What must be taken into account when calculating load ratings of rolling bearings?

When using the standardized calculation methods for ball bearings, roller bearings and needle roller bearings, the following points should be considered:

Basic static load rating:

1. The static load rating corresponds to a load at which the raceway of the bearing is permanently plastically deformed. These deformations can be seen under a microscope. The rolling elements have a small flattening  and an impression of the rolling element can be seen in the raceway. When this high load is applied very frequently, many imperfections occur on the rolling partners.
The flaws can affect the smooth running of the bearing. Plastic deformation also generates stresses in the material. In the worst case, the increased stress in the material and the poorer running smoothness can reduce the operating life.

2. In practice, the basic static load rating is often already exceeded by improper installation. This can happen, for example, when a deep groove ball bearing is pressed into a bore and the press-in force acts on the  bearing via the inner ring. After such an overload, the bearing still looks good externally, but the operating life may already be severely limited.

3. Damage to the bearing can also occur during transport of the fully assembled bearing. If a heavy rotor is transported over bad roads without a transport lock that completely relieves the bearing, the bearing can be      damaged by the transport alone.

Basic dynamic load rating:

1. The calculation of the basic dynamic load rating is an estimate. This estimate means that a certain percentage (90% according to the standard in the simplified procedure) will achieve the calculated service life.    Conversely, 10% may fail beforehand, a ratio that in practice is significantly undercut in the case of good quality. But no matter how high the ratio is, bearings can fail before reaching the service life even with the best quality.

2. If the service life calculation results in a service life of 1,000h or less, the result should be viewed with the greatest caution. The load on the bearing is then extremely high. The estimate is so inaccurate in this range that it should be backed up by suitable tests.

3. The service life calculation is based on the assumption that a lubricating film is built up by the rotation of the rolling elements. This lubricating film prevents direct metallic contact between the rolling elements and  the raceways. At speeds > 20 1/min or during pivoting movements, the lubricating film can be broken through and this considerably reduces the operating life. In such cases, it is also advisable to carry out suitable        tests to ensure that this is the case.

What must be considered when selecting a suitable grease?

Lubricating greases

The lubricant has various tasks. It should build up a separating, load-transmitting lubricating film between sliding and rolling parts and protect against corrosion.

Lubricating greases are thickened synthetic or mineral oils; metal soaps are usually used as thickeners. The grease may also contain other additives to improve its properties. The consistency of the grease depends largely on the type and mixing ratio of the thickener.

Decisive for the selection of a lubricating grease is the viscosity of the base oil, the consistency, the temperature application range, the corrosion protection properties and the load capacity.

Selection of lubricating greases

Rod ends requiring maintenance and roller bearings, as well as rolling bearings, are usually lubricated with high-performance lithium soap greases. These greases normally have a temperature range of -25°C to + 120°C. The greases can withstand a service temperature of + 120°C for short periods. Above 70°C continuous temperature, these
standard lithium soap-based greases, a reduction in grease operating life must be expected.

Sufficient service life values at higher temperatures can only be achieved with special greases.

However, it is essential to bear in mind that the operating limit of the contacting rolling bearing seals used in the standard is +110°C. For applications above this limit, it must be checked whether seals made of heat-resistant materials can be used.

Depending on the application (e.g. food industry), the lubricant must also be selected.

 

Examples of greases:

Shell:                Gadus S2 V100 2 / Alvania RL2

Kyodo:             Multemp. SRL  

Esso:                Polyrex EM      

Sinopec:           Great Wall No. 2

Klüber:             Isoflex LDS 18 Spec. A

 

Food greases:

Klüber:             Klübersynth UH1-14-151 (NSF H1 registriert)

Mobil:              FM 222 (NSF H1 registriert)

 

Special lubricating greases for high requirements:

Klüber:             Klüberplex BEM 34-132

Which fit is suitable for spherical plain bearings and rolling bearings?

Installation instructions and fits for shafts and spherical plain bearings as well as for shafts and rolling bearings.

Spherical plain bearings

Installation instructions

So-called impact caps (Fig. 1) are suitable for mounting small bearings with fits that are not too tight.  

When mounting on the shaft and in the housing at the same time, impact caps with one contact surface each for the inner and outer ring should be used (Image 2). The installation force must never act directly on the sliding surfaces. Presses are usually used for fitting bearings in larger quantities.

Larger bearings can generally no longer be installed when cold because the installation forces increase sharply with increasing bearing size. Bearings or housings are therefore heated before installation. The required temperature difference between the bearing ring and the respective counterpart depends on the diameter of the bearing seat and the fit interference. However, the bearings should not be heated higher than +110 °C if possible.

Heating devices or heating cabinets are used to heat the bearings. If the bearings are heated on electric heating plates, they must be turned several times to ensure uniform heating. Local overheating must be avoided at all costs

Fits

The outside diameter of spherical plain bearings, dimensional series K is toleranced with h6. Dimension series E see DIN EN 12240-1.

The bore Ø of the inner ring is toleranced with H7 for spherical plain bearings, dimensional series K. For dimensional series E, G and W see DIN EN 12240-1.

These fit guidelines do not release the user from the obligation to check the fit selection for his application!

 

Rolling bearing

Fits

 

Fits should ensure sufficient radial fastening of the rolling bearings to prevent sliding movements in the
contact surface. As a rule, this can only be achieved with tight fits.

The advantage of tight fits is that the relatively thin-walled bearing rings are supported over their entire circumference, which has a positive effect on the utilization of the service life.

However, this cannot always be realized since requirements such as the displaceability of a non-locating bearing or easy mounting and dismounting must also be considered.

The following influences must be considered for the fit selection:

 

Type and size of load

A distinction is made between circumferential load, point load and indeterminate load direction.

Circumferential load is present when the ring rotates and the load is stationary or when the load rotates and the ring is stationary, i.e., when during each revolution each point of the raceway is loaded once. Bearing rings with circumferential load tend to wander in the circumferential direction, which is why a tight fit must be provided in all cases. If this is not observed, fretting corrosion will occur because of the bearing ring wandering,

i.e., dry friction occurs between the contact surfaces and the two surfaces finally seize up. The greater the loads and impacts, the tighter the fit must be.

Point load is present when the load is stationary when the ring is stationary or when the load rotates when the ring is rotating. Bearing rings with point loads do not tend to wander. Therefore, a loose fit is permissible in this case.

Indeterminate load direction is present when both point load and circumferential load occur. Both bearing rings should have tight fits.

Temperature

The temperature gradient in the bearing position influences the fits, whereby the direction of the heat flow is important.

 

Examples

The following shaft tolerances (solid shafts) have proven effective for axial bearings with a cylindrical bore.

The following shaft tolerances (solid shafts) have proven effective for radial bearings with cylindrical bore.

The following housing tolerances have proven effective for radial bearings.

These fitting guidelines do not release the user from checking the fit selection for his application!

At what operating temperatures can spherical plain bearings and rod ends be used?

1. materials

Various material combinations are possible for spherical plain bearings and rod ends according to their sliding pairing and their application possibilities.

 

2. temperature

The temperature of the bearings results from the friction, the design, the lubricant type and quantity as well as the swivel frequency or bearing speed. However, possible external heating and heat dissipation to surrounding components also have an influence on the temperature of the bearings.

 

3. operating temperature

All Askubal spherical plain bearings and rod ends can be used without restriction in the operating temperature range from -10°C to +80°C.

3.2 At higher operating temperatures, the bearing load carrying capacity and thus also the operating life are reduced. Due to the different material combinations of the sliding pairings, it may be necessary to adjust the radial internal clearance to the temperature conditions, e.g. by widening the radial internal clearance, as a result of temperature-related changes in the volume of the materials.

3.3 The operating temperature of spherical plain bearings and rod ends requiring maintenance depends on the lubricant used and the lubricant film formation. For short periods, these spherical plain bearings and rod ends can be used up to +120°C. Sufficient lubrication and temperature-resistant lubricants are required.

3.4 The maintenance-free spherical plain bearings and rod ends can also be used for short periods up to an operating temperature of +120°C.

3.5 Sealed spherical plain bearings and rod ends (maintenance-required and maintenance-free) are temperature-resistant up to +100°C due to their seals.

3.6 Rolling bearing spherical plain bearings and rod ends with seals or shields can be used at operating temperatures up to +100°C within limits due to the seals and the lubricant.

 

At what operating temperatures can rolling bearings be used?

1. materials

 

Inner and outer rings as well as the rolling elements are mainly made of through-hardened rolling bearing steel. These are generally heat-treated so that they are dimensionally stable for short periods up to a limit temperature of +150°C.

2. temperature

 

The temperature of a rolling bearing is determined by the friction, the design, the type and quantity of lubricant and the bearing speed. However, possible external heating due to the ambient conditions and also heat dissipation to surrounding components also have an influence on the temperature of a bearing arrangement.

 

3.operating temperature

It is very difficult to calculate the operating temperature of a bearing in advance, since the dissipation of heat through the surrounding parts can hardly be determined. If no experience with similar bearing arrangements is available for new designs, especially at higher speeds or loads, testing is essential.

Without the influence of external heating, it is not to be expected that the operating temperature will rise above +100°C in the majority of all applications at medium speeds and medium loads.

Open rolling bearings (without seal or cover washer) can be used up to an operating temperature of +120°C with the appropriate lubricant.

Rolling bearings with seal or cover disc can be used at operating temperatures from -30°C to +110°C, limited by the seal and the lubricant.

Operating temperatures above +120°C require special heat treatment of the bearing parts. Bearings treated in this way are given a special marking (S1....S4).

If the operating temperature rises above the specified limit temperature over a long period of time, this can lead to a gradual structural transformation of the rolling bearing steel and thus to permanent dimensional and shape changes which have a negative effect on the function of the bearing.

 

4.temperature development

The friction in the bearing is converted into heat and causes an increase in temperature. As a rule, the temperature increases uniformly after commissioning until a constant operating temperature is reached. The running time until the almost constant operating temperature is reached depends mainly on the bearing size, the thermal conductivity of the housing and the shaft as well as the load, the speed and the lubrication. It is important that the operating temperature remains as constant as possible without external influences. Due to its larger surface area, the outer ring dissipates more heat to the surrounding structure than the inner ring. Therefore, the inner ring temperature may be slightly higher than the outer ring temperature. Under normal conditions, the difference is about 3°C to 10°C. It may therefore be necessary to select a larger bearing air group, e.g. C3, at higher operating temperatures and speeds.

Excessively high temperatures can also accelerate the degradation of the lubricant's lubricity and reduce bearing life.

Exceptionally high temperatures can be caused, for example, by misalignment, insufficient bearing clearance, excessive preload, inadequate lubrication, foreign matter or heat generation by seals.

What must be taken into account when calculating load ratings of spherical plain bearings and rod ends?

1. load ratings                                            

The load carrying capacity of spherical plain bearings and rod ends is expressed by the basic load ratings given in the dimension tables; these serve as authoritative characteristic and calculated values.
Load ratings from different manufacturers are not readily comparable with each other, since the materials selected for spherical plain bearings and rod ends can vary and therefore do not permit a 
uniform standardized definition.

 

2. static load rating C0

It corresponds to the maximum radial static load that a spherical plain bearing or rod end can support at standstill (load without rotary, swivel or tilting motion) without the sliding surfaces being destroyed. 
This assumes normal room and operating temperature and that the surrounding components are sufficiently stable to prevent deformation of the bearing.  

 

3. dynamic load rating C

Basic dynamic load ratings are used as calculation values for estimating the service life of spherical plain bearings and rod ends subjected to dynamic loads. They do not themselves provide any 
information about the effective dynamic load carrying capacity of the spherical plain bearing or rod end. For this purpose, the additional influencing factors such as load type, swivel or tilt angle, speed curve, max. permissible internal clearance or bearing friction, lubrication conditions, temperature, etc.must also be taken into account.

 

4. Calculation of the basic load ratings of spherical plain bearings

An exact calculation is not possible because of the actual flexing conditions and pressures that occur.
Approximately, the basic load ratings are determined as follows:

C0 or C = Projected bearing load bearing area x specific load characteristic value K0 or K
 

It should be noted that the basic load ratings are determined on the basis of purely radial loads, zero internal clearance, optimum flexing conditions and a stable installation situation.    

 

5. Static and dynamic loading of rod ends

In the case of rod ends, it should be noted that the basic static load rating refers to the maximum permissible radial load with the rod end housing under static load in the direction of tension along the thread axis, up to which no permanent deformation occurs on the weakest housing cross-section.

 

5.1 Load direction and type

In addition to the magnitude of the load, the direction and type of load must also be taken into account when using spherical plain bearings and rod ends. 
load direction and the type of load must also be taken into account. 

Load directions are differentiated as follows:
 

 

5.2   Load type

Load directions are distinguished as follows:

 

Depending on the type of load, the following load coefficients must be taken into account for the max. permissible radial housing load capacity Fr perm. along the thread axis:

The coefficients SB and SK are used to estimate the load limit.

The max. permissible housing load capacity Fr zul. is thus reduced as follows:

Fr perm. (KN) = stat. Load rating C0 (KN) x SB x (SK)

The load coefficient SK takes into account the weakening of the rod ends with external thread and lubrication hole or grease nipple. This can only serve as a rough guide value.
Coefficients for shock loads, such as those caused by increasing bearing clearance, or for additional loads caused by simultaneous rotary motion of the bearing are not included.

 

Note: If the component is to be loaded to the load limit, especially if the failure of the component poses a risk to life and limb or causes damage, the use must in any case be 
verified by practical tests. In addition, it should be noted that special requirements apply for use in aircraft. Our standard products do not meet these special requirements. 

 

In principle, when selecting rod ends and spherical plain bearings, care must be taken to ensure that 
the loads occurring in use are always well below the maximum permissible load limits.

 

 

6. static and dynamic load ratings of spherical plain bearings and rod ends supported by rolling bearings

Our spherical plain bearings and rod ends with integrated rolling bearing (ball or spherical roller bearing) are a special feature. The function of the bearings is comparable with self-aligning ball 
bearings or spherical roller bearings. 

The basic load ratings of the spherical plain bearings and rod ends with rolling bearings are derived 
from the standards for rolling bearings. 

 

Basic static load rating C0 

For spherical plain bearings and rod ends, the basic static load rating corresponds to the load at which the total permanent deformation of rolling elements and raceways is 0.0001 of the rolling element 
diameter.  

Basic dynamic load rating C

For spherical plain bearings and rod ends, the basic dynamic load rating corresponds to the load at which 90% of a larger quantity of identical bearings reach 1 million revolutions before they fail due to 
fatigue of the rolling surfaces.
 

 

 

 

 

Spherical plain bearings require maintenance / maintenance-free?

1.function

Spherical plain bearings are ready-to-fit bearing elements based on plain bearings, they transmit static and dynamic loads they ensure spatial adjustment movements between shaft and housing (self-aligning plain bearings). They allow swivel, tilting and rotary movements at relatively low sliding speeds.

The advantages are that manufacturing-related misalignments can be compensated,

Design-related misalignments are made possible. The edge pressure and thus excessive component stress is eliminated and larger manufacturing tolerances are made possible.

 

2. areas of application

Spherical plain bearings are used in a wide variety of applications such as:

  • Pneumatics - Hydraulics
  • Mechanical engineering
  • Rail vehicles
  • Motorcycle and vehicle construction
  • Aerospace technology and many more.

 

3. selection criteria

The question often arises as to when to use a maintenance-required spherical plain bearing and when to use a maintenance-free one.

 

Spherical plain bearings requiring maintenance are preferred for:

            - changing loads

            - Medium to large swivel movements

            - medium sliding speeds

 

Are only conditionally suitable for:

              - unilateral loads           

              - slow panning movements

 

Maintenance-free spherical plain bearings are preferred for:

            - unilateral loads

            - small to medium swivel movements

                                              

Are only conditionally suitable for:

            - changing loads

            - unilateral high impact loads                                                       

 

4. Construction structure

Maintenance required:

 

Sliding combination steel on bronze

Inner ring:        

With spherical outer shape is made of hardened rolling bearing steel.

Outer ring:

With hollow spherical inner shape is made of softer sliding material with integrated radial lubrication groove and lubrication hole.

Manufacture:

Cold forming of the originally cylindrical outer ring around the hardened inner ring.

Steel-on-steel sliding combination

Inner ring:

With spherical outer shape is made of hardened rolling bearing steel.

Outer ring:       

With hollow spherical inner shape is made of hardened rolling bearing steel is blasted after machining.

Manufacturing:

the inner ring is pressed into the blasted outer ring.

 

Maintenance-free:


Sliding combination of steel on PTFE metal fabric

Inner ring:

With spherical outer shape is made of hardened rolling bearing steel.

Outer ring:

With hollow spherical inner shape is made of softer material with integrated sliding film of PTFE embedded in metal mesh.

Manufacture:

Cold forming of the original cylindrical outer ring including the integrated sliding foil around the hardened inner ring.

When must which seal or cover washer be used?

Sealing the bearing

Seals for rolling bearings have the following task, they must prevent the penetration of dirt particles or moisture into the bearing and at the same time retain the lubricant in the bearing. The effectiveness should be guaranteed even under the most unfavorable conditions with a minimum of friction and wear, so that neither the service life nor the function of the bearings is impaired. 

 

Sealing types:

As a rule, two types of seals for rolling bearings can be considered, which differ in their mode of operation:

Non-abrasive sealing -ZZ design

Abrasive sealing -RS design

 

Bearing with cover plate (ZZ - design)

Deep groove ball bearings with 1 or 2 shields (Z or 2Z design) for shafts 3 to 120 mm. High sealing efficiency for standard applications (where the risk of contamination is low and the ingress of water, steam, etc. is not expected), low friction, for high speeds, ready for installation, greased, low noise. However, the seals do not replace an overpressure seal, for example.

Bearing with sealing washer (RS - design)

Deep groove ball bearing with 1 or 2 sealing washers (RS or 2RS design) for shafts'" 6 to 80 mm. High sealing effect in standard applications also against splash water or oil. However, the seals do not replace overpressure seals, for example. The application limit of the standard
contact (abrasive) seals used in standard applications is + 110 C°. Low-noise, ready-to-install, greased. maintenance-free operation.

Caution: Deep groove ball bearings with cover and sealing washer must not be washed out! When running in, a small amount of grease may escape with both types of seal.

 

 

 

 

What must be considered in the case of corrosion of rolling bearings, plain bearings and rod ends?

1. Main types of corrosion

Corrosion is a physicochemical reaction of a metal with its environment. The following elaboration deals only with the most important types of corrosion of the above mentioned machine elements.

There are many approaches to distinguish or classify types of corrosion.

Classification on the basis of the chemical process

  • general oxygen corrosion e.g. rust formation in iron
  • patina formation in copper
  • Hydrogen embrittlement
  • Hydrogen corrosion (acid corrosion) which will not be discussed in detail.

 

In engineering, it is also often distinguished by appearance and local location into:

  • Surface corrosion or pitting occurs on an open surface, e.g. due to weathering
  • Contact corrosion results from the bonding of two metals with different potentials.
  • Crevice corrosion due to the capillary action of a crevice, liquid is drawn into the crevice and remains there for a long time.
  • Vibration crack corrosion
  • Stress corrosion cracking
  • Pitting corrosion
  • Intergranular corrosion ( corrosion at the grain boundaries of the metal)
  • and many more

 

2. Salt spray test as a benchmark for rust formation (corrosion)

An exact prediction of how long it takes for rust to form as a typical product of oxygen corrosion is not possible, as this depends very much on the changing environmental conditions.

In order to be able to compare the rust formation of different materials or coatings, there is a standardized test according to DIN EN ISO 9227. The workpieces are suspended in a chamber during the test. The temperature in the chamber is 35°C +-2°C. A NaCL solution with defined concentration, quantity and pressure is sprayed.

The time taken for rust to form is measured and usually documented by photographs.

From the measured values, no direct conclusions can be drawn as to how long a component can be exposed to a certain atmosphere before red rust is formed. The standard itself points out that the test is only suitable for quality control of coatings.

The test was originally developed for the automotive industry in order to obtain a comparative statement on the corrosion resistance of components when driving on roads that are gritted with salt in winter. Understandably, this method cannot be used to predict corrosion resistance in sulfurous smog, as the type of corrosion in this case is different.

Examples of the salt spray test after 120h:

3. Measures against corrosion

In mechanical engineering, the following processes are very often used to reduce the tendency to corrosion.

 

3.1 Passive protective layer against corrosion

A simple way to prevent corrosion is to apply a corrosion-resistant layer to the metal. This can be organic layers (protective coatings) or galvanically applied metallic layers (e.g. chrome plating).

As a rule, these coatings work very well provided that they do not have holes or flaws through which the metal material to be protected comes into contact with the atmosphere. These flaws can be so small that they are not visible to the naked eye. In the long run, corrosion can start at such flaws and then infiltrate the coating.

In mechanical engineering, chromium is often used as a protective coating. If the coating is subjected to mechanical stress, a hard chrome plating is chosen. This should have a thickness of at least 7µm so that defects are largely excluded.

 

Disadvantages and weak points

Electroplated coatings are applied depending on the strength of the electric field created around the workpiece. Therefore, holes can only be coated to a depth equal to the hole diameter. At sharp edges, the coating thickness varies greatly and at the points where the workpiece is suspended to immerse it in the bath, no coating is applied.

In most cases, the coating is not as mechanically strong as the workpiece itself. Small particles (sand grains) can be pressed into the coating during operation and damage it.

Thermally applied coatings are usually not an option since the higher temperatures change the properties of the workpiece.

With all coatings, the dimensions of the workpiece are changed depending on the coating thickness.

Practice

The inner rings of spherical plain bearings can be provided with a hard chrome coating to improve corrosion protection. For technical reasons, the inner rings have no coating in the bore and on the flat surfaces.

The cast housings of housing units are painted to improve corrosion protection. In the case of housings, the fits for mounting the bearings and the screw-on surfaces are machined and therefore painted.

Conclusion

Corrosion protection by corrosion-resistant coatings can be effective, but rapidly lose their effect if the coating has flaws.

 

3.2 Galvanic coating with active protective layer

Base metals are applied to the workpiece as a layer. When the workpiece is wetted with water, an electrical voltage is created between the layer and the workpiece. The material of the layer has a higher voltage to the hydrogen (because it is less noble) than the steel of the workpiece. Therefore, the material of the coating is oxidized and the material of the workpiece is not changed during the process. 

The galvanic zinc coatings used in ASK actively protect the workpiece by means of electrochemical voltage and by means of an additional cover layer that provides additional passive protection. 

In this type of rust protection, the active protective layer is consumed over time. This can also be seen on the surface of the zinc layer, which forms so-called white rust (zinc oxide).

Once the zinc layer is used up, there is no corrosion protection.

 

Disadvantages and weak points

Electroplated coatings are applied depending on the strength of the electric field created around the workpiece. Therefore, holes can only be plated to a depth equal to the hole diameter. At sharp edges, the coating thickness varies greatly, and at the points where the workpiece is suspended in the bath, no coating is applied.

The layers cannot be applied in any thickness, otherwise they will flake off. In the case of galvanic applied zinc coatings, the upper limit is 10- 15µm.

In most cases, these coatings have little or no mechanical strength. Small defects are compensated by the active protection of the coating.

Galvanic zinc coatings usually receive a chromate layer as the last layer. By immersion in chromic acid, chromic acid salts (chromates) are formed with the zinc to form an almost transparent protective layer. In the past, these layers contained Cr VI. Cr VI is very toxic and in the EU directives 2002/95/EG (RoHS), 2002/96/EG (WEEE) the avoidance of Cr VI is required.

For zinc coatings applied in Europe, Cr VI is no longer a problem. In countries, where the environmental standards are not yet so high, it can still occur.

ASK therefore stipulates freedom from Cr VI as a matter of principle and has this verified by regular spot checks.   

For all coatings, the dimensions of the workpiece are changed depending on the coating thickness.

Practice

The most common application of such coatings is found in the housings of rod ends.

Conclusion

Corrosion protection by active coatings can be effective, but lose their effect over time when the coating is used up.

 

3.3 Thermal coating

Thermally applied coatings are usually not an option since the higher temperatures change the properties of the workpiece.

 

3.4 Selection of a base material with reduced tendency to corrosion

 

Apart from exotic materials (titanium, ceramics, etc.), the most suitable substitutes for chromium steel, free-cutting steel or heat-treatable steel are so-called stainless steels.

Stainless steel is the designation for steels with an alloy content of at least 10.5% Cr. They have a reduced tendency to corrode compared to normal steels.

In stainless steels, the improved corrosion properties are achieved by alloying elements such as Cr, Ni, Ti or Mo. (detailed information on this subject can be found on the pages of the steel manufacturers).

Corrosion resistance increases with increasing alloy content. The alloying elements create a protective oxide layer on the surface.

Improvement of corrosion resistance

For all stainless steels, resistance to corrosion is significantly improved by a smooth surface.

Deterioration of corrosion resistance

Depending on the alloy content, an acidic or alkaline environment can lead to corrosion.

Extreme deterioration due to ferritic particles

Extreme deterioration of resistance to corrosion is caused by ferritic particles on the surface of stainless steel. Ferritic particles on the surface lead to stress corrosion, which can also occur when mating components made of different materials. Starting from these nuclei, which are invisible to the naked eye, stainless steel can form red rust to a considerable extent.

In practice, iron particles can be transferred by tools. If a wrench is used to tighten steel screws first and then stainless steel screws, ferritic particles can be transferred to the stainless steel screw by the wrench.

Iron particles can also be transferred over longer distances by welding or grinding. Welding and grinding can release very small particles which are transported with the air flow over several 100 meters and then deposit on a stainless steel surface.

 

3.4.1 Stainless steels for normal stress levels

For housings for rod ends and outer rings for spherical plain bearings that do not have an increased stress level in operation, austenitic materials such as 1.4301 or 1.4305 have proven their worth.

 

3.4.2 Hardenable stainless steels

Among hardenable stainless steels suitable for inner and outer rings and rolling elements for rolling bearings or for inner rings of spherical plain bearings, the material 440C (1.4125 or 9Cr18, 9Cr18Mo, 8Cr17) is the most widely used. It is a martensitic chromium steel with increased carbon content which can be hardened by quenching.

The higher carbon content and the structure formed during quenching result in significantly reduced resistance to corrosion compared to non-hardenable chromium steels.

In practice, corrosion resistance can be improved somewhat by additionally applying a layer of hard chrome. This is only possible with plain bearings. In the case of rolling bearings, the chromium layer will flake off at the points where the component is overrun by rolling elements and the rolling bearing will be destroyed as a result.

The material 1.4108 has a significantly higher resistance to corrosion. This steel was produced for the aerospace industry and has an increased proportion of nitrogen.

In practice, it is hardly ever used in mechanical engineering because the material costs are many times higher than those of 440C.

 

3.5 Material pairing to prevent contact corrosion

Two connected components made of different metals that are wetted with an electrically conductive liquid form a galvanic element. Depending on the potential difference, a more or less strong current is generated.

The less noble metal becomes the anode and dissolves, while on the cathodic side any corrosion products are reduced in a reduction reaction.

In some cases this is desirable, (electroplating), in other cases it can cause significant damage. (Example aluminum sheet screwed with copper screws or combination of mild steel and stainless steel).

 

3.6 Replacement of copper and copper-containing alloys

Copper patina can occur on copper or copper-containing materials (bronze). Greenish copper compounds usually form. This often occurs when aggressive cleaning agents (food industry) are used.

The most effective measure is to replace copper-containing materials such as bronze or brass with materials that do not contain copper.
In rolling bearings, cages and parts of the seal can be made of materials containing copper.

In relubricatable plain bearings, the outer ring is often made of bronze. Replacement of the bronze is not readily possible in this case. As a rule, maintenance-free bearings with a PTFE sliding layer are selected in such cases.

In the case of maintenance-free bearings, a bronze mesh can be incorporated into the sliding lining in some designs. The bronze mesh can be replaced by a stainless steel mesh.

 

3.7 Annealing against hydrogen embrittlement

Can occur when quenched and tempered materials are galvanized or pickled for cleaning. During electrogalvanizing or pickling, atomic hydrogen is generated which diffuses into the metallic material and accumulates in the area of grain boundaries or flaws. At high stress levels in the material, the deposited hydrogen can then lead to brittle fractures.

In the case of bolts, it is assumed that hydrogen embrittlement can occur from strength class 10.9.
Strength class 10.9 corresponds to a tensile strength of 1000 N/mm² and a yield strength of 900 N/mm².

At ASK, all housings with a tensile strength above 900 N/mm² are annealed at temperatures above 200°C for several hours immediately after pickling or galvanizing. This diffuses the atomic hydrogen out of the metal.