Astronauts aboard the International Space Station spent considerable time over three days recently doing major in-flight maintenance (IFM) of the important and heavily-used Treadmill with Vibration Isolation and Stabilization (TVIS).

ISS astronaut Suni Williams bungee tethered to the TVIS treadmill

The TVIS needed all 50 roller bearings replaced, a new tread belt, 8 new belt slats, a new flywheel case, new electronics, and damage repair. Proper maintenance of the unit is critical, not just for astronaut health, but also because the Vibration Isolation system is necessary to use it without upsetting ISS instruments and microgravity experiments.

Performing any kind of mechanical maintenance in a weightless environment is challenging, but the number of bearings to replace and the extent of the repairs combine to make TVIS service far more difficult and time-consuming than on earth.

In order to help counter the damaging effects of prolonged weightlessness -- particularly loss of bone density -- each astronaut engages in just over an hour of physical training every day. The TVIS treadmill is used along with resistance exercises, making it a critical piece of equipment, in heavy use almost 5 hours every day.

Harnessed to the device with a bungee cord arrangement, astronauts experience approximately 60% of their earth weight on the TVIS.

In August 2009, astronauts are scheduled to receive a new treadmill to replaced the troubled TVIS -- the COLBERT (Combined Operational Load Bearing External Resistance Treadmill).

For cost control and ease of maintenance, COLBERT is based on a heavy-duty standard treadmill, but with major changes to its supporting structure to better isolate the unit and its vibrations from other ISS instruments and experiments.

Although the COLBERT's in-service design life is 150,000 miles, NASA said it expects the unit to see no more than 40,000 miles of use in its lifetime. During its service life, the bearings should need only to be regreased, not replaced.