GrafLube

GrafLube

Design Considerations

In this article, information is provided on GRAFLUBE Bearings & Sleeve Bearings. Essential design considerations are outlined in an attempt to help you select an adequate bearing and recommend the use of GRAFLUBE bearings for better machine design.

Design Notes

GRAFLUBEWhen selecting a material to be used as a plain bearing, be it a new, replacement or alternate application, consideration must be given to certain physical & chemical properties that will permit it to operate properly.

Generally, there are six properties accepted as the main criteria to which plain bearings should be compared:

  1. Load Capacity/ fatigue resistance - is the ability of the material to withstand the hydrodynamic pressures during service as well as repeated stresses and strains without cracking, flaking or being destroyed by some other means. GRAFLUBE bearings are designed to meet individual applications, with a wide range of bearing matrix materials to choose from, with yields strengths in excess of 450mPa GRAFLUBE can meet the most rigorous in service demands.

  2. graflubeEmbeddability - is the ability of the bearing lining material to absorb or embed within itself any foreign matter that may ingress between the journal and bearing.

    Poor embeddability permits particles to circulate around the bearing to score both the bearing and shaft. GRAFLUBE'S inherent design features allow for foreign particles to be embedded in the graphite lubricating system keeping the journal and bearing running surfaces clean and in good condition.

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  3. Malleability - or the ability of a material to creep or flow slightly under load, as in the initial stages of running, to allow the journal and bearing contours to conform with each other or to adjust erratic loading caused by mis - alignment. GRAFLUBE copper alloy bearing matrix provides excellent creep characteristic.

  4. Antiscoring Tendencies - the compatibility of the journal and bearing to resist galling and or seizing conditions of metal-tometal contact i.e. start up. GRAFLUBE Bearings are supplied with 1.6μm or better surface finish

  5. Corrosion Resistance - to the operating environment, including organic acids that may form in oils at operating conditions.

    GRAFLUBEGRAFLUBE bearings are general supplied in any of the copper / bronze alloy family matrix materials, which inherently provide excellent resistance to most organic acids and corrosive environments. Special matrix materials are available for harsher applications.

  6. Thermal Conductivity - the ability to absorb and disperse heat generated in the bearing. GRAFLUBE bearings' solid graphite lubrication system ensures that heat energy is dissipated from the mating surfaces through the superior thermal conductivity of graphite as well as the copper alloy matrix material. Generally, if a bearing material does not posses all of these characteristics to some degree, it will not function long as a bearing.

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  7. It should also be noted however, that few if any conventional bearing materials are outstanding in all these characteristics. At best using conventional bearing materials is a compromise to secure the best combination of properties needed for a specific application.

GRAFLUBE inherent design features answers the above criteria, when all operating parameters are taken into consideration and a GRAFLUBE solution determined, users can expect a significant increase in bearing life and a direct reduction in down time and running costs.

 

1.1 Mechanical Properties

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1.2 Service Life

graflubeThe service life of a sleeve type bearing, which depends on the amount of wear, is greatly effected by friction conditions - regardless if it is dry, low lubrication or fluid film lubrication. Wear may be estimated by the following experimental formula.

Theoretical estimation becomes almost useless when a rapid change in wear occurs due to lack of lubricant or invasion of foreign material.

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If the specific wear amount K is known, the wear amount W for a given running time T is determined and hence, the life of the bearing is calculated from an allowable wear amount of the bearing. Note: that the estimated life given y the above formula should be only used as a reference because other factors which also effect bearing life have not been taken into consideration. These factors include the influence of velocity and load, different modes of operation, type of lubricant used, clearance, surface roughness of matting material, invasion of foreign matter, etc. Experimental values of specific wear K for general radial journal sleeve bearings under different lubrication conditions are given here for reference.

graflubeNo lubrication: lack of oil....1 x 10-3-4
Low lubrication condition due to low speed etc....1 x 10-3-5
Relative good conditions due to greasing etc....1 x

GRAFLUBE (Solid lubricant bearings under low speed, non-lubricating and high load operation) ....10-7-8 The invasion of foreign particles or the lack of lubrication will lead general oil lubricated bearings to a sudden rise in specific wear amount K, followed by seizure. In contrast GRAFLUBE bearings are not sensitive to foreign particles and lack of lubrication and last longer in such conditions. Their excellent reliability and safety have been verified by R & D and by experience over an extended period of time.

1.3 PV Value

PV Value (product of bearing pressure P and velocity V....Kgf/cm2) is an important parameter in selecting a GRAFLUBE bearing. The friction calorie Q generated in unit area of bearing for unit time isobtained from the energy loss P and V, using the following formula.

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Assuming that the co-efficient of friction is a constant, the friction calorie Q, a criteria in selecting of bearings, is proportional to PV value. During continuous rotation of a shaft, the friction heat generated at the bearings and the eat lost by radiation maintains the bearing temperature at a constant balance. After an extended period of operation, however, the co-efficient of friction may increase due to changes in the sliding surface conditions. The existence of foreign particles, deteriorated lubricants, wear particles, material fatigue and other factors will influence the co-efficient of friction.

As a result, when the bearing temperature rises, eventual damage to sliding surfaces or seizure is considered to occur. The lower the bearing temperature stays, or in other words, the smaller the PV value is, the lighter the load condition of the bearing is considered to be. Safety design of bearings means that bearings are to be designed to lower PV value, which leads to a longer service life. Furthermore, under certain conditions, the PV value can exceed the maximum.

Calculated PV Values for Bearing Alloys assuming excellent lubrication.

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Calculated Dry Friction Values for Bearing Alloys

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1.3.1 Maximum Allowable PV Value

The maximum allowable PV value is defined as the maximum product of the pressure on a projected unit area of the bearing and the velocity allowable in the design of a bearing. Also specified are the maximum allowable load and velocity. The design value must be within the range enclosed by the solid lines in the following graph (FIG 1). The PV value is calculated from the formula below:

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In the case of a radial journal bearing, the projected area is obtained by multiplying the inner diameter of the bearing by the bearing length. In the case of a rotational operation, the following should apply.

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1.4 Bearing Load

In general, the bearing pressure is obtained by dividing the maximum load imposed on the bearing by the pressure supporting area of the bearing. The pressure supporting area is defined as the projected loaded area, which contacts with the shaft, projected in the direction of the load in cases of a cylindrical and spherical bearings.

1.5 Velocity

The main cause of generated heat is the work done at the friction surface of the bearing. It is known from experience that the rise in temperature at the friction surface is affected more by the velocity than by the pressure. With the same PV value, the larger the V value is the higher the bearing temperature will be. When used in a high velocity operation, it is recommended that the bearings should be designed and used in such a manner that the co-efficient of friction be reduced by positive supply of oil to enhance both cooling and lubricating effectiveness, in order to take advantage of their wear resistance and anti-seizure.

Direction of Motion and PV Value

In case of continuous, one directional, rotational motion of a radial journal, proper fluid film lubrication is easily accomplished for trouble-free operation. In case of a thrust bearing, however, careful consideration must be given in designing the lubrication method and oil grooves so as to insure proper lubrication of the sliding surface. More over, when intermitted operations, oscillating motion or reciprocating motion is encountered. The maximum allowable P values, indicated in the catalogue, are the values applicable to a rotational and oscillation motion of a thrust bearing; about half of the maximum allowable P value should be employed as indicated in the catalogue.

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Address: 42 - 44 Doyle Avenue, Unanderra,
NSW 2526 Australia

Phone: +61 2 4272 5344
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