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Technical

Shaft Pins for Quarter-Turn Valves

Many designs are employed to attach the closure member to the shaft of a quarter-turn valve. This connection must transmit loads primarily based on the actuating torque requirements. Other tensile or compression loads from the fluid pressure, temperature and flow are relatively small by comparison and for all practical purposes can be ignored.

This connection must transmit high torque loads, which are often fluctuating from turbulent fluid flow and changes in rotational direction with each opening and closing stroke. The connection must also be rigid and free of hysteresis as these valves often are used in modulating service. Even in on-off service, the closure member must repeatedly position accurately into the seat.

Figure 1 (below) depicts several of the most common shaft-to-disc connection methods. These are:

table 0

figure 1

ANALYSIS

The first four keyed connections (A through D) typically are not used because the installation and removal of the key from the closure member is often impossible in many valve designs. Even when the design is such that this connection can be used, there is hysteresis in the connection unless the keys are tapered, interference or shrink-fit. In either case, a hysteresis-free connection is: not well suited for mass production processes, not an interchangeable design, and not easily repaired in the field. As a result, these connections are not popular in quarter-turn valves,

The next two straight-centered pin connections (E and F) also have some major drawbacks. First, there is an amount of hysteresis when using the typical mass production machining fit dimensions. Use of precision fit dimension tolerances, interference or shrink-fit techniques is also costly and not suited for mass production and economy as well as parts interchangeability. Use of distorted locking, spiral-rolled and “C” form spring-type pins reduce the hysteresis initially but tend to loosen in service because of the highly reversing torque requirements. Next, the hole through the center of the shaft removes a large amount of the shaft’s cross-sectional area and causes stress concentration factors in the valve shaft of values greater than 2.1 to as much as 4.6 for most optimal pin diameters of about 20% to 40% of the shaft diameter. This reduces the strength of the shaft significantly and the size of the pin must be balanced with the shaft diameter based on the relative strengths of the shaft, closure member and pin materials to optimize the joint strength.

As a result, these first six connections are not often used for quarter-turn valves.

The next two straight-centered-pin connections (G and H) often are used as they are tight fitting (little or no hysteresis) and are suitable for mass production. Like the E and F centered-pin connections, these joints are susceptible to high shaft stress concentration factors. The relative size of the pin to the shaft should be - based on the relative strengths of the three materials (shaft, closure member and pin).

The taper pin of the type G connection makes this assembly non-interchangeable, as the shaft and closure member must be match-taper-reamed for the taper pin. Therefore, the shaft, closure member and taper pin(s) must all be replaced or repaired together and considered a single, matched set rather than three separate components. The tapered pin is often held in place with a secondary pin, tack weld, set screw, nut or threaded fastener, which keeps it from loosening in service.

The centered rivet pin of the type H connection is formed with the pin hole in the shaft being slightly larger than the diameter of the pin while the pin hole in the closure member is slightly greater than the pin hole in the shaft. When assembled in the valve body the entire valve assembly is placed in a press and the pin is loaded and deformed. As the pin is loaded axially in the press above the yield point of the pin, it increases in diameter to fill the pin holes in both the shaft and closure member. This connection is tight, does not have any hysteresis and is suitable for mass production. The drawback to this structure is that it cannot be disassembled without destroying the pin. Also, reassembly requires the use of a large pin press. As with all centered pin connections (E through H), this connection reduces the strength of the shaft significantly and the size of the pin must be balanced with the shaft diameter based on the relative strengths of the shaft, the closure member and the pin materials to optimize the joint strength. Finally, if coupled with brittle shaft or closure member materials (e.g., cast iron) it is easy to induce fractures.

The connections I through K employ pins that are located tangentially to the shaft. This connection removes less than half of the shaft material necessary for a centered pin and tremendously reduces the stress concentration factors. Although this arrangement creates stress concentrations in the shaft and closure member, the magnitude of the concentration is much lower. In this case, stress concentration factors in the shaft are only about 1.35 to 2.1 for pin diameters from 20% to as high as 58% of the shaft diameter. This means that the joint strength is increased, the pin to shaft diameter ratio is not as critical and stronger (larger) pins can be used without reducing the shaft strength.

The type I, straight tangential pin connection still has issues with hysteresis and mass production as with the other straight pin designs, but the types J and K (tangential taper pin and tangential taper wedge pin) are tight fitting connections.

The reason tangential wedge pins cause much lower shaft stress concentration factors than centered taper pins can easily be seen by comparing the amount of shaft area lost in both the installations. As displayed in Figure 2, the taper pin installation reduces the shaft cross-sectional area by 31.5% while the tangential wedge pin only reduces the cross-sectional area by 7.2%. Additionally, the diameter of the wedge pin can be increased for greater contact area with the shaft without changing the shaft cross-sectional area.

figure 2

DESIGN SELECTION

In the design of any product, there typically are many opposing requirements and traits that can make selection of a specific element unclear. When confronted with such a task, it is best to select several important design characteristics and rate the alternative designs on a three-level ranking system (e.g., poor, moderate and good). For this study, the following five design characteristics were selected:

  1. Suitability for high production techniques
  2. Minimum joint hysteresis
  3. Part interchangeability
  4. Low stress concentration factors
  5. Ease of field repair

The following table shows ratings for each design against the five characteristics.

table 1

figure 3

RESULTS

The tangential wedge pin (Figure 3), scored the highest in all of the characteristics and was selected as being the most desirable and practical design for the critical valve shaft-to-closure member connection for quarter-turn valves.


This email address is being protected from spambots. You need JavaScript enabled to view it. is engineering project manager at Val-Matic Valve. 


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