Barrett Hand BH8-280/282 Finger Cable Pretension
The third joint in each finger is driven by a brushless servomotor through opposing stainless steel cables that act like tendons transmitting torque from a pulley at the base of the finger out to a pulley at the fingertip joint. If you have purchased the joint-torque sensor option, the difference in tension between the tendon pair is used to determine the torque at the third joint. The fact that we measure the tensions differentially reduces the effect of actual pretension in the cable as long as the cable is not actually loose.
Under normal circumstances, the cables remain pretensioned indefinitely. But under heavy use over thousands of cycles, it can begin to relieve its pretension. You can easily readjust the pretension through Barrett Technology's patented cable tensioning mechanism as follows:
- Loosen the hex set screw with the right angle hex wrench provided in the maintenance kit. This screw is located adjacent to the termination of the joint 3 cable on each of the fingers as seen in Figure 11.
Figure 11 - Hex set screw
- Apply between 6oz-in and 8oz-in of clockwise torque to the tensioner screw located on the back of each Joint 3 housing as seen in Figure 12. A 2-mm hex torque wrench is provided in the maintenance kit for this purpose.
- CAUTION:
- The tendon is properly tensioned when all loose slack has been removed and you can feel the direct connection of the fingertip to its drive gears. DO NOT OVER-TIGHTEN THE TENDON! The pretensioning mechanism is stronger than the tendon and is capable of snapping it if over-tightened. Excessive pretension will change the frictional properties in the finger drives and may reduce the finger's range of motion.
- Re-tighten the hex set screw until it is snug against the tensioner screw.
- NOTE:
- It is advisable to completely remove the hex set screw to apply Loctite 222 to its threads before re-tightening it against the tensioner screw. This measure is especially important if the hand is under heavy use.
Figure 12 - Pretensioning the Tendon Cable
