Recently I was asked about safe torque levels when using electrically, pneumatically, or hydraulically powered screwdrivers or wrenches. These tools are often used in assembly jobs in the manufacturing industry.
Basically, torque is a measure of the turning force on an object. A person holds the tool in place while the tool delivers a specified amount of force, measured in English units, inch-pounds (Newton-meters [nM] in the metric world). As the tool delivers the force, the body braces against the force. When the specified force is reached, the machine stops abruptly. It is this jerking reaction force that causes the problem – over time this repeated force can cause musculoskeletal disorders (MSD). How much force, torque in this case, can a person safely handle? The amount of torque force that a person can tolerate over the course of day varies greatly. Overall, strength, age, sex, posture, grip size and type are all factors that determine tolerance to torque forces.
For healthy adults, we know the range of the maximum voluntary contraction (MVC), the measure of strength for this type of force. But that tells us only the maximum a person can generate. This is not a good indicator for someone repeatedly doing this type of work. For that, we need to modify the MVC with a percentage. 14% of MVC is used for intermittent static contractions and 8% for continuous static contractions over the course of day. So doing the math, I calculate that for 95% of women, the range is 6.7 inch-pounds to 14.6 inch-pounds, with 10.66 inch-pounds being the average. For 95% of men, the range is 13.6 inch-pounds to 21.3 inch-pounds, with 17.6 inch-pounds being the average.
What do you do if the torque tool generates more force than a person can comfortably handle over the course of the day? There are two approaches: engineering controls and administrative controls. Engineering controls should be the first line of defense. Here are a few options:
- Reaction arm for conventional tool: When a torque tool reaches its specified force, it abruptly stops. A reaction arm transmits the force to the frame rather than the human body. It is interesting that the industry recommends torque reaction arms for forces greater than 12 pounds; this is a pretty good estimate for males. For women, I recommend using these torque reaction arms for forces greater than 10 inch-pounds. There are many on the market, here is an example:
- Pulse tools: These tools apply the force by pulsing, and are very quiet and do not require a reaction arm. However, they are more expensive upfront and require more maintenance. In the long run, they may be cost-effective depending upon how they are used.
- Remember that posture matters. I advise the working surface should be set so the operator can be in an upright position, with good head posture.
- The grip should fit comfortably in the hand, and there should be no awkward angles of wrists and hands.
- Lighting should be adequate to do the job. Poor lighting can result in poor posture as people crane their necks to see better. However, overly bright environments can lead to eye fatigue.
- Limiting exposure is an administrative control that should be considered. Job rotation is a good strategy for limiting exposure. As an example of job rotation, a person would alternate between torque tool and non-torque tool tasks every two hours.
Torque tools are great in a manufacturing environment. With focus on engineering and administrative controls, they can be safe tools too. For more information, check out this torque tool resource from EHS Today, and hand tool safety article from the Canadian Centre for Occupational Health and Safety.