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The TE 77 High Frequency Friction Machine is a versatile reciprocating tribometer with a maximum stroke of 25 mm and maximum load of 1,000 N. It is now a well-established research and development tool for evaluation of lubricants, materials, coatings and surface treatments.
With the TE 77, sliding contact conditions can be matched to a number of machine elements. Specimens may either be of a standard format, or cut from real components, preserving surface finish and other properties.
The TE 77 was used for the inter laboratory tests for the development of ASTM G 133 "Standard Test Method for Linearly Reciprocating Ball on Flat Sliding Wear", which addresses the dry and lubricated wear of ceramics, metals and ceramic composites and also for ASTM G 181 "Standard Practice for Conducting Friction Tests of Piston Ring and Cylinder Liner Materials Under Lubricated Conditions".
Although not included in the inter laboratory test programs, the TE 77, in conjunction with selected adapters, can also accommodate tests specimens and provide test conditions as specified in the following standards:
ASTM D 5706 "Standard Test Method for Determining Extreme Pressure Properties of Lubricating Grease Using a High Frequency Linear-Oscillating Test Machine"
ASTM D 5707 "Standard Test Method for Measuring Friction and Wear Properties of Lubricating Grease Using a High Frequency Linear-Oscillating Test Machine"
ISO/DIN 12156-2 "Diesel Fuel Lubricity - Performance Requirement Test Method for Assessing Fuel Lubricity"
ASTM D 6079 "Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFRR)"
A large body of technical publications from existing users provides information on a wide range of non-standard research and development test procedures.
TE 77 High Frequency Friction Machine is supplied with its own floor standing bench and with a bench-top control unit incorporating a SUPERSLIM Serial Link Interface Module, which is connected to a host PC with COMPEND 2000 sequence control and data acquisition software installed. The system provides sequence control of load, frequency and temperature plus data acquisition of measured parameters.
Moving Specimen:
The moving specimen is mounted in a carrier. A number of geometries can be accommodated by using a range of simple clamping fixtures. The specimen is oscillated mechanically against the fixed lower specimen. The mechanical drive comprises a motor driven cam and scotch yoke assembly, providing pure sinusoidal motion. The drive mechanism runs inside an oil bath.
The stroke length is altered manually by adjusting splined eccentric cams on an splined eccentric shaft. Two fixed cams are provided as standard allowing strokes to be set from 0 to 12.5 mm and 12.5 mm to 25 mm, with a total of eleven discrete positions per cam. A continuously variable double cam arrangement is included, which allows continuous variation of the stroke in the range 0 to 12.5 mm.
The moving specimen is loaded against the fixed specimen through a lever mechanism actuated by a geared servomotor with in-line spring. The normal force is transmitted directly onto the moving specimen by means of the needle roller cam follower on the carrier head and the running plate on a loading stirrup. A strain gauge transducer is mounted on the lever at a point directly beneath the contact and this measures the applied load.
Fixed Specimen:
The fixed specimen is located in a stainless steel reservoir. Special inserts are available for mounting the ISO Fuel Lubricity Test specimens and other standard specimens. The reservoir is clamped to a block that is heated by four electrical resistance elements and the temperature is monitored by a thermocouple pressed against the side of the specimen or holder. The reservoir can be moved sideways on the heater block so that multiple tests can be performed on one fixed specimen.
The heater block is mounted on flexures, which are stiff in the vertical (loading) direction, but offer limited resistance to horizontal forces. Movement in the horizontal direction is resisted by a piezo-electric force transducer, which measures the friction forces in the oscillating contact.
The heated specimen carrier and piezo-electric force transducer are mounted on a common sub-base. This not only provides seismic isolation from machine vibrations at high frequencies but also permits other sub-base assemblies to be fitted to extend the operating range of the machine.
Friction Measurement:
The piezo electric transducer used to measure the friction force has a sensitivity of typically 43.5 pC/N and the output range is set to match expected friction levels in the contact. The maximum friction level is +/- 500 N.
A charge amplifier converts the measured force to a proportional voltage. This is followed by a low pass filter, which fixes the upper cut off frequency of the measuring system. This serves to suppress transducer resonance. Final scaling of the signal for voltage output takes place in a second stage amplifier.
During higher frequency (>1Hz) operation, the charge amplifier is operated a.c. coupled. This eliminates the effects of d.c. signal drift over long time periods. The signal is passed though a true rms/dc converter amplifier and the final output is the true mean friction force. The instantaneous friction signal is also made available for viewing on an oscilloscope or for data logging using the TE 77/HSD option.
For low frequency sliding (<1Hz), stick-slip, single pass sliding, work with the Energy Pulse Slide/Roll Adapter and also for calibration of the transducer, the charge amplifier is operated in Quasi-Static d.c. coupled mode. This gives signal decay times of up to 100,000s, sufficiently long when compared to typical measurement time scales for the zero not to have moved significantly during the measurement.
Electrical Contact Resistance Measurement:
The moving specimen carrier is electrically isolated from the drive shaft and therefore from the fixed specimen. This allows a millivolt potential to be applied across the contact using a Lunn-Furey Electrical Contact Resistance Circuit. The voltage signal is taken to a true rms/dc converter amplifier to give a time-smoothed average of the contact potential.
Variations in this voltage are indicative of the level of metallic contact, provided that both test specimens are conductors of electricity. This measurement may be used for observing the formation of chemical films from anti-wear and extreme pressure lubricants, the breakdown of non-conducting layers and coatings or the build-up of oxides.
The instantaneous value of contact potential is also available for display on an oscilloscope or for data logging using the TE 77/HSD option.
Temperature Measurement:
Many wear processes are driven by temperature, be they the formation of oxides on the surfaces, the transformation of microstructure, the formation or break-down of lubricant additive or other tribochemical films, the melting of the surface (the PV limit of the material) or thermal stress induced failure.
To be more specific wear occurs as the result of the dissipation of frictional energy in the contact and this is irresistibly accompanied by a rise in temperature. The frictional energy is generated by the combination of load and sliding speed and its distribution and dissipation is influenced by other contacting conditions such as size and relative velocity.
In the reciprocating contact of the TE 77, sliding velocities are deliberately maintained at low levels in order to minimise frictional heating and, in the case of lubricated tests, to promote boundary lubrication. Minimisation of frictional heating means that contact temperature can be controlled effectively by controlling the bulk temperature of the fixed specimen. The temperature is measured with a thermocouple pressed against the fixed specimen and control is by software PID with PWM output.
Wear:
This is not directly monitored on the TE 77 and assessments are made from wear scar sizes on the moving specimen and wear volumes on the stationary specimen. Specimen sizes are small enough to be placed in SEM and other surface analysis equipment for detailed chemical analysis of surface films.
With the optional TE 77/WEAR fitted, a continuous record can be made of the movement of the moving specimen relative to the fixed specimen. This measurement can be used as an indication of the combined wear of both surfaces and for identifying wear transitions.
The TE 77 has PC based sequence programmable control and data acquisition. This is provided by an integrated Serial Link Interface Module and COMPEND 2000 software running on a host PC, operating under Windows. Data is stored to hard disc in standard spread sheet compatible file formats (.csv or .tsv).
Tests are defined by a sequence of steps, each step containing set-point, data recording rates and alarm level information. Set-points may be adjusted by step change or ramp. The test sequence is followed unless interrupted by the operator or an alarm. Set-points may also be adjusted manually using on screen toggles.
Frequency Range:
The maximum frequency on the TE 77 is 50 Hz. It should be noted that the majority of standards relating to reciprocating tribometers call for a test frequency of 50 Hz or less. The frequency range of competing electro-magnetic oscillator driven reciprocating tribometers is often higher (up to 500 Hz), but the amplitude range is lower than the TE 77, with a maximum stroke range typically up to 4 mm (but not over the full frequency range). This is compared with 25 mm on the standard TE 77. The TE 77 will run at 50 Hz at 5 mm and 30 Hz at 15 mm stroke.
Whereas stroke length has no great significance for basic tests to evaluate the frictional and chemical film forming behaviour of lubricants, stroke length, hence sliding distance, are of great importance when it comes to wear generation. A stroke length of 10 mm is also specified in ASTM G 133.
Generation of Wear:
Wear is a direct function of sliding distance, hence, the rate of generation of wear is a direct function of the rate of accumulation of sliding distance.
25 mm Stroke at 20 Hz: 60 m per minute (TE 77)
15 mm Stroke at 30 Hz: 54 m per minute (TE 77)
5 mm Stroke at 50 Hz: 30 m per minute (TE 77)
4 mm Stroke at 50 Hz: 24 m per minute (Typical electro magnetic machine performance)
1 mm Stroke at 100 Hz: 12 m per minute (Typical electro magnetic machine performance)
The longer stroke capability of the TE 77 makes it a more effective wear generator than short stroke electro magnetically driven devices.
Entrainment and Wear Debris:
The ability of the moving specimen to “expose” all parts of the fixed specimen depends on the contact length being not more than half the stroke length. In other words, in the case of other devices, with a contact length greater than 2 mm and a maximum stroke of 4 mm, it is apparent that the centre portion of the fixed specimen will be in continuous contact with part of the moving specimen. This has serious implications for lubricant entrainment, for surface activation and for the discharge of wear debris from the contact. Debris can become entrapped and generate an unwanted third body wear mechanism. Because of this, the devices are not entirely satisfactory for adhesive wear tests with area contact specimens.
Contact Scale:
An important issue with regard to contact scale is how the wear is shared between the two contacting surfaces. Wear is a function of sliding distance. In the case of the moving specimen, the sliding distance is twice the stroke length x number of cycles. For a point on the fixed specimen, the linear wear is twice the contact length x number of cycles. In other words, the wear of the moving specimen is dependent on total sliding distance but the wear on the fixed specimen is dependent on the number of passes and the contact length. It follows that the ratio of wear between the two surfaces depends on both stroke and contact length. In order to model a real tribological contact, this contact scale parameter should be correctly modelled.
To get the model right, the contact length must be correctly scaled. Hence, running at 25 mm stroke modelling a contact length of 3 mm in an engine with 100 mm stroke, the ring specimen in the test machine should have a contact length of 0.75 mm. To model this at shorter stroke lengths would require a contact length of 0.09 mm, which is not practical.
The extra stroke length on the TE 77 means that quite a variety of large sliding specimens can be accommodated. This is particularly useful when working with test specimens cut from machine components. The test area is highly accessible and open to design of specialised specimen carriers.
Very Low Frequencies:
The TE 77 offers, by means of simple interchangeable gearboxes, a minimum frequency down to 0.01 Hz. Electro magnetically driven devices typically offer a minimum operating frequency of 1 Hz. This lower end speed range allows the TE 77 machine to be used for investigating stick-slip and friction-velocity (Stribeck) curve characteristics of lubricants and materials.
TE 77/WEAR is a high-resolution capacitance proximity measuring system. Output from the device is in the form of a DC voltage proportional to displacement, suitable for data acquisition on a PC.
A capacitance non-contact probe is mounted in the moving specimen carrier. The probe is mounted approximately 0.5 mm away from a reference surface mounted on the edge of the specimen bath. The capacitance of the gap is converted to a dc voltage by a charge amplifier. The voltage is passed through a true rms/dc converter to provide the mean gap value over the length of the stroke.
The variations in the gap due to wear, lubricant film formation, thermal expansion or a combination of these are picked up by the system. The fact that the gap is small ensures that temperature effects are limited to the thermal expansion over that length.
The measuring resolution is greatest when the temperature of the fixed specimen is held constant. The TE 77/WEAR system is ideal for establishing long-term wear rates and transition points.
This additional hardware option has two main uses on the TE 77:
High-speed data acquisition is implemented by means of a 16-bit sixteen channel multi-function ADC PCI bus card, with programmable data acquisition rates up to 250 kHz. Data is buffered and stored direct to hard disc with a separate file automatically created for each acquisition cycle. The high speed data file names are automatically inserted as hyperlinks in the standard machine data file so that the high speed data may be viewed at the relevant place in the test.
This option provides positional feedback for the moving specimen and is best used in conjunction with the TE 77/HSD to allow long stroke force-displacement data to be generated.
This gearbox mounts between the drive motor and camshaft, providing a 20:1 reduction in operating frequency. The low sliding speeds generated encourage the contact to operate in boundary lubrication conditions. The option is therefore useful when testing additives and lubricant formulations.
This gearbox mounts between the drive motor and camshaft, providing a 100:1 reduction in operating frequency. The very low sliding speeds generated ensure that the contact operates in boundary lubrication conditions. The option is used for studying static friction and stick-slip of lubricants for slide-ways, clutches and the like. Specimens are available for stick-slip testing.
The TE 77/HR is a replacement reciprocating specimen carrier with integral heating, designed to allow tests to be run with a differential temperature between the moving ring specimen and the fixed liner specimen. The ring sample is self-heating to 200°C and is normally used in conjunction with liner sample temperature controlled by the fixed specimen heater block to less than this value.
The TE 77/INERT Gas Enclosure is an anodised aluminium chamber that encloses the fixed specimen/force measurement and load yoke assemblies. Fittings are provided to clamp the enclosure onto the machine base. The enclosure is sealed against the base using foam strip and a customised rubber gasket is provided to seal the gaps around the loading yoke verticals.
The reciprocating specimen carrier is also sealed by use of a customised rubber bellows secured to the moving arm and the enclosure. A transparent lid is provided so that the test can be observed.
Inlet and outlet pipe fittings and a length of pipe are provided for connection to a customer's inert gas supply. A water manometer is used to measure the gas pressure in the enclosure. Ambient temperatures in the chamber are limited to 250°C.
This option is used for investigating the effects of ambient gas or moisture on friction and wear. Inert gases, water vapour and mildly corrosive gases may be used.
This replaces the standard fixed specimen heater block assembly with a cooler pad. A laboratory chiller circulates a water/glycol mixture, allowing temperatures from -30°C to ambient to be achieved. To avoid ice formation, this is best used in conjunction with the TE 77/INERT test enclosure with dry air.
On the TE 77 the heated specimen carrier and piezo-electric force transducer are mounted on a common sub-base. To carry out high temperature experiments this is replaced with the separate TE 77/800C sub-base assembly. Key components of this assembly are made from Inconel (for high temperature performance) and a thermal barrier ensures that the flexures and piezo-electric force transducer are not exposed to excessive temperature.
The standard reciprocating specimen carrier head is also replaced by a carrier made from Inconel. This has a "C" shape, permitting the specimens to be enclosed inside a shroud and the load to be transmitted through a roller bearing outside this enclosure.
Specimens can be mounted either in the standard sample bath for dry or lubricated tests up to 600°C or on a special platform for high temperature un-lubricated tests. A stainless steel shroud is provided to help reduce heat loss by radiation and convection at the high temperatures.
This option may be provided as a retrofit assembly comprising heater block and four 200 W heaters, flexures and piezo-electric force transducer all mounted on a separate sub-base. The heaters are wired into the same connector as the standard heaters once these have been removed.
The TE 77/PUMP drip feed system uses a variable speed peristaltic pump. The advantages of this system are that there is no cross-contamination between pump and fluid, fluids are not exposed to high shear rates, it is self-priming and safe under dry running. By selecting a range of tube bore diameters a very wide range of flow rates are achievable. With a single size of tube the pump has a 110:1 turn ratio.
The package includes the peristaltic pump controller and pump head, three sizes of pump tubing and universal pipe fittings. An adaptor is provided for use with the standard moving specimen carrier clamps. This allows a PTFE tube to be mounted on the carrier to direct lubricant onto the contact zone.
This option is used in additives screening and lubricant development programs to provide fresh lubricant to the contact over long time periods. This is particularly important if tests are being carried out at temperatures where oxidation or evaporation of the sample is accelerated.
This adapter replaces the standard reciprocating drive assembly with a piezo actuator drive system. This is for performing fretting tests at strokes from 10 to 100 microns with frequencies up to 100 Hz with control of mid-stroke position and amplitude to +/-0.2 microns. The system includes a high pre-load piezostack, servo amplifier and signal generator, capacitance displacement gauge, 250 kHz 16-bit 16 channel multi-function ADC (not required if TE 77/HSD is already installed) and C-flex mounted moving specimen carrier. Simultaneous high speed data acquisition of friction force and displacement allows force-displacement curves to be plotted.
The TE 77/PD Pin on Disc Adapter replaces the standard reciprocating head on the machine and allows the performance of conventional pin on disc tests, using the machines drive motor and automatic loading system.
The TE 77/EP Energy Pulse Adapter converts the sliding contact into a combined sliding/rolling contact. This recognises that it is the cyclic nature of the energy input into the contact that contributes to wear and permits lubricated contacts to withstand higher levels of energy dissipation than in a pure sliding case (where the energy pulse is continuous). The Energy Pulse is the product of the Friction Power Intensity and the contact time. The conditions of slide/roll can provide a close simulation of machine contacts such as cam/follower and meshing gear teeth.
The Energy Pulse Adapter uses the standard 15 mm stroke cam drive and loading system. It is supplied on an sub-base, interchangeable with the standard TE 77 heated specimen carrier and piezo-electric force transducer sub-base. The TE 77/EP test roller is mounted on the output shaft of a worm gearbox and runs in a heated lubricant reservoir. The reservoir is supported on flexures and a piezo-electric force transducer measures the horizontal (traction) forces in the contact. The enclosure is provided with an integral electric heater and thermocouple to enable tests with bulk fluid temperatures up to 100°C.
The worm gearbox with has a 2:1 speed reduction and the input shaft is connected via a timing belt drive of 1:2 ratio to the TE 77 motor output shaft. The rotation of the roller is synchronised therefore with the reciprocation of the upper plate and the drive does not interfere with the standard loading arrangement.
The upper plate specimen is secured to a pivoted yoke on the reciprocating drive. This ensures that the plate aligns with the roller with an even load distribution across the contact width. The load is transferred to the reciprocating arm by the usual method of the needle roller cam follower running against a plate on the lower side of the loading yoke. The ratio of slide to roll is varied simply by adjusting the stroke of the reciprocating arm.
There are a number of test procedures requiring low levels of normal load. These include tests on coatings and soft layers, the evaluation of the lubricity of fluids and the ISO/DIS 12156-2 Fuel Lubricity Test. The standard automatic loading system has a loading threshold of 5 N. The Low Load Adapter can apply loads down to fractions of a Newton, although the minimum resolvable friction forces are at a level of 2 N normal load.
The Low Load Adapter applies dead weight loading to a ball (point contact) moving test specimen. The adapter uses a balanced beam and a push rod running through a linear bearing in a modified specimen carrier. This ensures that the load remains vertically above the ball across the whole stroke. The design limits the stroke to 2 mm with this adapter.
This test bath is for use in conjunction with TE 77/LLA for performing tests using standard ISO Fuel Lubricity Test specimens.
The adapter is designed for lubricity tests under conditions as specified in ASTM D 6079 Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFRR), but under pressurized conditions. This allows wear tests to be performed on volatile fluids and fluids that are not liquid at atmospheris temperature and pressure.
The contact between the ring and the piston groove can give rise to problems of wear, scuffing, micro-welding and, in severe cases, seizure. The TE 77/MW adapter may be used for simulating this contact. The adapter uses the drive motor and control system of the standard TE 77. When in use, the normal reciprocating test system is disabled, the reciprocating drive mechanism removed from the machine and replaced with a vertical axis cam drive with a stroke of 3 mm. This provides the vertical reciprocation of the Ring Specimen Holder. The Piston Specimen Holder is placed above the Ring Holder and locates on two linear shafts, restrained by springs.
When the Ring Specimen strikes the Piston Specimen, the springs compress to provide the loading on the interface. The peak load is adjusted by pre-setting the spring compression. The Upper Piston Specimen Holder is connected to the motor drive shaft via timing belts and two gearboxes. This system imposes a +/- 90 degrees rotation on the Piston Specimen, at 10 rpm when the motor shaft is running at 1,200 rpm (20 Hz). The specimens are thus subjected to a combination of impact and relative rotary motion.
Both the Holders incorporate heaters to permit the temperature of the specimens to be controlled, with thermocouples for feedback to PID controllers. A test oil catching reservoir is provided underneath the Ring Sample Holder. Means of feeding the lubricant into the contact is not included, however, access is provided for either drip feed or oil mist delivery of lubricant samples.
TE 77/SRVF Fixed Specimen Bath, TE 77/SRVB Reciprocating Head for Ball Specimens and TE 77/SRVC Reciprocating Head for Cylinder Specimens, are three adapters allowing standard SRV fixed and moving specimens to be accommodated.
The following table provides a summary of the uses of the various optional accessories and adapter:
| TE 77/WEAR: | For dry or lubricated tests with on-line wear measurements |
| Used with standard specimen bath and heater block | |
| Replaces standard specimen head | |
| TE 77/HSD: | For data logging instantaneous friction force and contact resistance signals |
| TE 77/LVDT: | For positional feedback for moving specimen |
| TE 77/HSD+TE 77/GB/20 | For Stribeck Curve (low speed sliding) and stick-slip experiments |
| Used with standard specimen head and bath | |
| TE 77/HSD+TE 77/GB/100 | For Stribeck Curve (low speed sliding) and stick-slip experiments |
| Used with standard specimen head and bath | |
| TE 77/HR: | For piston ring on cylinder liner tests where differential ring/liner temperatures are considered important |
| Replaces standard specimen head | |
| TE 77/INERT: | Primarily for dry tests where control of test atmosphere is important |
| Used with standard specimen head and bath and TE 77/PELTIER | |
| TE 77/COOLER: | For tests at reduced temperatures |
| Replaces standard heater pad and best used in conjunction with TE 77/INERT | |
| TE 77/800C: | For dry or lubricated high temperature tests |
| Replaces standard specimen bath and heater block | |
| TE 77/PUMP: | For controlled lubricant feed to test specimens |
| Used with standard specimen arrangement | |
| TE 77/PIEZO: | For fretting tests with continuously variable stroke |
| Replaces standard specimen head and reciprocating drive assembly | |
| TE 77/PD: | For converting from reciprocating sliding to pin on disc configuration |
| Replaces standard specimen head, specimen bath and heater block | |
| TE 77/EP: | For lubricated tests involving combined sliding and rolling with variable lubricant entrainment |
| Replaces standard specimen head, specimen bath and heater block | |
| TE 77/LLA + TE 77/TB: | For performing diesel fuel lubricity tests |
| Replaces standard reciprocating head and best used in conjunction with TE 77/CAM/1 | |
| TE 77/PLA | For pressurized lubricity tests |
| TE 77/MW: | For evaluation of ring/groove interface |
| Replaces standard specimen head, specimen bath and heater block | |
| TE 77/SRVF+B+C: | For tests with standard Optimol SRV size specimens |
| Replaces standard specimen head and bath |
The two most important parameters to calibrate on the TE 77 are the normal load and the friction force. TE 77/CAL provides a pivoted beam with dead weights able to apply up to 1,000 N to the loading system and a pulley, cord and weights to apply a tangential force to the specimen bath to check the friction measurement.
Revised TE 77/06/09
Copyright © 2009 Phoenix Tribology Ltd.