Process for increasing torque generated by a clutch

Method for increasing the torque generating capabilities of a clutch are disclosed including the application of a film lubricant composition onto the engagement surfaces of a clutch. The composition imparts anti-galling properties to engaging surfaces of metallic pieces such as brakes or clutches.

FIELD OF THE INVENTION 
This invention relates to methods of increasing the torque between surfaces 
such as that produced by a clutch or brake without increasing the size of 
them. 
BACKGROUND ART 
When mating surfaces contact each other, galling may occur, i.e., there is 
a wearing away of the surface. It is most desirable to utilize techniques 
which utilize anti-galling compositions which control the friction between 
the surfaces and the wearing away of the surfaces. 
A clutch is a releasable coupling connecting the adjacent ends of two 
coaxial shafts. It is said to be engaged or, in, when the shafts are 
coupled, and disengaged, or out, when they are released. A clutch 
functions to selectively transfer torque from a driving member (input 
shaft) to a driven member (output shaft). 
Clutches are often characterized by the technique utilized to couple the 
input shaft to the output shaft and/or by the method of actuation. 
Coupling may be achieved by friction, mechanical engagement, an 
electromagnetic field, or a combination thereof. The coupling technique 
often influences the method of actuation which may include mechanical, 
electric, pneumatic, hydraulic or self-actuation, among others. Thus, 
electromagnetic clutches are typically electrically actuated due to the 
nature of the electromagnetic field which couples the driving member to 
the driven member. 
Electromagnetic clutches are primarily utilized in applications requiring 
variable slip between the input shaft and the output shaft. Rather than 
using only an electromagnetic field to couple the input shaft to the 
output shaft, some electromagnetic clutches utilize the attractive force 
created by the electromagnetic fields to generate a frictional force which 
couples the driving member to the driven member. These clutches are used 
in a wide variety of diverse applications including various mobile 
hydraulic systems such as those found on fishing boats, farm machinery, 
fire trucks, aerial lifts, and mining equipment, among others. Automotive 
applications utilize this type of clutch to couple accessories, such as an 
air-conditioning compressor, to the engine. Regardless of the particular 
type, electromagnetic clutches have similar principles of operation. 
In a vehicle, the basic friction type clutch comprises two discs, one, an 
engine flywheel and the other, generally the lighter of the two, the 
presser or pressure plate. The flywheel is bolted to a flange on the end 
of the crank shaft, while the other plate slides axially on the output 
shaft, except in as much as a spring or springs tending to press it 
against the flywheel. Such a clutch is engaged by its spring or springs 
and disengaged by a pedal-actuated linkage under the control of the 
driver. 
Of particular interest in the present case is the environment of a transfer 
case used in a modern four-wheel drive vehicle. In that environment, the 
metallic surfaces are present in an oily hot environment and there is a 
need for rapid, immediate and repeated engagement of the mating surfaces, 
e.g., clutch surfaces. Such oils are heavy aromatic oils having a high 
boiling point or synthetic transmission fluids such as Dexron or Mercon 
automatic transmission fluid and the like. 
Of particular interest, is the method used to generate torque. It is 
important to mention that there are two types of torque: 1) static torque, 
the "breakaway" torque required to slip a locked-up clutch; and 2) dynamic 
torque, which is applied during the period when the surfaces are sliding 
into engagement. 
The present invention is particularly useful in a dynamic torque 
environment such as that present in a transfer case of a vehicle. 
Generally there are three factors that determine the torque capacity of a 
clutch: 1) attractive engagement force between surfaces; 2) coefficient of 
friction; and 3) mean radius of the rubbing or friction faces. 
If the attractive force that moves the clutch into engagement exerts a 
force P normal to the discs, that is parallel to their common axis, the 
frictional force tending to prevent slip is .mu. times P where .mu. is the 
coefficient of friction of the rubbing faces. This frictional force is the 
sum of all of the small forces, each of which is acting at one of the 
almost infinite number of contact points between the friction faces of the 
discs. 
The resultant friction force is generally taken as acting tangentially at 
the mean radius, R, of the friction faces which, if they are lined, is 
usually a relatively narrow annular strip, or ring of pads, of the 
material having a high coefficient of friction. Consequently, the torque, 
Q, that is F.times.R, about the axis of the plates can be expressed most 
simply as .mu..times.P.times.R. 
Initially the axial force pressing the disks together is too small for the 
tangential frictional force to overcome the resistances to rotation of the 
driven shift, so there is slip at the friction faces, the relative 
velocity between which reduces until it becomes zero at the instant of 
full engagement. 
While the maximum torque a clutch can transmit is dependent upon the mean 
radius of the annulus of friction material, its rate of wear is determined 
by the area of that annulus. Consequently, although it is desirable to 
make both the inside and outside diameters of the annulus as large as 
practicable, if the inside diameter is too large, the area of the annulus 
will be too small to obtain an acceptable rate of wear. 
The force between surfaces and contacts, force of friction, depends upon 
several factors. The first is the materials of which the surfaces are 
composed. The second factor is the condition of the surfaces, whether 
rough, smooth or polished, clean or dirty, dry or oily. The third factor 
is the force acting between the surfaces and contact, but it is only the 
force perpendicular to the surface that affects the friction. This 
perpendicular force between the surfaces is called the normal force. 
The frictional force may be expressed in terms of the normal force, and a 
quantity that depends upon the kind and condition of the surfaces, the 
coefficient of friction, .mu.. Thus F (force of friction) equals .mu. 
(coefficient of friction) times P (normal force). These mechanical 
formulas demonstrate the relationship between the coefficient of friction 
and the torque produced by a clutch. As per the mathematical relationship, 
the torque capacity of a clutch can be increased by decreasing the 
coefficient of friction between the engaging surfaces or discs of the 
clutch. 
One can decrease the coefficient of friction by utilizing an anti-galling 
composition, thereby decreasing excessive wear of the engaging surfaces. 
Increasing the torque transmitting capabilities of a clutch has been a 
subject of indepth experimentation and testing as a result of the demand 
for higher torque production. Moreover, an increase in torque could 
possibly prevent clutch slipping problems, as the clutch would not slip if 
it transmitted more torque than its load. 
In the past, the following methods have been applied to the engagement 
surfaces in an unsuccessful effort to increase the dynamic torque, (1) 
concave, (2) machine turned, (3) plasma coating with a thin hard metal, 
(4) nitriding, (5) ion nitriding, (6) Teflon (trademark of DuPont for 
(polytetrafluoroethylene) coating, (7) copper infiltrate, (8) heat 
treating methods, (9) surface finish, (10) surface patterns. In addition, 
the following design methods have also been attempted: (1) including a 
larger diameter coil assembly winding wire, (2) increased magnetic flow 
areas, (3) multiple 6 pole clutch, (4) multiple disk clutch, (5) increase 
the envelope, diameter and length of the clutch assembly, (6) powdered 
metal housing and coil ring, (7) reduced tolerances and clearance between 
magnetic flux carriers, (8) a step in the face of the engagement surface. 
Lubricants have been suggested as a means of reducing the coefficient of 
friction and thus increasing the torque produced by a clutch. A lubricant 
primarily serves two purposes: one, to reduce the resistance to relative 
motion between two surfaces and contact under pressure and two, to reduce 
wear of the surfaces. 
In the absence of a lubricant, the coefficient of friction is high and 
fairly constant, and depends on the materials and fineness of finish, 
since there is actual contact between the two engaging surfaces. In 
contrast, with "boundary friction", some lubricant is present between the 
two engaging surfaces but not sufficient to completely separate the 
surfaces. Under boundary friction, the coefficient of friction is reduced, 
but the resistance to motion is still dependent on the load and the nature 
of the surfaces. 
A third type of friction is fluid friction, where the lubrication is 
force-fed such that it is possible for the engaging surfaces to be 
completely separated by the lubricant. In these circumstances the 
resistance to movement is caused by the viscosity, or resistance to 
shearing of the fluid itself. With most clutch surfaces, the friction 
conditions fall between boundary and fluid conditions. 
While oils are generally used as a lubricating fluid, oils differ 
considerably in their lubricating quality depending on the surrounding 
conditions. Animal and vegetable oils, such as sperm, rape and castor, are 
somewhat superior to mineral oils in the maintenance of a film at high 
pressures and low speeds. Oils, however, suffer from the tendency to gum 
at high temperatures. 
Another problem associated with the use of oil as a lubricant, is that the 
viscosity of the oil itself is a source of resistance, which in certain 
circumstances may make the total resistance with lubrication greater than 
without any lubrication. Moreover, in order for the resistance to motion, 
between the engaging surfaces, to be minimized, the oil film should have 
considerable thickness while having a low viscosity. These conditions are 
mutually exclusive, since it is not possible to confine a thin oil in a 
manner to maintain a thick film. In general, oil lubricants have several 
shortcomings when used for clutch applications. 
There is accordingly no commercially viable method for decreasing the 
coefficient of friction of a clutch such that it is capable of producing 
an increase in torque. While the use of oil lubricants has proved useful, 
there still remains a need for a process which effectively increases the 
torque capability of a clutch without having to increase the size of the 
clutch. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide a process for 
increasing the level of dynamic torque generated by engaging surfaces such 
as a clutch or brake through the use of an ionic film. 
Another object of the present invention is to provide a clutch assembly 
that generates increased levels of torque and which does not require a 
corresponding increase in the size of the clutch. 
A further object of the present invention is to provide a process which 
increases the wear resistance of the engagement surfaces of a clutch using 
a film lubricant composition. 
A still further object of the present invention is to provide a process for 
lowering the coefficient of friction of the engaging surfaces of a clutch 
such that it is capable of producing an increase in torque. 
It is a further object of the invention to provide a process for increasing 
the level of dynamic torque generated by engaging surfaces such as a 
clutch or brake by applying an anti-galling composition to the surfaces, 
particularly those present in a transfer case of a vehicle. 
In carrying out the above objects and other objects and features of the 
present invention, a process is provided for increasing the torque 
generating capabilities of a clutch by applying an ionic film lubricant 
composition onto the engagement surfaces of the clutch which acts to 
reduce the coefficient of friction between the engagement surfaces. 
The above objects and other objects, features and advantages of the present 
invention will be readily appreciated by one of ordinary skill in the art 
from the following detailed description of the best mode for carrying out 
the invention when taken in connection with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
The disclosed film lubricant composition reduces the coefficient of 
friction between the engaging surfaces of the clutch and thereby increases 
the torque production capabilities for a particular clutch. More 
specifically, an ionic film solution is applied to the engaging surfaces 
to effectuate a reduction in the coefficient of friction and improve 
resistance to corrosion, galling, wear and stress corrosion cracking 
between the two engaging surfaces. 
It is preferred that the film lubricant composition include a sulfate, an 
alkali metal silicate and a phosphate. The alkali metal silicate can be 
chosen from the following: sodium silicate, potassium silicate and lithium 
silicate. The film lubricant composition also preferably includes 
nitrogen, water, and acetic acid. 
Although not wishing to be bound by any particular theory, it is believed 
that the silicates and sulfates when used within the film lubricant 
composition and applied onto the engagement surfaces of the clutch, may 
form a glassy, greasy lubricating surface which greatly lowers the 
coefficient of friction. 
It is also believed, that as the temperature increases, a glassy film forms 
on the surface of the engagement surface which aids in keeping the 
engagement surface and edges smooth and hard, increasing the wear 
resistance of the engagement surfaces. The addition of alkali metal 
acetate within the film lubricant composition acts to buffer the surfaces 
and neutralize excess alkalinity. As temperature increases during the 
generation of friction, the acetate slowly decomposes forming acetic acid, 
carbon dioxide, and water which acts to lower the surface temperature and 
assist to some degree in the glassy surface formation with the silicates. 
The film lubricant composition may further, include phosphates which acts 
as a surfactant and corrosion inhibiter, forming a film of ferrous 
phosphate. The film lubricant composition may also include water soluble 
polymeric materials which are believed to interact to increase lubricity 
and strengthen the coating on the engagement surfaces. While a variety of 
polymeric materials may be used, such as acrylates, epoxies or polyamides, 
most preferred are acrylates and even more preferably 
polyacrylate/polyacrylamide copolymer. All of the plastic materials 
preferably should be stable at the operating environmental temperatures. 
A preferred film lubricant is sold by the trade name Tool-Tuff 101, a 
water-based ionic solution that is a product of Royal Purple Synthetic 
Lubricants, Inc. Although Tool-Tuff was formulated for use on cutting 
tools, this lubricant works effectively on clutch engagement surfaces to 
reduce the coefficient of friction. 
In general, it is believed that the film lubricant composition impregnates 
voids within the engagement surfaces of the clutch, providing a continuous 
lubricating film between the engagement surfaces. FIG. 1 and FIG. 2 depict 
the untreated and treated lock-up collar-clutch discs that were sectioned 
and prepared from a metallographic evaluation that was conducted. The 
clutch discs that were treated included an application of a film 
lubricant, specifically Tool-Tuff 101. In FIG. 1, the dark area represents 
pores which resulted from the powdered metallurgy experiments conducted on 
the untreated lock-up collar-clutch disks. In FIG. 2, the same powder 
metallurgical experimentation depicts a significant reduction in the pores 
present within the treated lock-up collar-clutch disk in an absence of any 
measurable coating at this magnification. 
Preferred compositions of anti-galling materials are depicted in Table 1. 
The pH is preferably alkaline (10-14), preferably 12-13, the composition 
preferably has a specific gravity of 1.31. 
TABLE 1 
______________________________________ 
ANTI-GALLING COMPOSITION 
MATERIALS RANGE (% by wt.) 
PREFERRED (% wt.) 
______________________________________ 
Alkali-hydroxide 
5-20% 10% 
Alkali sulfate 
0.5-5% 1% 
Alkali phosphate 
5-20% 12-14% 
Alkali silicate 
5-20% 13-15% 
Polymeric water soluble 
5-15% 9-10% 
material 
Water 40-60% 51-53% 
TOTAL 100% 100% 
______________________________________ 
Much experimentation has been conducted to increase the dynamic torque 
above previous levels as is commonly known in the clutch design and 
manufacturing industry. If one compares dry metal to metal surfaces with 
surfaces saturated with oil, the torque produced with dry surfaces is 
reduced up to 25%. This reduction of torque is due to the increase in 
coefficient of friction with the dry surfaces. However, previously known 
lubricants have several shortcomings. Accordingly, the present invention 
provides a method to increase dynamic torque without increasing the clutch 
size, that is, diametric and overall length of the clutch. 
One method to solve this problem is to eliminate or reduce substantially 
the galling or rough surface created by a full slip of the clutch which 
further reduces the coefficient of friction and increases the generated 
torque. Application of the disclosed film lubricant composition greatly 
improves the torque generating capabilities and prolongs the increase in 
torque over a longer period of time. 
The primary benefit of applying the disclosed film lubricant composition is 
that the film lubricant composition can be easily applied to the 
engagement surfaces of the clutch at a very low cost. 
It is preferable that the film lubricant composition is applied to the 
engaging surfaces typically by brushing, spraying, dipping or rubbing by 
hand. 
The engagement surfaces are preferably clean before application of the 
coating thereon. Accordingly, the preferred method of application includes 
cleaning at least one of the engagement surfaces of the clutch followed by 
applying the film lubricant composition onto the engagement surfaces of 
the clutch. The engagement surfaces are preferably cleaned with a cleanser 
selected from the group of cleansers consisting of soap, oil free 
degreasers and oil free solvents. 
Another preferred variation of the method includes cleaning at least one of 
the engagement surfaces followed by heating at least one of the engagement 
surfaces to approximately 200.degree. F. and then applying the film 
lubricant composition thereon. Before the engagement surfaces are actually 
brought into engagement it is preferred that the surfaces are 
appropriately cooled. 
Additionally, the preferred process includes engaging the engagement 
surfaces and applying coolant on the engagement surfaces. The coolant is 
preferably applied at least one minute after the surfaces are engaged. 
Another preferred method includes: washing the engagement surfaces of the 
clutch; drying the engagement surfaces; submerging the engagement surfaces 
in the film lubricant composition; cooling the engagement surfaces; and 
drying the engagement surfaces. 
It should be understood that while the forms of the invention herein shown 
and described include the best mode contemplated for carrying out the 
invention, they are not intended to illustrate all possible forms thereof. 
It will also be understood that the words used are descriptive rather than 
limiting, and that various changes may be made without departing from the 
spirit and scope of the invention discussed.