating Gudgeon Pin

38. Cross head & conn rod

39. Crosshead Guide

40. Crosshead View (different designs)

41. Cross head assly
Consists of X-head pin & guide shoes.
Made of forged steel, machined,ground and
super finished on journal.
Center part of pin is flattened out on four sides
and bored for piston rod end fixing.
The pin brg is highly stressed, motion is
oscillatory and load is always from same direction.
Difficult to achieve ideal lubrication.
Pin rests on brgs in the
conn rod yoke.
Pin takes the guide
slippers at the ends.
Brg surfaces are lined
with white metal and
grooved for lubrication.

42. SECTIONAL VIEW OF CROSSHEAD

43. X head
sectional
view

44. Crosshead View (improved design)

45. Cross head Assembly
(Exploded view of
improved design)
Center part of pin is flattened
out on upper portion for piston rod
palm seating.
Earlier type pin is small in
diameter and deflects under load –
load concentration at inner edge of
brgs. Is high. Here that is
overcome by increasing the size of
the brg area.

46. Seals the cylinder, transmits gas load to
conn rod.
Reversal of travel sets up inertia of
forces, so in high speed engine, light piston
Uncooled – heat from piston to rings-cyl
wall – jacket water.
Cooled – by water / lub oil.
Top part- crown, shape depending on
design of combustion chamber.
Crown made thick, slight tapper, less dia
towards top for expansion.
Upper part carries P rings(comp rings)
and lower part oil control rings.
Material- CI or Aluminum
Piston

47. Photo -
Piston rod
& stuffing
box

48. single casting of silicon aluminum alloy – light, hardened for wear resistance.
skirt acts as guide to transmit side thrust to liner.
crown cooled by lub oil circulated through coiled tubes inside piston.
oil supplied to main brg circulates via drilled passages in c/shaft to crank pin
brg, and then up to bore in conn rod – to gudgeon pin – piston coolig coil.
crown is recessed to allow opening of
inlet and exh v/vs.
the gudgeon pin floats, free to rotate.
piston skirt is hollow to allow and
shaped to locate conn rod at gudgeon
brg.
Generally 4 nos comp rings, 2 oil rings]
Top comp ring chrome plated.
Oil ring removes excess oil from liner,
passing oil through drain holes to c/case.
Trunk type Engine - Piston

49. Cut section view of Trunk Piston

50. Water cooled large piston(2 stroke)
steel crown, CI skirt.
Cast steel of heat resisting alloy
containing cr., Mo., for strength and resist
corrosion.
crown side thickened to accommodate
ring grooves, lower wear surface chrome
plated.
Cooling FW enters and leaves through
telescopic pipes.
Water outlet pipe inside set near crown
to ensure water remains full.
cooling space fitted with bolted cover and rubber seal to prevent leakage.
skirt is uncooled and acts as guide within liner. Two leaded bronze wearing
rings are caulked into the grooves to prevent possible damage to skirt / liner.
Sulzer RTA (early design)

51. Sulzer – RND Piston

52. Scavenge & Exhaust Ports uncovered

53. SULZER RD Piston

54. Sulzer RTA (Early Design)

55. Sulzer RTA

56. Piston crown & Skirt (Sulzer RTA – Oil
Cooled)

57. Sulzer RTA Piston (Sectional View)

58. MAN B&W

59. MAN B&W (Sectional View)

60. Telescopic pipes (for Piston Cooling)

61. Piston Cooling Stand pipes forTelescopic pipes
(Sulzer)

62. Oil cooled piston

Large engine –X head typeTrunk type engine

63. B&W Oil Cooled Piston

64. Piston Ring

65. Piston rings
Designed to fit in grooves.
Comp rings, oil control rings
Comp ring
– seals combustion space,
transmits heat to liner.
- Made of gray CI, special facing
for running-in.
- Special joint – 1)Butt or straight
cut, 2) Angle, 3) Lap or step.
- When free slightly larger
diameter than bore, when
squeezed presses against the
cylinder wall.
- The pr. Increases with gas pr
from back side of the ring.
Oil control rings
- On downward stroke scraps
most of the oil splashed into cyl
liner wall.

66. How gas pressure forces a compression ring
tighter against cylinder wall ?

67. Piston rings
 3 clearances are considered when installing
the rings on the piston are
1) Gap clearance(butt clearance)
2) Axial or side clearance
3) Back clearance( radial clearance)

68. Butt Clearance
Butt clearance :
Ring is placed at the
(least- worn portion)
liner and gap is
measured using
feeler gauge
If less, possibly in running engine,ring ends
touch, resulting in seizure.
If greater blow-by of gases occur which may
enter c/case

69. Axial and
Back
Clearances
Axial clearance: Distance between top of the ring and the
groove top (groove Height - ring thickness)
 If less the comb gases cannot get into back side,
resulting not having enough pr on liner wall and blow-by.
 If greater results in excess ring groove wear – as a
result of ring snapping to the top of the ring groove at the
end of exh stroke( in 4 stroke engine)

70. Piston rings
 If no back clearance the rings will take the
side thrust instead of piston trunk resulting
in high friction and possible piston seizure.
 Radial clearance:
Difference between
groove depth and
ring thickness. The
ring thickness
should be less than
groove depth.
The outer face of
the ring is slightly
outside the groove.

71. Material : Rings must have good strength,
elasticity and wear resistance with low friction,
and must maintain these properties at high
working temperatures.

 Cast and machined from perlitic grey cast iron
(additions such as chromium, molybdenum,
vanadium, titanium, nickel and copper)
 Some engines, spheroidal graphite iron is used,
which has greater hardness and tensile strength
Ring Material

72. Axial & Back Clearances
The ring has hardness slightly higher than that of
the liner material, so that wear between the ring
and liner is at the optimum low rate with the
easily replaceable ring suffering most wear.

73. Typical values would be :
0.10 – 0.15mm for four stroke engines.
0.2 – 0.25 mm for two stroke engines
The value may be doubled for the top ring
to Allow for groove distortion, carbon
deposits and thermal expansion.
The maximum acceptable clearance is
about 0.5mm
Allowable Piston ring Clearances

74. Piston Rings
Gas Pressure acting on rings at end of compression

75. Piston rings

76. Piston rings are manufactured using the pot
casting method, producing a short cylinder of oval
cross section from which the rings are machined.
This produces  a balanced and homogenous
casting around the entire circumference of the
ring.
Piston ring Manufacture

77. The Oval Pot Cam Turning Method
is more expensive but produces a
ring which retains its tension when
working in the engine.
 The rings are machined from the
cast oval pots in a cam turning lathe.
The lathe is equipped with an
interchangeable copying cam , which
controls the cutting tools.
By changing the shape or ovality of
the cam, the circumferential pressure
distribution around the ring can be
altered.
Oval Pot Cam
Turning Method

78. PISTON RINGS:
The piston rings must provide an effective seal of the
combustion space.
Under ideal condition the piston ring surfaces are in
complete contact over its entire depth and
periphery with the liner surface and ring width on
landing area.
The initial seal is estabIished between the ring and
the liner by a radial pressure exerted by the ring
when pressed on liner. after which the ring is
forced down on the grove landing surface by the
gas pressure.

79. The gas pressure is also throttIed at the back ring
through the small clearance space thus increasing
further the radial pressure against the liner.
The sealing of combustion space by a set of rings on
reciprocating piston follows labyrinth principle.
The gas pressure leaked in behind each piston ring is
successively throttled down to the pressure at the
underside of the piston.
In this way its natural tendency to leak out is
progressively diminished.

80. The number of rings, the ring section area, and the
contact areas are determined by consideration of
strength, pressure difference, volume of space to
be sealed, etc.
From the foregoing it will be clear that the pressure
on each ring is different, being the highest at the
top ring and diminishing successively at the
bottom ring.

81. The ring has hardness slightly higher than that of
the liner material, so that wear between the ring
and liner is at the optimum low rate with the easily
replaceable ring suffering most wear.
After manufacture of rings, some of them may be
coated or heated. Chromium plating or
Molybdenum spraying is done for resisting
scuffing attack on 4 stroke engines.

82. Copper plating, silver plating, phosphorus
treatment, Cadmium flashing and tin flashing can
prevent ring scuffing during initial running in.
Alloyed piston ring is for use in un-supercharged
and supercharged engines using heavy oil. The
above alloying with copper and molybdenum will
improve corrosion resistance and mechanical wear
properties.
Sulzer engine manufacturers for their
RD/RND/RTA type engines use this above
material for piston rings.

83. FORCES acting on a piston ring:
1.Radial direction:
2.The force exerted through the ring tension.
3.The gas pressure behind the ring, approximately
equal to the cylinder pressure in the case of top
piston ing.
4.The friction force between ring and groove.
5.A force created by the movement of the wedge of
oil between the liner and the ring.
6.A force due to the pressure of gas on the outer
surface of the ring again transmitted through the
oil film

84. 2. In Axial direction:
Gas pressure on the upper side face of the ring.
Gas pressure on the lower side face of the ring
The friction force between the ring and liner
surfaces.
The inertia force due to the weight of the ring, which
varies according to the acceleration or
deceleration of the piston

85. 3. WaIl pressure:
Generally, wall pressure is the ring tension over the
surface in contact with the liner. For most normal
applications rings are produced, which have
uniform pressure pattern. The wall pressure is
designed to vary according to the diameter of the
cylinder.
The depth of the ring grove should always be greater
than the ring radial thickness so that the piston
side loads are never transmitted to the liner wall
via the rings. An excessive clearance can allow the
build up of large hot gas volumes leading to
overheating and oil oxidation producing heavy
deposits.

86. RING MATERIAL
Most popular material is cast iron alloyed
principally with silicon, manganeese, chromium,
phosphorous, copper and molybdenum.
The presence of graphite in cast iron improves the
material property in sliding. The graphite acts as a
lubricant in the dry state.
The material must be resilient, strong to withstand
the pressure in the cylinder and sufficient resistant
to wear. The range of hardness in such a piston
ring material differs from 160-190 BrineIl.

87. The type of rings used in large engines is regarded as
soft. The use of these rings in engines with high
maximum pressure advantageous as it takes
bedding quickly over the liner surface during the
running in period.
Depending on the size of the ring a choice is made
between strength and wear resistance.

88. Ring clearances:
The 3 major clearances to take note are:
I 0 side or Axial clearance. This clearance allows the
ring to slide in the ring groove so that it can
maintain contact with the liner wall.

89. Typical values would be :
0.10 – 0.15mm for four stroke engines.
0.2 – 0.25 mm for two stroke engines
The value may be doubled for the top ring to Allow
for groove distortion, carbon deposits and thermal
expansion.
The maximum acceptable clearance is about 0.5mm

90. If the clearance is too small the ring will seize in the
groove causing ring collapse and “blow-by”.
It will also excessively restricts the gas pressure build
up behind the ring reducing the supporting
forces, which hold the ring against the liner wall.
This again may be sufficient to cause ring collapse.
If the clearance is too large then the ring will be
allowed o bend excessively reducing the contact
face height against the liner.
It will also allow the ring to hammer on the groove
landing, increasing wear.

91. 2 Butt clearance:
This clearance is necessary to allow circumferential thermal
expansion and permit fitting of ring to a solid piston.
The three most common shapes of ring end used to reduce gas
leakage
The plain gap is popular on 2- stroke engines for the top and
second rings as it is robust
The45°’Mitre gap rings: The mitre gap gives a reduced width
gas passage for the same circumferential clearance.
The lap Shape: The lap shape reduces the gas passage width
even further and also increases its length considerably there
by improving gas sealing but the ends are relatively fragile
and the rings are expensive to manufacture.
There fore, it is likely to be used for lower rings only.
Free gap :The gap clearance measured with the ring clear of the
cylinder liner.
Closed gap: Clearance measured with the ring in the unworn
part of the liner.

92. If the closed gap is large, poor sealing and blow by
results, while too small a clearance may permit
ring end to contact due to thermal expansion
leading to heavy ring pressure against the liner
and causing heavy wear.
The butt clearance is useful judge for ring wear and
loss of spring tension as the closed gap may be
measured through the scavange ports.
Ring wear is more commonly measured by removing
the ring and measuring the reduction in radial
thickness.

93. Back clearance
Ring pushed flush with the grooveface.

94. Two stroke Engine – Piston rod Stuffing Box
GENTLEMEN,
WHERE IS
PISTON ROD
STUFFING
BOX ?

95. Two stroke Engine

96. Piston rod stuffing box View through
Scavenge Trunking

97. Operational Information
The Two Stroke Crosshead Engine
The Stuffing Box
CUT SECTION OF STUFFING BOX

98. 1) Fitted in large crosshead
type engines.
2)Diaphragm is fitted to isolate
the lower end of the cylinder
from the crankcase in order to
prevent contamination of the
crankcase lubricating oil with
residue from combustion,
corrosion, wear, used cylinder oil
and exhaust gases which may be
blown past the piston rings.
P.Rod Stuffing Box

99.  1)Allows to use separate cylinder oil.
2)To seal off the scavenge space and also
for under-piston scavenging.
 3)Consists of inward sealing metallic
packing and oil scraper rings.
 4)Each ring consists of three or four
segments which are a good fit on to the
piston rod surface and are held by a coiled
garter spring.
 5)There must be sufficient
circumferential butt clearance between
segments. This clearance ensures that the
rings will bear against the piston even in
worn condition.
P.Rod Stuffing Box

100. Sectional view of Stuffing box (Sulzer Engine)

101. 1)Stuffing box housing is in two
parts are bolted together.
2) Housing have seven ring
grooves.(1+2+4)
3) The upper most set of rings
(1 set) scraps the sludge and
prevent it from entering from
scavenge box.
4) The middle rings (2 sets)
seals scavenge air, preventing air
passing down.
5) The lower rings (4) scraps the
lubricating oil off the piston rod.

P.Rod Stuffing box - Operation

102. 6)The scrapped (during
downward stroke) sludge -
(contaminated oil residue) should
be conveyed to the sludge tank
from under piston space
7) The scrapped (during
upward stroke) excess c/case oil is
by lower scrapper rings is
conveyed to c/case (after
purification). These rings could
be bronze with scraping edges or
cast iron back rings equipped
with scrapper lamellas.
P.Rod Stuffing Box – Operation (Cont’d)

103. Stuffing box – Operating principle

104. PISTON ROD
STUFFING
BOX (Exploded
View)
MAN B&W ENGIINE

105. Operational Information
The Two Stroke Crosshead Engine
The Stuffing Box
Stuffing box Photo

106. •Because the crankcase is separated from the cylinder and scavenge
space by the diaphragm plate on a two stroke crosshead engine,
provision must be made for the piston rod to pass through the plate
without oil from the crankcase being carried upwards, or used
cylinder oil contaminated from products of combustion being carried
downwards. It is also highly undesirable to allow the pressurized air
in the scavenge space to leak into the crankcase.
•The Piston rod passes through a stuffing box which is bolted into the
diaphragm plate. The stuffing box casing which can be split
vertically, as shown in the photo, contains a series of rings which are
each made up of three or four segments. On the outside of each set
of segments is a garter spring which provides the tension to hold the
ring segments against the piston rod. There is a clearance between
each segment to allow for wear. The rings are either bronze or can
comprise of replaceable cast iron lamella fitted into a steel backing
ring.
Stuffing box detail

107. •As the Piston rod passes up
through the stuffing box, the
oil from the crankcase is
scraped off by the lower sets
of rings and is returned via
drillings to the crankcase.
Any oil that passes this
primary set is scraped off by
another set of rings, and is
led away through a drain to a
tell tale open ended pipe into
a tundish outside the engine
from where it drains to a
recycling tank.
Stuffing box detail

108. •As the piston passes down
through the stuffing box, the
top set of scraper rings will
scrape off the contaminated
oil into the bottom of the
scavenge space, where it is
drained away via the
scavenge drains. However if
these rings are faulty, then
the oil may drain into the
recycling tank.
Stuffing box Detail

109. Stuffing box detail
•By observing the open
ended tell tale referred to
above, a guide to the
condition of the rings can
be ascertained. If a large
quantity of oil is draining
out, then the lower set of
rings are faulty. If air is
blowing out, then the
upper rings are worn.
Stuffing box detail

110. Stuffing box detail

111. •Oil in the recycling tank can be purified back to the crankcase.
However this is not necessarily a good idea. It may be contaminated
by used cylinder oil which if mixed with crankcase oil causes
anincrease in viscosity of the crankcase oil. calcium deposits inthe
bearings lead to damage and the oil may carboniseand deposit on
the underside of the piston crown when used as a piston coolant.
Often this contaminated oil is just landed ashore or burnt.
•Regular maintenance of the stuffing box will keep it in good
condition. checking garter spring tension, ring butt and axial
clearances, and replacing worn rings are all part of the overhaul
procedure.
•Excessive wear will take place if the crosshead guides are out of
alignment or if the guide clearances are excessive. Worn stuffing
boxes and excessive leakage can exacerbate theincidence of
scavenge fires and increase the risk of a crankcase explosion.
Stuffing box detail