Large pendulum valves having nominal diameters of 400 to 500 mm are extensively used as main valves for organic EL and other large deposition systems. The main parameter to consider while designing pendulum valves with nominal diameters of 400 mm or larger are to ensure resistance to the large forces (over 10,000 N (Newton)) generated when a differential pressure of 1 bar is applied to a valve plate and to keep costs down while satisfying performance requirements such as reduced vibration, reduced particles, easy maintenance and long maintenance cycle.
The developed mechanisms can be classified as:
- A mechanical lock mechanism –It makes use of cams, links and other mechanical elements to increase the thrust of an air cylinder serving as a driving source and apply that thrust to a valve plate to resist the differential pressure.
- A back pressure cancellation mechanism - When a differential pressure is applied to a valve plate, a part of the valve plate assembly slides in the direction of the differential pressure to relieve the force to the casing rather that resist directly the differential pressure.
Figure 1. The cylinder-in-valving element type in a released state
Figure 2. The cylinder-in-casing type, in a released state
This type of mechanism features a spring that is used for generating the driving force to force down the packing of the valve plate and a sliding o-ring or bellows used for sliding components in the valve body.
Traditional back pressure cancellation mechanisms are divided into:
- Cylinder-in-valving element
The main user requirements for large pendulum valves are:
- Valves need to be easily maintained without the need to remove the valve casing from the chamber
- Even when valves are continuously used exceeding the maintenance cycle, the valves suffer little damage and can be restored easily.
This paper describes the approach to meeting these requirements with the VFR series in terms of the basic principle of valve bodies and associated technologies in comparison with conventional technologies.
Valve Body Mechanism
Adoption of the Back Pressure Cancellation Mechanism
The mechanical lock type valve body mechanism makes use of cams, links and other thrust increasing parts, and some parts are unavoidably subject to locally intense surface pressures. Particle generation with this mechanism is difficult. The back pressure cancellation mechanism has no sliding parts in the driving force transmission route and so is superior to the mechanical lock type in terms of particle generation. Hence this mechanism was used for VFR series valves.
Consideration of Maintainability of Valve Body
The back pressure cancellation mechanism of the cylinder-in-valving element type requires removal of the casing from the chamber for maintenance of the ring cylinder. This is a key disadvantage and a reason why this type cannot satisfy user requirement. Hence the back pressure cancellation mechanism of the cylinder-in-casing type was used as the basic structure of VFR series valves.
Consideration of the rotary shaft sliding mechanism
The cylinder-in-casing type is developed on the assumption that the rotary shaft can slide axially when the valve plate is sealed. The release of the valve plate causes the rotary shaft to be held in a fixed position by the shaft holding spring, which is not shown in the figures, but is located around the rotary shaft. The sealing of the valve plate causes the counter plate to be pushed by the spring and moved downward in the figure. The rotary shaft slides axially.
Figure 3 shows the external forces acting on the rotary shaft assembly. The rotary shaft assembly is the assembly of the rotary shaft and the counter plate.
Figure 3. External forces acting on the rotary shaft assembly
Equations of balance among external forces on the rotary shaft assembly during sliding of the rotary shaft are formulated. Typical values for pendulum values with a nominal diameter of 500 are substituted in the equations.
F1: Force required to axially slide the rotary shaft assembly
F2: Radial load applied by the slide bearing to the rotary shaft assembly
F3: Axial friction force applied by the slide bearing to the rotary shaft assembly
F4: Actuating force of the spring to axially hold the rotary shaft assembly
µ2:Friction coeffi cient of the slide bearing
L1: Distance from the center of the valve plate to the center of the rotary shaft
L2:Distance between two slide bearings
Balance in the Y direction:-F1+F3+F4=0 (1)
Balance around the point O:-F1L1+F2L2=0 (2)
Friction force:F3=2µ2F2 (3)
From equations (1) to (3), we obtain:
F1=F4/[1-2µ2 (L1/L2)] (4)
F4= 589 [N] (to bear a weight twice as heavy as the 30 kg valve body)
L1/L2=2.18 (typical proportion)
L1/L2=4.15 (for low-profi le products)
The relationship between F1 and µ2 is shown in Figure 4
Figure 4. Relationship between F1 and µ2
The L1/L2 ratio can be approximately determined on the basis of market-leading products, and here we adopt the L1/L2 ratio of 2.18. A higher L1/L2 ratio means lower-profile products. To study the potential of designing lower-profile products in the future, the L1/L2 ratio of 4.15 is adopted for this calculation.
Adoption of a Neutral Axis Mechanism
A cylinder-in-valving element type was adopted with a neutral axis for the VFR series. A racket-shaped part, called an arm, is added to the valve body and connected to the valve plate via the arm and valve plate connection spring, which is mounted uniformly on the circumference. In this structure loads applied to the rotary shaft and the valve body do not increase divergently even when the friction coefficient of slide bearings worsens. The strength of the rotary shaft and the valve body is reduced and user requirements are satisfied.
Mechanical Sequence Circuit
The sealing mechanisms of the back pressure cancellation type, such as the cylinder-in-valving element type with a neutral axis that was adopted for VFR series, require two air cylinders. One is used for swinging the valving element like a pendulum from the closed position to the retreat position. The other is used for the up and down movement for sealing. The two cylinders need to be operated in a predetermined sequence, and they may be damaged if operated out of order.
An electromagnetic valve on the equipment side can control the operation sequence but a malfunction may occur when power is turned off or during maintenance and cause a problem. As a solution to this, the technique of integrating a mechanical sequence circuit that does not use electricity into a pendulum valve is gradually becoming the mainstream in recent years.
All the components except the speed controller were fabricated by and assembled them into a manifold block to make a compact sequence box that can be mounted as a unit. Figure 5 shows the mounted sequence box.
Figure 5. Mounted sequence box
This paper has described the element technologies of the valve body structure and the mechanical sequence circuit. These element technologies are intended to prevent trouble from occurring and it will take several years after the delivery of products to users to finally determine whether these technologies are effective or not.
This information has been sourced, reviewed and adapted from materials provided by ULVAC Technologies.
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