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New thermoplastic compounds for automotive cable liners.

Cable liners made from new thermoplastic compositions deliver higher efficiencies and longer cycle lives than cable liners made from auto industry standard materials.

Thermoplastic materials are used in control cables (such as push-pull cables) throughout automobiles and trucks, where their low friction and wear properties are crucial for performance. A typical automobile contains between 20 and 40 feet of these cables, for parts such as transmission shifts, brakes, hood and trunk releases, remote control mirrors, and accelerators.

The critical component is the antifriction liner--a tube of thermoplastic inside which the metal actuating cable slides. Liner materials include a variety resins for low temperature applications and polytetrafluoroethylene (PTFE) compounded with fillers for high temperature areas.

Cable manufacturers and end-users have expressed some dissatisfaction with the performance of these materials, and demands have increased for cables that will tolerate higher forces and still produce high actuator efficiency and long cycle life. Complex routing and higher under-the-hood temperatures have made the proper choice of antifriction liner material even more important.

This study examines the efficiency and cycle life of liners produced from two PTFE compounds that are considered to be the "high performance" industry standards and from eight thermoplastic compositions:

Industry standards:

PTFE/polyphenylene sulfide


Thermoplastic compositions:

Nylon 6/6

Polybutylene terephthalate (PBT)

Acetal copolymer

Ethylene-tetrafluoroethylene copolymer (ETFE)

ETFE/PTFE, 15% PTFE (FP-EL-4030)

ETFE/PTFE, 20% PTFE (FP-EL-4040)

ETFE/PTFE, 30% PTFE (FP-EL-4060)

PBT/PTFE (PDX-W-87470)

The goals of this research were to compare these materials under test conditions similar to actual use and harsh enough to cause failure in some of the liner materials; evaluate the effects of test load, cable tolerance, type of external lubricant, inner member construction, resin properties, and the addition of PTFE internal lubricant; identify promising new thermoplastic compositions for liner materials; and use the data to compile a database.

Testing Specifications

In the liner/member assembly, shown schematically in Fig. 1 [omitted], the metal cable inner member is the moving actuator in a finished cable. A film of external lubricant is applied to the metal before it is inserted into the liner. In a finished cable, the liner usually has a layer of metal strands wrapped around it, for strength and stiffness, and an outer jacket of thermoplastic resin.

The testing apparatus designed and built by ICI is shown in Fig. 2 [omitted]. An extruded tube of antifriction liner material is mounted over the 4-in-radius metal pulley and secured in the guides such that the contact angle is 180 degrees. A degreased metal inner cable member (lubricant added in some cases) is inserted into the liner. One end of the inner member is attached to the force transducer and the other end to the 20-lb test load.

The rotary motion of the speed-controlled motor is converted to a 1.5-in linear motion through a cam and linear bearing. The force transducer attached directly above the linear bearing measures the force required to lift the 20-lb test weight. Motor rpm [or cycles/min (cpm) of the test] is measured with an rpm gage. Another meter measures the number of cycles completed.

Liner failure is indicated by an electrical continuity circuit that ends the test by switching off the equipment when the circuit is completed. This occurs when the liner wears through, allowing the metal inner member to come in contact with the metal pulley.

The force required to lift the test load and the number of completed cycles are recorded at regular intervals. The force readings, taken at 4 cpm, are converted to percent efficiency using the equation:
 Test load
Percent efficiency = _______________ x 100
 Force to lift

For example, if the force reading is 30 lbs, the efficiency is

(20/30) X 100 = 67%

Traces of force vs. time are recorded to monitor the curve shape. "Slip-stick" behavior is also noted. Testing is continued at 60 cpm until the required number of cycles is completed (standard is 500,000) or the liner has worn through.

All liners had an inner diameter (ID) of 0.090 [+ or -] 0.003 inch and a wall thickness of 0.015 + 0.001 - 0.003 inch. The "flat-wrap" inner cable member was standard. It consisted of a "1X12" wire rope with a 0.098-in flat overwrap. A "1X7" wire rope inner member was also used. Both inner members had an outer diameter (OD) of 0.078 to 0.079 in.

The lubricants evaluated were a silicone oil type, SWS E-155; a lithium soap grease fortified with PTFE, NYE 739A; and a PTFE-thickened silicone grease, NYE 880.

All test results, unless otherwise noted, were obtained at standard test conditions-20-lb test load, "flat-wrap" inner member, and NYE 880 lubricant.

Test Load and Cable Tolerance

During test development it was necessary to determine the test load and the clearance between the cable OD and liner ID in order to minimize the dependence of efficiency upon test load throughout the test. FP-EL-4030 liners were used at 0.0075-in and 0.0040-in clearances. The initial efficiency for the larger clearance was greater and the efficiencies measured at that clearance were virtually constant over the 5.5- to 20.5-lb test load range, while the efficiencies at the 0.0040-in clearance decreased as the test load increased. Accordingly, a 0.008-in minimum clearance was established for all liner testing.

The efficiency/cycle life of industry standard PTFE/PPS liners was evaluated at both 10- and 20-lb test loads. While the initial efficiencies were similar, as the test proceeded to 100,000 cycles and beyond, the 20-lb test load produced higher efficiencies. As expected, the lower test load produced longer cycle life. However, neither test lasted beyond 400,000 cycles, so in both cases the liner material showed poor cycle life.

External Lubricant

The three lubricants were tested with each of the ten liner materials. NYE 880, the silicone/PTFE grease, consistently yielded greater cable efficiencies and in most cases produced longer average cycle life. The results for FP-EL-4060, ETFE internally lubricated with 30-wt% PTFE, presented in Fig. 3 [omitted], show that the use of the wrong lubricant (NYE 739A) can actually decrease both efficiency and cycle life. The use of NYE 880 increased efficiency from 67% to 78% at 250,000 cycles and increased cycle life from near 400,000 to well over 500,000 cycles.

The results for liners made from PTFE filled with PPS (PTFE/PPS) demonstrate that NYE 880 produced the highest cable efficiency, but had a cycle life shorter than the SWS E-155 silicone oil. This liner composition is widely used in applications where high temperature stability and high efficiency are required. Tests using lower test loads (3 to 9 lbs) and wire rope inner members have been reported to show cable efficiencies of 90% and cycle life in excess of 500,000 cycles. With the more stringent 20-lb test load that was used in this study, the liner composition was inconsistent in cycle life testing. Cycles-to-failure ranged from 2500 to 500,000, and only two of ten liners tested reached 500,000 cycles without failure. This is attributed to the fact that PTFE is a very soft material compared with the other liner materials evaluated

Resin Properties

The effect of Rockwell hardness on liner performance was demonstrated by the testing of liners made from neat resins and lubricated with NYE 880, where those made from softer resins exhibited lower average cycle life. Lubrication was necessary because without it all the thermoplastics underwent a pressure-velocity melting failure ("PV failure") within a few thousand cycles. The highest cable efficiencies achieved by these liners were in the 65% to 68% range, with efficiency declining to approximately 60% at 500,000 cycles.

It is worth noting that these unmodified resin liners exhibited "slip-stick" phenomena. During a pull stroke, the cable member would repeatedly stick to the liner and then release. The result in mild cases was a vibration in the force vs. time trace. In severe cases, the noise was very loud and the entire test stand vibrated.

PTFE Addition

The effect of adding lubricant grade PTFE to thermoplastics is well understood in cases of simple sliding wear. The PTFE lowers wear rate, reduces the coefficient of friction, and increases the limiting PV before melting occurs. Therefore, it should not be too surprising that PTFE addition improves performance in this case as well.

Internal lubrication with PTFE eliminated the PV failure previously noted when unmodified liners were tested, and cable efficiencies of 65% to 75% were achieved. Efficiencies at 250,000 cycles for liners with NYE 880 external lubrication Fig. 6) increased from about 60% for ETFE to 67% for ETFE with 15% and 20% PTFE (FP-EL-4030 and 4040) and to 76% for ETFE with 30% PTFE (FP-EL-4060). Figure 6 [omitted] also shows that PTFE addition dramatically improved the average cycle life of ETFE-based liners. For PBT-based liners, the cable efficiency increased from 67% in the neat resin to 78% when PTFE was added Fig. 7 [omitted], PDX-W-87470). The efficiency enhancement attained with

PTFE addition thus appears to vary from resin to resin.

New Compositions

Two new high-efficiency cable liner compositions, FP-EL-4060 and PDX-W-87470, are shown in Fig. 7 [omitted] to outperform the industry standard PTFE compositions in terms of cycle life. Both compositions maintain an efficiency >74% out to 500,000 cycles and >71 % efficiency out to 1,000,000 cycles under the demanding test conditions. Further testing of FP-EL-4060 liners with various test loads revealed an additional benefit of this material-consistent performance regardless of test load. The efficiency may even have increased slightly when the load was increased to 40 lbs.

FP-EL-4060 has a continuous use temperature range of 300 [degrees] F to 350 [degrees] F, making it ideal for high temperature applications. PDX-W-87470 can be used where cost is a consideration and the continuous service temperature does not exceed 200 [degrees] F to 250 [degrees] E

Inner Member Construction

Both FP-EL-4060 and PDX-W-87470 liners showed significantly higher efficiencies but much shorter wear life when a wire-rope or stranded inner member was used in place of the standard flat-wrap member. Cycle life of the PDX-W-87470 liner, however, still exceeded 500,000 cycles even with the stranded member.

Our understanding of the interactions between each component in the entire cable tribological or friction-wear system led to the supposition that surface hardness would be a more significant factor for stranded inner members than for flat-wrap inner members. This was confirmed by the results for the FP-EL-4040 liner used with a stranded member. The increase in surface hardness brought about by the one-third reduction in PTFE internal lubricant from FP-EL-4060 yielded a liner that maintains >85% efficiency past 1,000,000 test cycles. Different materials are therefore recommended for different cable members.

* Modification of thermoplastic resins with PTFE internal lubricant increases liner efficiency and cycle life. The degree of efficiency enhancement depends upon the amount of PTFE added and the properties of the base resin. Efficiencies of 65% to 75% were achieved without the use of an external lubricant.

* Liners made from two new thermoplastic compositions internally lubricated with PTFE, FP-EL-4060, or PDX-W-87470 outperformed PTFE/PPS and PTFE/glass industry standard liners, exhibiting cycle lives in excess of 1,000,000 cycles and efficiencies >75% at 500,000 cycles and >71 % at 1,000,000 cycles.

* The construction of the inner member does change the liner material required to achieve optimum performance. FP-EL-4040 and PDX-W-87470 are recommended for stranded inner members, whereas FP-EL-4060 and PDX-W-87470 are recommended for flat-wrap inner members.

* The type of external lubricant affects liner performance. Use of the PTFE/ silicone grease NYE 880 consistently gave better liner performance than either the silicone oil SWS E-155 or the PTFE-modified lithium soap grease NYE 739A.

* Cycle life of liners made from unmodified thermoplastics increases as the Rockwell hardness and abrasion resistance of the resins increase.

* An understanding of the interactions among all the factors discussed in this article as a tribological system is necessary for proper selection of liner compositions for a given requirement.
COPYRIGHT 1990 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:O'Brien, Gregory; Theberge, John; Williams, Edward Bennett
Publication:Plastics Engineering
Date:Sep 1, 1990
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