Introduction
The 2 million tons cement production line system of a company is composed of two sets with a diameter of 4.2m×13m grinding units. The supporting main reducer is JS150 type product developed in 1980s by a well-known gearbox company in China. The main technical parameters are as follows: the input power is 3550KW; Input speed 744r/min, speed ratio 47.2952:1, matching motor: type YR900-8, rated power: 3550KW, rated voltage: 6000V, rated current: 412.9A.

The two sets of mill systems have been running smoothly since they were put into production in 2010. On September 20, 2015, during the routine equipment inspection, the inspection workers found that the running sound of the main reducer of No.2 machine was slightly higher than that of No.1 machine and before. In view of the existing problems, the branch immediately set up a professional technical team, through a series of investigation and treatment, in a relatively short period of time, safe and effective accident will be eliminated in the bud.

Problem troubleshooting and maintenance plan

1.1 Preliminary judgment

First of all, the contact and wear condition of each tooth surface was observed by stopping the machine and uncovering the cover. Detailed understanding of the early lubrication situation, the oil pump oil volume, pressure, oil quality all normal; Verify the vibration, temperature, current and other curves of the central control instrument, and find no abnormality.

Then, when the professional VM22 vibrometer was used to detect the bearing positions of the reducer, it was found that the acceleration of the high-speed bearing positions near the coupling end was abnormal, which reached about 50m/s². It was judged that there would be a high safety risk if the reducer continued to operate.

The current period of the hidden danger is the peak season of the cement production of the company. Meanwhile, the company has no experience in the maintenance of the large reducer with the power double-split structure. If it is delayed or run with the hidden danger, it is very likely to lead to a major equipment accident. Finally, the company decided to set up a special technical maintenance team and organize the maintenance force of the company to complete the work.

1.2 Structure analysis of hidden trouble host

Verification of original technical data: this series reducer adopts the structure form of center drive, power double split, torque shaft load, two-stage reducer symmetrical arrangement. Main tooth surface after carburizing and hardening the grinding tooth, tooth shape and tooth to nap, gear accuracy for GB100095 level is above 6; rigid body shell structure for large welded structure, its internal bearing adopts multiple degrees of freedom thin-wall sliding bearing thrust bearing and gear shaft disk, the whole monitoring system USES PTl00 end platinum thermal resistance, through multi-channel temperature inspection instrument to display all the bearing temperature monitoring, centralized and thin oil lubrication system, lubrication of bearings and gears.

Through the analysis of the performance, structure and principle of the main engine, the maintenance task should be successfully completed. First, all work should be carried out in strict accordance with the maintenance standards. Second, find out the root cause of hidden danger, and deal with the problem thoroughly; Third, consider in advance some difficulties that may be encountered in dismantling the torsion shaft; Fourthly, all the equipment, especially the position of the double shunt gear cannot be shifted, because the original tooth surface match, bearing bush in the manufacturer has been strictly paired assembly; Fifth, emphasize all safety rules in the whole overhaul process.

1.3 Preparation of bearing fittings and determination of parts hoisting scheme

Through uncovered the high-speed shaft coupling end bearing end cover, and can be verified, the bearing of 23248 came 4 sn bearing, because the machine has been running for 5 years and high-speed bearing for inside and outside two bearing support, so the stock will be 2 pieces, no matter good or bad will be replaced in pairs, and must be as the international famous brand bearing, at the same time purchase returned to a comprehensive test of various technique parameters of bearing.

Because this equipment is in 5m height, using the steel beam ceiling structure, steel beam profiles using thin channel steel, cannot be used for hanging beam, and lift off the ceiling, dismantled and then restore, its integrity will be damaged, delay time and consumption cost, and the reducer top to the canopy and a certain lifting height. And then, the project team to find a professional crane mechanic, with respect to the position of the crane put, the size of the weight, the length of the arms, the size of the steel rope, the weight of the suspended parts after verification and calculation precision, make plan in advance, with company a 25 t and 2 t steel ball weight and 50 t car hoisting, does not need to be removed to ceiling can solve all broken parts of hoisting, effectively shortens the whole maintenance at least 1 ~ 2 days.

02 Specific implementation plans

2.1 Preparation of construction personnel and tools

Personnel preparation: 1 construction worker, 1 safety officer, 4 fitter, 3 lifting workers, and 1 welder.

Equipment preparation: 25T, 50T truck crane each 1, several steel rope, fitter tools including all kinds of wrench, hammer, polishing machine, hoist, oil top, lifting lug, oxygen, acetylene, etc.

Site preparation: before the shutdown, the main motor will be cut off, pull the switch, hang the safety sign, arrange and clean the place where all parts are placed after being disassembled.

2.2 Disassembly construction

First, unfasten all bolts of reducer shell, remove bolts of front and rear end cover, place 25T and 50T crane and jointly lift out the upper shell, and then remove the temperature control, vibration and other connections, and protect them to avoid damage, and then remove all lubricating oil pipes. Then remove the torque bar, input stage big teeth and high-speed shaft components, pay attention to the high speed teeth and the left and right input stage big teeth meshing position, mark with paint, use tools to pull out one by one the fixed pins at both ends of the torque bar, and number. After all dismantling, remove the motor junction box so that the torque bar can be removed at one time. Remove the torque bar, and then remove the top cover of the bearing bracket of one input stage big gear. Remove the input stage big gear on one side by using the crane, and mark the upward position with paint to facilitate the subsequent in-place reloading. After the big teeth of the input stage are removed at one side, the high-speed teeth and bearings can be easily pulled out as a whole, and the forklift is used to reverse into the professional installation shed to remove the coupling and bearings.

After the high-speed shaft bearing was disintegrated in the shed, it was found that the no-load end bearing only had a slightly larger clearance, which proved that the no-load end bearing was not the problem. However, when the bearing with load end of coupling is removed, it is found that the wear and pitting of the outer orbit of the bearing here are serious in the whole circle, so as to find the root cause of the change of acceleration during detection.

2.3 Reassembly of high-speed shaft components and problems encountered

After removing and cleaning all the original bearings according to the process, the installation of the new bearings is carried out. After installing the new bearings in a free state, the maximum clearance is 0.30 mm, manually moving the bearing raceway slewing freely, each working procedure are in line with the inspection standard, but will the high-speed bearing assembly, all the final assembly and reset to the deceleration machine, the outer roof reset after fastening, using artificial plate moving the ball cage found by coupling ball tighter orbit, the remaining three columns are loose. It is judged that the bearing clearance has changed after installation, that is, only the outer rail of the four raceways is the reason for the pitting corrosion fault.

The reasons for the above failures are, firstly, the errors caused by the hole machining tool of the original equipment may be the main reason. Secondly, it may be due to the micro deformation of the component stress change. If thoroughly handled, the whole reducer base must be sent to professional machine tools to re-find the processing, difficult, high cost; If not treated, the potential pitting corrosion of the bearing or burnt-out bearing fault will continue to appear. Finally, developed by mechanical maintenance of common bearing outer ring lead wire method, namely in the bearing and the bearing surface respectively placed lead wire, fitted with a pressure bearing bolt bearing seat cover, after fastening, loosen all bolt and remove the lead wire, its thickness is measured with micrometer, respectively, the average differential algebra is the interference between the bearing and the bearing clearance or value. According to the numerical value, it is determined whether to add copper skin to the joint surface of the bearing seat or copper skin to the outer ring of the bearing, which can initially eliminate the occurrence of pitting corrosion and wear caused by the tight bearing ball. In this process, it is calculated that 0.30mm thin copper sheet is added to both sides of the bearing seat. The test machine and the following months ‘load production tracking show that the bearing temperature and amplitude in the early hidden trouble are kept good, so as to confirm the correctness of this simple scheme.

2.4 Matters needing attention for parts reassembly

First of all, before installation, all parts and components of the lower box should be thoroughly cleaned, and the surface should be dried with compressed air to ensure the cleanliness of the installation. In particular, the bearing must not fall into iron filings, sand and other foreign bodies. At the same time, we should carefully check the same group marks and bearing marks on the end faces of bearings and gear components, and install them after confirmation, so as to avoid rework and damage of parts.

Secondly, for the input shaft component, the half coupling should be installed on the input shaft first. Attention should be paid to the O-type rubber sealing ring and bearing trap on the input shaft before installation. Lift the cleaned input shaft assembly horizontally, slowly put both ends of the bearing into the bearing seat hole (a small amount of grease is injected into the rolling bearing), cover the left and right marks of the gear end face, and install the torsion bar and upper cover back and tighten evenly. After the basic installation of the whole reducer in place, and then re-use the laser centering instrument or dial meter accurate alignment of the motor and reducer coupling radial and axial yaw shall not be greater than 0.1mm, it is recommended that the best control within 0.05mm.

In addition, for the motor and reducer is the use of elastic pillar pin coupling, must pay close attention to the processing error of apron. The maintenance operation to early ageing of aprons all change, and after looking for positive, bolts, ma chine motor bearing shell temperature suddenly appeared high jumping phenomenon, finally after screening, the main reason is due to the processing of synthetic error, namely the new aprons compared to the original length of synthetic increased by 2 mm, so the main motor shaft at fastening force of traction, caused by the axial displacement of the motor, the formation of trunnion bearing friction, fever.

03 Conclusions

The sudden hidden trouble of the equipment, the experience value of maintenance and the limited space of the equipment brought some difficulties to the rush repair. However, after our low-cost, safe, meticulous and solid work, the whole rush repair work only took 4 days. According to the follow-up observation after rush repair, its continuous and stable operation by October 2016 is enough to prove that it is a very successful low cost and safe rush repair example.

As a cement production enterprise, the long-term, continuous and stable operation of equipment is our constant pursuit of the goal, however, good at finding hidden dangers in the production process, and through their own efforts to timely and effectively deal with hidden dangers, is our modern new dry process equipment management work should have the ability.

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