| Thomas brought up a good point on evaluation of materials. From some
of my reference books on materials I put together a comparison of the
various material types. The summary is that glass reinforced plastics
are between 1/4th to 1/7th less strong than bronze in terms of their
tensile strength but the failure mode of seacocks should not be solely
because of tensile failure. Stress corrosion, stress cracking, and
impact strength are probably the more relevant.
Suffice it to say that bronze is probably the strongest material, but
the corrosion resistance and the relative flexibility of the plastics
would be superior in some circumstances.
The following 100 lines cover some of the properties of the various
materials;
The following is a synopsis of the mechanical properties of various
grades of bronze. I believe that most marine hardware used to be made
from NAVY M BRONZE (Unified Numbering System for Metals and Alloys
type C92200). However, my book lists the aluminum bronzes as having
the best resistance to salt water corrosion so take the last column
as the most probable material composition.
+-----------+-----------+------------+-----------+
| Manganese | Navy M |Leaded Ni-Sn| Aluminum |
| Bronze | Bronze | Bronze | Bronze |
--------------------+-----------+-----------+------------+-----------+
UNS | C67500 | C92200 | C92900 | C95400 |
Designation | | | | |
--------------------+-----------+-----------+------------+-----------+
Density | 0.302 | N/A | N/A | N/A |
(lb/in^3) | | | | |
--------------------+-----------+-----------+------------+-----------+
Thermal Coeff | 118 | 100 | 95 | 90 |
of Expansion | | | | |
(10E-7 in/in Fdeg) | | | | |
--------------------+-----------+-----------+------------+-----------+
Thermal Cond | 61 | 40.2 | 33.6 | 33.9 |
BTU-Ft/Hr-Ft^2-Fdeg | | | | |
--------------------+-----------+-----------+------------+-----------+
Modulus of | 15 | 15 | 14 | 15.5 |
Elast (10E6 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Yield Strength | 30-60 | 48 | 20 | 35-54 |
(10E3 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Tensile Strength | 65-84 | 40 | 47 | 85-105 |
(10E3 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Shear Strength | 42-48 | 180 | N/A | 47-50 |
(10E3 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Plain nylons are a thermoplastic resin from the Polyamide family. They
exhibit a crystalline structure and are available in a variety of
formulations. Nylon 6/6 is the most widely used, with Nylon 6 being
the second most used. Nylon 11 and 12 are used where better
dimensional stability and lower water absorption is needed. All nylons
are sensitive to UV degradation. Some of the relevant mechanical
properties are listed below.
+-----------+-----------+------------+-----------+
| Nylon 6/6 | Nylon 6 | Nylon 6/12 | Nylon 11 |
--------------------+-----------+-----------+------------+-----------+
Density | 0.0413 | 0.0408 | 0.0386 | 0.0376 |
(lb/in^3) | | | | |
--------------------+-----------+-----------+------------+-----------+
Thermal Coeff | 4.0 | 4.5 | 5.0 | 5.1 |
of Expansion | | | | |
(10E-7 in/in Fdeg) | | | | |
--------------------+-----------+-----------+------------+-----------+
Thermal Cond | 1.7 | 1.7 | 1.5 | N/A |
BTU-Ft/Hr-Ft^2-Fdeg | | | | |
--------------------+-----------+-----------+------------+-----------+
Flexural Modulus | 0.40 | 0.45 | 0.50 | 0.51 |
(10E6 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Tensile Strength | 12.0 | 11.8 | 8.8 | 8.5 |
(10E3 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Impact Strength | 1.0 | 1.0 | 1.0 | 3.3 |
(Ft-lb/in of notch) | | | | |
--------------------+-----------+-----------+------------+-----------+
Hardness | 121 | 119 | 114 | N/A |
Rockwell R | | | | |
--------------------+-----------+-----------+------------+-----------+
Finally many types of nylon are being reinforced with glass fibers.
The reason for this is general all mechanical properties will be
improved by a factor of two or more wit a glass load of 30-40%.
My guess is that "MARELON" is probably either a glass reinforced Nylon
6/12 because of its lower water absorption, or a Zytel clone because
of its higher impact resistance. The mechanical properties are listed
below;
+-----------+-----------+------------+-----------+
| Nylon 6/6 | Nylon 6 | Nylon 6/6 | Nylon 6/12|
| alloy | alloy | copolymer | |
--------------------+-----------+-----------+------------+-----------+
Company | RF-1006 HI| PF-1006 HI| VF-1006 HI| |
Designation | (LNP) | (LNP) |DuPont Zytel| |
--------------------+-----------+-----------+------------+-----------+
Thermal Coeff | 180 | 170 | 190 | 150 |
of Expansion | | | | |
(10E-7 in/in Fdeg) | | | | |
--------------------+-----------+-----------+------------+-----------+
Flexural Modulus | 1.10 | 1.00 | 0.90 | 1.20 |
(10E6 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Tensile Strength | 27.0 | 22.0 | 11.6 | 22.0 |
(10E3 psi) | | | | |
--------------------+-----------+-----------+------------+-----------+
Impact Strength | 3.0 | 3.2 | 4.5 | 2.4 |
(Ft-lb/in of notch) | | | | |
--------------------+-----------+-----------+------------+-----------+
|
| Another thing you want to keep in mind about nylon is that it swells
as it absorbs moisture, like wood, which causes problems when it
is used in bushing or bearing applications, and that it gets quite
brittle when cold.
John
|