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Lubricant Evaluation Laboratory
for Fine Wire Drawing

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Lubricant Evaluation Laboratory for Fine Wire Drawing

Printed as ref. #188 of the publications listing

by B. Avitzur, Metalforming Inc.
January 20, 1996


Table of Contents:

Background on Wire Drawing, Lubrication, Friction and Wear

  1. The Effect of Lubrication
  2. Schematic of the Draw Bench
  3. Criteria for the Evaluation

The Individual Procedures for Lubricant Evaluation

  1. Foreword
  2. Qualitative Comparisons of Lubricants
  3. Quantitative Determination of Friction Values
  4. Schematic Description of the Five Modules
  5. Force and Power Specifications for the Selection of the Hardware

Typical Case Study

References

Bibliography


 

Background on Wire Drawing, Lubrication, Friction and Wear

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:

  1. Reduced drawing force due to reduced values of the coefficient of friction
  2. Reduced wear on the die
  3. Reduced surface temperature on the die and on the wire
  4. Altered appearance of the wire surface
  5. 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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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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The Individual Procedures for Lubricant Evaluation

Foreword

We first define the following procedures, namely:

  1. Qualitative comparisons based on measurement of the drawing force, and
  2. 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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Typical Case Study

In Ref. [4], Evans and Avitzur presented a procedure for the evaluation of friction values during wire drawing, hydrostatic extrusion, and strip rolling. The procedure used Eq. (1b) to determine the value of friction as a function of experimentally determined optimal die angle. The experimental data as presented in literature by several investigator (Refs. [4]-[7]) was treated. Partial output is condensed in the following Table - 1. The complete data in Table - 1 of Ref. [4], provide several test runs at a range of different reductions for each line in the present table. The values of the coefficient of friction µ, and of the friction factor m in our table are the average of data values for a range of reductions. The STD values to the right of the values of µ and m are the "Standard deviation" values of these terms respectively.

TABLE - 1

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References

  1. Avitzur, B. "Metal Forming," Encyclopedia of Physical Science and Technology, Vol. 9, Academic Press Inc., 1992.
  2. Avitzur, B. "Handbook of Metal Forming Processes," John Wiley, New York, 1983.
  3. Styczynski, L., Shi, Jian-Jun, and Avitzur, B., "The Effect of Speed on the Individual Components of Wire Drag During Augmented Hydrostatic Extrusion of Fine Wire," Proceedings of the 1992 NSF Design and Manufacturing Systems Conf., Georgia, Inst. of Tech. Atlanta GA. Jan. 8-10, 1992, pp. 1117-1123.
  4. Evans, W. and Avitzur, B., "Measurement of Friction in Drawing Extrusion and Rolling," J. of Lub. Tech., Trans. ASME, Series F, Vol. 90, No. 1, Jan. 1968, pp. 72-80.
  5. Wistreich, J. G., "Investigation of the Mechanics of Wire Drawing," Proceedings, The Institution of Mechanical Engineers, Vol.169, 1955, pp. 654-670.
  6. Avitzur, B., "Hydrostatic Extrusion," Journal of Engineering for Industry, Trans. ASME, Series B, Vol. 87, No. 4, Nov. 1965, pp. 487-494.
  7. Pugh H. Ll. D., "Recent Developments in Cold Forging," Bullied Memorial Lectures, Vols. IIIA and IIIB, The University of Nottingham Press, 1965.

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Bibliography

  1. Bowden, F. P., and Tabor, D., "The Friction and Lubrication of Solids," Printed by Oxford University Press, Amen House, London E.C.4, First Edition 1950. 372 pages.
  2. Schey, J. A. (Editor), "Metal Deformation Processes: Friction and Lubrication," Printed by Marcel Dekker Inc., New York, 1970, 807 pages.
  3. Dowson, D., History of Tribology," Printed by Longman Group Limited, London, First published 1979, 677 pages.
  4. Blau, P. j., "Friction, Lubrication, and Wear Technology," ASM Handbook, Vol. 18, printed by the ASM International, 1992, 942 pages.

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