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渣浆泵困液现象和卸荷措施

作者:admin 发布日期:2021-03-06 10:35:03 关注次数:43

渣浆泵困液现象和卸荷措施
    为保证齿轮连续输送体和合时的运行平。必须使齿啮合的重数大于1即要求前一对齿尚未脱开前,后一对齿就进入啮合。因此,在一段时间中两对齿轮同时啮合,有两条啮合线,因而有一部分液体被困在两啮合线及两端盖形成的密封容器内。

当闭死容积由大变小时.被困在容积内的液体受到压,压力急剧升高,达到远大于排出压力(可以超过10倍以上)的程度。于是被液体从一切可以泄漏的缝中强行挤出,这时齿轮和轴承受到很大的脉冲径向力,功率损失增加,磨损加剧。当闭死容积由小变大时,剩余的被困液体压力下降,形成局部真空,使溶解在液体中的气体析出,或液体本身气化形成汽蚀,使泵产生振动和噪声。这种现象称为困液现象,困液观象对齿轮工作性能及寿命的危害很大。为消除困液现象,可以采取些卸荷措施,使闭死容积与吸腔或排出腔连通。当闭死容积由大变小时,与排出腔相通;当闭死容积由小变大时,与吸入腔相通。
四、齿轮泵的主要性能参数
1.流量
齿轮泵的理论流量常用近似公式计算。假定每转压出的被体量等于两个齿轮齿谷容积的总合。又假定齿谷体积等于齿的体积。由于齿高一般为2m(m为模数),故泵每转排容Vh按式(2-7)计算:其中
                      Vh= 2πDmbX 10-6
其中                             D= mz
式中Vh-----泵每转的排容,L;

m-------齿轮模数,mm;

b-------齿宽,mm;

D-----齿轮节圆直径, mm;

z------齿数。
泵每分钟的理论流量为每转排容V,与泵转速m的乘积,n是每分钟转速,z是齿数,即:

QT = Vhn=2πm2 zbnX 10-6
实际上齿谷体积比齿的体积稍大,所以加以修正,3.33代替π.:
                      Qt =6. 66m2zbnX 10-6
考虑容积效率影响后得实际排量为:
                      Q=6.66m2zbnηv*10-6

式中  ηv----容积效率,一般为0. 7~0.9,高压小流量ηv值取其中较小值。

影响齿轮泵流量的因素如下:

(1)转速。转速越高,在相同结构尺寸下流量越大,一般由选配电动机来确定。但转速过高时,齿间液体产生的离心力太大,会使齿谷中不能充满液体,影响泵的流量,故对节圆上的线速度有一定限制。
    (2)模数和齿数。在外形尺寸一定时,齿数越少,模数越大,则流量也越大。因此,齿轮泵中齿轮的齿数比一般传动齿轮的齿数少,而模数较大,常见齿数在8~14个范围内。但模数大、齿数少时流量脉动振幅大,一般中低压泵的齿轮模数见表2- 3。既要减少齿数,又要避免根切,一般采用修正齿轮,最小齿数可达到6
    (3)齿宽。齿宽与流量成正比。但齿宽越大,轴承所承受负荷增大,使泵尺寸增大而寿有缩短。
2.功率
齿轮泵的有效功率为:
                     Pe= pQX10-3

齿轮泵的轴功率为:

p= PQ/ηx 10-3
式中PeP-----  效功率和轴功率,kW;
     Q-----------实际流量, m3/s;

p------泵的全压力,Pa;

η-----泵效率,在0.6~0. 8之间。

3.齿轮泵的特性

2-9所示为Ch4.5型齿轮泵的特性,由图可看出泵流量Q、效率η及轴功率P与泵全压p的关系。

Liquid entrapment of slurry pump and unloading measures

In order to ensure the smooth operation of gear pump when continuously conveying liquid and meshing. The overlap coefficient of gear meshing must be greater than 1, that is to say, before the former pair of teeth is disengaged, the latter pair of teeth will enter the meshing. Therefore, in a period of time, two pairs of gears mesh at the same time, and there are two meshing lines, so a part of liquid is trapped in the sealed container formed by two meshing lines and two end covers.

When the closed volume changes from large to small, the liquid trapped in the volume is squeezed, and the pressure rises sharply, which is far greater than the discharge pressure of the pump (more than 10 times). As a result, the trapped liquid is forced out from all the gaps that can be leaked. At this time, the gears and bearings are subjected to a large pulse radial force, resulting in increased power loss and increased wear. When the closed volume changes from small to large, the pressure of the remaining trapped liquid drops, forming a local vacuum, which makes the gas dissolved in the liquid precipitate, or the liquid itself gasifies to form cavitation, which makes the pump produce vibration and noise. This phenomenon is called trapped liquid phenomenon, which is harmful to the working performance and service life of gears. In order to eliminate the phenomenon of trapped liquid, some unloading measures can be taken to connect the closed volume with the suction cavity or discharge cavity. When the closed volume changes from large to small, it is connected with the discharge chamber; when the closed volume changes from small to large, it is connected with the suction chamber.

4、 Main performance parameters of gear pump

1. Flow

The theoretical flow of gear pump is usually calculated by approximate formula. It is assumed that the extruded volume per revolution is equal to the total volume of two gear valleys. It is also assumed that the volume of the tooth Valley is equal to the volume of the tooth. As the tooth height is generally 2m (M is the modulus), the discharge capacity VH of the pump per revolution is calculated according to formula (2-7): where

Vh= 2πDmbX 10-6

Where d = MZ

Where VH -- discharge capacity of pump per revolution, l;

M -- gear module, mm;

B -- tooth width, mm;

D -- pitch diameter of gear, mm;

Z -- number of teeth.

The theoretical flow rate per minute of the pump is the product of the discharge capacity per revolution V and the pump speed M. n is the speed per minute and Z is the number of teeth

QT = Vhn=2πm2 zbnX 10-6

In fact, the volume of the tooth Valley is slightly larger than that of the tooth, so it is corrected to use 3.33 instead of π

Qt =6. 66m2zbnX 10-6

After considering the effect of volumetric efficiency, the actual displacement is as follows:

Q=6.66m2zbnηv*10-6

In the formula, η V --- volumetric efficiency, generally 0.7 ~ 0.9, and the value of η V for high pressure and small flow is the smaller one.

The factors affecting the flow of gear pump are as follows:

(1) Rotate speed. The higher the speed is, the greater the flow rate is under the same structure size, which is generally determined by the motor selection. However, when the speed is too high, the centrifugal force generated by the liquid between the teeth is too large, which will make the valley of the teeth can not be filled with liquid and affect the flow of the pump, so there is a certain limit to the linear speed on the pitch circle.

(2) Module and number of teeth. When the size is fixed, the less the number of teeth, the larger the modulus, the greater the flow. Therefore, the number of teeth in the gear pump is less than that of the general transmission gear, and the modulus is larger, and the common number of teeth is in the range of 8 ~ 14. But when the modulus is large and the number of teeth is small, the flow pulsation amplitude is large. Generally, the gear modulus of medium and low pressure pump is shown in table 2-3. In order to reduce the number of teeth and avoid undercutting, the modified gear is generally used, and the minimum number of teeth can reach 6.

(3) Tooth width. The tooth width is proportional to the flow rate. However, the larger the tooth width is, the greater the load on the bearing will be, which will increase the size of the pump and shorten its service life.

2. Power

The effective power of gear pump is:

Pe= pQX10-3

The shaft power of gear pump is:

p= PQ/ηx 10-3

Where PE, P -- effective power and shaft power, kW;

Q -------- actual flow, m3 / S;

P ------ total pressure of pump, PA;

η ----- pump efficiency, between 0.6 ~ 0.8.

3. Characteristics of gear pump

Figure 2-9 shows the characteristics of ch4.5 gear pump. The relationship between pump flow Q, efficiency η, shaft power P and pump total pressure P can be seen from the figure.



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