X-Ray Machine Snubbed by Air Dashpots
“If someone has already worked out all the problems in a particular component and it is now available off the shelf at a reasonable cost, why spend time designing it all over again? Why absorb the cost of setting up to manufacture it yourself? “These questions arose when the members of the engineering team at Phillips Medical System of Shelton CT were designing a radiographic / fluoroscopic (R/F) system. Their subsequent decision to buy rather than build, the necessary part reaffirmed the validity of the basic axiom: “Don’t reinvent the wheel.”
The component in question, a snubber, was purchased from Airpot Corporation. As a matter of fact, not one but six of the model 160 snubbers were installed on each machine; each snubber in conjunction with a limit switch. Together they govern the travel of three trays as they are shuttled back and forth in the film plane to provide just right framing of the X-Ray photo. Called the ECG trays- Esophagus, Cone, and Grid- they must move quickly from side to side to cover all or part of the X-ray film, as desired and to provide the proper scatter protection. Travel of the trays is provided by toothed belts, driven by miniature motors. Because of their small mass, these motors provide no drag when turned off. Therefore, most of the kinetic energy (3/4 ounce-inch) developed by the trays, travelling at 2 ½ ips, must somehow be absorbed at the end of their travel, just before they strike the small limit switches too hard and damage them The snubbers accomplish this energy dissipation. These dashpots have often proven their usefulness where vibration must damped out, velocity must be controlled , shock must be absorbed, in short, whenever mechanical motion must be precise, free to respond yet under total control and undisturbed by spurious inputs. Because of their precision construction and compatible materials, these dashpots offer multimillion cycle capabilities. A particular advantage of the typical pneumatic dashpot is that it does no depend on liquids for its action, thus requiring no seals that can dry out or begin to leak after a while. Air is the damping medium, whose supply is plentiful, and whose infinite heat sink capability instantly dissipates the heat of compression generated by the dashpots action. Another benefit accruing from specifying this type of dashpot is that starting and running friction are almost identical. There are no humps to overcome, no bouncing, no vibratory forces introduced by the dashpot itself. Friction between the carefully matched glass cylinder and graphite-carbon piston is so low, that galling is avoided and no lubrication is ever necessary.
The snubbers in the Phillips system are a standard item, except for the length of the cylinder, which was determined by the application. Extending from the piston is a semi-flexible polyurethane rod which helps to absorb some of the impact and bends sufficiently to accommodate any slight misalignment between the tray and the snubber mounting.
Although the engineering department had originally considered designing its own special snubber to slow down the ECG trays, this approach was abandoned when it was found that off-the-shelf dashpots would be able to handle the task. The designer of the ECG trays, commented the following about the basic concept that was used. “When you’re out to build a new piece of equipment, unhampered by subliminal influences of the earlier models, you have a wonderful opportunity to try new ideas. In your eagerness to create something ‘brand new’, however, you should not ignore the time tested, successful results of others. The Airpot dashpots are a typical case of ‘something borrowed that has worked to our advantage.”
Airpot Corporation manufactures instrument quality, precision Air Dashpots, Snubbers, Pneumatic Actuators, Airpel Anti-Stiction Air Cylinders and -AB Air Bearing Actuators. Airpot welcomes custom requirements and works with customers to adapt products to suit their individual application needs. Value added sub-assembly services are also a specialty. Airpot Corporation is a trusted supplier to many of the world’s most prestigious original equipment manufacturers. Visit www.airpot.com for detailed specifications. Call or email our technical staff for application assistance and ordering information. email@example.com 203-846-2021, 800-848-7681.
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