The overall goal of this procedure is to build a spring-transport mechanism for the transport of spinning rotor gauges. The design presented here has been tested for robustness and provides the user with better long-term stability of the calibration factor. Spinning rotor gauges or SRGs are often used as transfer standards but damage to the rotor's surface received during transport can change their calibration factor also known as the accommodation coefficient.
The advantage of using the spring-transport mechanism is that the rotor is immobilized and under vacuum during transport thus minimizing the possibility of changing the rotor's surface. Spring transport mechanisms have been used by a few national metallurgy institutes for more than 20 years but this example has been known to fail and the design today has remained unpublished. We realized that it's important both to build a robust spring-transport mechanism and to properly package it in a way that minimizes shock during transport.
The text protocol reviews several precursor steps to preparing the spring-transport mechanism which include fabricating the parts for the mechanism and cleaning out the vacuum components. For the assembly, wear nitrile, vinyl, or latex gloves and never accidentally handle the vacuum parts bare handed. Several tools are needed.
Be sure they are cleaned with ethanol or follow the procedure as described for vacuum parts. Additional parts include the valve, thimble, ball bearing, 18-millimeter 8-32 studs, thread adapters, nuts and lock washers, three-millimeter Allen wrench, and masking tape. With everything in order, proceed with the assembly.
Begin the assembly by closing the right angle valve. Looking through the valve ports, identify the M6 stud centered on the valve seat. Then, screw the adapter snugly onto the M6 stud.
Use a UHB clean wrench or pliers to get the adapter as snug as possible. Next, insert the 8-32 stud into the other end of the adapter and tighten it as snugly as possible. The 8-32 stud should extend through the adapter and push against the M6 stud.
By making certain that the 8-32 stud pushes firmly against the M6 stud on the valve seat, we ensure that the adapter will be held firmly in place. Place a lock washer on the 8-32 stud and tighten a nut onto it using a clean socket driver or pliers. Next, screw a second nut onto the 8-32 stud until it nearly touches the first.
Then, place a lock washer on the 8-32 stud by the standoff followed by the spring ball holder and tighten the holder until it compresses the lock washer. Now, check the length adjustment of the spring assembly. First, place the rotor in the custom thimble.
Then attach a magnet to the custom thimble exterior and fix it in place with masking tape. Next, slide the thimble over the spring ball holder until the ball holder touches the ball and measure the gap between the valve flange and the thimble flange. If the gap is in the range of two to six millimeters it is acceptable.
Otherwise rotate the standoff to adjust the gap. Three millimeters is ideal. Making certain the gap is within two to six-millimeter range ensures the mechanism will function as designed and will hold the bowl with sufficient force.
Next, remove the thimble flange assembly and set it aside. Then, tighten the nut against the standoff and open the valve. Continue the assembly by attaching the two tines to the flange.
Secure them with a lock washer and bolt provided by the commercial spinning rotor gauge. Remove the rotor by removing the masking tape and magnet and carefully allowing the rotor to roll out of the thimble. Now, rotate the tines so that they are square with one another and check their straightness by slipping the spinning rotor gauge head over the thimble.
The head should slide on freely. Then, give the two bolts a final tightening and recheck for alignment. Place the rotor back into the thimble and secure it with the magnet and masking tape.
Open the valve by turning it counterclockwise. The next step is to place a copper or silver-plated copper gasket on the valve port. Place the rotor into the thimble if it's not already there.
Now, slide the flange thimble assembly over the spring ball holder. Orient the flange so that the set screw in the tines will point downwards when the valve is attached to the chamber. Finally, using quarter 28 bolts and nuts secure the flange to the valve.
Then, close the valve and remove the magnet and tape holding the ball. This completes the spring-transport mechanism assembly. To use the spring-transport mechanism it must be properly mounted and the rotor must be properly suspended.
This should only be performed by those who know how to use the spinning rotor gauge, the controller electronics, and are familiar with high vacuum technology. The first step is to unpack the spring-transport mechanism from its shipping container. Then, remove the endcap and foil from the open port.
Next, using a new copper or silver-plated copper gasket and a quarter 28 bolt set, attach the open port to a DN40 port on the vacuum chamber. The orientation of the valve will depend on the orientation of the tines. When oriented properly, the tine with the set screw points downward.
Then, slip the head over the thimble and orient the thimble vertically to within two degrees of accuracy. Now, evacuate the vacuum chamber to less than one one-thousandth pascal. Then, slowly open the spring-transport mechanism valve.
Finally, attach the head to the controller, turn on the controller, and suspend the ball. After assembling the spring-transport mechanism, it was packed in foam inserts. The shipping container allows for 7.6 centimeters of foam surrounding the mechanism.
Ester foam was packed above and below and PE foam was packed on the sides. In this configuration, the shock experienced by the spring transport mechanism was measured at less than 100 g even when dropped from 152 centimeters. If the foam was compressed during packing, even by a small amount, the shock increased by about 40%In this case, the foam was compressed roughly 1-1/4 centimeter over a span of 22-3/4 centimeters.
After performing over 180 drop tests of one prototype model, a neutron radiograph image was made of the mechanism. The mechanism still worked, the ball remained secure to the thimble with the valve closed. Note that the valve on this prototype is smaller than the one in the protocol and is no longer manufactured.
Once all the parts have been procured, the spring-transport mechanism can be assembled in just a few minutes. It's important to procure the custom thimble given in our design. The commercial thimbles do not have the proper length or interior geometry to completely immobilize the ball.
Machine drawings of our custom parts are available as supplemental material. As described in our manuscript, it is critical to transport the spring-transport mechanism in a shipping container with at least 7.6 centimeters of foam packing surrounding it. This ensures that the rotor will remain immobilized during common shipping mishaps.
