Girder trolley

A system comprising:

• a door frame comprising a first beam about parallel to a second beam, the first beam and the second beam about parallel to a longitudinal axis;
• a first cross-beam connecting the first beam and the second beam, wherein a first hole having a first radius is disposed in the first cross-beam;
• a device comprising:
• a shaft having the longitudinal axis, a first end, a second end, and a second radius greater than the first radius;
• a connector attached to the first end of the shaft;
• a rod connected to the shaft, wherein:
• the rod comprises a third end, a fourth end and a third radius less than the first radius,
• the third end is connected to the second end of the shaft, and
• the rod is about parallel to the longitudinal axis;
• a spring surrounding the rod, wherein the spring is compressible or extendable along the longitudinal axis;
• and
• a stop connected to the rod at a position distal to the spring, wherein the stop resists longitudinal compression or extension of the spring;
• a girder connected to the connector, wherein:
• the rod is disposed through the hole,
• the spring is disposed between the cross-beam and the stop, and
• extension of the spring is constrained by the crossbeam;
• and
• a second cross-beam connecting the first beam and the second beam, wherein:
• the second cross-beam is about parallel to the first cross-beam,
• a second hole having a fifth radius is disposed in the second cross-beam,
• the fifth radius is greater than the second radius of the shaft,
• the shaft is disposed through the second hole, and
• the shaft is slidable through the hole during compression or extension of the spring.