Maintenance of mechanical equipment

The maintenance of machinery and equipment actually includes two aspects, namely, the maintenance of machinery and equipment and the repair of damaged machinery and equipment.

Generally speaking, the maintenance of machinery and equipment includes daily work, such as cleaning, painting and lubrication. Repairing damaged or identifying whether the running mechanical equipment is normal and preventing possible failures all belong to the scope of repair work. Below we will introduce it from two aspects: wear and lubrication and the repair of general mechanical equipment.

I. Wear and lubrication

All kinds of moving machines and parts will be worn in the work. In order to reduce the wear and tear of machines and parts, the usual measure is to lubricate the parts where friction occurs.

In the process of machine operation, two parts moving with each other have two forms of wear, namely natural wear and accidental wear.

Natural wear, that is, normal wear, refers to the wear of materials caused by friction, impact load, high temperature, chemical and electrochemical effects under normal working conditions. The degree of natural wear depends on the characteristics of the machine structure and the technological conditions when the machine works, the quality of the metal used to manufacture the parts, the machining accuracy and smoothness of the parts, the condition of the friction surface, the nature and type of lubricant, the size of unit pressure, the assembly accuracy and meshing accuracy of the machine, the operating conditions, the quality of maintenance and so on.

When the machine or its individual mechanism is temporarily damaged, the accidental wear is caused by the following factors: the structure of the machine is defective and the material quality of the parts is poor (cracks, casting holes, etc.). ), bad parts manufacturing and processing, violation of machine technical operation rules, incorrect assembly (installation) of parts or machines, violation of lubrication regulations, untimely repair or low quality, and irresistible disasters.

Fig. 10-25 shows the change of mating clearance of common journal and bearing assembly before and after wear.

Lubrication can reduce wear. Choosing a proper lubrication material and a reasonable lubrication system for a pair of worn parts will prolong their service life, otherwise improper lubrication will lead to accidents such as jamming and damage.

Fig. 10-25 Changes of mating clearance when journal and bearing components are evenly worn.

Viscosity, oiliness, composition and temperature of lubricant, moving speed of friction surface, unit pressure and structure between moving parts, etc. , all affect the friction coefficient and wear.

Sliding friction can be divided into the following forms:

(1) Dry friction occurs when two uneven hard surfaces that are not lubricated or slightly lubricated move relatively. In mechanical manufacturing, the dry friction coefficient of common metal parts is f = 0. 1 ~ 0.3. This form is very common in friction drives and various brakes. Dry friction will cause serious wear on the friction surface.

(2) Liquid Friction When the friction surfaces of a pair of friction parts are completely separated by a layer of lubricating material, friction occurs in the lubricating layer. Due to the internal friction of viscous liquid, the friction coefficient of the friction part is 0.003 ~ 0.0 10, which is close to the effect achieved by better rolling bearings. Because this pair of parts are separated by lubricating oil film when working, there will be no obvious wear after working for many years, so to prolong the life of sliding parts, liquid friction should be realized as much as possible.

(3) Semi-liquid friction and semi-dry friction are between dry friction and liquid friction, and the friction coefficient is between 0.0 1 ~ 0.3. Wear in this way is of course greater than liquid friction. However, in practical work, some friction parts cannot be completely lubricated by liquid. For example, on the friction surface with low relative motion speed, the gap between the shaft and the bearing is large, and the two parallel surfaces (such as piston rings) that slightly lubricate the shaft in the swing state partially occur between the teeth of gears, and the lubrication of balls and bushings of rolling bearings. Even sliding bearings are liquid friction during normal operation, and semi-liquid lubrication will occur during start-up and stop.

(4) Keep a thin oil film (below 0. 1μm) on the boundary friction surface.

The friction coefficient of different types of friction listed above can reach as much as 20 ~ 40 times. It can be seen that lubrication is important, and it is of great economic significance to choose a suitable lubricant.

Second, the mechanical equipment repair

In the factory, many worn and damaged parts are replaced from mechanical equipment every year. If these parts are repaired and reused, a lot of materials and funds can be saved. Therefore, the maintenance of mechanical parts is very important. As a mechanic, we must master the common repair methods of various main mechanical parts.

The wear and tear of mechanical equipment is a natural phenomenon, which has certain development laws, and these laws can be mastered. In order to keep the machinery and equipment in normal working condition, prevent accidents and grasp the initiative of production, it is necessary to check and maintain the machinery and equipment regularly according to the wear law in a planned way, so as to nip possible failures in the bud. Therefore, the factory should establish a planned pre-maintenance system. The planned pre-repair system is an annual plan for the repair cycle and workload of each repair level (repair work is divided into four levels: inspection, minor repair, medium repair and overhaul), and the mechanical equipment is inspected and repaired as planned. Of course, according to the results of disassembly and defect inspection, the actual maintenance workload may be different from the planned quantity, and the maintenance level may even be lowered. The repair cycle and workload of various mechanical equipment should be formulated according to the actual situation.

The data in table 10-4 can be used as a reference for making maintenance plans.

The work of each repair level is as follows:

(1) The inspection work is generally carried out by the operators and maintenance personnel on duty when the equipment is running. The main work is to check noise, vibration, leakage and lubrication, and carry out regular lubrication.

(2) Minor repairs shall be carried out by workshop maintenance personnel. Minor repairs include fastening parts, wiping friction surfaces, grinding valve plates, installing fillers, adjusting parts (such as adjusting bearings), checking oil rings, oil tanks, starting equipment, transmission shafts, transmission belts and chains, friction wheels, stuffing boxes, various seals and pipe fittings.

(3) In addition to all the work done by minor repairs, minor repairs also include the repair and replacement of individual parts, such as the recasting and grinding of bearing shells, the turning of optical journals, the replacement of brake pads, chains, gaskets and seals, and the repair of linings.

Table 10-4 Continuous running time and downtime of several mechanical equipment maintenance

(4) Overhaul and overhaul include cleaning and reassembly of mechanical equipment, replacement or repair of all worn parts, and adjustment of tolerance to meet technical requirements and repair basis. During the overhaul, mechanical equipment can also be modified.

Repairable damaged parts found in the inspection shall be repaired according to their structural characteristics and technical requirements. In order to shorten the downtime of mechanical equipment, the damaged parts can be replaced with spare parts first, and then repaired after the equipment resumes operation. The following introduces the common repair methods of mechanical parts wear.

The natural wear of mechanical parts and assemblies is the main form of wear, which is manifested in: destroying the initial matching performance of assemblies and changing the geometric size and shape of parts. In order to restore the working ability of these components, it is necessary to restore their initial mating characteristics, geometric dimensions and part shapes.

There are two ways to restore the working ability of the assembly: one is to restore only the matching properties; The other is to restore the fit and the geometric size and shape of the parts.

(1) A repair method that only restores the fitting characteristics of parts.

There are three repair methods to restore only the matching properties of components, such as gasket adjustment method, repair size method and supplementary parts method, as follows:

A. washer adjustment method

When repairing, the assembly is not processed or scraped, and the assembly clearance is restored from the worn maximum clearance to the initial clearance when assembling by reducing the adjusting gasket, as shown in figure 10-26. This repair method is called gasket adjustment method. The gasket adjustment method can only restore the fit clearance of the assembly, but can not restore the geometric size and shape of the parts.

Figure 10-26 Adjust the fit clearance between the journal and the bearing by adjusting the gasket.

B. Method of repairing dimensions

When repairing, only a part of the assembly is repaired, and its wear layer is removed to eliminate geometric deformation, while the other matching part is replaced with spare parts. In this way, the fitting clearance of the assembly can be restored to the original value, but the size of the repaired assembly is different from that of the new assembly, which is called repair size. This repair method is called repair size method.

Which part should be repaired during assembly? Which part should be replaced? This problem should be considered comprehensively. Generally speaking, more expensive parts are always repaired and cheaper parts are replaced.

Repair dimension method can not only restore the fit clearance of assembly, but also restore the geometric shape of parts, but can not restore the geometric size of parts.

C. supplementary parts method

When repairing with this method, a supplementary part (inner bushing or outer bushing) is installed on the worn part (bearing hole or journal) with a certain interference, and then it is machined to the original size to restore the fitting clearance of the assembly. So this method is actually a method to repair the size, but here two matching parts are still used together, and only one supplementary part is added, so it is called the supplementary part method.

(2) Repair methods to restore the matching characteristics of components and the geometric size and shape of parts.

The essence of this repair method is to cover the surface of worn parts with a layer of metal through various technological methods. After processing, the original geometric size and shape of the parts and the matching performance of assembly can be completely restored. There are several repair methods that fall into this category, such as cover welding, casting, spraying and electroplating. The commonly used surfacing methods are introduced below.

Cover welding or surfacing is to cover the surface of worn parts with molten metal to form a continuous and uniform coating. Its advantages are high connection strength and no need for special equipment and general welding tools. The disadvantage is that it is easy to cause thermal stress, crack, deformation and metallographic structure change. Cover welding can be used to repair parts made of ordinary carbon steel, high-quality carbon steel, low-alloy steel, cast iron, nonferrous metals and their alloys. For high alloy steel parts, the welding process has not been fully mastered, so it is generally not used. The welding material for surfacing is usually structural steel covered electrode (high quality carbon steel and low alloy steel). Arc welding or gas welding can be used for cover welding. If one layer is not thick enough, multi-layer welding can be covered. For important parts, in order to facilitate cladding processing, eliminate residual stress and improve metallographic structure, heat treatment (high temperature tempering, annealing or normalizing) should be carried out after cladding.