INSTRUCTION MANUAL FOR BEARINGS AND BUSHINGS RAR-12 AND RAR-27

          The new materials for creation of bearings and bushings of wide performance are alloys from system ZnAlCuMg, which are, in the western countries, widely used not only as a substitute for deficitated bronze, but also as a material of high tribological characteristics. Those alloys are in the USA part of the standards ASTM B669-89 and ASTM B791-88. RAR produces two types of alloys from this system, and they are called RAR-12 and RAR-27. Here is stated instruction manual for usage of them.

          1. HARDNESS OF SHAFT – If the hardness of shaft is 16-25 Rc, the RAR-12 should be used. For harder shadts (from 22 Rc and more), and particulary those with the hardness of 40-45 Rc –RAR-27 is used. In case where hardness of shaft is below 160 HB (below 16 Rc) it is not recommended to use these alloys.

          2. SLIDING VELOCITY AND USAGE IN BEARINGS – At all sliding bearings, sliding velocity and presure in bearing are in disproportion, the bigger the sliding velocity is – the smaller the presure in bearing must be. Their proportion is given in the following table. ANNOTATION: This statement stands if all other conditions stated in this document are fullfilled. Either way, you should try to have as smaller as possible presure in bearing, with specificed sliding velocity.

          3. THE TEMPERATURE OF BEARING – The highest allowed temperature of the bearing is 120°C. As the bearing is being heated, due to substantional rubbing, that rubbing should be reduced, for example, by having the proportion of sliding velocity (m/sec) and pressure (MPa) in the bearing above 0.01, and even better above 0.03, when the temperature of the bearing drops down to 40°C (of course, other stated recommendations should be fulfilled also).

          4. WEARING – Wearing is directly dependant of presure in the bearing. If that presure is below 8 Mpa, wearing of the bearing can be waived, with fullfilling other recommendations stated here.

          5. FINENEES OF FABRICATION – The quality of the bearings has a great influence on the intensity of wearing shafts and bearings. It is because of that that those surfaces must be very smoothly fabricated, with harshness not greater than 0.4-0.6×10^(-6) m. The intensity of wearing is significatly lowered with polished surfaces (harshness lesser than 0.2×10^(-6) m).

          6. GAP, SHAFT, BEARING – The diameter of the bearing – Dl – depends on the diameter of the shaft – DR – and can be calculated in the following way:
          – for lower temperatures (10-40 °C) Dl=1.0025 x DR
          – for higher temperatures (40-120 °C) Dl=1.0028 x DR
            (if the bearing is not contracted then Dl=1.0021 x DR)
          The bearing which is used on a higher temperature should be, before final refinement, thermaly stabilized (12h on 250°C).

          7. ALLOWANCE, BEARING/SHELL – Embeddment of RAR-bearings in its shell is done in the same way as the regular bronze bearings. Outer diameter of the bearing, Dls, is greater than the diameter of the shell Dk in which the bearing is being put (Dls=(1.0005 / 1.001) Dk). The bearing should not be put to its shell at the temperatures below 10°C, but with 15-30°C. If, by some reason, embedment is done on high temperatures, say between 60-120°C, then Dls becomes bigger, and can be calculated as Dls=(1.0007 / 1.0014) Dk.

          8. THICKNESS OF BEARING – The smallest thickness of the bearing can be calculated as followed:
Minimum thickness = 0.05 x diameter of shaft + 1.0 mm.

          9. LUBRICATION – For RAR bearing the same lubricants should be used as for the regular bronze bearings, under condition that they are not of animal or plant origin, but specificly of mineral origin. For bearings with diameters under 20mm no channals for lubrication are needed. For bearings with bigger diameters, depending of conditions of usage, can be used channals for lubrication of same characteristics as for the bronze bearings. Replacement of the lubricants is obligatory in certian time frames, depending on the wearing of lubricants. If the usage temperature is high, the replacements are more often.

          10. CORROSION – RAR alloys have very good resistability on corrosion, so it can be safetly used even in very poluted industrial and sea atmospheres.

          11. MECHANIC REFINEMENT – Due to the fact that the alloys RAR-12 i RAR-27, as a material for sliding bearings and other mechanic parts, are new on domestic market, we consider usefull to, as their manufacturer, give recommendations in a view of their mechanic refinement. First, we would like to emphasize is the fact that this alloys have substantial mechanic refinment ability, much better than most of the other alloys which are on a brass level of refinement. Second, parameters of mechanic refinement which are used with ZAMAK alloys can be used for this alloys as well, especiall for RAR-12. In most cases of mechanic refinement RAR-12 i RAR-27, the tools of highly carbonated steel can provide satisfactiory results. Still, the best results are provided by usage of tools of the fast cutting steel or ceramics. Cutters and similar tools should have big pikes of helicoid shape.
          DRILLING – For drilling of shallow holes, the drills made from carbonic steel are used, while, for drilling of deep holes, the fast cutting steel drills should be used, sharped with the angle of 150°, while their helicoid angle is 38°. While drilling, the wetting with waterized substance (emulsion).
          REFINEMENT ON BENCH – While refining on bench, the water substance should be used for lubrication and cooling. All angles on a knife should be 8°. The velocity of surface should be 90 m/min. The shift should be 0.15-0.2 mm/spin, and 0.05 mm/spin at the final refinement. The knife must be sharp. Under this condition of refinement, no stickiness should be attached to the knife, and the surface will have the high glow.

          12. ADVICES – If, with this manual, some aspects of usage that are common in user practice of RAR bearings are not covered, RAR® company is available for consultation and solvement of problems considering the usage of their bearings.

RECOMMENDED PROPORTION OF SLIDING VELOCITY AND PRESURE IN THE BEARING

MECHANIC PROPERTIES RAR-12 AND RAR-27