Periodic replacement of the rotation/drive belt on PG7000 series piston gauges is recommended. They can last 10+ years but old belt might result in improper engagement of the rotation system. Do not clean the belt with solvents as this is known to potentially decrease their lifespan significantly in some cases.
If you hear the rotation system trying to engage during normal use (should hear it "clicking") but the masses do not spin up this suggest the belts could have broke and need to be replaced. There are two belts that are stretched across three pulleys that are engaged and disengaged from the bottom of the mass bell as part of the rotation system.
To replace the belts remove all masses and the mass bell. Remove only the three outer screws on the metal pulley plate located directly around the piston mounting post as shown below in green. DO NOT REMOVE THE INNER SCREWS. There are slight differences in different PG7000 models but the process is the same:
Lift the plate up by hand or use something with a 90 degree leg (ex. hex/allen wrench) that can be inserted in the gap between the mounting post and carefully pull up on the plate. The plate will likely have some resistance as one of the pulleys is engaged with a drive system held in place by o-ring tension on its drive pins so might need to pull up near the side that is resisting removal:
Flip the pulley plate over and will see the three notched belt pulleys:
Spare belts are provide with each new system. Simply stretch the o-ring/belt across the outside edge of all three pulleys. One belt goes in the lower grooves and the other in the upper grooves.
When reinstalling the pulley plate you will need to rotate the pulley that has the two holes in it so that it so that it is orientated to line of with the drive motor pins. The plate wont sit flat until this is accomplished so should be easy to tell when the pulley sits over the drive pins properly. In some cases the o-rings provide enough tension that you will have to slightly press down on the plate to get it to seat over the o-ring but might also find the o-ring provide little to no obvious resistance:
Reinstall the three pulley plate screws and test that the rotation system engages properly when floating the piston. To do this it might be easiest to manually apply the minimum pressure for the installed piston (can see this when press the PC button) with just the piston and mass bell installed and manually float it. Can then hold down the rotation button to verify the rotation system engages and rotates the mass bell.