ISO TR 8285:1990 download free.Plain bearings – Evaluation of the tribological properties of polymer-based materials.
1 Scope
This Technical Report deals with the tribological testing of bulk pla8tic specimens under specified working conditions, i.e. load. sliding velocity and temperature, without lubrication. From the tt results, data are obtai,ud about the tübolotjivai behavluu# of steel-plastic rubbing pairs which can be used In practice for predicting the working capacity of a friction unit.
It specifies the test conditions, equipment and programmes that allow reproducibility of the results at particular conditions of the specimen preparation and testing.
This Technical Report specifies the specimen shape, conditions of the rubbed surfaces of the plastic and steel, tolerable run-out of the rotating counterface, heat transfer between the device housing and counterface, the range of variations In the friction process parameters in order to reach the reproducibility of the results In the tribological testing of plastics. It proposes tochnical aolutions for tho standard oquipmont usod for the tnbologlcal testing of plastics.
2 Symbols and units
See table 1.
3 DefinitIons
3.1 coeffIcient of friction, f: Ratio of the friction force resulting from the sliding of a plastic against a steel to the normal force pressing the two bodies together:
3.2 wear rate, wt: Ratio of the linear wear of the tested plastic block to the testing period:
3.3 wear Intensity. w,: Ratio of the linear wear to the sliding distance:
3.4 wear factor, k: Ratio of the wear intensity to the normal specific load:
4 Shape end manufacture of test specimens
To estimate the tribological properties of plastics in accordance with this Technical Report, a tribosystem should consist of a steel roller and a plastic block with mateable friction surfaces (see figure 1). The possible materiel combinations for the block and the roller are given in table 2.
4.1 Plastic block
The block can be made by moulding, injection moulding, turning or by milling, making use of the near surface structure effects formed by moulding or machining (see figures 2 and 3).
The block Is 10 mm high, 10 mm wide and 20 mm long (see figure 2).
The roughness of the plastic block depends on the machining conditions; in case of moulding or injection moulding, it depends on thu roughness of the moulding parts (R1 — 0,3 pm to 0,6 pm).
4.2 Test roller
A test roller complying with this Technical Report is a combination of a steel bushing and a heat-insulating bushing. The heat-insulating bushing Is made of textolite and Is pressed into the steel bushing. The dimensions of the mating parts are shown in figure 4.
The steel bushing Is made of steel with a carbon content of between 0,45 % and 0,8 %, of hardness 45 HRC to 50 HRCS for example stool complying with ISO 683 (all published parts).
The thermally Insulated roller Is designed for comparative tests; It provides reproducibility of the results when testing equipment of different designs Is employed.
For certificate testing, the temperature control of the test roller Is important. A schematic representation of the thermal regulation is shown in figure 5.
The surface roughness, R8, of the roller may have the following values, depending on the machining or preparation of the roller friction surfaces:
a) R0—0,63i.tmtoo,8pm;
b) R5 0,25 pm to0,32 pm; ci R5 = 0,04 pm to 0,05 pm.
Alternative b) Is preferred when estimating the tribological properties of plastics.
5 Preparation of test specimens
The specimens (block and roller) should be cleaned before the tests to remove various contaminants with liquids that do not dissolve the specimen material.
5.1 Polymeric block
The block surface should be cleaned with a liquid at a temperature of 23 °C ± 5 °C, at a relative humidity of 50 %. The blocks should be dried in air and then placed In a vacuum at 1O torr for 2 h.
Liquids easily volatile at 23 0C ± B °C that do not dissolve plastics or their components, for example petrol, acetone,
ethanol or cyclohexane, shall be used for cleaning. Cleaning should be carried out In a bath of one of the above liquids.
The recommendations of the plastics manufacturer should be taken into account when choosing a liquid for cleaning.
5.2 Test roller
One of the liquids given in 5.1 should be used for deaning the roller surface. The roller should be dried in an oven at 50 °C to 60 °C without ventilation.
6 Test equipment
Testing according to this Technical Report may be carried out with block-roller devices presently available. They need to be modernIzed and some technological limitations have to be observed to ensure the reproducibility of the results.
The technological limitations are as follows.
6.1 The comparative tests should be carried out with a heat- insulating bushing botwoen the teat shaft end roller (ace figures 4 and 6). The heat dissipation will Improve the reproducibility of the test results obtained with equipment of different designs.
6.2 The certification tests shall be carried out with thermally regulated rollers (see figure 5) or a multistep device with a thermally regulated shaft (see figure 7). The thermostatic regulation Is done to estimate the tribological properties of plastics at a constant counterface bulk temperature.
6.3 The radial run-out of the roller working surface should not be greater than 25 pm.
The test rig should allow
a) the smooth control and maintenance of the sliding velocity at a given level within 2,5 % of the existing value;
b) continuous loading of the specimen (block) within 2,5 % of the existing value;
c) measurement of the friction coefficient, temperature of the roller sliding surface, and the linear wear of the block.
The continuous control of the linear wear Is realized with a special instrument with an accuracy of ± 0,5 $.tm [the use of such an Instrument cuts the testing period of the specimens after running-in (see figure 6)1.
To measure average temperatures of the plastic and metal friction surfaces, a thermocouple is located within a through-hole in the plastic block 0,2 mm above the friction surface.
At the stage of thermostatic regulation, the required bulk temperature of the roller and the shaft in a multistop machine is controilod by variations In the oporoting oonditiono of tho liquid thermostat.
7 Test procedure
7.1 EnvIronmental conditions
Ambient conditions for the tests should be a temperature of 23°C ±5°C and a relative humidity of 40 % to 60 %. Devfalion from these conditions should be noted in the test report. In order to reproduce a required environmental regime, the pair tested should be placed In a confinod spoco.
7.2 Mounting of the specimen
Mount the roller with the pruptred sliding surface on the shaft of the test machine and fix it. Measure Its radial run-out with a mechanical indicator graduated to 1 pm.
Place the cleaned block of plastic in the holder without a clearance (see figure 6). The tangential friction force Is taken by the wall of the holder housing, which has a high bending strength.
The protruding part of the specimen shall measure at least 3 mm.
7.3 RunnIng-In of the block
The plastic specimen should be run In on the roller surface before the test. The purpose of the running-In procedure is to achieve full contact along the rubbed surface of the plastic block.
The running-In period depends on the accuracy of manufacture and relative mounting of the specimens, block materials, conditions of the running-in, friction transfer processes and secondary structures formed on the block surfaces. The friction transfer produces a plastic film 0,5 pm to 3 pm thick over the metallic surface.
With accurate manufacture and mounting of the block and roller, the running-In period lasts 3 h to 7 h. The basic criteria of the completion of the run-in are a steady-state wear, a constant friction coefficient, f, and full contact between the block and roller surfaces.
7.4 Test programmes
This Toohnlcol Roport opooifloa two spoclal programmes.
7.4.1 Programme C
The programme is used to compare the material properties. The tribological properties of plastics are estimated under conditions similar to operating regimes.
The loading-velocity regimes of testing are arbitrarily divided into low, medium and heavy (see table 3). Cach loading zone is confined to the maximum load and temperature in the contact zone associated with the compression strength o and the melting point Tm (or softening point 7) of the plastic.
In comparative testing, the thermal constant of the friction test unit, Q, should be taken into account:
Q is estimated experimentally and given in the test report. The value of Q is maintained within the range 7 x iO °C•ms/N to 12 x iO °C•m.s/N by means of a thermal Insulator between the test roller and the shaft.
At the first stage of the comparative programme, toad values are taken from table 3, i.e. one value for each loading zone. Next, minimum and maximum sliding velocities are calculated (see table 3) to compile a matrix for the loading-velocity regimes.
The matrix columns and rows ore used as thu programme. Tests are carried out three times at each regime with recording of the friction forces wear and temperateim in the contact 7one The value of wear measured during tests should be of one order higher than the error of the measurements.
After the specimen has been tested at one of the regimes. polish the surface of the test roller with emery cloth and clean It according to 5.6. Emery cloth is placed on a piece of wood of dimensions 200 mm x 50 mm x 10 mm with vulcanized rubber cemented onto It. Polishing1 carried out on the test machine or specifically designed test rig, should give the roller surface its Initial roughness (see 4.2) before starting testing at the next regime.
7.4.2 Programme P
Certification tests are carded out to obtain reference data for the tribological properties of plastics.
The certification tests cover from 9 to 27 loading-velocity regImes. The values of Pg. Uj and T1 are determined from table 3, and a matrix of the test results is compded:
The tests are carried out three times at each loading-velocity regime. Continuous measurements of the wear amount are taken with a special rig, having a error of not more than ± 0,5 pm (see figure 6).
Carry Out the tests at temperatures 7at the nine regimes with a single specimen. Then change the specimen and repeat the tests. When the regime is changed to a heavier one, do not clean the roller surface; this saves time of non-steady wear necessary for the formation of the friction transfer film.
When the specimen Is changed, scoure the roller surface with emery cloth to obtain the Initial roughness as In 4.2 and then oleen it according to 5.2.
8 Analysis and test report
8.1 General
The following test conditions should be included in the test report:
a) environmental conditions (temperature and humidity);
b) type of test programme and loading regimes;
c) details about the plastic (see 4.1), its compression strength and melting temperature (softening point of the polymer);
d) details about the roller (see 4.2) (steel grade, Rockwell hardness and surface roughness);
a) details about cleaning and running-In of the rubbed plastic and roller surfaces.
8.2 Test results
At steady friction and wear with the plastics, the following parameters shall be given as functions of normal specific load p, sliding velocity u, and temperature T:
a) average value of the friction coefficient;
b) wear rate of the plastics;
c) wear coefficient of the plastics.