Fire pump bearing box heat transfer calculation and temperature control measures

Abstract: In order to find out the causes of the different heating temperature of the bearing box of fire pump, the volumetric loss is the main factor that affects the degree of heat generation in the pump bearing box through the numerical calculation of the heat transfer through the heat transfer of the box body. Pump size has been improved.

In large fire pump operation, bearing housing often heat up to temperatures above 70ºC. High temperature will affect the lubricant performance. Tests showed that even with the same batch of products, the highest box temperature is high and low, by changing the type of bearing, adjust the radial thrust bearing installation gap can not control the temperature. Therefore, the root cause of temperature uncertainty needs to be found and effectively controlled. The main reason for overheating was found by numerical calculation of the heat transfer conjugate heat transfer (CHT) in the bearing housing. The problem of conjugate heat transfer can be divided into two computational regions, fluid-filled regions and solid regions. Energy flow through the diffusion process in the transfer between the two regions. Finite element method (FEM) is very suitable for purely solid heat transfer problem in discrete calculation method. Finite element based finite volume method (FVM) is more effective in conjugate heat transfer problem involving fluid. In this paper, the finite volume method.

1, heat transfer calculation results

Figure 1 is a computational mesh model that ignores the effects of oil in the tank on the heat transfer and the effect of heat transfer on air density differences. This problem is an axisymmetric problem, so a sector can be calculated, where a and b are the bearing mounting locations, Wire frame for the water, the rest for the box and shaft, box, shaft and air contact part of the air convection heat transfer boundary. Figure 2 is the calculation results, warm color is high temperature, cold color is low temperature. Although computers have strong computational and analytical abilities, the actual engineering problems are sometimes complicated and the relevant computational parameters have some similarities. This has implications for the accuracy of the calculations and should be fully understood in the calculation. When analyzing the results of calculations, take note of the effects of the uncertainty of the boundary conditions. In the calculation of bearing box heat transfer, water convection heat transfer is the forced convection heat transfer caused by the volume loss of the pump. Its heat transfer coefficient is related to the volume loss of the pump and should be controlled by the design. However, dimensional variations in manufacturing process lead to the uncertainty of volume loss, which leads to the uncertainty of water convection heat transfer coefficient. The calculation shows that the heat transfer coefficient varies widely. For a fire pump with a speed of more than ns = 76, the heat transfer coefficient equivalent to the flow rate when the volumetric efficiency is between 90% and 98% is usually 390W / m2 · ºC ~ 1240W / M2 · ℃ range of change. Air convection heat transfer is a natural convection heat transfer, its heat transfer coefficient and pump environment, and therefore there are uncertainties. Consider fire pumps are usually installed indoors, the speed of air flow changes will not be large, so the heat transfer coefficient will not change much. If the wind speed varies from 0m / s to 6.4 / s, the average heat transfer coefficient of the air is 5 W / m2 according to empirical formula. ºC to 25 W / m2 · ° C.

The water convection heat transfer coefficient is much larger than the air convection heat transfer coefficient, which is the dominant factor affecting the heat transfer, meanwhile the forced convection heat transfer can be controlled by the designer. Therefore, as shown in Figure 3, the effects of water convection heat transfer coefficient and mean air convection heat transfer coefficient on these two parameters were observed to see their effects on the calculation results. The temperature shown in Figure 3 is the maximum temperature of the bearing housing, which is usually located near the bearings away from the pump impeller. A single bearing heating power of 1000W, the ambient temperature is 20 º C. It can be seen that due to the water convection heat transfer coefficient of variation range than the average air convection heat transfer coefficient range is much larger, so water convection can affect the bearing box maximum temperature.

2, effective temperature control measures

In order to control the temperature, the volumetric efficiency can be properly reduced. If the maximum temperature is not more than 70ºC, the water convection heat transfer coefficient should not be less than 500 w / m 2 · ° C. Based on this, the pump volumetric efficiency and corresponding part size should be calculated.

Injection Molding

What is Precision CNC Milling

In general, computer numerical control milling machine (CNC) machining is a deductive manufacturing technique that programs 3-axis linkage, complex to 5-axis linkage, drives milling cutters to remove layers from solid blocks called blanks to produce finished parts. CNC milling is one of the main types of CNC Machining, which uses cutting tools that rotate at a speed of several thousand to tens of thousands of revolutions per minute (RPM) to precisely remove materials to obtain parts with complex surfaces. In addition to milling, CNC machining is also equipped with drills, boring tools, thread cutters and other tools to complete different part features at once. CNC machining produces parts based on a computer-aided design (CAD) model that is sent to a CNC machine through computer-aided manufacturing (CAM) software.

CNC Machining centers developed from CNC milling machines. The biggest difference compare to CNC milling machines is that the machining center has the ability to automatically exchange machining tools, by installing tools for different purposes on the tool magazine, the machining tools on the spindle can be changed through the automatic tool changing device in a single clamping device to achieve a variety of machining functions.

CNC milling accuracy

CNC milling centers and high-Precision Machining centers. Ordinary machining center, resolution of 1μm, maximum feed speed of 15 ~ 25m / min, positioning accuracy of about 0μm. High-precision machining center, resolution of 0.1μm, maximum feed speed of 15 ~ 100m / min, positioning accuracy of about 2μm. Between 2 and 10 μm, with a ± 5 μm more, can be called precision grade. FCE equipped with different grade and different size CNC milling center. Take in to account into both economic and quality in parallel.

CNC Finishing surface treatment

Post-processing is the final step in the CNC machining process. In a quick guide, we offer a lot of surface treatments to complete your final parts and get them to meet strict specific requirements. It should be noted that in CNC machining, post-processing is in optional, as the quality of the machined parts is already very high.

Anodized

The anodizing process allows the part to obtain excellent corrosion resistance, increasing the hardness and wear resistance of the surface. Anodizing is also the most common pre-painting treatment, which can help the painted surface to obtain good adhesion. We typically apply two types of anodizing in our production: Type II, corrosion resistant; Type III is thicker, adding a layer of wear resistance. Both anodizing processes can achieve a variety of color effects.

Polishing

Grinding polish, it offers the fastest turnaround parts and does not require post-processing. The surface finish of the grinding part is equivalent to 125 um in Ra, and the requirements can be increased to 63, 32 or 16 um Ra. Minor tool marks may still be visible on the last part.

Powder coatings

Powder Coating is thermoplastic powder spraying directly on the processed part. The sprayed parts are then baked in an oven to form a durable, abrasion- and corrosion-resistant plastic coating. In the powder coating process, its color, brightness, surface roughness can be customized.

Shot blasting

Shot peening is the high-speed spraying of beads of different hardnesses and sizes onto the surface of the part. To get different textures and brightness of the surface. Since the surface has been hit by similar forging beads, the hardness and wear resistance of the surface have also been enhanced.

Design of CNC machining

CNC machining is a universal machining process, especially parts that can be cut and machined can be achieved through CNC, from simple shape shapes to complex curved structures. However, as with every manufacturing technique, CNC machining has some design limitations. We break them down as follows to ensure that your products are optimally designed to better fit the CNC machining process.

General Tolerance

When designing parts, the size of each part is different according to the application environment, working conditions, etc., and the requirements for the machining accuracy of the workpiece are also different. Although CNC machining can achieve very high machining accuracy, we should also be aware that demanding machining accuracy and accuracy (strict product tolerances) usually mean Longer production times and higher costs. If a specific tolerance is not specified in the product design, we recommend choosing at the following levels.

The highest precision of our equipment is 0.001, and the recommended most precise machining requirements do not exceed +/-0.005mm

Interior angle

All internal vertical edges of our products need to have rounded corners instead of right angles. This is because the tools used in CNC milling are cylindrical, which means they cannot produce inner right-angled edges. The fillet required by this process is called the inner corner fillet. When designing parts, the larger the inner corner fillet, the more efficient the production process. Because of the larger fillets, larger diameter milling cutters can be used to increase milling speed, and fewer swaps can be made to improve accuracy.

Undercut

Undercut cannot be machined with standard milling cutters, so the use of retract slots for parts should be avoided when designing CNC machined parts. In particular, for non-standard inner contour shapes, customized machining tools are required, which will greatly increase processing time and costs. Secondly, if undercut can not be avoided, due to the limited length of cnc tools, the undercut can not be too deep. If they are too deep or the location is difficult to reach, they will not be able to be produced because the CNC tool cannot reach the machining location.

Chamber wall thickness

CNC machining requires your part design to meet minimum wall thickness requirements. In general, it is recommended to choose the thickest possible and avoid very thin or characteristic walls. This is because CNC tools processing thin-walled parts will cause vibration deformation, which may cause interruptions or damage, and the size is out of specification. The standard minimum wall thickness for CNC machined metals is 0.030" (0.76 mm) and for plastics is 0.060" (1.5 mm).

Thread

When designing parts, it is useful to choose the largest possible thread size because smaller taps have a higher risk of breaking during production. If possible, avoid using deep hole threads as they lead to higher production costs, especially when custom tools are required.

Metal

In principle, materials with high hardness are easier to process because better processing accuracy can be obtained, but they are limited by the hardness of processing tools, so the hardness of the generally recommended parts is 8 ~ 60HRC, and for metal materials, the hardness is greater than stainless steel Followed by cast iron, followed by copper, and finally aluminum, while the processing of ceramics, plastics, etc. belongs to the processing of non-metallic materials. Fast production time.

Polymer

Although CNC can process thermoplastics, the material properties of polymers still have many difficulties for CNC processing. First, due to poor thermal conductivity, many thermoplastics melt or bend when in contact with CNC milling machines or drill bits. Secondly, the processing of plastics, because the hardness is low, the size accuracy caused by the knife during product clamping and processing is not high. For those parts that do not have special requirements for the strength and hardness of the metal, thermoplastics can provide a cheaper alternative.

Cost optimization of machined products

Simplify product design

Simplifying product design while taking into account functionality usually saves production time and reduces production costs. Because the manufacture of complex structures and surfaces often requires complex longer passes, more layering to obtain the same fine contour quality. This means higher production costs.

Reduce cutter changes

Using the same milling machine machining features, larger and same internal fillets, can reduce the number of tools used to reduce machining time. For example, if a workpiece needs a 10mm end face milling cutter to process some features, it also needs a spherical milling cutter to process a curved surface, and a 2mm milling cutter to process a fine groove, which will require repeated tool changes and reduce the machining efficiency

Proper material selection

The choice of raw material for machining can have a significant impact on production time and costs for CNC machining. If possible, choose a material with good processability, such as brass or aluminum. For those applications that do not require metal hardness and strength, CNC machining of engineering plastics like PMMA and ABS also helps to reduce costs because the material blank is cheaper, and the processing efficiency is higher.

Tolerance and wall thickness

Higher tolerances and thinner wall thicknesses also increase THE CORRESPONDING CNC machining costs, as it takes time to achieve higher precision cutting. If your product or component can accept a larger size range, choose a lower tolerance to reduce production time and costs. The same is true for wall thickness: a larger wall thickness margin should be chosen.

Choice of surface treatment

Surface treatment is often the final stage of the CNC machining process, which can also affect the cost of your entire project. Choosing less surface treatment for your part or product can lead to better time and cost efficiency. FCE can recommend to you how to optimize the surface treatment to balance the conflict between cost and quality according to your final needs.

FCE Machining services

FCE facilities are equipped with the most advanced and highest precision 3, 4 and 5-axis CNC machines, which allows us to complete your order in record time

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