Printer Friendly

Transmission lip seal performance testing is key step toward product certification.

After all laboratory tests are complete, basic design configurations proven and material properties verified, the next step is to witness the performance of a lip seal "in action." Automatic transmission builders, in the past, relied on inhouse verification programs; or they contacted private testing laboratories to help predict performance and durability of the six to eight lip seals found in modem transmissions. But with new customer/supplier concepts, such as partnering, single source supply and product certification, the old way is not enough. Acadia is the first elastomer products supplier to the automotive industry to provide supplierbased performance testing of lip seals in their natural habitat. Their transmission seal tester (TST) puts lip seals through their operational paces, installed in the customer's own clutch housings. The bottom line: Accelerated life testing confirms that lip seals will perform reliably and provide durability over the operational life of the transmission.

Automatic transmission designers and manufacturers continually pursue new designs that advance automatic transmission state-of-the-art. From the outside, it does not look like much has changed., But inside, servos, pistons, seals, etc., have evolved and continue to evolve with one ultimate goal in mind; to accomplish all the sophisticated functions demanded by astute purchasers of automobiles and heavy duty equipment with components and parts that will outlive the vehicle.

Extremely crucial components, such as elastomeric seals, and in particular, lip-seals for clutch-applied pistons that regulate transmission functions, have met the developmental challenge. Original automatic transmission designs, dating back to the early 1940s, required only a few lip seals for clutches that provided just forward, reverse and neutral. Today's typical transmission requires approximately eight lip seals to power additional features such as low forward, overdrive, coast, etc.; all packaged in basically the same housing envelope.

A lip seal holds in place eight or more quarts of automatic transmission fluid (ATF) at approximately 300 [degree] F and assures that all functions are performed as intended. They perform in an environment that grows more hostile due to various additives in the ATF, higher temperatures, closer proximity designs and the need for faster, smooth response of apply/release clutch piston functions.

Lip seal variations

Space is not available to provide a complete dissertation on lip seal form, fit and function. But briefly, probably the most widespread use of lip seals is on accumulators, servos and clutch pistons in automatic transmissions. In terms of vehicle operation and customer awareness, the lip seal is the most critical component.

Each clutch piston has an inner and outer lip seal that seals the piston as it strokes to apply the clutch. Seal design alternatives vying for transmission applications include molded and machined seals, long lip and short lip seals and the newer design, one-piece piston/seal combinations.

Lip seals permit relaxed tolerances on piston groove diameter and width, bore diameter and finish, out of round, etc., which translates into lower manufacturing costs. The lip deflects to conform as needed. The full permissible tolerance range is usually not used because close fit is often required for piloting and other purposes not associated with sealing. Also, lip seals will compensate for a range of relative temperature changes in the piston and the bore.

A key feature of lip seals is their ability to provide zero leakage, where only a minute amount of leakage is desirable and needed for lubrication of the contact area of the lip.

It is apparent that configuration, leak characteristics, along with elastomer material selection and performance are all critical issues when designing or modifying a lip seal. The availability of the TST equipment accelerates analysis of all components of design, so that the optimum seal for the specific application can be selected with the confidence that it will perform as intended.

The Acadia TST

The Acadia lip seal test machine provides a cyclic pressure input of heated automatic transmission fluid into automatic transmission clutch housings. Fluid migration into the closed system is through "one-way" check valves installed in-line with each clutch housing.

The test chamber contains six clutch housings and provides an enclosure for liquid nitrogen, the cooling medium for the clutch housings. ATF expelled during testing is collected in stainless steel reservoirs and then weighed. A computer program converts the weight measurement into an equivalent volume value, and then produces tabular and graphic representations for leak rate analysis.

Basic operating constraints of the TST are an operating temperature range of -40 [degree] F to +250 [degree] F, a maximum ATF pressure of 400 psi and a maximum ATF flow rate of 16 gallons per minute. The clutch piston cycle rate can be as high as 60 cycles per minute, but this rate is highly dependent on fluid viscosity and controlling orifice sizes. To facilitate filling of the clutch housings and reducing transient pressure spikes, the slowest feasible cycle rate is established for most testing.

Leak rate analysis

The TST has six analytical weighing balances with 0.1 gram resolution and a maximum capacity of 6,100 grams. The balances are transducers that monitor leak rate by weight; weight is subsequently converted into volume by the computer program. Hard copy tabular and graphic representations of ATF leak rates are produced.

In operation, after all bench mark calibrations have been locked in, the clutch pistons are cycled in accordance with the needs of the specific tests. Typically, test cycles are from 100,000 to 250,000. However, for testing of transmissions used on heavy duty equipment, over a million cycles may be required.

A monitor screen displays test status information including environmental conditions, cycles, status, current weights, leak rates, cycle rates and abort parameters. The test operator can easily observe all critical parameters and respond if corrective action is needed.

Using set-up parameters, the computer calculates current weight by dividing the grams indication on each balance by the density of the specific ATF. Leak rate is the product of current weight, minus the weight from five minutes ago, all divided by five minutes. Leak rate calculation, for the production of the graphs, is a differential five point moving average. This filtering of the data results in a reduction of large leak rate spikes, while still providing a very good indication of actual leak rate.

TST in action

A test program completed on the TST will provide data for leak rate analysis, and will also furnish, for visual inspection, used lip seals that have performed for the equivalent of 100,000 miles or more. A program may be requested either by the manufacturer of the transmission, the seal design team, or Acadia's engineers. The subject seals may be based on entirely new design concepts, or may have just a slight modification, such as a change in shape that is intended to simplify installation. Tests are conducted:

* When comparison of performance of molded versus machined lip seals is required;

* To verify material changes that are made to lower cost or improve function;

* When configuration modifications are made to solve installation problems;

* On prototype bonded piston seals, proposed to replace standard loose lip seals;

* When failure analysis is required;

* During prototype development work. The following examples, abstracted from actual test programs, document obtainable results.

Example I

Five sets of short lip seals were obtained to determine the leak rate of the seals over the 100,000 cycle life of the test, and to provide for a visual inspection to determine wear and/or deterioration of the seals. ATF supplied by the customer was heated and cycled through clutch housings at 300 psi [+ or -] 15 psi at a rate of 25 cycles per minute, for 100,000 cycles. Then the lip seals and housings were removed and inspected.

Only two seals exhibited any leakage with approximately 0.12 mi/min. being the maximum encountered (see accompanying graphs). Final visual inspection showed minimal wear to the lip or heels of any of the seals. Two OD seals had small cuts on the sealing lip and two other seals showed slight abrasions on the sealing lip. One of each of the parts leaked, but neither exceeded 0. 12 ml/min. The last seal showed no abrasions and no leakage.

The tests determined that it was likely that the abrasions and cuts occurred while installing the seals in the fixtures, since some difficulty was encountered during the process. The short lip seals worked well with no seal exceeding the 1.0 mi/leak failure criteria, even with the damage to the sealing lips. The test results support the claim that the short lip seals should perform well under similar conditions in actual application.

Exmaple 2

This test program analyzed performance of prototype adhesives for one-piece seals used in transmissions for heavy duty equipment. After a total accumulation of 400,000 cycles, all adhesives were performing satisfactorily and all seals were exhibiting no leakage, except one bonded piston seal. which was removed. Test parameters were:
 Temperature: 194 [degree] F [+ or -] 1.5 [degree] F
 Pressure: 400 psi [+ or -] 15 psi
 Cycles/minute: 14.3


The leak rate history on the seal where the adhesive failed fluctuated between 3 and 10 ml/minute. The seal was removed, cleaned and reinstalled, but continued to leak until it was rotated at approximately 160,000 cycles.

The leak rate continued to fluctuate after a brief period of low leakage that occurred immediately following rotation. The leak rate then stabilized at an average of less than 1.0 ml/minute until the adhesive failed at 350,000 cycles.

Inspection of the failed bond revealed that the OD sealing lip had split resulting in a tear that opened the lip approximately 50 mm. The split was near the center of a fracture found in the stress concentration area at the base of the lip on the sealing side. The fracture existed for about a 140 degree arc on the part. The rubber tore back, exposing the adhesive.

Example 3

This test program was conducted to provide data on the comparative leak rate of molded and machined outer forward clutch lip seals. Six transmission clutch housings, and machined lip seals and molded lip seals for the outer forward clutches were delivered by the transmission builder. The six clutch housings were provided with fittings and check valves to allow migration of an ATF into and out of the housings.

The ATF was transmitted to the clutch housings at a temperature of 250 [+ or -] 5 [degree] F and a pressure of 300 [+ or -] 10 psi. The cycle rate was 60 [+ or -] I cycles per minute allowing the clutch housings to experience a pressure of from 0 [+ or -] 10 psi to 300 [+ or -] 10 psi.

Throughout the cycle test, the clutch housings were fixtured at 30 [+ or -] 10 degrees from horizontal to facilitate ATF draining into a stainless steel reservoir. Each reservoir was independently positioned on a 0.1 gram resolution analytical balance in order to record the fluid leak rate. The approximate conversion of weight of ATF to volume is: 1.0 grams is equivalent to 1.15 ml.

The leak rate analyses (see table) consisted of evaluating three machined lip seals in housings designated as #1. #2 and #3, and three molded lip seals in housings #4, #5 and #6. Following the predetermined test duration of 150.000 cycles, the lip seals were removed and molded lip seals were subsequently installed in housings #1, #2 and #3 and machined lip seals were utilized in #4. #5 and #6. The cyclic test was again performed for a duration of 150,000 cycles.

The greatest leak rate attained for any hourly collection period was 39.5 grams (45.4 ml) during cycle test 2 for a molded lip seal.

Conclusion

Automatic transmission state-of-the-art continues to advance as new elastomer materials and new, more effective seal configurations meet performance challenges. The Transmission Seal Tester allows accelerated testing of all aspects of lip seal design so that transmission designers can select, with confidence, the optimum seal for each specific application. Accelerated life testing confirms that lip seals will perform reliably and provide durability over the operational life of the transmission.
 Table 1 - cycle test number 1 - accumulated
 leak data (grams of AFT)
 Machined Molded
 housing housing
Time designation designation
(hrs.) Cycles 1 2 3 4 5 6
 0.5 1,880 0.0 0.2 0.0 0.6 6.5 5.2
 1.5 5,310 0.0 0.5 0.0 1.3 15.5 13.1
 2.5 8,910 0.0 0.8 0.0 2.3 23.8 22.3
 3.5 12,540 0.3 0.8 0.0 3.6 29.7 29.0
 4.5 16,510 0.3 0.8 0.0 4.3 34.2 33.7
 5.5 19,880 0.3 0.8 0.0 4.8 37.1 36.4
 6.5 23.610 0.3 0.8 0.0 5.3 39.1 39.0
 7.5 27,390 0.3 0.9 0.0 5.9 40.7 40.7
 8.5 34,410 0.7 1.1 0.5 6.7 42.4 42.2
10.5 37,950 0.7 1.1 0.5 7.4 42.6 42.2
20.5 73,900 1.0 1.1 0.5 11.5 45.7 44.3
30.5 109,700 1.0 1.1 0.5 12.9 53.5 49.3
40.5 145,180 1.0 2.0 0.5 14.0 59.3 60.8
41.5 148,750 1.0 2.0 0.5 14.0 59.4 61.9
41.8 150,000 1.0 2.0 0.5 14.1 59.4 62.3
 Table 2 - cycle test number 2 - accumulated
 leak data (grams of AFT)
 Molded Machined
 housing housing
Time designation designation
(hrs.) Cycles 1 2 3 4 5 6
 1.0 3,590 0.0 37.1 2.1 0.0 0.0 13.4
 2.0 7,200 0.0 67.6 5.1 0.0 0.0 17.6
 3.0 10,800 0.0 96.9 7.9 0.0 0.0 21.7
 4.0 14,400 0.0 126.8 10.7 0.0 0.0 25.5
 5.0 18,000 0.3 159.6 13.5 0.0 0.0 29.6
 6.0 21,600 1.4 196.0 16.0 0.0 0.0 33.7
 7.0 25,200 2.4 235.5 18.5 0.0 0.0 37.8
 8.0 28,200 3.4 271.4 20.7 0.0 0.0 41.8
 9.0 32,400 4.3 305.3 22.9 0.0 0.0 45.8
 10.0 36,000 5.2 336.2 24.7 0.0 0.0 53.6
 11.0 39,600 6.0 364.0 26.6 0.0 0.0 53.6
 12.0 43,200 6.8 391.5 28.4 0.0 0.0 57.4
13.0* 46,800 7.5 422.4 30.1 0.0 0.0 61.2
 14.0 50,400 8.1 439.2 30.1 0.0 0.0 61.3
 15.0 54,000 8.6 443.7 30.1 0.0 0.0 61.3
 16.0 57,600 8.7 444.6 30.1 0.0 0.0 61.5
 17.0 61,200 8.8 444.6 30.1 0.0 0.0 61.5
 18.0 64,800 8.9 444.6 30.1 0.0 0.0 61.6
 19.0 68,400 9.0 444.6 30.1 0.0 0.0 62.5
 20.0 72,000 9.0 444.6 30.1 0.0 0.0 63.1
 30.0 108,000 9.0 444.6 30.1 0.0 0.0 73.2
 40.0 144,000 21.3 444.6 30.1 0.0 0.0 87.2
 41.0 147,600 22.5 444.6 30.1 0.0 0.0 88.6
 41.7 150,000 23.2 444.6 30.1 0.0 0.0 889.2
 *Short interruption in tesst to change flow rate of pump to
cause more pronounced pulsing of piston and clutch
plates.
COPYRIGHT 1993 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
Printer friendly Cite/link Email Feedback
Author:Pyle, Jeff
Publication:Rubber World
Date:Mar 1, 1993
Words:2708
Previous Article:The unique design latitude of EPDM.
Next Article:Management.
Topics:


Related Articles
Equipment, sealing for HP/HT completions.
Advanced leak testing system secures diecaster's quality.
Contracts, licenses.
Rotational test machine helps predict real-world performance of elastomer seals.
Compound selection for dynamic shaft seals.
KCMA testing & certification.
Machine vision systems.
Effect of power steering fluid type on the performance of radial lip seals.

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters