Coated cartridge heater

An improved cartridge heater comprising a electrical resistance heating wire coil enclosed in a stainless steel cylindrical sheath and having bonded thereto a coating containing a solid lubricant and a binder. The solid lubricant providing for ease of removal of the cartridge heater from a bore hole into which it has been inserted for use and providing a barrier to corrosion between the cartridge heater and the metal of the apparatus in which the bore hole is located.

BACKGROUND OF THE INVENTION 
The present invention relates to electrical cartridge heaters and in 
particular to a cartridge heater having a protective low friction coating. 
Cartridge heaters are used to transmit heat to a wide range of equipment 
and apparatuses. Cartridge heaters are generally cylindrical and adapted 
to be inserted in appropriately dimensioned bore holes in apparatuses to 
be heated. A typical cartridge heater comprises a helically wound 
electrical heating element connected to two electrical leads and enclosed 
in a cylindrical or square metal sheath or casing, commonly made out of 
stainless steel. 
In use, it is desirable to obtain a close fit between the cartridge heater 
and the bore hole in which the heater is inserted to ensure maximum heat 
transfer from the cartridge heater to the apparatus. A common problem in 
using currently available cartridge heaters in tight fitting bore holes is 
that the heaters often seize in the bore holes after operation. It is 
believed that seizure of the cartridge heater in the bore hole is caused 
by oxidation between the stainless steel casing of the cartridge heater 
and the metal of the bore hole. Seizure of the cartridge heater in the 
bore hole may also be caused by warping of the cartridge heater in the 
bore hole. A seized cartridge must be driven out of the hole or in some 
cases drilled out. The heater is usually destroyed in this process and the 
equipment in which the cartridge heater was seized may be damaged in 
removing the cartridge. 
Attempts have been made to facilitate removal of cartridge heaters from the 
bore holes in which they have been used. Liquid lubricants, such as oils, 
greases and solutions of magnesium and water have been applied to the 
stainless steel casings of cartridge heaters prior to insertion into a 
bore hole to facilitate later removal. Although liquid lubricants are 
commonly used they prove to be inadequate. 
If liquid lubricants are to be used, the user must purchase the cartridge 
heaters and the lubricants separately. The user must apply the lubricant 
themselves which can be a messy operation and if not properly supervised 
can be done inadequately. Due to the close fit between the cartridge 
heater and the bore hole, insertion of the cartridge heater in the bore 
hole often results in the liquid lubricant being forced away from the 
surface of the cartridge heater, exposing the surface of the cartridge 
heater to the metal defining the bore hole such that the metal to metal 
contact would be subject to oxidation. Also, once the lubricated cartridge 
heater is inserted in the bore hole, the liquid lubricant would tend to 
migrate toward low spots exposing the metal surface of portions of the 
cartridge heater to direct contact with the metal defining the bore hole 
so as to allow oxidation. 
SUMMARY OF THE INVENTION 
The present invention comprises a cartridge heater having a solid lubricant 
bonded to an outer surface of the cartridge heater. The cartridge heater 
is typically cylindrical and comprises a resistance wire helically wound 
around a ceramic core or spirally threaded through the ceramic core and 
then enclosed in a stainless steel casing. A dielectric material separates 
the resistance wire from direct contact with the stainless steel casing. 
The cartridge heater is adapted to be inserted in a bore hole of an 
apparatus or piece of equipment to be heated. 
The solid lubricant is formulated with a resin binder and a solvent and 
then applied to the cartridge heater casing in a thin layer or coating. 
The coating may be applied by spraying, dipping, brushing or by other 
suitable means. Prior to application of the coating the surface of the 
cartridge heater is preferably treated by cleaning, degreasing, sand 
blasting or grinding to improve adherence of the coating to the surface. 
After the coating is applied to the cartridge heater casing, the coating 
is cured by baking. 
The cartridge heaters with the solid lubricant bonded thereon are delivered 
to the user ready for use. The user does not have to pretreat the 
cartridge heater with a messy lubricant prior to insertion in the selected 
bore hole. Once the cartridge heater is inserted in the bore hole, the 
coating of the solid lubricant separates the metal surfaces of the 
cartridge heater casing and the bore hole so as to prevent oxidation and 
reduces friction for ease in removal or release of the cartridge heater 
from the bore hole. Typically a cartridge heater is only removed when it 
fails. 
OBJECTS AND ADVANTAGES OF THE INVENTION 
Therefore, it is an object of the present invention to provide a cartridge 
heater that is easily removable from a bore hole in which it has been 
used; to provide such a cartridge heater that does not seize within the 
bore hole; to provide such a cartridge that does not oxidize within the 
bore hole; to provide such a cartridge heater having a lubricant bonded 
thereto; to provide such a lubricated cartridge heater that is readily 
packageable and transportable; to provide such a lubricated cartridge 
heater wherein the lubricant retains its lubricating properties even when 
the cartridge heater operates at temperatures up to 1200.degree. 
Fahrenheit. 
It is a further object of the present invention to provide a formulation 
including a lubricant that is readily bound to a cartridge heater; to 
provide such a formulation having an aqueous solvent; to provide such a 
formulation that may be applied to said cartridge heater by spraying 
dipping or the like; to provide such a formulation which may be applied to 
a cartridge heater in a relatively thin coat; to provide such a 
formulation that allows easy removal of the cartridge heater from a bore 
hole in which it has been used; to provide such a formulation which 
prevents oxidation between the metal of the cartridge heater and the metal 
defining the bore hole; to provide such a formulation which is relatively 
inexpensive to make, easy to apply, and particularly well adapted for its 
intended usages thereof. 
Other objects and advantages of this invention will become apparent from 
the following description taken in conjunction with the accompanying 
drawings wherein are set forth, by way of illustration and example, 
certain embodiments of this invention. 
The drawings constitute a part of this specification and include exemplary 
embodiments of the present invention and illustrate various objects and 
features thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As required, detailed embodiments of the present invention are disclosed 
herein; however, it is to be understood that the disclosed embodiments are 
merely exemplary of the invention, which may be embodied in various forms. 
Therefore, specific structural and functional details disclosed herein are 
not to be interpreted as limiting, but merely as a basis for the claims 
and as a representative basis for teaching one skilled in the art to 
variously employ the present invention in virtually any appropriately 
detailed structure. 
Referring to the drawings in more detail the reference numeral 1 generally 
represents a coated cartridge heater. The coated cartridge heater 
comprises an electrical resistance heating means such as a helically wound 
resistance wire or heating conductor 5 enclosed in a cylindrical sheath or 
casing 6 having an outer surface 7. The resistance wire 5 is wound about a 
core 9 which is made of an insulating material, particularly ceramic 
material. Ends of the resistance wire 5 are connected to electrical leads 
10 and 11 by electrical connection means such as connector pins 15 and 16 
which extend through bores 17 and 18 in the core 9. The leads 10 and 11 
extend out one end of the sheath 6. 
The helically wound resistance wire 5 is preferably made from a nickel 
chromium alloy but other alloys may be used. A layer of dielectric 
material 20 such as high purity magnesium oxide grain insulates the thin 
space between the helically wound resistance wire 5 and the cylindrical 
sheath 6. During assembly of the coated cartridge heater 1, the 
cylindrical sheath 6 can be swaged so as to compress the layer of 
dielectric material 20 to provide optimum thermal transfer and electrical 
resistivity. 
After the cylindrical sheath 6 has been swaged, a thin coating 25 of a 
lubricating formulation is applied to the cylindrical sheath 6 so as to 
form a dry film. The lubricating formulation generally comprises a solid 
lubricant, a binder or binding medium such as a resin binder and a 
solvent, preferably aqueous based. The components of the lubricating 
formulation are mixed together to form an emulsion and then applied to the 
cylindrical sheath 6 of the coated cartridge heater 1. 
The preferred solid lubricant is molybdenum disulfide. Molybdenum disulfide 
has a melting point of 1185.degree. Centigrade. It is believed that solid 
molybdenum disulfide retains its lubricating properties up to its melting 
point. Other potential solid lubricants for use in the lubricating 
formulation include fluoropolymers, ceramic materials, oxides and mineral 
powders such as graphite. Preferred fluoropolymers include 
tetrafluoroehtylene or polytetrafluoroethylene and fluorinated ethylene 
proylene It is foreseeable that the lubricating formulation may include 
more than one of the solid lubricants discussed above. It is foreseeable 
that other solid lubricants may be used in the lubricating formulation. 
Preferred resin binders that are compatible with an aqueous solvent include 
polyphenylene sulfide and polyimide. Other resin binders that may be used 
include polyamideimide, polyphenylene sulfide, polyvinylidene fluoride, 
and polyacrylate or alkyd binders. It is foreseeable that for high 
operating temperatures of the cartridge heater 1, the resin binder may be 
replaced by an inorganic binder, such as ceramic based materials. The 
binders provide cohesive forces that hold particles of solid lubricant 
together and also provide adhesive forces that bond the coating to the 
outer surface 7 of the cylindrical sheath 6. Above a maximum operating 
temperature for each binder, the binder apparently destabilizes breaking 
the cohesive and adhesive forces or bonds. 
Prior to application of the lubricating formulation, the outer surface 7 of 
the cylindrical sheath 6 is treated to ensure proper bonding of the solid 
lubricant to the outer surface 7 of the cylindrical sheath 6. The outer 
surface 7 of the cylindrical sheath 6 is cleaned and degreased. The outer 
surface 7 may also be sandblasted, ground, or treated by other means to 
provide a rough finish. 
The lubricating formulation is preferably applied to the outer surface 7 of 
the cylindrical sheath 6 by spraying and in particular by an electrostatic 
spraying process. However, it is foreseeable that the lubricating 
formulation may be applied by other means such as dipping or brushing. 
After the lubricating formulation has been applied to the outer surface 7 
of the cylindrical sheath 6, the coated cartridge heater 1 is baked at 
approximately 300.degree. to 400.degree. Fahrenheit for 10 to 15 minutes 
so as to partially cure the coating 25 of the lubricating formulation. 
Baking drives off the solvent and sets the binder such that the solid 
lubricant is bonded to the surface 7 of the cylindrical sheath 6. The 
coating 25, after it has been partially cured, has a thickness of 
approximately one one-thousandth of an inch. It is foreseen that the 
thickness of the coating 25 may vary. The coating 25 is fully cured during 
use when the coated cartridge heater 1 operates at a temperature which 
typically exceeds 400.degree. Fahrenheit. The coating 25 is only partially 
cured initially to avoid damage or discoloration to exposed leads 10 and 
11. 
A coated cartridge heater 1 having solid lubricant bonded thereon is 
readily packageable and transportable because the bonded solid lubricant 
is not readily brushed off or removable from the outer surface 7 of the 
cylindrical sheath 6. Upon receipt, a user may insert the coated cartridge 
heater 1, having solid lubricant bonded thereon, directly into a bore hole 
30 of an apparatus or piece of equipment 31 (not shown) adapted to receive 
the coated cartridge heater 1. The user does not have to lubricate the 
coated cartridge heater 1. Insertion of the cartridge heater 1 generally 
will not dislodge the coating 25 from the outer surface 7 of the 
cylindrical sheath 6. Also, storing the coated cartridge heater 1 for an 
extended period of time will not cause the coating to flake off. 
In use the coated cartridge heaters 1 may operate at temperatures of up to 
1200.degree. Fahrenheit, however most coated cartridge heaters 1 operate 
at temperatures from 400.degree. to 900.degree. Fahrenheit. The operating 
temperatures for the cartridge heaters 1 typically exceed the maximum 
operating temperature for the binders. The coated cartridge heaters 1 are 
typically removed from the bore holes 30 they are used in only when the 
heaters 1 fail. Upon removal of the coated cartridge heaters 1, it is 
observed that during use, the coating 25 typically destabilizes, degrades 
or breaks down into a flaky skin-like shell or a powdery form. However, 
the coating 25 in the flaky or powdery form continues to exhibit 
lubricating properties of the solid lubricant and functions as a release 
means for providing ease in removal from the bore hole 30. Because the 
coated cartridge heater 1 generally remains stationary after insertion in 
a piece of equipment, the coating 25 in the flaky or powdery form does not 
appear to migrate, therefore the entire outer surface 7 of the cylindrical 
sheath 6 remains lubricated allowing easy removal or release of the 
cartridge heater 1 from the respective bore hole 30. 
It is also believed that the coating 25 prevents oxidation between the 
stainless steel of the cylindrical sheath 6 and the metal of the apparatus 
or equipment 31 defining the bore hole 30 into which the coated cartridge 
heater 1 is to be inserted. Even when the coating 25 degrades into a flaky 
or powdery form, the coating 25 still apparently prevents such oxidation. 
The prevention of oxidation by the coating 25 contributes to the ease of 
removal of the cartridge heater 1 from the respective bore hole 30. 
It is to be understood that while certain forms of the present invention 
have been illustrated and described herein, it is not to be limited to the 
specific forms or arrangement of parts described and shown.