What is grinding of valve sealing surface?

What is the grinding of valve sealing surface?

Grinding, in the valve manufacturing process is a common finishing method for its sealing surface. Grinding can make the valve sealing surface to obtain high dimensional accuracy, geometric roughness and surface roughness, but can not improve the mutual position accuracy between the sealing surface surfaces. After grinding the valve sealing surface can usually reach the dimensional accuracy of 0.001-0.003mm; geometric shape accuracy (such as unevenness) of 0.001mm; surface roughness of 0.1-0.008.

20221111191807 33997 - What is grinding of valve sealing surface?

The basic principle of sealing surface grinding

The basic principle of seal surface grinding includes eight aspects: cleaning and inspection process, grinding process, grinding method, grinding motion, grinding speed, grinding pressure, grinding allowance and inspection process.

Cleaning and inspection process

Clean the sealing surface in the oil pan, use professional cleaning agent and check the damage of the sealing surface while washing. Fine cracks that are difficult to determine with the naked eye can be carried out by coloring detection method. After cleaning, the valve flap or gate valve should be checked with the sealing surface of the valve seat tightness, check with red and pencil. Use red Dan test red, check the seal surface seal shadow, to determine the seal surface fit; or use a pencil in the valve flap and seat sealing surface on a few concentric circles, and then the valve flap and seat close rotation, check the pencil circle wipe off the situation, to determine the seal surface fit. If the fit is not good, the standard plate can be used to test the valve flap or gate sealing surface and valve body sealing surface, respectively, to determine the grinding parts. 

Grinding process

Grinding process is essentially a cutting process without a lathe, valve head or seat pockmark or small hole depth is generally within 0.5mm, you can use the method of grinding for maintenance. The grinding process is divided into coarse grinding, medium grinding and fine grinding. 
Coarse grinding is to eliminate the sealing surface of the abrasion, indentation, corrosion points and other defects, so that the sealing surface to get a high degree of flatness and a certain degree of finish, to lay the foundation for the sealing surface of the medium grinding. Rough grinding using grinding head or grinding seat tools, using coarse sandpaper or coarse grinding paste, the particle size of 80#-280#, coarse particle size, large cutting volume, high efficiency, but the cutting pattern is deeper, the seal surface rougher. Therefore, coarse grinding is enough to remove the pockmarks on the valve head or valve seat as long as they are flat. Medium grinding is to eliminate the coarse grain on the sealing surface and further improve the flatness and finish of the sealing surface. The use of fine-grained sandpaper or fine-grained grinding paste, the particle size of 280 # – W5, fine particle size, small cutting volume, which is conducive to reducing the roughness; at the same time should be replaced with the corresponding research tools, research tools should be clean and clean. After the middle grinding, the contact plane of the valve should reach brightness. If you use a pencil on the valve head or seat a few lines, the valve head or seat against a light turn, the pencil line should be erased. 
Fine grinding is the last process of valve grinding, mainly to improve the finish of the sealing surface. Fine grinding can be used W5 or finer micro-part with machine oil, kerosene, etc. after dilution, with the valve head against the valve seat for grinding, without acting drama, which is more conducive to the sealing surface of the close. Grinding generally clockwise turn 60-100?about, and then the opposite direction turn 40-90?about, gently grinding for a while, must check once, to be ground to shine, and in the valve head and valve seat can be seen on a very thin line, the color reaches the black bright black bright, then gently grinding a few times with machine oil, wipe clean with clean gauze can be. After grinding, and then eliminate other defects, that is, should be assembled as soon as possible, so as not to destroy a good grinding of the valve head. 
Manual grinding regardless of coarse or fine grinding, are always throughout the lifting, putting down; rotation, reciprocation; light knock, change direction and other operations combined with the grinding process. The purpose is to avoid repetition of the abrasive track, so that the grinding tool and sealing surface to get uniform grinding, improve the flatness and finish of the sealing surface. 

Grinding method

What is the grinding method of valve sealing surface?
Valve sealing surface is divided into two kinds of sealing surface, flat and tapered, is the object of valve manufacturing must be ground.
Valve sealing surface grinding methods are manual grinding and mechanical grinding two.
(1). Manual grinding of the valve sealing surface
Manual grinding of the valve sealing surface using only simple grinding tools. Manual grinding is generally used when wet grinding, in the wet grinding process to often add thin abrasive, in order to wear the dull abrasive particles from the work surface, and constantly increase into the new abrasive particles, so as to get a high grinding efficiency. For sealing surfaces with particularly high accuracy and surface roughness requirements, sometimes dry grinding is also carried out using a pressed sand plate.
1). Manual grinding of the sealing surface of the valve body. The sealing plane of the valve body is located in the inner cavity of the valve body, which is difficult to grind. Usually use a disc with a square hole, placed on the sealing surface of the inner cavity, and then use a long handle with a square head to drive the grinding disc for grinding motion. There are cylindrical tabs or guiding shims on the grinding disc to prevent the grinding tool from leaving the ring sealing surface locally during the grinding process and causing uneven grinding.
Before manual grinding of the valve body sealing plane, the working surface of the grinding tool should be wiped clean with kerosene or gasoline, and remove the flying edge on the valve body sealing surface, burrs. Then apply a layer of abrasive on the sealing surface. Grinding tool into the valve body cavity, to carefully fit on the sealing surface, and then use the long-handle handle to make the research disc for positive and negative direction of the rotary motion. First clockwise rotation of 180 degrees, and then anti-clockwise rotation of 90 degrees, and so on repeatedly. Generally, more than ten times after the rotation of the abrasive in the abrasive will have been dull, so should often lift up the research plate to add new abrasive.
Grinding pressure should be uniform, and should not be too large. The pressure can be greater for rough grinding; it should be smaller for fine grinding. Care should be taken not to make the grinding tool partially disengage the sealing plane by applying pressure.
After grinding for a period of time, to check the unevenness of the work. At this point can be removed from the research tool, kerosene or gasoline will be wiped clean sealing surface, and then the disc-shaped inspection disc lightly placed on the sealing surface and gently applied by hand, remove the flat disc can be observed after the seal surface contact traces. When the ring-shaped sealing surface uniformly showed contact traces, and the ratio of the radial zui small contact width to the width of the sealing surface (i.e., the sealing surface and the test disk fit) to the value of the process, the unevenness can be considered qualified.
2). Manual grinding of the sealing plane of the gate and valve flap. The sealing plane of the gate, valve flap and valve seat can be manually ground by using a grinding plate. Before working, apply a layer of abrasive evenly on a clean plate, and rotate it by hand while making a linear motion after fitting the work to the plate. Because of the continuous change of the direction of the grinding movement, the abrasive grains are constantly grinding in the new line of defense, so the grinding efficiency can be improved.
In order to avoid uneven grinding plate, do not always grind in the middle of the plate, but should be in all the surface of the plate constantly changing parts, otherwise the grinding plate will soon lose flat accuracy.
Gate and some valve seat is covenant, the weight of the seal plane circumference is not uniform, grinding should be in its thin end (also known as the small head) plus a slightly larger pressure, so that the pressure on the ring sealing plane uniform, so as not to cause changes in the working covenant angle.
Throttle valve flap grinding can be used with holes in the annular disc research tool, the grinding method and the valve body sealing surface is basically the same.
3). Manual grinding of conical sealing surface. Grinding conical sealing surface need to use with a tapered research rod or sleeve. The taper of the grinding rod and sleeve should be consistent with the taper of the valve body sealing surface or valve sealing surface, respectively. Grinding plug body and plug research rod and research sleeve tapered surface to open a spiral shallow groove to accumulate excess abrasive. Grinding globe valve body, because the sealing cone is too short, poor stability. Therefore, usually in the valve body flange at the inner stop to add a guide disc, so that the research rod to maintain a smooth.
Grinding conical sealing surface, first wipe the research tool on a uniform layer of abrasive, lightly placed on the surface of the workpiece after hand pressure and rotate the research tool; rotate 3-4 weeks, the research tool can be pulled out some change the circumferential position and then grind. Re-grinding process should often add abrasive.
Grinding plug body research rod and research plug research sleeve taper should be consistent, otherwise the research conical sealing surface will be easy to occur between the leak.
(2). Mechanical grinding of the valve sealing surface
Due to the high efficiency of mechanical grinding, stable quality. Therefore, valve manufacturers mainly use machinery to grind the sealing surface. In the process of mechanical grinding sealing surface, we must first consider the complex grinding trajectory, reasonable movement, followed by the grinding efficiency.
There are five kinds of valve sealing surface grinding machines commonly used as follows.

  • 1). Axial pendulum lapping machine. The axial pendulum grinder is designed for grinding the sealing surface of the valve body, and can also be used to grind the sealing surface of the gate, valve seat and valve flap. The grinding track of this machine is ideal, especially suitable for small valve factories and valve maintenance departments. Japan, Germany, France, etc. commonly use this machine to grind valves. If you use the axial pendulum abrasive to grind the sealing surface of the valve body of large valves, you can first place a rectangular steel plate on the flange end of the corresponding end of the valve body, and then remove the spindle box of the grinder and install it directly on the steel plate, using the method of “ants gnawing the bones” to grind.
  • 2). Planetary type grinder. The planetary lapping machine is suitable for grinding the sealing plane of valve flaps, valve seats and gates. This machine is easy to use, can grind several workpieces at the same time, with high efficiency, and the geometric shape accuracy and surface quality of the workpiece after grinding is good.
  • 3). Vibratory grinding machine. Vibratory grinding machine is suitable for grinding the sealing plane of medium and small-sized globe valves and check valves. The machine can grind dozens of parts at a time, with high productivity. In addition, due to the high vibration frequency and small amplitude of the grinding disk, the relative motion of the workpiece and the grinding disk can be reduced. The direction of the relative movement of the workpiece and the grinding plate is constantly changing, so not only the geometric shape accuracy and finish of the workpiece after grinding is good, the grinding plate wear is also more uniform. In the vibratory grinding machine, the workpiece placed freely on the grinding plate has a short relative sliding between its own inertia and the vibrating grinding plate. The sliding of the workpiece on the grinding disc is irregular, and the collision between the workpieces often makes the grinding trajectory more complicated, but the grinding of the workpiece and the wear of the grinding disc are more uniform. Vibratory grinder is not suitable for grinding workpieces with high center of gravity or uneven weight distribution on the circumference.
  • 4). Multi-axis valve body grinding machine. Multi-axis valve body lapping machines are suitable for grinding the sealing plane of small globe valve covers. Mimicking manual grinding, it has high efficiency due to the automatic grinding process and the ability to grind more than 20 valve bodies simultaneously.
  • 5). Rotary plug grinding machine. The rotary plug grinder is suitable for grinding the conical sealing surface of the plug and the rotary plug body. The rotary plug grinder uses plugs and rotary plug bodies for matching grinding, with simple structure and easy operation, and its grinding motion is similar to manual grinding. Due to the high efficiency and good grinding quality of this grinder, its application is more common.

Grinding motion

When the relative movement of the grinding tool and the seal surface, the relative sliding path of each point on the surface of the seal to the grinding tool should be the same. And, the direction of relative motion should be constantly changed. The direction of motion constantly changes so that each abrasive grain does not repeat its own trajectory on the seal surface, so as not to cause obvious abrasion marks and increase the roughness of the seal surface. In addition, the direction of motion for the break change can not make the abrasive distribution is more uniform, so as to more uniformly cut the metal on the surface of the seal.
Despite the complexity of the grinding motion, the direction of motion varies greatly, but the grinding motion is always carried out along the surface of the grinding tool and the seal surface. Whether manual grinding or mechanical grinding, the geometric accuracy of the seal surface is mainly influenced by the geometric accuracy of the grinding tool and the grinding motion.

Grinding speed

The faster the speed of the grinding motion, the higher the efficiency of the grinding. The faster the grinding speed, the more abrasive grains pass on the surface of the workpiece per unit time, and the more metal is cut off.
Grinding speed is usually 10-240m/min. grinding precision requirements of the workpiece, grinding speed generally does not exceed 30m/min. valve sealing surface grinding speed and sealing surface materials, copper and cast iron sealing surface grinding speed of 10-45m/min; hardened steel and hard alloy sealing surface for 25-80m/min; austenitic stainless steel sealing surface for 10-25m/min. /min.

Grinding pressure

Grinding efficiency increases with the increase of grinding pressure, grinding pressure can not be too large, generally 0.01-0.4MPa.
Grinding cast iron, copper and austenitic stainless steel sealing surface, grinding pressure of 0.1-0.3MPa; hardened steel and carbide sealing surface for 0.15-0.4MPa. rough grinding to take a larger value, fine grinding to take a smaller value.

Grinding allowance

Since grinding is a finishing process, the cutting volume is very small. The size of the grinding allowance depends on the processing accuracy and surface roughness of the previous process. In order to ensure the removal of the previous process processing traces and correct the geometry of the seal error, the smaller the grinding margin, the better.
Seal surface grinding before the general should go through the fine grinding. After fine grinding of the seal surface can be directly fine grinding, its minimum grinding margin: diameter margin of 0.008-0.020mm; plane margin of 0.006-0.015mm. manual grinding or high material hardness to take a small value, mechanical grinding or low material hardness to take a large value.
The sealing surface of the valve body is inconvenient to grind processing, can be used for precision turning. Sealing surface after precision turning shall be rough grinding before precision grinding, the plane allowance of 0.012-0.050mm.

Inspection stage 

In the grinding process is always throughout the inspection stage, the purpose is to keep abreast of the grinding situation, so that the grinding quality to meet the technical requirements. It should be noted that different valve grinding should be used to adapt to the various forms of sealing surface grinding tools to improve grinding efficiency and ensure the quality of grinding. 
Valve grinding is a very detailed work, need to be in practice, and constantly experience, feel, improve, sometimes grinding very well, but after installation or leak steam and water, this is because in the grinding process there are grinding deviation imaginary, hand-held grinding rod is not vertical, skewed, or the size of the grinding tool angle deviation.

Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)

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