strange behavior when create constriant with joint_gear ?

[v-wewei]

Hi, guys.
I meet a strange problem recently. when I create some constraints between two joints for each finger in a robotics hand. Some of the joints show unreasonable behavior also when I click on my hand, the ring (fourth) finger got shaken at the same time, corresponding joint index pair is (16, 17).
the code is shown as blew.

cid_thumb = p.createConstraint(hithandId, 4, hithandId, 5, jointType=p.JOINT_GEAR,
jointAxis=[1, 0, 0],
parentFramePosition=[0, 0, 0],
childFramePosition=[0, 0, 0],
)
p.changeConstraint(cid_thumb, gearRatio=-1.0, erp=2.0, maxForce=1500)

cid_fore = p.createConstraint(hithandId, 8, hithandId, 9, jointType=p.JOINT_GEAR,
jointAxis=[1, 0, 0],
parentFramePosition=[0, 0, 0],
childFramePosition=[0, 0, 0],
)
p.changeConstraint(cid_fore, gearRatio=-1.0, erp=2.0, maxForce=1500)

cid_middle = p.createConstraint(hithandId, 12, hithandId, 13, jointType=p.JOINT_GEAR,
jointAxis=[1, 0, 0],
parentFramePosition=[0, 0, 0],
childFramePosition=[0, 0, 0],
)
p.changeConstraint(cid_middle, gearRatio=-1.0, erp=2.0, maxForce=1500)

cid_ring = p.createConstraint(hithandId, 16, hithandId, 17, jointType=p.JOINT_GEAR,
jointAxis=[1, 0, 0],
parentFramePosition=[0, 0, 0],
childFramePosition=[0, 0, 0],
)
p.changeConstraint(cid_ring, gearRatio=-1.0, erp=2.0, maxForce=1500)

cid_little = p.createConstraint(hithandId, 20, hithandId, 21, jointType=p.JOINT_GEAR,
jointAxis=[1, 0, 0],
parentFramePosition=[0, 0, 0],
childFramePosition=[0, 0, 0],
)
p.changeConstraint(cid_little, gearRatio=-1.0, erp=2.0, maxForce=1500)