The Effect of Lubrication
This report focuses on the design of a laboratory and its operation
for the evaluation of lubricants for fine wire drawing. Lubrication is
used mainly to reduce the resistance to sliding between the workpiece (the
wire) and the tool (the die). The reduction in resistance manifests itself
in several ways, among which are the following:
- Reduced drawing force due to reduced values of the coefficient of friction
- Reduced wear on the die
- Reduced surface temperature on the die and on the wire
- Altered appearance of the wire surface
- Improved drawability, deterred wire tearing, etc.
These and other effects are presented in Ref. [1], and in Chapter (3)
of Ref. [2]. Each one of these factors can be measured and serve as a criterion
for the evaluation of, and for the comparison among, lubricants. By any
criterion there is no ideal lubricant or single lubricant that is superior
to all others for all applications. For example, a lubricant that is best
for the drawing of steel wire with a carbide die may be a poor choice for
the drawing of copper wire, or even for steel wire with a diamond die.
Furthermore, even for an identical set of workpiece and tool, the lubricant
performing best during wire drawing may not be the best, and may even prove
very poor, for other processes such as rolling. Lubricants for large diameter
wire differ from those recommended for fine wire, etc. Lubricant evaluation
must be performed under conditions that are as close to actual production
conditions as possible. There are good reasons for
evaluating lubricants on production equipment, during production runs.
There are equally compelling reasons for the evaluation to be made under
controlled laboratory conditions with highly instrumented equipment. For
example, quantitative determination of friction value is made best with
highly instrumented equipment in the laboratory, while studies of wear
rates are delegated to the production floor during actual manufacturing
runs. We next study the selection of the evaluation method and the equipment
to be used.
The selection of the lubricant depends on other factors such as price,
toxicity, safety and residual film advantages and shortcomings.
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Schematic of the Draw Bench
The main tool for our lubricant evaluation laboratory is a highly instrumented
fine wire drawing machine, used to measure friction resistance by reading
the required drawing force. Fig. <1> displays a Photo and a schematic
of the five Modules of the draw bench. The sensors' analog readings of
the speed and drawing force are converted to digitized data and fed into
the computer. Drawing speed is programmed to increase gradually. The results
of each test are stored as a file in the computer. The data can then be
analyzed, manipulated and presented as an output in graphical or tabular
form, on the screen or as a hard copy printout.
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PHOTOGRAPH [
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Criteria for the Evaluation
The drawing tests provide data for the evaluation of, and comparisons
among, lubricants. Some evaluations can be made with simple straightforward
runs and minimal analysis. For example drawing the same wire under identical
conditions, but with two different lubricants, while reading the drawing
force, will indicate which lubricant reduces friction better. To determine
the effectiveness of the lubricant in reducing wear longer runs and periodic
monitoring of the die surface and contamination of the lubricant are required.
The criteria are listed above in the section entitled: "Effect
of Lubrication."
In Ref. [1] the study of flow through conical converging dies is presented,
showing the effect of processing parameters on the drawing force and drawing
stress. Specifically, Fig. 37 of Ref. [1] shows the characteristics of
the drawing stress as a function of die angle and reduction.
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Foreword
We first define the following procedures, namely:
- Qualitative comparisons based on measurement of the drawing force,
and
- Elaborate quantitative determination of the value of the friction factor
(m)
Both procedures depend on the use of a highly instrumented, computer
controlled, draw bench. These procedures are designed to distinguish between
good lubricants of comparable qualities. The differences between the friction
factors are not dramatic. These procedures fine tune the evaluation and
comparison process.
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Qualitative Comparisons of Lubricants
This procedure is the least ambitious, nevertheless it provides a basis
for comparison between lubricants. We draw wire of a chosen size at several
chosen reductions. Each spool is drawn at continuously increasing speeds
while the data on speed and drawing force are collected and stored in a
file. Alternate lubricants can be tested. Graphs of typical outputs are
presented in Figs. <2>.
In Figs. <2> the abscissa is the drawing speed, the ordinate is
the drawing stress, and the parameter is the lubricant. While no value
of friction is estimated, lubricant A is deemed to be more effective than
lubricant B in reducing friction resistance to sliding. To better understand
the friction phenomenon, and to enhance the confidence in the data, variations
in wire size, reduction, or even die angles can be explored.
Figure <2a>, represents results obtained from the drawing of moderate
to large diameter wire. An increase in speed results in lower drawing stress,
suggesting lower friction resistance to sliding at higher speeds. For ultrafine
wire (Fig. <2b>), the drawing stress increases with increasing speed,
mainly because of an increase in the back tension at higher speeds, as
described in Ref. [3].
The qualitative procedure for the evaluation of lubricants is useful
for wire of any size. This procedure requires the least amount of initial
expenditure, can promptly be implemented and is recommended as the first
step towards the establishment of a lubricant evaluation laboratory.
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Quantitative Determination of Friction
Values
To obtain an absolute quantitative value for friction, the value of
the friction factor m can be determined experimentally.
The friction factor m is treated as an input parameter,
which is determined by the effectiveness of the lubricant used, but is
also a function of the wire and die material and surface finish. Friction
is a complex factor and it can only be measured indirectly. Unlike wire
size, reduction or die angle, there is no tool (figuratively dubbed "frictio-meter")
that can provide readings for the value of m. However,
there is a direct dependence of the optimal semi-die cone angle 
opt on the value
of friction m , as described at the bottom left corner
of Fig. 3.34 of of Ref. [2]. This relation is
opt
[ (3/2) m ln( Ro / Rf ) ]1/2
Eq. (1a)
We can determine experimentally all the parameters of Eq. (1a) except
the value of m . We therefore adopt a procedure to determine
opt
experimentally and calculate the value of m
from the following expression for m as a function of reduction
and optimal angle opt.
m (2/3) 2opt / ln( Ro / Rf
) Eq. (1b)
See Eq. 3.4b of Ref. [2].
For this procedure we need a set of dies of identical size Rf
, but of semi-die angles varying from small to large.
Running identical reductions through a set of dies of varying die angles
will provide a plot of the characteristics of Fig. <3>.
In the hypothetical Fig. <3> the abscissa is the die angle, and
the ordinate is the drawing force. Experimental data points of the drawing
force for several dies of increasing die angles (denoted by *) are presented
and the best fit curve is plotted through them. The drawing force for very
small die angles is excessive due to the excessive length of contact between
the die and the wire, leading to high friction power losses. With increasing
die angles the length of contact shortens and the friction power losses
subside, causing a lower power loss. With very large die angles the length
of contact and the friction power losses diminish. However, distortion
and its related power losses, (also called shear or redundant power losses)
increase dramatically and cause the resumption of an increase in drawing
force after reaching a minimum. The angle that minimizes the total power
is called the optimal semi cone die angle opt . The value of the friction
factor m is calculated by Eq. (1b).
In Fig. <4>, reproduced from Fig. 3.19 of Ref. [2], the drawing
stress is presented as a function of die angle for a selection of reductions.
For different reductions the drawing force curves and the optimal die angles
are different. Curves for larger reductions are higher than those for smaller
reductions, exhibiting higher values for the optimal die angle.
The calculated differences in the values of the friction factor
m may be very little. Graphs for data from runs at different
speeds may provide the friction values as a function of speed.
Quantitative determination of the friction values m requires
a higher investment in tooling and it consumes more time for experimental
data acquisition. The pay-off is provided in the form of a numerical scale
for friction. One obstacle for the use of this procedure for very small
wire sizes, lies in the difficulty of producing a true conically shaped
die and measuring the die angle with precision.
Lower friction leads to lower drawing force and thus allows larger reductions
per pass without tearing. There are other beneficial effects that need
to be considered when evaluating or comparing lubricants. Lubricants can
be graded through their effect on die wear, wire surface damage, etc.
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Schematic Description of the Modules
Figure <1> presents a description of a lubricant evaluation draw
bench for fine wire drawing. The equipment is described as an assembly
of five major distinct modules.
Module #1, the central module, is the frame and bath
combination of the die and die holder. The wire and the die are immersed
in the bath containing the lubricant. Module #1 may also include a pump
to circulate the lubricant, a filter to clean the lubricant, and a temperature
control system.
Module #2 contains the pay-off spool which feeds the
wire into the drawing die.
Module #3 is the tensiometer, a standard sensor that
measures the tensile load on the emerging wire. All three rolls are idling
rolls, each is mounted on its own shaft with a low friction bearings. The
shaft of the central roll is free to move vertically. The vertical displacement
of the center roll is measured by a potentiometer and converted to digital
form by the data acquisition board, then it is presented on the computer
screen as a function of the drawing speed of the motor in module 4.
Module #4 contains the entire spool pick-up system.
The spool is mounted directly on the shaft of a 'step' motor that provides
the moment (and force) to draw the wire. The speed of the motor is controlled
through a signal from the computer, as provided by the operator. The speed
is programmed to rise monotonously up to a predetermined peak speed. The
spool pick-up motor is mounted on the transverse table that can move horizontally
parallel to the axis of symmetry of the pick-up spool. The transverse motion
table is driven by the transverse motion motor, whose speed is also controlled
by the computer. Limit switches reverse the direction of movement of the
transverse table automatically.
Module #5 comprises the computer control system, and
includes data collection, analysis, and display. The speed of both motors
and the measured tension are recorded into a file together with other pertinent
information for each run. Each run is fully controlled through the computer.
Data from each file alone or from several files together can be manipulated
through the computer, analyzed, saved and displayed in tabular and graphical
forms.
Module #6 (Optional) Comprises a lubricant circulation, filtration
and Temperature control. For various uses the system design may vary. This
module is not presented in the schematic of Fig. <1>.
Force and Power Specifications for the
Selection of the Hardware
To assist in making selections of the equipment size the following calculations
are helpful.
Drawing Force:
F = -
Rf2
o ln[ 1 - r% / 100]
Eq.(2)
Power Consumption:
W = F vf = -Rf2
o vf ln[ 1 - r% / 100]
Eq. (3)
where:
Rf, Ro are the final and original
radii of the wire
r% is percent reduction in area, r% = [ 1-
( Rf / Ro )2 ] *100
vf is the exit velocity of the wire
o is the flow strength
of the material of the wire
Please note that the drawing force, as estimated by Eq. (2), is independent
of the velocity because friction losses are ignored.
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