Theoretical and experimental substantiation of the applicability of PTFE-4 for sealing movable seals in vacuum milking systems

The article describes the results of theoretical calculations of the loads acting on the radial surface of the blade of a plate-type vacuum pump used in milking systems. On the basis of experimental studies of Fluoroplast-4, its use for sealing highly loaded movable seals in vacuum systems is substantiated. The results of experimental studies on the dependence of the actual contact area and specific pressure on the load of 0-100 N are presented. An analysis of the data showed that in the range of loads of 0 to 10 N, the dependence is almost linear. In the range of 20 to 100 N, the specific pressure reaches a limiting value and ranges from 47.3 to 57.8 MPa, i.e. it stabilizes. These data indicate that for sealing the most loaded and critical movable seals in milking vacuum systems, it is possible to use Fluoroplast-4.


Introduction
The automation and robotization of agriculture impose increased requirements on all systems of the technological process. Vacuum systems are one of the most demanded systems in milk production due to the fact that they are used to perform various technological processes: milking, milk transportation, equipment drive, degassing of working areas, etc. [1,2]. Therefore, to ensure the reliability of these systems, increased requirements are imposed on the methods of calculation, design and manufacture. The reliability and efficiency of technological equipment depend on the correctness of design solutions. It becomes necessary to substantiate the decisions made, especially at the stage of designing vacuum milking systems and its individual elements.
One of the main indicators that determine the efficiency and reliability of vacuum systems are ultimate vacuum and pumping speed. A decrease in these indicators causes a sharp deterioration in the quality of technological processes, which affects the quality of products and economic costs of production [3,4,5]. For example, insufficient vacuum in the milking system decreases the milk yield of cows which contributes to the development of mastitis. As the practice of operating vacuum systems shows, the main reasons affecting these indicators are gas leaks through the movable seals of the elements of vacuum systems. Such seals include a ball valve and a seal in the ball valve, a wedge and a wedge seal ring in the gate valve, a valve and its seat, end and radial surfaces of blades and a vane-type vacuum pump housing [6,7,8].
All this creates prerequisites for improving the performance of elements of the vacuum systems, through the use of modern technologies and high-tech materials. Fluoroplastic-4 polymer is used as a seal in ball valves; however, its use in other movable joints of vacuum systems is poorly studied, since 2 they all have different design characteristics and operating conditions. Therefore, the aim of this article is to study operating conditions of movable seals of vacuum systems, using the example of radial seals of a blade-housing of a vane-type vacuum pump and the use of fluoroplastic-4 material.

Materials and methods
Fluoroplast-4 is a high-molecular crystallized polymer, a high-tech unique material. It has become widespread due to the presence of unique physical and chemical properties: absolute chemical resistance, low friction (0.02 for steel), strength, stability and workability in the temperature range from 269 to + 260 0 С. Foreign analogues are Soreflon (France), GOSTflon (Germany), Teflon (USA), Fluon (England), Agloflon (Italy), Polyflon (Japan).
Research methods imply a theoretical calculation of possible loads in the zone of the movable seal blade-housing of a vane-type vacuum pump to study properties of the PTFE-4 material.
Since in terms of tightness of the movable seal, the most important indicator is the actual contact area, the laboratory unit was designed to determine it ( Figure 1).
1 -sample of PTFE 4; 2 -thrust bearing; 3 -base plate; 4 -bed; 5 -arrow; 6 -scale; 7 -screw; 8sleeve; 9 -spring Figure 1. The laboratory unit for determining the area of actual contact The experimental research technique using the designed laboratory unit was as follows. On the end surface of the sample (2.5 mm long; radius of rounding 2 mm), the long-drying polymer paint is applied. The sample is fixed between the thrust bearing and the base plate fixed on the bed. The arrow is set to zero on the scale. Using the screw, a force is created. It is transmitted through the sleeve to the spring with a known deformation ratio. The force from the spring is transmitted through the center bearing to the sample forcing it to deform in the contact zone with the center bearing leaving an imprint of the contact area.
Linear dimensions of the imprint of the contact of the material sample with the thrust bearing were measured using the digital caliper TSCHTSTS-I-150 0.01 GOST 166-89.

Results
One of the most critical movable seals are vane-type radial vane-casing vane-type seals, since their unsatisfactory operation affects characteristics such as pump performance and ultimate vacuum, which affects the quality characteristics of the vacuum system.
The main parameters affecting the performance of this movable seal is a contact area of the radial surface of the blade and the inner surface of the pump casing and the pressure in the contact zone. Analyze the diagram of the main forces acting in the contact zone of the blade with the pump casing ( Figure 2).  The pressure at the contact point will be influenced by all the projections of forces on the X-axis. Since the projection of the friction force is always directed tangentially to the radius of movement, its projection onto the X-axis will be zero, as well as the projection of the Coriolis force arising due to the eccentrically positioned rotor. Therefore, the resulting sum of forces Fs is determined by the vector sum of gravity mg and centrifugal force F1: Making transformations and passing from a vector sum to a scalar sum, we have: where: -plate mass, kg; -angular speed, degrees/sec -radius of the inner surface of the pump cylinder, m; -eccentricity, m; -blade rotation angle, degrees. Substituting the actual parameters for the vacuum pump РВН 40/350 in the formula, the graph of the dependence of force Fs on the angle of rotation of the pump rotor can be built ( Figure 3).