ASTM D7918-23

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7918 − 17a D7918 − 23
Standard Test Method for
Measurement of Flow Properties and Evaluation of Wear,
Contaminants, and Oxidative Properties of Lubricating
Grease by Die Extrusion Method and Preparation
This standard is issued under the fixed designation D7918; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers the determination and evaluation of flow properties, wear levels, contaminants, and oxidative
condition of new and in-service lubricating grease.
1.2 This test method provides guidance on evaluating in-service grease samples, NLGI grades 00 to 3, for wear, consistency,
contamination, and oxidation.
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3.1 Exception—The exception to this will be where units of references were developed by the developers of the test equipment
and necessary to report the results of the test.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D217 Test Methods for Cone Penetration of Lubricating Grease
D6300 Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products, Liquid Fuels, and
Lubricants
D6595 Test Method for Determination of Wear Metals and Contaminants in Used Lubricating Oils or Used Hydraulic Fluids by
Rotating Disc Electrode Atomic Emission Spectrometry
D7527 Test Method for Measurement of Antioxidant Content in Lubricating Greases by Linear Sweep Voltammetry
D7546 Test Method for Determination of Moisture in New and In-Service Lubricating Oils and Additives by Relative Humidity
Sensor
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.96.07 on Integrated Testers, Instrumentation Techniques for In-Service Lubricants.
Current edition approved Nov. 15, 2017Oct. 1, 2023. Published December 2017November 2023. Originally approved in 2015. Last previous edition approved in 2017 as
D7918 – 17.D7918 – 17a. DOI: 10.1520/D7918-17A.10.1520/D7918-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7918 − 23
D7718 Practice for Obtaining In-Service Samples of Lubricating Grease
D7843 Test Method for Measurement of Lubricant Generated Insoluble Color Bodies in In-Service Turbine Oils using
Membrane Patch Colorimetry
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions:
3.1.1 active grease sampling device, n—device designed to take an active sample of a lubricating grease from a bearing, gear, or
drive shaft located in a grease lubricated component. D7718
3.1.1.1 Discussion—
The Grease Thief (trademarked) Type 2 is a tool which meets this description and can be used to obtain an active grease sample
from a lubricated component. A full description and dimensions of this device can be found in Annex A2.
3.1.2 active sampling, v—to use a sampling device to actively gather an in-service lubricating grease sample from a
grease-lubricated component. D7718
3.1.3 calibration, n—the determination of the values of the significant parameters by comparison with values indicated by a set
of reference standards. D6595
3.1.4 consistency, n—of lubricating grease, the degree of resistance to movement under stress. D217
3.1.4.1 Discussion—
The term “consistency” is used somewhat synonymously with “penetration.” Generally, consistency refers to the worked
penetration of a grease.
3.1.5 linear sweep voltammetry, n—a test method designed to monitor the anti-oxidant additive content in lubricating greases.
D7527
3.1.6 lubricating grease, n—a semi-fluid to solid product of dispersion of a thickener in a liquid lubricant. D217
3.1.6.1 Discussion—
The dispersion of the thickener forms a two-phase system and immobilizes the liquid lubricant by surface tension and other
physical forces. Other ingredients are commonly included to impart special properties.
3.1.7 in-service lubricating grease, n—lubricating grease that has been applied as a lubricant to a gear, bearing, or drive screw for
any period of time. D7718
3.1.8 passive grease sampling device, n—a device designed to gather a sample from the equipment by being attached to the grease
reservoir at the purge point. D7718
3.1.8.1 Discussion—
The Grease Thief (trademarked) Type 1 is a tool which meets this description and can be used to obtain a representative grease
sample from a purge path. A full description and dimensions of this device can be found in Annex A1.
3.1.9 passive sampling, v—to use a passive grease sampling device to collect a purged sample of in-service lubricating grease from
a purge path. D7718
3.2 Definitions of Terms Specific to This Standard:
3.2.1 die extrusion index, n—an average of three unique test conditions of the in-service lubricating grease sample expressed as
a percent when compared to the average of three unique test conditions of the unused lubricating grease baseline.
The Grease Thief Type I and Type II is described in US Patent No. 7984661. Interested parties are invited to submit information regarding the identification of an
alternative(s) to this patented item to the ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical
committee, which you may attend. The sole source of the Grease Thief Type I and Type II are known to the committee is York Laboratories, LLC located at 410 Kings Mill
Rd., York, PA 17401. If you are aware of alternative suppliers, please provide this information to ASTM International Headquarters. Your comments will receive careful
consideration at a meeting of the responsible technical committee, which you may attend.
D7918 − 23
3.2.2 die extrusion tester, n—a device that measures the consistency of in-service lubricating greases and compares the
measurement to the values obtained from a baseline sample of lubricating grease.
3.2.3 ferrous debris level, n—the total amount of ferrous metal as measured by a Hall-effect sensor in the grease sample
independent of particle size.
3.2.4 full attenuation, n—the loss of intensity in signal strength.
3.2.5 grease colorimetry, n—the science of color measurement by the evaluation of the CIE LAB values of grease in the visible
light region of the electromagnetic spectrum.
3.2.6 grease ribbon, n—grease that is extruded onto a substrate.
3.2.7 indexing die, n—a single-use orifice die to measure the consistency as compared to the baseline.
3.2.8 spring cell calibration cylinder, n—a cylinder that houses a spring and a push rod to compress the spring.
3.2.9 substrate, n—the single-use strip onto which a ribbon of grease is extruded.
3.2.10 substrate segment, n—a peel-away section of the substrate containing a portion of the extruded grease; the segments are
pre-cut to hold 0.25 g of extruded grease ribbon.
3.2.11 speed 1, n—the slowest speed in which the grease is extruded from the indexing die.
3.2.12 speed 2, n—the fastest speed in which the grease is extruded from the indexing die.
3.2.13 speed 3, n—the intermediate speed at which the grease is extruded from the indexing die.
4. Summary of Test Method
4.1 Testing of a grease sample includes steps that characterize the ferrous wear within the sample, the consistency of the sample,
and properties related to its chemistry.
4.2 A grease sample of known volume is measured to determine the density of ferrous material in the sample.
4.3 A grease sample held with a defined geometry sample holder is placed into a temperature-controlled instrument and extruded
onto a substrate as a thin ribbon or strip of grease. The extrusion process provides a measurement of the consistency of the grease.
4.4 The test method requires the grease to be tested at three different rates to reflect the non-Newtonian nature of greases. Testing
at several different rates creates a series of step changes that are then compared to an unused baseline grease that has also been
tested under the same conditions.
4.5 While the flow properties are being measured, the grease is simultaneously deposited onto a thin-film substrate containing
substrate segments. Each substrate segment contains approximately 0.25 g of grease. The individual substrate segments are used
for further testing of wear, contamination, and oxidative properties.
4.6 The substrate is removed from the instrument and processed further to obtain information related to the chemistry and content
of the grease sample including linear sweep voltammetry and grease colorimetry.
5. Significance and Use
5.1 Trending the wear, contamination, consistency, and oxidative properties of a lubricating grease is a crucial part of
condition-monitoring programs. Changes in these properties or deviations from the new grease can be indicative of problems
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within the lubricated component, such as the mixing of incompatible thickener types, excessive wear or contaminant levels, or
significant depletion of antioxidant levels. These test methods also makes it possible to develop trends that can be used to predict
failures before they occur and allow for corrective action to be taken.
6. Interferences
6.1 Particulate Matter—When the in-service grease sample is heavily contaminated with particulate matter, it may be necessary
to perform a dilution of the sample in order to properly extrude the sample onto the substrate. Larger pieces of particulate matter
may also cause spikes in the load profile graph. The data from these spikes should be discounted from the average force
calculations because they are not indicative of changes in the flow properties of the sample.
6.1.1 Heavily contaminated grease samples are considered any grease that contains solid particulate of size or quantity that results
in clogging of the die, which inhibits the completion of the test.
NOTE 1—The specific steps to perform a dilution of grease samples is not addressed in this test method.
6.2 Sample Size—The passive sampling device shall be at least 70 % filled to perform the testing indicated within this test method.
If the sampling device is insufficiently filled, the instrument may not record a force reading over the entirety of the program.
6.3 Color—The grease colorimetry test may become insignificant if the color of the grease causes full attenuation and minimal
transmittance of the signal making it incomparable to the new grease.
7. Apparatus
7.1 Passive Grease Sampling Device—The device that holds the grease sample. The device is typically capable of holding
approximately 1.5 g to 2 g of grease when full. A full description and dimensions of this device can be found in Annex A1.
7.2 Active Grease Sampling Device—The device that holds an active grease sample from a grease-lubricated component. A full
description and dimensions of this device can be found in Annex A2.
7.3 Die Extrusion Tester —Designed for the passive grease sampling device, the die extrusion tester is a temperature-controlled
chamber consisting of a table that is controlled by a linear actuator that holds the substrate, a bracket that holds the passive grease
sampling device, and a second linear actuator to push the handle of the passive grease sampling device. Both the passive grease
sampling device and the second linear actuator are mounted perpendicular to the substrate table. A load cell is mounted between
the second actuator and the handle of the passive grease sampling device to measure the force during the extrusion. The die
extrusion tester is shown in Fig. 1.
FIG. 1 Die Extrusion Tester
Wurzbach, R. and Williams, L., “Die Extrusion Method For Comparing Changes in Grease Consistency and Flow Characteristics,” Viscosity and Rheology of In-Service
Fluids as They Pertain to Condition Monitoring, ASTM STP 1564, 2013, ASTM International.
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7.4 Constant Temperature Chamber—The test specimens in the passive grease sampling device should be heated in the
temperature-controlled chamber of the die extrusion tester prior to performing any test. The passive grease sampling device shall
have the indexing die firmly engaged on the threaded open end of the device.
7.5 Ferrous Debris Analyzer—An analyzer that allows the passive grease sampling device to be inserted completely in the coil
geometry, designed to measure the total amount of ferrous debris in the grease sample. By use of a Hall-effect sensor, the total
amount of ferrous debris is designed to measure quantitatively in parts per million (ppm). Upon receipt of the sample, it is the first
test performed on the grease.
7.6 Linear Sweep Voltammetry—Linear sweep voltammetry measures the amount of hindered phenols, amines, and zinc dithio
dialkyl phosphate (ZDDP) in a lubricating grease. Using 0.25 g of grease from the substrate, one substrate segment is inserted into
a vial and the antioxidant concentration remaining in the grease is measured and compared to an unused sample of the same grease.
7.7 Grease Colorimetry—The grease colorimetry test provides a spectrum in the 400 nm to 700 nm region of the visible light
spectrum. An optical spectroscopy cell is used and the substrate segment is placed into the same holder. The delta-E values within
the CIE LAB scale are recorded and a spectrum can be generated in a method similar to Test Method D7843.
7.8 Direct Imaging System—A particle sizing and identification component consisting of a CCD (charge-coupled device) video
chip, pulsed diode backlight, magnifying lens, fine focus dial, software, and readout system.
7.9 Relative Humidity Sensor Analyzer—The test specimens in the passive grease sampling device are extruded on the substrate
of the die extrusion tester, to create an approximate 0.25 g thin-film deposition on a plastic substrate. One plastic substrate is
weighed and placed in a clean, dry glass vial with a septum and sealing cap. The vial is placed into the heating chamber of the
relative humidity sensor analyzer, and a dry carrier gas is introduced to transfer the moisture driven from the sample by the heating,
and passed over the humidity sensor to calculate moisture content based on calculated total flow and original sample mass.
8. Reagents and Materials
8.1 Spring Cell Calibration Cylinder—This device is used to perform a calibration check on the die extrusion tester prior to use.
See Annex A3 for the full calibration procedure.
8.2 Indexing Die—Threads onto the end of the passive grease sampling device prior to performing the die extrusion test.
8.3 Substrate—The three-layered plastic substrate shall be long enough to hold the grease ribbon once it is extruded from the
indexing die. The substrate’s three layers consist of a rigid bottom layer of high-density polyethylene, a double-sided releasable
adhesive, and a top layer of low-density polyethylene. The top layer of the substrate is divided into peel-away segments that, when
fully covered with the grease ribbon, each contains approximately 0.25 g of grease.
9. Sampling
9.1 Good grease-sampling procedures are critical to good analyses, and samples should be taken in accordance with Practice
D7718.
10. Preparation of Test Specimen
10.1 Ferrous Debris Analyzer—Prior to analysis, the passive grease sampling tool shall be removed from the storage container.
The passive grease sampling tool shall be placed into the ferrous debris analyzer to determine the total amount of ferrous wear
present (in ppm).
10.2 Passive Grease Sampling Tool—Upon completion of the ferrous debris analyzer test, the protective cap shall be removed and
an indexing die shall be threaded onto the open end of the passive grease sampling device.
NOTE 2—If the in-service grease sample is received in an active grease sampling device, the stinger probe must be removed and replaced with the handle
of a passive grease sampling device prior to use.
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NOTE 3—If the in-service grease sample is received in any sampling container other than the passive or active grease sampling device, it must be
transferred into a passive grease sampling device by hand, with a clean, single-use glove and/or a clean, single-use 10 mL slip tip syringe.
11. Preparation of Apparatus
11.1 Preparation of the Die Extrusion Tester:
11.1.1 Heating the Chamber—Prior to the performance of any tests, a thermocouple is used to regulate the temperature of the
heating tray to achieve a temperature sufficient to pre-heat samples to meet the test temperature requirements for the trial specified
in 12.3.3. The samples shall be on the heating tray for at least 20 min to come to thermal equilibrium, which is verified with an
embedded thermocouple.
11.1.2 Validation Check—A check shall be performed with a spring cell calibration cylinder to confirm calibration prior to the
analysis of routine samples. The procedure and accuracy guidelines for the validation check are described in Annex A3. This
procedure shall be performed at the beginning of each shift or if the instrument has not been used for an extended period of time.
11.2 Preparation of the Optical Spectroscopy Cell—Check the mirror, optical path, and the tray for cleanliness.
11.3 This test method requires no additional preparation steps for the other instruments.
12. Testing Procedure
12.1 Procedure for Evaluation of Ferrous Debris Level:
12.1.1 Factory Calibration—The parameters for the analytical instrumentation are pre-set by the manufacturer of the equipment.
Daily validation checks must be performed and analyses of test specimens must be within the linear range of response.
12.1.2 Routine Standardization—A minimum two-point routine standardization shall be performed if the instrument fails the
validation check.
12.1.3 Set the instrument to grease mode and drop the sample into the coil.
12.1.4 Record the results in the nearest whole number in ppm ferrous content.
12.2 Analysis of Grease Samples by Die Extrusion:
12.2.1 Factory Calibration—The parameters for the analytical instrumentation are pre-set by the manufacturer of the equipment.
A calibration curve must be determined prior to use to calculate a K constant and an offset for the reference spring cell. Daily
validation checks must be performed and results must be within the linear range of response.
12.2.2 Routine Standardization—A minimum two-point routine standardization shall be performed if the instrument fails the
validation check. The procedure for the routine standardization is described in Annex A4.
12.3 Steps for Standard and In-service Sample Preparation Procedure:
12.3.1 Load a substrate, with the segments facing up.
12.3.2 Upon preparation of the sample, move the heated passive grease sampling device from the heating tray and immediately
place into the sample-holding bracket with the opening of the indexing die facing the same direction that the table moves. A new,
clean indexing die and substrate shall be used for each analysis.
12.3.3 The die extrusion tester shall be programmed to only perform the test if the sample is in the appropriate temperature range.
Therefore, if the sample loses too much heat in the loading process, it will need to be reheated. The appropriate temperature is
verified using a non-contact thermometer that has been modeled and calibrated by the manufacturers of the instrument to ensure
internal grease temperature of 30 °C 6 2 °C.
12.3.4 The die extrusion tester shall be programmed to run the following steps:
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12.3.4.1 The prime must be set so that it allows sufficient time for the die to be filled. Grease shall be exiting the die before
continuing to Rate 1.
12.3.4.2 Rate 1—0.15 mm/s 6 0.01 mm/s for 19 s 6 1 s.
12.3.4.3 Rate 2—0.73 mm/s 6 0.04 mm/s for 13 s 6 1 s.
12.3.4.4 Rate 3—0.44 mm/s 6 0.02 mm/s for 22 s 6 1 s.
12.3.5 Start the program, which will extrude the lubricating grease onto the substrate and collect the flow response data. This
methodology is good for grades 1, 2, and 3 greases. Consult the vendor manual for how to run grades 00 and 0.
NOTE 4—Die extrusion testing only uses the speeds that show a force response in the final calculation of the die extrusion index. The passive grease
sampling device must be at least 70 % full to run the die extrusion test.
12.3.6 Verify during the prime and prior to Rate 1 step that the load data shows an increase from the start load. Inspect the piston
for movement. Its movement should correspond to an increase in the load cell reading. If no change of the load cell reading is
observed during the prime while the piston is moving, immediately terminate the test by stopping the instrument. If the load cell
displays a zero reading during extrusion, it could be an indication of misalignment or instrument malfunction. Return to 12.3.1 and
repeat the test until a satisfactory validation step is achieved.
12.3.7 Once the test is complete, the data shall be transferred to a computer to be analyzed. For calculations see Section 13.
12.3.8 Remove the empty passive grease sampling device from the die extrusion tester and dispose in the trash.
12.3.9 Remove the substrate containing the grease ribbon for additional testing.
12.4 Procedure for Grease Colorimetry:
12.4.1 General Test Procedure—The test procedure will consist of an unused grease reference reading and the sample (in-service
grease) reading.
12.4.2 Prepare the grease according to 12.3.
12.4.3 Calibration of the Spectrophotometer:
12.4.3.1 Place the blank substrate segment into the drawer of the optical spectroscopy cell to collect the background spectrum.
12.4.3.2 Attach the surface head to the cell so that it is perpendicular to the substrate (or the separator is parallel to the side of
the cell containing the drawer).
12.4.3.3 Turn the i-Lab on using the power button in the center of the unit.
12.4.3.4 Using the arrow keys, highlight GreaseBk and press the power button to select.
12.4.3.5 Remove the protective cap from the i-Lab and place it onto surface head, which should already be attached to the cell.
12.4.3.6 Press the Power button again to obtain the background spectrum.
12.4.3.7 Remove the unit from the cell.
12.4.4 Procedure for Sample Data Collection:
12.4.4.1 Using the arrow keys, select the GreaseCIE method and press power.
12.4.4.2 Invert the unit back onto the surface head and press power again to obtain the spectrum of the sample.
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12.4.4.3 Repeat test as needed for multiple samples.
12.4.5 Procedure for Linear Sweep Voltammetry:
12.4.5.1 Prepare the grease sample according to 12.3.
12.4.5.2 Add one substrate segment with 0.25 g of grease to the vial and follow the procedure outlined in section 8.5.1 in Test
Method D7527 to perform a linear sweep voltammetry test.
NOTE 5—Step 8.5.1.3 in Test Method D7527 shall be replaced with a single grease strip from the substrate; instead of smearing the grease evenly on the
inside of the 7 mL vial, the user shall take one full strip of the grease substrate and curl it to fit inside the 7 mL vial. User may proceed following the
remainder of the steps in section 8.5.1 in Test Method D7527.
12.5 Analysis of Particle Concentration by Direct Imaging:
12.5.1 For each sample, the grease is extruded in a thin film and collected on a translucent polyethylene substrate as described
in 12.3.
12.5.2 The linear actuator that controls the moving table the plastic substrate sits upon is used to parade the grease ribbon
transversely across the fixed view of the high-resolution camera.
12.5.3 The grease ribbon width shall be equal to or greater than the frame width of the camera, and the length large enough to
capture a sufficient number of unique non-overlapping frames during video analysis.
12.5.4 Prior to analysis, the backlighting is adjusted to provide a consistent average background intensity between samples even
as grease color and opacity may vary.
12.5.5 The focus is also adjusted as necessary for each sample, with a fine vertical position dial.
12.5.6 The camera is used to capture a video recording of the grease ribbon as it passes.
12.5.7 The travel speed of the table is set to maximize the number of frames to be collected during video recording, while
minimizing overall test duration.
12.5.8 The video recording is then processed with particle sizing and counting software to determine the average particle area per
frame in the sample.
12.5.9 Out-of-focus frames, or frames that are otherwise compromised by grease distribution inconsistency, are excluded from the
count.
12.5.10 The results in μm are correlated to m
...