Parker O-Ring Handbook.pdf

Dynamic O-Ring Sealing
5-8
5.12 Calculate Rubbing Surface
The areas and lengths given in Table 5-4 are based on the
dimensions given in Design Table 5-2 at the end of this section.
If the application differs, use dimensions from the applicable
table, i.e. Table Design 5-1 for aerospace, and calculate the
area and length.
The following example illustrates the procedure:
Projected Area: Ap = (π /4) [A²max - (B-1)²min]
Ar = (π / 4) [(A-1)²max - B²min]
Rubbing Surface Length: Lp = π Amax
Lr = π Bmax
For Parker Size No. 2-113: Amax = 0.751 Bmin = 0.559
A-1max = 0.739 B-1min = 0.571
Bmax = 0.561
Projected Area:
Ap = (π/4) [(0.751 )²- (0.571)²] = 0.187 sq. in.
Ar = (π/4) [(0.739)² - (0.559)²] = 0.184 sq. in.
Rubbing Surface Length:
Lp = 0.751π = 2.36 in.
Lr = 0.561π = 1.76 in.
5.13 Methods To Reduce Friction
The foregoing formulas for estimating O-ring friction are
intended for applications in which standard O-ring compound
types are to be used in systems lubricated with hydraulic oil.
In pneumatic or other dynamic applications, Parker Seal
can help reduce friction in several ways. O-Lube and Super-
O-Lube greases are available from Parker distributors, and
O-rings may be ordered that have received special friction
reducing treatments. These include internally lubricated rings
and Lube Treated rings.
f —Friction Lb. per Inc hLength
of Rubbing Surface
c
Basis for Curves
1— Running Friction Due to
Squeeze and Hardness
(Durometer) Only
2—
15 Micro-Inch Finish
Chrome Plated Surface
4
3
2
1
3— AN6227 O-rings, 100,000 Cycles
Room Temperature,
Using MIL-H-5606
Hydraulic Oil
4— Speeds in Excess of
1 Ft. per Min.
Hardness 90° Shore A
80°
70°
0 5 10 15 20 25
Percent Seal Compression
Figure 5-9: Friction Due to O-ring Compression
Parker O-Ring Handbook
5.14 Friction and Wear
O-rings load a sealing surface due to their own resilience
compounded with any system pressure. When the surface to
be sealed moves relative to the O-ring, frictional forces are
set up producing two effects: one leads to wear and the other
reduces the useful load which a cylinder can transmit.
5.14.1 Friction
In dynamic applications difference must be made between
break-out and running friction. Break-out friction must be
overcome at the beginning of movement and also is known as
start-up friction. Once movement is established the frictional
forces drop to a lower level and gliding begins. This can be
clearly seen in reciprocating cylinders.
The running friction of seals depends on countless factors
making a mathematical analysis practically impossible. For
this reason it is diffi cult to make exact statements regarding
the level of friction which can be expected. The most
important factors are:
Related to the seal:
• Geometrical form including production tolerances and
resulting deformation;
• Hardness and surface fi nish;
• Friction values for dry and lubricated compound;
• Swell and temperature characteristics.
Related to the hydraulic fl uid:
• Tendency to build up a lubricating fi lm and its
distribution;
• Viscosity and temperature/viscosity relationship.
Friction Lb. per Square Inch
of Seal Projected Area
f —
h
Basis for Curves
1 — Running Friction
Due to Pressure
Only
2 —
15 Micro-Inch Finish
Chrome Plated Surface
70
50
30
10
0
1000 2000 3000
Fluid Pressure PSI
Figure 5-10: Friction Due to Fluid Pressure
3 — AN6227 O-rings, 100,000 Cycles
Room Temperature,
Using MIL-H-5606
Hydraulic Oil
4 — Speeds in Excess of
1 Ft. per Min.
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com