Failure Analysis on Bearings Used in Electrical Traction Drum Shearer

Abstract: Aiming at the early failure of rolling bearings in transmission system gearboxes of electrical traction drum shearers, the causes for early failure of bearings are analyzed from different aspects of host structure, force and clearance. Some corresponding solutions are put forward, which supplies guarantees for safe and reliable operation of electrical traction drum shearers. 

Key words: rolling bearing; drum shearer; failure; working clearance; solution

Question raising

The electric traction drum shearer (Figure 1) is a complex large-scale coal mining equipment that integrates mechanical, electrical, hydraulic, and auxiliary systems. It directly bears the heavy responsibility of coal cutting and loading in the coal mining working face and is one of the core equipment in the working face. From the perspective of the mechanical system alone, the mechanical transmission system of the electric traction shearer is a gear reducer, which converts the high speed and low torque of each electric motor (except for the electric motor used in the hydraulic leveling system) into low speed and high torque after passing through the gear reducer. Bearings are a key component in this process and are the key to ensuring the normal operation of the mechanical system of the shearer. The bearings in each gear reduction box of the electric traction drum shearer are all rolling bearings, mainly including ball bearings, cylindrical roller bearings, fully loaded roller bearings, self-aligning roller bearings, single row and double row tapered roller bearings. In terms of cost, bearings account for about one-third of the entire gearbox.

1- Left cutting section; 2- Left traction walking part; 3- Electric control system; 4- Right traction walking part; 5- Right cutting section

Figure 1 Structural diagram of electric traction shearer

The failure of bearings in electric traction drum shearers mostly occurs at the output end of the planetary mechanism of the cutting transmission device, the high-speed shaft of the cutting transmission device gearbox, and the walking transmission device of the traction part. Except for assembly factors and the quality of the bearings themselves, the early failure of all bearings in electric traction drum shearers is closely related to their actual working clearance. Therefore, the reasons for early failure of bearings in the main parts were analyzed, and corresponding solutions were proposed.

2. Output end bearing of planetary mechanism

2.1 Force analysis

The output end of the planetary mechanism of the cutting transmission device of a certain type of coal mining machine is connected to the drum connecting plate through a square tenon and cutting mechanism, and outputs high torque and low speed (usually 30-50 r/min), which is transmitted to the spiral drum. Due to the fact that this is the execution mechanism of the electric traction drum type shearer for direct coal cutting and loading, the force is complex and structurally equivalent to a short cantilever beam. Single row or double row tapered roller bearings are generally configured here: Single row tapered roller bearings can only achieve a certain internal clearance after installation; The internal clearance of the double row tapered roller bearing is adjusted by grinding its spacer ring. The assembly and adjustment of the axial clearance of tapered roller bearings is relatively complex, and the load and service life of the bearings are closely related to the actual working clearance. The common faults of tapered roller bearings here include abnormal sounds, scratches and wear on the inner and outer raceways and retaining edges, local fracture of the cage, wear and deformation of the rolling element, and loose sleeves of the entire bearing.

2.2 Failure Analysis

After analysis, it is believed that the main reasons for bearing failure are as follows.

(1) Improper adjustment of axial clearance. There are two conventional configurations for the planetary mechanism bearings of the cutting transmission device of the coal mining machine: first, as shown in Figure 2, both the upper and lower ends are single row tapered roller bearings; The second is to use cylindrical roller bearings at the lower end as shown in Figure 3, and double row tapered roller bearings with spacer rings at the upper end.

1- Drum connecting plate; 2- Square tenon; 3- Cooperate with sealing; 4- Floating seal; 5-Paired installation of tapered roller bearings

Figure 2 Configuration structure of paired installation of tapered roller bearings

1- Cylindrical roller bearings; 2- Double row tapered roller bearings; 3- Square tenon; 4- Cooperate with sealing; 5- Floating seal; 6- Drum connecting plate

Figure 3 Configuration structure of double row tapered roller bearings

Due to structural and assembly reasons, the load center distance a of single row tapered roller bearings installed face-to-face is much smaller than the center distance b of two sets of bearings (Figure 2). The small distance between the load centers of bearings is not conducive to bearing large overturning moments, and the radial force caused by torque loads and the deformation of bearings caused by them are greater than those of bearings installed back-to-back. In addition, when the face-to-face installation of bearings reaches the working temperature, the radial clearance will decrease, which is equivalent to increasing the preload. Usually, it is required that bearings still have a certain amount of remaining clearance during operation [2]. Reserve 50% between the end face of the upper tapered roller bearing ring and its pressure cover, which does not require high rigidity μ The clearance of m, if large, will seriously affect the fatigue life of tapered roller bearings. The clearance of the double row tapered roller bearing with spacer (Figure 3) can be ensured by the width dimension of the spacer during grinding, so that the two rows of tapered rollers and the inner and outer raceways are in line contact, otherwise the fatigue life of the bearing will be greatly reduced.

(2) Floating seals are often used at the outer end of bearings in large coal mining machines. Once the metal ring of the floating seal is damaged, its debris may enter the bearing, directly threatening its service life.

(3) The bearing is subjected to excessive impact load during coal cutting.

(4) The connection plate of the drum and the square tenon of the planetary mechanism of the cutting transmission device are loosely matched, causing the impulse caused by the drum rotation to directly impact the bearing at the output end of the planetary mechanism.

2.3 Solutions

(1) For the structure shown in Figure 2, double row tapered roller bearings installed in pairs from back to back should be used as much as possible for pre tightening without leaving any gaps. For the bearing shown in Figure 3, after assembly, the clearance needs to be adjusted to make the working clearance of the bearing close to zero or negative.

(2) While ensuring lubrication and cooling of the floating seal, try to avoid the impact of seal failure on the bearing as much as possible.

(3) Properly reduce the clearance between the large diameter drum connecting plate and the planetary mechanism tenon of the cutting transmission device to reduce the long-term impact of the drum rotation on the bearings.

3. Bearing of walking mechanism

Figure 4 shows the gearbox of the transmission device for the traction walking part of the coal mining machine. Although the speed of the gear at this location is very low, usually around 6-10 r/min, due to the gearbox running in the coal flow on the scraper machine, the working conditions are extremely harsh, and the shaft bears a large and complex force. Common faults include abnormal "creaking" sound, outer ring rupture, cage distortion and fracture, and the entire bearing loose sleeve.

Figure 4 Open traveling gearbox

3.1 Failure Analysis

(1) The axial clearance is too small. In order to adapt to the structure where there is a gap in the drainage direction of the guide shoe and scraper machine pin, the entire walking wheel assembly can generally slide 3-5 mm to the left/right. However, when sliding to a certain position, the bearing inner ring and shaft sleeve are limited by the inner surface of the guide shoe and cannot continue to slide. However, the bearing outer ring and walking wheel will continue to slide due to force (Figure 5). When the axial clearance of the bearing is zero and the walking wheel drives the outer ring to continue sliding, The bearing will generate negative clearance. When the negative clearance is large, additional additional loads will be generated inside the bearing, greatly reducing the fatigue life of the bearing.

The FAG 23132C3/W33 bearing is selected in the walking wheel component of a certain coal mining machine.

1- Inner ring of bearing; 2- The inner side of the guide shoe; 3- Bearing outer ring; 4- Walking wheel; 5- Shaft sleeve

Figure 5 Peripheral structure of walking wheel assembly

Ga/Gr=2.3Yo,    (1)

In the formula: Ga is the axial clearance, mm; Gr is the maximum radial clearance, Gr=0.22 mm; Yo is the calculation coefficient of the bearing (which is the parameter of the bearing itself), and according to the table, Yo=2.26. After calculation, Ga=1.14 mm, which is the initial axial clearance of the bearing. However, during assembly, due to the expansion of the inner raceway and the contraction of the outer raceway, the actual working clearance will be smaller than the initial clearance. When the structural design clearance b-a is much larger than the actual working clearance of the bearing (Figure 5), it indicates that the negative clearance during bearing operation is too large, which will seriously affect the operation of the bearing.

(2) Due to the high amount of coal powder at the walking wheel component, as the equipment wears out during operation, the sealing gap at both ends of the bearing of the walking wheel component increases. Coal powder may enter the bearing and contaminate the lubricating grease. In extreme cases, coal powder almost completely fills the bearing cavity, making it almost impossible to continue effective operation.

(3) The meshing transmission between the walking wheel component of the coal mining machine and the scraper pin row is a non conjugate gear rack transmission. When the coal mining machine is pulling and walking, the structure here is subjected to large and complex forces, especially due to geological conditions. When the coal mining machine is tilting and mining, the entire walking wheel component will bear a large axial force.

3.2 Solutions

(1) Select C5 series self-aligning roller bearings with large initial axial clearance to withstand large axial forces and dynamic loads.

(2) Select bearings with steel frame fluororubber (FPM) contact sealing rings on both sides (with a later code containing 2CS2) to avoid or minimize the entry of coal powder and dust into the bearings.

(3) For coal mining machines with high force on the walking mechanism, specially designed fully loaded roller bearings can be selected.

4. Cutting the bearing of the transmission part

The high-speed shaft component of the cutting transmission part (Figure 6) is the first stage of power transmission in the entire mechanical system of the cutting transmission part. It is connected to the spline of the cutting motor through a slender torque shaft, and the gear is symmetrically supported by cylindrical roller bearings or self-aligning roller bearings, and the power is transmitted to the next stage. The supporting bearing, cylindrical roller bearing or self-aligning roller bearing, maintains synchronous speed with the cutting motor (about 1475 r/min), and is the bearing with the highest speed in the cutting transmission part. Common faults of bearings here include abnormal sounds, rapid temperature rise, twisted and broken cages, and black and blue discoloration of the inner and outer ring end faces.

1- Cut the internal splines of the motor; 2- Torque shaft; 3- High speed shaft; 4- Reserve gaps; 5- Shell; 6- Cutting the motor and housing end; 7- Cutting motor

Figure 6 High speed shaft structure of cutting transmission part

4.1 Fault Analysis

(1) For the high-speed shaft structure of the double cutting electric motor, due to the impossibility of 100% synchronous operation of the two motors, there is a slight difference in the output torque of the two cutting electric motors, which causes uneven force on the gears corresponding to the two sets of bearings, thereby affecting the normal operation of the bearings.

(2) The operating conditions are high temperature and high speed. If the gap between the free end face of the bearing group and the pressure cover is too small, it will cause the outer ring end face of the bearing to come into contact with the pressure cover during actual operation. At this time, the bearing will bear additional force, which is very easy to burn out the bearing, especially cylindrical roller bearings.

4.2 Solutions

(1) During design, the gap between the free end of the bearing group and the gland can be increased by controlling the tolerance zone, such as increasing it to 1 mm.

(3) When space permits, a water cooling device can be added at the first axis component (the inner cavity of the first axis component and the housing) to reduce the temperature of the lubricating oil.

5. Conclusion

By analyzing the early failure of bearings in the gearbox of the mechanical system of the electric traction drum shearer, and based on clarifying the cause of failure, corresponding improvement measures are proposed, providing a guarantee for the reliable and normal operation of the electric traction drum shearer.

2024 March 1st Week KYOCM Product Recommendation:

Self-aligning Ball Bearing:

Self-aligning ball bearings have two rows of balls, a common sphered raceway in the outer ring and two deep uninterrupted raceway grooves in the inner ring. They are available open or sealed. The bearings are insensitive to angular misalignment of the shaft relative to the housing, which can be caused, for example, by shaft deflection.