Mahle GmbH (Ed.) Cylinder Components: Properties, applications, materials

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3.2 Design guidelines 53

Figure 3.8:

Test setup for testing fatigue

resistance of bearings

Figure 3.9:

Effect of bearing material

quality on specific bearing

load

54 3 Bearings

3.2.3 Wear resistance

To determine a ranking of bearing materials with regard to their wear resistance, a “block-on-

ring” machine (Figure 3.10) is used.

Wear resistance is particularly dependent on the hardness of the material. Galvanized tri-

metal bearings can therefore not be compared to bi-metal bearings, because the overlays

are very soft.

Figure 3.11:

Effect of bearing material on

wear resistance (bars show

the 90% confidence average)

Figure 3.10:

Principle of testing wear resistance of bearing

materials

Load: 267 N

Speed: 200 rpm

Duration: 5,000 cycles

Oil: SAE 30 at 120 °C

Ring: SAE 4620, 58-63 HRC, R

= 0.20 μm

3.3 Bearing geometry 55

3.2.4 Seizure resistance

Similarly, seizure resistance is evaluated on a pin-plate test machine under increasing load

(Figure 3.12). Hardness and the presence of soft phases in the material inﬂuence the rank-

ing.

Figure 3.12:

Principle of testing seizure

resistance of bearings, and

test results

Load: Increase in loading

in steps of 10 N

every 5 min

Speed: 850 rpm

Duration: Until seizure

Oil: G5, room tempera-

ture, 7 g/min

Washer: SAE 4340, 58-62

HRC, R

= 0.05 μm

3.3 Bearing geometry

3.3.1 Bearing diameter and width

Peak oil ﬁlm pressure (POFP) and minimum oil ﬁlm thickness (MOFT) are strongly associ-

ated with the bearing diameter and bearing width. The width/diameter ratio inﬂuences the

operating characteristics of the bearing. A larger bearing width reduces POFP and increases

the minimum oil ﬁlm thickness. A larger diameter has the same effect. For a given projected

bearing surface, the bearing with the higher width/diameter ratio experiences lower oil ﬁlm

56 3 Bearings

pressures, greater minimum oil ﬁlm thicknesses, and thus more advantageous load condi-

tions.

3.3.2 Oil grooves and holes

The lubricating oil enters the bearing through grooves and holes. They also have a signiﬁcant

inﬂuence on the function of the bearings. They are undesirable in loaded areas, because

they increase the maximum oil ﬁlm pressure and reduce the minimum oil ﬁlm thickness. If

the grooves and holes are poorly located, there is an increased risk of contact between the

sliding counterparts or cavitation damage to the bearing material.

3.3.3 Bearing clearance

Bearing clearance has a twofold effect on the properties of the oil ﬁlm. With less clearance,

the loads are better distributed, because the journal curvature is nearly identical to the defor-

mation of the bearing and generates a lower maximum oil ﬁlm pressure. On the other hand,

Figure 3.13:

Maximum oil film pressure

POFP as a function of bearing

clearance at various rated

power levels

Figure 3.14:

Minimum oil film thickness

MOFT as a function of bear-

ing clearance

3.3 Bearing geometry 57

lower clearances also generate more heat, which reduces the oil viscosity. The maximum oil

ﬁlm pressure increases more or less proportionately with greater clearance (Figure 3.13), and

the minimum oil ﬁlm thickness decreases (Figure 3.14). The recommended starting value for

diametric clearance is 0.1% of the bearing diameter.

3.3.4 Bearing and bushing fit

A properly designed ﬁt of the bearing in its housing ensures a reliable seat and good heat

transfer due to radial tension. This is achieved through correct design of the bearing overlap.

For bearings, this overlap results from the protrusion of the parting line height beyond the

housing radius, (Figure 3.15). For bushings, it is the difference in diameter between the hous-

ing ID and the bushing OD.

Computation of the overlap allows optimization of the assembly conditions. The calculated

radial pressures should be greater than 10 MPa, and the stresses in the bearing should not

exceed 450 MPa.

3.3.4.1 Eccentricity

Bearing eccentricity is the difference between the vertical and the horizontal diameter. The

eccentricity helps in generating adequate oil ﬁlm thicknesses, but also avoiding any heavier

contact of the journal on the sliding surface when the connecting rod closes in towards the

par

ting line region due to dynamic distortion. A simulation of the elasto-hydrodynamic lubri-

cation (EHL), using a special analysis program, allows the selection of the optimal eccentric-

ity for each application.

Figure 3.15: Development of radial pressure

58 3 Bearings

Figure 3.16:

Movement of the journal during a com-

bustion cycle

3.4 Numerical simulation

In the development of an engine component, time and costs play an important role. For this

reason, a great deal of effort is invested in analysis methods during development, in order to

evaluate components and adapt them, based on the results, prior to starting tests. Programs

are available for simulating the behavior of bearings, bushings, and thrust washers in combi-

nation with assembly and operating parameters.

3.4.1 Hydrodynamic lubrication (LOCUS)

To simulate the motion of the bearing journal in the bearing, the two-dimensional Reyn-

olds equation is solved numerically using the ﬁnite difference method. The most important

simpliﬁcation in this case is the assumption of a rigid housing. The primary results of the

simulation are the maximum oil ﬁlm pressure (POFP), which occurs in the gap between the

bearing journal and the bearing surface, and the minimum oil ﬁlm thickness (MOFT), which

corresponds exactly to the gap width. The data required for performing the analysis are the

operating parameters of the engine, the crankshaft and bearing geometries, and the proper-

ties of the lubricant.

The results of the analysis show the displacement of the bearing journal (Figure 3.16) and the

oil ﬁlm pressure and oil ﬁlm thickness during the entire engine cycle (Figure 3.17).

3.4 Numerical simulation 59

Figure 3.17: Connecting rod bearing load (POFP) and minimum oil film thickness (MOFT)

during one revolution of the crankshaft

3.4.2 Elasto-hydrodynamic lubrication (EHL)

To obtain more precise results, the same model is used for hydrodynamic lubrication, but

with the deformation of the housing due to the bearing load taken into consideration. The

rigidity of the housing is determined using a ﬁnite element model and is additionally entered

into the program. This results in more realistic values for oil ﬁlm thickness and maximum oil

ﬁlm pressure (Figure 3.18).

The use of the elasto-hydrodynamic theory assumes mixed lubrication, which takes into

consideration not only the hydrodynamic pressure but also pressure leading to metal to

metal contact.

One criterion for evaluating these analysis results is the minimum oil ﬁlm thickness. The

pressure on the roughness peaks (peak asperity contact pressure, PACP) can be used as an

additional variable for evaluation.

60 3 Bearings

3.4.3 Axial bearing simulation (ABAS)

To simulate the behavior of the contact surfaces of axial bearings by testing their geometric

properties under static load, a special analysis program has been developed. The analysis

process follows the schema shown in Figure 3.19.

The resulting maximum oil ﬁlm pressure and the resulting minimum oil ﬁlm thickness are

used to evaluate the effects of load, pad area, the ratio of inner to outer diameter, taper

conﬁguration etc. A parameter study that analyzes the inﬂuence of a taper conﬁguration and

loading on MOFT is shown in Figure 3.20.

3.4.4 Overlaps (PRESSFIT)

The behavior of the bearings and bushings depends on how securely these components

are installed in their housings. A proper ﬁt ensures that the part has a reliable seat and pro-

vides appropriate heat transfer. This secure seat is simulated based on the assembly of two

concentric tubes with overlap, using a dedicated analysis program. The data entered consist

of the geometric features of the assembly and the housing, the properties of the bearing

material, and the operating temperatures. The results are stresses and diametric overlaps or

clearances at different temperatures.

Figure 3.18:

Bearing load without and

with consideration of housing

deformation

3.5 Bearing materials 61

3.5 Bearing materials

Selection criteria for bearing materials include the load and the permissible stress of the

material. The capacity limits are determined for each material on the basis of simulations,

bench tests, and engine testing. They are lower for main bearings, due to potential alignment

errors.

For axial bearings, the selection of the material is based on empirical analysis, considering

the geometric and material factors. The speciﬁc area load should be less than the product of

“geometric factor x material factor.”

Figure 3.19:

Simulation process for optimi-

zation of an axial bearing

Figure 3.20:

Bearing load and minimum

oil film thickness in an axial

bearing, according to the

simulation

62 3 Bearings

3.5.1 Composition and properties of bearing materials

Table 3.2: Aluminum alloys

Descrip-

tion

Chemical composition of

the base alloy

[%]

Application/

properties

Process Minimum

hardness

(alloy/steel

back)

Specific

bearing

load

capacity

[MPa]

Design

Al Sn Si Cu other

MAS

92 6 1 Ni 1 Camshaft

bushings

Cast alumi-

num alloy

roll-plated

on steel

MAS 11:

45–70 HR 15T

40–70 HV

Steel:

82–99 HRB

150–235 HV

40 Bi-metal material

with fine precipitation

of the tin phase

in an aluminum

matrix, combined

with an intermediate

aluminum layer and

roll-plated onto a low-

carbon steel back

MAS

79 20 1 Plain

bearings,

bushings, and

thrust was-

hers; low load

capacity with

high seizure

resistance

and embeda-

bility

Cast alumi-

num alloy

roll-plated

on steel

MAS 15:

40–65 HR 15T

35–53 HV

Steel:

82–99 HRB

155–235 HV

40 Bi-metal material

with fine precipitation

of the tin phase

in an aluminum

matrix, combined

with an intermediate

aluminum layer and

roll-plated onto a low-

carbon steel back

MAS

79 20 1 Mn

0.25

Plain

bearings,

bushings,

and thrust

washers;

medium load

capacity with

high seizure

resistance

and embeda-

bility

Cast alumi-

num alloy

roll-plated

on steel

MAS 16:

62–69 HR 15T

48–62 HV

Steel:

82–99 HRB

155–235 HV

50 Bi-metal material

with fine precipitation

of the tin phase

in an aluminum

matrix, combined

with an intermediate

aluminum layer and

roll-plated onto a low-

carbon steel back

MAS

84 10 4 2 Plain

bearings,

bushings,

and thrust

washers;

medium load

capacity with

high wear

resistance

Cast

aluminum

alloy hot

roll-plated

on steel

MAS 17:

55–63 HR 15T

42–53 HV

Steel:

60–96 HRB

110–210 HV

50 Bi-metal material with

fine precipitation of

the tin and silicon

phase in an aluminum

matrix, roll-plated

onto low-carbon steel

with a galvanically

applied intermediate

layer of nickel