The basic characteristics of various failures of tapered roller bearings and their causes are pointed out

Abstract: Based on the failure analysis of tapered roller bearing, the failure forms of contact fatigue, wear, fracture, plastic deformation, corrosion and current corrosion are introduced in detail, and the basic characteristics and causes of various failures are pointed out, which provides an important reference for taking corresponding preventive measures.

Key words: tapered roller bearing; Contact fatigue; Wear; failure analysis

Tapered roller bearing has the advantages of large bearing capacity, bearing radial load and axial load at the same time, and long service life. It is widely used in automobile, railway truck, rolling mill, construction machinery, machine tool, aircraft landing gear, agricultural machinery, water pump, mining and other industries. In the process of production, installation and use, the phenomenon that the bearing loses its specified function due to some defects is called failure. The failure forms can be divided into contact fatigue, wear, fracture, plastic deformation, indentation, groove, corrosion, current corrosion and other forms according to the damage mechanism, of which contact fatigue and wear are the main failure forms.

The purpose of this paper is to analyze the failure forms of bearings, find out the failure causes, and take effective measures to improve the product quality.

1 Contact fatigue failure

Under the action of high contact stress, after several stress cycles, the rolling bearing will produce small pieces or small pieces of metal peeling off in the local area of the working surface of the ferrule or rolling element, forming pits or pits, which will cause vibration, increase noise, increase temperature, intensify wear, and lead to abnormal operation. This phenomenon is called contact fatigue failure, which is the main form of rolling bearing failure.

Due to the different materials, working conditions and lubrication environment, contact fatigue failure can be divided into pitting peeling, shallow peeling and hardened layer peeling.

1.1 Pitting and peeling

Pitting peeling is one of the common early failure forms of bearings. It has two types: non developmental and developmental. At the initial stage of bearing operation, the peak point contacts, and the welding surface is cut off to form small pockmarks, which generally do not expand. With the passage of time, it gradually disappears. This initial pockmarking is of running in property and does not affect the use of the bearing. "Developmental pockmarks" appear after long-term operation, and the pockmarks increase, expand, and peel off. When the pockmarks extend to a certain depth, the metal will peel off. Pockmarks are very harmful, and the causes are as follows:

(1) Surface defects cause stress concentration, such as material and heat treatment surface defects, grinding burn cracks, scratches, rust, foreign hard objects entering the contact surface, and corrosive media such as water, hydrogen, acid, etc.

(2) The rigidity of the mating parts with the bearing is insufficient or the accuracy is poor.

1.2 Shallow spalling

The initial fatigue crack starts from the maximum orthogonal shear stress under the contact surface, and then extends to the surface. The spalling is called shallow spalling. The development process is divided into crack source area, extension area and fault area.

The maximum shear stress of a bearing in good working condition is not on the contact surface, but at a certain depth below the contact surface. Early shallow fatigue will occur when subjected to large external load, material and heat treatment defects, low manufacturing accuracy, improper installation and large impact force.

1.3 Peeling of hardened layer

The workpieces with surface strengthening treatment are prone to initial cracks in the working process, which originate from the transition zone between the hardened layer and the core, resulting in the early failure of the hardened layer.

2 Wear failure

During the working process of the bearing, the rolling and sliding motion between the rolling body and the inner and outer raceways, and the sliding motion between the cage and the guide surface cause the continuous loss of metal on the working surface of the bearing, which is called the wear of the bearing.

The constant wear of the working surface of the bearing causes the change of the size and shape of the bearing parts, resulting in the increase of the bearing fitting clearance, the deterioration of the working surface and the loss of the rotation accuracy, which causes the increase of the working temperature, vibration, noise, friction torque, etc., and causes the bearing to not work normally. This phenomenon is called wear failure.

Wear failure is closely related to material properties, roughness, lubrication state, contact stress, relative sliding rate, surface friction coefficient, speed, temperature and environmental media.

The causes of wear failure are different, which are generally divided into five forms, such as adhesive wear, abrasive wear, corrosion wear, fretting wear and fatigue wear.

2.1 Adhesive wear

When the rolling element of the bearing moves with the inner and outer race raceways, the cage and the rolling element, and the cage and the guide surface, the phenomenon that the material on the contact surface is transferred from one surface to another due to the solid-phase welding is called adhesive wear.

The premise of adhesive wear is that the surface oil film is destroyed. The two bumps are in direct contact with each other, and the metal bonding and welding will occur at the contact point. In the relative movement, the bonding point is cut to form a new bonding point, and the metal will be transferred from one surface to another. According to the degree and characteristics of friction surface damage, adhesive wear can be divided into five forms: slight wear, smear, scratch, gluing and bite.

2.1.1 Slight wear

When the adhesive bonding strength is lower than that of the base metal of the two contact parts of the bearing, the shear failure occurs on the bonding surface, and the phenomenon that the metal transferred on the surface is extremely slight is called slight wear. It occurs in bearings with small load, low speed and good lubrication conditions, such as machine tool spindle bearing and instrument bearing.

The slight wear at the initial stage of the bearing will gradually smooth the roughness peak of the working surface of the bearing and work hardening, which is conducive to the normal operation of the bearing. This kind of wear needs long-term accumulation before it can fail.

2.1.2 Application

When the adhesive strength is greater than the strength of any base metal in the bearing parts, the soft metal adheres to the hard metal surface, such as the cage material adheres to the rolling body surface. This phenomenon is called smearing.

Coating refers to the pit formed by the metal on the contact surface at the tear off point, and the metal bump or stripe formed at the attachment point, which causes the contact surface to change sharply and loses accuracy, and causes vibration and noise to fail to work normally.

2.1.3 scratch

On the contact surface of the ferrule, the rolling body and the cage along the rolling or sliding direction, a thin and shallow plow mark type damage feature is formed, which becomes a scratch. The furrow is wide and deep, and the serious damage will become scratch.

Scratches mostly occur between the cage and the roller. If the clearance is too small, the cage deformation will clamp the roller, causing serious scratches. The metal coated on the rolling body will broach the cage pocket surface to form a furrow. Due to poor lubrication, the contact surface between the roller and the ferrule raceway slips, and the metal particles adhere between the two contact surfaces to form a furrow, which leads to failure.

2.1.4 Gluing

High speed and heavy load, serious insufficient lubrication, serious sliding between the roller and the ring raceway or retaining edge, too small bearing clearance, increased friction, too small clearance between the roller and the cage pocket or clamping, etc. will cause direct contact between metals to produce solid-phase welding. When the welding strength is greater than the strength of any matrix of the contact part, so that the shear force is higher than the welding strength, the local damage generated in the metal depth of one or both of the contact parts is called gluing.

2.1.5 Bite

The phenomenon that the relative movement between the contact surfaces of bearing parts stops due to the effect of friction is called seizure. This is a serious form of gluing wear.

The main causes of seizure are: overload, high speed, lubrication failure, no lubrication, too small clearance, fracture of cage, serious damage of roller and ring raceway surface, etc.

2.2 Abrasive wear

The friction surface of bearing parts is squeezed with foreign hard particles or metal fine particles separated from the contact surface wear and fatigue peeling. The phenomenon of metal peeling off on the friction surface during the friction process is called abrasive wear. For example, sand, dust, ash and other particles of construction machinery, rolling mill and agricultural machinery will enter the bearing working surface to produce abrasive wear.

The difference between abrasive wear and adhesive wear is that it is the cutting action of abrasive that cuts the metal from the friction surface.

2.3 Corrosion and wear

During the working process of the bearing, the wear phenomenon caused by the chemical and electrochemical reaction between the metal on the friction surface and the surrounding medium is called corrosion wear. There are two forms due to different media.

2.3.1 oxidation wear

The chemical reaction of bearing parts on the friction surface is the wear phenomenon caused by the interaction of oxygen or oxidation medium, which is called oxidation wear, such as wet environment, high temperature water, etc. Characteristics of oxidation wear: the wear products on the working surface are reddish brown ferric oxide or gray black ferric oxide.

2.3.2 hydrogen induced wear

During the working process of the bearing, the phenomenon that hydrogen continuously enters the working surface layer, resulting in accelerated wear is called hydrogen induced wear. For example, hydrogen is precipitated from lubricating oil and water during friction. Characteristics of hydrogen induced wear: crack sources appear in the friction surface at the same time, and powder wear debris is formed in a very short time.

2.4 Fretting wear

The phenomenon that the contact surfaces of bearing parts are worn due to the vibration type relative motion with small amplitude is called fretting wear. Characteristics of fretting wear: Fe3O4 or Fe2O3 is produced on the contact surface, and equidistant false indentation appears on the raceway of the ferrule.

Due to the influence of external vibration, the bearing swings back and forth in a small arc. For example, the spindle bearings of precision machine tools that have not been operated for a long time, cars and trains that have been parked for a long time often cause false indentation due to external vibration, resulting in work failure.

2.5 Fatigue wear

Fatigue wear and pitting spalling in contact fatigue failure are the same failure type.

3 Fracture failure

The fracture of bearing parts can be divided into overload fracture and fatigue fracture.

3.1 Overload fracture

Overload fracture occurs instantaneously, and the fracture is generally rough. The brittle fracture has no obvious plastic deformation, and the two segments can fit together. The ductile fracture is smooth and wavy. The fracture between them is brittle ductile mixed fracture.

The overload fracture of rolling element and ferrule is brittle fracture, and the fracture of cage and high-temperature bearing parts is ductile or brittle fracture.

3.2 Fatigue fracture

The phenomenon that the local area of the ferrule, cage and rolling element is broken due to the repeated action of the bending alternating stress exceeding the fatigue limit of the material is called fatigue fracture. The fracture surface of fatigue fracture can be divided into two regions: fatigue extension region and sudden break region.

Fracture failure is generally related to the following factors: excessive external load exceeds the strength limit of the material; Improper installation causes bearing parts to be skewed and partially worn; Fatigue spalling of ring raceway or roller forms crack source; Poor lubrication and serious wear caused by sliding friction; The interference between the bearing and the shaft and the hole will cause the ring to deform and cause fretting wear under external vibration; High speed rotation, external impact, vibration or roller impact on cage; Fracture caused by material and heat treatment defects.

4 Plastic deformation failure

4.1 Indentation

Due to incorrect installation, invasion of foreign particles, external impact and vibration, etc., indentation occurs on the surface of the ferrule or rolling element, resulting in increased vibration and noise of the bearing, increased temperature and wear, resulting in fatigue peeling and fracture, etc., which is called indentation failure. Indentation failure can be divided into true indentation and false indentation. The plastic deformation dent on the working surface of the ferrule and the rolling element due to the pressing of foreign objects is called the true indentation. Under the static state of the bearing, the wear or corrosion dent formed by the small arc swing at the contact between the rolling element and the ring raceway due to the external vibration, impact or invasion of corrosive medium is called false indentation. The causes of indentation formation are unreasonable installation, external vibration, foreign particles, invasion of static corrosion medium, etc.

4.2 Deformation failure

The failure caused by the plastic flow or overall plastic deformation on the surface of ferrule, rolling element and cage under the action of external force and the change of working surface or overall morphology is called deformation failure. Characteristics of deformation failure: there are plastic rheological traces along the rolling direction on the surface of rolling body or ferrule raceway; The cage is warped, distorted, and the pocket is elongated and becomes small; Overall deformation of ferrule and loss of accuracy; Rolling track distortion or rolling body shape distortion.

5 Other failures

5.1 Corrosion

The wear caused by the chemical or electrochemical reaction between the metal surface of bearing parts and the environmental medium is called corrosion failure. When the bearing works in the humid environment and the medium with acid and metal corrosion, the bearing parts will produce rust spots; Eventually leading to failure. In addition, the lubricant containing water or the additives containing acid and alkaline components will also lead to failure.

5.2 Electric corrosion

When the current passes through the bearing, the current passes through the thin lubricating oil film on the contact working surface of the raceway and the rolling element, causing sparks, and causing local surface melting is called electric erosion. The electric corrosion failure is visible to the naked eye, and the main features are: washboard like uneven, black dot like small round holes. Electric corrosion consists of 3 parts: modified layer + hardened layer + tempered normal part.

5.3 Clearance change failure

During the working process of the bearing, due to the influence of the external temperature and the change of its own internal factors, the original fit clearance is changed, resulting in the reduction of accuracy and even the phenomenon of seizure is called clearance change failure.

The first reason for this phenomenon is that during the operation of the bearing, the bearing parts are different in size, shape and expansion coefficient due to the work under overtemperature, resulting in the change of clearance and failure. The second is the change of the clearance due to the instability of the structure and stress. The bearing operates at a low operating temperature, but if the structure and stress (residual austenite) of the parts are in an unstable state, the size will change with the increase of the working time of the bearing. The light one will lose the operating accuracy, and the heavy one will bite.

More about GQZ Tapered Roller Bearing：

GQZ precious-quality tapered roller bearing with reasonable price for customers all over the world and first-class service and unrivaled products.

Tapered roller bearings are rolling element bearings that can support axial forces (i.e., they are good thrust bearings) as well as radial forces.

The inner and outer ring raceways are segments of cones and the rollers are tapered so that the conical surfaces of the raceways, and the roller axes, if projected, would all meet at a common point on the main axis of the bearing. This geometry makes the motion of the cones remain coaxial, with no sliding motion between the raceways and the outside diameter of the rollers. This conical geometry creates a linear contact patch which permits greater loads to be carried than with spherical bearings.