31cm Convolute Lift
Process
Note: Information here is intentionally vague due to the confidential nature of much work at Sandia National Labs.
Background:
The Z-machine is the world’s most powerful pulsed power facility. It compresses energy in space and time to create and study extreme environments in support of the country’s nuclear weapon stockpile. One of the ways this is done is through the study of special nuclear material (SNM) under extreme conditions. When using SNM a lid is installed to seal off the center section where the experiment happens, but this creates difficulties in accessing hardware such as the convolutes. Occasionally, the convolutes may need to be removed to be tested for nuclear contamination, but with plans to use larger and heavier convolutes, there is no way to do that with existing equipment. There is also an edge case in which the convolutes may be welded together, further increasing the total weight.
Given the background, a couple of design requirements were immediately obvious:
- Capable of lifting 250lbs with a minimum FOS of 3 on yield and 5 on buckling
- Able to reach 85" and lower to 45"
- Must be easy to disassemble and reassemble within the containment chamber.
- Must mount to the SNM lid and hold vacuum
- Can be easily operated in confined space
Driving Mechanism:
The mechanism is based around a multi-start thread that will be used to drive the weight up and down. The gearbox that drives the thread relies on a worm gear to transform rotational motion into linear motion. This should allow technicians to drive the lift with nothing but a gear, and still lift 250lbs. The entire mechanism is sandwiched between two aluminum plates, and it is the pressure of those plates which keeps the gears properly aligned. The worm gear is supported by two pillow blocks that are located on the top aluminum plate with pins. The worm wheel that interacts with the worm gear is squeezed between the two plates with thrust bearings and an O-ring to help with tolerances. That worm wheel then threads into a multi-start nut that interacts with the all-thread to push it up and down.
The CAD software used was Solidworks, and the design process took approximately 8 weeks. Many other options were explored, including a commercial off the shelf (COTS) lift and different mechanisms to solve the same problem. This was the ideal solution due to it's simplicity and
Confirming the Safety Requirements:
FEA using Abaqus was employed to evaluate the mechanical strength of the system. Several scenarios were simulated, including the convolutes slipping off the supporting plate, three convolutes being welded together, and a tangential force being applied from user error. The lowest FOS was 4 calculated under the extremely unlikely scenario in which 3 convolutes are welded together and they are misaligned on the plate all the way to the left. In this scenario there would be a 250lb force pushing down the thread while simultaneously applying a bending moment to it. Even in that scenario the lift would be capable of supporting 1000lbs.
Result
The lift has been manufactured and currently stands fully functional in the Z-machine facility at Sandia National Labs, ready to be used if the need ever presents itself. It has been verified to be capable of lifting the necessary weight with no visible deformation or unsteadiness, and can be disassembled and reassembled by two technicians rather quickly.
