Page 293 - The Tribology Handbook

B26 Mechanical piston rod packings

DESIGN OF PACKING ARRANGEMENT Piston rods

Number of sealing rings Rod material is chosen for strength or chemical resistance.
Carbon, low alloy and high chromium steels are suitable.
There is no theoretical basis for determining the number of For the harder packings (lead bronze and cast iron)
sealing rings. Table 26.1 gives values that are typical of hardened rods should be used; treatment can be flame or
good practice: induction hardening, or nitriding. Chrome plating or high
chromium steel is used for chemical resistance.
Table 26.1 The number of sealing rings for
various pressures Surface finish
Metal and filled PTFE packing 0.2-0.4 pm R, (8-16 pin
Pressure No. of sets of sealing rings
cla) .
up to 10 bar (150 p.s.i.) 3 Carbodgraphite and metal/graphite sinters 0.1-0.2 pm
R, (4-8 pin cla)
10-20 bar (150-250 p.s.i.) 4
Dimensional tolerances
20-35 bar (250-500 psi.) 5
f0.05 mm (+0.002 in)
Diameter
35-70 bar (500-1000 p.s.i.) 6 -0.05 mm (-0.002 in)
Taper over stroked length * 0.01 mm (0.0005 in)’
70-1 50 bar (1OOC-2000 p.s.i.) 8 Out-of-roundness 0.025 mm (0.001 in)
above 150 bar (2000 p.s.i.) 9-12
Notes: 1 With Type 4 packings increase number of sealing rings
by 50-100%.
‘2 With Type 5 packings four sets of sealing rings should
be adequate.
3 Above 35 bar (500 psi) use one or two pressure
breakers (Type 1) in addition, on the pressure side.

Packing materials

Table 26.2 The types of packing material and their applications

Rod Axial
Material Applications
hardness clearance
(1) Lead-bronze 250 BHN min 0.08-0.12 mm Optimum material with high thermal conductivity and good
(0.003-0.005 in) lubricated bearing properties. Used where chemical
conditions allow. Suitable for pressures up to 3000 bar
(50 000 psi.)

(2) Flake graphite 400 BHN min 0.08-0.12 mm Cheaper alternative to (1); bore may be tin coated to assist
grey cast iron (0.003-0.005 in) running in. Suitable up to 70 bar (1000 p.s.i.) for lubricated
operation
(3) White metal not critical 0.08-0.12 mm Used where (1) and (2) not suitable because of chemical
(Babbitt) (0.003-0.005 in) conditions. Preferred material for high chromium steel and
chrome-plated rods. Max. pressure 350 bar (5000 psi).
Max. temperature 120°C
(4) Filled PTFE 400 BHN min 0.4-0.5 mm Suitable for unlubricated and marginally lubricated operation
(0.015-0.020 in) as well as fully lubricated. Very good chemical resistance.
Above 25 bar (400 psi.) a lead bronze backing ring
(0.1/0.2 mm) clear of rod should be used to give support
and improved heat removal
(5) Reinforced p.f. not critical 0.25-0.4 mm Used with sour hydrocarbon gases and where lubricant may
resin (0.010-0.015 in) be thinned by solvents in gas stream

(6) Carbon-graphite 400 BHN min 0.03-0.06 mm Used with carbon-graphite piston rings. Must be kept oil free.
(0.0014.002 in) Suitable up to 350°C
(7) Graphite/metal 250 BHN min 0.08-0. 12 mm Alternative to (4) and (6)
sinter (0.003-0.005 in)
B26.3