Grease Sampling and Analysis for Robotic Gear Drives

Rich Wurzbach, CLS, LLA, ASNT PdM Level IIIMRG Labs

York, PA, USA

Robotic Gear Drives

Robot systems expanding use in manufacturing

Auto manufacturing a leading use of robotsFanuc is leader in market; 240,000 robots

deployed worldwide, 100,000 in N. AmericaFanuc robots are six-axis, include a grease

lubricated gear drive in each joint/axis

Robot Drive MaintenanceGrease in gear drive requires periodic

replacementLife of grease is dependent on cycle times,

degree of movement, load, temperature and thermal cycling, and contaminants

Manufacturer recommendations are uniform and time-based only

High cost of grease and downtime make good candidate for condition-based relubrication

Fanuc Robot Styles

Regreasing Operations

Gear Drive Access Points An entry and exit point for

each gearbox enclosure Entry is a Zerk fitting, with a

small threaded opening (M6) Exit is M12 threaded plug for

900 series and larger on most robots, some are smaller M6

Drain is removed with allen wrench

Grease gun is used at Zerk fitting to add new grease and to purge out old grease

Goal: Develop routine sampling method

Method should be minimally invasiveSamples should be available without

disassembling robotIdeally performed between regularly

scheduled purge grease changeoutsMethod should leave robot in a ready-to-run

state (purged of excess grease)

Development of the Grease Thief

Grease Thief originally developed for sampling Motor Operated Valve actuator gearboxes

Further development included methods for Electric Motors and Pillow Block bearings

Adaptation of existing design used for Fanuc gear drives

Grease Sampling-MOVs

• EPRI method using plastic tubing • Grease removed and visually inspected• Kits made utilizing known consistency

greases for comparison to obtained sample

Photos: Bolt, et. al. Machinery Lubrication Magazine. May 2003

Grease sampling devices• Allow grease to be captured from motor drain,

valve or pillow block• Maintain purge function with little/no

backpressure• Allow relief of grease if sampler fills • Design to optimize the analysis process

Grease Sampling-Motors

• T-handle and stinger probe for larger drains

• Grease relief fitting installed on drain of motor

Kit for Robot Drive Sampling

First version – Grease purge and captureGearbox must be filled in order to force

purgePressure from exiting grease fills Grease

ThiefExcess grease must be purged following

sampling, prior to return to service

Adapter for larger drains

Works for larger drains (M12) When used as an expander for smaller drains, determined to

be inadequate—creation of excessive backpressure and possible compromise of seals--REJECTED

Problems with original method• Use of reducers in some cases can constrict flow

in drain• Getting a sample requires filling the gearbox

completely with grease, then obtaining the sample, then purging back to appropriate volume

• Can consume large amount of grease in the sampling process

• May still be required for degraded greases that have hardened in service

New Kit for Robot SamplingLow consistency of grease allows for suction to be used

in most casesA sealing syringe, with the gearbox in the right position,

allows supply-side sampling in some but not all casesGrease Thief is threaded into purge drain and sample is

drawn in some smaller gears (J4-J6)Sampling method for Fanuc Gear Drives meets ASTM

D7718 – 11: Standard Practice for Obtaining In-Service Samples of Lubricating Grease

Syringe suction method Syringe with tapered tip is threaded

into Zerk fitting thread, cutting into plastic

Seal is made with reservoir, allowing suction of very fluid greases only

Orientation of joint is important in ensuring grease is close to zerk hole

Syringe is filled with about 10ml grease, so that the first 1-2ml dispensed into Grease Thief is after the purge volume

If grease has degraded or hardened, suction method may not work; purge method may be required

New Kit for Robot Sampling-Direct suction from Grease Thief on J4-J6

Samples submitted for Analysis• Samples are gathered

in or transferred to Grease Thief for uniform geometry

• Samples are protected in tube

• Labeling can include barcodes for definitive equipment ID

Additional Use – Tidy grease purge

Grease purge required with any relubrication task

Operating robot through degrees of movement with drain open

To avoid mess, plastic bags taped to drain, or channel constructed with tape to lead grease to floor

Additional Use – Tidy grease purge

Use of Grease Thief bodyThread size matches drain

thread for secure installationWith internals removed, bag is

tie-wrapped onto open end to capture all grease

All joints can be fitted for capture, and a single, multi-axis purge can be performed, saving time

Multi-point PurgeEmpty Grease Thief

bodies are threaded to the purge holes

Capture bag is tie-wrapped to the body

Robot is programmed for a multi-axis movement that optimizes purge of excess grease

Grease can be disposed prior to the installation of the drain plug

Analysis of Grease from Robots

• Tests to be performed include Wear, Consistency, Contamination, and Oxidation

• Oxidation used to evaluate relative stresses on the grease in each robot joint, and across robots in the facility

• Higher ambient temperatures, higher loads, and more rapid movements potentially age grease more rapidly

Analysis Techniques

Sample received fdM+ is run Grease Thief Analyzer is performed and substrate is made

Two strips are used to make a dilution to run RDE.

One Strip is used for FT-IR.

One Strip is Dissolved in RULER solution to run RULER.

Wear levels in Robot fleet-comparison

Die Extrusion Test

•Samplers loaded into fixture with load cell at pusher end

•Grease extruded through die to create ribbon on substrate

•Load profile at varying speeds developed for consistency evaluation

•Sample prepared for subsequent analyses

Consistency Monitoring

The graph shows the results from the Grease Thief Die Extrusion Test. The consistency has greatly dropped as compared to the grade 2 grease.

Anti-Oxidant Monitoring•Samples from similar gearboxes, same time in service

•Both have an adequate remaining anti-oxidant level to protect grease from oxidation

•Lower sample shows more rapid degradation of anti-oxidants, due to higher temperatures, contaminants, or other oxidation stressors.

•Differences in time of service or relubrication quantities or effectiveness could also affect anti-oxidant amount

Methods to identify grease mixing

• Grease Colorimetry spectrum identifies visible changes in the grease (400 – 700 nm)

• FT-IR looks at chemical bonds and can identify different thickener types and signs of oxidation.

• Molywhite and Vigogrease have very different signatures, and can be differentiated, or mixtures identified

Summary

• Grease samples can be used to evaluate wear condition and predict component failures

• Changeouts can be condition-based, compensating for non-uniform conditions of load, environment and service

• Mixing and improper grease use can be identified and corrected before damage occurs