Knob Twist Fixture


Longmate, Arthur R., 1996, Johnson & Johnson, Ergonomic Control Measures in the Health Care Industry, Occupational Ergonomics.

Task Prior to Abatement (Description)

Workers had to adjust the knob at the end of instruments in assembling intestinal staplers. Some of the adjustments required less than one complete knob rotation while other required extensive twisting, up to 10 twists, occasionally in both directions. In one case, the worker had to forcefully twist the knob loose to break weld adhesions which required torque up to 40 in-lb.

Task Prior to Abatement (Method Which Verified Hazard)

OSHA log revealed that about 67% of these injuries were related to different types of elbow tendinitis.

Task Prior to Abatement (Method Which Identified Hazard)

Increasing CTD incidence rate up to 2-3 times the overall plant average.

Ergonomic Risk Factor (Force)

The majority of the twisting required negligible torque, except one case which required substantial torque – up to 40 in-lb.

Ergonomic Risk Factor (Repetition)

The task was highly repetitive, sometimes over 10,000, 90-110 degree forearm rotations per 8 hour work shift.

Ergonomic Solution (Engineering Controls)

  1. An air-powered knob twist device was developed to eliminate manual twisting.
  2. A more sophisticated fixture with the following features was designed and built :
  3. Lateral movement of about 2-3 inches was provided for inserting the instrument.
  4. The fixture was designed to be extremely narrow in order to require minimum prime working space.
  5. Selectable number of turns to the nearest quarter turn.
  6. Selectable auto reverse feature
  7. Adjustable speed control
  8. Adjustable clutch to prevent over-torquing

Ergonomic Solution (Benefits)

  • All workers that perform this task now have reduced exposure to elbow tendinitis.
  • Workers with pre-existing conditions could work on many jobs which were previously restricted due to repetitive twisting.
  • Automatic twisting of the instrument in order to pre-stress it prior to final calibration.
  • Less physical stress to perform the task
  • Four fold increase in productivity in performing the task.
  • 10-15 % increase in productivity on most other jobs by using this new fixture.

Ergonomic Solution (Method Which Verified Effectiveness)

Within 6 months, new elbow trauma incidence cases were eliminated (95% reduction).


A 300 instrument fixture validation study was conducted to ensure the functionality of the instrument using the new fixtures. Another validation study was conducted to ensure that the torque achieved by the fixture was within process limits.